Crystal Structure and Physical Properties of the New Antimonide

Chem. Mater. 2010, 22, 6433–6437 6433 DOI:10.1021/cm102632a

Crystal Structure and Physical Properties of the New Antimonide Hf3Cu2Ge3.58Sb1.42 Mykhailo Guch, Cheriyedath Raj Sankar, Abdeljalil Assoud, and Holger Kleinke* Department of Chemistry, University of Waterloo, Waterloo, Ontario, Canada N2L 3G1 Received September 13, 2010. Revised Manuscript Received October 29, 2010

The antimonide Hf3Cu2Ge3.58Sb1.42 was obtained via a two-step synthesis: arc-melting of hafnium, copper, and germanium under argon was followed by a reaction of the thus obtained ingot with antimony under a vacuum in a silica ampule. This compound adopts a new structure type, space group P4/nmm, with lattice dimensions of a= 3.8023(6) A˚, c=24.575(4) A˚, V=355.29(10) A˚3. The structure of Hf3Cu2Ge3.58Sb1.42 comprises puckered double layers of the Hf and Ge/Sb atoms as well as 2¥[Cu] and 2¥[Ge/Sb] planar square nets, representing a new intergrowth structure of ZrSiS- and NdTe3-like slabs. The 2¥[Ge/Sb] square nets are composed of one disordered site that is statistically mixed occupied by 88% Ge and 12% Sb atoms. Electronic structure calculations predicted metallic properties, subsequently confirmed by an Seebeck and an electrical resistivity measurement. Introduction The PbFCl structure type1 is very flexible and adopted by many compounds with different types of bonding starting from typically ionic to metallic.2 ZrSiS3 is isopointal with PbFCl, but the formal Si2- anions participate in four planar hypervalent Si-Si bonds, whereas the topologically analogous F- anions do not form F-F bonds. Materials of the ZrSiS type with undistorted square nets are typically metallic and exhibit an ideal valence electron concentration of six per atom, such as Si2-.4 Peierls distortions of the square nets may occur with the formation of cis/trans or zigzag chains, i.e., two strong bonds and two nonbonding contacts instead of four hypervalent “half” bonds per atom, which often lead to narrow gap semiconductors. For example, the distorted variants of the ZrSiS3 and HfCuSi25 types, GdPS,6,7 CeAsSe,8,9 CeAgAs210 and PrAgAs2,11 possess semiconducting properties. Incommensurate superstructures like in LaSeTe2,12 originally described *To whom correspondence should be addressed. E-mail: kleinke@ uwaterloo.ca.

(1) Nieuwenkamp, W.; Bijvoet, J. M. Z. Kristallogr. 1932, 81, 469–74. (2) Pearson, W. B. Z. Kristallogr. 1985, 171, 23–39. (3) Onken, H.; Vierheilig, K.; Hahn, H. Z. Anorg. Allg. Chem. 1964, 333, 267–279. (4) Tremel, W.; Hoffmann, R. J. Am. Chem. Soc. 1987, 109, 124–140. (5) Andrukhiv, L. S.; Lysenko, L. A.; Yarmolyuk, Y. P.; Gladyshevskii, E. I. Dopov. Akad. Nauk Ukr. RSR A 1975, 645–648. (6) Hulliger, F.; Schmelczer, R.; Schwarzenbach, D. J. Solid State Chem. 1977, 21, 371–374. (7) Hulliger, F. Nature 1968, 219, 373–373. (8) Ceolin, R.; Rodier, N.; Khodadad, P. J. Less-Common Met. 1977, 53, 137–140. (9) Schlechte, A.; Niewa, R.; Prots, Y.; Schnelle, W.; Schmidt, M.; Kniep, R. Inorg. Chem. 2009, 48, 2277–2284. (10) Demchyna, R.; Jemetio, J. P. F.; Prots’, Y. M.; Doert, T.; Aksel’rud, L. G.; Schnelle, W.; Kuz’ma, Y. B.; Grin, Y. Z. Anorg. Allg. Chem. 2004, 630, 635–641. (11) Eschen, M.; Jeitschko, W. Naturforsch., B 2003, 58, 399–409. (12) Doert, T.; Polequin, B.; Fokwa, T.; Simon, P.; Lidin, S.; S€ ohnel, T. Chem.;Eur. J. 2003, 9, 5865–5872. (13) Norling, B. K.; Steinfink, H. Inorg. Chem. 1966, 5, 1488–1491. r 2010 American Chemical Society

as an undistorted ternay ordered variant of the NdTe313 type, are not uncommon, as recently summarized for tellurides with square net variants.14 Such changes leading to opening a band gap are very important with respect to the thermoelectric energy conversion.15-18 In addition, our research group is highly interested in exploring new compounds with anionic differential fractional site occupancies that can stabilize different structures,19-21 such as ZrGexSb2-x and HfGexSb2-x22 (both TiNiSi type, x e 0.2) and Ba2Cu4-xSeyTe5-y23 (new type) and/or dramatically change the physical properties. For example, the metallic antimonide Mo3Sb724 (Ir3Ge7 structure type)25 became semiconducting by a partial Sb/Te replacement;26 therefore, Mo3Sb5.4Te1.6 exhibits much improved thermoelectric properties, with ZT being close to unity above 1000 K.27,28 Similarly, an improvement of the Seebeck coefficient was achieved (14) Patschke, R.; Kanatzidis, M. G. Phys. Chem. Chem. Phys. 2002, 4, 3266–3281. (15) Rowe, D. M. CRC Handbook of Thermoelectrics; CRC Press: Boca Raton, FL, 1995. (16) Kleinke, H. Chem. Mater. 2010, 22, 604–611. (17) Toberer, E. S.; May, A. F.; Snyder, G. J. Chem. Mater. 2010, 22, 624–634. (18) Kanatzidis, M. G. Chem. Mater. 2010, 22, 648–659. (19) Yao, X.; Marking, G.; Franzen, H. F. Ber. Bunsenges. 1992, 96, 1552–1557. (20) K€ ockerling, M.; Franzen, H. F. Croat. Chem. Acta 1995, 68, 709–719. (21) Kleinke, H. Trends Inorg. Chem. 2001, 7, 135–149. (22) Soheilnia, N.; Assoud, A.; Kleinke, H. Inorg. Chem. 2003, 42, 7319–7325. (23) Mayasree, O.; Cui, Y.; Assoud, A.; Kleinke, H. Inorg. Chem. 2010, 49, 6518–6524. (24) Brown, A. Nature 1965, 206, 502–503. (25) H€aussermann, U.; Elding-Ponten, M.; Svensson, C.; Lidin, S. Chem.;Eur. J. 1998, 4, 1007–1015. (26) Dashjav, E.; Szczepenowska, A.; Kleinke, H. J. Mater. Chem. 2002, 12, 345–349. (27) Gascoin, F.; Rasmussen, J.; Snyder, G. J. J. Alloys Compd. 2007, 427, 324–329. (28) Xu, H.; Kleinke, M. K.; Holgate, T.; Zhang, H.; Su, Z.; Tritt, T. M.; Kleinke, H. J. Appl. Phys. 2009, 105, 053703/1–053703/5.

Published on Web 11/16/2010

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by partial substitution of As by Ge in Re3As729 in Re3Ge0.6As6.4 and Re3GeAs6.30 One focus of our current research efforts is the investigation of the M-Cu-E-Sb system (M = element of group 4; E=Si, Ge) with square nets or distorted variants thereof. Although the quaternary system was thus far unexplored, a few ternary M-Cu silicides, germanides, and antimonides31-33 can be found in the literature. Experimental Section Syntheses and Analyses. The syntheses commenced from the elements, handled in an argon-filled glovebox and originally acquired from Alfa Aesar: Hf powder, -100 mesh, 99.6%; Cu powder, -100 mesh, 99.9%; Ge powder, -100 mesh, 99.99%: Sb powder, -100 mesh, 99.5%. Hf3Cu2Ge3.58Sb1.42 was prepared in two steps. First, the elements Hf, Cu, and Ge in the molar ratio of 3:2:3.58 were pressed into a pellet, and then melted in arc-melting furnace under a flow of argon. The reaction mixture was then ground into fine powder and sealed under a vacuum of approximately 1
10-3 mbar in an evacuated silica tube with a stoichiometric amount of elemental Sb. To prevent a reaction with the silica tube, a ceramic crucible (Al2O3-based) was used inside the tube. The reaction mixture was heated to 1323 K within 48 h in a resistance furnace, kept at that temperature for 12 h, and then cooled down to the room temperature after the furnace was switched off. The reaction afforded the title compound in form of dark-gray crystalline powder; one crystal thereof was chosen for the single-crystal structure analysis. The X-ray powder diffraction (XRD) pattern of this dark-gray powder was obtained at room temperature on an Inel X-ray powder diffractometer (Cu KR1 radiation) equipped with a positionsensitive detector. After solving this structure via single-crystal structure determination, described below, all peaks of the powder diagram could be assigned to this new material, namely, Hf3Cu2Ge3.58Sb1.42. It is worthwhile to emphasize that the arc-melting step is crucial for the formation of Hf3Cu2Ge3.58Sb1.42: a combination of binaries and starting elements was found when the arc-melting step was omitted. Annealing at temperatures below 1173 K results in decomposition, indicating that Hf3Cu2Ge3.58Sb1.42 is an entropystabilized high-temperature phase. An energy-dispersive X-ray analysis (EDAX) was performed on the crystal that was used for the single-crystal determination, using the electron microscope LEO 1530 with an additional EDAX device, EDAX Pegasus 1200. The ratio of Hf, Cu, Ge and Sb was determined to be 32.7:18.3:35.6:13.5 (in atomic %). This compares well with the ratio of 30.0:20.0:35.8:14.2 obtained from the single crystal data. We noted earlier that the Hf:Sb ratio tends to be overestimated with this EDAX method.34 Any additional elements such as Al or Si that might have come from the reaction containers were not found. To study the phase range in Hf3Cu2Ge3.58Sb1.42, we prepared a set of reactions with different Ge/Sb ratios of 4:1, 3.7:1.3, 3.58:1.42, 3.4:1.6, and 3:2. Only the reaction with the 3.58:1.42 afforded (29) Furuseth, S.; Kjekshus, A. Acta Chem. Scand. 1966, 20, 245–250. (30) Soheilnia, N.; Xu, H.; Zhang, H.; Tritt, T. M.; Swainson, I.; Kleinke, H. Chem. Mater. 2007, 19, 4063–4068. (31) Sprenger, H.; Nickl, J. J. J. Less-Comm. Met. 1972, 27, 163–168. (32) Thirion, F.; Venturini, G.; Malaman, B.; Steinmetz, J.; Roques, B. J. Less-Comm. Met. 1983, 95, 47–54. (33) Kinzhibalo, V. V.; Kotur, B. Y.; Bochvar, N. R.; Lysova, E. V. Izv. Akad. Nauk SSSR, Neorg. Mater. 1986, 22, 606–606. (34) Assoud, A.; Kleinke, K. M.; Soheilnia, N.; Kleinke, H. Angew. Chem., Int. Ed. 2004, 43, 5260–5262.

Guch et al. the target compound without noticeable side products. The samples with the 3.7:1.4 and 3.4:1.6 ratios included smaller amounts of HfGeSb35 (ZrSiS type) and HfCuGe236 in addition to the target, whereass the target was not formed at all in the remaining two reactions. One can thus conclude that the phase width of the title compound has to be very small, possibly negligibly so. Attempts to prepare isostructural compounds by replacing Hf with Zr or Ti and Ge with Si did not afford the Hf3Cu2Ge3.58Sb1.42 type. Crystal Structure Determinations. A shiny needle-shaped crystal of the new compound was selected under an optical microscope and then mounted on a glass fiber. Single-crystal X-ray diffraction data were collected with the use of graphite-monochromatized Mo KR radiation at 298 K on a Bruker APEX CCD diffractometer. Data were collected by 0.3 ω scans, for an overall two groups of 600 frames (one with j=0 and one with j=90) with exposure times of 60 s each. The data were corrected for Lorentz and polarization effects. Absorption corrections were based on fitting a function to the empirical transmission surface by multiple equivalent measurements using the APEX2 program package from Bruker.37 Data refinement was done using the SHELXTL package.38 The unit cell parameters were established from the positions of the centered diffraction peaks taken from all frames. The tetragonal space group P4/nmm was selected based on the observed systematic extinctions. All atomic positions, initially identified as three Hf, one Cu, three Ge, and one Sb, were found by the direct method. The structure was refined smoothly to small residual factors of R1=0.0282 and wR2=0.0694 for all data. However, the significantly smaller temperature displacement parameters, Ueq, of Ge1 and Ge2 of 0.0051(3) A˚2 and 0.0013(4) A˚2, compared to the other atoms with up to 0.0088(4) A˚2, pointed toward mixed Ge/Sb occupancies. Thus both of these Ge sites were refined as statistic mixtures of Ge1/Sb1 (E1 site) and Ge2/ Sb2 (E2 site), respectively, yielding lower R values of R1=0.0246 and wR2=0.0567. The occupancies of the E1 and E2 sites were refined to be 88/12 and 82/18 atomic percent Ge/Sb, respectively. On the other hand, tentative refinements of the occupancies of the Cu1 and Ge3 sites yielded full occupancies within twice their standard deviations, namely 1.000(8) and 0.98(1), respectively. The crystallographic data of Hf3Cu2Ge3.58(5)Sb1.42 are summarized in Table 1. The atomic positions and displacement parameters are listed in Table 2. Calculations of the Electronic Structure. The first principles of LMTO method (linear muffin tin orbitals) was utilized for the electronic structures calculation using atomic spheres approximation (ASA).39,40 Therein the density functional theory is applied using the local density approximation (LDA) to treat correlation and exchange energies.41 The following wave functions were used: for Hf 6s, 6p and 5d, 5f (downfolded42); Cu 4s, 4p, and 3d; for Ge 4s, 4p, and 4d (downfolded); and for Sb 5s, 5p and 5d, 4f (downfolded). To model the electronic structure of Hf3Cu2Ge3.58Sb1.42, we treated the E1 and E2 sites as pure Ge sites, resulting in the (35) Dashjav, E.; Kleinke, H. Z. Anorg. Allg. Chem. 2002, 628, 2176– 2176. (36) Andrukhiv, L. S.; Lysenko, L. A.; Yarmolyuk, Y. P.; Gladyshevskii, E. I. Dopov. Akad. Nauk Ukr. RSR A 1975, 645–648. (37) M86-Exx078 APEX2 User Manual; Bruker AXS Inc.: Madison, WI, 2006. (38) Sheldrick, G. M. Acta Crystallogr., Sect. A 2008, 64, 112–122. (39) Andersen, O. K. Phys. Rev. B 1975, 12, 3060–3083. (40) Skriver, H. L. The LMTO Method; Springer: Berlin, Germany, 1984. (41) Hedin, L.; Lundqvist, B. I. J. Phys. C 1971, 4, 2064–2083. (42) Lambrecht, W. R. L.; Andersen, O. K. Phys. Rev. B 1986, 34, 2439– 2449.

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Table 1. Crystallographic Data of Hf3Cu2Ge3.58(5)Sb1.42 refined formula fw (g/mol) T of measurement (K) wavelength (A˚) cryst syst space group a = b (A˚) c (A˚) V (A˚3) Z Fcalcd (g/cm3) collected, unique, obsd reflns R1 and wR2 (all data)a R1 and wR2 (I > 2σ(I))a )

R1 = Σ Fo| - |Fc /Σ|Fo|; wR2 = [ Σ[w(Fo2 - Fc2)2]/Σ[w(Fo2)2] ]1/2. )

a

Hf3Cu2Ge3.58(5)Sb1.42 1095.55 298(2) 0.71073 Tetragonal P4/nmm 3.8023(6) 24.575(4) 355.29(10) 2 10.241 2670, 395, 383 0.0246, 0.0567 0.0240, 0.0563

Table 2. Atomic Coordinates and Equivalent Displacement Parameters for Hf3Cu2Ge3.58Sb1.42 atom

site

x

y

z

Ueq/A˚2

Hf1 Hf2 Hf3 Cu1 E1b E2c Ge3 Sb3

2c 2c 2c 4f 4f 2c 2c 2c

1/4 1/4 1/4 3/4 3/4 1/4 1/4 1/4

1/4 1/4 1/4 1/4 1/4 1/4 1/4 1/4

0.26298(3) 0.91304(3) 0.54311(3) 0.36463(6) 0.17395(5) 0.70218(7) 0.43203(7) 0.03917(4)

0.00795(19) 0.00576(19) 0.00522(19) 0.0084(3) 0.0080(4) 0.0057(5) 0.0070(4) 0.0056(2)

Figure 1. Crystal structure of (a) Hf3Cu2Ge3.58Sb1.42 in comparison to (b) HfGeSb and (c) PrSeTe2.

a

Ueq is defined as one-third of the trace of the orthogonalized Uij tensor. b Ge1/Sb1=88.3(14)%/11.7% c Ge2/Sb2=81.7(18)%/18.3%

hypothetical formula Hf3Cu2Ge4Sb. The integrations in k space were performed by an improved tetrahedron method15 based on 455 symmetry independent k points of the first Brillouin zone. Physical Property Measurements. A cold-pressed pellet with dimensions 13
2
2 (in mm) was made for these transport measurements. The thoroughly ground phase-pure sample was pressed using a force of 15 kN with a hydraulic press from Weber Pressen, followed by annealing at 1173 K for five days. This procedure, however, coincided with the formation of HfCuGe2 as a minor side product of approximately 10%, a consequence of the beginning decomposition of the metastable phase Hf3Cu2Ge3.58Sb1.42. The relative density of 85% did not change during the annealing. The electrical resistivity, F, and Seebeck coefficient, S, were measured simultaneously under low pressure helium atmosphere on the commercial apparatus ULVAC-RIKO ZEM-3 in the temperature range from 300 to 600 K. The Seebeck coefficient was measured three times with different temperature gradients between 10 and 20 K at each temperature step. The temperature difference was determined via a thermocouple. The electrical resistance was measured using a four-point method within the ZEM-3. An additional electrical resistance measurement from 10 K up to 300 K was carried out on a homemade device via a four-pointmethod as well to check for a low temperature metal-to-insulator phase transition. Silver paint (Ted Pella) was used to create the electric contacts. The resistances (R) were calculated from the voltage drops using Ohm’s law, that is, R = ΔV/I, with I = current. We calculated the resistivity F after measuring the distance between the contacts, L, and the cross-section area, A, according to F=RA/L.

Table 3. Selected Interatomic Distances for Hf3Cu2Ge3.58Sb1.42 interaction

no.

distance (A˚)

interaction

no.

distance (A˚)

Hf1-E1 Hf1-E2 Hf1-Cu1

4
4


2.8990(11) 2.8216(7) 3.1392(14)

Hf2-E1 Hf2-Sb3 Hf2-Sb3

4
4


2.8604(11) 2.9340(6) 3.0997(14)

Hf3-Cu1 Hf3-Ge3 Hf3-Ge3 Hf3-Hf3

4
4
4


2.9589(13) 2.730(2) 2.7571(6) 3.4233(9)

Cu1-E2 Cu1-Ge3 Cu1-Cu1

2
4
4


2.5120(14) 2.5216(16) 2.6886(4)

E1-E1

4


2.6886(4)

Sb3-Sb3

4


3.3069(13)

exclusively occupied by Hf atoms, one by Cu atoms, one by Ge atoms, and one by Sb atoms. The two remaining positions, E1 and E2, exhibit mixed occupancies of Ge and Sb atoms, with occupancy ratios of 0.88/0.12 and 0.82/0.18 for E1 (4f) and E2 (2c), respectively. Thus, both E sites are predominantly occupied by Ge atoms in the case of Hf3Cu2Ge3.58Sb1.42. On the basis of our experimental results, the phase width of Hf3Cu2Ge4-xSb1þx is negligibly small, which indicates that the mixed occupancies are required for the formation of this structure. Therefore, Hf3Cu2Ge3.58Sb1.42 can be classified as an anionic differential fractional site occupancies (DFSO) material.19,20 The crystal structure of Hf3Cu2Ge3.58Sb1.42 (Figure 1) may be regarded as an intergrowth of the ZrSiS type, adopted by, e.g., ZrGeSb43 and HfGeSb35 (fragment A), and the (ordered) NdTe3 type, adopted by PrSeTe244 (fragment B), comprising square nets of the E1 site (88% Ge, 12% Sb) within fragment A as well as square nets of the Cu atoms within fragment B.

Results and Discussion The crystal structure of Hf3Cu2Ge3.58Sb1.42 has eight independent crystallographic positions: three positions are

(43) Lam, R.; Mar, A. J. Solid State Chem. 1997, 134, 388–394. (44) Fokwa, B. P. T.; Doert, T.; Simon, P.; S€ ohnel, T.; Boettcher, P. Z. Anorg. Allg. Chem. 2002, 628, 2612–2616.

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Figure 2. Densities of states (left) and selected cumulated COHP (center, right) curves of Hf3Cu2Ge4Sb.

The fragment A, being topologically equivalent with HfGeSb, consists of square nets perpendicular to the c axis formed by the E1 atoms. Formally all the other atoms also form planar square nets, but their density is only half as big, √ i.e., the interatomic distances are longer by a factor of 2. Along the c axis, the layer sequence of fragment A is E2-Hf1-E1-Hf2-Sb3-Sb3-Hf2-E1-Hf1-E2, compared to Sb-Hf-Ge-Hf-Sb-Sb-Hf-Ge-Hf-Sb in HfGeSb. In a mnemonically useful approach, one may view the HfGeSb structure as comprising alternating anionic square nets 2¥[Ge]2- and puckered cationic double slabs 2¥[HfSb]2þ. The fragment B, on the other hand, is topologically equivalent with the undistorted PrSeTe2 type. Therein, the dense square layers are formed by Cu atoms (Te atoms in PrSeTe2), and the respective layer sequencies are Cu-E3-Hf3-Hf3-E3-Cu and Te-Pr-Se-Se-Pr-Te. Thus, the Hf atoms take the place of Se, and the E3 atoms replace Pr. The E1-E1 distances within the square nets are 2.69 A˚ (Table 3), and thus significantly longer than normal Ge-Ge single bond distances of 2.4-2.5 A˚ as found in elemental Ge (2.45 A˚),45 Hf2CuGe4 (2.50 A˚),32 Ba2GeP2 (2.42 A˚) and Ba2GeAs2 (2.44 A˚).46 Part of this elongation stems from the incorporation of 12% Sb, because Sb atoms are larger than Ge atoms (rGe=1.24 A˚, rSb=1.39 A˚ according to Pauling).47 However, the distance of 2.69 A˚ still implies a fractional bond order, as expected because of the square planar geometry that requires adoption of hypervalent bonds.48 The here observed E1-E1 bond length is very comparable to the Ge-Ge contacts of other square nets of Ge atoms, for example occurring in ZrGeSb (dGe-Ge = 2.72 A˚ ), 43 UGeS (2.70 A˚ ), 49 and ZrGeTe (2.71 A˚).50 The Cu-Cu distances within the square nets of Hf3Cu2Ge3.58Sb1.42 are equidistant with the E1-E1 (45) Cooper, A. S. Acta Crystallogr. 1962, 15, 578–582. (46) Eisenmann, B.; Jordan, H.; Schaefer, H. Z. Naturforsch. 1982, 37, 1221–1224. (47) Pauling, L. The Nature of the Chemical Bond, 3rd ed.; Cornell University Press: Ithaca, NY, 1960. (48) Papoian, G. A.; Hoffmann, R. Angew. Chem., Int. Ed. 2000, 39, 2408–2448. (49) Ptasiewicz-Bak, H.; Leciejewicz, J.; Zygmunt, A. Phys. Status Solidi A 1978, 47, 349–356. (50) Onken, H.; Vierheilig, K.; Hahn, H. Z. Anorg. Allg. Chem. 1964, 333, 267–279.

distances of 2.69 A˚. Denser square nets of Cu atoms are formed when the main group elements are smaller, such as in ZrCuSiP (dCu-Cu=2.52 A˚)51 and ZrCuSiAs (2.60 A˚).52 The stacking of the structure fragments occurs with monocapped square antiprisms around all the three crystallographically independent Hf atoms. The Hf1 atoms are coordinated by one Cu1 at a distance of 3.14 A˚, four E1 at 2.90 A˚, and four E2 atoms at 2.82 A˚. The Hf2 atoms are bonded to four E1 at 2.86 A˚, four Sb3 at 2.93 A˚, and one Sb3 at 3.10 A˚. Finally, the Hf3 atoms are surrounded by four Cu1 at 2.96 A˚, four Ge3 at 2.76 A˚, and one Ge3 at 2.73 A˚. Noting that E1 and E2 are comprised of roughly 85% Ge and 15% Sb atoms, the Hf-E distances are not surprisingly intermediate between the Hf-Ge3 and Hf-Sb3 distances. The Hf3-Ge3 bonds are typical, as comparisons with Hf2Ge (dHf-Ge =2.82 A˚),53 Hf5Ge3 (2.72 A˚),5 HfGe2 (2.78 A˚),54 and Hf2CuGe4 (2.78 A˚)32 reveal. The same is true for the Hf-Sb bonds that may be compared with those of Hf5Sb9, another variant of ZrSiS, ranging from 2.89 to 3.18 A˚.34 The layer sequency also leads to differences in the Hf-Hf contacts. The shortest Hf1-Hf1 and Hf2-Hf2 distances equal the a lattice parameters of 3.80 A˚, whereas shorter Hf3-Hf3 contacts of 3.42 A˚ occur between neighboring Hf3 layers within the 2¥[(Hf3)2(Ge3)2] double layers. In particular, the 3.42 A˚ contact is likely to have (weak) bonding character, as it does in Tl4HfTe4.55 Finally, an Sb3-Sb3 distance of 3.31 A˚ across neighboring Sb3 layers may be bonding as well, similar to the Sb-Sb contact in HfGeSb of also 3.31 A˚.35 Electronic Structure. Because of the numerous long but likely significant Hf-Hf and Sb-Sb contacts in addition to undistorted square nets with mixed Ge/Sb occupancies, we expected the title compound to be metallic. This expectation is supported by the densities of states of the Hf3Cu2Ge4Sb model (left part of Figure 2): a significant number of states occur at the Fermi level, EF, with some contributions from the Hf d orbitals, whereas the Cu d (51) Abe, H.; Yoshii, K. J. Solid State Chem. 2002, 165, 372–374. (52) Johnson, V.; Jeitschko, W. J. Solid State Chem. 1974, 11, 161–166. (53) Havinga, E. E.; Damsma, H.; Kanis, J. M. J. Less-Common Met. 1972, 27, 281–91. (54) Smith, J. F.; Bailey, D. M. Acta Crystallogr. 1957, 10, 341–2. (55) Sankar, C. R.; Bangarigadu-Sanasy, S.; Assoud, A.; Kleinke, H. J. Mater. Chem. 2010, 20, 7485–7490.

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Figure 3. Seebeck coefficient (left) and specific resistivity (right) of Hf3Cu2Ge3.58Sb1.42.

states are mostly concentrated in the region between -3 and -5 eV. Giving that the actual composition is slightly more Sb-rich, i.e., has more valence-electrons, the Fermi level of Hf3Cu2Ge3.58Sb1.42 should be 0.2 eV higher. This does not change the metallic character, because no band gap occurs in the vicinity of the Fermi level. The strong bonding character of the heteronuclear Hf-Ge and Hf-Sb and, to a lesser extent, the Hf-Cu interactions are revealed in their crystal orbital Hamilton populations, COHP56 (right part of Figure 2). Evidently, the homonuclear Hf-Hf, Ge-Ge, and Cu-Cu interactions contribute to stability of the structure as well (center of Figure 2). The strength of these interactions can be quantified in the integrated COHP values.57 The ICOHP values are -0.41 eV per Hf3-Hf3 bond of 3.42 A˚, whereas the Ge-Ge and Cu-Cu bonds of the square nets have ICOHP values of -1.16 eV and -0.75 eV, respectively. The Hf1Hf1 (3.80 A˚), Hf2-Hf2 (3.80 A˚) and Sb3-Sb3 (3.31 A˚) interactions are significantly weaker with ICOHP values of -0.35, -0.39, and -0.20 eV, respectively. For comparison, the Ge-Ge bond strength within the square nets of ZrGeSb are with -1.24 eV very comparable, whereas all Hf-Hf interactions here are weak compared with the ones in elemental hexagonal Hf with ICOHP values of -1.46 eV (3.13 A˚) and -1.36 eV (3.20 A˚). Physical Properties. The results of the electronic structure calculation point to metallic properties of Hf3Cu2Ge3.58Sb1.42. Corresponding, small values were measured both for the Seebeck coefficient and electrical resistivity, namely þ9 μV K-1 and 0.13 mΩ cm at room temperature (Figure 3). The Seebeck as well as the resistivity values

The new antimonide Hf3Cu2Ge3.58Sb1.42 has been synthesized. It crystallizes in its own structure type. According to our synthetic efforts, the mixed Ge/Sb occupancies are required for the formation of this structure, as was the case for the ZrGexSb2-x and HfGexSb2-x structures (TiNiSi type, x e 0.2). No evidence for a significant phase range was found. Hf3Cu2Ge3.58Sb1.42 is an entropy-stabilized high-temperature compound that decomposes gradually during annealing around 1173 K. Its crystal structure comprises infinite square nets of Cu and Ge atoms (with some Sb incorporation), and may be viewed as an intergrowth structure of the NdTe3 and the ZrSiS types, as realized in PrSeTe2 and HfGeSb, respectively. Electronic structure calculations of the model Hf3Cu2Ge4Sb indicated metallic character regardless of the exact Ge/Sb ratio, which was confirmed experimentally. The metallic properties as well as the low stability at intermediate temperatures inhibit the use of Hf3Cu2Ge3.58Sb1.42 as a thermoelectric material.

(56) Dronskowski, R.; Bl€ ochl, P. E. J. Phys. Chem. 1993, 97, 8617–8624. (57) Landrum, G. A.; Dronskowski, R. Angew. Chem., Int. Ed. 2000, 39, 1560–1585.

Supporting Information Available: Crystallographic information file (CIF). This material is available free of charge via the Internet at http://pubs.acs.org.

increase with increasing temperature, which is typical for metals. As the resistivity has the same linear trend measured from 10 K up to 300 K and from 300 to 600 K, no evidence for a metal-to-insulator transition was found. Conclusion

Acknowledgment. Acknowledgment is made to the Donors of the American Chemical Society Petroleum Research Fund for financial support of this research. H.K. is indebted to the Natural Sciences and Engineering Research Council of Canada for the Canada Research Chair award.