Structural evolution of LaBGeO5 transparent ferroelectric nano-composites

LaBGeO5 is a model material for understanding the formation of transparent ferroelectric nanocomposites (TFNs) that are being developed for several photonic applications. It is one of the few oxides that forms glass easily, devitrifies congruently, and then becomes ferroelectric upon crystallization. Through a two-step heat treatment we have successfully prepared this TFN with stillwellite-type nano-crystallites that are homogeneously formed in the bulk of the sample. The microstructures of the samples have been investigated with optical, scanning electron, and transmission electron microscopy; the crystallites undergo a ferroelectric transition at the Curie temperature that is less than the glass transition temperature. The changes in structure at the molecular level have been monitored by 11B high-field Magic Angle Spinning Nuclear Magnetic Resonance (MAS NMR). The 11B spectra exhibit two well-resolved lines, which can be attributed to planar BO3 and tetrahedral BO4 units, respectively. The relative fraction of the two units depends strongly on the degree of devitrification.