The First High-Pressure Chromium Oxonitridoborate CrB4 O6 N-an Unexpected Link to Nitridosilicate Chemistry.

CrB4 O6 N crystallizes in the non-centrosymmetric space group P63 mc (no. 186) with the lattice parameters a=5.1036(1), c=8.3519(3) Å, and a volume of 188.40(1) Å3 . It was synthesized in a high-pressure/high-temperature experiment at 7 GPa and 1673 K and represents the first high-pressure oxonitridoborate. It is built up of starlike-shaped entities of four BO3 N tetrahedra, connected via one common nitrogen atom that resembles the fourfold-coordinated nitrogen atoms in the homeotypic nitridosilicates MYbSi4 N7 (M=Sr, Ba). Building up a network with channels that contain the Cr3+ ions, CrB4 O6 N contains for the first time a tetrahedral building unit in contrast to trigonal planar B(O/N)3 entities in all other known oxonitridoborates. The structural relations as well as the results of spectroscopic measurements and calculations on the chromium oxonitridoborate are discussed.