THE CRYSTAL STRUCTURE OF LaNiSn

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1 Philips J. Res. 39, 77-81, 1984 R1082 THE CRYSTAL STRUCTURE OF by J. L. C. DAAMS and K. H. J. BUSCHOW Philips Research boratories, 5600 JA Eindhoven, The Netherlands Abstract The crystal structure of the compound has been determined; it is found to be homotypic with the orthorhombic op12 (C23) structure. is isotypic with e-tisi and shows great resemblance with the structure of CeCu2. PACS numbers: Hg. 1. Introduetion The growing interest being shown in the study of ternary intermetallic compounds is due to particular properties of ternary compounds not found in binary intermetallics. These properties comprise, for instance, the occurrence of superconductivity and magnetic ordering in one and the same compound 1), the observation of huge values for the magneto-optical Kerr rotation 2) and a pronounced first-order spinflop transition with a transition hysteresis of more than 30 k Oe "), Ternary compounds have also attracted much attention from the theoretical point of view 4,5). The Cls-type Heusier compounds, in particular, were found to have unusual features in their band structure, leading to 1000/0spin polarization of the conduction electrons. In a previous investigation 6) the occurrence of Heusler-type compounds based on was studied. It was found that the combination of and 3d elements with rare earths did not lead to the formation of such ternary compounds, but that other ternary compounds were formed instead. In the present investigation we have studied the crystal structure of the compound. 2. Experimental procedures and results The sample was prepared by are melting in purified argon gas, followed by vacuum annealing for two weeks at 800 C. After annealing the sample was pulverized and stress released by heating in vacuum at 500 "C for three hours. X-ray diagrams were obtained by means of a Philips PW 1700 diffraction Philips Journalof Research Vol. 39 No

2 J. L. C. Daams and K. H. J. Buschow system. CuKa radiation was used in combination with a graphite monochromator. We indexed the X-ray diffraction diagram of on the basis of a primitive orthorhombic unit cell with the lattice constants a = Á,. b = Á and c = Á. From the systematic extinctions we derived TABLE I. Observed and calculated diffraction angles (J (in degrees) and intensities I (in arbitrary units) of the intermetallic compound. observed calculated h k (J I (J I Philips Jourooi of Research Vol.39 No

3 The crystal structure of Pnma (62) 7) as the most probable space group. In table I the indexing together with the observed reflecting angles are compared with the calculated angles. 3. Structure determination The crystal structure was determined by using the trial structure obtained by means of a cell-filling program 8). After seven refinement cycles we reached convergence with an ultimate reliability factor R = /0 based on intens i- ties. The atomic position parameters are: 4 in 4 (c) 4 in 4 (c) 4 in 4 (c) x = x = x = y = Y = y = z = z = z = The value of B occurring in the expression of the Debije-Waller temperature factor was found to be equal to cm 2 for the sites, cm" for the sites and ern" for the sites. Table I also gives a comparison of the observed intensities and the calculated intensities. In table IIthe interatomie distances up to 4 Á are given. A schematic representation of the structure is given in fig. 1, where the viewing direction is almost perpendicular to the (100) plane. TABLE II Number of neighbours and interatomie distances in. atom neighbour number distance (Á) Phlllps Journalof Research Vol.39 No

4 J. L. C. Daams and K. H. J. Buschow Fig. I. Schematic representation of the crystal structure of. 4. Discussion The formation of the compound is accompanied by a substantial contraction of the atomic volumes compared to the pure starting materials (36.9 A3, 10.9 A3 and 26.9 A3 for, and respectively). From the lattice constants of given above one finds a volume per unit cell equal to A3, which is more than 9070 smaller than the value calculated on the basis of the atomic volumes of the elements. The crystal structure of can be regarded as a distortion of the hexagonal -In structure 9). As in the other members of the R compounds, the distortion is relatively strong, due to the large difference in metallic radii between the constituent metal atoms. Our values for the lattice constants are in good agreement with those given by Dwight 10). Dwight reports that his value for a falls significantly below that expected on the basis of the linear relationship between the values of a and the radii of the R component in the remainder of the R compounds. By contrast, the a value found by us is in good agreement with the linear relationship mentioned. Closer inspection of Dwight's data showed that the values for a and c of are apparently interchanged in his plot, so that deviations from the linear behaviour remain restricted to the b and the c axis. If one compares the atomic positions found in the course of the present investigation for with those of TiSi 11,12) one notices substantial differences between the two compounds. Owing to this difference in the atomic position, a slight shift in coordination of the atoms in has taken place with respect to TiSi. In the latter compound the Ti, and Si atoms are coordinated by 15, 12 and 9 atoms respectively. As can be derived from the data listed in table Il, in the coordination of, and corresponds to 16, 10 and 10, respectively. These changes in coordination are in 80 Philips Journalof Research Vol. 39 No

5 The crystal structure of accordance with the metallic radii of the constituent elements. The and atoms are larger than the Ti and Si atoms, respectively. For the former atoms the coordination number has increased. By contrast, in is surrounded by larger atoms than in TiSi. For therefore the coordination number has decreased. However there is a more striking resemblance between the structure and the structure of CeCu2. CeCu2 has a bodycentered orthorhombic unit cell with lattice constants a = A, b = A and c = A, space group 74 Imma 12-14). REFERENCES I) J. R. Remeika, G. P. Espinosa, A. S. Cooper, H. Barz, J. M. Rowerll, D. B. McWan, J. H. Vandenberg, D. E. Moncton, Z. Fisk, L. D. Woolf, H. C. Hamaker, M. B. Maple, G. Shirane and W. Thomlinson, Solid State Commun. 34, 923 (1980). 2) P. G. van Engen, K. H. J. Buschow, R. J ongebreur and M. Erman, Appl. Phys. Lett. 42, 202 (1983). 3) T. T. M. Palstra, J. A. Mydosh, G. J. euwenhuys, F. R. de Boer and K. H. J. Buschow (to be published). 4) J. Kübler, A. R. Williams and C. B. Sommers, Phys, Rev. (B) 28,1745 (1983). 5) R. A. de Groot, F. M. Mueller, P. G. van Engen and K. H. J. Buschow, Phys, Rev. Lett. 50, 2024 (1983). 6) P.. G. van Engen, K. H. J. Buschow and M. Erman, J. Magn. Mat. 30, 374 (1983). 7) N. F. M. Henry and K. Lonsdale (eds), International tables for crystallography, Vol. 1, Kynoch Press, Birmingham, ) W. P. J. Fontein, J. M. M. Verbakei and J. H. N. van Vucht, Philips J. Res. 34, 238 (1979). 9) W. B. Pearson, The crystal chemistry and physics of metals and alloys, John Wiley and Sons Inc., New York, 1972; cf. p ) A. E. Dw ig h t, J. Less-Common Met. 93,411 (1983) 11) C. B. Shoemaker and D. P. Shoemaker, Acta Cryst. 18,900 (1965). 12) W. B. Pearson, A handbook of lattice spacings and structures of metals and alloys, Vol. 2, Pergamon Press, 1965; cf. p ) A. C. rson and D. T. Cromer, Acta Cryst, 14, 73 (1961). 14) E. Hovestreydt, N. Engel, K. Klepp, B. Chabot and E. Parthé, J. Less-Common Met. 85, 247 (1982). Phlllps Journalof Research Vol.39 No

THE MAGNETO-OPTICAL PROPERTIES OF HEUSLER ALLOYS OF THE TYPE Coz_xCuxMnSn

Philips J. Res. 42, 429-434, 1987 R 1165 THE MAGNETO-OPTICAL PROPERTIES OF HEUSLER ALLOYS OF THE TYPE Coz_xCuxMnSn by P.P.J. VAN ENGE~EN and K.H.J. BUSCHOW Philips Research Laboratories, 56 JA Eindhoven,