Structures, Phase Transition, and Crystal Water of Fe2–xYxMo3O12

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Structures, Phase Transition, and Crystal Water of Fe2 xYxMo3O12 Z. Y. Li, W. B. Song, and E. J. Liang* School of Physical Science & Engineering and Key Laboratory of Materials Physics of Ministry of Education of China, Zhengzhou University, Zhengzhou 450052, China ABSTRACT: Materials with the formula Fe2 xYxMo3O12 (x = 0.2, 0.4, 0.5, 0.6, 0.8, 1.0, 1.2, 1.4, 1.6, and 1.8) have been synthesized, and their structures, phase transitions, hygroscopicity, and thermal expansion properties have been studied by Raman spectroscopy, X-ray diffraction, and differential scanning calorimetry. It is found that Fe2 xYxMo3O12 crystallize in a single monoclinic for x e 0.4 and a single orthorhombic structure for x g 0.5. The monoclinic-to-orthorhombic phase transition temperature can be effectively decreased by increasing the contents of Y3+ so that Fe1.5Y0.5Mo3O12 crystallizes already in orthorhombic at room temperature and keeps this structure till very low temperatures (lower than 103 K). Two kinds of water species are found to present in Fe2 xYxMo3O12, one having little influence on the motions of the polyhedra, whereas another interacting strongly with the polyhedra and even causing a cell volume to contract. The amount of crystal water decreases with reducing the content of Y3+ so that the motions of the polyhedra in Fe2 xYxMo3O12 for 0.5 e x e 0.8 are not obviously influenced by the crystal water. It is shown that the orthorhombic Fe2 xYxMo3O12 exhibit negative (x > 1.0), near zero (x = 1.0), and low (x < 1.0) thermal expansion properties after the removal of the crystal water.

1. INTRODUCTION The discovery of negative thermal expansion (NTE) of ZrW2O8 in a large temperature range (0.3 1050 K) triggered considerable interests in recent years.1 An increasing number of materials with NTE have been discovered, and a variety of applications have been explored since then.2 6 Of the NTE materials found, compounds with the general formula A2M3O12 present large chemical flexibility, which means that the A3+ cation can be a transition metal or rare earth that accepts an octahedral position, while M6+ is W6+ or Mo6+.7 9 A2M3O12 may crystallize in either a monoclinic or an orthorhombic structure depending on the A3+ cation size.10 13 Both are framework structures consisting of AO6 octahedra and MO4 tetrahedra.10,14 17 Only the orthorhombic corner-shared polyhedral network shows significant negative thermal expansion behavior. The orthorhombic structure has an open framework structure with A O M linkages, which can accommodate for transverse thermal vibrations responsible for negative thermal expansion, whereas the monoclinic modifications have an edge-shared structure, which are more densely packed and cannot accommodate for transverse thermal vibrations.17 19 The correlation between the crystal structure and thermal expansion in the A2M3O12 series has been reported.20 25 The coefficient of thermal expansion (CTE) for the orthorhombic phase could be generally tailored by choosing larger or smaller A3+ cations.26 A2M3O12 with larger A3+ cations exhibits generally a more negative overall linear CTE than the ones with smaller A3+ because smaller oxygen oxygen repulsion in octahedra with larger r 2011 American Chemical Society

A3+ cations may result in the polyhedra in A2M3O12 being more distorted on heating, allowing the compounds with larger A3+ cations to exhibit stronger NTE in all crystallographic directions .9,16 Though the materials of the A2M3O12 with a larger A3+ cation size exhibit larger NTE with lower anisotropy over a wide temperature range, the openings in the network become large enough to admit water molecules that inhibit the flexibility as well as the internal vibrations of the polyhedra, which are essential for negative thermal expansion.7,17,24,27 In a previous paper,17 we have studied the effect of water species on the phonon modes in orthorhombic Y2Mo3O12 by Raman spectroscopy and revealed that both high-energy optical phonons (symmetric and asymmetric stretching vibrations) contribute to the NTE in Y2Mo3O12, together with the well-known low-energy (soft) modes (translational and librational motions) and the presence of water species hinders the motions of the polyhedra and, therefore, the NTE. It is, therefore, important to reduce the hygroscopicity of this family of materials with larger A3+ cations. On the other hand, the materials of the A2M3O12 family with a smaller A3+ cation size give rise to a smaller NTE or even crystallize in a monoclinic structure at room temperature whose NTE is only possible after monoclinic-to-orthorhombic phase transition at higher temperatures.12 Al2Mo3O12, Cr2Mo3O12, and Fe2Mo3O12 are examples that crystallize in monoclinic at Received: March 1, 2011 Revised: July 8, 2011 Published: August 05, 2011 17806

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The Journal of Physical Chemistry C room temperature and transform from monoclinic to orthorhombic around 473, 658, and 780 K, respectively.12,24 To reduce the monoclinic-to-orthorhombic phase transition temperature to below room temperature remains still a great challenge for this family of materials with a smaller A3+ cation size. In this paper, we synthesized materials with the formula Fe2 xYxMo3O12 with x = 0.2, 0.4, 0.5, 0.6, 0.8, 1.0, 1.2, 1.4, 1.6, and 1.8 with the aim to solve the problems mentioned above. The structures, hygroscopicity, and phase transitions of the new compounds are studied by Raman spectroscopy, X-ray diffraction (XRD), differential scanning calorimetry (DSC), and thermogravimetry (TG). It is found that Fe2 xYxMo3O12 forms singlephase structures, monoclinic for x e 0.4 and orthorhombic for x g 0.5 at room temperature, instead of composites of Y2Mo3O12 and Fe2Mo3O12, though both are stable compounds. The phase transition temperature can be effectively reduced with increasing the contents of Y3+. Two kinds of water species, one of those interacting strongly with the polyhedra, are found in Fe2 xYxMo3O12 for x > 0.8, whereas only one kind of water species, which does not hinder the motions of the polyhedra, is observed for 0.5 e x e 0.8.

2. EXPERIMENTAL SECTION Commercial chemicals of Fe2O3, Y2O3, and MoO3 (99.9% purity) were mixed according to the stoichiometric ratios of destination materials of Fe2 xYxMo3O12 (x = 0, 0.2, 0.4, 0.5, 0.6, 0.8, 1, 1.2, 1.4, 1.6, 1.8, and 2). The mixtures were ground in a mortar for 2 h, sintered in a tubular furnace at 1173 K for 8 h, and cooled down slowly. X-ray diffraction (XRD) measurements were carried out with  Pert PRO X-ray diffractometer. A Renishaw MR-2000 an X Raman spectrometer with 532 nm laser wavelength excitation and a Jobin-Yvon T64000 Raman spectrometer with 514.5 nm laser wavelength excitation were used for Raman spectroscopic studies. Temperature dependence of the Raman spectra was recorded by using a TMS 94 heating/freezing stage from Linkam Scientific Instruments Ltd., with an accuracy of (0.1 K. The differential scanning calorimetry (DSC) study was done on an Ulvac Sinku-Riko DSC, model 1500M/L, in the temperature range of 303 873 K, with the heating and cooling rates of 10 K/min. The linear thermal expansion coefficient was measured with a dilatometer (LINSEIS DIL L76). 3. RESULTS AND DISCUSSION 3.1. Phase Transition. Figure 1 shows the XRD patterns of Fe2 xYxMo3O12 for x = 0, 0.2, 0.4, 0.5, 0.6, 0.8, 1, 1.2, 1.4, 1.6, 1.8, and 2. It is well known that Fe2Mo3O12 (x = 0) and Y2Mo3O12 (x = 2) are both stable compounds and crystallize in monoclinic and orthorhombic structures at room temperature, respectively. The XRD peaks shift obviously to smaller angles with increasing the contents of Y3+, indicating a progressive enlargement of the lattice constants by substitution of Fe3+ by Y3+ in Fe2Mo3O12 or vice versa in Y2Mo3O12 due to the difference in the cation radius of Fe3+ (64.5 pm) and Y3+ (90 pm). This suggests that the synthesized compounds form a single-phase Fe2 xYxMo3O12 (0 < x < 2) instead of a composite of (2 x)Fe2Mo3O12 and xY2Mo3O12. However, detailed examination of the XRD patterns shows that the compounds adopt a monoclinic for x e 0.4 (ICDD-PDF No. 00-031-0642) and an orthorhombic structure (ICDD-PDF No. 00-033-0661) for x g 0.5, evidenced by the

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Figure 1. X-ray diffraction patterns of Fe2 xYxMo3O12 at room temperature: (a) x = 0.0, (b) x = 0.2, (c) x = 0.4, (d) x = 0.5, (e) x = 0.6, (f) x = 0.8, (g) x = 1.2, (h) x = 1.6, (i) x = 2.0.

Figure 2. Raman spectra of Fe2Mo3O12 at different temperatures.

disappearance of the XRD peak at 26.67° (indicated by the arrow in Figure 1). The Raman spectra of Y2Mo3O12 3 xH2O (x = 0 3) has been reported previously.12 Figure 2 shows the temperature dependence of the Raman spectra of Fe2Mo3O12. By comparison with the Raman spectrum of Y2Mo3O12, the Raman modes between 900 and 1050, 750 900, and 300 400 cm 1 can be identified as internal symmetric stretching, asymmetric stretching, and bending vibrations of the MoO4 tetrahedra, whereas those below 300 cm 1 are translational and librational motions of the polyhedra. With the temperature increasing, the Raman bands at 994 and 839 cm 1 become weak gradually and vanish when the temperature is increased to 793 K. This indicates a monoclinic-toorthorhombic phase transition of Fe2Mo3O12 occurring between 783 and 793 K, in accordance with its phase transition temperature reported be Tyagi et al.12 and by Ari et al.24 Therefore, the Raman spectrum at 793 K is characteristic for the orthorhombic Fe2Mo3O12. We would like to point out that the phase transition is a sluggish process that may already start from the temperatures well below 783 K, as indicated by the progressive changes of the stretching modes at 995 cm 1 and the translational and librational modes below 300 cm 1. The latter are main contributions for negative thermal expansion, at least in part of its axes. Figure 3 shows the temperature dependence of the Raman spectra of Fe1.8Y0.2Mo3O12. The Raman band at about 990 cm 1 appears clearly at room temperature, confirming that Fe1.8Y0.2Mo3O12 crystallized in a monoclinic structure at room temperature, which is in accordance with the XRD measurement in Figure 1. With the temperature increasing, this Raman band becomes weak gradually and vanishes when the temperature is increased to 603 K. This indicates a monoclinic-to-orthorhombic 17807

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Figure 3. Raman spectra of Fe1.8Y0.2Mo3O12 at different temperatures. Figure 5. Raman spectra of Fe1.5Y0.5Mo3O12 at different temperatures.

Figure 4. Low wavenumber Raman spectra of Fe1.8Y0.2Mo3O12 at different temperatures.

phase transition for Fe1.8Y0.2Mo3O12 occurring between 583 and 603 K. Figure 4 shows the temperature dependence of the lowerenergy Raman modes of Fe1.8Y0.2Mo3O12. The Raman band at 52 cm 1 at room temperature shifts to lower wavenumbers (to 36 cm 1 at 583 K) with increasing temperature, as indicated by the solid line. It vanishes at the phase transition temperature accompanied by the appearance of a new Raman band at about 50 cm 1, which also shifts to lower wavenumbers (to 47 cm 1 at 673 K), as indicated by the dotted line. Compared to the behaviors of the low wavenumber modes in ZrW2O8 and HfW2O8,28 the appearance of librational/translational motions related to this mode should make a large contribution to the negative thermal expansion of the orthorhombic phase. The sharp peak indicated by the arrow should be the Raman line from the air. Figures 3 and 4 show that the phase transition temperature of Fe1.8Y0.2Mo3O12 is reduced by about 180 K with respect to that of Fe2Mo3O12. We have measured the temperature dependence of the samples with further increasing the contents of Y3+. It is found that the phase transition temperature of Fe2 xYxMo3O12 can be further lowered to well below room temperature when x > 0.4. Figure 5 shows the temperature dependence of the Raman spectra of Fe1.5Y0.5Mo3O12. When compared to the roomtemperature Raman spectra of Fe2Mo3O12 and Fe1.8Y0.2Mo3O12, the Raman band at about 990 cm 1, which is characteristic for the monoclinic phase, does not appear for Fe1.5Y0.5Mo3O12, suggesting that Fe1.5Y0.5Mo3O12 crystallized in an orthorhombic structure already at room temperature. To find the operating temperature range of the orthorhombic phase, we searched for its orthorhombic-to-monoclinic phase transition by decreasing the temperature. However, no phase transition was found with decreasing temperature from 873 to 103 K. It can be deduced

Figure 6. Typical DSC scans for Fe2 xYxMo3O12 (x = 0, 0.2, 0.4, 0.5, and 0.6).

that the material of Fe1.5Y0.5Mo3O12 keeps the orthorhombic structure till very low temperatures ( 0.4, in accordance with our Raman spectral analyses above. 3.2. Hygroscopicity. Figure 7 shows the Raman spectra of Fe2 xYxMo3O12 for x = 0.0, 0.2, 0.4, 0.5, 0.6, 0.8, 1, 1.2, 1.4, 1.6, and 1.8. It is found that a distinct change of the Raman spectra occurs between x = 0.8 and 1.0 in addition to the monoclinic-toorthorhombic phase transition with increasing x. An abrupt disappearance of the sharp feature of the Raman band at about 780 cm 1 (asymmetric stretching mode) occurs when x g 1.0, being replaced by broad features with lower intensity, and at the same time, an obvious shift of the Raman band at about 970 cm 1 (symmetric stretching mode) to lower wavenumbers happens. These distinct changes are indications of water species residing in the microchannels of Fe2 xYxMo3O12. Such water species hinder not only the librational/translational motions of the polyhedra but also the stretching vibrations of M O in the polyhedra. It is demonstrated that the compounds of Fe2 xYxMo3O12 with x g 1.0 are highly hydroscopic at room temperature, resembling Y2Mo3O12.17 Figure 8 shows the temperature dependence of the Raman spectra of Fe0.2Y1.8Mo3O12. The Raman spectra change very slowly with increasing temperature from 293 to 403 K, the 17808

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Figure 7. Raman spectra of Fe2 xYxMo3O12 for x = 0.0, 0.2, 0.4, 0.5, 0.6, 0.8, 1, 1.2, 1.4, 1.6, and 1.8 at room temperature.

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Figure 9. Typical DSC curves for Fe2 xYxMo3O12 with x = 0.6, 0.8, 1.0, 1.2, 1.6, and 2.0.

Figure 8. Raman spectra of Fe0.2Y1.8Mo3O12 at different temperatures.

bending mode moving successively from 346 to 339 cm 1 and the symmetric (950 cm 1) and asymmetric (823 cm 1 with a weak shoulder) stretching modes to higher wavenumbers (957 and 825 cm 1). However, distinct changes of the Raman spectra take place when the temperature is increased to 408 K, the symmetric and asymmetric stretching Raman modes as well as the bending modes splitting into sharp peaks. The changes of the Raman spectra are closely correlated with the release of water species with increasing temperature. It can be inferred from Figure 8 that there are two kinds of water species in Fe0.2Y1.8Mo3O12: one, which can be removed below 408 K, has little influence on the motions of the polyhedra, as revealed by the moderate changes of the Raman spectra with temperature, whereas another, which can only be removed at or above 408 K, interacts strongly with the polyhedra. We assume that the latter water species are those residing within the microchannels. The openings of the microchannels become larger as more Fe3+ ions are being replaced by Y3+, allowing water molecules to enter. Considering the results obtained in Figure 7, we deduce that the critical substitution value for the microchannels to admit water hindering obviously the motions of the polyhedra is x > 0.8. In other words, an orthorhombic Fe2 xYxMo3O12 whose polyhedron’s motions are not hindered obviously by crystal water species requires 0.4 < x e 0.8. Figure 9 shows the DSC plots of Fe2 xYxMo3O12 with x = 0.6, 0.8, 1.0, 1.2, 1.6, and 2.0. The DSC plots show clearly that there exist two endothermic processes, as revealed by the different slopes for x > 0.8, while only one endothermic process presents for x e 0.8, corresponding, respectively, to the releases of the two different kinds of water species. The release of water species in one step for x e 0.8 and two steps for x > 0.8 is confirmed by the thermogravimetric plots that give the weight losses of the samples with increasing temperature, as shown in Figure 10. The water admitting and releasing properties of Fe2 xYxMo3O12 with

Figure 10. Thermogravimetric plots of Fe2 xYxMo3O12 with x = 0.6, 0.8, 1.2, 1.6, and 2.0.

Figure 11. Relative length change of Fe1.5Y0.5Mo3O12 and Fe1.2Y0.8Mo3O12 with temperature.

x > 0.8 resemble that of Y2Mo3O12.14,17,26 The number of water species residing in the microchannels for each molecular formula is calculated from the thermogravimetric measurements to be 2.9, 2.0, and 1.3 for hydrated Y2Mo3O12, Fe0.4Y1.6Mo3O12, and Fe0.8Y1.2Mo3O12, respectively. However, though the water species may be present mainly outside the microchannels in Fe2 xYxMo3O12 with 0.4 < x e 0.8, a minor amount of crystal water in the microchannels cannot be excluded. 3.3. Thermal Expansion Properties. Figure 11 shows the relative length changes of Fe1.5Y0.5Mo3O12 and Fe1.2Y0.8Mo3O12 with temperature. The linear CTEs are measured to be about 7.85  10 6 (300 500 K) and 2.89  10 6 (500 900 K) for Fe1.5Y0.5Mo3O12 and 6.15  10 6 (300 500 K) and 2.47  10 6 (500 900 K) for Fe1.2Y0.8Mo3O12. Structure refinements by the Rietveld method from the XRD data show that Fe1.5Y0.5Mo3O12 and Fe1.4Y0.6Mo3O12 expand on a (Ra = 5.35 17809

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Figure 12. Relative length change of Fe0.4Y1.6Mo3O12 with temperature.

 10 6 and 5.565  10 6) and b (Rb = 1.92  10 6 and 1.70  10 6) axes and contract on the c axis (Rc = 0.74  10 6 and 1.07  10 6) with increasing temperature from 500 to 1000 K. These give rise to linear CTEs of 2.18  10 6 and 2.06  10 6 for Fe1.5Y0.5Mo3O12 and Fe1.4Y0.6Mo3O12, respectively. Both methods show a decrease of the CTE with increasing the content of Y3+. Because these materials are hydroscopic, they show low thermal expansion only after complete remove of water species. With further increasing the contents of Y3+, the CTE continues to decrease so that materials of FeYMo3O12 and Fe0.8Y1.2Mo3O12 exhibit a near zero thermal expansion (Rl = 0.05  10 6) and negative thermal expansion (Rl = 0.23  10 6) after the removal of water species, respectively. Figure 12 shows the relative length changes of Fe0.4Y0.6Mo3O12 with temperature. The arrows in Figure 12 indicate the temperature increasing and decreasing directions. It shows a clear contraction with the temperature increasing after the removal of water species and expansion with the temperature decreasing. The CTE is measured to be 4.05  10 6. Figure 13a,b shows the building blocks of a unit cell of the Y2Mo3O12 structure showing the microchannels viewed from different directions. In a unit cell, there are eight Y1 O8 Mo2 linkages nearly along the c axis in the bc plane with an angle of about 144°, two Y1 O3 Mo2 linkages along the b axis with an angle of about 149°, four Y1 O6 Mo9 linkages tilted closer to the bc than to the ab plane with an angle of about 149.5°, and four Y1 O7 Mo2 linkages along the a axis with an angle of about 173°. The outshoot of the O3, O6 and O8 anions in the microchannels makes them a higher probability to interact with water species. Because of the steric hindrance of the YO6 octahedra and MO4 tetrahedra, water species seem less probable to interact with the O7 in Y1 O7 Mo2 linkages. This analysis is in agreement with the number of crystal water species measured per molecular formula. When Y3+ cations are partially replaced by Fe3+, the microchannels become smaller and hence the number of crystal water species reduces with decreasing the contents of Y3+. The negative and low thermal expansion properties of the orthorhombic Fe2 xYxMo3O12 can be understood by the librational/translational motions or the rotations of the polyhedra, as revealed by the appearance of the Raman mode at about 50 cm 1 (Figure 4). However, a contribution from the high-energy optical phonon (stretching) modes, particularly those appearing only after remove of the crystal water species, cannot be excluded, as in the cases of Y2Mo3O12 and ZrW2O8.17,28 It is noticed from Figure 8 that Fe0.2Y1.8Mo3O12 gives rise to two intense asymmetric stretching modes at about 826 and 795 cm 1. They can be assigned to Mo O asymmetric stretching vibrations from the MoO4 tetrahedra sharing corners with YO6 and FeO6 octahedra, respectively. Because both of these asymmetric modes and the

Figure 13. Building block of a unit cell of orthorhombic Y2Mo3O12 comprising corner-sharing YO6 octahedra and MoO4 tetrahedra, where large, medium, and small spheres are, respectively, Y, Mo, and O.

symmetric stretching mode at about 970 cm 1 are suppressed by the crystal water species and gain larger intensities as the crystal water species are released, they arise probably from the stretching vibrations of the MoO4 tetrahedra involved in Y1(Fe1) O8 Mo2 linkages, Y1(Fe) O6 Mo9 linkages and Y1(Fe) O3 Mo2 linkages. Besides gaining intensities, the symmetric stretching modes shift obviously to higher wavenumbers as the crystal water species release. This is an indication that the interaction of the crystal water species with the O anions draws part of the electron density away from the Mo O bonds to form possibly hydrogen bonds with the water species, leading to a weakening of the Mo O bonds. The Mo O bonds become stronger when the crystal water species are removed due to the recovering of the electron density. Because of the existence of the water species, the lateral motions of the O3, O6 and O8 are largely limited, which makes the rotation of the polyhedra difficult. However, when the water species are released, the rotation of the polyhedra will be much easier. This explains why low or negative thermal expansions of Fe2 xYxMo3O12 are only observed after complete remove of the crystal water species.

4. CONCLUSIONS Materials with the formula Fe2 xYxMo3O12 (x = 0.2, 0.4, 0.5, 0.6, 0.8, 1.0, 1.2, 1.4, 1.6, and 1.8) have been synthesized, and 17810

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The Journal of Physical Chemistry C their structures, phase transitions, hygroscopicity, and thermal expansion properties have been studied for the first time. It is found that Fe2 xYxMo3O12 crystallize in a single monoclinic structure when x e 0.4 and in a single orthorhombic structure when x g 0.5. The monoclinic-to-orthorhombic phase transition temperature can be effectively decreased with increasing the contents of Y3+ so that Fe1.5Y0.5Mo3O12 crystallizes already in the orthorhombic phase at room temperature and keeps this structure till very low temperatures ( 1.0), near zero (x = 1.0), and low (x < 1.0) thermal expansion properties that may be related to the appearance of the librational/translational motions of the polyhedra at about 50 cm 1 as well as the high-energy optical phonon modes after the complete removal of the crystal water.

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’ AUTHOR INFORMATION Corresponding Author

*Phone: +86 371 67767838. Fax: +86 371 67766629. E-mail: [email protected].

’ ACKNOWLEDGMENT This work was supported by the National Science Foundation of China (No. 10974183) and the Fund for Sci & Techn Innovation Team of Zhengzhou (2011-3). ’ REFERENCES (1) Mary, T. A.; Evans, J. S. O.; Vogt, T.; Sleight, A .W. Science 1996, 272, 90–92. (2) Mittal, R.; Chaplot, S. L. Prog. Mater. Sci. 2006, 51, 211–286. (3) Barrera, G. D.; Bruno, J. A. O.; Barron, T. H. K.; Allan, N. L. J. Phys.: Condens. Matter 2005, 17, R217–R252. (4) Grima, J. N.; Zammit, V.; Gatt, R. Xjenza 2006, 11, 17–29. (5) Miller, W.; Smith, C. W.; Mackenzie, D. S.; Evans, K. E. J. Mater. Sci. 2009, 44, 5441–5451. (6) Liang, E. J. Recent Pat. Mater. Sci. 2010, 3, 106–128. (7) Evans, J. S. O.; Mary, T. A.; Sleight, A. W. J. Solid State Chem. 1997, 133, 580–583. (8) Evans, J. S. O.; Mary, T. A.; Sleight, A. W. Physica B 1998, 241 243, 311–316. (9) Forster, P. M.; Yokochi, A.; Sleight, A. W. J. Solid State Chem. 1998, 140, 157–158. (10) Forster, P. M.; Sleight, A. W. Int. J. Inorg. Mater. 1999, 1, 123–127. (11) Evans, J. S. O.; Mary, T. A. Int. J. Inorg. Mater. 2000, 2, 143–151. (12) Tyagi, A. K.; Achary, S. N.; Mathews, M. D. J. Alloys Compd. 2002, 339, 207–210. 17811

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