Structural Variations in the Solid‐Solution Series LnxCa2−xAl[Al1+xSi1−xO7] with 0 ≤ x ≤ 1 and Ln = La, Eu, Er

Abstract

AbstractGehlenite, Ca2Al[AlSiO7], has melilite‐type structure with space group $P{\bar 4}{\rm 2}{\rm _{1}}m$. 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 xAl = 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 $P{\bar 4}{\rm 2}{\rm _{1}}m$, without changes in the Wyckoff‐positions of the ions compared to gehlenite.