Ca6.3Mn3Ga4.4Al1.3O 18 - A novel complex oxide with 3D tetrahedral framework

Abstract A new Ca6.3Mn3Ga4.4Al1.3O18 compound has been prepared by solid state reaction in a dynamic vacuum of 5×10−6 mbar at 1200 °C. The crystal structure of Ca6.3Mn3Ga4.4Al1.3O18 was studied using X-ray powder diffraction ( a = 15.07001 ( 5 ) A , SG F432, Z = 8 , R I = 0.031 , R P = 0.068 ), electron diffraction and high resolution electron microscopy. The Ca6.3Mn3Ga4.4Al1.3O18 structure can be described as a tetrahedral [(Ga0.59Mn0.24Al0.17)15O30]18.24− framework stabilized with embedded [(Ca0.9Mn0.1)14MnO6]18.24+ polycations, which consists of an isolated MnO6 octahedron surrounded by a capped cube of (Ca0.9Mn0.1) atoms. The Ca6.3Mn3Ga4.4Al1.3O18 structure is related to the structure of Ca7Zn3Al5O17.5, 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 ( x ≈ 0.17 , 0.15 ) positions in a random way according to the F432 space symmetry. The analogy between the Ca6.3Mn3Ga4.4Al1.3O18 crystal structure and the structure of the “fullerenoid” Sr33Bi24+δAl48O141+3δ/2 oxide is discussed. Ca6.3Mn3Ga4.4Al1.3O18 adopts a Curie–Weiss behavior of χ(T) above T ≈ 50 K with a Weiss temperature Θ = - 60 K and μ eff = 10.57 μ B 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.