Investigation of the Order-Disorder Rotator Phase Transition in KSiH3 and RbSiH3
Artikel i vetenskaplig tidskrift, 2017

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.


R. Nedumkandathil

Stockholms universitet

A. Jaworski

Stockholms universitet

A. Fischer

Universität Augsburg

Carin Österberg

Chalmers, Fysik, Kondenserade materiens fysik

Yuan-Chih Lin

Chalmers, Fysik, Kondenserade materiens fysik

Maths Karlsson

Chalmers, Fysik, Kondenserade materiens fysik

J. Grins

Stockholms universitet

A. J. Pell

Stockholms universitet

M. Eden

Stockholms universitet

U. Haussermann

Stockholms universitet

Journal of Physical Chemistry C

1932-7447 (ISSN) 1932-7455 (eISSN)

Vol. 121 9 5241-5252





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