Ca6.3Mn3Ga4.4Al1.3O18—A novel complex oxide with 3D tetrahedral framework

A new Ca_6.3Mn₃Ga_4.4Al_1.3O₁₈ compound has been prepared by solid state reaction in a dynamic vacuum of 5×10⁻⁶ mbar at 1200 °C. The crystal structure of Ca_6.3Mn₃Ga_4.4Al_1.3O₁₈ was studied using X-ray powder diffraction (, SG F432, Z=8, R_I=0.031, R_P=0.068), electron diffraction and high resolution electron microscopy. The Ca_6.3Mn₃Ga_4.4Al_1.3O₁₈ structure can be described as a tetrahedral [(Ga_0.59Mn_0.24Al_0.17)₁₅O₃₀]^18.24− framework stabilized with embedded [(Ca_0.9Mn_0.1)₁₄MnO₆]^18.24+ polycations, which consists of an isolated MnO₆ octahedron surrounded by a capped cube of (Ca_0.9Mn_0.1) atoms. The Ca_6.3Mn₃Ga_4.4Al_1.3O₁₈ structure is related to the structure of Ca₇Zn₃Al₅O_17.5, but appears to be significantly disordered due to the presence of two orientations of oxygen tetrahedra around the cationic 0,0,0 and x,x,x () positions in a random way according to the F432 space symmetry. The analogy between the Ca_6.3Mn₃Ga_4.4Al_1.3O₁₈ crystal structure and the structure of the “fullerenoid” Sr₃₃Bi_24+δAl₄₈O_141+3δ/2 oxide is discussed. Ca_6.3Mn₃Ga_4.4Al_1.3O₁₈ adopts a Curie-Weiss behavior of χ(T) above with a Weiss temperature and per formula unit. At lower temperatures, the χ(T) deviates from the Curie-Weiss law indicating a strengthening of the ferromagnetic component of the exchange interaction.