Negative thermal expansion in AOMO(4) compounds.

摘要

Several members of the AOMO₄ family have been evaluated for their potential as negative thermal expansion (NTE) materials. In these systems the A and M pentavalent cations are coordinated by oxygen to form octahedra and tetrahedra, respectively, and their overall connectivity consists of a three-dimensional framework built of comer-shared polyhedra. The structures of tetragonal NbOPO₄, TaOPO₄, monoclinic NbOPO₄, orthorhombic TaOVO₄, and a solid solution member of the Ta-V-As-O system were investigated as a function of temperature using diffraction techniques.; All structural work was carried out using X-ray powder diffraction measurements and in some cases high-resolution powder neutron diffraction. Dilatometry and differential scanning calorimetry were employed for preliminary thermal analysis to gain an understanding of the bulk thermal expansion and transition temperatures. Computer simulations were compiled using two separate computer programs, Distance Least Squares and CRUSH, both of which give an indication of the flexibility of the framework structures studied.; A phase transition was found for most of the compounds studied. Negative thermal expansion was seen in the high temperature polymorphs of NbOPO ₄ and TaOPO₄ and also in the orthorhombic TaOVO₄ structure.; One mechanism for negative thermal expansion has been identified for all these NTE compounds. This is based on the transverse motion of the polyhedral bridging oxygen atoms.