The Ternary Rare Earth Chromium Nitrides Ce₂CrN₃ and Ln₃Cr_10−xN₁₁ with Ln = La, Ce, Pr

The title compounds have been prepared by annealing cold-pressed pellets of the binary nitrides LnN and CrN. Well developed crystals were obtained by recrystallization of the binary or prereacted ternary nitrides in a Li₃N flux. Their structures were determined from single-crystal diffractometer data. C e₂CrN₃ has a U₂CrN₃ type structure: Immm , a = 379.0(1), b = 340.4(1), c = 1251.7(2) pm, Z = 2, R = 0.012 for 383 structure factors and 16 variables. The atomic positions of this structure are similar to those of U₂IrC₂ and K₂NiF₄. The structure may be rationalized to a first approximation with the formula (Ce⁺⁴)₂[CrN₃]⁸⁻. The chromium atoms are in a distorted square-planar nitrogen coordination. The CrN₄-squares are linked via corner-sharing nitrogen atoms, thus forming infinite, straight - N - CrN₂- N - CrN₂- chains. The cubic structure of La₃Cr_10−xN₁₁ (a = 1298.2(1) pm ), Ce₃Cr_10−xN₁₁ (with a small homogenity range; a = 1284.3(1)-1286.1(3) pm ), and Pr₃Cr_10−xN₁₁ (a = 1289.1(2) pm ) was determined for the lanthanum compound: Fm 3̄ m , Z = 8, R = 0.027 for 189 F values and 18 variables. One chromium site was found to have an occupancy of only 80.9(5)% resulting in the composition La₃Cr_9.24(1)N₁₁. The nitrogen atoms occupy four atomic sites. Three of these have octahedral environments (6 La, 3 La + 3 Cr, 2 La + 4 Cr), the fourth one is surrounded by eight chromium atoms forming a cube. The chromium atoms are tetrahedrally coordinated by nitrogen atoms, and these CrN₄-tetrahedra are linked via common corners and edges to form a three-dimensionally infinite polyanionic network. In addition the chromium atoms with oxidation numbers of about 2 to 3 form numerous Cr - Cr bonds, which allow to rationalize the Pauli paramagnetism of the compound.