High-pressure behavior of $\mathrm{(Cs,K)Al_{4}Be_{5}B_{11}O_{28}}$ (londonite): A single-crystal synchrotron diffraction study up to 26 GPa

The compressional behavior and the P-induced deformation mechanisms at the atomic scale of (Cs,K)Al$_{4}$Be$_{5}$B$_{11}$O$_{28}$ (londonite, a ~7.31 Å and space group $P^{-}_4$3m) were investigated by in situ single-crystal synchrotron X-ray diffraction with a diamond anvil cell up to 26 GPa. No phase transition was observed within the P-range investigated: this material exhibits isotropic compression (i.e., with cubic symmetry) in response to the applied pressure. Fitting the P–V data with a Birch-Murnaghan isothermal equation of state, we obtained: V$_0$=390.8(3) Å$^3$, K$_{P0}$=212(7) GPa (β$_0$=1/K$_{P0}$=0.0047(1) GPa$^{−1}$) and K′=4.6(6). A series of structural refinements, based on the high-pressure intensity data, were performed. The stiffness of londonite (similar to that of carbides) is governed by its close-packing structure, and in particular by the very low compressibility of B- and Be-tetrahedra and the modest compressibility of the Al-octahedra. The Cs-polyhedra are the most compressible units of the structure. The effects of pressure can be accommodated by intrapolyhedral compression or deformation, leading to a modest bulk compression. The high amount of boron in londonite (B$_2$O$_3$ ~50 wt%) makes its synthetic counterpart a potential neutron absorber. In addition, the high content of Cs makes londonite-type materials as potential hosts for nuclear waste