The mechanism of phase transitions and luminescence properties of azide perovskites.

The temperature-dependent IR and Raman spectroscopy has been used to study the phase transitions in Na/K-chromium-azide frameworks with tetramethylammonium (TeMA+) cations, [(CH3)4N]2[NaCr(N3)6] and [(CH3)4N]2[KCr(N3)6], which crystallize in a perovskite-like structure. The phase transition occurring within 310-315 K in both compounds leads to a symmetry change from Pa-3 to Fm-3m and its major contributor is an order-disorder process of the organic cations followed by an adjustment of the azide bridges. The luminescence properties arise from the optically active Cr3+ ions and the studies reveal some admixture of low-symmetry component of the octahedral crystal environment of the Cr3+ sites.