Cation ordering in Ca2La8(SiO4)6O2

The distribution of La3+ and Ca2+ over the cation sites in Ca2La8(SiO4)6O2 was determined by single-crystal X-ray diffraction. Ca2La8(SiO4)6O2 has the apatite structure, and all available evidence indicates that the space group is P63m, thus precluding a completely ordered structure. The 6h lattice sites are occupied by La3+. In contrast, the 4f sites are occupied equally by La3+ and Ca2+ ions. Consideration of the properties of the La3+ and Ca2+ ions suggests that this distribution is thermodynamically favored for this composition. A simple Ising model suggests ordered columns. These would not be precluded by space group P63m, if the correlation between adjacent columns were random.