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

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.