Crystal structure and stability ofβ-Na2ThF6 at non-ambient conditions

Abstract

The crystal structure and stability ofβ-Na2ThF6 at non-ambient conditions have been studied with calorimetricanalysis, second harmonic generation measurements, and19F and 23Na magic-angle spinning nuclear magnetic resonance (MAS NMR) spectroscopy, as well assynchrotron x-ray single-crystal and powder diffraction. The twinned structure(P321,Z = 1) is built of chains of capped trigonal prisms around the Th atoms formed along thec-axis through sharing of the basal faces. More distorted capped trigonalprisms around the Na atoms share their basal and equatorial faceswith each other. The twin operation is a two-fold rotation around thec-axis.β-Na2ThF6 is stable in the temperature and pressure ranges of 100–954 K and 0.0001–6.4 GPa,respectively. The Na–F distances are more compressible than the Th–F distances. Ahypothetical ferroelastoelectric and ferrobielastic  phase transition is discussed.