Octahedral oxide glass network in ambient pressure neodymium titanate

Abstract

Rare-earth titanates form very fragile liquids that can be made into glasses with useful optical properties. We investigate the atomic structure of 83TiO₂-17Nd₂O₃ glass using pair distribution function (PDF) analysis of X-ray and neutron diffraction with double isotope substitutions for both Ti and Nd. Six total structure factors are analyzed (5 neutron + 1 X-ray) to obtain complementary sensitivities to O and Ti/Nd scattering, and an empirical potential structure refinement (EPSR) provides a structural model consistent with the experimental measurements. Glass density is estimated as 4.72(13) g cm⁻³, consistent with direct measurements. The EPSR model indicates nearest neighbor interactions for Ti-O at ${\overline{r}}_{\mathit{TiO}}$ = 1.984(11) Å with coordination of $n_{\mathit{TiO}}$ = 5.72(6) and for Nd-O at ${\overline{r}}_{\mathit{NdO}}$ = 2.598(22) Å with coordination of $n_{\mathit{NdO}}$ = 7.70(26), in reasonable agreement with neutron first order difference functions for Ti and Nd. The titanate glass network comprises a mixture of distorted Ti-O₅ and Ti-O₆ polyhedra connected via 71% corner-sharing and 23% edge-sharing. The O-Ti coordination environments include 15% nonbridging O-Ti₁, 51% bridging O-Ti₂, and 32% tricluster O-Ti₃. This structure is highly unusual for oxide glasses melt-quenched at ambient pressure, as it consists of Ti-O_x predominantly in octahedral (with nearly no tetrahedral) coordination.