Tracking the Progression of Anion Reorientational Behavior between α-Phase and β-Phase Alkali-Metal Silanides, MSiH3, by Quasielastic Neutron Scattering.

Abstract

Quasielastic neutron scattering (QENS) measurements over a wide range of energy resolutions were used to probe the reorientational behavior of the pyramidal SiH3 - anions in the monoalkali silanides (MSiH3, where M = K, Rb, and Cs) within the low-temperature ordered β-phases, and for CsSiH3, the high-temperature disordered α-phase and intervening hysteretic transition region. Maximum jump frequencies of the β-phase anions near the β-α transitions range from around 109 s-1 for β-KSiH3 to 1010 s-1 and higher for β-RbSiH3 and β-CsSiH3. The β-phase anions undergo uniaxial 3-fold rotational jumps around the anion quasi-C 3 symmetry axis. CsSiH3 was the focus of further studies to map out the evolving anion dynamical behavior at temperatures above the β-phase region. As in α-KSiH3 and α-RbSiH3, the highly mobile anions (with reorientational jump frequencies approaching and exceeding 1012 s-1) in the disordered α-CsSiH3 are all adequately modeled by H jumps between 24 different locations distributed radially around the anion center of gravity, although even higher anion reorientational disorder cannot be ruled out. QENS data for CsSiH3 in the transition region between the α- and β-phases corroborated the presence of dynamically distinct intermediate (i-) phase. The SiH3 - anions within i-phase appear to undergo uniaxial small-angular-jump reorientations that are more akin to the lower-dimensional β-phase anion motions rather than to the multidimensional α-phase anion motions. Moreover, they possess orientational mobilities that are an order-of-magnitude lower than those for α-phase anions but also an order-of-magnitude higher than those for β-phase anions. Combined QENS and neutron powder diffraction results strongly suggest that this i-phase is associated chiefly with the more short-range-ordered, nanocrystalline portions (invisible to diffraction) that appear to dominate the CsSiH3.