A profound analysis of Rb2[PH] and Cs2[PH] and the role of [PH]2− ions during temperature-induced solid–solid phase transitions

•The temperature-induced solid–solid transformation of Rb2[PH] and Cs2[PH] is identified as the orientation disordering of PH2− ions above 80 K.•Characterization is performed from neutron diffraction, IR-spectroscopy and ab-initio molecular dynamics simulations.•A conclusive picture of the molecular mechanism is elaborated.

The temperature-induced solid–solid transformation of Rb2[PH] and Cs2[PH] is characterized from both experiment and theory. Neutron diffraction, IR-spectroscopy and ab-initio molecular dynamics simulations reveal an asymmetric shift of the lattice constants at 80 K. The molecular mechanism of the structural transformation as identified from IR-spectroscopy and ab-initio molecular dynamics simulations is closely connected to the orientation of the [PH]2− moieties which undergo a partial order–disorder phase transition.