Order in the ζ' phase of the silver–gallium alloys

Abstract

The ¢' phase of the system Ag-Ga, which occurs around 75 at. % silver, is isomorphous with the ~ phase of the system Ag-Zn, which occurs around 50 at.% silver. A ternary single-phase region exists between the two binary phases. Edmunds & Qurashi (1951) determined the crystal structure of the AgZn ~ phase to be partially ordered and to belong to space group C3. Crystallographic aspects of the relation between the AgZn ~ phase and the ordered body-centred cubic fl' phase from which it is derived have been examined by Kitchingman (1962) who showed that the order of the ~ phase is closely related to that of the fl' phase. :No body-centred cubic phase exists in the Ag-Ga system, and in view of this and the composition difference of the ¢ Ag-Zn and ¢' AgGa alloys, the ordering of the ¢' AgGa structure has been determined.The AgGa alloy investigated contained 72 at. % silver. The lattice parameters at 20 °C determined from powder photographs with Cu Ka radiation were: a =7.7677 _+0.0002, c =2.8778 _+ 0-0001 ~.Intensity measurements of 33 reflexions were obtained from a flat powder sample with a Phillips X-ray diffractometer and filtered CuKa radiation. Structure factors derived from line intensities were compared with those calculated assuming a random distribution of atoms in the unit cell, using the atomic coordinates of the ~ AgZn structure given by Edmunds & Qurashi. Since the residual index R for the observed and calculated structure factors was 0.13 over all the observed reflexions the structure was refined. Order parameters were introduced and refined together with the atomic parameters using the modified least-squares method of Qurashi (1949). The refinement showed that the atomic coordinates of the AgZn $ phase could be used for the AgGa ~' phase without any correction. The order parameters indicated that the atomic position at the origin (0, 0, 0) is occupied exclusively by gallium atoms whilst the remaining gallium atoms and silver atoms occupy the two positions (½, ~ 2 3-, ¼) and (½, 9, ~). The remaining silver atoms occupy the six positions derived by symmetry operations about the position (0.350, 0.032, 0.750). Table 1 shows the agreement between the observed and calculated structure factors using the refined parameters, the residual index R being 0.059 over all the observed reflexions.

Structures of the ζ and ζ′ phases in the Ag–Ga system

“…Stratton and Kitchingman determined the structure of -Ag 2 Ga based on Xray powder diffraction of a sample containing 28 at.%Ga [7]. They confirmed that the structure was isomorphous with the AgZn -phase with the space group P3 determined by Edmunds and Qurashi [15].…”

“…Since the first phase diagram of the Ag-Ga system was published by Weibke et al in 1936 [1], several investigators have contributed to the updating of this phase diagram [2][3][4][5][6][7][8][9]. The Ag-Ga phase diagram evaluated by Baren [10] based on previous studies was collected in the Binary Alloy Phase Diagram, 1990 edition.…”

Phase relations of the Ag–Ga–N system

“…About 11 of them with the compositions of 6,13,17,20,24,28,30,32,34,40 and 50 at.% Ga are prepared by arc melting the pure metals Ag (99.95%) and Ga (99.99%) in an atmosphere of high purity argon. Each sample is arc-melted four times with the button being turned over.…”

“…Therefore, the high-angle diffraction peaks (5 2 1) and (4 2 2) are collected to determine the lattice parameters. The diffraction angles and the corresponding interplanar distances of (5 2 1) and (4 2 2) are listed in Table 5 for the samples containing 20,24,28,30,32,34,40 at.% Ga. Using these data, the lattice parameters and cell volumes of the samples are obtained and also listed in Table 5.…”

“…Since Weibke and Wiegels [8] reported the first phase diagram of the Ag-Ga system, many investigations have been conducted on this system [9][10][11][12][13][14][15][16][17][18][19][20][21][22][23]. Baren [24] made an evaluation of the phase diagram on the basis of several experimental investigations of the phase equilibria [11][12][13] and gave a partial phase diagram in the Ag-rich region of the Ag-Ga system.…”