Partial proton ordering during phase transition in Friedel's salt

Friedel`s salt, also known as Cl-Hydrocalumite, (Ca ₂[Al(OH) ₆]· Cl·2H ₂ O), is one of the calcium aluminates phases (AFm phases). It can accommodate radioactive chlorine isotopes, serving as a potential material in radioactive waste applications. Earlier X-Ray diffraction and nuclear magnetic resonance (NMR) experiments demonstrated the existence of the first-order phase transition between 303 K and 313 K. The phase transition involves an orientational ordering of water molecules while lowering the temperature. However, such behavior was not properly reproduced in computer simulations using classical molecular dynamics. Here we apply density functional theory and ab initio molecular dynamics to study structural properties and rotational dynamics of water molecules in the interlayer space of Cl-AFm. The transition is characterized by the partial proton ordering in the low-temperature phase, which was not previously observed in experiments and computer simulations.