Structural and Dynamical Trends in Alkali-Metal Silanides Characterized by Neutron-Scattering Methods

Published

September 2, 2016

Author(s)

Wan Si NMN Tang, Mirjana NMN Dimitrievska, Jean-Noel Chotard, Wei Zhou, Raphael Janot, Alexander V. Skripov, Terrence J. Udovic

Abstract

Structural, vibrational, and dynamical properties of the mono- and mixed-alkali silanides (MSiH₃, where M=K, Rb, Cs, K_0.5Rb_0.5Cs_0.5, and Rb_0.5Cs_0.5) were investigated by various neutron experiments, including neutron powder diffraction (NPD), neutron vibrational spectroscopy (NVS), neutron scattering fixed window scans (FWS) and quasielastic neutron scattering measurements (QENS). Structural characterization showed that the mixed compounds exhibit disordered (alpha) and ordered (beta) phases for temperatures above and below around 200-250 K, respectively, in agreement with their mono-alkali correspondents. Vibrational and dynamical properties are strongly influenced by the cation environment, in particular there is a red-shift in band energies of the librational and bending modes for increasing lattice size due to changes in bond lengths and force constants. Additionally, slightly broader spectral features are observed in the case of the mixed compounds, indicating the presence of structural disorder caused by the random distribution of the alkali-metal cations within the lattice. FWS measurements upon heating show that there is a large increase in reorientational mobility upon going through the order-disorder (β-α) phase transition, and measurements upon cooling the α-phase reveal the known strong hysteresis for reversions back to the β-phase. Interestingly, at a given temperature among the different alkali silanide compounds, the relative reorientational mobilities of the SiH₃- anions in the α- and β-phases tend to decrease and increase, respectively, with increasing alkali-metal mass. This dynamical result may provide some insights concerning the enthalpy-entropy compensation effect previously observed for these potentially promising hydrogen storage materials.

Citation

Journal of Physical Chemistry C

Volume

120

Issue

38

Pub Type

Journals

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Keywords

Alkali-Metal Silanides, Neutron powder diffraction, neutron vibrational spectroscopy, fixed window scans, quasielastic neutron scattering, Reorientational Dynamics, vibrational properties

Citation

Tang, W. , Dimitrievska, M. , Chotard, J. , Zhou, W. , Janot, R. , Skripov, A. and Udovic, T. (2016), Structural and Dynamical Trends in Alkali-Metal Silanides Characterized by Neutron-Scattering Methods, Journal of Physical Chemistry C, [online], https://tsapps.nist.gov/publication/get_pdf.cfm?pub_id=921363 (Accessed May 10, 2024)