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Synthesis, Structure and Thermal Contraction of a New Low-Temperature Polymorph of ZrMo2O8 Simon Allen, Nicholas R. Warmingham, Richard K. B. Gover, and John S. O. Evans* Department of Chemistry, University of Durham, Science Laboratories, South Road, Durham DH1 3LE, United Kingdom Received May 23, 2003 Revised Manuscript Received June 30, 2003 The observation that inorganic materials such as ZrW2O8 can undergo significant volume contraction (socalled “negative thermal expansion” or NTE) over a wide temperature range has caused significant interest into both the theory of the phenomenon and its potential commercial exploitation.1,2 Cubic ZrW2O8, for example, has been shown to contract continually, isotropically, and reversibly on heating from 2 K to over 1000 K,3-5 with a coefficient of thermal expansion of -9.1 × 10-6 K-1 between 2 and 350 K. This can be compared to positive expansion coefficients of “normal” materials such as Al2O3 (+8 × 10-6 K-1), Si (+2.5 × 10-6 K-1), and Cu (+16.6 × 10-6 K-1) around room temperature. A number of potential applications arise, the most obvious being the use of these materials as components of composite bodies with temperature-invariant size. The origin of this unusual behavior has been shown by both theory and experimental studies to be due to the coupled librations of the semirigid ZrO6 and WO4 polyhedra that make up the structure.5-8 In the simplest picture these can be thought of as giving rise to transverse vibrations of Zr-O-W bridging oxygen atoms; as the magnitude of these vibrations increases, the metal-metal distances decrease. While this insight has prompted the investigation and discovery of NTE in several families of framework materials, most notably the AM2O7 materials, the Sc2(WO4)3 family, and siliceous zeolites,9-12 its magnitude and isotropic nature in ZrW2O8-related phases makes these potentially the most technologically important. * To whom correspondence should be addressed. Fax: (+44)191384-4737. E-mail:
[email protected]. (1) Sleight, A. W. Annu. Rev. Mater. Sci. 1998, 28, 29-43. (2) Evans, J. S. O. J. Chem. Soc., Dalton Trans. 1999, 3317-3326. (3) Evans, J. S. O.; Mary, T. A.; Vogt, T.; Subramanian, M. A.; Sleight, A. W. Chem. Mater. 1996, 8, 2809-2823. (4) Mary, T. A.; Evans, J. S. O.; Vogt, T.; Sleight, A. W. Science 1996, 272, 90-92. (5) Evans, J. S. O.; David, W. I. F.; Sleight, A. W. Acta Crystallogr. B 1999, 55, 333-340. (6) Pryde, A. K. A.; Hammonds, K. D.; Dove, M. T.; Heine, V.; Gale, J. D.; Warren, M. C. Phase Transitions 1997, 61, 141-153. (7) Mittal, R.; Chaplot, S. L. Phys. Rev. B 1999, 60, 7234-7237. (8) Ernst, G.; Broholm, C.; Kowach, G. R.; Ramirez, A. P. Nature 1998, 396, 147-149. (9) Korthuis, V.; Khosrovani, N.; Sleight, A. W.; Roberts, N.; Dupree, R.; Warren, W. W. Chem. Mater. 1995, 7, 412-417. (10) Evans, J. S. O.; Mary, T. A.; Sleight, A. W. J. Solid State Chem. 1998, 137, 148-160. (11) Evans, J. S. O.; Mary, T. A. Int. J. Inorg. Mater. 2000, 2, 143151. (12) Lightfoot, P.; Woodcock, D. A.; Maple, M. J.; Villaescusa, L. A.; Wright, P. A. J. Mater. Chem. 2001, 11, 212-216.
ZrW2O8 is itself a metastable material but can be made from the binary oxides ZrO2 and WO3 at high temperature followed by rapid quenching, though lower temperature synthetic routes have also been devised.13-16 Once formed, the material is kinetically stable to around 1050 K. ZrW2O8 also undergoes a phase transition to a more dense structure (with a significantly lower coefficient of NTE) at around 0.2 GPa.17-19 Such pressures can be caused by internal stresses in composite materials and could limit potential applications.20 This makes investigation of structurally related phases such as ZrMo2O8 and ZrW2-xMoxO8 solid solutions, which do not undergo this transition, an attractive goal. Cubic ZrMo2O8 is not, however, an easy material to prepare. When conventional solid state techniques are used, the more thermodynamically stable trigonal R or monoclinic β polymorphs are formed.21-23 These do not show negative thermal expansion. The breakthrough in this area came when Lind and co-workers described that controlled decomposition of a ZrMo2O7(OH)2‚2H2O precursor phase first prepared by Clearfield in 197224 could, under certain conditions, lead to cubic ZrMo2O8.25-27 In this paper we describe how the key to this process is the formation of a previously uncharacterized metastable intermediate polymorph of ZrMo2O8 (referred to here as LT-ZrMo2O8). We have determined the structure of this material, which allows unique insight into the pathway from the precursor phase, via the new intermediate polymorph, to the cubic phase, providing a rare rationalization of a complex solid state reaction pathway. This allows us to propose a mechanism for the conversion and determine directly the optimal temperatures for the formation of the cubic material by quantitative variable temperature powder diffraction. Experimental Details. Precursor ZrMo2O7(OH)2‚ 2H2O was prepared using either the chloride or perchlorate routes described by Clearfield and Lind.24,26 In a typical reaction a solution of ZrOCl2‚8H2O (98% Avocado) and (NH4)6Mo7O24‚4H2O (98+% Avocado) in (13) Graham, J.; Wadsley, A. D.; Weymouth, J. H.; Williams, L. S. J. Am. Ceram. Soc. 1959, 42, 570. (14) Chang, L. L. Y.; Scroger, M. G.; Phillips, B. J. Am. Ceram. Soc. 1967, 211-215. (15) Kameswari, U.; Sleight, A. W.; Evans, J. S. O. Int. J. Inorg. Mater. 2000, 2, 333-337. (16) Closmann, C.; Sleight, A. W.; Haygarth, J. C. J. Solid State Chem. 1998, 139, 424-426. (17) Jorgensen, J. D.; Hu, Z.; Teslic, S.; Argyriou, D. N.; Short, S.; Evans, J. S. O.; Sleight, A. W. Phys. Rev. B 1999, 59, 215-225. (18) Evans, J. S. O.; Hu, Z.; Jorgensen, J. D.; Argyriou, D. N.; Short, S.; Sleight, A. W. Science 1997, 275, 61-65. (19) Evans, J. S. O.; Jorgensen, J. D.; Short, S.; David, W. I. F.; Ibberson, R. M.; Sleight, A. W. Phys. Rev. B 1999, 60, 14643-14648. (20) Holzer, H.; Dunand, D. C. J. Mater. Res. 1999, 14, 780-789. (21) Auray, M.; Quarton, M.; Tarte, P. Acta Crystallogr. Sect. C-Cryst. Struct. Commun. 1986, 42, 257-259. (22) Klevtsova, R. F.; Glinskaya, L. A.; Zolotova, E. S.; Klevtsov, P. V. Dokl. Akad. Nauk SSSR 1989, 305, 91-95. (23) Auray, M.; Quarton, M.; Tarte, P. Powder Diffr. 1989, 4, 2930. (24) Clearfield, A.; Blessing, R. H. J. Inorg. Nucl. Chem. 1972, 34, 2643-2663. (25) Lind, C.; Wilkinson, A. P.; Hu, Z. B.; Short, S.; Jorgensen, J. D. Chem. Mater. 1998, 10, 2335. (26) Lind, C.; Wilkinson, A. P.; Rawn, C. J.; Payzant, E. A. J. Mater. Chem. 2001, 11, 3354-3359. (27) Lind, C.; Wilkinson, A. P.; Rawn, C. J.; Payzant, E. A. J. Mater. Chem. 2002, 12, 990-994.
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6 M HCl (Fisher) (or ZrO(ClO4)2 and HClO4), with a 63% Zr excess over the 1:2 Zr:Mo ratio, was used. To prepare the 3-g sample of LT-ZrMo2O8 used for neutron diffraction studies, precursor prepared by the chloride route was heated at 573 K for 8 h. The peach-colored material so obtained was sealed in a vanadium can and time-offlight diffraction data were collected on the HRPD diffractometer of the ISIS pulsed neutron source at the Rutherford Appleton Laboratory, U.K. X-ray diffraction data of all materials were collected on either a Bruker d8 or Siemens d5000 diffractometer. The d8 system was equipped with a Ge(111) monochromator giving Cu KR1 (λ ) 1.540598 Å) radiation and a M. Braun PSD-50M linear position-sensitive detector. Data were collected from room temperature to 950 K using an Anton-Paar HTK1200 furnace and from 100 to 500 K using an Anton-Paar TTK450 cryofurnace. Temperature calibration at high temperatures was checked using an Al2O3 internal standard and at low temperatures using both an internal standard and the positions of the Al reflections of the sample holder.28,29 X-ray diffraction data for structure solution of LT-ZrMo2O8 at room temperature were obtained by dehydrating the material on the diffractometer at 473 K under a dynamic vacuum and cooling the material back to room temperature while maintaining the vacuum. The unit cell was indexed using a local modification of the ito code.30 Structure solutions and refinements were performed using the Topas software suite.31 For final cycles of refinement of the LT structure, 1 X-ray and 3 neutron data sets were refined simultaneously. For neutron data sets 9 background terms, 3 cell parameters (equated), and a scale factor were refined. For X-ray data 9 background terms, 6 pseudo-Voigt peak shape parameters, cell parameters, and sample height were refined. 19 structural coordinates and 6 temperature factors were used. hkl-dependent peak broadening (see main text for details) was modeled using a spherical harmonic function with 9 parameters and scaled to each data set using a 1/cos(θ) dependence for X-ray and a dependence on (d spacing)2 for neutron diffraction data. A final agreement factor of wRp ) 4.725% (all data sets) was obtained (6.44/4.10/28.31/11.16% for 168/90/30° neutron banks and X-ray data, respectively; we note that the poor counting statistics on the 30° bank give rise to high wRp valuessthese data have extremely low weight in the combined refinement). Bragg R-factors were 1.47(400)/1.45(199)/4.74(13)/4.70(207)%, where the value in parentheses is the number of reflections in each of the four data sets. ZrMo2O7(OD)2.2D2O was prepared by heating a 2-g sample of the LT form and 15 mL of D2O (99% Aldrich) in a 23-mL Teflon-lined Parr autoclave at 373 K for 1 day. 1.43 g was sealed in a standard vanadium can and neutron diffraction data collected on the Vega and Sirius diffractometers at the KEK pulsed neutron source at the National Laboratory for High Energy Physics, Tsukuba, Japan. The H:D ratio in the material was determined by EI mass spectrometry of the emitted (28) Taylor, D. Br. Ceram. Trans. J. 1984, 83, 92-98. (29) Wang, K.; Reeber, R. R. Philos. Mag. A 2000, 80, 1629-1643. (30) Visser, J. W. J. Appl. Crystallogr. 1969, 2, 89. (31) Bruker AXS Ltd. Topas V2.0: General Profile and Structure Analysis Software for Powder Diffraction Data ed.; Bruker AXS: Karlsruhe, 2000.
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water as the material was heated, and the H/D scattering length was adjusted accordingly. D2O/OD sites were determined by Rietveld refinement of combined X-ray and neutron diffraction data. Results and Discussion. In situ variable temperature X-ray diffraction studies and TGA experiments show that crystalline ZrMo2O7(OH)2‚2H2O loses three molecules of water at around 400 K. At this point peaks in the powder diffraction pattern from the crystalline precursor phase disappear and are replaced by a new set of rather broad peaks which did not match any known phases or combination of phases in the Zr/Mo/O system. By cooling the material back to room temperature at this point and collecting high-quality diffraction data, we have indexed this pattern to an orthorhombic cell of dimensions a ) 5.879, b ) 7.316, and c ) 9.139 Å. This basic cell has been confirmed by electron diffraction measurements.32 Despite the extremely broad nature of reflections in the diffraction data, the structure has been successfully solved directly from its powder pattern using a simulated annealing protocol within the Topas software suite.31 Due to the broad nature of many of the peaks, reliable determination of systematic absences and therefore space group symmetry proved impossible from the raw data. The cell volume of 389 Å3 is approximately half that for cubic ZrMo2O8 (761 Å3) and did suggest that if similar structural units were present, the cell contents were likely to be Zr2Mo4O16. Structure solution was therefore initially attempted in space group P1 by simulated annealing of the positions of 6 metal atom sites (Zr and Mo are approximately isoelectronic so no attempt was made to distinguish metal sites at this stage; this process models ∼66% of the total scattering of the unit cell). After several thousand cycles of stochastic movement of metal atom sites followed by full Rietveld refinement of the trial model, a solution with reasonable agreement to the X-ray data was obtained. In this solution the relative position of metal sites was reminiscent of the positions of Ti and O in the rutile structure (vide infra). Simulated annealing was therefore performed in which rigid MoO4 tetrahedra were introduced at 4 of these 6 metal sites and allowed to rotate/translate in space until good agreement with the experimental data was achieved. At this point the symmetry of the model was approximately Pmn21 and the orientation of MoO4 tetrahedra had created an octahedral coordination of the Zr site by oxygen atoms. To confirm the structural model, and in particular oxygen coordinates, a combined X-ray/neutron Rietveld refinement was performed using time-of-flight diffraction data collected on the HRPD diffractometer of the ISIS neutron source and laboratory X-ray data. Due to the extreme broadness of certain reflections in the diffraction data (presumably caused by anisotropic (32) Electron diffraction data confirm the basic cell, but give some evidence for a tripled superstructure in the [012] direction. No evidence for this can be observed in either powder X-ray or neutron data sets, suggesting the effect is either too short range or too subtle to be observed in the inherently size/strain broadened diffraction data; such discrepancies between electron and X-ray diffraction data are not uncommon. The basic cell has therefore been used for structural analysis. A similar orthorhombic cell has recently been reported from electron diffraction data for ZrW1.6Mo0.4O8 by Zhao et al.; here, no such superstructure was reported, and no structural investigation was performed (Zhao, X. H.; Wang, Q.; Ma, H. Chem. Res. Chin. Univ. 2002, 18, 233-236).
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Figure 1. Final Rietveld plots of (a) X-ray and (b) time-of-flight neutron diffraction data (90° bank shown). Data have been plotted as a function of d spacing for ease of comparison. (c) shows a polyhedral view of the structure. ZrO6 octahedra are shown in green; MoO4 tetrahedra are shown in yellow; small red spheres are oxygen atoms. The spherical harmonic function used to model excess broadening in the diffraction data is also shown. Note that the direction of minimal broadening runs parallel to the direction of the chains of Figure 3g.
sample size/strainsvide infra), reflection peak shapes were modeled using a sixth-order spherical harmonic function to describe hkl-dependent broadening. The same function was applied with appropriate scaling to the X-ray data and three neutron data sets and led to a significant improvement in the quality of the fit. We note that by application of the same spherical harmonic correction to describe hkl-dependent peak widths of both X-ray and neutron peak shapes, the total number of refined peak shape parameters used to describe the data is comparable to that typically used in Rietveld refinements employing conventional peak shape descriptions.33 There are a number of potential origins of the observed peak broadening: anisotropic size/strain or a (33) To minimize possible correlations between the spherical harmonic function and structural parameters, spherical harmonic coefficients were initially derived from a model-independent Pawley fitting of all four data sets simultaneously. An optimal set of coefficients was obtained by a simulated annealing method in which coefficients of the spherical harmonic function and peak intensities were set to random values, all variables allowed to refine to convergence using the Pawley method (Pawley, G. S. J. Appl. Crystallogr. 1981, 357-61) and then re-randomised. This process was repeated several thousand times. The form of the spherical harmonic function changed only slightly when performing Rietveld refinement of the data and was also equivalent to spherical harmonics derived directly from Rietveld fitting.
Figure 2. Volume thermal expansion of LT-ZrMo2O8 between 100 and 500 K.
lowering of the crystallographic symmetry. Given the extreme width of the peaks, conclusive evidence about symmetry lowering is hard to obtain from powder data. Trial refinements in lower symmetry space groups led to insignificant reductions in profile R-factors and we therefore chose to retain the broadened orthorhombic model. Both size- and strain-dependent peak broadening
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Figure 3. Schematic view of the transformation of ZrMo2O7(OH)2‚2H2O into LT-ZrMo2O8. Topological views of ZrMo2O7(OH)2‚ 2H2O and LT-ZrMo2O8 are shown in (a) and (b) in which oxygen atoms have been omitted for clarity and Zr/Mo sites are shown as green/yellow spheres, respectively. Chains of edge-sharing ZrO6(OH) pentagonal bipyramids and MoO4(OH)(H2O) octahedra run parallel to the c-axis (perpendicular to the plane of the page in (a); these chains are highlighted as rectangles). The sequence of figures on the right shows the transformation of these chains. (c) shows the chains as polyhedra; (d) shows an atomic view of the chains with water molecules highlighted in blue; (e) shows the topology formed by removing H2O + OH + H and the breaking of one Mo-O bond; (f) shows the actual atomic arrangement in LT-ZrMo2O8; and (g) shows a polyhedral view of this arrangement. Step (e) to (f) requires a regularization of the coordination polyhedra in (e) and their coupled rotation in three dimensions.
terms were investigated, slightly better agreement factors being achieved in the former case, which is presented here. We do not believe the data warrant the inclusion of both terms. It seems reasonable to assume that low-temperature dehydration leads to stacking faults in the structure and an anisotropic domain size; evidence of such faults is observed in TEM images. A view of the structure of LT-ZrMo2O8 is included in Figure 1. Fractional coordinates and bond lengths are available as Supporting Information. (Fractional coordinates, bond distances and angles, and bond valence sums are given in Tables 1 and 2 in the Supporting Information.) The material can be described as containing corner-sharing ZrO6 and MoO4 tetrahedra. Each ZrO6 octahedron shares all six of its corners with a MoO4 tetrahedron. Each tetrahedron shares only three of its four corners with an octahedron. Pairs of MoO4 tetrahedra exist in the structure, reminiscent of those in R-ZrW2O8, with one oxygen atom per pair of tetrahedra being strictly one-coordinate. The 3D arrangement of these polyhedra can be related to the rutile structure. By placing three-connected MoO4 tetrahedra at the positions of the three-coordinate oxygen atoms in the rutile structure, one generates the corner-sharing polyhedral network of LT-ZrMo2O8. A network of cornersharing polyhedra such as this is one of the features often found in NTE materials. Variable temperature X-ray diffraction experiments (Figure 2) show that LT-ZrMo2O8 does indeed display a bulk volume contraction with a linear coefficient of expansion,34 Rl ) -1.2 × 10-6 K-1, between 100 and 500 K and is itself a new NTE material. The successful identification of LT-ZrMo2O8 as a new polymorph of ZrMo2O8 and its structural determination allows mechanistic insight into the pathway by which decomposition of ZrMo2O7(OH)2‚2H2O permits the formation of cubic ZrMo2O8 instead of the more thermodynamically stable trigonal R form. Such insight in the (34) Defined here as one-third of the volume expansion. R values for a/b/c of -7.2, +7.5, and -5.1 × 10-6 K-1 were obtained for individual cell edges.
solid state is rare. We have found that the dehydration of ZrMo2O7(OH)2‚2H2O is fully reversible, allowing us to rehydrate the material with D2O and determine the positions of hydroxyl and water groups in the precursor for the first time. Again, this has been performed by combined Rietveld refinement of X-ray and time-of-flight neutron diffraction data and further details will be published elsewhere. Interestingly, the peaks in the X-ray diffraction pattern of the rehydrated ZrMo2O7(OD)2‚2D2O phase are essentially as sharp as those in the as-synthesized material, showing that the stacking faults or anisotropic strain causing peak broadening in the LT phase are removed on rehydration. Location of the hydrogen sites and the reversibility of the dehydration process leads us to suggest the topotactic mechanism for the formation of LT-ZrMo2O8 shown in Figure 3. The dehydration/rehydration can at least conceptually be broken down into (a) the loss of coordinated H2O molecules from MoO6 octahedra, (b) the loss of OH + H groups from ZrO7 and MoO5 polyhedra and the breaking of an O-Mo bond to give six- and four-coordinate Zr/Mo sites, and (c) the coupled twisting of the resultant polyhedra to form the LT structure. We note that the direction of minimum broadening in the LT material, as indicated by the size and shape of the spherical harmonic function used to model hkl-dependent broadening of the diffraction data, suggests that broadening is lowest in directions parallel to the chain directions shown in Figure 3. This is consistent with the proposed topotactic pathway since for size broadening stacking faults could occur between adjacent chains in the structure, while polyhedral rearrangements which could lead to strain broadening are largest perpendicular to the chain direction. These effects would lead to hkl-dependent broadening in corresponding directions. Determination of the structure of LT-ZrMo2O8 also allows one to follow the sequence of transformations ZrMo2O7(OH)2‚2H2O f LT-ZrMo2O8 f cubic-ZrMo2O8 quantitatively by multiphase Rietveld refinement of variable temperature X-ray diffraction data for the first
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Figure 4. Phase fraction of ZrMo2O7(OH)2‚2H2O, LT-ZrMo2O8, cubic-ZrMo2O8, and trigonal-ZrMo2O8 derived by Rietveld refinement as the precursor is heated under dynamic vacuum in 10 K steps from 300 to 1000 K at an overall heating rate of ∼16 K/h. Essentially phase pure cubic material can be made at 750 K. Under these conditions trigonal-ZrMo2O8 starts to decompose above 920 K.
dard have confirmed that all the transformations are between crystalline phases and that no significant amount of amorphous phase is formed at any stage during the transformation pathway. The structure of LT-ZrMo2O8 also provides some insight into why the cubic phase is kinetically favored over the thermodynamic trigonal form by this synthetic route. Portions of the LT and cubic structures are shown in Figure 5. The topologies of the two structures are such that significant sections of the cubic structure are “preformed” in the LT phase. The kinetics of the transformation LT f cubic are therefore presumably considerably faster than those for the more drastic atomic rearrangements required to form the trigonal phase. Figure 5. Polyhedral view of portions of the LT (top) and the cubic form (bottom) of R-ZrW2O8.
time. This process is time-, temperature-, and sampledependent and extremely difficult to follow by other means. Figure 4 shows the outcome of one such experiment and highlights the fact (as demonstrated by Lind26) that the temperature range in which the metastable cubic phase can be stabilized before the thermodynamic trigonal phase forms is rather narrow. Precise control of both time and temperature is thus critical to the successful formation of the cubic phase. Refinements using a known quantity of an internal intensity stan-
Acknowledgment. We would like to acknowledge the EPSRC for funding (GR/N00524), Dr. Wuzong Zhou and Prof. Russell Morris at the University of St Andrews for electron diffraction data, Takashi Kamiyama of the Institute of Materials Structure Science, High Energy Accelerator Research Organization, Japan, and Richard Ibberson of the Rutherford Appleton Laboratory for assistance with neutron data collections. Supporting Information Available: Tables of fractional coordinates, bond distances and angles, and bond valence sums (PDF). This material is available free of charge via the Internet at http://pubs.acs.org. CM034397S
