| Abstract
| - A new manganese oxide, Ba4Ca1-xMn3+xO12-δ, closely related to the hexagonal perovskitefamily has been synthesized. It crystallizes in the P6̄m2 space group with a = 5.8003(3) Åand c = 38.958(1) Å. This structure derives from the rare hexagonal 16L polytype and exhibitsordered oxygen deficient vacancies, so that it can be described as the stacking along c⃗ ofcubic (c) and hexagonal (h) [BaO3] layers, with oxygen deficient cubic (c‘) [BaO2] layers,according to the sequence (hhhccc‘cc)2. This framework forms octahedral layers occuped bymanganese and calcium and tetrahedral layers occupied by manganese. These structuralresults, together with the chemical composition, confer to manganese an unusual mixedvalency.
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