Controllable thermal expansion and phase transition in Yb2−xCrxMo3O12

Solid State Sciences 11 (2009) 325–329

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Controllable thermal expansion and phase transition in Yb2xCrxMo3O12 M.M. Wu a, X.L. Xiao b, Z.B. Hu b, Y.T. Liu a, D.F. Chen a, * a b

China Institute of Atomic Energy, Beijing 102413, China College of Chemistry and Chemical Engineering, Graduate University of Chinese Academy of Sciences, Beijing 100049, China

a r t i c l e i n f o

a b s t r a c t

Article history: Received 2 May 2008 Received in revised form 13 July 2008 Accepted 12 August 2008 Available online 19 August 2008

The thermal expansion and phase transition of solid solutions Yb2xCrxMo3O12 have been investigated by X-ray powder diffraction and differential thermal analysis. The XRD patterns and the results of Rietveld refinement of Yb2xCrxMo3O12 indicate that the solid solution limit was in the composition range of 0.0  x  0.4 and 1.7  x  2.0. Yb2xCrxMo3O12 (0.0  x  0.4) has an orthorhombic structure and exhibits negative thermal expansion between 200  C and 800  C. Yb2xCrxMo3O12 (1.7  x  2.0) crystallizes in monoclinic below the phase transition and above, transforms to orthorhombic. Both monoclinic and orthorhombic compounds Yb2xCrxMo3O12 (1.7  x  2.0) present positive thermal expansion. Orthorhombic Yb2xCrxMo3O12 exhibit anisotropic thermal expansion with the contraction of a and c axes, and the linear thermal expansion coefficients range from negative to positive with increasing chromium content. Partial substitution of Yb3þ for Cr3þ exhibits depressed monoclinic to orthorhombic phase transition. Ó 2008 Elsevier Masson SAS. All rights reserved.

Keywords: Phase transition X-ray diffraction Molybdates Negative thermal expansion

1. Introduction Although most materials expand on heating, a growing number of compounds contract as the temperature is raised. Negative thermal expansion (NTE) has been observed in many orthorhombic tungstates and molybdates of A2M3O12 [1–6]. The orthorhombic structure has an open framework structure with A–O–M linkages, which can accommodate for transverse thermal motions responsible for NTE [4,7]. Such materials have potential applications both as pure phases and as components of composite materials with overall coefficients of thermal expansion adjusted to a desired value. It is reported that controllable thermal expansion coefficient of A2M3O12 can be obtained by partial chemical substitution of the A site by another trivalent cation [6]. Previous studies on Er2xLnxW3O12 (Ln ¼ Ce, Sm and Nd) have indicated that substitution of larger Ln3þ for smaller Er3þ could enhance the NTE of orthorhombic Er2xLnxW3O12 [8–10]. The reason is that the distortions of Er(Ln)O6 octahedra necessary for NTE are enhanced. These findings incite us to study whether the NTE could be reduced through the partial substitution of a smaller cation for a larger one. Yb2Mo3O12 crystallizes in an orthorhombic symmetry (Pnca) and exhibits NTE with the linear thermal expansion coefficient of 6.103  106  C1 in the 200–800  C temperature range [4]. Cr2Mo3O12 exhibits positive thermal expansion in the temperature range of 25–800  C by dilatometer [11]. Thus compound Yb2xCrxMo3O12 with controllable thermal expansion coefficient is expected * Corresponding author. Tel.: þ86 010 69358015; fax: þ86 010 69357787. E-mail address: [email protected] (D.F. Chen). 1293-2558/$ – see front matter Ó 2008 Elsevier Masson SAS. All rights reserved. doi:10.1016/j.solidstatesciences.2008.08.002

to be obtained through careful adjustment of x. At the same time, it has been reported that Cr2Mo3O12 undergoes phase transition from monoclinic (P21/a) to orthorhombic (Pnca) at 380  C [11]. The phase transition temperature of A2Mo3O12 increases with increasing electronegativity of the A cation [5,12]. It will be interesting to study the thermal expansion property and phase transition of Yb2xCrxMo3O12 through the substitution of Yb3þ with Cr3þ. We herein will report our work on solid solutions Yb2xCrxMo3O12. 2. Experimental Polycrystalline samples Yb2xCrxMo3O12 were prepared by the conventional solid-state reaction. Stoichiometric amounts of Yb2O3 (purity 99.9%), Cr2O3 (purity 99.9%) and MoO3 (purity  99.0%) were thoroughly mingled. Yb2xCrxMo3O12 (0.0  x  0.4) was heated at 750–800  C for 48 h with an intermediate regrinding, and Yb2xCrxMo3O12 (1.7  x  2.0) was heated at 750  C for 12 h. All samples were quenched in air from the reaction temperature. Phase components of the prepared samples were determined by room temperature XRD on MSAL-XD2 using Cu Ka radiation at Laboratory of Inorganic Materials of Graduate University of Chinese Academy of Sciences. Thermal expansion properties of Yb2xCrxMo3O12 were investigated by high-temperature XRD on PAN X’ Pert PRO MPD with Cu Ka radiation at Beijing Normal University. The heating speed was 30  C/min and each temperature was kept for 5 min before data was collected. XRD data were refined using the Rietveld program FULLPROF [13]. TG curves of Yb2xCrxMo3O12 (x ¼ 0.2 and 0.4) were collected in air over the temperature range of 25–450  C using STA409C

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thermal analyzer at Tsinghua University. The heating rate was 10  C/min. Al2O3 powder was used as a reference material. DTA/TG measurements for Yb2xCrxMo3O12 (x ¼ 1.8 and 2.0) were obtained on a TA instruments SDT Q600 with the heating temperature from 25  C to 450  C at 10  C/min in air at Peking University. 3. Results and discussion 3.1. Phase formation Molybdates Yb2xCrxMo3O12 has been systematically studied on phase formation by XRD. The room temperature XRD patterns and Rietveld refinement results indicate that Yb3þ and Cr3þ can mutually substitute in finite composition of 0.0  x  0.4 with orthorhombic (Pnca) structure and 1.7  x  2.0 with monoclinic (P21/a) structure. For 0.4 < x < 1.7, samples were mingled with monoclinic and orthorhombic phases, and it is failed to obtain single phase by changing reaction time and/or temperature. The solubility of Yb3þ in Cr3þ-sites or Cr3þ in Yb3þ-sites can be easily understood in terms of the difference in ionic radii between Yb3þ (0.86 Å) and Cr3þ (0.63 Å). Yb2xCrxMo3O12 (0.0  x  0.4) is Yb2Mo3O12-type structure and can be easily hydrated at room temperature. This behavior was observed in the isostructural tungstates and molybdates A2M3O12 [3,4,14,15]. The hydrated (25  C) and unhydrated (200  C) XRD patterns of Yb1.6Cr0.4Mo3O12 are shown in Fig. 1. Great changes of some peak intensities and peak shapes obviously can be found in the unhydrated pattern compared with the hydrated one. 3.2. Thermal analysis The TG and DTG curves of molybdates Yb2xCrxMo3O12 (x ¼ 0.2 and 0.4) are shown in Fig. 2(a) and (b). TG and DTG studies of Yb2xCrxMo3O12 (x ¼ 0.2 and 0.4) indicate the two stage water loss between 40  C and 180  C, similar to those observed in some other hygroscopic molybdates A2Mo3O12 [3,4]. The TG slope changes at about 107  C for Yb1.8Cr0.2Mo3O12 and 122  C for Yb1.6Cr0.4Mo3O12. However, only one inclination can be observed in the TG curve of isostructural tungstates A2W3O12, and the water was released in 60–120  C temperature range [14,15]. Marinkovic et al. [3] explained that weight loss at the first stage of A2Mo3O12 is owing to

Fig. 1. XRD patterns of hydrated (25  C) and unhydrated (200  C) Yb1.6Cr0.4Mo3O12.

Fig. 2. (a) TG curves of Yb2xCrxMo3O12 (x ¼ 0.2, 0.4, 1.8 and 2.0); (b) DTG curves of Yb2xCrxMo3O12 (x ¼ 0.2 and 0.4).

the release of water associated with the amorphous phase, and the second stage is related to the crystalline water in A2Mo3O12 microchannels. However, only one kind of water molecule exists in tungstates A2W3O12. The weight loss of Yb1.8Cr0.2Mo3O12 and Yb1.6Cr0.4Mo3O12 is 5.4% and 5.0%, respectively, and the corresponding number of water molecules per formula unit is 2.54 and 2.27, respectively. TG studies of Cr2Mo3O12 and Cr1.8Yb0.2Mo3O12 (Fig. 2(a)) don’t show any weight changes, indicating no water of hydration/adsorbed moisture in the samples. Some members of A2Mo3O12 undergo a volume-reducing displacive phase transition from orthorhombic to monoclinic at low temperature [5,12]. For Sc2Mo3O12, the phase transition is accompanied by 1.4% decrease in the volume per formula unit [16]. The transition temperature for Cr2Mo3O12 was reported to be in the 350–400  C temperature range [5]. In the present work, phase transition temperature detected by DTA of Cr2Mo3O12 and Cr1.8Yb0.2Mo3O12 are 401  C and 204  C, respectively. The DTA curve of Cr2Mo3O12 is shown in Fig. 3. It is reported that the phase transition temperature of A2Mo3O12 decreases with decreasing electronegativity of A3þ cation [12], which was confirmed by our work . The effect of substitution of Yb3þ for Cr3þ apparently inhibits the undesirable orthorhombic to monoclinic transition until low temperature, which can be attributed to the smaller electronegativity value of Yb3þ (1.1) than that of Cr3þ (1.66). The phase transition is a displacive ferroelastic to paraelastic transition [12].

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Fig. 3. DTA curve of Cr2Mo3O12.

3.3. Thermal expansion studies The incorporation of water in Yb2xCrxMo3O12 (0.0  x  0.4) at room temperature would destroy its framework structure and prevent negative thermal expansion. Therefore, thermal expansion behaviors of Yb2xCrxMo3O12 (0.0  x  0.4) were studied at and above 200  C. High-temperature XRD data were collected in the temperature range of 200–800  C for Yb2xCrxMo3O12 (x ¼ 0.2 and 0.4) and 25–800 C for Yb2xCrxMo3O12 (x ¼ 1.8 and 2.0). Yb2xCrxMo3O12 (x ¼ 0.2 and 0.4) remains orthorhombic in the whole testing temperature. Yb2xCrxMo3O12 (x ¼ 1.8 and 2.0) crystallizes in monoclinic (P21/a) below the phase transition and transforms to orthorhombic (Pnca) above the transition. Fig. 4 shows the XRD patterns of Yb0.2Cr1.8Mo3O12 at the various temperatures. Monoclinic Yb0.2Cr1.8Mo3O12 transforms to an orthorhombic lattice with the disappearance of the diffraction peak at 2q ¼ 26.6 (A). The presence or absence of that peak in the XRD patterns of 180  C and 250  C clearly indicates the presence of monoclinic or orthorhombic structures. The XRD patterns of monoclinic and orthorhombic samples were refined using the atomic positions of monoclinic Fe2Mo3O12 [17] and orthorhombic Sc2Mo3O12 [16] as starting models, respectively. The XRD patterns of Yb2xCrxMo3O12 (x ¼ 0.2 and 0.4) recorded at elevated temperature indicate no change in XRD pattern except the shift in position of the reflections toward higher 2q values with

Fig. 4. Comparison of XRD patterns of Yb0.2Cr1.8Mo3O12 at different temperatures.

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increasing temperature, due to the temperature-induced unit cell contraction. Representative Rietveld refinement of Yb1.6Cr0.4Mo3O12 at 200  C is given in Fig. 5. The cell parameters and volumes of Yb2xCrxMo3O12 (x ¼ 0.2 and 0.4) as a function of temperature are plotted in Fig. 6. A linear contraction of a and c axes and an expansion/contraction of b-axis are observed over the entire temperature range, indicating the anisotropic thermal expansion. The linear thermal expansion coefficient (al ¼ aV/3) of Yb1.8Cr0.2Mo3O12 and Yb1.6Cr0.4Mo3O12 are 4.729  106  C1 and 1.691 106  C1, respectively. Axial and linear coefficients of thermal expansion for Yb2xCrxMo3O12 (x ¼ 0.2 and 0.4) are listed in Table 1. For comparison, the corresponding coefficients of Yb2Mo3O12 are also tabulated [4]. Temperature dependence of the cell dimensions of monoclinic and orthorhombic Yb2xCrxMo3O12 (x ¼ 1.8 and 2.0) are shown in Fig. 7. The volume per formula unit of Yb0.2Cr1.8Mo3O12 increases 1.6% from 180  C to 250  C accompanying the monoclinic to orthorhombic phase transition. X-ray diffraction studies clearly show a dramatic change in the unit cell volume with temperature, consistent with a phase transition at about 204  C. Comparing Fig. 6 with Fig. 7, it can be observed that the lattice parameters and unit cell volumes of orthorhombic Yb2xCrxMo3O12 contract with increasing Cr3þ content, resulting from the fact that the radius of Cr3þ is smaller than Yb3þ. Unit cell volume of Yb2xCrxMo3O12 (x ¼ 1.8 and 2.0) shows an increase from 25  C to 800  C, whereas a low or near zero thermal expansion is observed above the phase transition. Cr2Mo3O12 exhibits near zero thermal expansion in the 450–800  C temperature range with al ¼ 0.708  106  C1, which is agreeable to that reported by Ari et al. [18] (al ¼ 0.67  106  C1) studied using synchrotron XRD. However, Cr2Mo3O12 is reported to exhibit strong NTE above transition temperature with the coefficient of 9.39  106  C1 by dilatometer [11]. The different coefficient of thermal expansion (CTE) of orthorhombic Cr2Mo3O12 might be resulted from the microcracks during the dilatometeric measurement. Below 180  C Yb0.2Cr1.8Mo3O12 has a monoclinic structure and exhibits positive thermal expansion with al ¼ 14.475  106  C1 and above 250  C it transforms into orthorhombic structure exhibiting low thermal expansion with al ¼ 1.134  106  C1. The axial and linear CTEs of monoclinic and orthorhombic Yb2xCrxMo3O12 (x ¼ 1.8 and 2.0) can also be found in Table 1. Comparing the CTEs, it can be found that a and c axes exhibit NTE for all the orthorhombic phases, but ab and al change from negative to positive with increasing Cr3þ content.

Fig. 5. Rietveld refinement of XRD pattern of Yb1.6Cr0.4Mo3O12 at 200  C. Observed (), calculated (solid line), Bragg position (j) and difference plots (lower trace) are shown.

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Fig. 6. Cell parameters and unit cell volumes of Yb1.8Cr0.2Mo3O12 (-) and Yb1.6Cr0.4Mo3O12 (C) vs. temperature.

Orthorhombic and monoclinic structures of A2Mo3O12 are very similar, both consisting of corner-sharing network of AO6 octahedra and MoO4 tetrahedra [16]. There are only a single A site (8d) and two crystallographically distinct Mo atoms in orthorhombic structure, with Mo1 on the two-fold axis (4c) and Mo2 on a general position (8d). Whereas four distinguishable A sites and six Mo sites can be observed in the monoclinic structure. This is the major difference between orthorhombic and monoclinic structures of A2Mo3O12. Each AO6 octahedron shares corner with six MoO4 tetrahedra, and each MoO4 tetrahedron shares corner with four AO6 octahedra. Thus, each oxygen atom bridges two metal atoms and constitutes the A–O–Mo bridging bond. The mechanism of thermal expansion in orthorhombic Yb2xCrxMo3O12 is attributed to the rocking motion of polyhedra related to the transverse vibrations of two-fold coordinated oxygens [3,19]. This vibrational motion of Yb(Cr)–O–Mo will pull Yb(Cr) and Mo atoms closer together and result in overall lattice shrinkage. However, in Yb2xCrxMo3O12 structure, the rocking motions of polyhedra necessary for NTE cannot occur without some slight distortion of the polyhedra [1,3]. The forces keeping polyhedra regular and rigid are primarily the oxygen–oxygen repulsive interactions, which become less important as the polyhedra become larger. The polyhedra distortion can easily occur with the increasing polyhedra

Table 1 Coefficients of thermal expansion for Yb2xCrxMo3O12 Yb2xCrxMo3O12

aa

ab

ac

al

(106  C1) (106  C1) (106  C1) (106  C1) x ¼ 0.0 (200–800  C) [4] 9.712 8.530 x ¼ 0.2 (200–800  C) x ¼ 0.4 (200–800  C) 5.378  13.063 x ¼ 1.8 (25–180 C)  1.112 x ¼ 1.8 (250–800 C) 9.172 x ¼ 2.0 (25–350  C) 2.424 x ¼ 2.0 (450–800  C)

3.268 0.203 3.512 16.539 5.069 10.243 6.293

5.431 5.886 3.201 21.530 0.551 13.770 1.738

6.103 4.729 1.691 14.475 1.134 9.922 0.708

Fig. 7. Cell parameters of Yb0.2Cr1.8Mo3O12 (C) and Cr2Mo3O12 (B) with rising temperature. The cell volume of the monoclinic phase has been halved for comparison with the orthorhombic.

size. Fluctuating distortions of the Yb(Cr)O6 octahedron are easier in the structure of Yb2xCrxMo3O12 (x ¼ 0.2 and 0.4) and hence negative thermal expansion can be found. With more Cr3þ substituted for Yb3þ, the octahedral distortions in Yb2xCrxMo3O12 (x ¼ 1.8 and 2.0) are attenuated with strong oxygen–oxygen repulsion, giving rise to positive thermal expansion. Thus, the al of orthorhombic Yb2xCrxMo3O12 becomes from negative to positive with increasing chromium content. Owing to the large difference in the ionic radii of Yb3þ and Cr3þ, the solid solution Yb2xCrxMo3O12 were only obtained in finite composition through traditional solidstate reaction. If the solid solution limit can be expanded by trying other synthesizing method, compounds with zero thermal expansion are expected to be obtained. 4. Conclusions Molybdates Yb2xCrxMo3O12 (1.7  x  2.0) can experience monoclinic to orthorhombic phase transition with rising temperature, and partial substitution of Yb3þ for Cr3þ decreases the phase transition owing to the smaller electronegativity value of Yb3þ than Cr3þ. High-temperature XRD reveals that Yb2xCrxMo3O12 (0.0  x  0.4) exhibits negative thermal expansion between 200  C and 800  C, while monoclinic and orthorhombic Yb2xCrxMo3O12 (1.7  x  2.0) possess positive thermal expansion between 25  C and 800  C. The thermal expansion coefficients for the orthorhombic Yb2xCrxMo3O12 can be controllable. It is found that high chromium content leads to the al of orthorhombic Yb2xCrxMo3O12 changing from negative to positive, which can be explained by the rigidity of Yb(Cr)O6 octahedron. Acknowledgements Financial support from Chinese Academy of Sciences (One Hundred Persons Award) is greatly appreciated.

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