(La₃Z_x)Al and (Ce₃Z_x)Al with Z = C, N, O: preparation, physical properties and chemical bonding of metal-rich perovskites

Abstract

The inverse cubic perovskites (La₃O)Al, (La₃N)Al, (Ce₃O)Al, and (Ce₃N)Al are reported together with the solid solution series (Ce₃C_1–xN_x)Al. The crystal structure of (La₃N)Al is analyzed in detail based on single crystal X-ray diffraction data (space group Pm-3m, a = 509.04(1) pm, Z = 1, R_gt(F) = 0.008, wR(F²) = 0.018). Combined X-ray powder diffraction and thermal analysis studies on samples with various N and O contents indicate only small tolerance of the ternary compounds towards Z = N, O deficiency and small solubility of Z in hexa gonal α-Ce₃Al. Indications for the existence of a cubic β-Ce₃Al with Cu₃Au structure type could not be derived from any experiment. All studied phases are metals, the Ce compounds contain the rare-earth metal in Ce(4f¹) states according to X-ray absorption spectroscopy and measurements of the magnetic susceptibilities. The Ce moments order antiferromagnetically with T_N decreasing and scaling with increasing unit cell dimension of the compounds. Electronic band structure calculations reveal a trend to increased mixing of Z and La states on going from Z = O via N to C. Formation of a band gap even in the formally electronically balanced (La₃C)Al is mainly prohibited by insufficient charge transfer between the metal species.