J. MATER. CHEM., 1994, 4( 12), 1893-1895 Crystal Structure of U,Pt,Sn: A New Derivative of the Tetragonal U,Si,-type Structure P. Gravereau: F. Mirambet,“ 6. Chevalier,*aF. Weill,” L. Fournes? D. Laffargue??”F. Boureeb and J. Etourneaua a Laboratoire de Chimie du Solide du CNRS, 351 Cours de la Liberafion, 33405 Talence Cedex, France ” Labomtoire Leon Bri//ouin, (CEA-CNRS), Centre d’Efudes de Saclay, 9I 79I Gif-sur-Yvetfe, France The crystal structure of the new ternary stannide U,Pt,Sn has been investigated by both X-ray powder diffraction and electron diffraction. It crystallizes in a tetragonal unit cell with a =768.1(1) pm and c =739.1(1)pm. This crystal structure is a new superstructure of the tetragonal U,Si,-type which appears on account of the existence of short Pt-Pt distances.The crystal structure of U,Pt,Sn is described taking into consideration those of UPt and UPt,Sn. Recent investigations have been devoted to the new family of ternary compounds U2M,X containing 3d, 4d or 5d transition- metal element as M and indium or tin as the X The magnetic properties of these compounds are strongly influenced by the nature of M. For instance in the series U2M,Sn with M =Ru, Rh or Pd, U,Ru,Sn is a Pauli paramag- net whereas U2Rh2Sn and U2Pd,Sn order antiferromag- netically at TN=25( 1) and 42( 1)K, re~pectively.~,~These properties are correlated likely to the number of d electrons of the M component which partly governs the strength of the 5f( U)-rzd(M) hybridization. We have previously shown that the stannides U2M,Sn with M =Fe, Co, Ni, Ru, Rh and Pd crystallize in the tetragonal ordered version of the U,Si,-type structure (P4/rnbnz space group).A compound containing platinum U,Pt,Sn has already been reported with the ideal U,Si,-type [a=767.8(5) pm and c=369.9(2) prn]., Recently, we have claimed that its X-ray diffraction pattern could not be indexed in the P4/mbm space group.4 In order to solve this discrepancy, we have investigated the crystal structure of U,Pt,Sn again by both X-ray powder diffraction and electron diffraction. In the present work we show that U,Pt,Sn exhibits a superstructure corresponding to a new deformation of the U,Si,-type structure. Experimenta1 U,Pt,Sn was prepared by direct melting of the constituents using an induction levitation furnace under a purified argon atmosphere, followed by an annealing treatment under vacuum at 800°C for 2 weeks. Microprobe examination was used to check both the homogeneity and the composition of the sample.This analysis is based on the measurements of U Ma1, Pt La, and Sn La, X-ray radiations which are compared to those of UPt,Sn used as reference. This procedure indicates a good homogeneity of the sample and the experimental atomic percentages [U, 40.6( 2)%; Pt, 39.7( 2)”/0 and Sn, 19.7(2)’?/0]are in agreement with the atomic composition of U2Pt,Sn (u,40%; Pt, 40%; Sn, 20%). The lattice parameters have been obtained by a least-squares refinement method with the help of Guinier X-ray powder data (Cu-Ka,) using Si( 5N) as an internal standard.The crystal structure of U,Pt,Sn has been refined by the Rietveld profile method.6 The data were collected on a Philips PW 1050 diffractometer using Bragg-Brentano geometry with Cu-Ka radiation and a take-off angle of 6”. The pattern was scanned in steps of 0.02 (28) from 20” to 120” with a constant counting time of 40 s. The electron diffraction investigation was carried out on a JEOL 2000 FX microscope, operating at 200 kV, equipped with a double tilt specimen stage. For this experiment, the melted or annealed sample was crushed in methanol and a drop of the suspension was deposited on a holey carbon support film. Results and Discussion The X-ray powder pattern of U,Pt,Sn shows the presence of reflections which could not be indexed on the basis of the tetragonal U3Si2 unit cell as for U,Fe,Sn (Fig.l).’ A precise examination of these reflections reveals the presence of a tetragonal unit cell having a c parameter twice as large as that determined for other U,M,Sn stannides. At room tem- perature, the unit-cell parameters of U,Pt,Sn, deduced from X-ray powder pattern, are: a=768.1(1) pm and c= 739.1(1) Pm. Electron Diffraction Study Fig. 2 shows some selected-area electron diffraction patterns obtained on the U,Pt,Sn sample; the zone axis is [OOl], [OiO] and [Olj], respectively, for Fig. 2(a),(h) and (c). These patterns are indexed on the basis of the tetragonal U,Si, unit cell. However, some reflections, indicated by arrows in Fig.2(b) and (c),remain unexplained by this indexation, which demonstrates the existence of a superstructure. To account for these additional reflections, the c parameter of 1J,Pt,Sn must be doubled in comparison to the c parameter of U3Si,. ei : ’ ’ . : : ’ -ctcect+.. : : : : : : 20 30 40 50 60 70 a 2adegrees Fig. 1 Rietveld refinement of the X-ray powder pattern of U,Pt,Sn (+indicates the reflections which cannot be indexed in the U,Si, unit cell) (a1 Fig. 2 Electfon diffraction patterns of U2Pt2Sn along [OOl] (a),[OiO] (h) and [012] (c) zone axis (arrows indicate the superstructure; (hkl) indices are given in the ideal U3Si2unit cell) Also, this study reveals no systematic extinction for (hkl) reflections, which is consistent with a P Bravais lattice.Nevertheless Fig. 2(b) shows that for (h01) reflections an additional condition must be considered: h +I # 2n. This obser- vation indicates the presence of an n-type glide plane. The corresponding extinctions along the a*,b* or c* axes [Fig. 2(u) and (c)] vanish on account of the double diffraction phenomenon. Structure Determination A tiny single crystal was isolated from a crushed annealed U,Pt,Sn sample. A study by a Buerger precession and a Weissenberg camera confirmed the tetragonal symmetry. Systematic extinctions were observed for (hOl) with h +1# 2n leading to three possible space groups: P4,/mnm7 Pan2 and P4,nm. Unfortunately, the poor quality of the single crystal did not allow it to be used for crystal-structure determination. A structural model deriving from the U,Fe,Sn type’ can be found in a unit cell with a doubled c parameter and P4,/mnrn symmetry.This hypothesis has been refined by the Rietveld profile method on X-ray powder data (Fig. 1). As currently obtained for Rietveld studies of intermetallic compounds, both an extra-lorentzian contribution to the pseudo-Voigt profile function is observed (presence of mechanical constraints induced by the melting procedure used for the synthesis of the compounds)’ and some slightly negative values for the isotropic thermal parameters B of U and Pt atoms (micro- absorption induced by surface roughness). Refinements with free or fixed B parameters showed no significant variation of the atomic coordinates obtained (<1 esd).They are summar- ized in Table 1 and were obtained with the reliability factors R,, =0.100 and R,=0.058. The relevant interatomic distances are given in Table 2. The projection of the structure of U,Pt,Sn onto the xy plane is shown in Fig. 3. The Sn and Pt atoms are located, respectively, inside [U,] and [u,] distorded prisms. Important features distinguish this structure from the ideal U3Si, type: (i) U atoms form zig-zag chains running along the c axis; (ii) each Pt atom is coordinated by six U atoms, forming a distorded [u,] trigonal prism; moreover Pt atoms J. MATER. CHEM., 1994, VOL. 4 Table 1 Atomic parameters of U2Pt2Sn (esds calculated according to ref. 15) atom site x 1’ B/A2 U(1) U(2)Pt Sn 4f 4g8j 4d 0.3407(4) 0.1860(4) 0.1281(4) 0 0.3407(4) 0.8140(4) 0.1281(4) 1I2 0 0 0.221618) 114 0.2 0.2 0.3 0.4 Table 2 Selected interatomic distances (pm) in tetragonal U2Pt2Sn 346.1(4) 404.1 (4) 382.5(4) 382.5(4) 421.6(4) 421.6(4) 370.7( 4) 370.7(4) 283.1(6) 288.3(6) 302.7( 6) 295.0(6) 342.9(2) 335.7(2) Sn-4U( 1) 342.9(2) 283.1(6) 4U(2) 335.7(2) 302.7( 6) 4Pt 302.8(4) 288.3(6) 295.0(6) 278.3(8) 327.6(8) 302.8(4) U 00 0 0.5 Pt o k0.22 0 f0.28 Sn @ a.25 Fig.3 Crystal structure of U2Pt2Sn(projection onto the ry plane) are located near one triangular face of this prism; (iii) along the c axis, the Pt atoms form short (327.6 pm) and long (411.5 pm) Pt-Pt distances through the basal face of the [U,] prism, thus forcing the U atoms to move perpendicularly to the Pt-Pt bond; (iv) Sn and Pt atoms are not located on the same atomic plane perpendicular to the c axis.All these structural characteristics seem to be due to the presence of the Pt atom in the [u,] trigonal prism. U,Pt,Sn can be considered as a ternary ordered substitution of the Zr3Al, phase.’ The three different zirconium sites observed in Zr,Al, are occupied by uranium and tin atoms in U,Pt,Sn, whereas the platinum atoms are located at the aluminium site. Some deformation of the [U,] prism has been observed in the structure of the binary compound UPt which crystallizes in the monoclinic PdBi phase deriving from the orthorhombic CrB phase.’ In this case, the displacement of the platinum atom inside the [u,] prism is responsible for the doubling of the original u and c parameters of the orthorhombic CrB phase.The origin of this behaviour has been explained by J. MATER. CHEM., 1994, VOL. 4 veIocity/mm s-' 44-20 24 6 Fig. 4 *I9Sn Mossbauer spectrum of U,Pt,Sn at room temperature Parthe by considering the size of the [RE,] prisms found in the structure of many equiatomic binary compounds REX (RE=rare-earth metal and X =Si, Ge, Ga, transition-metal element) built by linking the trigonal prisms." For example, the compounds REPt show compressed [RE,] prisms with the ratio h/l< 1 (h=height of prism and I= average edge length of the triangular prism base). This compression leads to a deformation of the prism as in UPt or U2Pt,Sn.Short Pt-Pt distances also exist in the ternary stannide UPt,Sn which adopts the hexagonal ZrPt,Al type structure." In this case, the Pt atoms form pairs having 285pm as interatomic distance. Each uranium atom has seven uranium nearest neighbours in U2Pt2Sn: five in the xy plane and two along the c axis (Fig. 3 and Table 2). As a result, the U sublattice can be considered as a three-dimensional framework where one U-U distance (346.1 pm) is short. Note that in other ternary stannides known in the uranium-platinum-tin system, the U-U distances are much longer: 468 and 455.5 pm for UPtSn and UPt2Sn.11-'2 Another interesting comparison concerns the U-Pt and U-Sn distances observed in the ternary stannides U,Pt,Sn, UPtSn and UPt,Sn.The U-Sn distances in U,Pt,Sn (4 x 342.9 pm) are comparable to those determined in UPt,Sn (6 x 346.2 pm) but larger than that observed for UPtSn (6 x 330.9 pm). On the contrary, the U-Pt distances are shorter in U,Pt,Sn (2 x 283.1 pm) than in the other Pt-based ternary stannides, suggesting that 5f( U)-5d( Pt) hybridization may be important. l19Sn Mossbauer Spectroscopy At room temperature, the lI9Sn Mossbauer spectrum of U,Pt,Sn exhibits one quadrupole doublet because the Sn site possesses tetragonal point symmetry (Fig. 4). The Mossbauer parameters are S=2.07(2) mm s-', A=0.34(2) mm s-l and r=0.85(2) mm s-'. Note that U,Pt,Sn exhibits a smaller quadrupole splitting than those observed for the other U,M,Sn ternary stannides with M =Fe, Co, Ni, Ru, Rh, Pd.4 This can be explained by the amplitude of the deformation of the [U,] prism surrounding Sn atoms in U,Pt,Sn (Fig.3). Conclusion The U,Pt,Sn stannide crystallizes in a new ordered version of the tetragonal U3Si,-type structure. The occurrence of this crystal structure can be related to the shifting of the Pt atoms inside the trigonal [U,] prims, forming one of the building units of the crystallographic arrangement. This observation appears to be a feature for the compounds based on platinum since our preliminary study performed on U,Pt,In reveals that it is isostructural to U,Pt,Sn. Note: The crystal structure of U,Pt,Sn was reported by us at '24iemes Journees des Actinides' (15-19 April 1994, Obergurgl, Austria).13 At this conference a similar superstruc- ture was noted by other authors for both U2Pt,Sn and U2Ir,Sn.l4 References 1 F.Mirambet, P. Gravereau, B. Chevalier, L. Trut and J. Etourneau, J. Alloys Compounds, 1993,191, L1. 2 M. N. Peron, Y. Kergadallan, J. Rebizant, D. Meyer, J. M. Winand, S. Zwirner, L. Havela, H. Nakotte, J. C. Spirlet, G. M. Kalvius, E. Colineau, J. L. Oddou, C. Jeandey and J. P. Sanchez, J. Alloys Compounds, 1993,201, 203. 3 Z. Zolnierek and A. Zaleski, Proc. 232me JournCes des Actinides, Schwarzwald (Germany), 1993, Abstract 06.6. 4 F. Mirambet, B. Chevalier, L. Fournes, P. Gravereau and J. Etourneau, J. Alloys Compounds, 1994,203,29. 5 F. Mirambet, Thesis, University of Bordeaux I, no.1050, 1993. 6 J. Rodriguez-Carvajal, Collected Abstracts of Powder Diffraction Meeting, Toulouse, France, 1990, 127. 7 P. Gravereau, H. Guengard, F. Mirambet, L. Trut, J. (irannec, B. Chevalier, A. Tressaud and J. Etourneau, Muter. Sci. forum, to be published. 8 C. G. Wilson and F. J. Spooner, Acta Crystallogr., 1960,13,358. 9 A. Dommann and F. Hulliger, Solid State Comnun., 1988, 65, 1093. 10 E. Parthe, in Structure and Bonding in Crystals, ed. M. 0.Keeffe and A. Navrotsky, Academic Press, New York, 1981, vol. 11, p. 256. 11 Z. Zolnierek, J. Mugn. Magn. Muter., 1988,76-77,231. 12 K. H. J. Buschow, D. B. de Mooij, T. T. M. Palstra, G. J. Nieuwenhuys and J. A. Mydosh, Philips J. Res., 1985.40,313. 13 P. Gravereau, F. Mirambet, B. Chevalier, F. Weill, L. Fournks, D. Laffargue and J. Etourneau, 24iemes Journees des Actinides, Obergurgl, Austria, 1994, Abstract PB12. 14 L. C. J. Pereira, J. M. Winand, F. Wastin, J. Rebizant and J. C. Spirlet, 24iemes JournCes des Actinides, Obergurgl, Austria, 1994, Abstract PB9. 15 J. F. Berar and P. Lelann, J. Appl. Crystallogr., 1991, 24, 1. Paper 4/04299T; Received 14th July, 1994