High‐Pressure Syntheses, Crystal Structures, and Properties of the Transition Metal Borates β ‐ M B 4 O 7 ( M = Fe, Co)

The tetraborates β ‐FeB 4 O 7 and β ‐CoB 4 O 7 were synthesized under high‐pressure/high‐temperature conditions of 10.5 GPa/900 °C ( β ‐FeB 4 O 7 ) and 7.5 GPa / 1250 °C ( β ‐CoB 4 O 7 ) in a Walker‐type multianvil apparatus. Single crystal structure determinations of both compounds revealed an isotypy to β ‐MB 4 O 7 (M = Mn, Ni, Cu, Zn) and completed the series β ‐MB 4 O 7 from M = Mn–Zn. β ‐FeB 4 O 7 and β ‐CoB 4 O 7 crystallize in the space group Cmcm ( Z = 4) with the parameters a = 1088.3(2), b = 652.4(2), c = 519.5(1) pm, V = 0.3688(2) nm 3 , R 1 = 0.0376, wR 2 = 0.1020 (all data), and a = 1087.0(2), b = 646.8(2), c = 517.4(2) pm, V = 0.3638(2) nm 3 , R 1 = 0.0224, wR 2 = 0.0561 (all data), respectively. The compounds consist exclusively of corner‐sharing BO 4 ‐tetrahedra, forming a network with channels of four‐ and six‐membered rings. Inside the six‐membered rings, the metal ions are coordinated with five oxygen ions. β ‐CoB 4 O 7 shows distinct dichroic behavior.