Magnetic, electrical and lsrEu Mossbauer properties of EuPt Ge Rainer Pottgen,"" Reinhard K. Kremer," Walter Schnelle," Ralf Mullmand and Bernd D. Moselb "Max-Planck-Institut fur Festkorperforschung, Heisenbergstrasse I, 0-70569 Stuttgart, Germany bInstitutfur Physikalische Chemie, Universitat Miinster, Schlossplatz 417, 481 49 Miinster, Germany The title compound was prepared from the elements in a tantalum tube at 1170 K. EuPtGe [single-crystal X-ray data: space group P2,3,Z =4, a =654.63( 9) pm, R, =0.0208,378 F2 values and 11 parameters] crystallizes with the LaIrSi type structure, an ordered ternary version of the SrSi, type. Magnetic susceptibility and '"Eu Mossbauer measurements show no magnetic order down to 4.2 K. The experimentally determined magnetic moment, peXp=7.80(5) p,JEu compares well with the free ion value, peff=7.94 pu,/Eu for Eu2+, in accordance with the isomer shift 6 = -10.4 mm s-' which is typical for divalent Eu.EuPtGe is a metallic conductor with a specific resistivity at room temperature of 145 pt2 cm. Recently, the crystal structure and properties of EuAuGe' and the related intermetallics EuAgGe, EuPdGe and EuZnGe have been In the course of our systematic studies on crystal structures and physical properties of equiatomic europium transition-metal germanides and indide~,~ we have now investigated EuPtGe. In the present paper we report on single-crystal X-ray data, magnetic susceptibility, electrical resistivity, specific heat and '"Eu Mossbauer investigations of this intermetallic compound.X-Ray powder data of EuPtGe have been published previously by Evers et aL6 Experimental Starting materials for the preparation of EuPtGe were ingots of europium (Johnson Matthey), platinum powder (Degussa) and germanium lumps (Wacker), all with stated purity >99.9%. The elements were mixed in the ideal atomic ratio and sealed in a tantalum tube under an argon pressure of about 800 mbar. The argon was purified over molecular sieves, titanium sponge (at 900K) and an oxisorb ~atalyst.~ The tantalum tube was sealed in a quartz ampoule to prevent oxidation and heated at 1320 K for five days; then cooled to 1170 K and annealed for a further four weeks. Modified Guinier powder patterns' of the samples were recorded with Cu-Ka, radiation using silicon (a=543.07 pm) as an internal standard.The indexing of the diffraction lines was facilitated by intensity calculations9 taking the positional parameters of the refined structure. The lattice constant (Table 1) was obtained by a least-squares fit of the powder data. It is in good agreement with the data obtained on the four-circle diffractometer [a =654.00(1) pm] as well as the powder data from Evers et al. of a=655.1(1) pm.6 Single-crystal intensity data were collected on a four-circle diffractometer ( Enraf-Nonius CAD4) with graphite-monochro- mated Ag-Ka radiation and a scintillation counter with pulse height discrimination. The magnetic susceptibilities of polycrystalline pieces of EuPtGe were determined with an MPMS SQUID magnet-ometer (Quantum Design, Inc.) between 4.2 and 300 K with magnetic flux densities up to 2 T.The specific resistivities were measured on a small block (0.8 x 1.0 x 1.2 mm3 cut with a diamond saw from a larger ingot) with a conventional four-point setup. Four very thin gold wires (diameter 0.1 mm) were glued to the block using a silver epoxy resin. Cooling and heating curves measured at a constant current between 4.2 and 300 K were identical within error bars. The specific heat capacity of a EuPtGe powder sample (about 600mg) was measured between 20 and 270K in an adiabatic scanning calorimeter designed for small samples." The powder was sealed in a Duran glass ampoule under 4He exchange gas. The resolution of the measurement was 0.15% of C, at 100 K.'"Eu Mossbauer experiments were performed between 4.2 and 300K in a conventional Mossbauer spectrometer with a "'Sm :EuF, source at 300 K. Results Powderous EuPtGe is dark grey and stable in air for several months. No decomposition was visible. Single crystals are light grey with a metallic lustre, with an irregular platelet-like shape. Structure refinement Single crystals of EuPtGe were selected by mechanical fragmen- tation from the annealed sample. They were investigated by Buerger precession photographs in order to establish both symmetry and suitability for intensity data collection. The precession photographs indicated a primitive cubic cell. The systematic extinctions (hOO only observed for h =2n) led to the non-centrosymmetric group P2,3, in agreement with the results reported for the prototype LaIrSi" as well as the X-ray powder refinement for EuPtGe.6 Details on the data collection and crystallographic data are listed in Table 1.The atomic positions of the previous powder investigation6 were taken as starting values. The structure was refined with SHELXL-93,12 assuming anisotropic displacement parameters for all atoms. The refined atomic parameters are listed in Table 2, interatomic distances in Table 3. A final difference Fourier synthesis revealed no significant residual peaks. Additional data on the structure refinement are available.? In order to check for the correct handedness (since P2,3 is a non- centrosymmetric space group), SHELXL-93I2 automatically calculates the absolute structure parameter, which should be 0 for the correct and + 1 for the inverted absolute structure.Our refinement (see Tables 1-3) resulted in an absolute struc- ture parameter of 0.01(3), indicating the correct handedness. A refinement of the inverted structure, on the other hand, resulted in an absolute structure parameter of 0.97(6), higher standard deviations (by a factor of 2) for the positional parameters and higher residuals of R, =0.0399 and R2=0.0939. t Details may be obtained from: Fachinformationszentrum Karlsruhe, D-76344 Eggenstein-Leopoldshafen (Germany), by quoting the Registry No. CSD-401561. J. Muter. Chem., 1996, 6(4), 635-638 635 Table 1 Crystal data and structure refinement for EuPtGe empirical formula formula mass T/KWavelengthlpm crystal system space group unit-cell dimensions/pm cell volume/nm3 z Dclgcmcrystal size/pm3 absorption correction transmission ratio (max, min) absorption coefficient/mm F (000) 8 range for data collection/degrees scan mode range in hkl total no reflections independent reflections reflections with I >241) refinement method data/restraints/parameters goodness-of-fit on F2 final R indices [I >20(1)] R indices (all data) extinction coefficient largest difference peak and hole/e nm absolute structure parameter Table 2 Atomic coordinates and anisotropic displacement parameters' (pm2) for EuPtGe (Ueq is defined as one third of the trace of the orthogonalized U,, tensor) atom P2,3 X u11 u12 Eu 4e 0 13207( 6) 114(1) 11(1)Pt 4e 041819(4) 123( 1) -10( 1) Ge 4e 0 83351( 12) 135(3) 4(3) 'The anisotropic displacement factor exponent takes the form -2n2 [(ha*)2U,,+ +2hka*b*UI2] x=y=z U,,=U22=U33=Ueq, U12= u13=u23 Table 3 Interatomic distances (pm) in the structure of EuPtGe, all distances shorter than 600 (Eu-Eu), 535 (Eu-Pt, Eu-Ge), 470 (Pt-Ge) and 405 (Pt-Pt, Ge-Ge) are listed (standard deviations are all equal to or less than 0 1 pm) Eu 1 Pt 3244 Pt 3 Ge 2380 3 Pt 327 7 1 Eu 3244 3 Ge 3328 3 Eu 3277 1 Ge 3385 3 Eu 3879 3 Ge 3746 3 Pt 3879 Ge 3 Pt 2380 6 Eu 401 1 3 Eu 3328 1 Eu 3385 3 Eu 3746 Magnetic susceptibility The inverse magnetic susceptibility of EuPtGe as a function of temperature is displayed in Fig 1 Above 100K EuPtGe follows the Curie-Weiss law No magnetic order could be observed down to 4 2 K The experimental magnetic moment peXp=7 80( 5) pg/Eu, calculated from the slope of the 1/x us T plot according to pexp=283[~(T-@)]~/~,is close to the free ion value peff=7 94 pB for Eu2+ The paramagnetic Curie temperature (Weiss constant) of 0=20( 1)K was determined by linear extrapolation of the high temperature part of the 1/x us T plot to 1/x=O At about 70 K, the 1/x us T plot deviates from linearity This slight deviation is most probably due to a minor amount of an impurity of ferromagnetic EuO (T,= 69 K),13-" although the Guinier powder patterns showed single-phase EuPtGe In order to quantify the nature and 636 J Muter Chem, 1996,6(4), 635-638 EuPtGe 419 64 293 (2) 56 086 cubic P213 (no 198) a =654 63( 9) 0 28054( 7) 4 9 936 25 x 50 x 75 from $-scan data 0 855,O 471 44 14 692 2-26 a19 O,<h,<10,O,<k,<10,-10,<1< 10 1256 378 (R,,,=O0436) 371 (R,,,,,=O0291) full-matrix least-squares on F2 378/0/11 1138 R, =0 0202, R, =0 0425 R1 =0 0208, R2 =0 0427 0 0079(7) 1630 and -1328 OOl(3) 0 50 100 150 200 250 300 T/K Fig. 1 Temperature dependence of the reciprocal magnetic susceptibil- ity of EuPtGe measured at a magnetic flux density of 2 T amount of this impurity, specific heat measurements were performed A similar magnetic behaviour was recently also observed for isotypic EuPdSi by Adroja et a1l6 Specific heat measurements The specific heat capacity C,(T) shows a small, but clearly visible A-type anomaly with a peak at 69 0 K The shape and temperature of this anomaly is consistent with the presence of a small amount of a EuO impurity in the EuPtGe sample l3 l5 A rough estimate of the size of the anomaly and a comparison with the data in ref 15 yields a content of about 1-2 mass% of EuO in the EuPtGe powder No further anomalies are visible in the CJT) curve '"Eu Mossbauer spectroscopy The Mossbauer spectrum of EuPtGe at 4 2 K and the theoreti- cal fit is shown in Fig 2 The curve can be fitted well by a single Eu" site at 6 = -10 40( 5) mm s-l subject to quadrupole interaction with an axially symmetric field gradient e2qeffQ = 4( 1)mm sC1, and an Eu"' impurity of area 5% at 0 5 mm S".l The parameters are nearly constant up to room temperature, I I-15100.0 I I I I J -37.a -18.9 0 t18.9 t37.a vlmm s-1 Fig. 2 '"Eu Mossbauer spectrum of EuPtGe at 4.2 K relative to EuF, 6(300 K) = -10.46(3) mm s-'. Between 300 and 4.2 K no magnetic order can be detected. Very similar "'Eu Mossbauer properties have recently been reported by Adroja et ~1.'~for isotypic EuPdSi. The latter silicide has an isomer shift of 6(300 K)= -10.15 mm s-' and shows no magnetic ordering down to 4.2 K. Electrical conductivity In Fig. 3 the temperature dependence of the specific resistivity of EuPtGe is presented.The specific resistivity decreases with decreasing temperature as is typical for metallic conductors. In agreement with the magnetic susceptibility and '"Eu Mossbauer investigations, no phase transition is observed in the temperature range investigated. At room temperature EuPtGe has a specific resistivity of 145 pR cm. For comparison, europium and platinum have specific resistivities at room temperature of 90 pQ cm and 10.9 pQ cm, re~pective1y.l~ Discussion The present single-crystal investigation on EuPtGe fully con- firms the previous powder investigation.6 EuPtGe (see Fig. 4) crystallizes with the LaIrSi-type structure," an ordered ternary version of SrSi2.'s-20 The LaIrSi structure" is also formed by the related compounds NdIrSi,'l RPtSi (R=Ca, Sr, Ba, RPdSi (R =Sr, Ba, Eu),~~ EU),'~,'~ RPtGe (R =Ca, Sr, Ba, Eu)~ and RIrP (R =Ba, Eu).~~ Among these intermetallics, the rare- earth-metal containing compounds exhibit interesting physical properties.While LaRhSi and LaIrSi2' become supercon- ducting at low temperatures, NdIrSi" and EuIrPZ4 order ferromagnetically. A detailed description of this structural family was already given by Klepp and ParthC for the prototype LaIrSi" as well as by Evers et al. for the silicide series with Pd and Pt.6*22323 The europium atoms in EuPtGe have the high coordination number (CN) of 20 with six Eu, seven Pt and seven Ge atoms 145 r 135 5 125 G.d Q 115 105 I 1 I I 1 I95 1 0 50 I00 1% 200 250 300 TIK Fig.3 Temperature dependence of the specific resistivity of EuPtGe Fig. 4 Perspective view of the cubic EuPtGe structure. The three- dimensional [PtGe] polyanion is outlined. in their coordination shell. This is typical for such intermetallic compounds. The coordination polyhedron for this position was already shown and discussed in detail by Klepp and ParthC for the prototype La1rSi.l' The Eu-Eu distances of 401.1 pm are about twice the metallic radius [r(Eu) for CN 12=204.2 pm2']. The Eu-Pt distances range from 324.4 pm to 387.9 pm with an average distance of 353.0 pm, somewhat longer than the sum of the metallic radii [r(Pt) for CN 12= 138.7 pm2'] of 342.9 pm. The Eu-Ge distances (332.8-374.6 pm) have an average value of 351.5 pm, which is also longer than the sum of the metallic radii [r(Ge) for CN 12=136.9 pm''] of 341.1 pm.Both Pt and Ge atoms have CN 10 with three Ge (Pt) and seven Eu atoms in their coordination shell. The Pt-Ge distance of 238.0 pm is significantly shortened when compared to the average Pt-Ge distance of 260.1 pm for the four Ge neighbours of the Pt atoms in CaPtGe' with the TiNiSi-type structure. Similar short Pt-Ge distances have recently been observed in the cerium compounds Ce,Pt,Ge, (240.6-277.8 pm)26 and Ce,Pt,,Ge,, (232.0-264.0 ~m).'~ A comparison of the interatomic distances indicates strong Pt-Ge bonding in EuPtGe. Considering that the europium atoms are by far the most electropositive component in EuPtGe, they will have largely transferred their valence elec- trons to the three-dimensional [PtGe] polyanion.Therefore the compound can be described to a first approximation as Eu2+ [PtGe]'-. The three-dimensional [PtGe] polyanion is outlined in Fig. 4. At this point it is interesting to compare the structure of EuPtGe to that of the binaries EuG~,~,~~ sinceand EuP~~,~~ very often the equiatomic RTX compounds adopt ordered structures of the binary border phases. The latter germanide may be considered as a Zintl compound and the formula may be written as Eu2+(Ge-),. Considering the 8-N rule, the germanium anions are thus isoelectronic with arsenic. Indeed, the germanium sublattice in EuGe, consists of puckered hexa- gons with triply-connected germanium atoms, like in a-arsenic.30 These two-dimensional networks are separated by the europium atoms.In EuPtGe, half of the germanium atoms are replaced by platinum. Although the size and electronegativ- ity of Pt and Ge are similar, the structure is different to that of EuGe,. Each platinum and germanium atom remains triply- connected; however, the network is now three-dimensional and the europium atoms are embedded in the [PtGe] polyanion (see Fig. 4). If all the germanium atoms in EuGe, are replaced by platinum, the situation is totally different. EuP~,~~ is a Laves phase with the MgCu,-type structure. In EuPt, each platinum atom has six platinum neighbours within the tetra- hedral Pt sublattice. J.Mater. 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