Pressure-induced structural changes in the network-forming isostatic glassGeSe4: An investigation by neutron diffraction and first-principles molecular dynamics
| Autor: | Henry E. Fischer, Mauro Boero, Assil Bouzid, Keiron J. Pizzey, Anita Zeidler, Stefan Klotz, Craig L. Bull, Guido Ori, Philip S. Salmon, Carlo Massobrio |
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| Rok vydání: | 2016 |
| Předmět: | |
| Zdroj: | Physical Review B. 93 |
| ISSN: | 2469-9969 2469-9950 |
| Popis: | The changes to the topological and chemical ordering in the network-forming isostatic glass ${\mathrm{GeSe}}_{4}$ are investigated at pressures up to $\ensuremath{\sim}14.4$ GPa by using a combination of neutron diffraction and first-principles molecular dynamics. The results show a network built from corner- and edge-sharing Ge(${\mathrm{Se}}_{1/2}$)${}_{4}$ tetrahedra, where linkages by ${\mathrm{Se}}_{2}$ dimers or longer ${\mathrm{Se}}_{n}$ chains are prevalent. These linkages confer the network with a local flexibility that helps to retain the network connectivity at pressures up to $\ensuremath{\sim}8$ GPa, corresponding to a density increase of $\ensuremath{\sim}37%$. The network reorganization at constant topology maintains a mean coordination number $\overline{n}\ensuremath{\simeq}2.4$, the value expected from mean-field constraint-counting theory for a rigid stress-free network. Isostatic networks may therefore remain optimally constrained to avoid stress and retain their favorable glass-forming ability over a large density range. As the pressure is increased to around 13 GPa, corresponding to a density increase of $\ensuremath{\sim}49%$, Ge(${\mathrm{Se}}_{1/2}$)${}_{4}$ tetrahedra remain as the predominant structural motifs, but there is an appearance of 5-fold coordinated Ge atoms and homopolar Ge-Ge bonds that accompany an increase in the fraction of 3-fold coordinated Se atoms. The band gap energy decreases with increasing pressure, and midgap states appear at pressures beyond $\ensuremath{\sim}6.7$ GPa. The latter originate from undercoordinated Se atoms that terminate broken ${\mathrm{Se}}_{n}$ chains. |
| Databáze: | OpenAIRE |
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