Cs(Be2Li)Al2Si6O18, a cesium-stuffed host-guest structure, and its structure-property variations with temperature and pressure

The solid-state structure of (Cs,Na)[(Be2Li)Al2Si6O18], a beryl-type material, was studied in-situ on single crystal and powder samples at hydrostatic high pressure conditions (P <= 9.8 GPa) as well as under temperature variation (83 <= T <= 1173 K) by means of Raman spectroscopy, X-ray and synchrotron diffraction. The investigations show an inconspicuous, easily overlooked transformation from R (3) over barc to non-centrosymmetric R3c space-group symmetry on increasing pressure (P-c similar to 4 GPa) and decreasing temperature (T-c = 312 +/- 5 K), as revealed by Raman band splitting as well as discontinuities in the evolution of Raman spectra, thermal expansivity coefficients and anomalous compression behavior. Distortions to acentric symmetry originate from the Cs and Na guest atoms inside the channels parallel the c-axis. The occurrence of diffuse scattering planes in all measured data sets indicates substitutional defects inside the channels following an approximate similar to 4.6 angstrom periodicity, without any remarkable ordering between adjacent channels. The determined thermal expansivity follows an inverse behavior compared to other beryl-type structures for the R3c phase. The bulk moduli for the two polymorphs compare, together with the observed compressional anisotropy, to other complex beryl-type framework structures. However, the symmetry reduction at high pressure leads to independent displacements of the framework oxygen positions, and as a consequence, to the increase of the coordination number from [6 + 6] to [6 + 9] for one of the Cs sites.