Phases in the ZrxTa1 –x(O,N)y system, formed by ammonolysis of Zr–Ta gels: preparation of a baddeleyite-type solid solution phase ZrxTa1 –xO1 +xN1 –x, 0≤X≤1

Abstract

Phase formation in the system Zr_xTa₁–x(O,N)_y has been studied by ammonolysis of Zr-Ta gels, prepared by the sol–gel technique, at temperatures between 700 and 1000 °C. The starting gels and observed phases were characterised by X-ray powder diffraction (XRPD), scanning and transmission electron microscopy (SEM and TEM) and thermogravimetric (TG) analysis. Oxynitride phases of compositions Zr_xTa_1–xO_{1 +x}N_1–x, 0≤x≤1, with the baddeleyite-type structure, were prepared at 800 °C. The unit-cell volume increased linearly from 127.8 ų for TaON (x= 0) to 140.9 ų for ZrO₂(x= 1). The structure was verified for the composition Zr_0.4Ta_0.6O_1.6N_0.4(x= 0.4) by a Rietveld refinement (R_F= 3.5%) using Cu-Kα₁, XRPD data. An orthorhombic oxynitride phase was observed in preparations at 700 °C for 0.26 ≤x≤ 0.90 in Zr_xTa_1–xO_1+xN_1–x. Unit-cell parameters and powder X-ray reflection intensities agree with an orthorhombic ZrO₂ type structure. According to X-ray data, a cubic solid solution phase with a fluorite related subcell is present in materials prepared at 900 °C for 0.26≤x≤ 0.68. However, electron microdiffraction patterns suggest a metrically monoclinic unit cell with a = 6.1 Å, b= 14.1 Å, c= 7.1 Å and β= 125°. The Ta₃N₅, type of structure was found to incorporate up to ca. 18 atom% Zr at 900 and 1000 °C.