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 diffusi on profile in the centered phase. The core of such a diffusion couple can then be isolated to obtain single-phase (and compact) material. Th is 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 wa s characterized by computational simulation, metallography, EPMA and X RD. By means of powder XRD and chemical nitrogen analysis it was possi ble to obtain more precise data on the two subnitride phases. Eta-Hf3N 2-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 f or the first time.