Pressure-induced transformations in α- and β-Ge3N4: 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 25 GPa, 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-Ge3N4 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-Ge3N4 phase, that contains octahedrally coordinated Ge4+. Experimentally determined bulk modulus values for α-Ge3N4 (K0=165(10) GPa, K0'=3.7(4)) and β-Ge3N4 (K0=185(7) GPa, K0'=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=24 GPa); K=161(20) GPa, assuming K'=4. Above 45 GPa, both α- and δ-Ge3N4 polymorphs become amorphous, as determined by X-ray diffraction and Raman scattering