Associating Borate and Silicate Chemistry by Extreme Conditions:  High-Pressure Synthesis, Crystal Structure, and Properties of the New Borates RE3B5O12 (RE = Er−Lu)

The rare-earth borates RE3B5O12 (RE = Er−Lu) were prepared by use of high-pressure and high-temperature conditions in a Walker-type multianvil apparatus at 10 GPa and 1100 °C. All compounds crystallize in a novel structure type and possess a new composition, unknown in borate chemistry until now. Interestingly, the crystal structure is homeotype to the beryllo-silicate mineral semenovite ((Fe2+, Mn, Zn, Ti)RE2Na0 - 2(Ca, Na)8(Si, Be)20(O, OH, F)48), representing an outstanding example for the realization of complex silicate structures in the field of borates by high pressure. Next to a marked pseudosymmetry, the compounds represent new examples of the rare crystallographic space group Pmna with only 22 exponents known so far. Fortunately, it was possible to establish single-crystal data for all compounds. The results of temperature-resolved in situ powder-diffraction measurements, quantitative analysis (ICP, EDX), DTA-TG, vibrational spectroscopy, and magnetic investigations of the new compounds are also presented.