HIGH-PRESSURE SYNTHESIS OF CUBIC PEROVSKITE CALAMGMNO5.5+X

Oxygen-deficient perovskite CaLaMgMnIV 1-xMnVI xO5.5+ x (indicated by P) has been prepared at 930°C for 10 mn using high oxygen pressure (60 kbar). X-ray diffraction study indicates that P has a simple cubic unit cell (a = 3.923Å). The average oxidation state of Mn in P was determined by redox titration before (4.13) and after (4.03) thermal treatments at 300, 900, and 1100°C and was confirmed by thermogravimetry analysis. Two paramagnetic configurations are observed from the EPR spectrum: One isotropic signal characterized by g = 1.994, ΔH = 950 G, and an anisotropic signal with g⊥ = 1.978, g{norm of matrix} = 1.938. They correspond respectively to octahedrally coordinated Mn(IV) and to tetrahedrally coordinated Mn(VI). According to magnetic susceptibility measurements, the constant spin-only moment observed between 77 and 650 K (μeff = 3.8 μB) is consistent with a Mn(IV)-4A2g ground term-(μeff = 3.87 μB). Such behavior can be explained by the low Mn(VI) content in P. © 1990.