Comprehensive Interpretation of a Substitution Effect on an Order

Aug 3, 2007 - Temperatures of order−disorder phase transition (Ttrs) associated with the ... properties of a novel cubic ZrW1.80V0.20O7.90 solid sol...
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J. Phys. Chem. B 2007, 111, 10118-10122

Comprehensive Interpretation of a Substitution Effect on an Order-Disorder Phase Transition in A1-xMxW2O8-y (A ) Zr, Hf; M ) Trivalent Cations) and Other ZrW2O8-Based Solid Solutions Yasuhisa Yamamura,*,† Kenichi Masago,‡ Masayuki Kato,‡ and Toshihide Tsuji‡ Department of Chemistry, Graduate School of Pure and Applied Sciences, UniVersity of Tsukuba, Tsukuba, Ibaraki 305-8571, Japan, and School of Materials Science, Japan AdVanced Institute of Science and Technology, 1-1 Asahidai, Nomi, Ishikawa 923-1292, Japan ReceiVed: April 30, 2007; In Final Form: June 15, 2007

Hf1-xLuxW2O8-y solid solutions up to x ) 0.04, based on a negative thermal expansion material HfW2O8, were synthesized by a solid sate reaction method. X-ray diffraction experiments of these solid solutions from 90 to 560 K indicated thermal contraction with increasing temperature. Temperatures of order-disorder phase transition (Ttrs) associated with the orientation of WO4 tetrahedra were determined from disappearance of a characteristic diffraction peak (310). The Ttrs of the solid solutions drastically decreased with increasing Lu content. Saturated order parameters (ηs) associated with the orientational order of the WO4 pairs were estimated from the characteristic diffraction peak at sufficient low temperature. These behaviors of Hf1-xLuxW2O8-y are consistent with those of Zr1-xMxW2O8-y (M ) Sc, Y, In, Lu). The drastic suppression of Ttrs in Hf1-xLuxW2O8-y can be interpreted in the framework of a model proposed for Zr1-xMxW2O8-y, which states the existence of a local nanoregion including the WO4 pairs having the frozen-in orientational disorder. To understand the substitution effect on the order-disorder phase transition comprehensively, classification based on the saturated order parameter ηs of the phase transition of AW2O8 (A ) Hf, Zr)-based solid solutions was carried out and discussed.

Introduction Many studies on ZrW2O8 have been carried out in the past decade since the compound shows isotropic thermal contraction with increasing temperature, the so-called negative thermal expansion (NTE).1,2 The NTE of ZrW2O8 results from a framework structure composed of a ZrO6 octahedron and WO4 tetrahedron units and a strong bond between metal (Zr, W) and oxygen atoms.1,2 These structural characters bring another cooperative phenomenon to ZrW2O8; an order-disorder phase transition at 440 K.1-5 This phase transition is associated with the disordering orientation of an unshared vertex of the WO4 unit. The previous structural1-3 and thermodynamic4,5 investigations revealed that two adjacent WO4 units lay in tandem on the [111] body diagonal line in a unit cell and that the WO4 pair had only two orientations ([111] or [1h1h1h]). The tandem WO4 pairs change the orientation concertedly in the disordering process. The phase transition of ZrW2O8 can be described with the 3-D Ising model3 assuming that the one pair of WO4 is an Ising spin. The phase transition is not favorable for practical use of ZrW2O8 since the thermal expansion coefficient changes slightly at the phase transition temperature. If the phase transition temperature of ZrW2O8 could be shifted to below room temperature without a change in the noble NTE property, ZrW2O8 would be applicable for a variety of practical uses above room temperature. To control the phase transition temperature, four solid solutions have been investigated so far: Zr1-xHfxW2O8,6-8 Zr(W1-xMox)2O8,9-14 Zr1-xMxW2O8-y (M ) Sc, Y, In, Lu),8,15-19 and Zr1-xSnxW2O8.20 Although there * Corresponding author. E-mail: [email protected]. † University of Tsukuba. ‡ Japan Advanced Institute of Science and Technology.

have been some reports on each solid solution, no overall discussion on the phase transition of the ZrW2O8-based solid solutions has been carried out yet, in spite of being helpful in planning a synthetic strategy of new ZrW2O8-based solid solutions. One of the solid solutions shows a significant composition dependence of the phase transition temperature. The solid solution is Zr1-xMxW2O8-y (M ) Sc, Y, In, Lu)8,15-19 substituted for the Zr(IV) site by the trivalent cations M(III). In Zr1-xMxW2O8-y (M ) Sc, Y, In, Lu) solid solutions, the phase transition temperature (Ttrs) of ZrW2O8 decreases drastically with the substitution of just a few percent M content. The depression of Ttrs reaches ca. 80 K (approximately 20% Ttrs in ZrW2O8) in Zr0.96Sc0.04W2O8-y.15 The phenomena of Zr1-xMxW2O8-y (M ) trivalent cation) are far different from those of the other solid solutions based on ZrW2O8. Zr0.7Sn0.3W2O8, for example, shows only a 9% decrease in Ttrs in spite of 30% substitution.20 For the drastic lowering of Ttrs, we have proposed a model where the substituted M forms a nanoscale region under a nonequilibrium state where the disordered orientation of the tandem WO4 pairs is frozen-in even at a lower temperature than Ttrs.16 The small regions behave just like nonmagnetic impurities in a site-diluted magnetic system, leading to the drastic lowering of Ttrs in Zr1-xMxW2O8-y (M ) Sc, Y, In, Lu).16,17 The model is supported by the depression of a characteristic diffraction peak (310) to the orientational order of tandem WO4 tetrahedra15-17 and explains the substitution effect on Zr1-xMxW2O8-y (M ) Sc, Y, In, Lu) successfully.16 The present model16 is, however, applied only to Zr1-xMxW2O8-y solid solutions and has not been validated for other ZrW2O8-type compounds thus far.

10.1021/jp073323r CCC: $37.00 © 2007 American Chemical Society Published on Web 08/03/2007

Substitution Effect on ZrW2O8-Based Solid Solutions

Figure 1. Powder X-ray diffraction patterns of Hf1-xLuxW2O8-y (x ) 0.02, 0.04) at room temperature, together with that of HfW2O8.21

J. Phys. Chem. B, Vol. 111, No. 34, 2007 10119

Figure 2. Lattice parameters of Hf1-xLuxW2O8-y (x ) 0.02, 0.04) as a function of temperature, together with that of HfW2O8.21

In the present paper, we discuss the following two subjects. One is the study of the substitution effect upon HfW2O8 by trivalent cations to extend our model for Zr1-xMxW2O8-y solid solutions. HfW2O8 is the only compound that is isostructural to ZrW2O8 at room temperature,1,2 and it undergoes an orderdisorder phase transition associated with the orientation of the WO4 pairs at 468 K.21 To understand Hf1-xMxW2O8-y and Zr1-xMxW2O8-y solid solutions simultaneously leads to the verification of our model. The other subject is the substitution effect on the order-disorder phase transition in the whole series of ZrW2O8-based solid solutions studied so far. Experimental Procedures The samples of Hf1-xLuxW2O8-y (x ) 0, 0.02, 0.04) were synthesized by a solid state reaction method. Starting materials were HfO2 (Soekawa Tech., 99.9%), WO3 (Kojundo Chemical Lab, 99.99%), and Lu2O3 (Kojundo Chemical Lab, 99.99%). These materials were mixed at the required proportion thoroughly in an agate mortar and pressed into pellets. The pellets covered with platinum foil were sintered in air at 1473 K for 20 h and quenched in liquid nitrogen. All samples were characterized by XRD using Cu KR radiation at 40 kV and 200 mA (RINT 2500V, Rigaku), having a low-temperature attachment capable of controlling temperature within (1 K. The XRD data were collected by a step scanning method in the 2θ range from 10 to 100°. The powder patterns were obtained in a vacuum from 90 to 560 K, where the temperature was kept constant at each temperature. After XRD measurements, the surface temperature of the sample was calibrated against the system temperature by using another K-type thermocouple attached directly to the surface of the sample, and then a correction curve of the sample temperature was calculated. These corrected temperatures were used as the sample temperature in these experiments. Results and Discussion Phase Transition of Hf1-xLuxW2O8-y. X-ray diffraction patterns of Hf1-xLuxW2O8-y (x ) 0.02, 0.04) solid solutions were observed at room temperature and are shown in Figure 1, together with that of HfW2O8.21 They ensure that the Hf1-xLuxW2O8-y (x ) 0.02, 0.04) solid solutions are of a single phase and isostructural to HfW2O8. Lattice parameters of the samples were determined using about 30 diffraction peaks between 40 and 100° by a least-squares method after correcting 2θ with Nelson-Riley’s method.22 All the obtained lattice parameters of Hf1-xLuxW2O8-y (x ) 0.02, 0.04) solid solutions are depicted against the temperature in Figure 2, together with that of HfW2O8.21 The lattice parameters decrease with increasing temperature. The temperature dependences indicate the

Figure 3. Lattice parameters of Hf1-xLuxW2O8-y (x ) 0,21 0.02, 0.04) at 121 K, together with those of Zr1-xLuxW2O8-y.17

negative thermal expansion of the solid solutions over the experimental temperature region in this study. Small changes in the temperature dependence are seen around 430 K (Lu2%) and 400 K (Lu4%). These small changes are attributed to the order-disorder phase transition associated with the disordering of the WO4 orientation. Figure 3 shows the lattice parameters of Hf1-xLuxW2O8-y plotted against the Lu content at 121 K, together with those of Zr1-xLuxW2O8-y.17 Since the temperature of 121 K is much lower than the phase transition temperature, the effect of the phase transition on the lattice parameter is negligible. The lattice parameter decreases linearly with increasing Lu content. The composition dependence of Hf1-xLuxW2O8-y is in agreement with that of Zr1-xLuxW2O8-y. The linearity ensures that Hf1-xLuxW2O8-y forms a solid solution at least up to x ) 0.04. The lattice parameters of the Hf1-xLuxW2O8-y (x ) 0.02, 0.04) solid solutions are smaller than those of HfW2O8, although the ionic radius of the substituted Lu3+ ion is larger than that of the Hf4+ ion.23 A similar reduction is also seen in the Zr1-xMxW2O8-y (M ) Sc, Y, In, Lu) system. Considering the difference in ionic radius, the reduction implies the existence of a local region having a smaller lattice volume than the mother compound, as shown in Zr1-xMxW2O8-y (M ) Sc, Y, In, Lu).15-17 On the other hand, the lattice parameters of the Hf1-xLuxW2O8-y (x ) 0.02, 0.04) solid solutions are the same magnitude as those of HfW2O8 in the high-temperature phase, as shown in Figure 2. The coincidence suggests that the local region disappears or merges into the high-temperature structure above Ttrs. A similar behavior is also seen in Zr1-xLuxW2O8-y.17 The low-temperature phase of HfW2O8 has some characteristic diffraction peaks (e.g., 310 and 111 superlattice reflections).1,2 Since the square root of the integrated intensity is proportional to the crystal structure factor, that of the 310 reflection corresponds to a degree of order associated with the orientation of WO4 pairs in HfW2O8. This intensity increases from zero to a saturated value at sufficiently low temperatures

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Figure 4. Enlarged plots of the powder X-ray diffraction patterns of Hf1-xLuxW2O8-y (x ) 0.02, 0.04) at 121 K, together with that of HfW2O8.21

Yamamura et al.

Figure 6. Order-disorder phase transition temperatures of Hf1-xLux W2O8-y (x ) 0,21 0.02, 0.04) against Lu content, together with those of Zr1-xLuxW2O8-y (x ) 0, 0.02, 0.04).17

Figure 5. Temperature dependence of the order parameters in Hf1-xLuxW2O8-y (x ) 0.02, 0.04), together with that of HfW2O8.21

below Ttrs with a decreasing temperature. Figure 4 shows the diffraction peaks of the 310 and 311 reflections of Hf1-xLuxW2O8-y at 121 K. The saturated intensity of the 310 peak decreases clearly with an increasing Lu content, while the 311 peak does not change. The decrease means that the degree of orientational order of the WO4 pairs for Hf1-xLuxW2O8-y becomes smaller than that of HfW2O8 by Lu substitution. If the intensity of the 310 reflection is normalized by the saturated value at low temperatures, its change from zero to unity can be regarded as a so-called order parameter of the order-disorder phase transition assuming complete order at a sufficiently low temperature. In the previous work for ZrW2O8,16 we introduced an order parameter by using the following equation:

η ) x{(I 310/I210)Zr1-xMxW2O8-y}T/{(I310/I210)ZrW2O8}saturated

Figure 7. (a) Normalized order-disorder phase transition temperatures, Ttrs(A1-xLuxW2O8-y)/Ttrs(AW2O8), where A ) Zr17 or Hf, as a function of Lu content. (b) Saturated order parameters, ηs, of Zr1-xLuxW2O8-y17 and Hf1-xLuxW2O8-y (x ) 0, 0.02, 0.04) against Lu content.

where I310 is the integrated intensity of the 310 diffraction peak and I210 the integrated one of the 210 diffraction peak, which is the largest fundamental peak. The denominator is the saturated value of (I310/I210) of ZrW2O8 at sufficiently low temperatures. The saturated order parameters (η) of ZrW2O8 can be regarded as being a unity because ZrW2O8 shows the complete order effectively.3 This equation is applicable also to the HfW2O8 system. The η of Hf1-xLuxW2O8-y are estimated from the observed diffraction patterns and are plotted against temperature in Figure 5. Each η of solid solutions varies continuously from a saturated value (ηs) below ca. 200 K to zero with increasing temperature. The temperature reaching η ) 0 corresponds to the phase transition temperature (Ttrs). The saturated values (ηs) of Hf1-xLuxW2O8-y (x ) 0.02, 0.04) solid solutions are smaller than that of HfW2O8 (ηs ) 1). The smallness suggests that the orientation of the WO4 pairs does not order completely even at sufficiently low temperatures and that a part of the orientation remains disordered. The order of ηs of Hf1-xLuxW2O8-y is consistent with that of Zr1-xMxW2O8-y (M ) Sc, Y, In, Lu).16,17 The temperatures of the order-disorder phase transition (Ttrs) were estimated from Figure 5 by extrapolating the η curve to

zero. Figure 6 shows the Ttrs of Hf1-xLuxW2O8-y as a function of Lu content, together with that of Zr1-xLuxW2O8-y.17 The Ttrs of Hf1-xLuxW2O8-y decreases linearly with an increasing Lu content. The slope of Hf1-xLuxW2O8-y in Figure 6 is in agreement with that of Zr1-xLuxW2O8-y.17 For comparison, the normalized Ttrs values {Ttrs(A1-xLuxW2O8-y)/Ttrs(AW2O8)} of Hf1-xLuxW2O8-y and Zr1-xLuxW2O8-y are plotted in Figure 7a as a function of Lu content. There is little difference in the composition dependence of the normalized Ttrs values between Hf1-xLuxW2O8-y and Zr1-xLuxW2O8-y solid solutions. This agreement suggests that the effect of trivalent cation substitution on Ttrs is independent of the kind of mother compound. Figure 7b shows the saturated order parameters, ηs, of Hf1-xLuxW2O8-y and Zr1-xLuxW2O8-y17 as a function of Lu content. The ηs of Hf1-xLuxW2O8-y decreases linearly with increasing the Lu content, and its composition dependence of Hf1-xLuxW2O8-y is almost the same as that of Zr1-xLuxW2O8-y17 within experimental error, although the slope of Zr1-xLuxW2O8-y seems to be slightly steeper than that of Hf1-xLuxW2O8-y. The decrease in ηs means the reduction of the number of the WO4 pairs with the ordered orientation even at a sufficiently low

Substitution Effect on ZrW2O8-Based Solid Solutions

Figure 8. Normalized order-disorder phase transition temperatures of Hf1-xLuxW2O8-y (x ) 0,21 0.02, 0.04) plotted against their saturated order parameters, ηs, together with those of Zr1-xMxW2O8-y (M ) Sc, Y, In, Lu).16,17

temperature. In other words, some local regions including the WO4 pairs with the orientational disorder are generated locally in the crystal by Lu substitution. Indeed, this situation is confirmed by broadening the full width at half-maximum (fwhm) of the 310 superlattice peak in Hf1-xLuxW2O8-y. This broadening of the fwhm suggests that there is something to disturb the periodic order in the stacking of the diffraction planes and to cause distortion in the solid solution. That is, the periodic order is disturbed by the local regions including the WO4 pairs in the orientationally disordered state.16 The normalized Ttrs values {Ttrs(A1-xLuxW2O8-y)/Ttrs(AW2O8)} of Hf1-xLuxW2O8-y are plotted against their saturated order parameters ηs in Figure 8, together with those of Zr1-xMxW2O8-y (M ) Sc, Y, In, Lu) reported previously.16,17 In Zr1-xMxW2O8-y (M ) Sc, Y, In, Lu), Ttrs is proportional to ηs, and all of the Ttrs values are on a universal line.16,17 The data points of Hf1-xLuxW2O8-y are also on the universal line and maintain linearity in the Ttrs-ηs relation. Therefore, the effect of Lu substitution on the Hf1-xLuxW2O8-y system can be understood by our model associated with the existence of a local region including the WO4 pairs having an orientational disorder.16 If we assume the local region to form a sphere around the substituted Lu site, it is easy to estimate the size of the local region per one substituted Lu from the lattice parameter, the saturated order parameter, and the Lu content. The averaged radius of the local region is estimated to be about 1.6 nm in Hf1-xLuxW2O8-y at 121 K. This value of Hf1-xLuxW2O8-y is in agreement with that of Zr1-xLuxW2O8-y and is a middle value between Zr1-xScxW2O8-y and Zr1-xInxW2O8-y. The volume of the spherical region corresponds to about two to three unit cells. The large regions play the role of a cluster of nonmagnetic impurities in a site-diluted magnetic system, leading to the drastic lowering of the phase transition temperature in Hf1-xLuxW2O8-y with just a few percent Lu content. Classification of the Phase Transition of ZrW2O8Based Solid Solutions. Four types of ZrW2O8-based solid solutions have been reported previously: Zr1-xHfxW2O8,6-8 Zr(W1-xMox)2O8,9-14 A1-xMxW2O8-y (A ) Zr, Hf; M ) Sc, Y, In, Lu),8,15-19 and Zr1-xSnxW2O8.20 Their phase transition temperatures (Ttrs) are shown in Figure 9. Although the composition dependences in Ttrs are remarkably different, those solid solutions can be classified into two categories, considering the saturated order parameter. Zr1-xHfxW2O8 and Zr1-xSnxW2O8 having the same substituted valence as the mother compound are assigned to the first category. In these solid solutions, the saturated order parameter equals unity (ηs ) 1). That is, the orientation of the WO4 pairs is ordered completely at sufficiently low temperatures.

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Figure 9. Phase transition temperatures of Zr(Hf)W2O8-based solid solutions; Zr1-xHfxW2O8,6 Zr1-xSnxW2O8,20 Zr(W1-xMox)O8,11 Zr1-xMxW2O8-y (M ) Sc, Y, In, Lu),15-17 and Hf1-xLuxW2O8-y. Solid symbols belong to the first category (ηs ) 1; ηs is the saturated order parameter) and open symbols to the second category (ηs < 1).

ZrW2O8 and HfW2O8 form a complete solid solution.6 The Ttrs values of Zr1-xHfxW2O8 shift continuously from Ttrs(ZrW2O8) to Ttrs(HfW2O8) with increasing Hf content, while any saturated order parameters equal unity and are independent of the Hf content.6 The same behavior was seen in Zr1-xSnxW2O8.20 The solid solution takes complete ordering of WO4 pairs at sufficiently low temperatures, and their Ttrs values decrease with an increase in the Sn content. The unity of the saturated order parameter suggests that the suppression of Ttrs in Zr1-xSnxW2O8 is interpreted in the same line as Zr1-xHfxW2O8. Assuming the existence of SnW2O8 having the same crystal structure and order-disorder phase transition as ZrW2O8, the Ttrs value of Zr1-xSnxW2O8 possibly changes from Ttrs(ZrW2O8, 440 K) to Ttrs(SnW2O8, ca. 300 K is virtual Ttrs estimated by extrapolating Ttrs of Zr1-xSnxW2O8) with an increasing Sn content. If there is another solid solution of ZrW2O8 substituted for the Zr4+ site by another tetravalent cation, its order-disorder phase transition temperature possibly varies against the M4+ content in the same trend as Zr1-xHfxW2O8 and Zr1-xSnxW2O8. On the other hand, Zr(W1-xMox)2O8 and A1-xMxW2O8-y (A ) Zr, Hf; M ) Sc, Y, In, Lu) belong to the second category, where the saturated order parameter is less than unity (ηs < 1). The Ttrs values of Zr(W1-xMox)2O8, which is a solid solution between ZrMo2O8 and ZrW2O8, decrease with increasing Mo content.9 ZrWMoO8 (x ) 0.5) undergoes an order-disorder phase transition at 270 K.11 This phase transition at x ) 0.5 depends on the cooling rate11 and disappears in the quenched sample. The quenched ZrWMoO8 is frozen into a glassy state with the orientational disorder of the tetrahedral MoO4 or/and WO4 by rapid cooling and is able to maintain a high-temperature phase structure at sufficiently low temperatures.11,14 The frozenin orientational disorder of the tetrahedron is intrinsic in nature for the MoO4 in the framework of ZrW2O8 (or ZrMo2O8) since all MoO4 tetrahedra maintain the orientational disorder in cubic ZrMo2O8,10,11,13 which undergoes no orientational phase transition. In Zr(W1-xMox)2O8, the frozen MoO4 or/and WO4 act as a nonmagnetic impurity in a site-diluted magnetic system, leading to a lowering of the phase transition temperature.16 In A1-xMxW2O8-y (A ) Zr, Hf; M ) Sc, Y, In, Lu), the substitution effect on the phase transition has been interpreted previously through the same analogy as Zr(W1-xMox)2O8. The difference between Zr(W1-xMox)2O8 and A1-xMxW2O8-y is the size of the region with the frozen-in orientational disorder of the tetrahedra. In Zr(W1-xMox)2O8, the substituted Mo6+ produced one pair of tetrahedra with the orientational disorder. In contrast, the trivalent cation makes a local region with eight

10122 J. Phys. Chem. B, Vol. 111, No. 34, 2007 to 12 pairs of the tandem WO4 with the orientational disorder, leading to the drastic lowering of the phase transition temperature. Why is the substitution effect of the trivalent cation (M3+) much larger than that of Mo6+? The answer is concerned strongly with the following three factors: the large ionic radius of M3+, the formation of the oxygen vacancy, and the substitution of Zr4+ (Hf4+) at the center of the octahedron (ZrO6). Stabilization of the local region with the orientational disorder of WO4 is related closely to a local distortion of the lattice,16 which results from the introduction of the trivalent cation with a larger ionic radius than Zr4+ (Hf4+)23 and the formation of an oxygen vacancy due to the charge compensation to maintain the electroneutrality condition. The oxygen vacancy may lie near the substituted M3+ sites, which are distributed randomly over the solid solution. The former pushes the oxide ion around the substituted M3+, and the latter yields a reduction in volume. A balance between the counteracting two factors is very important for the stabilization of the solid solution since Ga3+ and Gd3+ with a smaller ionic radius23 than Zr4+ are not able to form the solid solution according to our preliminary experiment. The two factors yield the local distortion around the substituted M3+ lying at the center of the octahedron. Since the octahedron shares its oxygen atom with the six WO4 molecules, the local distortion affects directly the orientation of the surrounding six WO4 pairs at least. The local distortion is relieved gradually by being separated from the M3+ site, and its influence reaches the region 1.3-1.7 pm in diameter.16,17 In this way, the local region is formed by M3+ substitution. The local region behaves in the same way as the cluster of nonmagnetic impurities in the dilute magnetic system, leading to the lowering of Ttrs under the socalled percolation theory.16 Conclusion Hf1-xLuxW2O8-y solid solutions were synthesized, and their X-ray powder diffraction patterns were measured in the temperature region from 90 to 560 K. The temperature dependence of the characteristic diffraction peak provides the corresponding order parameter (η) associated with the orientational order of the tandem WO4 pairs, fixing the phase transition temperature. The Ttrs decreased significantly with an increasing Lu content. The saturated order parameter (ηs) at a sufficiently low temperature was less than unity, meaning that there was an existence of a region with the frozen-in orientational disorder of the WO4 pairs. The features of Hf1-xLuxW2O8-y in this study are in agreement with those of Zr1-xLuxW2O8-y. The relation between Ttrs and ηs in Hf1-xLuxW2O8-y is consistent with that of Zr1-xMxW2O8-y. It is concluded that the M3+ substitution effect on Hf1-xLuxW2O8-y is interpreted in the framework of our proposed model for Zr1-xMxW2O8-y, which states the existence of a local nanoregion including the WO4 pairs with an orientational disorder. Including Hf1-xLuxW2O8-y, four types of ZrW2O8-based solid solutions have been investigated so far. These are classified into two categories in view of the magnitude of ηs. The first category (ηs ) 1) consists of Zr1-xHfxW2O8 and Zr1-xSnxW2O8, where the end members of the solid solution undergo the phase transition inherently, and Ttrs shifts from one end to the other

Yamamura et al. with composition. The Zr1-xHfxW2O8 and Zr1-xSnxW2O8 solid solutions are substituted for the Zr4+ site by tetravalent cations (Hf4+, Sn4+). The substitution effect of the tetravalent cation acts directly on the framework of ZrW2O8 but is limited to an indirect contribution to the orientation of the WO4 pairs. On the other hand, Zr(W1-xMox)2O8 and A1-xMxW2O8-y (A ) Zr, Hf; M ) Sc, Y, In, Lu) belong to the second category (ηs < 1), where the tetrahedra with orientational disorder remain even at sufficiently low temperatures. These solid solutions behave just like nonmagnetic impurities in a site-diluted magnetic system, leading to the suppression of Ttrs. The substitution effect in the second category acts directly on the orientation of the tetrahedra (WO4 or/and MO4) pairs. The difference between Zr(W1-xMox)2O8 and A1-xMxW2O8-y lies in the number of the orientationally disordered tetrahedra formed by cation substituiton. That is, the M3+ substitution affects not only the WO4 pairs but also the framework of ZrW2O8 directly, while the Mo6+ substitution influences the on-site tetrahedra pairs only. The classification based on ηs leads to an overall understanding of the order-disorder phase transition in ZrW2O8-based solid solutions. References and Notes (1) Mary, T. A.; Evans, J. S. O.; Vogt, T.; Sleight, A. W. Science 1996, 272, 90. (2) Evans, J. S. O.; Mary, T. A.; Vogt, T. M.; Subramanian, A.; Sleight, A. W. Chem. Mater. 1996, 8, 2809. (3) Evans, J. S. O.; David, W. I. F.; Sleight, A. W. Acta Crystallogr., Sect. B: Struct. Sci. 1999, 55, 333. (4) Yamamura, Y.; Nakajima, N.; Tsuji, T. Solid State Commun. 2000, 114, 453. (5) Yamamura, Y.; Tsuji, T.; Saito, K.; Sorai, M. J. Chem. Thermodyn. 2004, 36, 525. (6) Nakajima, N.; Yamamura, Y.; Tsuji, T. J. Therm. Anal. Calorim. 2002, 70, 337. (7) Yamamura, Y.; Nakajima, N.; Tsuji, T.; Koyano, M.; Iwasa, Y.; Katayama, S.; Saito, K.; Sorai, M. J. Ceram. Soc. Jpn. 2004, 112, 291. (8) Tsuji, T.; Yamamura Y.; Nakajima, N. Thermochim. Acta 2004, 416, 93. (9) Closmann, S.; Sleight, A. W.; Haygarth, J. C. J. Solid State Chem. 1998, 139, 424. (10) Lind, C.; Wilkinson, A. P.; Hu, Z.; Short, S.; Jorgensen, J. D. Chem. Mater. 1998, 10, 2335. (11) Evans, J. S. O.; Hanson, P. A.; Ibberson, R. M.; Duan, N.; Kameswari, U.; Sleight, A. W. J. Am. Chem. Soc. 2000, 122, 8694. (12) Kameswari, U.; Sleight, A. W.; Evans, J. S. O. Int. J. Inorg. Mater. 2000, 2, 333. (13) Allen, S.; Evans, J. S. O. Phys. ReV. B 2003, 68, 134101. (14) Allen, S.; Evans, J. S. O. J. Mater. Chem. 2004, 14, 151. (15) Nakajima, N.; Yamamura, Y.; Tsuji, T. Solid State Commun. 2003, 128, 193. (16) Yamamura, Y.; Nakajima, N.; Tsuji, T.; Kojima, A.; Kuroiwa, Y.; Sawada, A.; Aoyagi, S.; Kasatani, H. Phys. ReV. B 2004, 70, 104107. (17) Yamamura, Y.; Kato, M.; Tsuji, T. Thermochim. Acta 2005, 431, 24. (18) Morito, Y.; Takahashi, K.; Wang, S.; Abe, H.; Katoh, A.; Hashimoto, T. J. Ceram. Soc. Jpn. 2002, 110, 807. (19) Hashimoto, T.; Kuwahara, J.; Yoshida, T.; Nashimoto, M.; Takahashi, Y.; Takahashi, K.; Morito, Y. Solid State Commun. 2004, 131, 217. (20) Meyer, C. D.; Bouree, F.; Evans, J. S. O.; Buysser, K. D.; Bruneel, E.; Driessche, I. V.; Hoste, S. J. Mater. Chem. 2004, 14, 2988. (21) Yamamura, Y.; Nakajima, N.; Tsuji, T. Phys. ReV. B 2001, 64, 184109. (22) Nelson, J. B.; Riley, D. P. Proc. Phys. Soc. London 1945, 57, 160. (23) Shannon, R. D. Acta Crystallogr., Sect. A: Found. Crystallogr. 1976, 32, 751.