L Journal of Alloys and Compounds 322 (2001) 127–134 www.elsevier.com / locate / jallcom Structural disorder and magnetism of the semiconducting clathrate Eu Ga Ge 8 16 30 * B.C. Chakoumakos , B.C. Sales, D.G. Mandrus Solid State Division, Oak Ridge National Laboratory, Oak Ridge, TN 37831-6393, USA Received 18 January 2001; accepted 21 February 2001 Abstract The temperature dependence of the atomic displacement parameters for an isotopically enriched Eu Ga Ge crystal determined from 8 16 30 refinements of neutron diffraction data show the anomalously large values for the Eu atom in the large cage persist from 295 to 12 K. The ˚ Eu site in the large cage is modeled by splitting it into four fractionally occupied positions, displaced 0.4 A from the cage center. Fourier maps of this Eu site support this view in showing residual nuclear density with distinct peaks in the directions of the split-atom positions, and lesser density at the cage center position. The Ga and Ge atoms appear to be fully disordered on the three distinct framework sites. In comparing the isostructural X Ga Ge (X5Eu, Sr, Ba) compounds, the positional disorder around the large cage site increases 8 16 30 progressively with decreasing size of the caged atom. Below 35 K, Eu Ga Ge orders ferromagnetically, with the preferred direction of 8 16 30 the Eu moments along (100). The projected saturation value of the Eu moment, 7 m , equals the free ion value.  2001 Elsevier B Science B.V. All rights reserved. Keywords: Atomic displacement parameters; Semiconductor clathrate; Ferromagnet; Neutron diffraction; Eu Ga Ge ; Ba Ga Ge 8 16 30 8 16 30 1. Introduction effects transport properties for evaluating thermoelectric behavior. In a companion paper [17], we report heat Clathrate-type compounds [1–7] of the group IV ele- capacity and magnetic, electrical and thermal transport ments [8], are providing new directions for thermoelectric measurements for single crystals of X Ga Ge (X5Ba, 8 16 30 materials research. Compounds isotypic with the type I Sr, Eu). hydrate structure X (H O) of the ice clathrates (Fig. 1) 8 2 46 [9] are of particular interest, because they exhibit cubic crystal structures, a generous range of solid solution, n- 2. Experimental type semiconducting behavior, relatively high Seebeck coefficients and electrical conductivity, and glass-like For the Eu Ga Ge single-crystal growth, about 2 g of 8 16 30 thermal conductivity [7,10–13]. Recent X-ray structural Eu-153 metal initially were reduced from several frag- studies of various clathrates of this type and related ones ments of a EuO crystal. Stoichiometric amounts of the are expanding the range of known substitutions and Eu-153 metal, Ga shot (99.999%) and Ge (99.9999%) revealing new structural variants; see for example Refs. were loaded in a helium dry box into a carbonized silica [14–16] and references therein. Those with the structural tube. The tube was sealed under high vacuum, placed in a formula X E can have E5Si, Ge, Sn, Al, Ga, In, and furnace and heated to completely melt the charge, cooled 8 46 X5Na, K, Rb, Cs, Sr, Ba, Eu [7,8,10]. Here we compare at 1–28C/min to 6258C, held at 6258C for several days, and contrast the anomalously large ‘rattling’ of the guest and finally furnace cooled to room temperature. The atom in the oversize XE cage in the X Ga Ge (X5 resulting boule consisted of large (5–10 mm) single crystal 24 8 16 30 Ba, Sr, Eu) compounds by means of neutron crystal- grains of Eu Ga Ge . The Ba Ga Ge crystals were 8 16 30 8 16 30 lography, reporting new structural results for Eu Ga Ge grown by a similar recipe, except that BaGe was first 8 16 30 2 and Ba Ga Ge and explore how the structural disorder prepared by arc-melting Ba (99.999%) and Ge 8 16 30, (99.9999%), then combined with the Ga in the carbonized silica tube, and the charge was cooled to 9008C. Several *Corresponding author. E-mail address: kou@ornl.gov (B.C. Chakoumakos). single crystals of the appropriate size (typically 23232 0925-8388 / 01 / $ – see front matter  2001 Elsevier Science B.V. All rights reserved. PII: S0925-8388(01)01169-0