Pressure-induced transformations in α- and β-Ge ₃N₄: In situ studies by synchrotron X-ray diffraction

Metastable high-pressure transformations in germanium nitride (α- and β-Ge₃N₄ polymorphs) have been studied by energy- and angle-dispersive synchrotron X-ray diffraction at high pressures in a diamond anvil cell. Between P=22 and 25GPa, the phenacite-structured β-Ge₃N₄ phase (P6₃/m) undergoes a 7% reduction in unit-cell volume. The densification is primarily concerned with the a-axis parameter, in a plane normal to the hexagonal c-axis. Based on results of previous LDA calculations and Raman spectroscopic studies, we propose that the structural collapse is due to transformation into a new metastable polymorph (δ-Ge₃N₄) that has a unit-cell symmetry based upon P3, that is related to the low-pressure β-Ge ₃N₄ phase by concerted displacements of N atoms away from special symmetry sites in the plane normal to the c-axis. No such transformation occurs for α-Ge₃N₄, due to the different stacking of linked GeN₄ layers. All three polymorphs (α-, β- and δ-Ge₃N₄) are based on tetrahedrally coordinated Ge atoms, unlike the spinel-structured γ-Ge ₃N₄ phase, that contains octahedrally coordinated Ge ⁴⁺. Experimentally determined bulk modulus values for α-Ge ₃N₄ (K₀=165(10) GPa, K₀′=3. 7(4)) and β-Ge₃N₄ (K₀=185(7) GPa, K ₀′=4.4(5)) are in excellent agreement with theoretical predictions. The bulk modulus for the new δ-Ge₃N₄ polymorph is only determined above the β-δ transition pressure (P=24GPa); K=161(20) GPa, assuming K′=4. Above 45GPa, both α- and δ-Ge₃N₄ polymorphs become amorphous, as determined by X-ray diffraction and Raman scattering.