Pressure-induced transformations in alpha- and beta-Ge3N4: in situ studies by synchrotron X-ray diffraction

摘要

Metastable high-pressure transformations in germanium nitride (alpha- and beta-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 beta-Ge3N4 phase (P6(3)/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 (delta-Ge3N4) that has a unit-cell symmetry based upon P3, that is related to the low-pressure beta-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 alpha-Ge3N4, due to the different stacking of linked GeN4 layers. All three polymorphs (alpha-, beta- and delta-Ge3N4) are based on tetrahedrally coordinated Ge atoms, unlike the spinel-structured gamma-Ge3N4 Phase, that contains octahedrally coordinated Ge4+. Experimentally determined bulk modulus values for alpha-Ge3N4 (K-0 = 165(10) GPa, K-0' = 3.7(4)) and beta-Ge3N4 (K-0 = 185(7) GPa, K-0' = 4.4(5)) are in excellent agreement with theoretical predictions. The bulk modulus for the new delta-Ge3N4 polymorph is only determined above the beta-delta transition pressure (P = 24 GPa); K = 161(20) GPa, assuming K' = 4. Above 45 GPa, both alpha- and delta-Ge3N4 polymorphs become amorphous, as determined by X-ray diffraction and Raman scattering. (C) 2003 Published by Elsevier Inc. [References: 28]