Partial breakdown of translation symmetry at a structural quantum critical point associated with a ferroelectric soft mode

We report that complete suppression of a phonon-driven structural phase transition causes partial breakdown of a three-dimensional translation symmetry in a well-defined sublattice. This state is revealed for a dielectric compound, Ba1-xSrxAl2O4, that comprises an AlO4 network incorporated into a hexagonal Ba(Sr) sublattice. Pair distribution function analyses and inelastic neutron scattering experiments provide clear-cut evidence of the AlO4 network forming a continuum of Al-O short-range correlations similar to glasses, whereas the Ba(Sr) sublattice preserves the original translational symmetry. This glassy network significantly dampens the phonon spectrum and transforms it into the broad one resembling those typically observed in glass materials.