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Crystal structure and physical properties of a novel ternary compound La15MoxGe9
Chemical Physics Letters 730 (2019) 612–616
Contents lists available at ScienceDirect
Chemical Physics Letters journal homepage: www.elsevier.com/locate/cplett
Research paper
Crystal structure and physical properties of a novel ternary compound La15MoxGe9 Joanna Blawata,
⁎,1
, Weiwei Xieb, Robert J. Cavac, Tomasz Klimczuka,
T
⁎
a
Faculty of Applied Physics and Mathematics, Gdansk University of Technology, Narutowicza 11/12, 80-233 Gdansk, Poland Department of Chemistry, Louisiana State University, Baton Rouge, LA 70803, USA c Department of Chemistry, Princeton University, Princeton, NJ 08544, USA b
H I GH L IG H T S
polycrystalline sample of the new compound La MoGe was synthesized. • The crystallizes in a new version of Mn Si -type structure. • LaLa MoGe is the first compound in ternary system La-Mo-Ge. • The MoGe • specific heat measurement shows no phase transition down to 2 K. 15
15
9
15
9
5
9
3
A B S T R A C T
We present the synthesis, structural characterization and physical properties measurements of a new compound, La15MoxGe9 in Mn5Si3-type derived structure with interstitial Mo. The sample was synthesized by arc-melting method. La15MoxGe9 crystallizes in non-centrosymmetric hexagonal structure P63mc (#186) with lattice parameters a = b = 15.495(5) Å and c = 6.917(2) Å. The refinement on single X-ray diffraction data show that interstitial atom Mo occupies two different Wyckoff positions (2a, 2b) with 0.19(2) Mo/per f.u. The specific heat properties were well studied. The obtained Sommerfeld coefficient and Debye temperature are: γ = 89.2(9) mJ/(mol K2) and ΘD = 190(5) K, respectively. La15MoxGe9 is the first compound in the ternary system La-Mo-Ge.
1. Introduction Understanding the relationship between crystal structure and physical properties remains challenging especially for complex intermetallic compounds. Due to a huge variety of combining elements and crystallographic structures, the prediction of the chemical stability and physical properties of a new compound are extremely difficult and usually based on some empirical rules. One of the approaches to a new material with targeted property is choosing a crystal structure which is favorable for some specific feature and tuning the electronic instabilities in a compound [1–3]. Hexagonal Mn5Si3-type structure has been intriguing for solid state materials community for decades due to the various interesting properties and applications, such as different kinds of magnetic ordering like antiferromagnetism or ferromagnetism [4,5], the superconductivity in Zr5Sb3 [6], and the Dirac semimetal state in Ca3P2 [7]. Structurally, the main component of its hexagonal unit cell (space group P63/mcm, 193) with a chain of octahedra provides sufficient
space to accommodate another atom inside. This encapsulation can cause changes in lattice parameters and interatomic distances without deconstruction of the crystal structure. That structural flexibility offers an ideal platform for synthesis of many new compounds with a wide range of interstitial atoms and thus also for tuning physical properties among the same crystal structure (P63/mcm, 193) [8]. Especially interesting seems to be a La5Ge3Z family, where Corbett and Guloy successfully synthesized new compounds using nineteen different interstitial atom with a wide range from p- to d-blocks [9]. A few years later, they showed that La5Ge3Z has another intriguing feature – the La5Ga3 can encapsulate atoms not into all octahedra but only into one third of them and form a new family with stoichiometry La15Ge9Z (=La5Ge3Z1/ 3). In this way, they successfully synthesized nine new compounds, which are gathered in Fig. 1(a) [10]. Moreover, they also observed the loss of the inversion center in the superlattice compounds – all La15Ge9Z-members crystallize in noncentrosymmetric hexagonal structure P63mc (#186). What is more, another unusual feature reported by Corbett and Guloy was a fact that it is possible to synthesize a
⁎
Corresponding authors. E-mail addresses: [email protected] (J. Blawat), [email protected] (T. Klimczuk). 1 Current affiliation: Department of Chemistry, Louisiana State University, Baton Rouge, LA 70803, USA. Department of Physics and Astronomy, Louisiana State University, Baton Rouge, LA 70803, USA. https://doi.org/10.1016/j.cplett.2019.06.065 Received 9 March 2019; Received in revised form 23 June 2019; Accepted 25 June 2019 Available online 26 June 2019 0009-2614/ © 2019 Elsevier B.V. All rights reserved.
Chemical Physics Letters 730 (2019) 612–616
J. Blawat, et al.
Fig. 1. (a) The summary of reported compounds in La15ZGe9 family, where Z is an interstitial atom. Green color means compounds which Corbett and Guloy synthesized, red color represent these Z atoms for which they reported the unsuccessful synthesis and the blue color shows compounds in La15ZGe9 type structure with atom Z in two different Wyckoff positions. (b) The chains of octahedral going by 2a and 2b sites with measured distances between La atoms for La15MoxGe9, La15NbxGe9 and La15LixGe9. The drawings and measurements were done using Vesta software [17]. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
Bruker D8 diffractometer with Cu Kα1 radiation (λ = 1.5406 Å). The results were analyzed by means of LeBail refinement [14] using a FullProf software [15]. Peak shapes were modelled with the Thompson–Cox–Hastings pseudo–Voight profile convoluted with axial divergence asymmetry. The sample with nominal stoichiometry La15Mo0.5Ge9 had the highest purity and hence we used it for further measurements. Small pieces of the polycrystalline sample were mounted on a kapton loop and examined by single X-ray diffraction method on a Bruker Apex II diffractometer with Mo radiation Kα (λ = 0.71073 Å). The crystal structure was refined with SHELTX [16] package using direct methods and refined by full-matrix least-squares on F2. The specific heat was performed in Quantum Design Physical Properties Measurement System (PPMS) by means of a standard 2τ relaxation method. The polished sample was studied in temperature range 2–300 K without applied magnetic field.
new compound with some substoichiometric amount of interstitial atoms if Z is a small element from p-block (for example B, O, C) [8–10]. On the other hand, they claimed that atoms Z always occupy 2b site (2/ 3, 1/3, z) solely. But recently, a few new compounds in RE15Ge9C family, where RE is rare earth metal have been reported and the neutron diffraction data showed that for two of them (Ce15Ge9C and Pr15Ge9C) carbon also occupies 2a site (0, 0, z). The occupancy of 2a site is significantly lower than for 2b site, but the existence cannot be ignored [5,11]. This fact also points out to additional instability in crystal and electronic structure in La15Ge9Z family. Recently, a new member La15Ge9Li1.50 was discovered, which extended the interstitial Z to sblock with occupying both 2a and 2b sites [12]. Similarly, 2b site is fully occupied while 2a is partially occupied. It is worth noting that a La15Ge9Z – type compounds can encapsulate higher amount of interstitial atoms. Recently we reported a new compound La15NbxGe9, where Nb also occupy two different Wyckoff positions, and we showed that it is possible to synthesize a new compound with significantly bigger interstitial atom Z [13]. In that case, our motivation was also looking for new superconducting materials using superconducting elements in a superconductivity-favored structure [3]. Herein, we synthesized a new compound with Mo as the interstitial Z. The heat capacity measurements show no superconductivity above 1.85 K, which is similar with La15NbxGe9.
3. Results and discussions 3.1. Crystal structure The single X-ray diffraction refinement reveals that La15MoxGe9 crystallizes in non-centrosymmetric hexagonal space group P63mc (#186) with lattice parameters a = b = 15.495(5) Å and c = 6.917(2) Å [unit cell volume V = 1438(1) Å3], slightly larger than we obtained for La15NbxGe9 [13]. All the details of collecting data and refinement are presented in Tables 1 and 2. The crystal structure is displayed in Fig. 2. The main components of La15MoxGe9 crystal structure are chains of octahedra formed by lanthanum atoms. One of them is built by La2 and La3 and is going by Wyckoff position 2a (0, 0, z), while the second one is surrounded by La1 and centered on 2b site (2/3, 1/3, z). The occupancies have been refined to obtain the reasonable R1 and wR2 and electron density residuals peaks and holes close to zero. All of these lanthanum octahedra are bridged by germanium atoms. The La4 atoms form honeycomb structures around the chains in ab-plane and a zig-zag line along the crystallographic c-axis shown in Fig. 2. In Fig. 1(b) we compare dimensions of the octahedra for all known
2. Experimental parts The polycrystalline sample of La15MoxGe9 was synthesized using arc-melting method under a high-purity Zr-gettered argon atmosphere. Lanthanum (99.9%), molybdenum (99.9%) and germanium pieces (99.99%) pure metals were melted together in stoichiometry 15:1:9 and 15:0.5:9 (x = 0.5 and 1). The buttons were flipped over and re-melted 4 times to promote homogeneity. The total mass of the sample did not exceed 300 mg, and the weight losses were lower than 1%. The as-cast buttons were wrapped in tantalum foil and annealed in a high temperature high vacuum furnace at 1180 °C for 2 days. Powder X-ray diffraction measurements were performed using a 613
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Table 1 Single crystal La15Mo0.19Ge9.
crystallographic
data
and
structure
refinement
Formula
La15Mo0.19Ge9
F. W. (g/mol) Space group, Z a (Å) b (Å) c (Å) V (Å 3) Absorption correction Extinction coefficient Θ range (deg) hkl ranges
2755.67 P63mc (#186), 2 15.495(5) 15.495(5) 6.917(2) 1438(1) Numerical 0.00034(7) 2.629–33.129 −21 ≤ h ≤ 20 −23 ≤ k ≤ 23 −10 ≤ l ≤ 8 1818, 0.0794 1259 53 0.0474, 0.0939 0.979 1.952, −3.857
No. reflections, Rint No. independent reflections No. parameters R1, wR2 (all I) Goodness of fit Largest diff. peak and hole (e−/Å3)
Table 3 Comparison between octahedral volumes for three compounds: La15Mo0.19Ge9, La15Nb0.12Ge9 and La15Li1.5Ge9. All volumes were measured using VESTA software [17]. Crystallographic data are from Refs. [13] and [12] for La15Nb0.12Ge9 and La15Li1.5Ge9, respectively.
for
Wyckoff
Occupancy
x
y
z
Ueq
La1 La2 La3 La4 Ge1 Ge2 Ge3 Mo1 Mo2
6c 6c 6c 12d 6c 6c 6c 2b 2a
1 1 1 1 1 1 1 0.0432 0.1490
0.9189(2) 0.2522 (1) 0.5854(1) 0.0169 (1) 0.7996(3) 0.1322(4) 0.4658 (2) 1/3 0
0.0811(1) 0.7478(1) 0.4146(1) 0.3416(7) 0.2004(2) 0.8679(2) 0.5341(3) 2/3 0
0.2432(3) 0.1938(3) 0.2253(3) 0.4668(7) 0.2551(8) 0.1966(10) 0.2072(2) 0.0020(1) 0.1620(9)
0.0117(3) 0.0134(1) 0.0010(1) 0.0236(1) 0.0103(7) 0.0125(2) 0.0098(6) 0.0077(4) 0.1628(1)
La15Mo0.19Ge9
La15Nb0.12Ge9
La15Li1.5Ge9
Octahedra with Z atom in 2a site Octahedra with Z atom in 2b site Octahedra without Z atom in 2b site
28.41 26.63 30.22
28.06 26.57 29.55
28.61 27.04 30.21
Table 4 Selected interatomic distances for La15Mo0.19Ge9.
Table 2 Atomic coordinates and isotropic displacement parameters of La15Mo0.19Ge9. Ueq is defined as one-third of the trace of the orthogonalized Uij tensor (Å2). Atom
Volume (Å3)
Atom 1
Atom 2
Distance (Å)
La 1 La1 La1 La1 La1 La2 La2 La2 La2 La2 La2 La3 La3 La3 La3 La3 La3 La4 La4 La4 La4 La4 La4 La4
La1 Ge1 Ge2 La4 Mo2 La2 Ge1 Ge2 Ge3 Mo1 La4 La2 La3 Ge1 Ge3 Mo1 La4 La4 Ge1 Ge1 Ge2 Ge2 Ge3 Ge3
3.772(5) 3.201(5) 3.115(4) 3.890(3) 2.75(4) 3.770(5) 3.338(6) 3.222(6) 3.107(4) 2.55(3) 3.745(3) 3.905(3) 3.776(4) 3.122(4) 3.211(6) 2.90(3) 3.761(3) 3.488(1) 3.208(5) 3.302(4) 3.202(5) 3.301(4) 3.222(5) 3.279(5)
Fig. 2. Crystal structure of La15MoxGe9. The drawings were made using VESTA software [17] with crystallographic data obtained from single X-ray diffraction refinement. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.) 614
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niobium atoms and hence the volume of octahedra is bigger. The purity of synthesized sample was confirmed by powder X-ray diffraction measurements. Fig. 3(a) shows the LeBail refinement of the PXRD pattern for La15MoxGe9. The analysis confirms that this compound crystallizes in a hexagonal crystal structure (P63mc #186) with lattice parameters a = b = 15.5041(7) Å and c = 6.9198(6) Å. The obtained figures of merit are Rp = 9.7%, Rwp = 13.2%, Rexp = 9.97% and χ2 = 1.75. In Fig. 3(b) are shown superstructure reflections which are characteristic for the La15ZGe9-type structure.
compounds in the group La15ZxGe9 in which atom Z occupies two different Wyckoff positions and in Table 3 are gathered the volumes of octahedra, while the main interatomic distances for La15MoxGe9 are gathered in Table 4. The measurements were performed using the Vesta software [17]. The distances for La15MoxGe9 between La1-La1 and La2La2 along a-b direction are almost the same and equal to 3.772(5) Å and 3.770(5) Å, respectively, but in dimensions along c direction we can observe differences. Distance between La1-La1 is equal to 4.087(2) Å, while distances between La2-La3 are 3.905(3) Å and 4.273(3) Å. According to the symmetry in La15ZGe9-type structure compounds only one third of octahedra formed by La2 and La3 atoms can encapsulate interstitial atoms, so the differences between distances along the c direction could be caused by chemical pressure. The same dependence is observed for La15NbxGe9 and La15LixGe9. It is worth noting that all volumes of octahedra are the biggest for La15LixGe9 (see Table 3) and the smallest for La15NbxGe9, while the atomic radii for Li is the smallest, and the biggest for Nb. It is expected that the biggest atom will make the biggest volume of octahedra, but besides the atomic radii the occupancy of the interstitial atoms should also be considered. The compound with Li contains significantly larger occupancy of the interstitial atoms than the compounds with Mo and Nb. It is also worth noting that atomic radii for Mo is slightly smaller than for Nb, but it caused that the La15ZGe9-type unit cell can encapsulate more molybdenum than
3.2. Heat capacity The heat capacity measurements were performed to reveal the physical properties of the new compound La15MoxGe9. Fig. 4(a) shows the temperature dependence of heat capacity in the range 2–300 K. At room temperature the heat capacity reaches the value expected by Dulong-Petit law 3nR = 611 J/mol K, where R is a gas constant (R = 8.31 J/mol K) and n is the number of atoms per formula unit. Low temperature data of heat capacity divided by T (C/T) versus T2 are presented in Fig. 4(b) and the fit to the equation C/T = γ + βT 2 (blue line) where γ is the Sommerfeld coefficient, and β is the phonon specific heat parameter. The first term γT represents an electronic contribution (Cel) and βT 3 is the phonon contribution (Cph) to the specific heat [18]. The obtained parameters are: γ = 89.2(9) mJ/(mol K2), and β = 7.5(1) mJ/(mol K4) is related with Debye temperature through the formula:
θD =
3
12π 4nR 5β
The Debye temperature was estimated to be θD = 188 K. The obtained value of the Sommerfeld coefficient is larger than reported for the simple metals, however when we consider the large number of atoms (per formula unit) we estimated γ = 89.2(9) mJ/(mol K2) = 3.6 mJ/(mol-at K2). Similarly high value of the Sommerfeld coefficient was found for nonmagnetic intermetallic compounds: La15NbxGe9 (2.6 mJ/(mol-at K2)) [13] and Y8Co5 (4.8 mJ/(mol-at K2)) [19]. Fig. 4(c) presents Cph/T3 versus T to estimate the Einstein temperature (θE). A broad local maximum is found at Tmax ∼ 11 K, which gives the Einstein temperature value of θE ∼ 5Tmax = 55 K. The fit to the overall temperature dependence of heat capacity (Fig. 4(a)) were done using these estimated values of Debye and Einstein temperatures. The blue line in Fig. 4(a) represents the fit of the data by: C (T ) = γT + kCD (T )+(1 − k ) CE (T ) , where γT is the contribution due to conduction electrons and the next two terms stand for lattice contribution described by Einstein (CE) and Debye (CD) models and the parameter k is a mutual weight of phonon modes of the two types:
CD (T )=9nR (
T 3 ) ΘD
CE (T ) = 3nR (
∫0
ΘD
x 4 e x dx (e x − 1)2
ΘE 2 Θ Θ ) exp( E )[exp ⎛ E ⎞ − 1]−2 T T ⎝T ⎠
In the equations, n is the number of atoms per formula unit, ΘD and ΘE stand for Debye and Einstein temperature, respectively. The obtained from the fit parameters are: ΘD = 190(5) K, ΘE = 54(8) K and k = 0.94(2). The Debye and Einstein temperatures are very close to those we estimated from the low temperature analysis as is shown in Fig. 4(b) and (c). In Fig. 4(a) the solid and dashed red lines represent the lattice contribution described by Debye and Einstein model, respectively. It is clear that the Debye phonon contribution to the specific heat is dominating and the theoretical calculations should shed more light on this problem.
Fig. 3. (a) The LeBail refinement powder X-ray diffraction pattern for La15MoxGe9. The red points, black line and blue line represent the experimental data (Iobs), calculated intensities (Icalc) and difference between Iobs and Icalc, respectively. The green marks show the expected Bragg reflections for the space group P63mc (#186). On panel (b) the black stars show supercell reflection in this structure. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
4. Summary We successfully synthesized a new intermetallic compound 615
Chemical Physics Letters 730 (2019) 612–616
J. Blawat, et al.
Fig. 4. Heat capacity versus temperature for La15MoxGe9 (a) the solid blue line shows the fit to the combined Debye (solid red line) and Einstein model (dashed red line) (b) C/T versus T2 and the solid blue line is the fit to the equation C/T = γ + βT2 (c) specific heat plotted as C/T3 vs T to reveal the Einstein temperature. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
La15MoxGe9. The crystal structure was studied using single and powder X-ray diffraction. The new compound crystallizes in a hexagonal structure P63mc (#186) with partially occupied Mo position. Moreover, Mo occupies two inequivalent Wyckoff sites. Despite the fact that the compound contains two superconducting elements (La and Mo), La15MoxGe9 does not show any superconductivity down to 1.85 K. It is worth noting that it is the first compound in the ternary La-Mo-Ge system.
jallcom.2011.03.092. [6] B. Lv, X.Y. Zhu, B. Lorenz, F.Y. Wei, Y.Y. Xue, Z.P. Yin, G. Kotliar, C.W. Chu, Superconductivity in the Mn5Si3-type Zr5Sb3 system, Phys. Rev. B. 88 (2013) 134520, , https://doi.org/10.1103/PhysRevB.88.134520. [7] L.S. Xie, L.M. Schoop, E.M. Seibel, Q.D. Gibson, W. Xie, R.J. Cava, A new form of Ca3P2 with a ring of Dirac nodes, APL Mater. 3 (2015) 083602, https://doi.org/10. 1063/1.4926545. [8] J.D. Corbett, E. Garcia, A.M. Guloy, W.-M. Hurng, Y.-U. Kwon, E.A. Leon-Escamilla, Widespread interstitial chemistry of Mn5Si3-type and related phases. Hidden impurities and opportunities, Chem. Mater. 10 (1998) 2824–2836, https://doi.org/10. 1021/cm980223c. [9] A.M. Guloy, J.D. Corbett, The lanthanum-germanium system. Nineteen isostructural interstitial compounds of the La5Ge3 host, Inorg. Chem. 32 (1993) 3532–3540, https://doi.org/10.1021/ic00068a025. [10] A.M. Guloy, J.D. Corbett, La15Ge9Z interstitial derivatives with an ordered superstructure of the Mn5Si3 structure type. Property trends in a series of homologous intermetallic phases, Inorg. Chem. 35 (1996) 4669–4675, https://doi.org/10.1021/ ic9515158. [11] S. Tencé, O. Isnard, F. Wrubl, P. Manfrinetti, A neutron diffraction study of the R15Ge9C compounds (R=Ce, Pr, Nd), J. Alloys Compd. 594 (2014) 148–152, https://doi.org/10.1016/j.jallcom.2014.01.115. [12] G. Nam, H. Jo, K.M. Ok, J. Kim, T.-S. You, Crystal structure, 7Li NMR, and structural relationship of two rare-earth metal richer polar intermetallics: La15Ge9Li1.50(16) and La7Ge3: structural relationship of La15Ge9Li1.50 and La7Ge3, Bull. Korean Chem. Soc. 37 (2016) 1344–1353, https://doi.org/10.1002/bkcs.10872. [13] J. Bławat, M. Roman, W. Xie, R.J. Cava, T. Klimczuk, La15NbxGe9: a superstructure of the Mn5Si3 structure type with interstitial Nb atoms, J. Solid State Chem. 265 (2018) 50–54, https://doi.org/10.1016/j.jssc.2018.05.007. [14] H.M. Rietveld, A profile refinement method for nuclear and magnetic structures, J. Appl. Crystallogr. 2 (1969) 65–71, https://doi.org/10.1107/S0021889869006558. [15] J. Rodríguez-Carvajal, Recent advances in magnetic structure determination by neutron powder diffraction, Phys. B Condens. Matter. 192 (1993) 55–69, https:// doi.org/10.1016/0921-4526(93)90108-I. [16] G.M. Sheldrick, Crystal structure refinement with SHELXL, Acta Crystallogr. Sect. C Struct. Chem. 71 (2015) 3–8, https://doi.org/10.1107/S2053229614024218. [17] K. Momma, F. Izumi, VESTA 3 for three-dimensional visualization of crystal, volumetric and morphology data, J. Appl. Crystallogr. 44 (2011) 1272–1276, https:// doi.org/10.1107/S0021889811038970. [18] E.S.R. Gopal, Specific Heats at Low Temperatures, Springer US, Boston, MA, 1966, , https://doi.org/10.1007/978-1-4684-9081-7. [19] T. Klimczuk, V.A. Sidorov, A. Szajek, M. Werwiński, S.A.J. Kimber, A.L. Kozub, D. Safarik, J.D. Thompson, R.J. Cava, Structure and paramagnetism in weakly correlated Y8Co5, J. Phys. Condens. Matter. 25 (2013) 125701, https://doi.org/10. 1088/0953-8984/25/12/125701.
Declaration of Competing Interest The authors declared that there is no conflict of interest. Acknowledgements This project was supported by National Science Centre (Poland), grant number: UMO-2016/22/M/ST5/00435. The work at LSU was supported by Beckman Young Investigator (BYI) program. References [1] G.A. Papoian, R. Hoffmann, Hypervalent bonding in one, two, and three dimensions: extending the Zintl-Klemm concept to nonclassical electron-rich networks, Angew. Chem. Int. Ed. 39 (2000) 2408–2448, https://doi.org/10.1002/15213773(20000717)39:14<2408::AID-ANIE2408>3.0.CO;2-U. [2] D.C. Fredrickson, Electronic packing frustration in complex intermetallic structures: the role of chemical pressure in Ca2Ag7, J. Am. Chem. Soc. 133 (2011) 10070–10073, https://doi.org/10.1021/ja203944a. [3] W. Xie, H. Luo, B.F. Phelan, T. Klimczuk, F.A. Cevallos, R.J. Cava, Endohedral gallide cluster superconductors and superconductivity in ReGa5, Proc. Natl. Acad. Sci. 112 (2015) E7048–E7054, https://doi.org/10.1073/pnas.1522191112. [4] A. Marcinkova, C. de la Cruz, J. Yip, L.L. Zhao, J.K. Wang, E. Svanidze, E. Morosan, Strong magnetic coupling in the hexagonal R5Pb3 compounds (R=Gd–Tm), J. Magn. Magn. Mater. 384 (2015) 192–203, https://doi.org/10.1016/j.jmmm.2015. 02.015. [5] F. Wrubl, K.V. Shah, D.A. Joshi, P. Manfrinetti, M. Pani, C. Ritter, S.K. Dhar, Superstructure and magnetic properties of R15X9C compounds (R=rare earth; X=Si and Ge), J. Alloys Compd. 509 (2011) 6509–6517, https://doi.org/10.1016/j.
616
Contents lists available at ScienceDirect
Chemical Physics Letters journal homepage: www.elsevier.com/locate/cplett
Research paper
Crystal structure and physical properties of a novel ternary compound La15MoxGe9 Joanna Blawata,
⁎,1
, Weiwei Xieb, Robert J. Cavac, Tomasz Klimczuka,
T
⁎
a
Faculty of Applied Physics and Mathematics, Gdansk University of Technology, Narutowicza 11/12, 80-233 Gdansk, Poland Department of Chemistry, Louisiana State University, Baton Rouge, LA 70803, USA c Department of Chemistry, Princeton University, Princeton, NJ 08544, USA b
H I GH L IG H T S
polycrystalline sample of the new compound La MoGe was synthesized. • The crystallizes in a new version of Mn Si -type structure. • LaLa MoGe is the first compound in ternary system La-Mo-Ge. • The MoGe • specific heat measurement shows no phase transition down to 2 K. 15
15
9
15
9
5
9
3
A B S T R A C T
We present the synthesis, structural characterization and physical properties measurements of a new compound, La15MoxGe9 in Mn5Si3-type derived structure with interstitial Mo. The sample was synthesized by arc-melting method. La15MoxGe9 crystallizes in non-centrosymmetric hexagonal structure P63mc (#186) with lattice parameters a = b = 15.495(5) Å and c = 6.917(2) Å. The refinement on single X-ray diffraction data show that interstitial atom Mo occupies two different Wyckoff positions (2a, 2b) with 0.19(2) Mo/per f.u. The specific heat properties were well studied. The obtained Sommerfeld coefficient and Debye temperature are: γ = 89.2(9) mJ/(mol K2) and ΘD = 190(5) K, respectively. La15MoxGe9 is the first compound in the ternary system La-Mo-Ge.
1. Introduction Understanding the relationship between crystal structure and physical properties remains challenging especially for complex intermetallic compounds. Due to a huge variety of combining elements and crystallographic structures, the prediction of the chemical stability and physical properties of a new compound are extremely difficult and usually based on some empirical rules. One of the approaches to a new material with targeted property is choosing a crystal structure which is favorable for some specific feature and tuning the electronic instabilities in a compound [1–3]. Hexagonal Mn5Si3-type structure has been intriguing for solid state materials community for decades due to the various interesting properties and applications, such as different kinds of magnetic ordering like antiferromagnetism or ferromagnetism [4,5], the superconductivity in Zr5Sb3 [6], and the Dirac semimetal state in Ca3P2 [7]. Structurally, the main component of its hexagonal unit cell (space group P63/mcm, 193) with a chain of octahedra provides sufficient
space to accommodate another atom inside. This encapsulation can cause changes in lattice parameters and interatomic distances without deconstruction of the crystal structure. That structural flexibility offers an ideal platform for synthesis of many new compounds with a wide range of interstitial atoms and thus also for tuning physical properties among the same crystal structure (P63/mcm, 193) [8]. Especially interesting seems to be a La5Ge3Z family, where Corbett and Guloy successfully synthesized new compounds using nineteen different interstitial atom with a wide range from p- to d-blocks [9]. A few years later, they showed that La5Ge3Z has another intriguing feature – the La5Ga3 can encapsulate atoms not into all octahedra but only into one third of them and form a new family with stoichiometry La15Ge9Z (=La5Ge3Z1/ 3). In this way, they successfully synthesized nine new compounds, which are gathered in Fig. 1(a) [10]. Moreover, they also observed the loss of the inversion center in the superlattice compounds – all La15Ge9Z-members crystallize in noncentrosymmetric hexagonal structure P63mc (#186). What is more, another unusual feature reported by Corbett and Guloy was a fact that it is possible to synthesize a
⁎
Corresponding authors. E-mail addresses: [email protected] (J. Blawat), [email protected] (T. Klimczuk). 1 Current affiliation: Department of Chemistry, Louisiana State University, Baton Rouge, LA 70803, USA. Department of Physics and Astronomy, Louisiana State University, Baton Rouge, LA 70803, USA. https://doi.org/10.1016/j.cplett.2019.06.065 Received 9 March 2019; Received in revised form 23 June 2019; Accepted 25 June 2019 Available online 26 June 2019 0009-2614/ © 2019 Elsevier B.V. All rights reserved.
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Fig. 1. (a) The summary of reported compounds in La15ZGe9 family, where Z is an interstitial atom. Green color means compounds which Corbett and Guloy synthesized, red color represent these Z atoms for which they reported the unsuccessful synthesis and the blue color shows compounds in La15ZGe9 type structure with atom Z in two different Wyckoff positions. (b) The chains of octahedral going by 2a and 2b sites with measured distances between La atoms for La15MoxGe9, La15NbxGe9 and La15LixGe9. The drawings and measurements were done using Vesta software [17]. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
Bruker D8 diffractometer with Cu Kα1 radiation (λ = 1.5406 Å). The results were analyzed by means of LeBail refinement [14] using a FullProf software [15]. Peak shapes were modelled with the Thompson–Cox–Hastings pseudo–Voight profile convoluted with axial divergence asymmetry. The sample with nominal stoichiometry La15Mo0.5Ge9 had the highest purity and hence we used it for further measurements. Small pieces of the polycrystalline sample were mounted on a kapton loop and examined by single X-ray diffraction method on a Bruker Apex II diffractometer with Mo radiation Kα (λ = 0.71073 Å). The crystal structure was refined with SHELTX [16] package using direct methods and refined by full-matrix least-squares on F2. The specific heat was performed in Quantum Design Physical Properties Measurement System (PPMS) by means of a standard 2τ relaxation method. The polished sample was studied in temperature range 2–300 K without applied magnetic field.
new compound with some substoichiometric amount of interstitial atoms if Z is a small element from p-block (for example B, O, C) [8–10]. On the other hand, they claimed that atoms Z always occupy 2b site (2/ 3, 1/3, z) solely. But recently, a few new compounds in RE15Ge9C family, where RE is rare earth metal have been reported and the neutron diffraction data showed that for two of them (Ce15Ge9C and Pr15Ge9C) carbon also occupies 2a site (0, 0, z). The occupancy of 2a site is significantly lower than for 2b site, but the existence cannot be ignored [5,11]. This fact also points out to additional instability in crystal and electronic structure in La15Ge9Z family. Recently, a new member La15Ge9Li1.50 was discovered, which extended the interstitial Z to sblock with occupying both 2a and 2b sites [12]. Similarly, 2b site is fully occupied while 2a is partially occupied. It is worth noting that a La15Ge9Z – type compounds can encapsulate higher amount of interstitial atoms. Recently we reported a new compound La15NbxGe9, where Nb also occupy two different Wyckoff positions, and we showed that it is possible to synthesize a new compound with significantly bigger interstitial atom Z [13]. In that case, our motivation was also looking for new superconducting materials using superconducting elements in a superconductivity-favored structure [3]. Herein, we synthesized a new compound with Mo as the interstitial Z. The heat capacity measurements show no superconductivity above 1.85 K, which is similar with La15NbxGe9.
3. Results and discussions 3.1. Crystal structure The single X-ray diffraction refinement reveals that La15MoxGe9 crystallizes in non-centrosymmetric hexagonal space group P63mc (#186) with lattice parameters a = b = 15.495(5) Å and c = 6.917(2) Å [unit cell volume V = 1438(1) Å3], slightly larger than we obtained for La15NbxGe9 [13]. All the details of collecting data and refinement are presented in Tables 1 and 2. The crystal structure is displayed in Fig. 2. The main components of La15MoxGe9 crystal structure are chains of octahedra formed by lanthanum atoms. One of them is built by La2 and La3 and is going by Wyckoff position 2a (0, 0, z), while the second one is surrounded by La1 and centered on 2b site (2/3, 1/3, z). The occupancies have been refined to obtain the reasonable R1 and wR2 and electron density residuals peaks and holes close to zero. All of these lanthanum octahedra are bridged by germanium atoms. The La4 atoms form honeycomb structures around the chains in ab-plane and a zig-zag line along the crystallographic c-axis shown in Fig. 2. In Fig. 1(b) we compare dimensions of the octahedra for all known
2. Experimental parts The polycrystalline sample of La15MoxGe9 was synthesized using arc-melting method under a high-purity Zr-gettered argon atmosphere. Lanthanum (99.9%), molybdenum (99.9%) and germanium pieces (99.99%) pure metals were melted together in stoichiometry 15:1:9 and 15:0.5:9 (x = 0.5 and 1). The buttons were flipped over and re-melted 4 times to promote homogeneity. The total mass of the sample did not exceed 300 mg, and the weight losses were lower than 1%. The as-cast buttons were wrapped in tantalum foil and annealed in a high temperature high vacuum furnace at 1180 °C for 2 days. Powder X-ray diffraction measurements were performed using a 613
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Table 1 Single crystal La15Mo0.19Ge9.
crystallographic
data
and
structure
refinement
Formula
La15Mo0.19Ge9
F. W. (g/mol) Space group, Z a (Å) b (Å) c (Å) V (Å 3) Absorption correction Extinction coefficient Θ range (deg) hkl ranges
2755.67 P63mc (#186), 2 15.495(5) 15.495(5) 6.917(2) 1438(1) Numerical 0.00034(7) 2.629–33.129 −21 ≤ h ≤ 20 −23 ≤ k ≤ 23 −10 ≤ l ≤ 8 1818, 0.0794 1259 53 0.0474, 0.0939 0.979 1.952, −3.857
No. reflections, Rint No. independent reflections No. parameters R1, wR2 (all I) Goodness of fit Largest diff. peak and hole (e−/Å3)
Table 3 Comparison between octahedral volumes for three compounds: La15Mo0.19Ge9, La15Nb0.12Ge9 and La15Li1.5Ge9. All volumes were measured using VESTA software [17]. Crystallographic data are from Refs. [13] and [12] for La15Nb0.12Ge9 and La15Li1.5Ge9, respectively.
for
Wyckoff
Occupancy
x
y
z
Ueq
La1 La2 La3 La4 Ge1 Ge2 Ge3 Mo1 Mo2
6c 6c 6c 12d 6c 6c 6c 2b 2a
1 1 1 1 1 1 1 0.0432 0.1490
0.9189(2) 0.2522 (1) 0.5854(1) 0.0169 (1) 0.7996(3) 0.1322(4) 0.4658 (2) 1/3 0
0.0811(1) 0.7478(1) 0.4146(1) 0.3416(7) 0.2004(2) 0.8679(2) 0.5341(3) 2/3 0
0.2432(3) 0.1938(3) 0.2253(3) 0.4668(7) 0.2551(8) 0.1966(10) 0.2072(2) 0.0020(1) 0.1620(9)
0.0117(3) 0.0134(1) 0.0010(1) 0.0236(1) 0.0103(7) 0.0125(2) 0.0098(6) 0.0077(4) 0.1628(1)
La15Mo0.19Ge9
La15Nb0.12Ge9
La15Li1.5Ge9
Octahedra with Z atom in 2a site Octahedra with Z atom in 2b site Octahedra without Z atom in 2b site
28.41 26.63 30.22
28.06 26.57 29.55
28.61 27.04 30.21
Table 4 Selected interatomic distances for La15Mo0.19Ge9.
Table 2 Atomic coordinates and isotropic displacement parameters of La15Mo0.19Ge9. Ueq is defined as one-third of the trace of the orthogonalized Uij tensor (Å2). Atom
Volume (Å3)
Atom 1
Atom 2
Distance (Å)
La 1 La1 La1 La1 La1 La2 La2 La2 La2 La2 La2 La3 La3 La3 La3 La3 La3 La4 La4 La4 La4 La4 La4 La4
La1 Ge1 Ge2 La4 Mo2 La2 Ge1 Ge2 Ge3 Mo1 La4 La2 La3 Ge1 Ge3 Mo1 La4 La4 Ge1 Ge1 Ge2 Ge2 Ge3 Ge3
3.772(5) 3.201(5) 3.115(4) 3.890(3) 2.75(4) 3.770(5) 3.338(6) 3.222(6) 3.107(4) 2.55(3) 3.745(3) 3.905(3) 3.776(4) 3.122(4) 3.211(6) 2.90(3) 3.761(3) 3.488(1) 3.208(5) 3.302(4) 3.202(5) 3.301(4) 3.222(5) 3.279(5)
Fig. 2. Crystal structure of La15MoxGe9. The drawings were made using VESTA software [17] with crystallographic data obtained from single X-ray diffraction refinement. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.) 614
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niobium atoms and hence the volume of octahedra is bigger. The purity of synthesized sample was confirmed by powder X-ray diffraction measurements. Fig. 3(a) shows the LeBail refinement of the PXRD pattern for La15MoxGe9. The analysis confirms that this compound crystallizes in a hexagonal crystal structure (P63mc #186) with lattice parameters a = b = 15.5041(7) Å and c = 6.9198(6) Å. The obtained figures of merit are Rp = 9.7%, Rwp = 13.2%, Rexp = 9.97% and χ2 = 1.75. In Fig. 3(b) are shown superstructure reflections which are characteristic for the La15ZGe9-type structure.
compounds in the group La15ZxGe9 in which atom Z occupies two different Wyckoff positions and in Table 3 are gathered the volumes of octahedra, while the main interatomic distances for La15MoxGe9 are gathered in Table 4. The measurements were performed using the Vesta software [17]. The distances for La15MoxGe9 between La1-La1 and La2La2 along a-b direction are almost the same and equal to 3.772(5) Å and 3.770(5) Å, respectively, but in dimensions along c direction we can observe differences. Distance between La1-La1 is equal to 4.087(2) Å, while distances between La2-La3 are 3.905(3) Å and 4.273(3) Å. According to the symmetry in La15ZGe9-type structure compounds only one third of octahedra formed by La2 and La3 atoms can encapsulate interstitial atoms, so the differences between distances along the c direction could be caused by chemical pressure. The same dependence is observed for La15NbxGe9 and La15LixGe9. It is worth noting that all volumes of octahedra are the biggest for La15LixGe9 (see Table 3) and the smallest for La15NbxGe9, while the atomic radii for Li is the smallest, and the biggest for Nb. It is expected that the biggest atom will make the biggest volume of octahedra, but besides the atomic radii the occupancy of the interstitial atoms should also be considered. The compound with Li contains significantly larger occupancy of the interstitial atoms than the compounds with Mo and Nb. It is also worth noting that atomic radii for Mo is slightly smaller than for Nb, but it caused that the La15ZGe9-type unit cell can encapsulate more molybdenum than
3.2. Heat capacity The heat capacity measurements were performed to reveal the physical properties of the new compound La15MoxGe9. Fig. 4(a) shows the temperature dependence of heat capacity in the range 2–300 K. At room temperature the heat capacity reaches the value expected by Dulong-Petit law 3nR = 611 J/mol K, where R is a gas constant (R = 8.31 J/mol K) and n is the number of atoms per formula unit. Low temperature data of heat capacity divided by T (C/T) versus T2 are presented in Fig. 4(b) and the fit to the equation C/T = γ + βT 2 (blue line) where γ is the Sommerfeld coefficient, and β is the phonon specific heat parameter. The first term γT represents an electronic contribution (Cel) and βT 3 is the phonon contribution (Cph) to the specific heat [18]. The obtained parameters are: γ = 89.2(9) mJ/(mol K2), and β = 7.5(1) mJ/(mol K4) is related with Debye temperature through the formula:
θD =
3
12π 4nR 5β
The Debye temperature was estimated to be θD = 188 K. The obtained value of the Sommerfeld coefficient is larger than reported for the simple metals, however when we consider the large number of atoms (per formula unit) we estimated γ = 89.2(9) mJ/(mol K2) = 3.6 mJ/(mol-at K2). Similarly high value of the Sommerfeld coefficient was found for nonmagnetic intermetallic compounds: La15NbxGe9 (2.6 mJ/(mol-at K2)) [13] and Y8Co5 (4.8 mJ/(mol-at K2)) [19]. Fig. 4(c) presents Cph/T3 versus T to estimate the Einstein temperature (θE). A broad local maximum is found at Tmax ∼ 11 K, which gives the Einstein temperature value of θE ∼ 5Tmax = 55 K. The fit to the overall temperature dependence of heat capacity (Fig. 4(a)) were done using these estimated values of Debye and Einstein temperatures. The blue line in Fig. 4(a) represents the fit of the data by: C (T ) = γT + kCD (T )+(1 − k ) CE (T ) , where γT is the contribution due to conduction electrons and the next two terms stand for lattice contribution described by Einstein (CE) and Debye (CD) models and the parameter k is a mutual weight of phonon modes of the two types:
CD (T )=9nR (
T 3 ) ΘD
CE (T ) = 3nR (
∫0
ΘD
x 4 e x dx (e x − 1)2
ΘE 2 Θ Θ ) exp( E )[exp ⎛ E ⎞ − 1]−2 T T ⎝T ⎠
In the equations, n is the number of atoms per formula unit, ΘD and ΘE stand for Debye and Einstein temperature, respectively. The obtained from the fit parameters are: ΘD = 190(5) K, ΘE = 54(8) K and k = 0.94(2). The Debye and Einstein temperatures are very close to those we estimated from the low temperature analysis as is shown in Fig. 4(b) and (c). In Fig. 4(a) the solid and dashed red lines represent the lattice contribution described by Debye and Einstein model, respectively. It is clear that the Debye phonon contribution to the specific heat is dominating and the theoretical calculations should shed more light on this problem.
Fig. 3. (a) The LeBail refinement powder X-ray diffraction pattern for La15MoxGe9. The red points, black line and blue line represent the experimental data (Iobs), calculated intensities (Icalc) and difference between Iobs and Icalc, respectively. The green marks show the expected Bragg reflections for the space group P63mc (#186). On panel (b) the black stars show supercell reflection in this structure. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
4. Summary We successfully synthesized a new intermetallic compound 615
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Fig. 4. Heat capacity versus temperature for La15MoxGe9 (a) the solid blue line shows the fit to the combined Debye (solid red line) and Einstein model (dashed red line) (b) C/T versus T2 and the solid blue line is the fit to the equation C/T = γ + βT2 (c) specific heat plotted as C/T3 vs T to reveal the Einstein temperature. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
La15MoxGe9. The crystal structure was studied using single and powder X-ray diffraction. The new compound crystallizes in a hexagonal structure P63mc (#186) with partially occupied Mo position. Moreover, Mo occupies two inequivalent Wyckoff sites. Despite the fact that the compound contains two superconducting elements (La and Mo), La15MoxGe9 does not show any superconductivity down to 1.85 K. It is worth noting that it is the first compound in the ternary La-Mo-Ge system.
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