A contribution about the crystal chemistry of alkaline earth oxochromates(V): Sr3(CrO4)2 and Sr18Ca3Cr14O56

The crystal structure of (I) Sr3(CrO4)2 was examined by X-ray single crystal technique. It belongs to the Ba3(PO4)2-type, space group D3d5-R3-barm, a = 5.562; c = 20.221A; Z = 3. The replacement of Sr2+ against Ca2+ leads to a compound of the formula (II) Sr18Ca3Cr14O56, space group C3v6-R3c, a = 11.004; c = 39.256 A; Z = 3. The Sr2+ and Ca2+ ions occupy the alkaline earth positions in an ordered manner. So the formulation Sr2.57Ca0.43(CrO4)2 should be avoided although it belongs to the β-Ca3(PO4)2 = Ca3(CrO4)2-type. The crystal chemistry of (I) and (II) is discussed. (author)