- Email: [email protected]
β-HfCuGe—A new polymorph of HfCuGe with a novel structure type
Journal of Solid State Chemistry 199 (2013) 66–70
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Journal of Solid State Chemistry journal homepage: www.elsevier.com/locate/jssc
b-HfCuGeFA new polymorph of HfCuGe with a novel structure type Leslie M. Schoop a,b,n, Jared M. Allred a, Ni Ni a, D. Hirai a, Julia Krez b, Michael Schwall b, Huiwen Ji a, Mazhar N. Ali a, R.J. Cava a a b
Department of Chemistry, Princeton University, Princeton, NJ 08544, USA Graduate School Material Science in Mainz, 55099 Mainz, Germany
a r t i c l e i n f o
a b s t r a c t
Article history: Received 18 September 2012 Received in revised form 25 October 2012 Accepted 3 November 2012 Available online 13 December 2012
The structure and elementary physical properties of a new intermetallic compound, b-HfCuGe, are reported. b-HfCuGe has a tetragonal structure (space group I4/mmm) with lattice constants of ˚ The structure, which consists of double layers of Hf stacked with a ¼3.7634(11)A˚ and c ¼13.499(4) A. edge-sharing CuGe4 squares, is not typical for intermetallic compounds and appears to be a new structure type. The compound is a weak paramagnet and a normal metal down to 0.4 K. & 2012 Elsevier Inc. All rights reserved.
Keywords: Intermetallics b-HfCuGe Single crystal diffraction
1. Introduction
2. Experimental details
There are four previously reported compounds in the Hf–Cu–Ge ternary system. The 1:1:1 compound HfCuGe crystallizes in the TiNiSi structure type (space group Pnma) [1], Hf2CuGe4 crystallizes in space group Cmcm [2], Cu4Hf3Ge2 crystallizes in the Fe2P structure type with the space group P62m [3] and HfCuGe2 crystallizes in space group P4/nmm [4]. Among these, HfCuGe2 is a layered compound with alternating layers of Hf–Ge– Cu2–Ge–Hf–Ge2. Hf2CuGe4 also consists of layers but with a different stacking order. Fe2P-type Cu4Hf3Ge2 consists of alternating Cu–Ge and Cu–Hf layers. In none of these compounds are two layers of the same element adjacent. In the previously reported form of HfCuGe, which has the TiNiSi structure type, the layers include all three atom types. To the best of the authors’ knowledge the properties of those phases have not yet been reported. Here we report the crystal structure and properties of a new phase in the Hf–Cu–Ge ternary system with the composition 1:1:1. This polymorph of HfCuGe crystallizes in a new structure type that is quite different from the known Hf–Cu–Ge compounds. The layers consist of edge sharing CuGe4 squares alternating with two Hf metal layers; both structural components are unusual for ternary intermetallic compounds.
Samples of b-HfCuGe were prepared by three different methods. The first of these was arc melting and annealing. Stoichiometric amounts of Hf (99.6%), Cu (99.99%) and Ge (99.9999%) pieces were arc melted in an argon atmosphere. A Zr sponge was co-heated in the system to bind remaining oxygen. The buttons
n Corresponding author at: Department of Chemistry, Princeton University, Princeton, NJ 08544, USA. E-mail address: [email protected] (L.M. Schoop).
0022-4596/$ - see front matter & 2012 Elsevier Inc. All rights reserved. http://dx.doi.org/10.1016/j.jssc.2012.11.001
Table 1 Crystallographic data and details of data collection. Formula sum
b-HfCuGe
Crystal system Space group Formula weight (g/mol) Density (g/cm3) ˚ a (A)
Tetragonal I4/mmm (No. 139) 314.65 10.925 3.7643(11)
˚ c (A) V (A˚ 3)
13.499(4)
Z Temperature (K) F000 Unique reflections
4 100(2) 532 92 þ 2.062 to 2.600
Difference e density (e/A˚ 3) Extinction coefficient R1 (all reflections) R1 ðF0 4 4sðF0 ÞÞ wR2 Rint=RðsÞ m ðcm1 Þ GooF
191.28(10)
0.0042(6) 0.0200 0.0186 0.0413 0.0168/0.0085 802.17 1.272
L.M. Schoop et al. / Journal of Solid State Chemistry 199 (2013) 66–70
were melted three times, and turned over after each melt process to yield homogeneous samples. The polycrystalline ingots obtained were annealed in an evacuated quartz tube at 900 1C for 2 week. In the second method the samples were synthesized from similarly stoichiometric mixtures of the elements in an induction furnace. The samples were heated until liquid and then cooled within 3–4 h to room temperature. The obtained ingots were annealed at 900 1C for 2 week. In the third method, powders of the elements were mixed, pressed in to a pellet, and reacted in an evacuated quartz tube at 700 1C for 3 day. The samples with the highest phase purity were obtained by the second method. Higher annealing temperatures (1100 1C) yielded the polymorph of HfCuGe with the TiNiSi structure. The b-HfCuGe phase also forms if the mixture of elements employed is different from 1:1:1. In this case, as expected, additional impurity phases were present. Table 2 Position coordinates and thermal parameters for b-HfCuGe. Atom
Wyck
x
y
z
Occ.
Uiso (A˚ 2)
Hf Ge Cu
4e 4e 4c
0.5000 0.5000 0.5000
0.5000 0.5000 0.0000
0.17038(5) 0.37334(15) 0.5000
1 1 1
0.0024(3) 0.0039(5) 0.0015(5)
67
Single crystals were obtained by crushing polycrystalline samples in liquid nitrogen. Single crystal X-ray diffraction data were collected on a Bruker Kappa APEX II using graphite-monochromated Mo K a ˚ at 100 K. Unit cell determination and radiation ðl ¼ 0:71073 AÞ refinement, and data integration were performed with Bruker APEX2 software. The crystal structure was determined using SHELXL-97 [5] implemented through WinGX [6]. Powder X-ray diffraction patterns were obtained on a Bruker D8 Focus X-ray diffractometer operating with Cu K a radiation and a graphite diffracted beam monochromator. The powder studies were performed to confirm that the bulk samples consist of the reported phase. The powder data were fit with the structure determined from the single crystal data, using the FULLPROF program [7]. The crystal structure reported in this paper is from the single crystal studies. To establish the existence and preliminary composition of the new compound, Energy Dispersive X-ray (EDX) spectroscopy analysis and Scanning Electron Microscopy (SEM) images were taken on a FEI Quanta 200 FEG Environmental SEM system. A back scattering electron detector was used. EDX analysis was performed on different locations of a polycrystalline sample in order to look for impurity phases. Thermogravimetry (TGA) and Differential Scanning Calorimetry (DSC) measurements were taken on a Nietsch STA 449 Fe Jupiter system. The measurements were taken in an argon flow with a heating rate of 10 K/min from 30–1200 1C. Resistivity, heat capacity and magnetic susceptibility measurements were performed with a Physical Property Measurement System (PPMS) from Quantum Design. For these measurements, polycrystalline pieces were employed. The magnetic susceptibility, defined as M/H, where M is the magnetization and H is the applied field, was independent of the magnitude of H to greater than 5 T. The susceptibility of the compound is sufficiently small that temperature dependent measurements in 5 T applied field were employed, and the magnetization of the blank sample holder had to be subtracted from the observed M values.
3. Results and discussion
Fig. 1. Crystal structure of b-HfCuGe.
b-HfCuGe appears as a gray-metallic bulk ingot. It is stable in air and water. Single crystal diffraction studies resulted in the determination of the structure in the tetragonal space group I4/mmm (No. 139) with the lattice constants a¼3.7643(11) A˚ ˚ The crystallographic data are given in Table 1 and c¼13.499(4) A. and structural details are given in Table 2 [8]. The reflection intensity statistics could not reliably predict the space group of this novel structure, so it was initially solved in the space group I4. Once the atomic positions were determined they were used to test the structure for higher symmetry. The two most likely candidates were I4 mm and I4/mmm. Since the former is noncentrosymmetric and does not significantly improve the refinement, the latter was chosen. Afterwards the data was rescaled and the model refined
Fig. 2. Coordination of the different atoms in b-HfCuGe: (a) Hf-coordination, (b) Cu-coordination, and (c) Ge-coordination. For interatomic distances see Table 3.
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L.M. Schoop et al. / Journal of Solid State Chemistry 199 (2013) 66–70
assuming I4/mmm symmetry. The thermal parameters were refined isotropically because in anisotropic refinements the z-direction components refined to 0. The refined 1:1:1 stoichiometry was confirmed by the EDX analysis. The crystal structure of b-HfCuGe is shown in Fig. 1. It consists of two types of layers. The layers can be described as Hf–[GeCu2Ge]–Hf and they consist of squares of each atom type. These layers stack in an alternating ˚ significantly fashion. The Hf–Hf distance in these layers is 3.421 A, larger than the 3.132 A˚ distance found in Hf metal (which is HCP). The overall tetragonal body centered stacking is such that successive Hf–[GeCu2Ge]–Hf layers are shifted by (1/2, 1/2); in other words the stacking sequence can be represented as (Hf– [GeCu2Ge]–Hf)–(Hf–[GeCu2Ge]–Hf)c–, where the subscript c represents the (1/2, 1/2) layer shift. Fig. 2 shows the different coordination polyhedra of Hf, Cu and Ge. Ge is coordinated by a capped square antiprism. Hf is coordinated by a square pyramid of Ge but the next nearest neighbors are not much more distant (see Table 3), such that the coordination can alternatively be considered as prismatic. Cu is coordinated by a distorted cubeoctahedron consisting of all three atom types. Each atom type coordinates Cu in a square plane. Fig. 3 compares the crystal structures of b-HfCuGe, HfCuGe in the TiNiSi structure type, and HfCuGe2, with emphasis on the Cu–Ge coordination. In the other structures Cu is tetrahedrally coordinated by Ge instead of the CuGe4 square plane found in b-HfCuGe. HfCuGe2 has more Ge per formula unit than
Table 3 Distances in b-HfCuGe. Atom1–Atom2
˚ Distance (A)
Hf–Ge (5x) Hf–Hf (4x) Hf–Cu (4x) Cu–Ge (4x) Cu–Cu (4x) Ge–Ge
2.7264(8) 3.4214(9) 2.9719(9) 2.5428(15) 2.6618(6) 3.7643(11)
b-HfCuGe. If one compares the two different structure types it can be seen that additional Ge is found in between the Hf layers and the Cu coordination is tetrahedral. One can also view the structure of b-HfCuGe in terms of stacking of planar squares of the three atoms types. There are many ternary intermetallic structures known that are based on the stacking two-dimensional square layers, e.g. the BaAl4 and TiAl3 structure types [9], but none appear to have a similar arrangement or sequence of the layers. Cu typically forms in tetrahedral geometry in germanide intermetallics rather than the square planes seen here, which are more typical of oxides. Further, the fact that an electropositive element like Hf forms adjacent layers is unusual in this ternary structure. This creates a potential interstitial region between two Hf layers where the accommodation of an electronegative element could be favored [10]. In order to confirm that such an adventitious element is not present in b-HfCuGe, several tests were performed. Although there was no significant electron density found in difference maps, the crystal structure was re-refined with the potential adventitious elements C, O and N in the tetrahedral sites between the Hf layers. In all cases this led to an increased R value. In addition, we attempted to synthesize the b-HfCuGe structure in the presence of those elements intentionally added. With O and N present, the crystal structure did not form. In the presence of C it was possible to synthesize the compound as part of a multiple phase mixture, but no evidence such as variable lattice parameter or improved phase purity was found to indicate that C was included in the phase or was important for its formation. If one looks at the distances in the structure more carefully, then the size of the tetrahedral interstitial site between the Hf layers can be determined. The distance between the interstitial site and the four neighboring Hf atoms is 2.17 A˚ ˚ which is too (the distance to the nearest Ge atoms is 2.51 A), small to fit C or N, since the distance is significantly smaller than in known compounds (HfC: 2.32 A˚ [11] HfN: 2.36 A˚ [12]). It would be large enough, however, to fit H (HfH: 2.053 A˚ [13]). H is not expected to be present in significant amounts in materials prepared in vacuum at high temperatures, but such
Fig. 3. Comparison of different Hafnium–Copper–Germanides with emphasis on the Cu–Ge coordination: (a) b-HfCuGe, (b) HfCuGe in the TiNiSi structure type, and (c) HfCuGe2.
L.M. Schoop et al. / Journal of Solid State Chemistry 199 (2013) 66–70
Fig. 4. DSC measurements of mixed Hf, Cu and Ge powders.
69
temperature of this range was also observed in a DSC measurement of the reaction of Zr, Cu and Ge powders (not shown); the higher temperature portion of the peak was missing in this measurement. Cu3Ge was often found as an impurity in samples prepared by arcmelting. The reaction of the powders at 700 1C was not complete and showed several impurities, however, b-HfCuGe was found to be the main phase. The TGA scan (not shown) did not show any significant changes in mass over the temperature range of the DSC measurements. Fig. 5 shows the temperature dependent resistivity of polycrystalline b-HfCuGe. The phase is metallic. The resistivity changes linearly with temperature above 50 K. The room temperature resistivity is 44:7 mO cm and the residual resistivity is 4:8 mO cm, yielding a residual resistivity ratio of approximately 10. b-HfCuGe was tested for superconductivity down to 0.4 K and was found to be a normal metal. The low temperature heat capacity follows the cV ¼ gT þ bT3 law, where g describes the electronic contribution to the heat capacity and b is the phonon contribution, which can be related to the Debye Temperature YD . For b-HfCuGe we found g ¼ 2:25 mJ=ðmole K2 Þ and YD ¼ 250 K. The magnetic susceptibility is very low and temperature independent. Measurements obtained a value of 2:5 105 emu= ðOe moleÞ. Considering the core diamagnetism of the elements ðHf ¼ 16 106 emu=ðOe moleÞ, Cu ¼ 12 106 emu= ðOe moleÞ Ge ¼ 7 106 emu=ðOe moleÞ [15]), one can estimate a resulting paramagnetic susceptibility of w0 ¼ 1 105 emu= ðOe moleÞ. This low value is consistent with the low value of g, suggesting that there are very few states at the Fermi level. This leads to the conclusion that b-HfCuGe is an intrinsic Pauli paramagnet.
4. Summary We have reported a new polymorph of HfCuGe and discussed its crystallographic and physical properties. b-HfCuGe crystallizes in a structure with adjacent Hf layers and a square planar network of Cu and Ge. It shows metallic conductivity and is an intrinsic Pauli paramagnet with a low density of states at the Fermi level. The presence of adjacent Hf layers suggests that b-HfCuGe may form a hydride under the appropriate conditions. Fig. 5. Resistivity vs Temperature for b-HfCuGe. The insert shows the low temperature heat capacity data in a C/T vs T2 plot.
Acknowledgments sites may be suitable for hydrogen insertion at low temperatures and high pressures. In order to understand further the conditions under which the new phase forms, we investigated the reactions of mixed powders of the starting elements through the DSC method. Fig. 4 shows the cooling and heating curve for one such experiment. Through the temperatures at which they occur, most of the observed peaks can be associated with reactions that occur in the Cu–Ge binary phase diagram [14]. The melting point of the eutectic in the liquidus curve of Cu–Ge is 644 1C at a composition of 63.5 at% Cu. This fits the endothermic peak at 642.5 1C. Furthermore the formation and crystallization of the Cu3Ge phase can be seen in the heating and the cooling curves. The orthorhombic phase with 76.5 at% Cu melts congruently at 747 1C. (The solidification appears at a lower temperature in the measurement due to the nucleation barrier.) The broad exothermic peak at 700 1C likely represents the formation of b-HfCuGe. We have tested this hypothesis by successfully synthesizing the compound from the powders at this temperature. We believe that the broad peak at 700 1C in the DSC scan is likely a combination of two peaks because a peak at the lower
This research was supported by the Department of Energy, Division of Basic Energy Sciences, Grant DE-FG02-98-ER45706.
References [1] W. Jeitschko, A. Jordan, P. Beck, Trans. Met. Soc. AIME. 245 (2) (1969) 335–339. [2] F. Thirion, G. Venturini, B. Malaman, J. Steinmetz, B. Roques, J. Less Common Metals 95 (1) (1983) 47–54. URL: /http://www.sciencedirect.com/science/ article/pii/0022508883903831S. [3] H. Sprenger, J.J. Nickl, J. Less Common Metals 27 (2) (1972) 163–168. URL: /http://www.sciencedirect.com/science/article/pii/0022508872900276S. [4] L. Andrukhiv, L. Lysenko, Y. Yarmolyuk, E. Gladyshevs’kii, Dopov. Akad. Nauk Ukr. RSR, Ser. A: Fiz.–Tekh. Mat. Nauki (7) (1975) 645–648. [5] G.M. Sheldrick, Acta Cryst. Sec. A 64 (1) (2008) 112–122. URL: /http://dx.doi. org/10.1107/S0108767307043930S. [6] L. Farrugia, J. Appl. Cryst. 32 (4) (1999) 837–838. [7] Juan, Rodriguez-Carvajal, Phys. B Condens. Matter 192(1–2) (1993) 55–69. URL: /http://www.sciencedirect.com/science/article/pii/ 092145269390108IS. [8] Further details may be obtained from Fachinformationszentrum Karlsruhe, 76344 Eggenstein-Leopoldshafen, Germany (Fax: þ49-7247-808-666; E-mail: crysdata(at)fiz-karlsruhe.de, /http://www.fiz-karlsruhe.de/requestS for deposited data.html) on quoting the CSD number 425142.
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[9] P. Villars, L. Calvert, Pearson’s Handbook of Crystallographic Data for Intermetallic Compounds, American Society for Metals, Materials Park, OH, 1991, No. 2. ¨ anorganische und allgemeine [10] J. Nuss, U. Wedig, M. Jansen, Zeitschrift fur Chemie 637 (13) (2011) 1975–1981. [11] K. Nakamura, M. Yashima, Mater. Sci. Eng.: B 148 (1) (2008) 69–72.
[12] F. Glaser, D. Moskowitz, B. Post, Trans. AIME 197 (1953) 1119. [13] S. Sidhu, Acta Cryst. 7 (5) (1954) 447–449. [14] R. Olesinski, G. Abbaschian, J. Phase Equilibria 7 (1986) 28–35, http://dx.doi. org/10.1007/BF02874979. URL: /http://dx.doi.org/10.1007/BF02874979S. [15] C. O’Connor, Prog. Inorganic Chem. (1982) 203–283.
Contents lists available at SciVerse ScienceDirect
Journal of Solid State Chemistry journal homepage: www.elsevier.com/locate/jssc
b-HfCuGeFA new polymorph of HfCuGe with a novel structure type Leslie M. Schoop a,b,n, Jared M. Allred a, Ni Ni a, D. Hirai a, Julia Krez b, Michael Schwall b, Huiwen Ji a, Mazhar N. Ali a, R.J. Cava a a b
Department of Chemistry, Princeton University, Princeton, NJ 08544, USA Graduate School Material Science in Mainz, 55099 Mainz, Germany
a r t i c l e i n f o
a b s t r a c t
Article history: Received 18 September 2012 Received in revised form 25 October 2012 Accepted 3 November 2012 Available online 13 December 2012
The structure and elementary physical properties of a new intermetallic compound, b-HfCuGe, are reported. b-HfCuGe has a tetragonal structure (space group I4/mmm) with lattice constants of ˚ The structure, which consists of double layers of Hf stacked with a ¼3.7634(11)A˚ and c ¼13.499(4) A. edge-sharing CuGe4 squares, is not typical for intermetallic compounds and appears to be a new structure type. The compound is a weak paramagnet and a normal metal down to 0.4 K. & 2012 Elsevier Inc. All rights reserved.
Keywords: Intermetallics b-HfCuGe Single crystal diffraction
1. Introduction
2. Experimental details
There are four previously reported compounds in the Hf–Cu–Ge ternary system. The 1:1:1 compound HfCuGe crystallizes in the TiNiSi structure type (space group Pnma) [1], Hf2CuGe4 crystallizes in space group Cmcm [2], Cu4Hf3Ge2 crystallizes in the Fe2P structure type with the space group P62m [3] and HfCuGe2 crystallizes in space group P4/nmm [4]. Among these, HfCuGe2 is a layered compound with alternating layers of Hf–Ge– Cu2–Ge–Hf–Ge2. Hf2CuGe4 also consists of layers but with a different stacking order. Fe2P-type Cu4Hf3Ge2 consists of alternating Cu–Ge and Cu–Hf layers. In none of these compounds are two layers of the same element adjacent. In the previously reported form of HfCuGe, which has the TiNiSi structure type, the layers include all three atom types. To the best of the authors’ knowledge the properties of those phases have not yet been reported. Here we report the crystal structure and properties of a new phase in the Hf–Cu–Ge ternary system with the composition 1:1:1. This polymorph of HfCuGe crystallizes in a new structure type that is quite different from the known Hf–Cu–Ge compounds. The layers consist of edge sharing CuGe4 squares alternating with two Hf metal layers; both structural components are unusual for ternary intermetallic compounds.
Samples of b-HfCuGe were prepared by three different methods. The first of these was arc melting and annealing. Stoichiometric amounts of Hf (99.6%), Cu (99.99%) and Ge (99.9999%) pieces were arc melted in an argon atmosphere. A Zr sponge was co-heated in the system to bind remaining oxygen. The buttons
n Corresponding author at: Department of Chemistry, Princeton University, Princeton, NJ 08544, USA. E-mail address: [email protected] (L.M. Schoop).
0022-4596/$ - see front matter & 2012 Elsevier Inc. All rights reserved. http://dx.doi.org/10.1016/j.jssc.2012.11.001
Table 1 Crystallographic data and details of data collection. Formula sum
b-HfCuGe
Crystal system Space group Formula weight (g/mol) Density (g/cm3) ˚ a (A)
Tetragonal I4/mmm (No. 139) 314.65 10.925 3.7643(11)
˚ c (A) V (A˚ 3)
13.499(4)
Z Temperature (K) F000 Unique reflections
4 100(2) 532 92 þ 2.062 to 2.600
Difference e density (e/A˚ 3) Extinction coefficient R1 (all reflections) R1 ðF0 4 4sðF0 ÞÞ wR2 Rint=RðsÞ m ðcm1 Þ GooF
191.28(10)
0.0042(6) 0.0200 0.0186 0.0413 0.0168/0.0085 802.17 1.272
L.M. Schoop et al. / Journal of Solid State Chemistry 199 (2013) 66–70
were melted three times, and turned over after each melt process to yield homogeneous samples. The polycrystalline ingots obtained were annealed in an evacuated quartz tube at 900 1C for 2 week. In the second method the samples were synthesized from similarly stoichiometric mixtures of the elements in an induction furnace. The samples were heated until liquid and then cooled within 3–4 h to room temperature. The obtained ingots were annealed at 900 1C for 2 week. In the third method, powders of the elements were mixed, pressed in to a pellet, and reacted in an evacuated quartz tube at 700 1C for 3 day. The samples with the highest phase purity were obtained by the second method. Higher annealing temperatures (1100 1C) yielded the polymorph of HfCuGe with the TiNiSi structure. The b-HfCuGe phase also forms if the mixture of elements employed is different from 1:1:1. In this case, as expected, additional impurity phases were present. Table 2 Position coordinates and thermal parameters for b-HfCuGe. Atom
Wyck
x
y
z
Occ.
Uiso (A˚ 2)
Hf Ge Cu
4e 4e 4c
0.5000 0.5000 0.5000
0.5000 0.5000 0.0000
0.17038(5) 0.37334(15) 0.5000
1 1 1
0.0024(3) 0.0039(5) 0.0015(5)
67
Single crystals were obtained by crushing polycrystalline samples in liquid nitrogen. Single crystal X-ray diffraction data were collected on a Bruker Kappa APEX II using graphite-monochromated Mo K a ˚ at 100 K. Unit cell determination and radiation ðl ¼ 0:71073 AÞ refinement, and data integration were performed with Bruker APEX2 software. The crystal structure was determined using SHELXL-97 [5] implemented through WinGX [6]. Powder X-ray diffraction patterns were obtained on a Bruker D8 Focus X-ray diffractometer operating with Cu K a radiation and a graphite diffracted beam monochromator. The powder studies were performed to confirm that the bulk samples consist of the reported phase. The powder data were fit with the structure determined from the single crystal data, using the FULLPROF program [7]. The crystal structure reported in this paper is from the single crystal studies. To establish the existence and preliminary composition of the new compound, Energy Dispersive X-ray (EDX) spectroscopy analysis and Scanning Electron Microscopy (SEM) images were taken on a FEI Quanta 200 FEG Environmental SEM system. A back scattering electron detector was used. EDX analysis was performed on different locations of a polycrystalline sample in order to look for impurity phases. Thermogravimetry (TGA) and Differential Scanning Calorimetry (DSC) measurements were taken on a Nietsch STA 449 Fe Jupiter system. The measurements were taken in an argon flow with a heating rate of 10 K/min from 30–1200 1C. Resistivity, heat capacity and magnetic susceptibility measurements were performed with a Physical Property Measurement System (PPMS) from Quantum Design. For these measurements, polycrystalline pieces were employed. The magnetic susceptibility, defined as M/H, where M is the magnetization and H is the applied field, was independent of the magnitude of H to greater than 5 T. The susceptibility of the compound is sufficiently small that temperature dependent measurements in 5 T applied field were employed, and the magnetization of the blank sample holder had to be subtracted from the observed M values.
3. Results and discussion
Fig. 1. Crystal structure of b-HfCuGe.
b-HfCuGe appears as a gray-metallic bulk ingot. It is stable in air and water. Single crystal diffraction studies resulted in the determination of the structure in the tetragonal space group I4/mmm (No. 139) with the lattice constants a¼3.7643(11) A˚ ˚ The crystallographic data are given in Table 1 and c¼13.499(4) A. and structural details are given in Table 2 [8]. The reflection intensity statistics could not reliably predict the space group of this novel structure, so it was initially solved in the space group I4. Once the atomic positions were determined they were used to test the structure for higher symmetry. The two most likely candidates were I4 mm and I4/mmm. Since the former is noncentrosymmetric and does not significantly improve the refinement, the latter was chosen. Afterwards the data was rescaled and the model refined
Fig. 2. Coordination of the different atoms in b-HfCuGe: (a) Hf-coordination, (b) Cu-coordination, and (c) Ge-coordination. For interatomic distances see Table 3.
68
L.M. Schoop et al. / Journal of Solid State Chemistry 199 (2013) 66–70
assuming I4/mmm symmetry. The thermal parameters were refined isotropically because in anisotropic refinements the z-direction components refined to 0. The refined 1:1:1 stoichiometry was confirmed by the EDX analysis. The crystal structure of b-HfCuGe is shown in Fig. 1. It consists of two types of layers. The layers can be described as Hf–[GeCu2Ge]–Hf and they consist of squares of each atom type. These layers stack in an alternating ˚ significantly fashion. The Hf–Hf distance in these layers is 3.421 A, larger than the 3.132 A˚ distance found in Hf metal (which is HCP). The overall tetragonal body centered stacking is such that successive Hf–[GeCu2Ge]–Hf layers are shifted by (1/2, 1/2); in other words the stacking sequence can be represented as (Hf– [GeCu2Ge]–Hf)–(Hf–[GeCu2Ge]–Hf)c–, where the subscript c represents the (1/2, 1/2) layer shift. Fig. 2 shows the different coordination polyhedra of Hf, Cu and Ge. Ge is coordinated by a capped square antiprism. Hf is coordinated by a square pyramid of Ge but the next nearest neighbors are not much more distant (see Table 3), such that the coordination can alternatively be considered as prismatic. Cu is coordinated by a distorted cubeoctahedron consisting of all three atom types. Each atom type coordinates Cu in a square plane. Fig. 3 compares the crystal structures of b-HfCuGe, HfCuGe in the TiNiSi structure type, and HfCuGe2, with emphasis on the Cu–Ge coordination. In the other structures Cu is tetrahedrally coordinated by Ge instead of the CuGe4 square plane found in b-HfCuGe. HfCuGe2 has more Ge per formula unit than
Table 3 Distances in b-HfCuGe. Atom1–Atom2
˚ Distance (A)
Hf–Ge (5x) Hf–Hf (4x) Hf–Cu (4x) Cu–Ge (4x) Cu–Cu (4x) Ge–Ge
2.7264(8) 3.4214(9) 2.9719(9) 2.5428(15) 2.6618(6) 3.7643(11)
b-HfCuGe. If one compares the two different structure types it can be seen that additional Ge is found in between the Hf layers and the Cu coordination is tetrahedral. One can also view the structure of b-HfCuGe in terms of stacking of planar squares of the three atoms types. There are many ternary intermetallic structures known that are based on the stacking two-dimensional square layers, e.g. the BaAl4 and TiAl3 structure types [9], but none appear to have a similar arrangement or sequence of the layers. Cu typically forms in tetrahedral geometry in germanide intermetallics rather than the square planes seen here, which are more typical of oxides. Further, the fact that an electropositive element like Hf forms adjacent layers is unusual in this ternary structure. This creates a potential interstitial region between two Hf layers where the accommodation of an electronegative element could be favored [10]. In order to confirm that such an adventitious element is not present in b-HfCuGe, several tests were performed. Although there was no significant electron density found in difference maps, the crystal structure was re-refined with the potential adventitious elements C, O and N in the tetrahedral sites between the Hf layers. In all cases this led to an increased R value. In addition, we attempted to synthesize the b-HfCuGe structure in the presence of those elements intentionally added. With O and N present, the crystal structure did not form. In the presence of C it was possible to synthesize the compound as part of a multiple phase mixture, but no evidence such as variable lattice parameter or improved phase purity was found to indicate that C was included in the phase or was important for its formation. If one looks at the distances in the structure more carefully, then the size of the tetrahedral interstitial site between the Hf layers can be determined. The distance between the interstitial site and the four neighboring Hf atoms is 2.17 A˚ ˚ which is too (the distance to the nearest Ge atoms is 2.51 A), small to fit C or N, since the distance is significantly smaller than in known compounds (HfC: 2.32 A˚ [11] HfN: 2.36 A˚ [12]). It would be large enough, however, to fit H (HfH: 2.053 A˚ [13]). H is not expected to be present in significant amounts in materials prepared in vacuum at high temperatures, but such
Fig. 3. Comparison of different Hafnium–Copper–Germanides with emphasis on the Cu–Ge coordination: (a) b-HfCuGe, (b) HfCuGe in the TiNiSi structure type, and (c) HfCuGe2.
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Fig. 4. DSC measurements of mixed Hf, Cu and Ge powders.
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temperature of this range was also observed in a DSC measurement of the reaction of Zr, Cu and Ge powders (not shown); the higher temperature portion of the peak was missing in this measurement. Cu3Ge was often found as an impurity in samples prepared by arcmelting. The reaction of the powders at 700 1C was not complete and showed several impurities, however, b-HfCuGe was found to be the main phase. The TGA scan (not shown) did not show any significant changes in mass over the temperature range of the DSC measurements. Fig. 5 shows the temperature dependent resistivity of polycrystalline b-HfCuGe. The phase is metallic. The resistivity changes linearly with temperature above 50 K. The room temperature resistivity is 44:7 mO cm and the residual resistivity is 4:8 mO cm, yielding a residual resistivity ratio of approximately 10. b-HfCuGe was tested for superconductivity down to 0.4 K and was found to be a normal metal. The low temperature heat capacity follows the cV ¼ gT þ bT3 law, where g describes the electronic contribution to the heat capacity and b is the phonon contribution, which can be related to the Debye Temperature YD . For b-HfCuGe we found g ¼ 2:25 mJ=ðmole K2 Þ and YD ¼ 250 K. The magnetic susceptibility is very low and temperature independent. Measurements obtained a value of 2:5 105 emu= ðOe moleÞ. Considering the core diamagnetism of the elements ðHf ¼ 16 106 emu=ðOe moleÞ, Cu ¼ 12 106 emu= ðOe moleÞ Ge ¼ 7 106 emu=ðOe moleÞ [15]), one can estimate a resulting paramagnetic susceptibility of w0 ¼ 1 105 emu= ðOe moleÞ. This low value is consistent with the low value of g, suggesting that there are very few states at the Fermi level. This leads to the conclusion that b-HfCuGe is an intrinsic Pauli paramagnet.
4. Summary We have reported a new polymorph of HfCuGe and discussed its crystallographic and physical properties. b-HfCuGe crystallizes in a structure with adjacent Hf layers and a square planar network of Cu and Ge. It shows metallic conductivity and is an intrinsic Pauli paramagnet with a low density of states at the Fermi level. The presence of adjacent Hf layers suggests that b-HfCuGe may form a hydride under the appropriate conditions. Fig. 5. Resistivity vs Temperature for b-HfCuGe. The insert shows the low temperature heat capacity data in a C/T vs T2 plot.
Acknowledgments sites may be suitable for hydrogen insertion at low temperatures and high pressures. In order to understand further the conditions under which the new phase forms, we investigated the reactions of mixed powders of the starting elements through the DSC method. Fig. 4 shows the cooling and heating curve for one such experiment. Through the temperatures at which they occur, most of the observed peaks can be associated with reactions that occur in the Cu–Ge binary phase diagram [14]. The melting point of the eutectic in the liquidus curve of Cu–Ge is 644 1C at a composition of 63.5 at% Cu. This fits the endothermic peak at 642.5 1C. Furthermore the formation and crystallization of the Cu3Ge phase can be seen in the heating and the cooling curves. The orthorhombic phase with 76.5 at% Cu melts congruently at 747 1C. (The solidification appears at a lower temperature in the measurement due to the nucleation barrier.) The broad exothermic peak at 700 1C likely represents the formation of b-HfCuGe. We have tested this hypothesis by successfully synthesizing the compound from the powders at this temperature. We believe that the broad peak at 700 1C in the DSC scan is likely a combination of two peaks because a peak at the lower
This research was supported by the Department of Energy, Division of Basic Energy Sciences, Grant DE-FG02-98-ER45706.
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