Giant Thermal Vibrations in the Framework Compounds Ba1-xSrxAl2O4

TL;DR

This study investigates giant thermal vibrations in Ba1-xSrxAl2O4, revealing persistent soft modes and anomalously large atomic displacements across a wide temperature range, especially with increased Sr substitution.

Contribution

It provides new insights into the thermal vibrations and soft mode behavior in Ba1-xSrxAl2O4, highlighting the impact of Sr substitution on structural dynamics.

Findings

Large isotropic atomic displacement parameters ($B_{iso}$) persist at low temperatures.

Soft modes related to structural fluctuations survive over a wide temperature range.

Sr substitution enhances thermal vibrations and soft mode effects.

Abstract

Synchrotron X-ray diffraction (XRD) experiments were performed on the network compounds Ba1-xSrxAl2O4 at temperatures between 15 and 800 K. The ferroelectric phase of the parent BaAl2O4 is largely suppressed by the Sr-substitution and disappears for $x\geq0.1$. Structural refinements reveal that the isotropic atomic displacement parameter ($B_{\rm iso}$) in the bridging oxygen atom for $x\geq0.05$ is largely independent of temperature and retains an anomalously large value in the adjacent paraelectric phase even at the lowest temperature. The $B_{\rm iso}$ systematically increases as $x$ increases, exhibiting an especially large value for $x\geq0.5$. According to previous electron diffraction experiments for Ba1-xSrxAl2O4 with $x\geq0.1$, strong thermal diffuse scattering occurs at two reciprocal points relating to two distinct soft modes at the M- and K-points over a wide range of temperatures below 800 K [Y. Ishii et al., Sci. Rep. 6, 19154 (2016)]. Although the latter mode disappears at approximately 200 K, the former does not condense, at least down to 100 K. On the contrary, it still survives at low-temperature. The anomalously large $B_{\rm iso}$ observed in this study is ascribed to these soft modes existing in a wide temperature range.