Correlation between AO₆ Polyhedral Distortion and Negative Thermal Expansion in Orthorhombic Y₂Mo₃O₁₂ and Related Materials

Abstract

The Pbcn orthorhombic phase of Y2Mo3O12 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, αl, is −9.02 × 10−6 K−1 averaged over T = 20−450 K. From a thorough analysis of the structure of Y2Mo3O12, we find that the YO6 octahedra and MoO4 tetrahedra are increasingly distorted with increasing temperature. The inherent volume distortion parameter (υ) of AO6 has been introduced to quantitatively evaluate polyhedral distortion and it is observed that this parameter is strongly correlated with the linear coefficient thermal expansion (αl) of different members of the A2M3O12 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.