Ge-O coordination in cesium germanate glasses

Area of Science:

• Materials Science
• Solid State Chemistry
• Glass Science

Background:

• Understanding the structure-property relationships in oxide glasses is crucial for materials development.
• The germanate anomaly, a deviation in thermophysical properties of germanate glasses, remains incompletely understood.
• Cesium germanate glasses offer a unique system to probe structural changes with alkali oxide addition.

Purpose of the Study:

• To investigate the local atomic structure of cesium germanate glasses using neutron diffraction.
• To determine the coordination number of germanium (Ge) in relation to oxygen (O) as a function of Cs2O content.
• To elucidate the structural origins of the germanate anomaly and the low alkali anomaly.

Main Methods:

• High-resolution neutron diffraction experiments were conducted on a series of cesium germanate glasses (2-30 mol% Cs2O) and pure GeO2 polymorphs.
• Real-space analysis of diffraction data was employed to extract structural information.
• A compositional model was developed to describe the Ge-O coordination number dependence.

Main Results:

• Neutron diffraction successfully identified changes in Ge-O coordination in pure GeO2 and cesium germanate glasses.
• The Ge-O coordination number increased with Cs2O content, reaching a maximum of 4.36 ± 0.03 at 18 mol% Cs2O, then decreased.
• The rise in coordination is primarily attributed to the formation of GeO5 units, not GeO6 units.

Conclusions:

• The study provides direct structural evidence linking the germanate anomaly to changes in Ge-O coordination, specifically the formation of GeO5 units.
• The low alkali anomaly is a long-range phenomenon, not solely dependent on the formation of non-bridging oxygens at low alkali concentrations.
• Neutron diffraction is a powerful tool for characterizing the local structure of oxide glasses.