title: Giant Volume Change and Topological Gaps in Temperature- and Pressure-Induced Phase Transitions: Experimental and Computational Study of ThMo2 O8.
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.
authors.affiliation: Institute of Energy and Climate Research (IEK-6), Forschungszentrum Jülich GmbH, 52428, Jülich, Germany.|Institut für Kristallographie, RWTH Aachen University, 52066, Aachen, Germany.
authors.forename: Bin
authors.initials: B
authors.lastname: Xiao
authors.affiliation: Institute of Energy and Climate Research (IEK-6), Forschungszentrum Jülich GmbH, 52428, Jülich, Germany.
authors.forename: Philip
authors.initials: P
authors.lastname: Kegler
authors.affiliation: Chemische Kristallographie fester Stoffe, Institut für Anorganische Chemie und Kristallographie, Universität Bremen, Leobener Straße, 28359, Bremen, Germany.
authors.forename: Thorsten M
authors.initials: TM
authors.lastname: Gesing
authors.affiliation: Chemische Kristallographie fester Stoffe, Institut für Anorganische Chemie und Kristallographie, Universität Bremen, Leobener Straße, 28359, Bremen, Germany.
authors.forename: Lars
authors.initials: L
authors.lastname: Robben
authors.affiliation: Institute of Energy and Climate Research (IEK-6), Forschungszentrum Jülich GmbH, 52428, Jülich, Germany.|JARA High-Performance Computing, Schinkelstraße 2, 52062, Aachen, Germany.|Department of Chemistry, Thomas Young Centre, Imperial College London, London, SW7 2AZ, UK.
authors.forename: Ariadna
authors.initials: A
authors.lastname: Blanca-Romero
authors.affiliation: Institute of Energy and Climate Research (IEK-6), Forschungszentrum Jülich GmbH, 52428, Jülich, Germany.|JARA High-Performance Computing, Schinkelstraße 2, 52062, Aachen, Germany.
authors.forename: Piotr M
authors.initials: PM
authors.lastname: Kowalski
authors.affiliation: Institute of Energy and Climate Research (IEK-6), Forschungszentrum Jülich GmbH, 52428, Jülich, Germany.|JARA High-Performance Computing, Schinkelstraße 2, 52062, Aachen, Germany.
authors.forename: Yan
authors.initials: Y
authors.lastname: Li
authors.affiliation: Institute of Energy and Climate Research (IEK-6), Forschungszentrum Jülich GmbH, 52428, Jülich, Germany.
authors.forename: Vladislav
authors.initials: V
authors.lastname: Klepov
authors.affiliation: Institute of Energy and Climate Research (IEK-6), Forschungszentrum Jülich GmbH, 52428, Jülich, Germany.|JARA High-Performance Computing, Schinkelstraße 2, 52062, Aachen, Germany.
authors.forename: Dirk
authors.initials: D
authors.lastname: Bosbach
authors.affiliation: Institute of Energy and Climate Research (IEK-6), Forschungszentrum Jülich GmbH, 52428, Jülich, Germany. e.alekseev@fz-juelich.de.|Institut für Kristallographie, RWTH Aachen University, 52066, Aachen, Germany. e.alekseev@fz-juelich.de.|JARA High-Performance Computing, Schinkelstraße 2, 52062, Aachen, Germany. e.alekseev@fz-juelich.de.
authors.forename: Evgeny V
authors.initials: EV
authors.lastname: Alekseev
keywords: actinides; high-pressure chemistry; high-temperature conditions; negative thermal expansion; phase transitions; structural polymorphs