The Pbcn orthorhombic phase of Y 2 Mo 3 O 12 has been examined through high-resolution X-ray powder diffraction (10-450 K), heat capacity determination (2-390 K), and differential scanning calorimetry (103-673 K). No phase transition was found over this temperature range. The overall thermal expansion is negative, and the average linear thermal expansion coefficient, R l , is -9.02 Â 10 -6 K -1 averaged over T = 20-450 K. From a thorough analysis of the structure of Y 2 Mo 3 O 12 , we find that the YO 6 octahedra and MoO 4 tetrahedra are increasingly distorted with increasing temperature. The inherent volume distortion parameter (υ) of AO 6 has been introduced to quantitatively evaluate polyhedral distortion and it is observed that this parameter is strongly correlated with the linear coefficient thermal expansion (R l ) of different members of the A 2 M 3 O 12 family. We attribute the negative thermal expansion to the reduction of the mean Y-Mo nonbonded distances and Y-O-Mo bond angles with increasing temperature, the joint action of high-energy optical and low-energy translational and librational modes.

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In the search for materials with negative thermal expansion, the HfMgMo3O12 phase has been synthesized and characterized for the first time. It is established that this phase belongs to a new class of ceramics with the general formula A4+B2+M6+3O12 that crystallizes in orthorhombic symmetry of space group Pnma (62) or Pna21 (33). No evidence is observed for any phase transition of orthorhombic HfMgMo3O12 through differential scanning calorimetry and high‐temperature X‐ray diffraction measurements in the investigated temperature range (298–1013 K). This new phase exhibits unusually low thermal expansion (αl = 1.02 × 10−6 K−1) as determined by high‐temperature X‐ray diffraction using a synchrotron source. (© 2008 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

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