Structural Differences between the Glass and Crystal Forms of the Transparent Ferroelectric Nanocomposite, LaBGeO5, from Neutron Diffraction and NMR Spectroscopy

A combination of pulsed neutron diffraction and high-field 11B nuclear magnetic resonance (NMR) spectroscopy was used to obtain structural information on the xLa2O3·(1 – x)­(0.33B2O3·0.67GeO2) (x = 0, 0.20, 0.25, 0.30) glass system, as well as a crystallized sample of composition LaBGeO5. LaBGeO5 is of interest as a ferroelectric nanocomposite, as it crystallizes isochemically from the glass. The crystal structure is composed of 4-coordinated boron and germanium, together with 9-coordinated lanthanum. Our results support a model for the glass structure that is substantially different: both NMR and neutron diffraction show that the glasses contain large fractions of 3-coordinated boron as well as 4-coordinated boron, and the neutron diffraction results show clear evidence for a significant fraction of germanium with a coordination number greater than 4. Moreover, the La coordination number in the glasses is estimated as 7.4, with a typical La–O bond length of 2.52 Å, based on detailed modeling of the neutron correlation function. The composition dependence of the boron and germanium coordination numbers provides evidence that both boron and germanium undergo non-monotonic changes in coordination similar to those observed in the “borate anomaly” and “germanate anomaly”.