PREPARATION OF BINARY SINGLE-PHASE LINE COMPOUNDS VIA DIFFUSION COUPLES - THE SUBNITRIDE PHASES ETA-HF3N2-X AND ZETA-HF4N3-X

It is shown that diffusion couples allow the preparation of pure line compounds, i.e. compounds with a very narrow homogeneity region, which are otherwise difficult or even impossible to prepare in single-phase form. With appropriate reaction conditions even a line compound forms a relatively large core in a symmetric diffusion couple of ''finite'' plane-sheet diffusion geometry due to the formation of a flat diffusion profile in the centered phase. The core of such a diffusion couple can then be isolated to obtain single-phase (and compact) material. This was shown for the phases eta-Hf3N2-x and zeta-Hf4N3-x, which have a homogeneity region of only 2.5 at.% and < 1 at.% N, respectively, in the temperature range of 1200-1700-degrees-C. The diffusion process was characterized by computational simulation, metallography, EPMA and XRD. By means of powder XRD and chemical nitrogen analysis it was possible to obtain more precise data on the two subnitride phases. Eta-Hf3N2-x has hexagonal lattice parameters of a = 0.32124(1) and c = 2.32790(10)nm for [N]/([Hf] + [Zr]) = 0.58(1) and 2.5 wt%Zr, and zeta-Hf4N3-x has hexagonal lattice parameters of a = 0.32084(1) and c = 3.10800(9)nm for [N]/([Hf] + [Zr]) = 0.62(1) and 3.1 wt%Zr. By this technique a single-phase sample of a zeta-phase, zeta-Hf4N3-x, could be prepared for the first time.