Investigation of the OrderDisorder Rotator Phase Transition in KSiH3 and RbSiH3

The beta-alpha (order -disorder) transition in the silanides ASiH(3) (A = K, Rb) was investigated by multiple techniques, including neutron powder diffraction (NPD, on the corresponding deuterides), Raman 10 spectroscopy, heat capacity (C-P), solid-state H-2 NMR spectroscopy, and quasi-elastic neutron scattering (QENS). The crystal structure of alpha-ASiH(3) 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 (beta) 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 C-p(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 parameters and the broadening and eventual disappearance of libration modes in the Raman spectra. In alpha-ASiH(3), the SiH3- anions undergo rotational diffusion with average relaxation times of 0.2-0.3 ps between successive H jumps. The first -order reconstructive phase transition is characterized by a large hysteresis (20-40 K). H-2 NMR revealed that the a -form can coexist, presumably as 2-4 nm (sub-Bragg) sized domains, with the,6-phase below the phase transition temperatures established from C-P, measurements. The reorientational mobility of H atoms in undercooled alpha-phase is reduced, with relaxation times on the order of picoseconds. The occurrence of rotator phases alpha-ASiH(3) near room temperature and the presence of dynamical disorder even in the low-temperature beta-phases imply that SiH3- ions are only weakly coordinated in an environment of A(+) cations. The orientational flexibility of SiH3- can be attributed to the simultaneous presence of a lone pair and (weakly) hydridic hydrogen ligands, leading to an ambidentate coordination behavior toward metal cations