Investigation of the Order–Disorder Rotator Phase Transition in KSiH3 and RbSiH3

Abstract

The β–α (order–disorder) transition in the silanides ASiH3 (A = K, Rb) was investigated by multiple techniques, including neutron powder diffraction (NPD, on the corresponding deuterides), Raman spectroscopy, heat capacity (Cp), solid-state 2H NMR spectroscopy, and quasi-elastic neutron scattering (QENS). The crystal structure of α-ASiH3 corresponds to a NaCl-type arrangement of alkali metal ions and randomly oriented, pyramidal, SiH3– moieties. At temperatures below 200 K ASiH3 exist as hydrogen-ordered (β) forms. Upon heating the transition occurs at 279(3) and 300(3) K for RbSiH3 and KSiH3, respectively. The transition is accompanied by a large molar volume increase of about 14%. The Cp(T) behavior is characteristic of a rotator phase transition by increasing anomalously above 120 K and displaying a discontinuous drop at the transition temperature. Pronounced anharmonicity above 200 K, mirroring the breakdown of constraints on SiH3– rotation, is also seen in the evolution of atomic displacement paramet...