Low-Temperature X-ray Crystal Structure Analysis of the Cage-Structured Compounds MBe13 (M = La, Sm, and U)

TL;DR

This study investigates the low-temperature crystal structures of MBe13 compounds (M = La, Sm, U), confirming their cage structure stability and identifying a low-energy phonon mode through X-ray diffraction and analysis.

Contribution

It provides detailed low-temperature structural data and phonon mode analysis for MBe13 compounds, expanding understanding of their lattice dynamics.

Findings

MBe13 compounds retain their cage structure down to 7 K.

Debye temperature estimated between 600-750 K.

Presence of a low-energy phonon mode with ~160 K characteristic temperature.

Abstract

The beryllides MBe13 (M = rare earths and actinides) crystallize in a NaZn13-type cubic structure, which can be categorized as a cage-structured compound. In this study, powder X-ray diffraction measurements have been performed on LaBe13, SmBe13, and UBe13 in the temperature range between 7 and 300 K in order to investigate their crystallographic characteristics systematically. They keep the NaZn13-type cubic structure down to the lowest temperature. We estimated their Debye temperature to be 600 - 750 K from analyses of the temperature dependence of a lattice parameter, being in good agreement with the values reported previously. Rietveld refinements on the obtained powder patterns revealed that the M atom in the 8a site is located in an almost ideal snub cube formed by 24 Be atoms in the 96i site, whose caged structure is unchanged even at the low temperatures. In addition, it is argued from the temperature variation of an isotropic mean-square displacement parameter that the MBe13 compounds commonly have a low-energy phonon mode, which can be described by a model assuming an Einstein oscillation of the M atom with a characteristic temperature of ~ 160 K.