Giant Volume Change and Topological Gaps in Temperature‐ and Pressure‐Induced Phase Transitions: Experimental and Computational Study of ThMo2O8

By applying high temperature (1270 K) and high pressure (3.5 GPa), significant changes occur in the structural volume and crystal topology of ThMo₂O₈, allowing the formation of an unexpected new ThMo₂O₈ polymorph (high‐temperature/high‐pressure (HT/HP) orthorhombic ThMo₂O₈). Compared with the other three ThMo₂O₈ polymorphs prepared at the ambient pressure (monoclinic, orthorhombic, and hexagonal phases), the molar volume for the quenched HT/HP–orthorhombic ThMo₂O₈ is decreased by almost 20 %. As a result of such a dramatic structural transformation, a permanent high‐pressure quenchable state is able to be sustained when the pressure is released. The crystal structures of the three ambient ThMo₂O₈ phases are based on three‐dimensional (3D) frameworks constructed from corner‐sharing ThO_x (x=6, 8, or 9) polyhedra and MoO₄ tetrahedra. The HT/HP–orthorhombic ThMo₂O₈, however, crystallizes in a novel structural topology, exhibiting very dense arrangements of ThO₁₁ and MoO₄₊₁ polyhedra connecting along the crystallographic c axis. The phase transitions among all four of these ThMo₂O₈ polymorphs are unveiled and fully characterized with regard to the structural transformation, thermal stability, and vibrational properties. The complementary first principles calculations of Gibbs free energies reveal the underlying energetics of the phase transition, which support the experimental findings.