Negative thermal expansion correlated with polyhedral movements and distortions in orthorhombic Y2Mo3O12

The internal polyhedral distortions have been reported experimentally in orthorhombic Y2Mo3O12 as a negative thermal expansion (NTE) material. To reveal the relationship between NTE and polyhedral movements, distortions, the vibrational properties of Y2Mo3O12 have been studied using first-principles calculations. The lowest optical branch corresponding to translational mode of the O bridge atom in YOMo linkage has the largest negative Grüneisen parameter and therefore contributes most to the NTE behavior. The different vibrational eigenvectors of oxygen atoms relative to Y or Mo atoms can cause internal polyhedral to distort unevenly. Herein, an extended 3D model of the connected unit YO6MoO4 based on the YOMo linkage presents a simultaneous dynamic process, i.e. the YO6 octahedra and MoO4 tetrahedra distort unevenly, along with both polyhedra being closer which makes the volumetric contraction. This model is helpful to improve the mechanisms of NTE and may be applied in the whole A2M3O12 family.