The A 2 TiX 6 (A = Rb, Cs, X = F, Cl, Br, I) vacancy double perovskites hold promise as substitutes in solar cells for APbX 3 . Phonon calculations show that Rb 2 TiF 6 and Rb 2 TiI 6 are unstable in cubic form and may experience a phase transition.
Guo et al. (Thu,) studied this question.