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

Abstract

By applying high temperature (1270 K) and high pressure (3.5 GPa), significant changes occur in the structural volume and crystal topology of ThMo2 O8 , allowing the formation of an unexpected new ThMo2 O8 polymorph (high-temperature/high-pressure (HT/HP) orthorhombic ThMo2 O8 ). Compared with the other three ThMo2 O8 polymorphs prepared at the ambient pressure (monoclinic, orthorhombic, and hexagonal phases), the molar volume for the quenched HT/HP-orthorhombic ThMo2 O8 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 ThMo2 O8 phases are based on three-dimensional (3D) frameworks constructed from corner-sharing ThOx (x=6, 8, or 9) polyhedra and MoO4 tetrahedra. The HT/HP-orthorhombic ThMo2 O8 , 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 ThMo2 O8 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.

The Role of Acidity in the Synthesis of Novel Uranyl Selenate and Selenite Compounds and Their Structures

“…[43]. Deformation of the structure or occurrence of corrugation in layers often occurs through factors, such as the application of pressure during synthesis [30,37,44], non-spherical interlayer ions [45], or the unideal coordination of layer ions [30,46], where the structures or layers deform accordingly to accommodate these. In the case of Rb 2 [(UO 2 ) 2 (SeO 4 ) 3 ], the relatively simplistic formula would suggest a less distorted structure, however the occurrence of UO 7 and SeO 4 polyhedra results in the latter of the described examples.…”

Rich Non-centrosymmetry in a Na–U–Te Oxo-System Achieved under Extreme Conditions

“…Additionally, this method is also particularly suitable for studying temperature−pressure-induced phase transition behaviors. 30 The hightemperature/high-pressure experiments were performed using the piston cylinder module of a Voggenreiter LP 1000-540/50 (Forschungszentrum Julich, IEK-6) in a 0.5 in. piston cylinder talc− Pyrex assembly.…”

Advances and perspectives of actinide chemistry fromex situhigh pressure and high temperature chemical studies

“…A pertinent example that illustrates the structural flexibility of thorium and how it can be manipulated through HP/HT conditions is exemplified by ThMo 2 O 8 . 49 At ambient pressure, using specific high temperature conditions ThMo 2 O 8 can adopt three different structural types, monoclinic (SG P 2 1 / c ), orthorhombic (SG Pbca ) and hexagonal (SG P 3̄). 49 When exposed to conditions of over 3.5 GPa and 1000 °C an orthorhombic variant (SG Pna 2 1 ) can be generated culminating in a reduction of molar volume by 20% compared to that of the ambient pressure monoclinic variant.…”