Structural variations in the solid-solution series Ln(x)Ca(2-x)Al Al1+xSi1-xO7 with 0 <= x <= 1 and Ln = La, Eu, Er

Gehlenite, Ca2Al[AlSiO7], has melilite-type structure with space group P (4) over bar2(1)m. It contains two topologically distinct positions coordinated tetrahedrally by oxygen. One is completely occupied by Al3+, whereas the other one contains Al3+ and Si4+. Normally, the Al3+ molar fraction in the second tetrahedrally coordinated position does not exceed x(A1)=0.5, i.e. the so-called Loewenstein-rule is obeyed. In this contribution the structural variations in the melilite-type compounds of the compositions LaxCa2-xAl[Al1+xSi1-xO7], EuxCa2-xAl[Al1-xSi1-xO7] and ErxCa2-xAl[Al1-xSi1-xO7] are discussed. All members of the solid solution except the end-members violate Loewenstein's rule. Rietveld refinements against X-ray powder diffraction patterns confirm that the compounds have space group P (4) over bar2(1)m, without changes in the Wyckoff-positions of the ions compared to gehlenite.