Structural, electrical, and optical properties of \(Ba1

This article was downloaded by: [Birla Tech Inst ] On: 13 March 2014, At: 20:20 Publisher: Taylor & Francis Informa Ltd Registered in England and Wales Registered Number: 1072954 Registered office: Mortimer House, 37-41 Mortimer Street, London W1T 3JH, UK

Phase Transitions: A Multinational Journal Publication details, including instructions for authors and subscription information: http://www.tandfonline.com/loi/gpht20

Structural, electrical, and optical properties of (Ba1-xNd2x/3)TiO3 ceramics a

a

a

b

b

M. Ganguly , S.K. Rout , P.K. Barhai , C.W. Ahn & I.W. Kim a

Department of Applied Physics, Birla Institute of Technology, Mesra, Ranchi 835215, India b

Department of Physics and Energy Harvest-Storage Research Center, University of Ulsan, Ulsan 680-749, Republic of Korea Published online: 08 Jun 2013.

To cite this article: M. Ganguly, S.K. Rout, P.K. Barhai, C.W. Ahn & I.W. Kim (2014) Structural, electrical, and optical properties of (Ba1-xNd2x/3)TiO3 ceramics, Phase Transitions: A Multinational Journal, 87:2, 157-174, DOI: 10.1080/01411594.2013.798411 To link to this article: http://dx.doi.org/10.1080/01411594.2013.798411

PLEASE SCROLL DOWN FOR ARTICLE Taylor & Francis makes every effort to ensure the accuracy of all the information (the “Content”) contained in the publications on our platform. However, Taylor & Francis, our agents, and our licensors make no representations or warranties whatsoever as to the accuracy, completeness, or suitability for any purpose of the Content. Any opinions and views expressed in this publication are the opinions and views of the authors, and are not the views of or endorsed by Taylor & Francis. The accuracy of the Content should not be relied upon and should be independently verified with primary sources of information. Taylor and Francis shall not be liable for any losses, actions, claims, proceedings, demands, costs, expenses, damages, and other liabilities whatsoever or howsoever caused arising directly or indirectly in connection with, in relation to or arising out of the use of the Content. This article may be used for research, teaching, and private study purposes. Any substantial or systematic reproduction, redistribution, reselling, loan, sub-licensing, systematic supply, or distribution in any form to anyone is expressly forbidden. Terms &

Downloaded by [Birla Tech Inst ] at 20:20 13 March 2014

Conditions of access and use can be found at http://www.tandfonline.com/page/termsand-conditions

Phase Transitions, 2014 Vol. 87, No. 2, 157–174, http://dx.doi.org/10.1080/01411594.2013.798411

Structural, electrical, and optical properties of (Ba1xNd2x/3)TiO3 ceramics M. Gangulya, S.K. Routa*, P.K. Barhaia, C.W. Ahnb and I.W. Kimb a

Department of Applied Physics, Birla Institute of Technology, Mesra, Ranchi 835215, India; Department of Physics and Energy Harvest-Storage Research Center, University of Ulsan, Ulsan 680-749, Republic of Korea

b


(Received 14 February 2013; final version received 18 April 2013) In this paper, we report the obtention of barium neodymium titanate (Ba1xNd2x/3) TiO3 ceramics with (0.00 x 0.10) by the solid-state reaction method. Structural studies suggested a phase transition from tetragonal to cubic structure with increase in Nd3þ ion content, supported by Rietveld refinement method. Photoluminescence properties revealed that the introduction of Nd3þ ions created structural disorder by means of A-site vacancies and displacement of M–O bond leading to shallow defects. Optical band gap value calculated from ultraviolet–visible spectra decreased with increase in Nd3þ ion concentration. A drastic decrease in grain size of undoped barium titanate was observed with introduction of Nd3þ ions through scanning electron microscopic images. Dielectric properties showed a gradual decrease in the Curie temperature with increase in Nd3þ ion content along with pinching effect. Normal ferroelectric character was obtained within the doping limit. P–E hysteresis loop showed a decrease in remnant polarization and coercive field. However, the composition x ¼ 0.08 and 0.10 showed paraelectric behavior. Keywords: barium titanate; neodymium; Rietveld; dielectric; photoluminescence

1. Introduction Perovskite-type oxides like barium titanate (BT; BaTiO3) has gained renewed interests in the past few decades due to its application in multilayer ceramic capacitors, electromechanical system, electro-optical system, pyroelectric detectors, piezoelectric actuators, micro electro mechanical system, ferroelectric random access memory devices, etc.[1] A significant modification in the properties of this material can be made through rare earth doping, which has again attracted immense attention. They are frequently added to control the temperature coefficient of capacitance, grain growth, insulating electrical resistance, and reliability.[2–6] The largest applications are as positive temperature coefficient of resistance material and dielectric material in multilayer ceramics capacitor, though pure BT is an insulator and no positive temperature coefficient of resistance effect can be observed.[7–9] Since the ionic radii of trivalent rare earths (R3þ ) range from 0.8 to 1.3 A, these dopants could occupy either Ba2þ (1.6 A) or Ti4þ (0.6 A) ionic sites, depending on the ionic radius and Ba/Ti ratio.[4] The microstructural development and electrical properties are strongly dependent on the site preference of rare earth impurity including neodymium (Nd) in BaTiO3 lattice.[3–6] As the ionic radius of rare earth element increases, the behavior is increasingly donor type, suggesting an increasing fractional occupation of Ba site. For the larger ions such as Nd3þ ions, the donor behavior is more pronounced in *Corresponding author. Email: [email protected] Ó 2013 Taylor & Francis

158

M. Ganguly et al.

the samples with an excess of TiO2. The cations of intermediate size seem to occupy both A and B sites to varying degrees, depending on their radii.[6] A detailed work on few rare earth substituted BaTiO3 compounds has been done by Narang et al. [10] and Nd3þ ions substituted BaTiO3 compounds showed the most remarkable dielectric properties among all the rare earth ones. The compounds were also reported to be possibly used for microwave applications. A vast study on only Nd3þ ions doped BaTiO3 has been carried by Qizhen et al. [11] through sol–gel method. Enhanced microstructural and dielectric properties could be obtained, but very few detailed reports are available in Nd3þ ions doped BaTiO3 through charge-compensated mechanism that generates A-site-deficient compounds. This paper offers an account of the structural, dielectric as well as optical behavior of such A-site-deficient Nd-doped BaTiO3 compounds. Nd3þ ions are incorporated at the A site, where it behaves as a donor according to the equation by Kr€oger–Vink:


BaTiO3

Nd2 O3 ! 2NdBa þ V00Ba þ 3Oxo

ð1Þ

Creation of oxygen vacancies is also responsible for semiconductive behavior according to the following equation [12,13]: ½BaO11 VxO þ ½BaO12 x !½BaO11 VO þ ½BaO12 0

ð2Þ

0 ½BaO11 VO þ ½BaO12 x !½BaO11 V O þ ½BaO12 ½NdO7 x þ ½NdO6 VxO !½NdO7 0 þ ½NdO6 VO

ð3Þ

½NdO7 þ ½NdO6 VO !½NdO7 0 þ ½NdO6 V O x 0 x ½TiO6 þ ½TiO5 VO !½TiO6 þ ½TiO5 VO ½TiO6 x þ ½TiO5 VO !½TiO6 0 þ ½TiO5 V O x

ð4Þ ð5Þ ð6Þ ð7Þ

Since the substitution at the A site is off valent (larger radii Ba2þ ion being substituted by smaller radii Nd3þ ion), so to obtain charge neutrality vacancy is created. Equation (1) implies that for every two rare earth cation substitution at the A site, three alkaline cations get replaced creating one positively double charge vacancy, provided the charges are to be taken to the perfect lattice. The number of this vacancy increases with increase in doping concentration.[14] Here, Nd-doped BaTiO3 ceramics have been prepared with doping concentration ranging from 0% to 10% according to the following equation: BaTiO3

1 xBaCO3ðsÞ þ TiO2 ðsÞ þ 2x=3Nd2O3 ðsÞ ! ðBa1x Nd2x=3 ÞTiO3ðsÞ

ð8Þ

þ CO2ðgÞ ð0:00 x 0:10Þ Excluding the composition x ¼ 0.00, all the other compositions were nonstoichiometric, exhibiting A-site-deficient perovskite-type solid solutions according to the following structural formula: Ba1x Nd2x=3 &x=3 TiO3 where & denotes A-site vacancy in the perovskite structure.[15] This paper aims to shed some light on all such aspects of Nd-doped BaTiO3 compounds that are rarely available, facilitating further research.

Phase Transitions

159


2. Experimental details (Ba1xNd2x/3)TiO3 (x ¼ 0.00, 0.02, 0.04, 0.06, 0.08, 0.10) ceramics were prepared through solid-state reaction technique from reagents BaCO3 (99% Pure, Merck India Ltd), TiO2 (99% Pure, Merck India Ltd) and Nd2O3 (99.99% Pure, Sigma-Aldrich, USA). Powders were mixed in an appropriate amount and grinded with distilled water in an agate mortar. The homogeneous powder was milled in a FRITSCH ‘Pulverisette 5’ planetary mill for 10 h with zirconium balls (5 mm diameter) and then heated at 1200 C for 12 h. Final calcination was done at 1400 C for 4 h. Structural characterization was done by x-ray diffraction study (XRD) using CuKa radiation from 15 to 80 with a step size of 0.02 and scanning rate of 1 per minute. The Fourier transform infrared (FTIR) spectra were recorded at room temperature by the standard KBr pellet technique in a Perkin Elmer Fourier Transform Infra Red Spectrophotometer (Spectrum 1000), Japan. Raman spectroscopic investigations followed thereafter (Seki Technotron with excitation 514 nm). Photoluminescence (PL) study was done using a Hitachi Fluorescence Spectrophotometer (model-F2500) to study the luminescence efficiency of the doped ceramics. Ultraviolet–visible (UV–vis) spectra were done to calculate the band gap using a Cary 5G (Varian, USA) spectrophotometer. Calcined powders were pressed uniaxially under pressure 60 kg/cm2 to form diskshaped pellets of 10 mm diameter and 2 mm thickness with 5% polyvinyl alcohol as the binder. Sintering of the pellets was done at 1400 C for 10 h for x ¼ 0.00, while 1430 C for 4 h for x ¼ 0.02–0.10. To obtain properly optimized dense pellets, this difference in the sintering temperature had to be maintained. Bulk densities calculated according to Archimedes’ principle were found to be greater than 95% of their respective theoretical values. Microstructural study was done using a scanning electron microscope (JSM-6390LV). Dielectric study of the sintered and densified pellets was done over a wide temperature range from 15 to 573 K by HP4294A system. A Sawyer–Tower circuit at room temperature measured the ferroelectric P–E hysteresis loops.

3. Result and discussion Figure 1 shows the XRD pattern of all the compositions of Ba1xNd2x/3TiO3 ceramic powders calcined at 1400 C for 4 h. The diffraction pattern for all such composition for the peak (002), (200) within the 2u range of 44o to 46o is shown for clarity. The presence of (002), (200) peak confirms the phase for the compositions x ¼ 0.00–0.06 to be tetragonal structure similar to that of undoped BaTiO3 while a cubic phase for x 0.08 is detected at room temperature. Cubic phase for Nd-doped BaTiO3 has been reported by Qizhen et al.[11] Shifting of peaks toward higher angle is noticed with increase in the content of Nd3þ ions. Hence, there is a decrease in the lattice parameter with increase in doping concentration. The radius of Ba2þ ion is 1.6 A for coordination number 12, while that of Nd3þ ion is 1.27 A for coordination number 7. Hence, it is obvious that substitution of lower radii Nd at the higher radii Ba site results in decrease in cell parameters and hence in cell size. Structural refinement was carried out for the compositions x ¼ 0.00, 0.04, and 0.10 using the Rietveld refinement program ‘Full Prof’ and the final output is shown in Figure 2 (a)–(c). The initial parameters are taken from the standard Wycoff position Table 1. The refinement produced satisfactory agreement factors and lattice parameters, which are listed in Table 1.


160

M. Ganguly et al.

Figure 1. Room temperature XRD patterns of Ba1xLa2x/3TiO3 powders along with shifting and merging of peak of Ba1xNd2x/3TiO3 with increase in doping concentration.

FTIR technique was employed to study the influence of additives in the ceramics. As the ferroelectricity in perovskite ferroelectric materials depends greatly on the vibration of the crystal lattice, so FTIR is an effective technique to study the correlation between physical properties and microstructure of such materials.[16] This is due to antisymmetric stretching vibrations of metal–oxygen bonds. The FTIR spectrum of all the compositions at room temperature is given in Figure 3, while the inset shows shift of Ti–OI absorption peak. A strong absorption peak for pure BaTiO3 (x ¼ 0.00) is obtained at 524.64 cm1. This corresponds to the stretching normal vibration of Ti–OI octahedron. [17–19] This mode is very important because the direction of stretching normal vibration is along with that of spontaneous polarization in BaTiO3 with tetragonal phase.[17] Absorption peaks for the same mode for compositions x ¼ 0.02, 0.04, 0.06, 0.08, and 0.10 are obtained at 526.37, 528.49, 530.42, 532.35, and 538.24 cm1, respectively. Thus, a shift toward higher energy is observed. Since the radius of Nd3þ ion is smaller than Ba2þ ion, it is expected that when it replaces Ba, cell parameters and hence the cell

Table 1. Results of Rietveld refinement of X-ray diffraction data of Ba1xNd2x/3TiO3 measured at room temperature for the tetragonal region with space group P4mm for x ¼ 0.00, 0.04, while for the cubic region with space group Pm-3m for x ¼ 0.10. Parameters

Lattice parameter (a ¼ b) (A) Lattice parameter (c) (A) Volume (A3) Rp Rwp Rexp Goodness of fit (x2) (Rwp/Rexp)

x ¼ 0.00

x ¼ 0.04

x ¼ 0.10

3.9957 4.0256 64.270 7.95 10.60 7.71 1.90

3.9889 4.0098 63.802 6.72 8.56 7.03 1.48

3.9914 3.9914 63.590 6.54 8.31 7.00 1.41

161


Phase Transitions

Figure 2. Observed ( ), calculated () and residual X-ray powder diffraction pattern of Ba1xNd2x/ 3TiO3 compositions (a) x ¼ 0.00, (b) x ¼ 0.04, and (c) x ¼ 0.10 revealed from Rietveld’s powder structure refinement analysis. Peak positions of the phases are shown at the base line as small marker. Positional parameters for the compositions x ¼ 0.00 and 0.04 (P4mm) are as follows: Ba/ Nd at 1a(0,0,0), Ti at 1b(0.5,0.5,0.524), O1 at 1b(0.5,0.5,0.005), and O2 at 2c(0.5,0,0.427) and for the composition x ¼ 0.10 (Pm-3m) are as follows: Ba/Nd at 1a(0,0,0), Ti at 1b (0.5,0.5,0.5), and O at 3c(0,0.5,0.5).

M. Ganguly et al.


162

Figure 3. FTIR spectra of Ba1xNd2x/3TiO3 at room temperature.

size must decrease. Decrease in cell size shortens the distance between Ti4þ and O2 (ions), enhancing the bond strength. As explained by Jin et al. [20] according to Equation (9), we have BT Ti OV BNT Ti OV

sffiffiffiffiffiffiffiffiffiffiffi rffiffiffiffiffiffiffiffiffiffiffiffisffiffiffiffiffiffiffiffiffiffiffi BT mTiO Ti BT Of Ti O f ¼ ¼ BNT mTiO TiBNT Of Ti O f

ð9Þ

Here, BT ¼ BaTiO3; BNT ¼ Ba1xNd2x/3TiO3. The variation of Ti–O vibration is only related to force constant fTi–O. Smaller radius of Nd3þ ion than that of Ba2þ ion results in the shortening of Ti–O bonds of BNT.

Phase Transitions

163

Table 2. Tolerance factor and shift in absorption peak of Ba1xNd2x/3TiO3 ceramics. Composition


x ¼ 0.00 x ¼ 0.02 x ¼ 0.04 x ¼ 0.06 x ¼ 0.08 x ¼ 0.10

Absorption peak (cm1)

Tolerance factor (t)

524.64 526.37 528.49 530.42 532.35 538.24

1.060 1.055 1.050 1.045 1.039 1.034

Hence, they become stronger than that of BT, so Ti OfBNT > Ti OfBT V and Ti OV BNT > Ti OBT .[21] A theoretical explanation can be given through the tolerance factor (t), which is unity for an ideal perovskites (cubic). The value of this factor can be calculated by using the following equation [19]: RO þ RA t ¼ pffiffiffi 2ðRO þ RB Þ

ð10Þ

For Ba1xNd2x/3TiO3 the tolerance factor is given as t¼

RO þ ð1 xÞRBa2þ þ ð2x=3ÞRNd3þ pffiffiffi 2ðRO þ RTi4þ Þ

ð11Þ

Taking the ionic radii of O2, Ba2þ, Ti4þ, and Nd3þ as 1.4, 1.6, 0.6, and 1.27 A, respectively, the values of tolerance factors obtained from Equation (7) along with respective absorption peak positions are listed in Table 2. Successive decrease in the values of tolerance factor with a tendency toward unity is noticed with increase in Nd3þ ion content. Clearly the tetragonality in the structure decreases with doping as obtained through XRD. Hence, the cell volume decreases and increase in bond energy is observed. Optical modes of cubic phase of BaTiO3 transform according to the 3F1uþ1F2u irreducible representation. The detailed mechanism behind the origin of Raman spectra in tetragonal BaTiO3 and doped cubic BaTiO3 is given in our earlier works.[22] Figure 4 shows the Raman spectra for the series of samples. Mode assignment shows appearance of A1(LO3) mode at 721 cm1, a feature of tetragonal BaTiO3, which arises for phonons propagating along the z-axis.[23] The E(LO) mode also arises simultaneously for phonons propagating in a–b plane. Presence of B1 mode at 305 cm1 along with overlapping of E (TO) and E(LO) is clearly observable up to the composition 0.00 x 0.06. This mode then disappears for the composition x 0.08. This confirms the transition in phase taking place, as observed from XRD study, from tetragonal (space group P4mm) to cubic (Pm-3m).[22–25] A split in the mode at 721 cm1 is observed with Nd substitution (at 721 cm1 and 837 cm1) whose intensity is found to increase with increase in its content. It had been reported that mode frequency of this mode changes in a series of complex perovskites as a function of perovskite unit cell and with changes in ionic radii. Though neither A nor B ions move in this vibration, the mode still reflects changes in the perovskite structure. In this mode only the oxygen ions move. Doping BaTiO3 with Nd results in presence of

M. Ganguly et al.


164

Figure 4. Raman spectra of Ba1xNd2x/3TiO3 at room temperature.

different radii ions at the A site. This results in formation of inequivalent oxygen octahedral that changes the spacing between A and B ions. A change in their bonding also takes place. Thus, mode frequency changes with size of the ion.[26,27] Increase in Nd3þ ion content results in the formation of more inequivalent oxygen octahedral, which may lead to the increase in intensity of the peak at 837 cm1 and broadening of the peak at 721 cm1. Asymmetric, broad, intense modes are obtained at 518 cm1 and centered at 219 cm1 [A1(TO)] are observed. A decrease in the intensity and broadening in the A1(TO3) mode at 518 cm1 is clearly noticeable at x 0.08. The A1(TO2) mode is noticed to shift toward higher energy values along with a broadening effect. Merging of the A1(TO1) and A1(TO2) modes are featured for the composition x 0.08. All these are

Phase Transitions

165


Table 3. Variation in FWHM and peak position of A1 mode of Ba1xNd2x/3TiO3 w.r.t. x at room temperature.

Composition

Peak position of A1(TO3) mode (cm1)

x ¼ 0.00 x ¼ 0.02 x ¼ 0.04 x ¼ 0.06 x ¼ 0.08 x ¼ 0.10

518 520 520 521 512 512

FWHM of A1(TO3) mode (cm1) 55.27 50.24 50.77 51.87 67.85 67.86

Peak position of A1(LO3), E(LO) mode (cm1)

FWHM. of A1(LO3), E(LO) mode (cm1)

721 722, 840 723, 837 723, 837 730, 840 730, 840

23.65 28.73, 14.34 34.18, 14.08 36.83, 11.53 46.37, 9.04 49.44, 9.72

suggestive to decrease in tetragonality with increase in Nd3þ ion content, leading to the phenomenon of transition to cubic phase at the composition x ¼ 0.08. Some antiresonance effect is obtained at around 160 cm1, which may be an interference effect of coupling between sharp A1(TO1 and TO2) modes.[24] The half-width variation and peak positions for A1 and E(LO) modes (after Gaussian fit) with respect to (w.r.t.) composition are given in Table 3. For the A1(TO3) mode, a trend of shifting of peaks toward higher energy side with mild broadening w.r.t. Nd3þ ion content is disrupted at the composition x ¼ 0.08. A pronounced broadening along with shifting in peaks toward lower energy values is observed for x 0.08. This may be due to transition in phase occurring at the respective composition.[26,27] For the doped compositions, though broadening with peak shifting toward higher energy values is observed for the peak at 721 cm1 but not much pronounced shifting is observed for the peak at 840 cm1. Broadening in the modes may be due to introduction of disorder in the lattice of the doped samples due to Nd substitution. As already explained, size of the A-site cation modifies the movement of ‘O’ atom along the B-O-B axis. B-site ordering decreases off-centering the Ti4þ ion. This disorder increases with increase in Nd3þ ion content, and hence, the full width at half-maximum (FWHM) of the mode increases. The microstructures of the polished and thermally etched sections of Ba1xNd2x/3TiO3 are shown in Figure 5 (a) and 5(b). Disk/plate like morphology within large-sized grains (50 mm) is obtained for undoped BaTiO3 (x ¼ 0.00). These may be ferroelectric domains of BaTiO3, which are clearly visible at higher resolution. The doped ones are found to be dense with clearly visible grains of much smaller size than that of undoped BaTiO3 and with well-defined grain boundary. With increase in Nd3þ ion content though grain density and average grain size increases from 1 mm at x ¼ 0.02 to 5 mm at x ¼ 0.10, but the homogeneity or regularity in grain shape decreases. Thus, it can be considered that doping Nd in BaTiO3 inhibits grain growth but with increase in its content grain size and density increases successively. Similar behavior has been reported by Qizhen et al.[11] Also the substitution of Nd at the Ba site results in the formation of A-site vacancies. These vacancies have been reported to inhibit grain growth.[28] Microstructural features are mainly governed by transport of matter during heating process. Solid-state synthesis method starts with milling of raw materials, during which a reduction of average particle size occurs. Upon heat treatment, diffusion starts at the contact points of the particles, which over prolonged sintering lead to formation of necks between grains. This is basically an elastic deformation due to the reduction in surface energy at the contact interface. During this period equilibrium is reached between the surface and interfacial tension. The formation of neck favors matter transport at long


166

M. Ganguly et al.

Figure 5. SEM micrographs of (a) BaTiO3 and (b) Ba1xNd2x/3TiO3..

distances. Evolution of larger grains takes place with the smaller ones disappearing and pores shrink increasing the density.[29] Here in the prepared ceramics, with increase in Nd3þ ion content, diffusion increases by increasing matter transport over long distances leading to formation of larger-sized grains. Variation in the kinetics of matter transport over long distances results in irregular shape of the grains at higher Nd3þ ion content. Dielectric behavior as a function of temperature of the sintered pellets of all the compositions at different frequencies is shown in Figure 6. A dramatic decrease in Tc-t (cubic to tetragonal transition temperature) while increase in the Tt-o (tetragonal to orthorhombic) and To-r (orthorhombic to rhombohedral) transition temperatures are observed with increase in Nd3þ ion concentration. At x ¼ 0.06, Tt-o and To-r merges. At x ¼ 0.08 the Tc-t shifts below room temperature. Hence, it can be concluded that undoped BT and doped composition 0.02 x 0.06 have tetragonal symmetry, while compositions x 0.08 have cubic symmetry at room temperature. This is in consistent with the room temperature XRD and Raman data of the ceramics. Frequency-dependent dielectric behavior is not observed. Decrease in the value of dielectric constant is observed with increase in frequency. Higher values of dielectric constant at lower frequency may be due to space charge polarization.[30]

Phase Transitions

167


Decrease in Tc-t with substitution and increase in content of rare earth element in BaTiO3 have been well reported in literature.[30–32] This is basically an indication of decrease in tetragonality. Replacing Ba2þ ions by Nd3þ ions results in A-site deficiency to maintain charge neutrality. This increases with increase in doping content. Also tetragonal unit cell parameters decrease. Shrinkage of unit cell takes place off centering the Ti4þ ion out of the octahedral site. Thus, the coupling between the TiO6 octahedra weakens. This results in a strong decrease in Tc-t (Tc-t is directly related to the displacement of cation from the center of the octahedral site to its position in the polar phase). [17,22,30–32] Pinching effect is observed in the ceramics, where Tc-t, Tt-o, and To-r transition temperatures move toward each other, with increase in Nd3þ ion concentration. The Tc-t shifts toward lower value, while To-t and To-r move toward higher ones and finally merge to give a broad transition. Such shifting of peaks is observed up to the composition x ¼ 0.06, while for x 0.08, merging of peaks results in broad transition. No trace of any kind of relaxor behavior is observed within the doping limit.

Figure 6. Dielectric constant and dielectric loss as a function of temperature of Ba1xNd2x/3TiO3 at various frequencies.


168

M. Ganguly et al.

Figure 6. (Continued)

The P–E hysteresis loops of pure and doped ceramics at room temperature for all the compositions are shown in Figure 7. These hysteresis loops were measured using a maximum applied electric field of 1 kV/mm at a frequency of 50 Hz. It is noticed that the hysteresis loops approach saturation under such an electric field. The values of the remnant polarizations obtained are 4.82, 1.81, 2.19, and 2.35 mC/cm2, and coercive fields are 0.28, 0.17, 0.16, and 0.11 kV/mm, respectively, for x ¼ 0.00–0.06, while for x ¼ 0.08 and 0.10, both the values dropped to almost zero giving a linear P–E behavior. Clearly, a steady decrease in the ferroelectric behavior of the samples is obtained with increase in amount of Nd3þ ion content. Though a regular decrease in the coercive field (Ec) values with increase in Nd3þ ion content is noticed, but an irregular behavior in the remnant polarization (Pr) is obtained. For compositions x ¼ 0.08 and 0.10, a linear P–E behavior suggests the structure to be normal paraelectric at room temperature. This further supports the fact that these two compositions are cubic at room temperature. No further investigation was done. Existence of ferroelectric loop was the only concern of study. A lower remnant polarization and smaller coercive field had been reported to be due to increased domain pinning by residual vacancies.[32,33] As already explained these prepared ceramics are A-site deficient. Hence, under the applied electric field, it is quite


Phase Transitions

169

Figure 7. P–E hysteresis loop of Ba1xNd2x/3TiO3.

natural that these A-site vacancies (V00 Ba) along with some oxygen vacancies may hop to lower free energy sites such as domain walls and interfaces with electrodes. This generally weakens the defect mobility and contributing to domain pinning that should decrease the remnant polarization and coercive field values. Now, with increase in Nd3þ ion content vacancy increases, as a result an enhanced domain pinning effect should further decrease their values. Though in this case such behavior for coercive field value is noticed, but the remnant polarization value is found to increase for compositions x ¼ 0.02–0.06, and then for x 0.08, it decreases. This may be an effect of the grain size. Remnant polarization increases with increase in grain size. For the compositions x ¼ 0.02–0.06, an increase in grain size is noticed that dominates the decrease ferroelectricity effect, resulting in increase in the remnant polarization values. Beyond this limit, the studied compositions with higher Nd3þ ion content (x ¼ 0.08 and 0.10) are paraelectric; hence, the trend is disrupted, giving almost zero values. It had also been reported that the random electric field around the defects lowers the barrier energy required for domain nucleation, decreasing the coercive field.[34] The PL emission spectra of calcined powders of undoped and doped BaTiO3 for all compositions were examined at room temperature irradiated with ultraviolet radiation of 250 nm with a 290-nm filter. The PL curves shown in Figure 8 represent five PL components, violet maximum below 418 nm, blue maximum below 448 nm, green maximum below 493 nm, yellow maximum below 577 nm, and red maximum below 657 nm in allusion to the regions where the component’s maxima appear.[35] Undoped BaTiO3 shows a broad peak at 400 nm, and some small peaks at 451, 470, 484, and 494 nm. Undoped BaTiO3 has been reported to show PL peak in the UV region at 396 nm, which is quite close to our observations.[36] Luminescence of pure BaTiO3 is generally considered to be due to self-trapped excitons. The electronic band structure of BaTiO3 has low-lying narrow conduction bands from Ti-3d orbitals and valence bands from O-2p orbitals.[13] The energy corresponding

M. Ganguly et al.


170

Figure 8. Room temperature photoluminescence emission spectra of Ba1xNd2x/3TiO3..

to 250-nm radiation makes a direct transition of electrons followed by some decay mechanism, resulting in the luminescence.[37–40] To exhibit room temperature PL, a system must have at least two types of differently charged clusters creating a polarization within the structure and/or some localized states existing in the band gap that directly affects the degree of order–disorder. Introduction of defect is a mechanism to create disorder in a system. This defect may be a vacancy. When Nd is doped in BaTiO3, defect is introduced in the structure in the form of A-site vacancy. Symmetry is broken due to formation of


Phase Transitions

171

Figure 9. Room temperature UV–vis absorbance spectrum of Ba1xNd2x/3TiO3.

[TiO5Voz] intermediate complex clusters (where VOz ¼ VOx ; VO ; VO ), leading to oxygen vacancies, as explained in Equations (6) and (7). Again substitution of smaller radii Nd at the larger radii Ba-site may result in a displacement of Ti ion at the B site. Displacements in Ti–O, Ba–O/Nd–O bonds may also have taken place, ensuring Jahn–Teller effect.[41] The energy levels responsible for the p–d transitions arising for O-2p and Ti-3d states are reduced within the band gap. Hence, the narrow bands in the Nd3þ ion doped BaTiO3 PL spectra can be related to f–f transitions of Nd3þ ions. A detailed work on the PL properties of Nd-doped CaTiO3 has been done by Marques et al.[12] As reported by them, the visible luminescence of Nd3þ ions in appropriate host materials correspond to 4f n-15d–4f n transitions, when excited with ultraviolet radiation. The ground-state of Nd3þ ion is [Xe] 4f46s2 and its PL properties are ascribed to 4f25d1–4f3 transitions. An energy level diagram depicting the transitions is also provided, showing the respective emissions. Clearly the decay of electrons from ground state 4I9/2 to 4G11/2 state leads to the visible emission around 400 nm, while decay from 4I9/2 to 2K15/2 þ 2D3/2 þ 2G9/2 state leads to the photon emission in the range 451–494 nm. If the degree of local order in a system is such that structurally inequitable sites can be distinguished by different types of electronic transitions, then these different types of electronic transitions are linked to a specific structural arrangement. The red component


172

M. Ganguly et al.

represents the less energetic electronic transitions and is thus linked to states that are deeply inserted in the band gap. Conversely, the blue component, more energetic, is linked to shallow defects in the band gap.[42] Since the prepared materials are found to belong to the zone of violet, blue and green maximum, it is clear that formation of shallow defects in the band gap of BaTiO3 takes place with doping of Nd. The DR spectra of all the prepared ceramics after Kubelka–Munk treatment are shown in Figure 9. The detailed theory behind is given in our earlier works.[22,43] The Egap values were evaluated by extrapolating the linear portion of the curve or tail and for the respective compositions and are found to decrease with increase in Nd3þ ion content. The obtained Egap values for the doped ceramics can be associated with the structural disorder introduced into the lattice due to creation of A-site vacancy and distortions in the [TiO6] clusters. A-site vacancies introduce shallow defects in the band gap of BaTiO3 decreasing the value. This vacancy increases with increase in doping content and results in formation of larger number of shallow defects. Distortion in the TiO6 octahedron increases along with increase in Nd content. Hence, band gap decreases further.[44] The band gap of pure BaTiO3 in normal conditions lies between 3.2 and 3.5 eV. But here a low value is obtained. This may be due to oxygen vacancies, lattice defects, and/or local bond distortion, which yield localized electronic levels in the band gap of the material. [45] Deep holes (between the valence band and conduction band with small Egap values) are responsible for the green, yellow, orange, and red PL emission at room temperature, while the shallow holes (between the valence band and conduction band with high Egap values) promote the violet and blue emissions.[44] Clearly, formation of shallow defects is the reason behind optical characteristics. 4. Conclusions In summary, we have obtained successfully the ferroelectric/optical ceramics (Ba1xNd2x/3) TiO3 (with x ¼ 0.02, 0.04, 0.06, 0.08, 0.10) prepared by solid-state reaction route. XRD patterns and Rietveld refinement confirm a compositionally induced phase transition from tetragonal to cubic symmetry at the composition x ¼ 0.08. The same is supported through FTIR, Raman spectroscopy, and temperature-dependent dielectric study. FTIR study reveals distortion in the TiO6 octahedra due to Nd substitution. Decrease in intensity, followed by a disappearance and split in the tetragonal Raman modes occurred with successive Nd substitution. Nd3þ ion incorporation causes local distortion and breaks partially the translation symmetry in BT. Presence of Nd is found to strongly modify the microstructure. An increase in grain size is observed with increase Nd3þ ion content, suggesting greater diffusivity and increase in matter transport. Dielectric study shows normal ferroelectric character. The paraelectric to ferroelectric transition temperature (Tc-t) is observed to shift below room temperature at the composition x 0.08, supporting the structural studies. No frequency-dependent behavior is observed up to 10% doping content. P–E hysteresis loops show a successive decrease in remnant polarization and coercive field with Nd substitution in coherence with respective microstructural features. A paraelectric behavior was traced for the composition x ¼ 0.08 and 0.10. Domain pinning due to oxygen and A-site vacancies reduces the remnant polarization and coercive field values. PL emission spectra recorded at room temperature show structural distortion due to substitution and formation of vacancies that generates asymmetry and self-trapped excitons. The narrow bands in the Nd-doped compositions were ascribed to f–f transitions arising from Nd3þ ions. UV–vis study reveals a successive decrease in band gap values, which may be due to formation of shallow defects in the forbidden gap.

Phase Transitions

173

Acknowledgments The authors M. Ganguly, S.K. Rout, and P.K. Barhai gratefully acknowledge the financial support of major research project [39-865/2010(SR)] funded by the UGC, Government of India.


References [1] Moulson AJ, Herbert JM. Electroceramics: materials, properties, and applications. London: Chapman-Hall; 1990. [2] Kishi H, Mizuno Y, Chazono H. Base-metal electrode-multilayer ceramic capacitors: past, present and future perspectives. Jpn J Appl Phys. 2003;42:1–15. [3] Hwang JH, Han YH. Dielectric properties of erbium doped barium titanate. Jpn J Appl Phys. 2001;40:676–679. [4] Tsur Y, Dunbar TD, Randall CA. Crystal and defect chemistry of rare earth cations in BaTiO3. J Electroceram. 2001;7:25–34. [5] Sakabe Y, Hamaji Y, Sano H, Wada N. Effects of rare-earth oxides on the reliability of X7R dielectrics. Jpn J Appl Phys. 2002;41:5668–5678. [6] Smyth DM. The defect chemistry of metal oxides. Oxford University Press, Oxford, New York; 2000. [7] Sunatori H, Okamoto T, Takata M. Hot spot in La doped BaTiO3 ceramic rod. J Ceram Soc Jpn. 2003;111:217. [8] Glinchuk MD, Bykov IP, Korneinko SM, Laguta VV, Slipenyuk AM, Belous AG, V’yunov AO, Yancheviski OY. Influence of impurities on the properties of rare earth doped barium titanate ceramics. J Mater Chem. 2000;10:941–947. [9] Kishi H, Mizuno Y, Chazono H. Base-metal electrode-multilayer ceramic capacitors: past, present and future perspective. Assoc Asia Paci Phys Soc Bull. 2004;14:1. [10] Narang SB, Kaur D. Dielectric investigations of lanthanides (ln ¼ sm, nd and gd) substituted barium titanate microwave ceramics. Int Ferrro. 2009;105:87–98. [11] Qizhen S, Bin C, Hui W, Jing T, Zhuguo C, Yudong H. Nd-doped barium titanate ceramics with various Ti/Ba ratios prepared by sol-gel method. Sc China Ser B Chem. 2005;48:60–64. [12] Marques VS, Cavalcante LS, Sczancoski JC, Paris EC, Teixeira JMC, Varela JA, De Vicente FS, Joya MR, Pizani PS, Li MS, Santos MRMC, Longo E. Synthesis of (Ca,Nd)TiO3 powders by complex polymerization, Rietveld refinement and optical properties. Spectrochimica Acta Part A. 2009;74:1050–1059. [13] Cavalcante LS, Gurgel MFC, Paris EC, Sim~oes AZ, Joya MR, Varela JA, Pizani PS, Longo E. Combined experimental and theoretical investigations of the photoluminescent behavior of Ba(Ti,Zr)O3 thin films. Acta Materialia. 2007;55:6416–6426. [14] Yu Z, Ang C, Guo R, Bhalla AS. Dielectric properties of Ba(Ti1xZrx)O3. Mater Lett. 2007;61:326. [15] Alioune K, Guehria-Laidoudi A, Simon A, Ravez J. Study of new relaxor materials in BaTiO3–BaZrO3–La2/3TiO3 system. Solid State Sci. 2005;7:1324–1332. [16] Sczancoski JC, Cavalcante LS, Badapanda T, Rout SK, Panigrahi S, Mastelaro VR, Varela JA, Li M Siu, Longo E. Structure and optical properties of [Ba1xY2x/3](Zr0.25Ti0.75)O3 powders. Solid State Sci. 2010;12:1160–1167. [17] Perry CH, Khanna BN, Rupprecht G. Infrared studies of perovskite titanates. Phys Rev. 1964;135 A:408. [18] Nyquist RA, Kagel RO. Infrared spectra of inorganic compounds. New York: Academic Press; 1971. [19] Ostos C, Mestres L, Martınez-Sarrion ML, Garcıa JE, Albareda A, Perez R. Synthesis and characterization of A-site deficient rare-earth doped BaZrxTi1xO3 perovskite-type compounds. Solid State Sci. 2009;11:1016–1022. [20] Jin X, Sun D, Zhang M, Zhu Y, Qian J. Investigation on FTIR spectra of barium calcium titanate ceramics. J Electroceram. 2009;22:285–290. [21] Wang D, Yu R, Feng S, Zheng W, Takei T, Kumada N, Kinomura N. Hydrothermal synthesis of perovskite-type solid solution of (1x)BaTiO3xLa2/3TiO3. Solid State Ionics. 2002;151:329–333. [22] Ganguly M, Rout SK, Park HY, Ahn CW, Kim IW. Structural, dielectric and optical characterization of cerium doped barium titanate. Phys Express. 2013;3:1–12.


174

M. Ganguly et al.

[23] Badapanda T, Rout SK, Cavalcante LS, Sczancoski JC, Panigrahi S, Longo E, Li M. Siu, Optical and dielectric relaxor behavior of Ba(Zr0.25Ti0.75)O3 ceramic explained by means of distorted clusters. J Phys D: Appl Phys. 2009;42:175414. [24] Liu Y, Feng Y, Wu X, Han X. Microwave absorption properties of La doped barium titanate in X-band. J Alloys Compd. 2009;472:441–445. [25] Dobal PS, Dixit A, Katiyar RS. Effect of Lanthanum substitution on the Raman spectra of barium titanate thin films. J Raman Spectrosc. 2007;38:142–146. [26] Rout D. Structural, electrical and Raman spectroscopic studies of lead ytterbium tantalate based ceramics [PhD thesis]. Chennai: IIT Madras; 2006. [27] Correa M, Kumar A, Priya S, Katiyar RS, Scott JF. Phonon anomalies and phonon-spin coupling in oriental PbFe0.5Nb0.5O3 thin films. Phys Rev B. 2011;83:014302, 1–10. [28] Jo SK, Park JS, Han YH. Effects of multidoping of rare-earth oxides on the microstructure and dielectric properties of BaTiO3. J Alloys Compd. 2010;501:259–264. [29] Badapanda T, Senthil V, Rout SK, Cavalcante LS, Sim~ oes AZ, Sinha TP, Panigrahi S, de Jesus MM, Longo E, Varela JA. Rietveld refinement, microstructure, conductivity and impedance properties of Ba[Zr0.25Ti0.75]O3 ceramic. Curr Appl Phys. 2011;11:1282–1293. [30] Ikeda T. Some studies on the ternary system (Ba-Pb-Ca)TiO3. J Phys Soc Jpn. 1958;13:335. [31] Xinle Z, Zhimei M, Zuojiang X, Guang C. Preparation and characterization on nano-sized barium titanate powder doped with lanthanum by sol-gel process. J Rare Earths. 2006;24:82–85. [32] Kumar S, Varma KBR. Influence of lanthanum doping on the dielectric, ferroelectric and relaxor behaviour of barium bismuth titanate ceramics. J Phys D: Appl Phys. 2009;42 (075405):9. [33] Ganguly M, Parida S, Sinha E, Rout SK, Himansu AK, Hussain A, Kim IW. Structural, dielectric and electrical properties of BaFe0.5Nb0.5O3 ceramic prepared by solid-state reaction technique. Mater Chem Phys. 2011;131:535–539. [34] Viehland D, Chen YH. Random-field model for ferroelectric domain dynamics and polarization reversal. J Appl Phys. 2000;88(11):6696–6707. [35] Longo VM, Cavalcante LS, de Figueiredo AT, Santos LPS, Longo E, Varela JA, Sambrano JR, Paskocimas CA, De Vicente FS, Hernandes AC. Highly intense violet-blue light emission at room temperature in structurally disordered SrZrO3 powders. Appl Phys Let. 2007;90 (091906):1–3. [36] Sahoo T, Tripathy SK, Nandy S, Pandey B, Verma HC, Chattopadhyay KK, Ananda S. Microstructural and photo luminescence studies on hydrothermally synthesized Ce-doped barium titanate nanocrystals. Mater Sci Eng B. 2006;131:277–280. [37] Yu J, Sun J, Chu J, Tang D. Light emission properties in nanocrystalline BaTiO3. Appl Phys Lett. 2000;77:2807. [38] Meng J, Huang Y, Zhang W, Du Z, Zhu Z, Zou G. Photoluminescence in nanocrystalline BaTiO3 and SrTiO3. Phys Lett A. 1995;72:205. [39] Cordona M. Optical properties and band structure of SrTiO3 and BaTiO3. Phys Rev A. 1965;140:651. [40] Zhang MS, Yin Z, Chen Q, Zhang W, Chen W. Optical and structural properties of pure and Ce-doped nanocrystals of barium titanate. Prog Crystal Growth Char Mater. 2000;40:33–42. [41] Orhan E, Pontes FM, Leite ER, Pizani PS, Varela JA, Longo E. Experimental and theoretical investigation of the room-temperature photoluminescence of amorphized Pb(Zr,Ti)O3. Chem Phys Chem. 2005;6:1530. [42] Rout SK, Cavalcante LS, Sczancoski JC, Badapanda T, Panigrahi S, Li MS, Longo E. Photoluminescence property of Ba(Zr0.25Ti0.75)O3 powders prepared by solid state reaction and polymeric precursor method. Physica B. 2009;404:3341–3347. [43] Morales AE, Mora ES, Pal U. Use of diffuse reflectance spectroscopy for optical characterization of un-supported nanostructures. Rev Mex Fis. 2007;53:18–22. [44] Moreira ML, Gurgel MFC, Mambrini GP, Leite ER, Pizani PS, Varela JA, Longo E. Photoluminescence of barium titanate and barium zirconate in multilayer disordered thin films at room temperature. J Phys Chem A. 2008;112:8938–8942. [45] Longo VM, Figueiredo AT, de Lazaro S, Gurgel MF, Costa MGS, Paiva-Santos CO, Varela JA, Longo E, Mastelaro V, de Vicente RFS, Hernandes AC, Franco RWA. Structural conditions that leads to photoluminescence emission in SrTiO3: an experimental and theoretical approach. J Appl Phys. 2008;104(023515):1–11.