The Nitrido Borates Ln3B2N4 (Ln = La–Nd) and La5B4N9: Syntheses,Structures, and Properties

Single crystals of the lanthanoide nitrido borates Ln₃B₂N₄ (Ln = La–Nd) and La₅B₄N₉ have been obtained from reactions of lanthanoide metal powder, lanthanoide nitride powder, and hexagonal boron nitride in calcium chloride melts. The isotypic compounds Ln₃B₂N₄ belong to the space group Immm (#71), Z = 2, with the lattice parameters for La₃B₂N₄: a = 362.94(3), b = 641.25(6), c = 1097.20(8) pm; Ce₃B₂N₄: a = 356.20(3), b = 631.90(6), c = 1071.91(8) pm; Pr₃B₂N₄: a = 353.46(4), b = 630.04(13), c = 1079.04(23) pm and Nd₃B₂N₄: a = 351.52(4), b = 627.01(15), c = 1075.59(23) pm. The structure of La₅B₄N₉ has been determined in the space group Pbcm (#57), Z = 4, with a = 988.25(5); b = 1263.48(7), c = 770.33(4) pm. These two structure types resemble three kinds of nitrido borate anions, the oxalate analogue B₂N₄ of Ln₃B₂N₄, and the carbonate analogue BN₃ together with the six‐membered ring system B₃N₆ of La₅(BN₃)(B₃N₆). In contrast to the valence compound La₅B₄N₉ the compounds (Ln³⁺)₃(B₂N₄)^8–(e^–) contain one electron in the conduction band, yielding temperature independent paramagnetism for La₃B₂N₄. The calculated electronic structure is developed through the formation of B₂N₄^8– ions by dimerisation of two BN₂ units.