Advanced luminescent materials exhibiting downconversion are important components of next-generation photovoltaic devices aimed at minimizing the spectral mismatch losses in solar cells. We showed that germanates with the general formula ALa9(GeO4)6O2 (A = Li, Na, K, Rb) can effectively downconvert high-energy photons from the visible and near-IR parts of the spectrum into far-IR photons. The optical transitions of many rare-earth ions are influenced by small changes in the crystal structure of their hosts. In order to control luminescent properties of these materials, neutron diffraction experiments are essential. The experiments will help to determine the oxygen and alkaline metal positions, cation distributions, and to refine oxygen deficiency. Obtained structural models will be used for DFT calculations aimed at explaining the differences in luminescent properties of these compounds.