Experimental and Theoretical Investigation of Glass Ceramics: The Transparent Ferroelectric Nanocomposite LaBGeO5

Abstract

The LaBGeO5 glass-ceramic composite is a transparent ferroelectric nanocomposite (TFN) material that has come under attention for its ferroelectric properties. LaBGeO5 crystals-in-glass can be formed through controlled devitrification of the glass, as well as through laser irradiation. While structural models of the glass have been proposed, they have only considered differences in the borate environment between the glass and the crystal.

Understanding the structure of the LaBGeO5 glass is an important first step towards understanding the formation of the crystals-in-glass. To study the structure of the LaBGeO5 glass, we made use of a combination of nuclear magnetic resonance (NMR) spectroscopy, neutron diffraction, density functional theory (DFT) calculations, and several other techniques. These techniques provided complementary data on both local order within the glass, as well as connectivity between different local structural units.

B-11 NMR spectroscopy was used to establish the presence of both trigonal and tetrahedral borate units, and to identify the trigonal borate species. B-11\{B-10\} heteronuclear NMR spectroscopy was used to probe the connectivity between the borate units. Neutron diffraction was used to provide evidence of the presence of high-coordinate germanate units, as well as evidence of changes in the lanthanum--oxygen coordination number between the glass and the crystal. La-139 NMR spectroscopy was applied to glasses for the first time in order to provide data on the La-O environment. The use of O-17 NMR spectroscopy provided data regarding the connectivity of all local structural units. The O-17 NMR data support a highly interconnected structure for the LaBGeO5 glass, and indicated that the glass network is homogeneous.

DFT calculations were carried out to probe the stress environment in laser-written LaBGeO5 crystals-in-glass. Our calculations support an anisotropic stress environment with both tensile and compressive stresses being present.

Our results support a model of the structure of the LaBGeO5 glass that is radically different than the crystal structure. The new structural model should inform future studies of the LaBGeO5 laser-written crystals-in-glass. Our data suggest that the nucleation mechanism in the LaBGeO5 glass is heterogeneous, and we propose future work to test this hypothesis.