Structural Variations in the Solid‐Solution Series Ln x Ca 2−x Al[Al 1+x Si 1−x O 7 ] with 0 ≤ x ≤ 1 and Ln = La, Eu, Er

Gehlenite, Ca 2 Al[AlSiO 7 ], 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 Al 3+ , whereas the other one contains Al 3+ and Si 4+ . Normally, the Al 3+ molar fraction in the second tetrahedrally coordinated position does not exceed x Al = 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 La x Ca 2−x Al[Al 1+x Si 1−x O 7 ], Eu x Ca 2−x Al[Al 1+x Si 1−x O 7 ] and Er x Ca 2−x Al[Al 1+x Si 1−x O 7 ] 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.