Structural, compositional, optical, thermal and

Indian Journal of Pure & Applied Physics Vol. 51, December 2013, pp. 851-859

Structural, compositional, optical, thermal and magnetic analysis of undoped, copper and iron doped potassium hydrogen tartrate crystals V Mathivanan1,2 & M Haris1* 1 2

Department of Physics, Karunya University, Karunya Nagar, Coimbatore 641 114, Tamil Nadu

Department of Physics, United Institute of Technology, Periyanaickenpalayam, Coimbatore 641 020 *E-mail: [email protected] Received 5 October 2012; revised 18 April 2013; accepted 17 September 2013

Crystals of pure potassium hydrogen tartrate having molecular formula KHC4H4O6, copper-mixed potassium hydrogen tartrate having molecular formula (K)1−x(Cu)xH C4H4O6 where (x =0.04) and iron-mixed potassium hydrogen tartrate having molecular formula (K)1−x(Fe)x HC4H4O6 where (x= 0.04) were grown using gel method. The materials were characterized using energy dispersive X-ray analysis, Fourier transform infrared spectroscopy, powder X-ray diffraction, magnetic property test and thermal analysis. The presence of tartrate ion and external mode vibrational frequencies has been identified and discussed. Doping of copper and iron in the pure potassium hydrogen tartrate single crystals found to influence the size, perfection, morphology, crystal structure and thermal stability of crystals. From powder X-ray diffraction studies, the unit cell volume of pure potassium hydrogen tartrate crystal = 857.41, Cu2+ doped crystal = 790.64 Å3 and Fe2+ doped crystal =704.03 Å3. The magnetic properties of pure, Cu2+ and Fe2+ doped potassium hydrogen tartrate crystals reveal the magnetic susceptibility and magnetic moment of the grown crystals. The magnetic susceptibility of pure potassium hydrogen tartrate crystal =26.5908×10−6 emu, Cu2+ doped crystal =20.4545×10−6 emu and Fe2+ doped crystal =22.3636×10−6 emu. Similarly, the magnetic moment of pure potassium hydrogen tartrate crystal =2.525 BM, Cu2+ doped crystal = 2.215 BM and Fe2+ doped crystal =2.316 BM. The thermal analysis of the pure and doped samples reveal that there are two stages of decomposition. The first one is the major stage of decomposition and the second one is the minor stage of decomposition which is confirmed through DSC curves. Also, we can see that there are no co-ordinated water molecules for pure, Cu2+ and Fe2+ doped potassium hydrogen tartrate crystals. Keywords: Gel growth, FTIR, Powder XRD, Magnetic properties, Thermal properties, Potassium hydrogen tartrate

1 Introduction Metal tartrate compounds deserve special attention because of their many interesting physical properties such as dielectric, piezoelectric, ferroelectric and optical second harmonic generation1-4. The characteristics of tartrate compound crystals are utilized for their use in transducers, linear and nonlinear mechanical devices5. They are also used in nonlinear optical devices, optical transmission characteristics, fabrication of crystal oscillators and controlled laser emission6-9. Rare-earth metals have attracted considerable attention on account of their luminescent and magnetic properties10. Many researchers have reported the vibrational studies of rochelle salt and metal tartrate crystals10-13. Alkali metals, inspite of having important metallic properties such as high electric and thermal conductivity, have received less attention to the growth and characterization of these elements as metallic tartrate. Alkali metals are more metallic in character than alkaline earth transition metals because the valence

shell electron of the former is loosely bound to their atoms14. The effect of dopant on the absorption spectrum of pure and doped potassium hydrogen tartrate crystals has been investigated from FTIR studies. A number of salts of tartaric acid (TA) [HOOC(CHOH),COOH], viz. sodium potassium tartrate tetrahydrate (rochelle salt), sodium ammonium tartrate tetrahydrate, lithium ammonium tartrate monohydrate and lithium thallium tartrate monohydrate, are well known ferroelectric compounds. The vibrational spectra and ferroelectric phase transitions of these compounds have been investigated. The phenomenological behaviour of the phase transition cannot be understood without correct frequency assignments. However, the assignment of the OH stretching frequency of the alcoholic hydroxy group of the tartrate ion and that of water of crystallization in these compounds is uncertain owing to their overlap11. The vibrational spectra of pure potassium hydrogen tartrate, lithium and potassium

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INDIAN J PURE & APPL PHYS, VOL 51, DECEMBER 2013

doped potassium hydrogen tartrate crystals have been reported already12. The effect of dopants on various properties of single crystals is of great interest from both solid state science as well as technological point of view. Amongst the metal tartrates, potassium hydrogen tartrate is having interesting physical properties such as dielectric, piezoelectric, ferroelectric and optical second harmonic generation. The addition of magnetic ions like copper and iron with potassium hydrogen tartrate crystals affects the growth kinetics, morpholology, shape, size, perfection, unit cell dimensions, magnetic and thermal properties. Practically no information on copper and iron doped potassium hydrogen tartrate crystals grown by gel technique is available. The substituted ions that has been selected for the modified potassium hydrogen tartrate are copper and iron. This is because the ionic radii of copper (0.73 Å) and iron (0.645 Å) are smaller than that of potassium (1.38 Å). Hence, their entry into the pure potassium hydrogen tartrate crystal is quite probable. The reason for doping two particular different transition metal ions viz. copper and iron in the pure potassium hydrogen tartrate crystal is to find out their composition and variation in unit cell dimensions. Also, to compare and contrast the magnetic and thermal behaviour of Cu2+ and Fe2+ doped potassium hydrogen tartrate crystals with respect to pure crystals. In the present paper, a systematic and complete analysis of copper doped and iron doped potassium hydrogen tartrate crystals have been studied. The vibrational spectral analysis of the pure and doped crystals are explained from FTIR studies. The unit cell dimensions of pure and doped potassium hydrogen tartrate crystals are found out from powder XRD studies. Magnetic susceptibility and thermal analysis of the samples have been made to find the effect of dopant on pure crystals. 2 Experimental Details Single crystals of pure KHC4H4O6, copper doped and iron doped KHC4H4O6 have been grown by gel method at room temperature. This technique consists of incorporating one reactant in the gelling mixture and then diffusing another reactant into the gel. This leads to high supersaturation, the initiation of nucleation and finally crystal growth. Silica gel (sodium meta silicate solution) is used as the growth medium. The required quantity of double distilled

water is added with the sodium meta silicate to obtain a specific gravity 1.05 gml−1. The required quantity of tartaric acid (1 M to 2 M) is added to form the gel medium. The pH of the mixture is 5. The gel setting time was found to be strongly dependent on pH and environmental temperature. It would take about 24 h for gel to set in summer (35°-40°C) whereas it would take even 14 days for the gel to set in winter (10°-15°C). After confirming the gel setting, an aqueous solution of potassium chloride of a particular molarity was poured over the gel carefully along the walls of a test tube so as to avoid any gel breakage. The diffusion of K+ ions through the narrow pores of the silica gel leads to reaction between these ions and HC4H4O6− ions present in the gel as lower reactant. The following reaction is expected to take place leading to the formation of pure potassium hydrogen tartrate crystals: KCl + C4H6O6 ĺKHC4H4O6 + HCl To grow doped potassium hydrogen tartrate crystals, potassium chloride solution was first mixed with an aqueous solution of copper nitrate (having oxidation state of +2) of a particular molarity. The diffusion of K1+ and Cu2+ ions though the narrow pores of the silica gel leads to reaction between these ions and HC4H4O6− ions present in the gel as lower reactant. The reaction leads to the formation of copper doped potassium hydrogen tartrate crystals. Similar procedure was followed to grow iron doped potassium hydrogen tartrate crystals (having oxidation state of +2) using aqueous solution of iron sulphate. 3 Results and Discussion 3.1 Compositional analysis

The atomic and weight % of pure and doped KHC4H4O6 crystals are presented in Table 1. It is observed that the dopants Cu2+ and Fe2+ have entered into the lattice site of pure potassium hydrogen Table 1 — Atomic and weight % of pure and doped potassium hydrogen tartrate crystals Name of the crystal

Pure crystals KHC4H4O6 Cu2+ doped KHC4H4O6 Fe2+ doped KHC4H4O6

Element

K Cu Fe K Cu K Fe

Experimental Theoretical Atomic Weight Atomic Weight % % % % 100 0 0 95.60 4.40 95.30 4.70

100 0 0 92.56 7.44 95.22 4.78

100 0 0 96 4 96 4

100 0 0 93.65 6.35 94.38 5.62

MATHIVANAN & HARIS: OPTICAL, THERMAL AND MAGNETIC ANALYSIS OF CRYSTALS

tartrate crystals. The entry of Cu2+ and Fe2+ ions into the lattice of pure potassium hydrogen tartrate might be due to the fact that the ionic radii of Cu2+(0.73Å) and Fe2+(0.645 Å) are smaller and in close approximation of that of K+(1.38 Å) ion. Doping of Cu2+ and Fe2+ ions into pure potassium hydrogen tartrate crystals is observed to influence perfection, size, colour, morphology and internal crystallographic feature. Pure potassium hydrogen tartrate crystals and Cu2+ doped crystals were found to be more perfect (in terms of transparency and morphological development). This might be due to the slower reaction nature of K+ and Cu2+ ions with HC4H4O6−ions, respectively and hence the size of the crystals is small. The Fe2+ doped potassium hydrogen tartrate crystals are less perfect due to the higher

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reactive nature of Fe2+ ions with HC4H4O6−ions and hence the size of the crystals is large. The photographs of the pure and doped crystals are shown in Fig. 1. The summary of the single crystals grown by gel method is presented in Table 2. The size of the pure KHC4H4O6 crystal is found to be 8 × 4×3 mm3 and the growth rate is slow. The size of the Cu2+ doped KHC4H4O6 is found to be 8.5×4.5×3 mm3 and the growth rate is slow. The size of the Fe2+ doped KHC4H4O6 crystal is found to be 40×3×2 mm3 and the growth rate is fast. 3.2. Optical studies

Figure 2 shows the FTIR spectrum for pure, Cu2+ doped KHC4H4O6 and Fe2+ doped KHC4H4O6 crystals. The observed vibrational frequency and their

Fig. 1 — Crystals grown in gel (a) Pure KHC4H4O6 (b) Cu2+doped KHC4H4O6 (c) Fe2+ doped KHC4H4O6


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Table 2 — Summary of the experiments of single crystal growth of pure and doped KHC4H4O6 Name of the crystal

Constant parameters Gel R.d. = 1.05 Gel pH =5 Pure KHC4H4O6 Gel age = 24 hours LR concentration =2M Gel R.d. = 1.05 Cu2+ doped Gel pH =5 KHC4H4O6 Gel age = 24 hours LR concentration =2M Gel R.d. = 1.05 Fe2+ doped Gel pH =5 KHC4H4O6 Gel age = 24 hours LR concentration =2M *All experiments performed at the temperature (L.R.) – tartaric acid ; R.d. – Relative density*

Variable parameters

Results

U.R. concentration: 1 Molar 2 Molar

Silvery white crystals of orthorhombic structure

U.R.concentration: 1 Molar (96% +4% Cu2+) 2 Molar (96%+4% Cu2+)

Bluish transparent crystals of orthorhombic structure

U.R. concentration: 1 Molar (96% +4% Fe2+) 2 Molar (96% +4% Fe2+)

Slightly brownish transparent needle shaped structure

range 30°-38°C ; Upper reactant (U.R)-Potassium chloride and lower reactant

Fig. 2 — FTIR spectrum of (a) Pure KHC4H4O6, (b) Cu2+ doped KHC4H4O6 and (c) Fe2+ doped KHC4H4O6 crystals


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Table 3 — FTIR assignments for pure and doped potassium hydrogen tartrate crystals Pure KHC4H4O6 3315.43 1568.52 1413.90 1337.12 1212.44 1162.39 1068.00 904.20 791.42 672.22 572.17 to 485.48

Wave number (cm−1) Cu doped KHC4H4O6 Fe2+ doped KHC4H4O6

Assignments

2+

3324.40 1603.23 904.01 790.99 690.44 581.96 to 405.75

3320.72 1563.85 1407.64 1065.48 904.30 793.22 667.99 568.19 to 470.09

Ȗ (OH) of acid Ȗas(COO−) Ȗ s(COO−) OH plane bending ȕCH C−O stretch į(C-H) + ʌ (C-H) Ȗ (C−C) Ĳcoo Ĳcoo Metal –oxygen stretching

assignment are presented in Table 3. FTIR spectrum reveals the presence of O-H bonds, C-O bond, C-H bond, C-C bond and carbonyl C=O bonds15−19. From the spectrum, it was found that although the radiations are absorbed at the same frequency by all the three crystals, the percentage of transmittance of Cu2+ doped KHC4H4O6 crystal is higher than the percentage of transmittance of pure and Fe2+ doped KHC4H4O6 crystals. This may be due to the absence of IR absorbing impurities in Cu2+ doped KHC4H4O6 crystals. The number of peaks in copper and iron doped potassium hydrogen tartrate crystals have been decreased when compared to pure crystals. In the frequency range 1100-1400 cm−1 there are three extra peaks(1162.39 cm−1,1212.44 cm−1,1337.12 cm−1) found in the pure potassium hydrogen tartrate crystals. This confirms the +2 oxidation state in the case of copper and iron doped potassium hydrogen tartrate crystals. 3.3 Structural analysis

The powder X-ray diffractograms of pure and doped potassium hydrogen tartrate crystals are shown in Fig. 3. The crystallinity of both pure and doped crystals is quite clear from diffractograms because of the occurrence of sharp peaks at specific Bragg’s angles. From the diffractograms, it is clear that the entry of Cu2+ and Fe2+ ions in the modified composition of potassium hydrogen tartrate crystals leads to shift in the positions of peaks. The indexed XRD data for pure and doped potassium hydrogen tartrate crystals are presented in Table 4. The calculation of cell parameters reveals that both pure and doped crystals belong to orthorhombic crystal system having space group P212121. A comparison of crystallographic data of pure and doped crystals are

Fig. 3 — Powder X-ray diffractograms of (a) Pure KHC4H4O6 and (b)Cu2+ doped KHC4H4O6 and (c) Fe2+ doped KHC4H4O6 crystals


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Table 4 — Indexed XRD data for pure and doped potassium hydrogen tartrate crystals Pure KHC4H4O6 Cu2+ doped KHC4H4O6 Fe2+ doped KHC4H4O6 hkl 2ș hkl 2ș hkl 2ș 102 112 120 113 300 132 410

23.3000 23.7512 24.7170 28.0300 31.3039 36.7210 37.9800

102 120 111 300 132 321 410

23.4485 24.4153 27.6955 31.000 36.2921 37.6988 38.3651

012 121 113 122 300 321 104

24.4636 24.9155 28.0000 28.3000 31.4884 34.3407 38.1000

Table 5 — Comparative powder XRD data for pure and doped Potassium hydrogen tartrate crystals Chemical formula

Inter axial angles

o

Pure KHC4H4O6

Į=ȕ=Ȗ=90

(K)0.96(Cu)0.04H C4H4O6

Į=ȕ=Ȗ=90o

(K)0.96(Fe)0.04H C4H4O6

Į=ȕ=Ȗ=90o

Unit cell Unit cell dimensions volume (Å) (Å3) a=9.625, b=8.545, c=10.425 a=9.845, b=10.045, c=7.995 a=9.945, b=7.932, c=8.925

Table 6 — Change in mass with respect to applied magnetic field for (a) Pure KHC4H4O6 (b) Cu2+ doped KHC4H4O6 and (c) Fe2+ doped KHC4H4O6 crystals Name of the crystal

Pure KHC4H4O6

Cu2+ doped KHC4H4O6

Fe2+ doped KHC4H4O6

857.41

Magnetic field in k Gauss

Mass in kg

1 2 3 4 5 1 2 3 4 5 1 2 3 4 5

0.2669 0.2667 0.2665 0.2661 0.2656 0.1781 0.1780 0.1778 0.1775 0.1772 0.3486 0.3485 0.3483 0.3480 0.3475

790.64

704.03

given in Table 5. It is clear that doping has brought about a change in the cell dimensions due to the change in bond lengths resulting into a change in cell volume20. 3.4 Magnetic properties

The pure and doped KHC4H4O6 crystals are finely ground, crushed and the resulting powders were packed in a Gouy tube of known magnetic susceptibility. These experiments were repeated five times and the change in weight was calculated for the given magnetic field. The readings of Gouy balance was recorded when the values became steady. These values are given in Table 6. The magnetic susceptibility of the samples are found out by using the equation mg =(A/2)x H2,where ‘m’ is the mass of the substance; ‘A’ is the area of cross section of the glass tube; ‘H’ is the magnetic field between the polepieces and ‘x’ magnetic susceptibility of the substance. A graph is drawn between m and H2 and the slope gives Ax/2g. Hence the susceptibility ‘x’ is calculated. This is shown in Fig. 4. The slope is found out at the linear region of the graph. The magnetic moment µ of pure and doped KHC4H4O6 crystals are found out by using the formula µ = 2.828 (x×T)1/2 BM, Where T is the room temperature in terms of Kelvin. The susceptibility and magnetic moment of the pure and doped crystals are given in Table 7.

Fig. 4 — Graph between m and H2 for (a) Pure KHC4H4O6, (b) Cu2+ doped KHC4H4O6 and (c) Fe2+ doped KHC4H4O6 crystals 3.5 Thermal analysis

The thermal behaviour (TGA) of pure and doped KHC4H4O6 crystals are shown in Fig. 5. Thermograms of pure KHC4H4O6, Cu2+ doped KHC4H4O6 and Fe2+ doped KHC4H4O6 crystals show that there is no loss in weight up to 250°C, 235°C and 245°C, respectively21. Hence, the materials are thermally stable, which indicates no possibility of co-ordinated water molecules in these crystals. The first stage of decomposition in the case of pure KHC4H4O6 crystals


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starts from 254°C and continues up to 310.5°C resulting in weight loss of about 57% which indicates the major decomposition of the material. The second stage of decomposition starts from 786°C and continues up to 1000°C resulting in weight loss of about 16.2% which indicates the minor decomposition of the material. Comparison of the observed and calculated weight losses suggests chemical formula for the grown crystal to be KHC4H4O6. In the case of Cu2+ doped KHC4H4O6, the first stage of decomposition starts from 240°C and continues up to 320°C resulting in the weight loss of about 56.9% which indicates the major decomposition of the material. The second stage of decomposition starts from 785°C and continues up to 1000°C resulting in the weight loss of about 16.1% which indicates the minor decomposition of the material. Similarly, in the case of Fe2+ doped KHC4H4O6 the first stage of decomposition starts from 255°C and continues up to 325°C resulting in the weight loss of about 57.1% which indicates the major decomposition of the material. The second stage of decomposition starts from 779˚C and continues up to 1000°C resulting in a weight loss of about 15.9% which indicates the minor decomposition of the material. The decomposition process of pure and doped KHC4H4O6 crystals are presented in Table 8. DSC is a thermoanalytical technique in which the difference in the amount of heat required to increase the temperature of a sample and reference is measured Table 7 — Susceptibility x and magnetic moment µ for pure and doped KHC4H4O6 crystal Name of the crystal

Pure KHC4H4O6 crystals Cu2+ doped KHC4H4O6 Fe2+ doped KHC4H4O6 crystals

Magnetic susceptibility x × 10−6 emu

Magnetic moment µ BM

26.5908 20.4545 22.3636

2.525 2.215 2.316

Fig. 5 — TGA of (a) Pure KHC4H4O6, (b) Cu2+ doped KHC4H4O6 and (c) Fe2+ doped KHC4H4O6 crystals

Table 8 — Decomposition process of pure and doped KHC4H4O6 crystals Temperature range (o C)

Weight loss % Observed Calculated

Reaction

Pure KHC4H4O6 254-310.5 786-1000

57.0 16.2

57.27 15.9

KHC4H4O6ĺKHCO2 KHCO2 ĺKO

Cu2+ doped KHC4H4O6 240-320 785-1000

56.9 16.1

57.4 15.82

(K)0.96(Cu)0.04 H C4H4O6 ĺ (K)0.96(Cu)0.04 HCO2 (K)0.96(Cu)0.04 HCO2 ĺ (K)0.96(Cu)0.04 O

Fe2+ doped KHC4H4O6 255-325 779-1000

57.1 15.9

58.2 15.78

(K)0.96(Fe)0.04 H C4H4O6 ĺ (K)0.96(Fe)0.04 HCO2 (K)0.96(Fe)0.04 HCO2 ĺ (K)0.96(Fe)0.04 O

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KHC4H4O6 crystals. The endothermic peaks at 286.68°C and 841.67°C show the first and second stage of decomposition for copper doped KHC4H4O6 crystals. Similarly, the endothermic peaks at 290.46°C and 849.57°C show the first and second stage of decomposition for iron doped KHC4H4O6 crystals.

Fig. 6 — DSC of (a)Pure KHC4H4O6, (b)Cu2+ doped KHC4H4O6 and (c) Fe2+ doped KHC4H4O6 crystals

as a function of temperature. Both the sample and reference are maintained at nearly the same temperature throughout the experiment. The result of a DSC experiment is a curve of heat flux versus temperature. These curves may be exothermic or endothermic used to calculate enthalpies of transition. The glass transition temperature, crystallization temperature and melting temperature can be calculated. The DSC analysis was done between 0° to 1000°C at a heating rate of 10°C min−1 in nitrogen atmosphere and the DSC trace for pure and doped KHC4H4O6 crystals is shown in Fig. 6. Two endothermic peaks are shown in each case. The endothermic peaks at 285.27°C and 836.08°C show the first and second stage of decomposition for pure

4 Conclusions Pure potassium hydrogen tartrate crystals and copper, iron doped potassium hydrogen tartrate were grown as single crystals in silica gel medium. The optimum conditions for better size and quality of crystals are: gel pH =5, gel age =24 h, gel relative density = 1.05, upper reactant concentration = 1 molar, lower reactant concentration = 2 molar, growth temperature =33° to 38°C. Entry of Cu2+ and Fe2+ into the lattice of pure potassium hydrogen tartrate crystals as dopant influence the size, perfection, morphology, transparency and internal crystal structure. The FTIR spectroscopy reveals the presence of O−H bonds, C−O bond, C−H bond, C−C bond and carbonyl C=O bonds. Doping of foreign ions Cu2+ and Fe2+ into the pure crystal of potassium hydrogen tartrate affects the internal crystallographic features. The unit cell dimensions of pure crystals are worked out to be a=9.625Å, b=8.545Å, c=10.425Å while that of cu2+ doped crystals are a=9.845Å, b=10.045Å, c=7.995Å and that of Fe2+ doped crystals are a=9.945Å, b=7.932Å, c=8.925Å. The interfacial angles of pure and doped crystals are Į=ȕ=Ȗ=90°. The magnetic moment of pure potassium hydrogen tartrate crystals = 2.525 BM, Cu2+ doped crystals = 2.215 BM and Fe2+ doped crystals = 2.316 BM. Thermal studies of pure and doped potassium hydrogen tartrate crystals indicate no possibility of co-ordinated water molecules. There are two stages of decomposition of the material. The first one is the major stage of decomposition and the second one is the minor stage of decomposition which is confirmed through DSC curve. Acknowledgement Authors express their thanks to Dr M Sekar, Department of Chemistry, Sri Ramakrishna Mission Vidyalaya College of Arts and Science, Coimbatore for the timely help during the course of this work. The authors also wish to thank Mr Raja, Karunya University, Coimbatore, for taking the EDAX and powder XRD spectrum of the samples. Dr M Harris wishes to thank Mr T V Manimaran, Deputy Manager, Marketing, G Plast Private limited,


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