PHYSICAL REVIEW B VOLUME 51, NUMBER 18 1 MAY 1995-II Magnetic properties of some equiatomic ternary rare-earth compounds S. K. Dhar Solid State Physics, Tata Institute of Fundamental Research, Homi Bhabha Road, Bombay 400005, India P. Manfrinetti and A. Palenzona Istituto di Chimica I'isica, Uniuersita di Genova, Corso Euro@a, I'alazzo delle Scienze, I-16132 Genoa, Italy (Received 16 December 1994; revised manuscript received 3 February 1995) We have studied CePbCu, CeZnIn, and YbZnIn using heat-capacity (3 — 25 K), electrical-resistivity (1. 4 — 300 K), and magnetic-susceptibility (2 — 300 K) measurements. For the sake of comparison, some measurements were also made on LaPbCu and GdPbCu. CePbCu orders antiferromagnetically at 8.3 K while magnetic ordering below 3 K is indicated in CeZnIn. Contrary to an earlier report in the litera- ture, we find that YbPbCu does not exist and that the crystal structure of YbZnIn, in which the Yb ions are in the divalent state, is of the CeCu2 type, not the CaIn& type. INTRODUCTION Interesting and diverse magnetic behavior has been re- ported in the literature on the rare-earth-based equiatom- ic ternary compounds. For example, CeGeGa (Ref. l) and possible CePdSb (Ref. 2) are dense Kondo ferromag- nets, while hybridization-induced pseudogap formation at the Fermi level at low temperatures occurs in the non- magnetic CeRhSb (Ref. 3) and CeNiSn. ' A massive heavy-electron state with a Sommerfeld coeScient of about 8 J/molK has been reported in the low-carrier- density compound YbPtBi which undergoes an antiferro- magnetic phase transition near 0. 4 K. In YbNiSb, which is isostructural to YbPtBi and orders at 0.8 K, an unusually large magnetic-Geld dependence of the heat capacity and a large decrease in the electrical resistivity below 150 K are observed. It is, therefore, of interest to investigate the magnetic behavior of other rare-earth- based equiatomic ternary compounds known in the litera- ture. With that in mind we have studied in the present work the compounds CeZnIn, YbZnIn, and CePbCu. GdPbCu and the nonmagnetic LaPbCu were also studied for the sake of comparison with the properties of CePbCu. The synthesis of Rznln and RPbCu (R =La — Yb) was reported by Mazzone et al. They reported that the crystal structure of these compounds were related to the hexagonal hP6, CaIn2 type (space group P63/mmc ). The two atoms of the rare earth occupy the b position while the other two atoms occupy the f position at random. An ordered arrangement of the non-rare-earth atoms at the f site, however, was not ruled out by them. The lat- tice parameters of RZnIn and RPbCu followed the usual behavior due to the lanthanide contraction of trivalent rare-earth ions, but for the compounds containing Yb the lattice volume was larger, indicating a fluctuating or di- valent state of the Yb ions. EXPERIMENT The compounds CeZnIn, YbZnIn, LaPbCu, and CePbCu, each weighing about 5 g, were prepared by melting the constituents in sealed tantalum crucibles in a high-frequency induction furnace. The purity of the rare earths is 99. 9 wt % and that of the non-rare-earth constit- uents is 99. 999 wt % (Koch-Light Ltd. , U. K.). The sam- ples were melted three times after inverting the crucible each time and they were slowly cooled to room tempera- ture. Differential thermal analysis (DTA) on different small samples prepared in the same way showed that all the four compounds are congruently melting with melt- ing points of 1125, 855, 1140, and 1155'C respectively. Micrographic examination showed that the samples are homogeneous with little grain separation, well crystal- lized, and brittle. A sample of GdPbCu of nearly 1. 5 g was made by arc melting. Since Mazzone et al. reported the existence of YbPbCu, we also tried to prepare this compound to study the magnetic behavior of Yb. How- ever, despite several attempts using sealed tantalum cru- cibles, we always obtained nonhomogeneous alloys both in the as-cast state and after annealing at various temper- atures. DTA also showed several thermal effects. On contacting the authors of Ref. 8 we came to know that the published misprinted result actually concerned YPbCu and not YPbCu. We therefore conclude that YbPbCu does not exist. RESULTS AND DISCUSSION The results of our x-ray diffraction on CeZnIn, LaPbCu, and CePbCu are in reasonably good agreement with those reported in Ref. 8. For the latter two com- pounds the lattice parameters a =4. 670 and 4. 661 A and c=7.971 and 7. 773 A are in excellent agreement with the values reported in Ref. 8. For CeZnIn our values of a =4. 760 and c =7. 642 A lead to a volume contraction of 0. 9% compared to the value in Ref. 8. In the case of arc-melted GdPbCu our values of a = 4. 596 and c =7.425 A rnatch closely with the values listed in Ref. 8. All four compounds have the CaIn2-type structure. On the other hand, we find that YbZnIn crystallizes in the orthorhombic CeCu2-type structure and not in the CaInz type as reported previously by Mazzone et al. The lat- tice parameters are a =4. 734, b =7. 370, and c =8. 191 A. 0163-1829/95/51(18)/12464(4)/$06. 00 51 12 464 1995 The American Physical Society