Sohr, G.; Többens, D.M.; Schmedt auf der Günne, J.; Huppertz, H.: HP-CsB5O8: Synthesis and Characterization of an Outstanding Borate Exhibiting the Simultaneous Linkage of All Structural Units of Borates. Chemistry - A European Journal 20 (2014), p. 17059-17067
The new cesium pentaborate HP-CsB5O8 is synthesized under high-pressure/high-temperature conditions of 6 GPa and 900 °C in a Walker-type multianvil apparatus. The compound crystallizes in the orthorhombic space group Pnma (Z=4) with the parameters a=789.7(1), b=961.2(1), c=836.3(1) pm, V=0.6348(1) nm^3, R1=0.0359 and wR2=0.0440 (all data). The new structure type of HP-CsB5O8 exhibits the simultaneous linkage of trigonal BO3 groups, corner-sharing BO4 tetrahedra, and edge-sharing BO4 tetrahedra including the presence of threefold-coordinated oxygen atoms. With respect to the rich structural chemistry of borates, HP-CsB5O8 is the second structure type possessing this outstanding combination of the main structural units of borates in one compound. The structure consists of corrugated chains of corner- and edge-sharing BO4 tetrahedra interconnected through BO3 groups forming octagonal channels. Inside these channels, cesium is 13+3-fold coordinated by oxygen atoms. 11B MQMAS NMR spectra are analyzed to estimate the isotropic chemical shift values and quadrupolar parameters. IR and Raman spectra are obtained and compared to the calculated vibrational frequencies at the Γ-point. The high-temperature behavior is examined by means of temperature-programmed powder diffraction.