Positional ordering in the light RE-Ga systems (RE=rare earth element) D. Gur and G. Kimmel a) Department of Materials Engineering, Technion-lsrael Institute of Technology, Haifa 32000, Israel (Received 30 November 1995; accepted 6 July 1998) Crystal data and results of structure refinement, using powder X-ray diffraction and Rietveld analysis, for e-ordered structures near 75 at. % Ga are reported. The structures are based on the AlB 2 -type with deviation from stoichiometry due to pairwise substitution of rare earth (RE) atoms by gallium. Weak reflections which are forbidden in the AlB 2 -type structure were observed and indicate the existence of positional ordering of Ga 2 pairs located in the basal planes preserving the hexagonal symmetry (S.G. P6lmmm). This ordering results in a new unit cell with parameters a',c', in which a'=aV3 and c'=c. The experimental data of the cells parameters are: a = 7.467(2)A, c=4.276(3)A for Nd^Ga^ (x=0.18), and a=7.400(3)A, c=4.184(2)A for Gd 1 _^Ga 2 + 2 j (x=0.165). © 1999 International Centre for Diffraction Data. [S0885-7156(98)00204-8] I. INTRODUCTION The structure of e solid solutions with the formula Rj -jGa 2 + 2 ^ in the light rare-earth (RE)-Ga systems was de- termined to be AlB 2 -type (Kimmel et al, 1985). In the basal planes of these solid solutions, RE atoms are substituted ran- domly by Ga pairs designated as Ga 2 , distinguished from the Gal atoms at z= \ (see Figures 1, 3). The symmetry of S.G. P6/mmm (No. 191) is maintained. In previous works, the suggested position for the Ga 2 atoms was 6(/)x,2x,0, but, in recent work in the the La-Ga system, the positions were determined to be 6(y')x,0,0 (Kaplan and Kimmel, 1992). At a concentration near 80 at. % Ga, an ordered structure has been detected in the La-Ga system. X-ray diffraction of e solid solutions which were annealed at 480 °C and quenched to room temperature, indicated a change in symmetry to an orthorhombic structure with the space group Pmmm, (No. 47) (Zevin et al, 1984; Kaplan and Kimmel, 1990, 1996). Pairwise substitution suggests two kinds of ordering (Zevin etal., 1984): (1) Position ordering, in which the pair-centroid occupies special positions. (2) Orientation ordering, in which pairs are aligned along special directions. In the orthorhombic structure reported by Zevin et al. (1984) and Kaplan and Kimmel (1990, 1996) both types of ordering exist at once. Position ordering: The Ga 2 pairs substitute for the RE atoms along alternate rows (Zevin etal, 1984), and, in ad- dition, with a certain periodicity in the rows. Additional dif- fraction peaks in the pattern have been detected due to this effect, resulting in Pmmm symmetry even without lattice dis- tortion (Kaplan and Kimmel, 1996). Orientation ordering: The Ga 2 pairs are aligned parallel to each other, causing an orthorhombic distortion of the original hexagonal structure. Figure 1 shows a projection on the basal plane of the hexagonal e structure. When the Ga concentration increases, the ordering takes place causing a transition from Pdlmmm to Pmmm. The present work deals with the detection of pure positional ordering (the first type) in Gd-Ga and Nd-Ga systems. In this ordering the hexagonal symmetry is pre- served but with a larger unit cell, whereas in the Gd-Ga phase this was the only sort of ordering found in the e regime (66.7-75 at. % Ga). II. EXPERIMENT A. Samples preparation and data collection Mixtures of rare earth (99.9% pure) and Ga metal (99.99% pure) ingots were arc melted under an argon atmo- sphere in a water-cooled copper crucible with several remelt- ings to ensure good mixing of the components. The samples were homogenized at 650 °C for 48 h and then annealed at temperatures of 400 °C for a couple of days. These tempera- tures are well above and just below the detected phase tran- sition temperature (about 420 °C), respectively (Peleg et al, 1981, 1985; Kimmel and Kaplan, 1991). Scanning electron microscopy combined with energy dispersive spectroscopy analysis were used to obtain phase composition. The samples were very brittle and easily pulverized into a powder with particle size less than 10 /xm. Bulk powder specimens for X-ray diffraction were prepared by mixing the powder with vaseline paste to prevent oxidation and to reduce preferred orientation. The estimated volume fraction of the vaseline "'Present address: Nuclear Research Center-Negev, Division of Research, P.O. Box 9001, Beer-Sheva, 84190 Israel. a. e AlBs-type b. The Orthorhombic structure Figure 1. Projection on (001) plane of the e phase. In (001) plane (Z = 0) large circles are lanthanide (RE) atoms, and the Ga 2 pairs which subtitude the lanthanides are small filled circles linked by the segments. In (002) plane (Z= j) small open circles are Ga atoms. When gallium concentration in- creases, ordering takes place, the left figure is the disordered structure, and the right one is with orientational and positional order leading to the ortho- rhombic structure (Zeven et al, 1984). 122 Powder Diffraction 14 (2), June 1999 0885-7156/99/14(2)/122/4/$6.00 i 1999 JCPDS-ICDD 122