Mössbauer and Magnetic Studies in Cyano-Bridged Fe−Mn Bimetallic ...

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In accord with XRD, Mössbauer spectroscopy and IR spectra study, the structure is consistent suffer ... [10]: S. Iijima, Z. Honda, S. Koner, F. MizutaniJ. Magn.

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Physics Procedia 25 (2012) 363 – 368

2012 International Conference on Solid State Devices and Materials Science

Mössbauer and Magnetic Studies in Cyano-Bridged FeíMn Bimetallic Complexes Qing Lina.b*, Jianmei Xub, Chenglong Leia, Hui Zhanga, Zhonghui Yea, Haifu Huanga, Ruijun Wangb, YuanFu Hsiac, Yun Hea* a

b

College of Physics and Technology, Guangxi Normal University, Guilin 541004, China Department of Information Technology, Hainan Medical College, Haikou 571101, China c Department of Physics, Nanjing University, Nanjing 210093 , China

Abstract Prussian blue compound K0.1Mn1.4[Fe(CN)6]·yH2O has been synthesized. In accord with XRD, Mössbauer spectroscopy and IR spectra study, the structure is consistent suffer structural disorder with the face-centered structure (fcc) Prussian blue lattice. The compounds exhibits spontaneous magnetic ordering at low temperatures; in the temperature range 5-100 K, the magnetic susceptibilities of the compound can be fit to the Curie-Weiss law and Weiss paramagnetic Curie temperature of Ԧ=11K.The increasing large line widths of the doublet show a dynamic behavior which means that the electric field gradient is moving from one direction to another direction in the crystal. Until the sample was cooled down to 11K, we have detected the magnetic splitting in the compound.

© 2012 Published by by Elsevier Elsevier Ltd. B.V.Selection Selectionand/or and/orpeer-review peer-reviewunder under responsibility Garry Lee 2011 Published responsibility of of [name organizer] Open access under CC BY-NC-ND license. Keywords: Molecule-based magnet, Cyano-bridged complex,ferromagnetism ,Magnetic interaction ;

1. Introduction In recent decades, design and synthesis of molecule-based magnets, involving many fields such as chemistry, physics, materials and life sciences, has become one of the hot research projects on the physics and chemistry nowadays[1-4]. In that field cyano-bridged complex have become a hot topic due to its higher magnetic ordering temperature, people found that cyano-bridged complex have excellent magnetic properties , high ferrimagnetic or ferromagnetic phase transition temperature and interesting optical * Corresponding author. Tel.: +86-0773-5825256; fax: +86-0773-5837252. E-mail address: [email protected](Y. He).; [email protected](Q.Lin).

1875-3892 © 2012 Published by Elsevier B.V. Selection and/or peer-review under responsibility of Garry Lee Open access under CC BY-NC-ND license. doi:10.1016/j.phpro.2012.03.097

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Qing Lin et al. / Physics Procedia 25 (2012) 363 – 368

properties by looking for high-Tc ferromagnets temperature molecular[5-6].In this paper,molecular magnetic material K0.1Mn1.4[Fe(CN)6]·yH2O was selected and have been studied for their magnetic properties through elemental analysis, IR, Mossbauer spectrum and magnetic measurements, etc. We report a detailed investigation of the ferromagnetism behavior in Prussian blue compound K0.1Mn1.4[Fe(CN)6]·yH2O. 2. Experiment section 2.1. Materials and physical measurements MnCl2 and K3Fe(CN)6 are reagent grade, and without further purification. For the characterization of the samples we used Perkin Elmer Corporation Spectrum One FT-IR Spectrometer Fourier transform infrared spectrometer (KBr pellet), with the radiation spectrum of the 4000~400cm-1; Perkin Elmer Corporation PE2400 II elemental analysis device; Perkin Elmer TGA detector. Magnetization measurements were measured by a Quantum Design MPMS-7S superconducting quantum interference device (SQUID) magnetometer; The powder diffraction data are measured using the Bruker D8 ADVANCE x-ray diffractometer instrument (Cu-ka). As the radioactive source for Mössbauer measurements, 57Co (Pd) moving in a constant acceleration mode was used. And the room temperature spectrum was fitted with the MössWin3.0 software. The isomer shifts are reported relative to a-Fe at room temperature. 2.2. Synthesis Polycrystalline samples of K0.1Mn1.4[Fe(CN)6]·yH2O have been prepared in co-precipitation method following the literature method[7]. A mixture of aqueous solutions of MnCl2 (100 ml, 20mmol )was poured in aqueous solution of K3[Fe(CN)6] (100 ml, 10mmol). Then the reaction solution was left to stand at room temperature for an appropriate period of time so that the completion of those reactants as soon as that were complete. A light brown precipitation was obtained, and precipitation then was filtered, washed many times with demineralized water and finally dried under IR lamp for about 30 minutes. 3. Results and discussion 3.1. Characterization X-ray diffraction experiment of the compound was carried out on Cu KĮ. The X-ray diffraction pattern (at 293 K)for the compound is shows in Figure.1 ( the numbers beside the peaks are the d-values). The lattice parameters determined in the refinement using (here the method or program used) is 10.484 Å, and the space group is Fm3m. Figure.2 displays the XRD diffraction pattern (at 89K), compared with the room temperature XRD patterns, indicating no structural phase transition.The space group is Fm3m. The FT-IR spectrum of the compound shows two CN stretching bands at 2105.47cm-1 and 2163.87cm1 in the regions 2200-2000cm-1 indicating the existence of two types of bridging cyanide group in this compound [7]. Meantime, the broad peak at 3413.76cm-1 and peak at 1607.64cm-1 correspond to the v(OH) of the crystal water.

Qing Lin et al. / Physics Procedia 25 (2012) 363 – 368

Fig. 1. The X-ray diffraction pattern.(at 293 K)

Fig. 2. The X-ray diffraction pattern.(at 89K)

3.2. Magnetism analyses The magnetic susceptibility of the compound was measured from 5 K to 300 K in 0.5 K Oe field, as shown in Figure. 3 in the forms of Ȥm vs T and ȤmT vs T curves. where Ȥm is the magnetic susceptibility per K0.1Mn1.4Fe unit. With the decrease of the temperature, ȤmT increases slowly down to 30 K and then sharply, reaching a maximum value of 7.7 abr.unit at 11 K. Below 11 K, ȤmT decreases rapidly, which may be due to the interlayer antiferromagnetic interaction, the field saturation of the magnetization,and the zero-field splitting effect of the Mn(II) ions in axially elongated octahedral surroundings.This kind of behaviour is a characteristic of a ferromagnet. For a ferrimagnetic compound ȤmT vs T curve undergoes to a minima before rising around magnetic ordering temperature. 8 1.0

0.8

0.4 4

XT(abr.unit)

X(abr.unit)

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2 100

Temperature(K)

Fig. 3. Temperature dependence of xm and xmT curves of the sample

Fig. 4. Temperature dependence of xm-1 curve of the sample

The xm-1 value obey the Curie-Weiss law(as shown in Figure.4) and Weiss paramagnetic Curie temperature of Ϊ=11K, indicating there exists a ferromagnetic ineraction in the compound. The results indicate that magnetic property of the sample change from paramagnetism to ferrimagnetism.The antiferromagnetic coupling interaction of the bridged segment in the material could be determined.

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The ferrimagnetic behavior is further characterized by the measurements of hysteresis behavior as shown in Figure.5.It shows a weak coercive field Hc, typical of a soft ferromagnet. The small coercive field is consistent with the presence of magnetically isotropic ions such as Mn(II) and Fe(III) in a highly symmetrical cubic environment. The coercive field and remanent magnetization for the sample are smaller than K0.2Co1.4[Fe(CN)6]˜6.9H2O[4],these data are characteristic for bulk ferromagnetic or ferromagnetic type of ordering. 1.0

M (emu/g)

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-200

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H (kO e )

Fig. 5. Hysteresis curve of the sample at 4 K

Fig. 6. Mössbauer spectrum of the sample (at 11 K)

3.3. Mössbauer Spectroscopy The Mössbauer spectra of K0.1Mn1.4[Fe(CN)6]·yH2O have been measured as a function of temperature and selected paramagnetic spectra (as shown in Figure.6-8).The experimental data shown in these figures clearly indicate the presence of two quadrupole doublets, which may be assigned to iron(II) and iron(III) ions.Summarized in Table I, and with the parameters obtained for the magnetically ordered spectra.The asymmetrical lattices and unpaired charge electron give rise to QS. Thus a doublet is observed. The FeIII anion in Prussian blue analogs usually exhibits a QS ranging 0.2-1.2 mm/s in those three-dimension compounds. Relative large QS values were observed for cyano-bridged complexes having low-dimension structure as a result of some distortion from cubic symmetry of the [Fe(CN)6]3- unit [10-11]. The negative isomer shift (IS) values indicate that the configuration of FeIII ion is low-spin state, which is known to be a common spin state of FeIII in various [Fe(CN)6]3- complexes [8-12]. Table I. Mössbauer parameters of K0.1Mn1.4[Fe(CN)6]·yH2O at various temperatures Doublet Sextet IS (mm/s) QS (mm/s) Area (%) IS (mm/s) Hhf (T) Area (%) 293 -0.18 0.22 100 133 -0.13 0.65 100 103 -0.11 0.69 100 73 -0.12 0.73 100 53 -0.11 0.76 100 33 -0.11 0.77 100 23 -0.09 0.79 100 13 -0.11 0.82 100 11 -0.11 0.84 64 -0.05 10.47 36 The isomer shift (IS) values relative to a-Fe at room temperature (with error of +0.02 mm/s). QS, quadrupole splitting; T, temperature.

Temp. (K)

Qing Lin et al. / Physics Procedia 25 (2012) 363 – 368

Room temperature Mossbauer spectrum(293 k) with the isomer shift parameter(-0.18mm/s) and quadrupole splitting(QS = 0.22 mm / s) characteristic for low-spin Fe(III) ions ( in Figure.8), the result of the low-spin (S = 1 / 2) of the Fe (III) ion of valence electrons on the electric field gradient (EFG) caused by non-zero contribution[14-16].The increasing large line widths of the doublet ( in Figure. 7.8) show a dynamic behavior which means that the electric field gradient is moving from one direction to another direction in the crystal [13]. This behavior which is described in the model of Tjon and Blume is ascribable to magnetic relaxation [13]. The singlet is assigned to the low-spin FeIII(II) (S = 1/2) ions with octahedral symmetrical surroundings (the isolated [Fe(CN)6]3-) . As a result of magnetic relaxation, the dynamic behavior happens also in the case of FeIII(II) site. However, in this case the quadurupole splitting contributed by unpaired charge electron is not longer observed, because the velocity of dynamic behavior is in the fast relaxation limit [13]. As shown in Figure. 5,until the sample was cooled down to 11K, we have detected the magnetic splitting in the compound.

Fig. 7. Mössbauer spectrum of the sample (at 13,23,33 and 53 K)

Fig. 8. Mössbauer spectrum of the sample (at 73, 103, 133 and 293 K)

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Qing Lin et al. / Physics Procedia 25 (2012) 363 – 368

4. Conclusion We have reported a detailed investigation of magnetic properties of Prussian blue compound K0.1Mn1.4[Fe(CN)6]·yH2O. In accord with XRD, Mössbauer spectroscopy and IR spectra study, the structure is consistent suffer structural disorder with the face-centered structure (fcc) Prussian blue lattice. The compounds exhibits spontaneous magnetic ordering at low temperatures; in the temperature range 5100 K, the magnetic susceptibilities of the compound can be fit to the Curie-Weiss law and Weiss paramagnetic Curie temperature of Ϊ=11K.The increasing large line widths of the doublet show a dynamic behavior which means that the electric field gradient is moving from one direction to another direction in the crystal. Until the sample was cooled down to 11K, we have detected the magnetic splitting in the compound.

Acknowledgements This work was financially supported by the National Natural Science Foundation of China (NO.11164002,20971029,G19835050);Natural Science Foundation of Guangxi (NO.0991092).

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Research Assistant: LinQing(1981-) He received his BS (2004) from Jilin University and his MS(2010) from Guangxi Normal University.His research interests include photofunctional materials science.

Prof. He Yun. (1962-) His current research interest is the design and demonstration of a novel type of functionalized magnetic phenomenon using molecule-based magnets.

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