Bulk thermal expansion for tungstate and molybdates of the type A 2 M 3 O 12
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Bulk thermal expansion for tungstate and molybdates of the type A2 M3 O12 T. A. Mary and A. W. Sleight Department of Chemistry and Center for Advanced Materials Research, Oregon State University, 153 Gilbert Hall, Corvallis, Oregon 97331-4003 (Received 23 April 1998; accepted 16 July 1998)
Bulk thermal expansion properties of 19 members of the A2 M3 O12 family of tungstates and molybdates were determined from room temperature to 800 ±C. The observed behavior ranges from strong negative thermal expansion (a 211 3 1026 K21 ) in Sc2 W3 O12 to near zero thermal expansion in Al1.68 Sc0.02 In0.30 W3 O12 .
I. INTRODUCTION
We have recently reported1–3 unusual thermal expansion properties for an isostructural A2 M3 O12 family. For most members of this family, M is W or Mo and A is a trivalent cation ranging in size from Al to the smaller rare earths. However, the A cation can also be Zr or Hf, in which case M3 is WP2 or MoP2 . The structure for members of this family is ideally orthorhombic, but some members have a structure collapse phase transition to a monoclinic structure as temperature is decreased.4 Because crystallites of materials of this family exhibit anisotropic thermal expansion, values of thermal expansion derived from diffraction data can differ significantly from values obtained from size changes measured on ceramic bars. The strongest intrinsic negative thermal expansion behavior based on diffraction data we have seen in this A2 M3 O12 family is 26.8 3 1026 K21 for Lu2 W3 O12 .5,6 II. EXPERIMENTAL
All samples were prepared by calcination of appropriate quantities of binary oxides which had been ground together with an agate mortar and pestle. Final calcination temperatures were 1200 ±C for tungstates and 1000 ±C for molybdates. Phase purity was confirmed by x-ray diffraction. Thermal expansion was determined from room temperature to 800 ±C using a Netzsch dilatometer with fused silica components.
obtained in certain solid solutions (Fig. 6). When several curves are shown for a particular composition, these reflect repeated measurements on the same sample. The variation is presumably the result of irreversible changes in microstructure on thermal cycling. The abrupt change from positive-to-negative thermal expansion for In2 W3 O12 occurs at a temperature previously proposed to be the monoclinic-to-orthorhombic phase transition for this compound.4 This transition is below room temperature in the case of Al2 W3 O12 .4 Substitution of 50% of Al for In completely suppresses this transition in the 25 to 800 ±C region. Substitution of 50% of the In by Sc and Er suppresses this phase transition to about 60 and 135 ±C, respectively (Figs. 7 and 8). Both Sc2 W3 O12 and Sc2 Mo3 O12 show strong negative thermal expansion behavior, with the effect being about twice as strong in the tungstate (Figs. 4 and 5). Attempts to prepare Ga2 W3 O12 and Ga2 Mo3 O12 have apparently never succeeded, but it is possible to partially substitute Ga on the A site leading to phases such a
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