Lutetium Trigermanide LuGe3: High-Pressure Synthesis

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Cite This: Inorg. Chem. XXXX, XXX, XXX−XXX

Lutetium Trigermanide LuGe3: High-Pressure Synthesis, Superconductivity, and Chemical Bonding Julia-Maria Hübner, Matej Bobnar, Lev Akselrud, Yurii Prots, Yuri Grin, and Ulrich Schwarz*

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Max-Planck-Institut für Chemische Physik fester Stoffe, Nöthnitzer Straße 40, 01187 Dresden, Germany ABSTRACT: LuGe3 was obtained under high-pressure and high-temperature conditions at pressures between 8(1) and 14(2) GPa and at temperatures in the range from 1100(150) to 1500(150) K. The high-pressure phase is isotypic to DyGe3 and decomposes at ambient pressure and T = 690 K mainly into (cF8)Ge and LuGe2−x. Chemical bonding analysis of LuGe3 reveals two-center electrondeficient Ge−Ge bonds, multicenter polar Lu−Ge interactions, and lone pairs on germanium. Magnetic susceptibility, specific heat, and electrical conductivity measurements indicate transition into a superconducting state below Tc = 3.3(3) K.

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between 8(1) and 14(2) GPa plus annealing between 1100(110) and 1500(150) K for 1 h before temperature quenching under load. Calibration of the pressure and temperature had been conducted prior to the experiments by observing resistance changes of bismuth,9 as well as thermocouple-calibrated runs. X-ray Diffraction (XRD) Data Collection and Processing. Phase identification was realized by powder XRD experiments with a Huber Image Plate Guinier Camera G670, employing Cu Kα1 radiation, λ = 1.540562 Å (Figure 2). High-resolution XRD experiments were conducted using synchrotron radiation (λ = 0.399972 Å) at room temperature at beamline ID22 of the European Synchrotron Radiation Facility (ESRF; Grenoble, France). The lattice parameters were refined using LaB6 as an internal standard. All crystallographic calculations including diffraction peak position finding, lattice parameters, and crystal structure refinements (Tables 1 and 2) were performed with the WinCSD program package, version 2018.10 Thermal Analysis. Differential scanning calorimetry (DSC) experiments were performed in corundum crucibles operated under an argon atmosphere and using a Netzsch DSC 404C apparatus. The heating and cooling rates were set to 5 K min−1. Metallographic Analysis. For metallographic analysis, samples were polished by using disks with diamond powders (grain sizes 6, 3, and 0.25 μm) in paraffin and investigated with a Philips XL 30 scanning electron microscope (LaB6 cathode). Energy-dispersive Xray spectroscopy (EDXS) was performed with an attached EDAX Si(Li) detector. The measurements resulted in Lu24.2(5)Ge75.8(5), in line with the target composition Lu25Ge75. Physical Property Measurements. Measurements of the magnetic susceptibility were carried out using a polycrystalline sample of cylindrical shape (diameter 1.95 mm; length 3.0 mm) on a SQUID magnetometer (MPMS XL-7, Quantum Design). The sample used for the electrical resistivity (ρ) and specific heat capacity (Cp) experiments was a cuboid (1.97 mm × 1.65 mm × 1.0 mm) cut from a polycrystalline pellet. The electrical resistivity was determined

INTRODUCTION Compounds of the semiconducting elements silicon or germanium with electropositive partners of the alkaline-, alkaline-earth-, and rare-earth-metal groups form a rich variety of binary phases with significant contributions of ionic and classical covalent two-center, two-electron (2c2e) interactions. The interdependence of the chemical bonding and electron count mostly follows the Zintl−Klemm concept.1 The application of high-pressure methods to these binary systems led to the discovery of new structural patterns, especially for tetrel-rich compositions.2−6 These metastable high-pressure phases often exhibit multicenter interactions and combine covalent partial structures of the p-block element with metal-type electrical conductivity. In the series of rare-earth metals, lutetium, usually occurring as Lu3+ in compounds, provides the absence of a magnetic momentum, setting the stage for the occurrence of superconductivity in these so-called covalent metals. Hitherto, the superconducting lutetium tetrel compounds LuSi3 and LuGe2 have been discovered.6,7 In this study, we describe the high-pressure and high-temperature synthesis of LuGe3 and its superconducting properties.

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EXPERIMENTAL SECTION

Synthesis. Sample handling (except for the high-pressure experiments) including the preparation of precursors was performed in argon-filled gloveboxes (MBraun, H2O, O2 < 0.1 ppm). High pressures were applied by the transformation of uniaxial forces of the hydraulic press into quasi-hydrostatic conditions by a Walker-type module8 operated with MgO octahedra having an edge length of 14 mm. Graphite sleeves envelope boron nitride crucibles containing the sample and facilitate resistive heating under load. The new germanium compound was prepared in two steps. First, a mixture of lutetium (Lamprecht, 99.9%) and germanium (Chempur, 99.9999+%) with an atomic ratio of 1:3 was arc-melted. This was followed by high-pressure and high-temperature synthesis at pressures © XXXX American Chemical Society

Received: June 4, 2018

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DOI: 10.1021/acs.inorgchem.8b01510 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry

elevated temperatures in the range from 1100(150) to 1500(150) K yields the new phase LuGe3. On the basis of the crystallographic data for (cF8)Ge,19 LuGe2−x,21 and LuGe3 (see below), the formation of LuGe3 is associated with a volume decrease between 7.8% and 9.6% (the spread results from the uncertainty in the reported compositions for LuGe2−x). This implies that the formation of LuGe3 at elevated pressures is a consequence of Le Chatelier’s principle. At ambient pressure, the compound starts to decompose upon heating to approximately 690(5) K, as evidenced by DSC measurements (Figure 1). The first subtle exothermic anomaly

Table 1. Data Collection, Structure Refinement and Crystallographic Information for LuGe3a composition space group, Pearson symbol structure type unit cell param a/Å b/Å c/Å V/Å3 formula units, Z measurement range measd points/reflns R(I)/R(P)/GOF

LuGe3 Cmcm (No. 63), oC16 DyGe3 3.9755(2) 20.3770(8) 3.8697(2) 313.47(4) 4 1.000° ≤ 2θ ≤ 37.914°, 0.002° step width 18456/668 0.0848/0.0279/1.010

a

The calculated estimated standard deviations take into account the local correlations in Rietveld refinements.23.

Table 2. Wyckoff Sites, Site Occupancy Factors (SOFs), Relative Atomic Coordinates, and Equivalent Displacement Parameters Beq for LuGe3a atom Lu Ge1 Ge2 Ge3

site 4c 4c 4c 4c

SOF 1.0 1.0 1.0 1.0

x/a 0 0 1 /2 1 /2

y/b 0.4188(1) 0.0421(3) 0.1905(3) 0.3120(3)

z/c 1

/4 /4 1 /4 1 /4 1

Beq 0.83(3) 1.04(10) 0.56(9) 0.65(9)

a

The calculated estimated standard deviations take into account the local correlations in Rietveld refinements.

by a direct-current four-probe method (alternating-current transport option; 1.9−320 K). The measurements were conducted using a Physical Property Measurement System (PPMS; Quantum Design). The inaccuracy of ρ was estimated to be ±20% because of the intricate contact geometry. The heat capacity was measured by a relaxation method (PPMS, Quantum Design) between 1.9 and 100 K in fields up to 1 T. Electronic Structure Calculations. Quantum-chemical calculations on LuGe3 were performed using the TB-LMTO-ASA (tightbinding linear muffin-tin orbital atomic-sphere approximation) program package with an exchange correlation potential (LDA) according to von Barth and Hedin.11,12 Experimentally obtained lattice parameters and atomic coordinates were used for the calculations. The radial scalar-relativistic Dirac equation was solved to get the partial waves. The calculation within the ASA includes corrections for the neglect of interstitial regions and partial waves of higher order.13 In the case of LuGe3, an addition of empty spheres was not necessary. The following radii of the atomic spheres were applied for the calculations: r(Lu) = 1.954 Å, r(Ge1) = 1.603 Å, r(Ge2) = 1.527 Å, and r(Ge3) = 1.527 Å. A basis set containing Lu(6s,5d,4d) and Ge(4s,4p) states was employed for the selfconsistent calculations, with Lu(6p) and Ge(4d) functions being downfolded. The electron localizability indicator was evaluated in its ELI-D representation with a module implemented within the TB-LMTOASA program package.14,15 The topology of ELI-D and the electron density was analyzed with the program DGrid.16 The calculated electron density was integrated in basins, bound by zero-flux surfaces in the ELI-D gradient field. This technique followed the procedure proposed by Bader for electron density analysis17 and provided electron counts for each ELI-D basin. The electron counts revealed basic information for the description of the bonding situation.15,18

Figure 1. DSC of LuGe3 taken upon heating (red curve) and cooling (blue curve) in the range of 300−1300 K with a heating rate of 5 K min−1 at ambient pressure. Inset: DSC curve of LuGe3 between 300 and 825 K illustrating the onset of the exothermal decomposition at 690(5) K.

upon heating (Figure 1, inset) corresponds to the onset of decomposition, transforming LuGe3 mainly into (cF8)Ge and

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RESULTS AND DISCUSSION Compression of lutetium−germanium mixtures Lu25Ge75 to pressures between 8(1) and 14(2) GPa combined with

Figure 2. Powder XRD pattern (Cu Kα1 radiation) of LuGe3 after heating to 825 K at ambient pressure, which causes decomposition into (1 + x)(cF8)Ge and LuGe2−x. B


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Inorganic Chemistry LuGe2−x,19,20 which represent the stable boundary phases in this composition region of the phase diagram. Next, the reaction is exothermal, and the cooling curve shows no evidence for the re-formation of LuGe3 (Figure 1, inset). These findings consistently indicate that LuGe3 is a true high-pressure phase, which is metastable at ambient conditions. The mixture (cF8)Ge + LuGe2−x resulting from decomposition represents a germanium-rich eutectic that melts at Tm = 1135(5) K (see the large signal in Figure 1), in full accordance with the established ambient-pressure phase diagram.7 Comparisons of the XRD pattern of LuGe3 to those of related compounds evidence a DyGe3-type structure.21 The same atomic arrangement is also realized in the high-pressure phases TmGe3 as well as the related PrGe3.36 and NdGe3.25.22 Later, full-profile refinements take into account the local correlations of Rietveld data23 for calculation of the estimated standard deviations (Figure 3 and Tables 1 and 2). The results

Figure 4. Crystal structure of LuGe3. Red lines denote short Ge−Ge distances in the chain and in the double layers, and gray ones mark slightly longer distances between adjacent dumbbell atoms.

Table 3. Selected Interatomic Distances in LuGe3 atom

distance/Å

atom

distance/Å

Lu−4Ge1 Lu−2Ge3 Lu−2Ge2 Lu−2Ge1 Lu−2Lu Lu−2Lu Lu−2Lu

2.886(2) 2.948(4) 2.951(4) 3.202(5) 3.832(3) 3.8697(2) 3.9755(2)

Ge1−2Ge1 Ge1−2Ge3 Ge1−2Ge2 Ge2−1Ge3 Ge2−4Ge3 Ge3−1Ge2 Ge3−4Ge2

2.585(5) 3.549(6) 3.620(6) 2.475(7) 2.7744(2) 2.475(7) 2.7744(2)

Table 4. Ge−Ge Distances in Selected Compounds Comprising Germanium Chains or Dumbbells distance/Å

Figure 3. Synchrotron powder XRD pattern of LuGe3 and results of the Rietveld refinement. High-angle data with low peak intensities above 2θ = 27° are omitted for clarity.

structure type

chain

Eu5Ge326 Gd3Ge427 RE3Ge528 (RE = Y, Sm, Gd, Tb, Ho) CaGe329

Cr5B3 Tm3Ge4 Y3Ge5

2.632(2)a 2.570(4)a

SrGe34

CaGe3

BaGe330

CaGe3

YGe324 DyGe321 TmGe322

DyGe3 DyGe3 DyGe3

compound

show a larger displacement parameter for Ge1, which may hint to defects on the germanium position or strong anisotropic displacement along [010]. Although some other DyGe3 phases exhibit significant germanium deficiency, e.g., YGe3−x (x = 0.31),24,25 EDXS measurements of LuGe3 (see above) basically confirm the 1:3 composition. Thus, the large displacement in LuGe3 is attributed to the low connectivity of these atoms within the zigzag chains (Figure 4) and their chemical bonding properties (see below). The other germanium atoms Ge2 and Ge3 form double layers of condensed digermanium dumbbells. Most interatomic distances d(Ge−Ge) in LuGe3 (Table 3) are significantly longer than those in elemental diamond-type germanium (2.449 Å), pointing to a deviation from conventional 2c2e bonding in the binary compound. Lutetium is surrounded by 10 Ge atoms in the form of an irregular polyhedron with d(Lu−Ge) in the range from 2.886(2) to 3.202(5) Å. The observed Ge−Ge distances of LuGe3 fall into a range covered by other compounds comprising similar germanium units (Table 4). The selected data only take into account values originating from full structure refinements. The Ge1− Ge1 distance of 2.585(5) Å in the chain is in good agreement

a

within dumbbell

between dumbbells

2.560(2)

CaGe3

2.72(1) 2.505(5) 2.541(3)

2.549(1) 2.5994(8) 2.5325(9) 2.5432(7) 2.511(6) 2.513(5) 2.44(1) 2.442(8) 2.474(2)

2.7631(5) 2.8231(7) 2.7811(3) 2.8431(6) 2.797(3) 2.840(3) 2.82(1) 2.803(1) 2.7830(4)

Averaged value for the two-bonded Ge atoms in the structure motif.

with the values observed in similar 1D germanium units but is significantly shorter than the one in YGe3. The distance of 2.475(7) Å within the Ge2−Ge3 dumbbells falls into the range of other DyGe3 arrangements but is significantly shorter than the values for corresponding units in other structure motifs. Four longer contacts of 2.7744(2) Å complete the formation of a total of five near germanium neighbors in the germanium double layers; their values are similar to those in CaGe3-type and other DyGe3-type germanium compounds. Overall, C


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Inorganic Chemistry distances d(Ge−Ge) clearly increase with the number of Ge− Ge contacts (Figure 5), except for two-bonded Ge atoms.

Figure 6. Calculated total electronic DOS for LuGe3.

The interactions between the Lu and Ge atoms are deduced from analysis of the electronic density according to the quantum theory of atoms in molecules (QTAIM).17 The zeroflux surfaces in the distribution of the electron density gradient define boundaries that limit basins of QTAIM atoms. Integration of the electron density in all associated basins yields the populations of these QTAIM atoms and allows for the estimation of effective charges. The calculated moderate charge transfer reveals clearly that lutetium is the cationic component (effective charge 0.87+), whereas the germanium species carry negative charges (0.43−, 0.21−, and 0.23− for Ge1, Ge2, and Ge3, respectively). This observation follows the electronegativity values of the components. The Ge2 and Ge3 atoms have small effective charges, which indicates that their interaction in the structure is more driven by covalent character than by charge transfer. The shapes of the Lu and Ge QTAIM atoms are far from spherical, indicating the formation of directed bonds. n the present study, the electron localizability indicator14,35 was computed in order to clarify the bonding situation in direct space. Analysis of the electron localizability indicator (ELI-D) sheds more light on the bonding situation (Figure 7). The first striking feature of the ELI-D distribution is the absence of local maxima (attractors), which would visualize the outer (valence) shell of lutetium. This finding signals cationic behavior and is in agreement with the calculated QTAIM charge of lutetium, albeit the value of the charge transfer is smaller than 2 and, thus, undersized with respect to that observed in other rare-earth metal compounds.36,37 The second characteristic feature is the presence of local maxima around the Ge cores. Their location resembles the formation of Zintl-like anions. Between the Ge1 atoms, a local double maximum indicates the formation of zigzag chains parallel to the c axis (a similar feature occurs in YGa2).37 Indeed, the Ge1−Ge1 bond is a two-center interaction with a population of just 0.7 electrons (Figure 7, bottom, red basin). Furthermore, the local maxima of ELI-D above and below the Ge1 core resemble lone pairs, in accordance with twobonded germanium in an anionic zigzag chain. The ELI-D distribution around the Ge2 and Ge3 atoms clearly indicates the formation of germanium dumbbells. Following the Zintl count for diatomic units, a triple bond would be required between Ge atoms, with a single negative charge corresponding to [Lu3+][(2b)Ge12−][(3b)Ge2−][(3b)Ge3−] × 1p+, indicating an electron demand in the system. This finding

Figure 5. Average interatomic distances d(Ge−Ge) versus the number of Ge−Ge contacts between 2.5 and 3 Å of selected alkaline-earth or rare-earth germanides.4,21,22,27−33 The values for the two-connected Ge atoms in the Ge1 chains as well as the average value for the five-connected framework atoms Ge2 and Ge3 fall into the typical ranges observed for similar structure motifs.4,21,22,27−30,33

With respect to the electron balance, the two-bonded chain atoms Ge1 may be considered as normal Ge2− species in accordance with the Zintl concept. The 1 + 4 connectivity of germanium within the layers (NC = 5 for Ge2 and Ge3) bears some similarity to that of bromine in BrF5. Here, the axial bond is considered to be a 2c2e interaction, and the four equatorial contacts are considered to be two three-center, four-electron bonds. Upon application of this concept to the germanium layers in LuGe3, a formal charge of 1− for Ge2 and Ge3 is implied. Together with the two-bonded (Ge1)2−, such a situation would resemble the electron count Lu3+ [(2b)Ge2−][(5b)Ge−]2 × 1p+, i.e., a deficit of one electron. Chemical Bonding and Electronic Structure. An earlier analysis of the chemical bonding in the isotypic compound DyGe321 was performed by means of the extended Hückel method operating in reciprocal space.34 In essence, the study revealed that the Ge atoms in the kinked chains follow the Zintl rule and form Ge2− species, making this building unit isoelectronic to a sulfur chain (with a bond order of 0.61 for germanium). For the double layers, the involvement of the Dy atoms increases the electron demand to (Ge2−Ge3)− for optimal 2c2e bonding in the dumbbells (bond order of 0.7). In addition, neighboring germanium dimers interact via electrondeficient multicenter bonding (bond order of 0.37). IThe calculated electronic density of states (DOS) for LuGe3 reveals a distribution with a pseudogap above the Fermi level at about 1 eV (Figure 6). The region of the DOS below the Fermi level is mostly formed by p states of germanium with contributions of mainly d states of the rare-earth metal. The middle region is constituted by f states of lutetium together with p states of germanium. The DOS distribution of the d states of lutetium is strongly structured in the regions of the contribution of p states of germanium, suggesting their participation in the bonding. D


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Figure 8. Magnetic susceptibility χ of LuGe3 for μ0H = 2 mT at temperatures between 1.8 and 6 K. The characteristic difference between zero-field and field cooling indicates the Meissner effect and, thus, the superconducting transition.

Consistently, the specific heat Cp(T) of LuGe3, measured between 0.41 and 10 K (Figure 9a), indicates a superconducting transition slightly above 3 K. The midpoint of the critical temperature Tc,mid = 3.1 K in H = 0 is obtained by an equal-entropy construct. In the normal state at μ0H = 200 mT, the specific heat is well-described by Cp(T) = γNT + βT3 + δT5, in which γNT is the Sommerfeld electronic specific heat and βT3 + δT5 refers to the first terms of the harmonic lattice approximation of the phonon contribution.40 The fit of the Cp T−1 versus T2 plot resulted in γN = 4.29(2) mJ mol−1 K−2, β = 0.24(1) mJ mol−1 K−2, and δ = 0.11(1) × 104 mJ mol−1 K−2. From β, the Debye temperature θD = 320 K was calculated.40,41 The Debye temperature can be used to appraise the strength of the electron−phonon coupling by McMillan’s formula,42 calculated for LuGe3 as 0.50 ≤ λe−p ≤ 0.60 (assuming a repulsive screened Coulomb potential μ* between 0.1 and 0.15). This finding indicates that LuGe3 is a superconductor with weak electron−phonon coupling. The electronic specific heat Ce in the superconducting state is derived by subtracting the phonon contribution to the specific heat data. An equal-entropy construction for H = 0 is shown in Figure 9a. This electronic contribution was fitted using the Bardeen−Cooper−Schrieffer theory (BCS) expression,40 and the least-squares fits yield γ0 = 0.40(1) mJ mol−1 K−2 and the energy gap Δ0 = 0.26 (2) meV = 3 kBT at the Fermi level. The calculation of 2Δ0/kB Tc = 3.87 gives a value that is in accordance with the 3.52 BCS value for weak coupling.40 The size of the jump of the electronic specific heat Ce at Tc, ΔCe/γNTc = 1.43, is in great agreement with the BCS value of ΔCe/γNTc = 1.44. This is another sign of LuGe3 being a weakly coupled superconductor. The application of a magnetic field suppresses Tc quickly, as illustrated by the field-dependent specific-heat data in Figure 9b. The electrical resistivity ρ(T) of LuGe3 (Figure 10) at room temperature and zero field amounts to ρ300 K = 75.8 μΩ cm, with the residual resistance ratio ρ293 K/ρ4 K = 8.7. This value implies an adequate sample quality and is in line with that reported for other polycrystalline high-pressure trigermanides.43

Figure 7. Electron localizability indicator in LuGe3. Top: Zintl-like arrangement of the ELI-D attractors around the Ge atoms. Middle: Electron-deficient character of the Ge1−Ge1 bonds and additional electron-deficient two-center Ge2−Ge3 interactions in the ac planes. Bottom: Calculated shapes and populations of the bond- and lonepair-like basins.

agrees with the position of the pseudogap in the calculated electronic DOS (see above), and, indeed, the Ge2−Ge3 twocenter bond within the dumbbell holds two electrons (Figure 7, bottom, pink basin). However, besides the local maximum on the line between Ge2 and Ge3, two other local maxima are located on the outer sides of the dumbbell (Figure 7, top), evidencing that the real bonding pattern is different and more complex. The additional bonds between the Ge2 and Ge3 dumbbell atoms in the ac plane (Figure 7 middle) conform to electrondeficient two-center or multicenter interactions. The lone-pairlike basins of the local maxima near all germanium species contain about two electrons. However, their basins have common surfaces with the lutetium cores, indicating polar multicenter interactions (Figure 7, bottom, blue, pink, and green basins). In total, decreasing the population of the twocenter Ge−Ge interactions in the chains and between adjacent Ge2 dumbbells reduces the electron deficiency and, thus, approximates a Zintl-related bonding situation. The same tendency applies to binary phases in the system Y−Ga.37 In contrast, Eu−Ga compounds realize an alternative solution by reducing the electron count of the lone-pair-like basins.38 Because a superconducting transition of a DyGe3-type rareearth-metal germanide has been observed at Tc,mid = 2.0 K in diamagnetic YGe3,39 the magnetic susceptibility χ of LuGe3 was measured. In an external field μ0H = 2 mT, the data reveal a sharp diamagnetic transition with a critical temperature Tc = 3.3 K (Figure 8). The Meissner volume fraction exceeds 1, indicating bulk superconductivity. The ratio of χ zfc/χ fc observed during field cooling is small, pointing to type II superconductivity. E


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Figure 10. Temperature-dependent electrical resistivity of LuGe3 at zero-field. The solid red line represents the fit to the Bloch− Grüneisen expression. Inset: Low-temperature electrical resistivity of LuGe3 in external fields μ0H between 0 and 0.1 T.

within the dumbbells. Each Ge atom shows a lone-pair-like feature, visualizing multicenter polar Lu−Ge interactions. Thus, LuGe3 represents a covalent metal that shows weakly coupled BCS-type superconductivity below Tc = 3.3(3) K, as is consistently indicated by magnetization, specific heat, and resistivity measurements.

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ASSOCIATED CONTENT

Accession Codes

CCDC 1856164 contains the supplementary crystallographic data for this paper. These data can be obtained free of charge via www.ccdc.cam.ac.uk/data_request/cif, or by emailing [email protected], or by contacting The Cambridge Crystallographic Data Centre, 12 Union Road, Cambridge CB2 1EZ, UK; fax: +44 1223 336033.

Figure 9. (a) Electronic specific heat of LuGe3 (Ce/T vs T), with the solid line constituting a fit of the BCS equation. (b) Specific heat of LuGe3 (Cp/T vs T2) in different external magnetic fields.

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AUTHOR INFORMATION

Corresponding Author

In the normal state, the zero-field electrical resistivity of LuGe3 shows a positive slope, denoting metallic behavior. Taking into account the Mattheisen rule, the normal-state resistivity is well-described within the Bloch−Grüneisen model40,41,44 up to room temperature. The best fit yields the residual resistivity ρ0 = 8.32(5) μΩ cm, the electron−phonon coupling constant A = 1.2(2) μΩ cm K, the Debye temperature θD = 299(8) K, and the coefficient of the cubic term k = 2.1(8) × 10−4 μΩ cm K−2. This Debye temperature is in satisfying agreement with the value of θD = 320 K received from the specific heat measurement. In zero field, a drop in the resistivity is visible, indicating the transition to the superconducting state at Tc = 3.6 K. The application of a small magnetic field (Figure 10, inset) suppresses the transition into the superconducting state. In conclusion, we have synthesized the new compound LuGe3 under high-pressure and high-temperature conditions. The germanide is metastable at ambient conditions and crystallizes in a DyGe3-type structure, containing double layers of condensed Ge2 dumbbells as well as germanium zigzag chains. The two-center Ge−Ge bonding is characterized by electron-deficient interactions except classical 2c3e bonds

*E-mail: [email protected]. ORCID

Yuri Grin: 0000-0003-3891-9584 Ulrich Schwarz: 0000-0002-7301-8629 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We gratefully acknowledge support by Liudmilla Muzica in high-pressure syntheses. We thank Marcus Schmidt and Susann Scharsach for DSC measurements as well as Ulrich Burkhardt, Sylvia Kostmann, and Petra Scheppan for metallographic characterizations. The assistance of Wilson Mogodi with synchrotron XRD measurements at beamline ID22 of the ESRF is appreciatively recognized.

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REFERENCES

(1) (a) Zintl, E.; Brauer, G. Z. Ü ber die Valenzelektronenregel und die Atomradien unedler Metalle in Legierungen. Z. Phys. Chem. 1933, 20B, 245−271. (b) Zintl, E. Intermetallische Verbindungen. Angew. Chem. 1939, 52, 1−6.

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Lutetium Trigermanide LuGe3: High-Pressure Synthesis

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