6
This work has been digitalized and published in 2013 by Verlag Zeitschrift für Naturforschung in cooperation with the Max Planck Society for the Advancement of Science under a Creative Commons Attribution 4.0 International License. Dieses Werk wurde im Jahr 2013 vom Verlag Zeitschrift für Naturforschung in Zusammenarbeit mit der Max-Planck-Gesellschaft zur Förderung der Wissenschaften e.V. digitalisiert und unter folgender Lizenz veröffentlicht: Creative Commons Namensnennung 4.0 Lizenz. New Magnesium Compounds /fcE2Cu2Mg (RE = Y, La - Nd, Sm, Gd - Tm, Lu) with Mo2FeB2 Type Structure Ratikanta Mishra, Rolf-Dieter Hoffmann, and Rainer Pöttgen Department Chemie, Ludwig-Maximilians-Universität München, Butenandtstraße 5 -1 3 (Haus D), 81377 München, Germany Reprint requests to R. Pöttgen. E-mail: [email protected] Z. Naturforsch. 56 b, 239-244 (2001); received January 25, 2001 Intermetallic Compounds, Magnesium, Crystal Structure The title compounds were synthesized by reaction of the elements in sealed tantalum tubes in a water-cooled sample chamber in a high-frequency furnace. These magnesium intermetallics crystallize with the tetragonal Mo2FeB2 type structure, space group PMmbm. The lattice pa rameters of all compounds were refined from X-ray powder data. Single crystal X-ray data yielded a = 792.09(6), c = 396.31(8) pm, wR2 = 0.0396, 315 F2 values for La2Cu2Mg, a = 778.30(5), c = 384.04(5) pm, wR2 = 0.0954, 214 F2 values for Nd2Cu2Mg, and a = 762.65(5), c = 374.09(3) pm, wR2 = 0.0566, 186 F2 values for Y2Cu2Mg with 12 variable parameters for each refinement. The /?£, 2Cu2Mg structures can be described as an intergrowth of distorted A1B2 and CsCl related slabs of compositions RECui and REMg. Chemical bonding in La2Cu2 Mg was investigated on the basis of extended Hückel calculations and compared to isotypic La2Cu2ln. This structure was also refined from single crystal X-ray data: PMmbm, a = 780.8(2), c = 400.1(2) pm, wR2 = 0.0351, 211 F2 values and 12 variable parameters. Introduction The ternary systems rare earth metal (RE)- transition metal (7)-magnesium (cadmium) have drawn attention in recent years [1 - 8 ]. So far, the equiatomic series RETMg (T = Pd, Ag, Au) [1,6] and 7?£AuCd [8 ] have been reported. These com pounds crystallize with the ZrNiAl type structure [9 - 11] for the trivalent rare earth atoms but they adopt the TiNiSi type structure [12] in the case of divalent europium and ytterbium atoms. Some of the RETMg and RETCd compounds show remarkably high magnetic ordering temperatures, e.g. Tc = 28 K for EuAuCd [8 ] or Tc = 62 K for GdPdCd [13]. With lower magnesium and cadmium content the series RE2 T2Mg (T = Ni, Pd, Pt) [2, 4, 5] and RE2 T2 Cd (T = Ni, Pd, Pt, Au) [4, 7, 8 ], respectively, with Mo2FeB2 type structure [14] and Gd2 Ni2Cd [2 ] with Mn2AlB2 type structure [15] have been syn thesized. In continuation of our systematic studies of these ternary systems we have now investigated the series RE2 C\i2 Mg. The syntheses and structure refinements are reported herein. We also refined the structure of isotypic La2 Cu2ln [16] in order to get precise atomic parameters for the electronic struc ture calculations. Experimental Synthesis Starting materials for the preparation of ft£ 2Cu2Mg were ingots of the rare earth elements (Johnson Matthey, Chempur, > 99.9%), copper wire (Johnson Matthey, 0 1 mm), and a magnesium rod (Johnson Matthey, 0 16 mm, > 99.95%). In the first step, the rare earth ingots were cut into smaller pieces and arc-melted [17] to buttons (about 500 mg) under an argon pressure of about 600 mbar and kept in small Schlenk tubes prior to the reactions. The argon was purified before by pass ing over molecular sieves, silica gel, and titanium sponge (900 K). The rare earth buttons were subsequently mixed with pieces of the copper wire and turnings of the mag nesium rod in the ideal 2:2:1 atomic ratio and sealed in tantalum tubes (about 1 cm3) under an argon pressure of about 800 mbar. The tantalum tubes were placed in a water-cooled quartz glass sample chamber of a high-frequency gen erator (KONTRON Roto-Melt, 1.2 kW) under purified flowing argon [18]. They were first heated for 2 min with the maximum power output (about 1500 K) and subse quently annealed at about 900 K for another 4 h. The tubes were finally quenched by radiative heat loss within the water-cooled sample chamber. The reaction between the three elements was visible by a heat flash. After the 0932-0776/01/0300-0239 $ 06.00 (c) 2001 Verlag der Zeitschrift für Naturforschung, Tübingen • www.znaturforsch.com K

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  • This work has been digitalized and published in 2013 by Verlag Zeitschrift für Naturforschung in cooperation with the Max Planck Society for the Advancement of Science under a Creative Commons Attribution4.0 International License.

    Dieses Werk wurde im Jahr 2013 vom Verlag Zeitschrift für Naturforschungin Zusammenarbeit mit der Max-Planck-Gesellschaft zur Förderung derWissenschaften e.V. digitalisiert und unter folgender Lizenz veröffentlicht:Creative Commons Namensnennung 4.0 Lizenz.

    New Magnesium Compounds /fcE2Cu2Mg (RE = Y, La - Nd, Sm, Gd - Tm, Lu) with Mo2FeB2 Type StructureRatikanta Mishra, Rolf-Dieter Hoffmann, and Rainer Pöttgen Department Chemie, Ludwig-Maximilians-Universität München,Butenandtstraße 5 -1 3 (Haus D), 81377 München, Germany

    Reprint requests to R. Pöttgen. E-mail: [email protected]

    Z. Naturforsch. 56 b, 239-244 (2001); received January 25, 2001 Intermetallic Compounds, Magnesium, Crystal Structure

    The title compounds were synthesized by reaction of the elements in sealed tantalum tubes in a water-cooled sample chamber in a high-frequency furnace. These magnesium intermetallics crystallize with the tetragonal Mo2FeB2 type structure, space group PMmbm. The lattice parameters of all compounds were refined from X-ray powder data. Single crystal X-ray data yielded a = 792.09(6), c = 396.31(8) pm, wR2 = 0.0396, 315 F 2 values for La2Cu2Mg, a = 778.30(5), c = 384.04(5) pm, wR2 = 0.0954, 214 F 2 values for Nd2Cu2Mg, and a = 762.65(5), c = 374.09(3) pm, wR2 = 0.0566, 186 F 2 values for Y2Cu2Mg with 12 variable parameters for each refinement. The /?£,2Cu2Mg structures can be described as an intergrowth of distorted A1B2 and CsCl related slabs of compositions RECui and REMg. Chemical bonding in La2Cu2Mg was investigated on the basis of extended Hückel calculations and compared to isotypic La2Cu2ln. This structure was also refined from single crystal X-ray data: PMmbm, a = 780.8(2), c = 400.1(2) pm, wR2 = 0.0351, 211 F 2 values and 12 variable parameters.

    Introduction

    The ternary systems rare earth metal (RE)- transition metal (7)-magnesium (cadmium) have drawn attention in recent years [1 - 8 ]. So far, the equiatomic series RETMg (T = Pd, Ag, Au) [1,6] and 7?£AuCd [8 ] have been reported. These compounds crystallize with the ZrNiAl type structure [9 - 11] for the trivalent rare earth atoms but they adopt the TiNiSi type structure [12] in the case of divalent europium and ytterbium atoms. Some of the RETMg and RETCd compounds show remarkably high magnetic ordering temperatures, e .g . Tc = 28 K for EuAuCd [8 ] or Tc = 62 K for GdPdCd [13].

    With lower magnesium and cadmium content the series RE2T2Mg (T = Ni, Pd, Pt) [2, 4, 5] and RE2 T2Cd (T = Ni, Pd, Pt, Au) [4, 7, 8 ], respectively, with Mo2FeB2 type structure [14] and Gd2Ni2Cd [2 ] with Mn2AlB2 type structure [15] have been synthesized. In continuation of our systematic studies of these ternary systems we have now investigated the series RE2C\i2M g. The syntheses and structure refinements are reported herein. We also refined the structure of isotypic La2Cu2ln [16] in order to get precise atomic parameters for the electronic structure calculations.

    Experimental

    Synthesis

    Starting materials for the preparation of ft£2Cu2Mg were ingots of the rare earth elements (Johnson Matthey, Chempur, > 99.9%), copper wire (Johnson Matthey,0 1 mm), and a magnesium rod (Johnson Matthey,0 16 mm, > 99.95%). In the first step, the rare earth ingots were cut into smaller pieces and arc-melted [17] to buttons (about 500 mg) under an argon pressure of about 600 mbar and kept in small Schlenk tubes prior to the reactions. The argon was purified before by passing over molecular sieves, silica gel, and titanium sponge (900 K). The rare earth buttons were subsequently mixed with pieces of the copper wire and turnings of the magnesium rod in the ideal 2:2:1 atomic ratio and sealed in tantalum tubes (about 1 cm3) under an argon pressure of about 800 mbar.

    The tantalum tubes were placed in a water-cooled quartz glass sample chamber of a high-frequency generator (KONTRON Roto-Melt, 1.2 kW) under purified flowing argon [18]. They were first heated for 2 min with the maximum power output (about 1500 K) and subsequently annealed at about 900 K for another 4 h. The tubes were finally quenched by radiative heat loss within the water-cooled sample chamber. The reaction between the three elements was visible by a heat flash. After the

    0932-0776/01/0300-0239 $ 06.00 (c) 2001 Verlag der Zeitschrift für Naturforschung, Tübingen • www.znaturforsch.com K

  • 240 R. M ishra et al. • Ternary M agnesium C om pounds ^ E ^ C i^ M g

    Table 1. Lattice parameters of tetragonal /?£2Cu2Mg compounds and La2Cu2In with ordered UsSi2 type structure.

    Compound a (pm) c (pm) da V (nm3) Ref.

    Y2Cu2Mg 762.65(5) 374.09(3) 0.491 0.2176 this workLa2Cu2Mg 792.09(6) 396.31(8) 0.500 0.2486 this workLa2Cu2Mg 791.9(1) 397.0(1) 0.501 0.2490 [2 0 ]Ce2Cu2Mg 786.8(1) 387.5(2) 0.493 0.2399 this workCe2Cu2Mg 787.41(9) 387.23(7) 0.492 0.2401 [2 0 ]Pr2Cu2Mg 782.82(8) 385.12(6) 0.492 0.2360 this workNd2Cu2Mg 778.30(5) 384.04(5) 0.493 0.2326 this workSm2Cu2Mg 771.45(6) 379.52(5) 0.492 0.2259 this workGd2Cu2Mg 765.31(8) 377.22(7) 0.493 0.2209 this workTb2Cu2Mg 762.57(9) 373.94(9) 0.490 0.2175 this workDy2Cu2Mg 759.93(9) 371.30(9) 0.489 0.2144 this workHo2Cu2Mg 758.67(5) 369.87(4) 0.488 0.2129 this workEr2Cu2Mg 756.18(3) 367.93(3) 0.487 0.2104 this workTm2Cu2Mg 754.1(1) 365.3(4) 0.484 0.2077 this workLu2Cu2Mg 749.6(4) 359.9(4) 0.480 0 .2 0 2 2 this workLa2Cu2In 780.8(2) 400.1(2) 0.512 0.2439 this workLa2Cu2In 778.9(2) 398.3(3) 0.511 0.2416 [16]

    annealing procedures the light-gray samples could readily be separated from the tantalum tubes. No reactions of the samples with the tubes could be detected. All samples were obtained in amouts of about 1 g. They are stable against air and moisture as compact buttons as well as fine-grained powders. Single crystals exhibit metallic luster.

    X-ray investigations

    The samples were characterized through powder diffractograms (Stoe Stadi P) using Cu-Kq i radiation and silicon (a = 543.07 pm) as an external standard. The tetragonal lattice parameters (Table 1) were obtained from least-squares fits of the powder data. The correct indexing of the patterns was ensured by intensity calculations [ 19] taking the atomic positions from the structure refinements. The lattice parameters determined from the powders and the single crystals agreed well. The X-ray powder data of Er2Cu2Mg with experimental and calculated intensities are presented as an example in Table 2. For La2Cu2In, La2Cu2Mg, and Ce2Cu2Mg we found perfect agreement with the previously published data [16, 20].

    Irregularly shaped single crystals of Y2Cu2Mg, La2Cu2Mg, La2Cu2In, and Nd2Cu2Mg were isolated from the annealed samples by mechanical fragmentation. They were first examined on a Buerger precession camera equipped with an image plate system (Fujifilm BAS- 2500) in order to establish both symmetry and suitability for intensity data collection.

    Single crystal intensity data were collected at room temperature by use of a four-circle diffractometer (CAD4) with graphite monochromatized Mo-KQ (71.073 pm) radiation and a scintillation counter with pulse height dis-

    Table 2. X-ray powder data (Cu-KQi radiation) for Er2Cu2Mg. The experimental intensities were obtained on a powder diffractometer taking the peak heights and calculated intensities were generated with Lazy- Pulverix [19].

    hkl 20o dc (A) do (A) Ic Io

    110 16.56 5.3470 5.3486 20 25200 23.51 3.7809 3.7808 14 13001 24.16 3.6793 3.6805 6 9210 26.33 3.3817 3.3819 18 19220 33.49 2.6735 2.6734 5 7201 33.97 2.6368 2.6368 25 29211 36.05 2.4898 2.4893 100 100310 37.58 2.3912 2.3913 24 21221 41.73 2.1628 2.1626 2 2320 43.10 2.0973 2.0970 6 3311 45.18 2.0050 2.0052 1 1400 48.09 1.8904 1.8904 5 5002 49.50 1.8397 1.8398 12 10410 49.66 1.8340 1.8343 14 9321 50.01 1.8220 1.8222 2 4330 51.22 1.7823 1.7820 11 6112 52.57 1.7396 1.7394 2 2420 54.20 1.6909 1.6908 1 1202 55.50 1.6542 1.6543 3 3411 55.98 1.6414 1.6412 18 9212 56.93 1.6160 1.6161 5 5331 57.42 1.6040 1.6034 11 6312 63.78 1.4581 1.4580 12 6322 67.67 1.3830 1.3834 3 2511 68.11 1.3755 1.3755 7 3440 70.36 1.3367 1.3369 2 < 1402 71.50 1.3184 1.3184 4 2521 71.90 1.3119 1.3120 12 5412 72.75 1.2988 1.2988 12 4530 72.87 1.2968 1.2969 2 3332 73.99 1.2801 1.2800 9 3600 75.34 1.2603 1.2604 4 1531 78.09 1.2231 1.2228 9 2

    crimination. Scans were taken in the to/20 mode, '/'-scan data were used for an empirical absorption correction followed by a spherical absorption correction. All relevant crystallographic data and experimental details for the intensity data collections are listed in Table 3.

    Structure refinements

    The isotypy of Y2Cu2Mg, La2Cu2Mg, and Nd2Cu2Mg with the Mo2FeB2 type [14] was already evident from the X-ray powder data. The atomic parameters of Ce2Cu2Mg[4] were taken as starting values and the three structures were successfully refined using SHELXL-97 (full-matrix least-squares on F02) [21] with anisotropic atomic displacement parameters for all atoms. As a check for the correct composition, the occupancy parameters were re-

  • R. M ishra et al. • Ternary M agnesium C om pounds REjCniMg 241

    Table 3. Crystal data and structure refinement for Y2Cu2Mg, La2Cu2ln, La2Cu2Mg, and Nd2Cu2Mg; space group PA/mbm and Z = 2.

    Empirical formula Molar mass Unit cell dimensions Calculated density Crystal size ( p m )Transm. ratio (max/min) Absorption coefficient F(000)9 Range Range in hkl Total no. reflections Independent reflections Reflections with I > 2a(I) Data/parameters Goodness-of-fit on F 2 Final R indices [/ > 2ct(/)]

    R Indices (all data)

    Extinction coefficient Largest diff. peak and hole

    Y2Cu2Mg329.21 g/mol Table 1 5.03 g/cm3 20 x 70 x 701.4336.0 mm-1 2962° to 30°+5,+10, ±5 433186 (flint = 0.0676)122 (Ra =0.0781) 186/12 1.015R\ =0.0322 wR2 = 0.0469 R \ =0.0766 wR2 = 0.0566 0 .002(1)2.09 and -1.52 e/Ä3

    , - i

    La2Cu2ln519.72 g/mol Table 1 7.08 g/cm3 40 x 40 x 120 1.5230.2 mm'4422° to 35°+3, ±12, ±6 775211 (Rm = 0.0387) 200 (Ra = 0.0247) 211 /1 2 1.196R\ =0.0181 wR2 = 0.0345 R\ =0.0211 wR2 = 0.0351 0.0055(4)0.51 and-0.59 e/A3

    La2Cu2Mg429.21 g/mol Table 15.73 g/cm3 20 x 100 x 100 1.9825.2 mm-1 3682° to 35°+7, ±12, ±6 1437315 (/?int = 0.0551) 261 (R„ =0.0345) 315/12 1.057R\ =0.0231 wR2 = 0.0372 R\ =0.0355 wR2 = 0.0396 0.0006(3)0.85 and -1.07 e/A3

    Nd2Cu2Mg 439.87 g/mol Table 1 6.28 g/cm3 10 x 80 x 801.5630.9 mm-1 3802° to 30°-10 < h < 5, ±10, ±5 725214 (flint = 0.0686)158 (Ra =0.0569)214/121.084Al =0.0369 wR2 = 0.0827 R1 =0.0700 wR2 = 0.0954 0.003(2)1.70 and-2.56 e/A3

    Table 4. Atomic coordinates and isotropic displacement parameters (pm2) for Y2Cu2Mg, La2Cu2ln, La2Cu2Mg, and Nd2Cu2Mg (space group PA/mbm). Ueq is defined as one third of the trace of the orthogonalized Uij tensor.

    Atom Wykoff site

    X y 2 Ueq

    Y2Cu2MgY Ah 0.1716(1) 1/2 + x 1/2 151(4)Cu 4 g 0.3784(2) 1/2 + x 0 163(5)Mg 2 a 0 0 0 171(16)La2Cu2InLa 4 h 0.17838(4) 1/2 + x 1/2 165(1)Cu 4 g 0.38273(8) 1/2 + x 0 195(2)In 2 a 0 0 0 191(2)La2Cu2MgLa 4 h 0.17368(4) 1/2 + x 1/2 196(1)Cu Ag 0.38050(9) 1/2 + x 0 212(2)Mg 2 a 0 0 0 231(8)Nd2Cu2MgNd Ah 0.1723(1) 1/2 + a; 1/2 158(4)Cu Ag 0.3783(3) 1/2 + x 0 172(7)Mg 2 a 0 0 0 171(23)

    fined in a separate series of least-squares cycles. All sites were fully occupied within two standard deviations. Final difference Fourier syntheses revealed no significant residual peaks (see Table 3). The positional parameters and interatomic distances of the four refinements are listed in

    Tables 4 and 5. Listings of the observed and calculated structure factors are available*.

    Electronic structure calculations

    Three-dimensional semi-empirical band structure calculations were based on an extended Hiickel Hamiltonian [22, 23], whereas off-site Hamiltonian matrix elements were evaluated according to the weighted Wolfs- berg-Helmholz formula [24], minimizing counterintuitive orbital mixing. All exchange integrals, orbital exponents, and weighting coefficients [25, 26] are listed in Table 6. The eigenvalue problem was solved in reciprocal space at 165 k points within the irreducible wedge of the Brillouin zone by using the YAEHMOP code [27],

    Discussion

    Eleven new magnesium compounds ftE^C^Mg (RE = Y, Pr, Nd, Sm, Gd-Tm, and Lu) have been prepared for the first time. These intermetallics crystallize with the Mo2FeB2 type structure [14], a ternary ordered version of U3Si2 [28, 29]. As an example we present the La2Cu2Mg structure in Fig. 1. It may

    “Details may be obtained from: Fachinformationszentrum Karlsruhe, D-76344 Eggenstein-Leopoldshafen (Germany), by quoting the Registry No’s. CSD-411709 (La2Cu2Mg), CSD- 411708 (La2Cu2In), CSD-411710 (Nd2Cu2Mg), and CSD- 411711 (Y2Cu2Mg).

  • 242 R. M ishra et al. • Ternary M agnesium Com pounds 7?E2Cu2Mg

    Table 5. Interatomic distances (pm), calculated with the lattice parameters taken from X-ray powder data of Y2Cu2Mg, La2Cu2In, La2Cu2Mg, and Nd2Cu2Mg. Standard deviations are all equal or less than 0.3 pm. All distances within the first coordination sphere are listed.

    Y2Cu2Mg La2Cu2In La2Cu2Mg Nd2Cu2Mg

    Ln:2 Cu 291.1 301 .6 304.9 297.24 Cu 294.0 309'.2 308.3 301.34 Mg/In 338.9 350LO 353.6 346.31 Ln 370.1 393 .9 389.1 379.22 Ln 374.1 400i.l 396.3 384.04 Ln 399.6 406 .I 414.1 407.5Cu:1 Cu 262.3 259 .0 267.7 267.92 Ln 291.1 301 .6 304.9 297.24 Ln 294.0 309'.2 308.3 301.32 Mg/In 303.1 312 .6 315.9 309.3Mg/In:4 Cu 303.1 312 .6 315.9 309.38 Ln 338.9 350i.O 353.6 346.3

    Table 6. Extended Hiickel parameters.

    Atom Orbital Hü (eV) 6 Cl f 2 c2

    La 6s -4.637 1.3186p -3.233 1.1425d -7.316 3.153 0.594 1.338 0.612

    Cu 4s -7.755 1.5154p -4.331 1.2323d 20.188 6.676 0.487 2.768 0.657

    Mg 3s -6.886 1.0763p -4.169 0.862

    In 5s 10.141 1.9345p -5.368 1.456

    be considered as an intergrowth of distorted AIB2 and CsCl related slabs of compositions LaCu2 and LaM g. Binary LaMg [30] indeed crystallizes with the CsCl type structure with a lattice parameter of 396.5 pm. The related slab in La2Cu2Mg, however, is tetragonally distorted with a' = 414.1 pm and c' = 396.3 pm, resulting in a c'/a' ratio of 0.957. Thus, with respect to binary LaMg, only the a' parameter is significantly enlarged. This is discussed in more detail below. The reverse trend is observed for the LaCu2 slab. Here, the c'la' ratio of 0.957 is significantly enlarged when compared with A1B2 type LaCu2 (a = 434.6, c = 380.7 pm, d a = 0.876) [31]. Also the pseudo-hexagonal angle of 114° significantly deviates from the ideal value. This has a

    Fig. 1. Projection of the La2Cu2Mg structure onto the xy plane. All atoms lie on mirror planes at z = 0 (thin lines) and z = 1/2 (thick lines). The A1B2 and CsCl related slabs are emphasized.

    Y La Ce Pr NdPmSmEuGd Tb Dy Ho ErTmYb Lu

    Fig. 2. Plot of the lattice parameters of the /?£'2Cu2Mg and /?£2Cu2In compounds with tetragonal Mo2FeB2 structure. The solid lines serve as a guide to the eye.

    drastic effect on the Cu-Cu distances within these AIB2 related fragments: 251 pm in binary LaCu2 and 268 pm in the LaCu2 slab of La2Cu2Mg. In fee copper [32] each copper atom has twelve neighbors at 255.6 pm.

    The lattice parameters decrease from the lanthanum to the lutetium compound (Fig. 2) as expected from the lanthanoid contraction. So far no RE2T2X compounds are known to be formed with the possibly divalent europium and ytterbium. No pronounced deviation from the smooth curve was observed indicating that the rare earth metal atoms are trivalent in all /^ C ^ M g compounds.

  • R. M ishra e t al. • Ternary M agnesium Compounds /?£'2Cu2Mg 243

    DOS DOS

    Fig. 3. Density-of-states (DOS) for La2Q i2Mg and La2Q i2ln. The lanthanum contributions are shaded and the Fermi levels are drawn as dotted lines.

    This is different to the corresponding nickel series[5], where mixed-valent behavior was observed for Ce2Ni2Mg [2]. In the series we observedlarger lattice parameters for the magnesium compounds when compared with the isotypic indium compounds /^ N i^ In [5] although the metallic and the covalent radius of magnesium are smaller than those of indium [33, 34]. This effect is even more pronounced for the copper compounds: the a parameters of the magnesium compounds are significantly larger, while the c parameters are slightly smaller (as expected from the atomic size) than those of the indium compounds (Fig. 2).

    In order to analyze the course of the lattice parameters in more detail, we performed extended Hiickel band structure calculations on La2Cu2Mg and isotypic La2Cu2ln. The density-of-states (DOS) curves for both compounds are displayed in Fig. 3. Near the Fermi level the DOS is heavily dominated by the lanthanum states. The copper ^-states (not shown in that Figure) stay localized around -20 eV and the indium(magnesium) states are spread out over the plotted energy range.

    The course of the La-La, La-Cu, and Cu-Cu crystal orbital overlap populations (COOP) is quite similar for both compounds, i.e. 0.118, 0.158, and 0.233 for La2Cu2ln and 0.125, 0.170, and 0.218 for La2Cu2Mg. Also these are the highest overlap

    populations in both compounds. Differences occur for the La-In/Mg, Cu-In/Mg, and In-In/Mg-Mg COOPs, i. e. 0.158, 0.211, and 0.102 for La2Cu2ln, and 0.096, 0.147, and 0.069 for La2Cu2Mg. Based on these COOP values we can conclude that bonding interactions with indium contributions are stronger than those where magnesium is involved. This might have been deduced already from the course of the electronegativities, i.e. 1.31 for magnesium and 1.78 for indium on the Pauling scale [34].

    The distances within the ab plane are governed by the Cu-Cu and Cu-In (Cu-Mg) interactions. The shorter distances in the indium compound go along with higher overlap populations. Thus, the decrase of the a lattice parameters of the indium compounds is dictated by electronic effects, while the c lattice parameter depends mere on geometrical effects. This bonding pattern is very similar to the one discussed recently for the series andRE2N i2ln [5] but more pronounced for the copper compounds discussed herein.

    Acknowledgments

    Special thanks go to the Alexander von Humboldt Foundation for a research stipend to R. M. This work was financially supported by the Fonds der Chemischen Industrie and by the Deutsche Forschungsgemeinschaft.

    [1] A. Iandelli, J. Alloys Compd. 203, 137 (1994).[2] C. Geibel, U. Klinger, M. Weiden, B. Buschinger,

    F. Steglich, Physica B 237-238, 202 (1997).[3] F. Canepa, S. Cirafici, F. Merlo, M. Pani, C. Ferde-

    ghini, J. Magn. Magn. Mater. 195, 646 (1999).

    [4] R. Pöttgen, A. Fugmann, R.-D. Hoffmann, U. Ch. Rodewald, D. Niepmann, Z. Naturforsch. 55b, 155 (2000).

    [5] R.-D. Hoffmann, A. Fugmann, U. Ch. Rodewald, R. Pöttgen, Z. Anorg. Allg. Chem. 626,1733 (2000).

  • 244 R. M ishra et al. • Ternary M agnesium Compounds /?£ '2C u2Mg

    [6] R. Pöttgen, R.-D. Hoffmann, J. Renger, U. Ch. Rodewald, M. H. Möller, Z. Anorg. Allg. Chem. 626, 2257 (2000).

    [7] D. Niepmann, R. Pöttgen, B. Künnen, G. Kotzyba, J. Solid State Chem. 150, 139 (2000).

    [8] R. Mishra, R. Pöttgen, R.-D. Hoffmann, D. Kac- zorowski, C. Rosenhahn, B. D. Mosel, H. Pi- otrowski, P. Mayer, Z. Anorg. Allg. Chem., in press.

    [9] P. I. Krypyakevich, V. Ya. Markiv, E. V. Melnyk, Dopov. Akad. Nauk. Ukr. RSR, Ser. A 750 (1967).

    [10] A. E. Dwight, M. H. Mueller, R. A. Conner, Jr., J. W. Downey, H. Knott, Trans. Met. Soc. AIME 242, 2075 (1968).

    [11] M. F. Zumdick, R.-D. Hoffmann, R. Pöttgen, Z. Naturforsch. 54b, 45 (1999).

    [12] C. B. Shoemaker, D. P. Shoemaker, Acta Crystallogr. 18, 900 (1965).

    [13] R.-D. Hoffmann, Th. Fickenscher, R. Pöttgen, K. Latka, Solid State Sciences, to be submitted.

    [14] W. Rieger, H. Nowotny, F. Benesovsky, Monatsh. Chem. 95, 1502(1964).

    [15] H. J. Becher, H. Krogmann, E. Peisker, Z. Anorg. Allg. Chem. 344, 140 (1966).

    [16] Ya. M. Kalychak, V. I. Zaremba, V. M. Baranyak, P. Yu. Zavalii, V. A. Bruskov, L. V. Sysa, O. V. Dmytrakh, Inorg. Mater. 26, 74 (1990).

    [17] R. Pöttgen, Th. Gulden, A. Simon, GIT-Labor- fachzeitschrift 43, 133 (1999).

    [18] R. Pöttgen, A. Lang, R.-D. Hoffmann, B. Künnen,G. Kotzyba, R. Müllmann, B. D. Mosel, C. Rosenhahn, Z. Kristallogr. 214, 143 (1999).

    [19] K. Yvon, W. Jeitschko, E. Parthe, J. Appl. Crystallogr. 10, 73 (1977).

    [20] B. J. Gibson, R. K. Kremer, R.-D. Hoffmann, R. Pöttgen, J. Solid State Chem., to be submitted.

    [21] G. M. Sheldrick, SHELXL-97, Program for Crystal Structure Refinement, University of Göttingen, Germany (1997).

    [22] R. Hoffmann, J. Chem. Phys. 39, 1397 (1963).[23] R. Hoffmann, Solids and Surfaces: A Chemist’s

    View of Bonding in Extended Structures, VCH, Weinheim, New York (1988).

    [24] J. H. Ammetter, H.-B. Bürgi, J. C. Thibeault, R. Hoffmann, J. Am. Chem. Soc. 100, 3686 (1978).

    [25] J. P. Desclaux, At. Data Nucl. Data Tables 12, 3110 (1973).

    [26] P. Pyykkö, L. L. Lohr (Jr.), Inorg. Chem. 20, 1950 (1981).

    [27] G. A. Landrum, YAEHMOP (Yet Another Extended Hückel Molecular Orbital Package), Version 2.0, 1997; available on: http://overlap.chem.comell.edu: 8080/yaehmop.html

    [28] W. H. Zachariasen, Acta Crystallogr. 2, 94 (1949).[29] K. Remschnig, T. Le Bihan, H. Noel, P. Rogl,

    J. Solid State Chem. 97, 391 (1992).[30] H. Nowotny, Z. Metallkd. 34, 247 (1942).[31] A. R. Storm, K. E. Benson, Acta Crystallogr. 16,

    701 (1963).[32] J. Donohue, The Structures of the Elements, Wiley,

    New York (1974).[33] L. Pauling, The Nature of the Chemical Bond and

    the Structure of Molecules and Crystals, Cornell University Press, Cornell (1960).

    [34] J. Emsley, The Elements, Oxford University Press, Oxford (1999).