New Ternary Alkaline Earth Metal Cerium (IV) Nitrides: CaCeN₂ and SrCeN₂

CaCeN2 and SrCeN2 were prepared by reactions of Li2CeN2 with Ca3N2 or Sr2N in a nitrogen atmosphere at 1020 K. According to measurements of the magnetic susceptibilities both compounds contain Ce-IV. The crystal structures were determined by full- profile Rietveld refinements of the X-ray powder diffraction patterns. CaCeN2 crystallizes in a rocksalt-type structure with disordered Ca and Ce (space group Fm (3) over barm, a = 499.21(1) pm, R-profile = 0.061, R-Bragg = 0.034). The low temperature modification of SrCeN2 crystallizes in the alpha- NaFeO2 type structure (space group R (3) over barm, a = 362.18(4) pm, c = 1795.8(2) pm, R-profile = 0.085, R-Bragg = 0.031). At elevated temperatures an order-disorder phase transition leads to HT-SrCeN2 (space group Fm (3) over barm, a = 515.01(2) pm, quenched from 1273 K) with a cubic unit cell and complete disorder of Sr and Ce