Thermally‐Induced Reentrant Order‐Disorder‐Order Transitions in ALa₉(GeO₄)₆O₂ Apatites

Oxyapatites A 10 (GeO 4 ) 6 O 2 (A – alkaline, rare‐earth elements) are considered promising materials for applications in solid‐state lighting and non‐contact thermometry. The temperature dynamics of the oxygen sublattice have a crucial influence on the luminescent properties of these optical hosts. Here, the structural evolution with temperature of germanates with the general formula ALa 9 (GeO 4 ) 6 O 2 (A – Li, Na, K, Rb) is investigated, using X‐ray and neutron powder diffraction methods. These compounds crystallize in space‐group P 6 3 / m . NPD experiments helped to determine the oxygen and alkaline metal positions. Thermally‐induced reentrant structural transitions relating to the oxygen in the apatite channel are discovered for the first time. In the example of Eu‐doped RbLa 9 (GeO 4 ) 6 O 2 and KLa 9 (GeO 4 ) 6 O 2 , it is demonstrated that the luminescence of the Eu 3+ ion is influenced by small changes in the crystal structure.