Pressure-induced polymorph selectivity in the Ln(OH)₂Cl system

Trivalent lanthanide (Ln) species are often utilized as analogues for trivalent actinide experiments, reducing the cost and risk of experimental research. The monoclinic Ln(OH)₂Cl series consists of remarkably inert structures, credited to their highly linked frameworks. This makes their actinide analogues ideal candidates for use in long-term nuclear fuel storage. The later lanthanides, however, favorably crystallize in an orthorhombic structure that is very similar to the monoclinic. Here, we computationally model the high-pressure behavior of this lanthanide series to investigate the monoclinic and orthorhombic polymorphism using first-principles total-energy calculations. A phase transition is predicted by DFT calculations, with the transition pressure dependent on the ionic radius of the central Ln and the monoclinic β angle. Increased coordination is also afforded under hydrostatic pressure, likely improving chemical resistance and improving potential as a long-term storage option. The structural relationship to this phase change is explored using density of states and elastic tensor calculations.

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