Neutron spectroscopy of the atomic dynamics of La$_2$Zr$_2$O$_7$ at fluorite-pyrochlore structural transformations

The formation of the structural and dynamic properties of La$_2$Zr$_2$O$_7$ in the process of crystallization at the isothermal annealing of initially amorphous precursors obtained by the coprecipitation of corresponding salts has been studied by neutron spectroscopy. The existence of vibrational states characteristic of hydrogen, which is in one or another of the possible chemical states and is incorporated into a solid matrix, has been detected in the spectra of amorphous and fluorite phases. The DFT calculation of the phonon density of states has been performed to analyze the energy structure of experimental phonon spectra for various phases of the La$_2$Zr$_2$O$_7$ compound. The amount of hydrogen in the fluorite phase has been estimated.