The two alkali silanides ASiH3 (A = K and Rb) were investigated by means of quasielastic neutron scattering, both below and above the order–disorder phase transition occurring at around 275–300 K. Measurements upon heating show that there is a large change in the dynamics on going through the phase transition, whereas measurements upon cooling reveal a strong hysteresis due to undercooling of the disordered phase. The results show that the dynamics is associated with rotational diffusion of SiH3 – anions, adequately modeled by H-jumps among 24 different jump locations radially distributed around the Si atom. The average relaxation time between successive jumps is of the order of subpicoseconds and exhibits a weak temperature dependence with a small difference in activation energy between the two materials, 39(1) meV for KSiH3 and 33(1) meV for RbSiH3. The pronounced SiH3 – dynamics explains the high entropy observed in the disordered phase resulting in the low entropy variation for hydrogen absorption/desorption and hence the origin of these materials’ favorable hydrogen storage properties.

The β−α (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 spectroscopy, heat capacity (C p ), solid-state 2 H NMR spectroscopy, and quasi-elastic neutron scattering (QENS). The crystal structure of α-ASiH 3 corresponds to a NaCl-type arrangement of alkali metal ions and randomly oriented, pyramidal, SiH 3 − moieties. At temperatures below 200 K ASiH 3 exist as hydrogen-ordered (β) forms. Upon heating the transition occurs at 279(3) and 300(3) K for RbSiH 3 and KSiH 3 , 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 SiH 3 − 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 α-ASiH 3 , the SiH 3 − 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). 2 H NMR revealed that the α-form can coexist, presumably as 2−4 nm (sub-Bragg) sized domains, with the β-phase below the phase transition temperatures established from C p measurements. The reorientational mobility of H atoms in undercooled α-phase is reduced, with relaxation times on the order of picoseconds. The occurrence of rotator phases α-ASiH 3 near room temperature and the presence of dynamical disorder even in the low-temperature β-phases imply that SiH 3 − ions are only weakly coordinated in an environment of A + cations. The orientational flexibility of SiH 3 − 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.

The hydrogen storage materials ASiH3 (A = K and Rb) represent complex metal hydrides built from metal cations and pyramidal SiH3− ions. At room temperature, SiH3− moieties are randomly oriented because of dynamical disorder (α‐modifications). At temperatures below 200 K, ASiH3 exist as ordered low‐temperature (β) modifications. The vibrational properties of β‐ASiH3 were characterized by a combination of Raman spectroscopy and inelastic neutron scattering. Internal modes of SiH3− are observed in the spectral range 1800–1900 cm−1 (stretching modes) and 890–1000 cm−1 (bending modes). External modes are observed below 500 cm−1. Specifically, SiH3− librations are between 300–450 cm−1 and 270–400 cm−1 for A = K and Rb, respectively, SiH3− translations are between 95 and 160 cm−1, K+ translations are in the range 60–100 cm−1 and Rb+ translations in the range 50–70 cm−1. The red‐shift of libration modes for A = Rb is associated with a 15–30% reduction of the libration force constants of SiH3− ions in β‐RbSiH3. This correlates with a lower temperature for the β–α order–disorder phase transition (278 vs 298 K). Libration modes become significantly anharmonic with increasing temperature but are maintained up to at least 200 K. The vibrational properties of ASiH3 compare well to those of alkali metal borohydrides ABH4 (A = Na–Cs). Copyright © 2016 John Wiley & Sons, Ltd.

The accuracy of power system grounding analysis is critically dependent on the resistivity of the local soil surrounding the grounding system. This paper develops and discusses a new closed-form formula based on field theory relating soil resistivity to measured resistance which relaxes the requirement that rod electrodes have a large spacing compared to their buried length. The paper also demonstrates that the interpretation of practical resistivity measurements based on the new equation is significantly more accurate than the one based on the classical equation.

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