Pathways to metastable nitride structures

The structure types of spinel and willemite-II are related by a Bain correspondence. gamma-Si3N4 can, therefore, transform into wII-Si3N4 by a diffusionless mechanism. We calculate the activation barrier for this process from the spinel into the energetically more favorable wII modification Of Si3N4 and Ge3N4 to 0.92 eV and 0.56 eV/f.u., respectively. We predict that the spinel modification is a suitable precursor for the synthesis of the wII modification, which otherwise is metastable throughout the enthalpy-pressure phase diagram Of Si3N4 and Ge3N4 and thus not accessible via standard pressure techniques using equilibrium thermodynamics. To allow a phase identification we provide the calculated Raman-spectra of both wII-Si3N4 and wII-Ge3N4 and compare them with those of the spinel modifications. (C) 2003 Elsevier Inc. All rights reserved.