Two novel Zintl compounds Na12Ge8Sn and Na15Ge8SnP: Single crystal and electronic structures

Two novel Zintl compounds Na12Ge8Sn and Na15Ge8SnP: Single crystal and electronic structures

Available online at www.sciencedirect.com Solid State Sciences 10 (2008) 525e532 www.elsevier.com/locate/ssscie Two novel Zintl compounds Na12Ge8Sn ...

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Available online at www.sciencedirect.com

Solid State Sciences 10 (2008) 525e532 www.elsevier.com/locate/ssscie

Two novel Zintl compounds Na12Ge8Sn and Na15Ge8SnP: Single crystal and electronic structures Yongkwan Dong, Chinmoy Ranjan, Francis J. DiSalvo* Baker Laboratory, Department of Chemistry and Chemical Biology, Cornell University, Ithaca, NY 14853-1301, USA Received 2 August 2007; received in revised form 8 October 2007; accepted 11 October 2007 Available online 22 October 2007

Abstract Two mixed anionic Zintl phases, Na12Ge8Sn and Na15Ge8SnP, have been synthesized and structurally characterized. Na12Ge8Sn crystallizes ˚ , c ¼ 21.8483(7) A ˚ , V ¼ 4354.3(2) A ˚ 3, Z ¼ 8, and R/wR ¼ 0.0481/0.1213) and Na15in the tetragonal space group P41212 (#92, a ¼ 14.1172(2) A ˚ , b ¼ 12.890(3) A ˚ , c ¼ 11.111(2) A ˚ , V ¼ 2508.7(9) A ˚ 3, Z ¼ 4, Ge8SnP crystallizes in the orthorhombic space group Cmcm (#63, a ¼ 17.517(4) A þ 4 and R/wR ¼ 0.0313/0.0669). Both compounds adopt new structure types and are made up of isolated Na cations, anionic (Ge4) tetrahedral clusters, and Sn4 or P3 anions. The structure of Na12Ge8Sn can be described as a square packing of two kinds of columns, highly distorted Sn centered Na icosahedra chains ð1N ½Na9 Sn5þ Þ and highly distorted Na(1) centered anti-rectangular prism (four (Ge4)4 and four Na, 5 8þ 1 1 saw tooth-like chains, which are made up of N ½Na3 ðGe4 Þ2  ) chains. The structure of Na15Ge8SnP is composed of cationic N ½Na15 SnP Sn centered Na bicapped hexagonal prisms and P centered tricapped trigonal prisms, extending along the c axis with anionic (Ge4)4 tetrahedral clusters between the chains. The electronic structures have been studied using density functional theory. Ó 2007 Elsevier Masson SAS. All rights reserved. Keywords: Zintl phase; Crystal and electronic structure; X-ray diffraction

1. Introduction A few sodium containing nitrido-germanates and -phosphates have been recently reported (NaGe2N3, NaPN2, NaP4N7, and Na3P6N11) [1e4]. While condensed PN4 tetrahedral building units are found in nitrido-phosphates, multiple types of GeeN units are known, such as dumbbell (GeN4 2 ) [5], tetrahedral (GeN8 ) [1,6], and edge-sharing bow-tie type (Ge2N10 4 6 ) [7] units. On the other hand, most gallium or indium compounds containing electropositive metals (mainly group 2) form isolated Zintl anions instead of cations bonded to N anions [8e15]. These Zintl anions vary from isolated M (Ga or In) atoms to threedimensional networks of corner-sharing tetracapped tetrahedra. In contrast, NaSnN shows a novel layered Zintl anion [SnN] with SneN bonding [16]. For these reasons, we were interested to explore the chemistry of similar Ge containing Zintl anions. * Corresponding author. Tel.: þ1 607 255 7238; fax: þ1 607 255 4137. E-mail address: [email protected] (F.J. DiSalvo). 1293-2558/$ - see front matter Ó 2007 Elsevier Masson SAS. All rights reserved. doi:10.1016/j.solidstatesciences.2007.10.016

Zintl compounds composed of group 13 (triels), group 14 (tetrels), and electropositive elements such as alkali or alkaline-earth metals have been studied because of their structural diversity and interesting chemical bonding [17e19]. Among them, NaeSn and NaeGe binary phases have been extensively investigated by crystallographic, theoretic, and vibrational methods [20e28]. However, while heteroanionic systems (oxychalcogenides, oxynitrides, chalcogenide nitrides, oxyhalides, etc.) are well known, mixed anionic Zintl phases are relatively few [29e32]. No NaeGeeSn ternary or NaeGeeSneP quaternary compounds have been previously reported. The title compounds were discovered in attempts to prepare germanium-containing nitrides. As is sometimes found in such nitride syntheses, the phases formed contain no nitrogen. However, in this case we found compounds that contain two separate metal based anions: isolated anionic (Ge4)4 tetrahedral clusters and Sn4 anions. Since these phases are structurally interesting, we present here the single crystal and electronic structures of Na12Ge8Sn and Na15Ge8SnP.

Y. Dong et al. / Solid State Sciences 10 (2008) 525e532

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2. Experimental section 2.1. Synthesis All manipulations were performed in an argon-filled glovebox because the products were expected to be moisture sensitive. All chemicals were used as obtained: NaN3 powder (99%, Aldrich), Ge powder (99.999%, CERAC), red P powder (99.999%, JohnsonMatthey), Na (ACS reagent, stick, dry, Aldrich), and Sn (99.9%, JohnsonMatthey). The surface of Na was scraped to remove any surface oxides. Nb tubes were cleaned in a mixture of concentrated H2SO4, HNO3, and HF (45:40:15 by volume. Caution: This solution is extremely corrosive and contact with skin may cause severe burns and in extreme cases can even be fatal.) [33]. The reactants were sealed in a crimped Nb metal tube, using a Centorr Associates arc furnace. The Nb tubes were then sealed inside a silica tube under vacuum to protect them from oxidation during heat treatment. After completing the heating, the Nb tubes were cut open and excess Na was removed by sublimation for about 8 h at 340  C under dynamic vacuum. For Na12Ge8Sn, the starting materials of 0.0325 g of NaN3, 0.0363 g of Ge, 0.0920 g of Na and 0.0594 g of Sn (atomic ratio of Na/Ge/Sn was 10:1:1.) were loaded into a Nb tube which was made by welding one end shut in an argon atmosphere. The reaction tube was heated gradually to 700  C for 24 h and held at this temperature for 120 h. The tube was cooled to room temperature at 3  C/h over approximately 224 h. Irregular shaped black crystals were found in the reaction mixture.

Table 1 Details of X-ray data collection and refinement for Na12Ge8Sn and Na15Ge8SnP Formula weight, amu Space group ˚ a, A ˚ b, A ˚ c, A ˚3 V, A Z T, K Radiation Linear absorption coefficient, mm1 Density, calc. g/cm3 Crystal size, mm3 q limits,  No. of unique data with F2o No. of unique data with F2o > 2sðF2o Þ wR2ðF2o > 0Þ R1 (on Fo for F2o > 2sðF2o Þ) Goodness-of-fit on F2 Min. and Max. ˚ 3) residual e-density (e/A

Na12Ge8Sn 975.29 D44-P41212 (#92) 14.1172(2)

Na15Ge8SnP 1075.23 D17 2h-Cmcm (#63) 17.517(4) 12.890(3) 21.8483(7) 11.111(2) 4354.3(2) 2508.7(9) 8 4 165.0(1) 165.0(1) Graphite monochromated ˚) Mo Ka (l(Ka) ¼ 0.7107 A 12.226 10.731 2.975 0.17  0.09  0.07 1.72  q  30.72 6786 (Rint ¼ 0.0515)

2.847 0.09  0.07  0.04 1.96  q  34.61 2842 (Rint ¼ 0.0510)

5551

2023

0.1213 0.0481 1.084 2.268 and 5.097

0.0669 0.0313 1.024 1.706 and 0.888

For Na15Ge8SnP, the starting materials of 0.0325 g of NaN3, 0.0363 g of Ge, 0.0155 g of P, 0.1035 g of Na and 0.1187 g of Sn (atomic ratio of Na/Ge/P/Sn was 10:1:1:2) were loaded into a Nb tube. After sealing the remaining end of the Nb tube as mentioned above, the reaction tube was protected by an evacuated silica tube. The reaction tube was heated gradually to 650  C for 24 h and held at this temperature for 96 h. The tube was cooled to 100  C at 3  C/h and the furnace was turned off cooling to room temperature for approximately 12 h. Irregular shaped black crystals were found in the product mixture. 2.2. Microprobe analysis and preparation of bulk samples Microprobe analysis of the irregular shaped black crystals was made with an EDAX (Thermonoran) equipped scanning electron microscope (Jeol JXA-8900R). Since both compounds are highly moisture sensitive, they were transferred from an argon-filled glovebox to the microprobe using Table 2 ˚ 2) for Atomic coordinates and equivalent isotropic displacement parameters (A Na12Ge8Sn and Na15Ge8SnP Atoms Na12Ge8Sn Na(1) Na(2) Na(3) Na(4) Na(5) Na(6) Na(7) Na(8) Na(9) Na(10) Na(11) Na(12) Ge(1) Ge(2) Ge(3) Ge(4) Ge(5) Ge(6) Ge(7) Ge(8) Sn(1) Sn(2)

Wyckoff notation

x

y

z

Ueqa

8b 8b 8b 8b 8b 8b 8b 8b 8b 8b 8b 8b 8b 8b 8b 8b 8b 8b 8b 8b 4a 4a

0.0011(3) 0.0752(3) 0.0786(3) 0.1035(2) 0.1344(3) 0.1381(3) 0.1462(3) 0.3548(3) 0.3745(3) 0.3836(3) 0.4178(4) 0.4343(3) 0.0528(1) 0.1409(1) 0.1810(1) 0.2291(1) 0.2769(1) 0.3234(1) 0.3809(1) 0.4519(1) 0.0102(1) 0.5025(1)

0.4816(3) 0.2310(3) 0.2379(3) 0.0633(3) 0.3797(3) 0.0210(3) 0.1754(3) 0.3346(3) 0.4293(4) 0.1170(3) 0.2705(3) 0.2769(2) 0.2839(1) 0.4218(1) 0.3722(1) 0.2613(1) 0.2236(1) 0.0848(1) 0.0856(1) 0.2007(1) 0.0102(1) 0.5025(1)

0.2876(2) 0.0221(2) 0.4970(2) 0.3792(2) 0.1554(2) 0.1253(2) 0.2453(2) 0.2333(2) 0.3747(2) 0.3258(2) 0.4811(2) 0.0149(2) 0.3625(1) 0.4124(1) 0.3037(1) 0.3911(1) 0.1146(1) 0.1865(1) 0.0758(1) 0.1510(1) 0 0

0.038(1) 0.042(1) 0.049(1) 0.030(1) 0.038(1) 0.039(1) 0.040(1) 0.044(1) 0.053(1) 0.033(1) 0.054(1) 0.030(1) 0.028(1) 0.039(1) 0.031(1) 0.021(1) 0.021(1) 0.034(1) 0.025(1) 0.023(1) 0.016(1) 0.016(1)

0.1306(1) 0.1895(1) 0.0841(1) 0.4067(1) 0.2515(1) 0 0.3535(1) 0.2475(1) 0.2508(1) 0 0

0.4124(1) 0.1312(1) 0.1486(1) 0.3964(2) 0 0.5667(2) 0.1629(1) 0.2568(1) 0.0551(1) 0 0.3435(1)

0.1168(1) 0.0002(1) 0.2500 0.2500 0.0030(1) 0.2500 0.1366(1) 0.2500 0.2500 0 0.2500

0.023(1) 0.036(1) 0.032(1) 0.036(1) 0.021(1) 0.020(1) 0.015(1) 0.015(1) 0.015(1) 0.016(1) 0.013(1)

Na15Ge8SnP Na(1) 16h Na(2) 16h Na(3) 8g Na(4) 8g Na(5) 8f Na(6) 4c Ge(1) 16h Ge(2) 8g Ge(3) 8g Sn 4a P 4c a

Ueq is defined as one third of the trace of the orthogonalized Uij tensor.

Y. Dong et al. / Solid State Sciences 10 (2008) 525e532

a specially designed portable antechamber to prevent decomposition of the samples [34]. Analyses of the crystals indicated the presence of Na, Ge and Sn for Na12Ge8Sn and presence of Na, Ge, Sn, and P for Na15Ge8SnP. No other elements were detected with Z  10 in either compound. After the structure determination, a bulk powder sample of Na12Ge8Sn was obtained from a stoichiometric reaction of the elements. Na, Ge, and Sn were weighed out in a 12:8:1 molar ratio and loaded into a Nb tube which was welded closed. The Nb tube was sealed inside a silica tube under vacuum. The reaction tube was heated gradually to 700  C for 24 h, kept at this temperature for 120 h, and then cooled to room temperature by shutting the furnace power off. This bulk powder sample of Na12Ge8Sn was studied on a Scintag 2000 theta-theta diffractometer with Cu Ka radiation. Due to its air sensitive nature, the powder diffraction sample was prepared in an argon-filled glovebox and covered with Mylar film. The X-ray powder diffraction pattern could be completely fit with the pattern calculated from the Na12Ge8Sn crystal structure, with the exception of a few unidentified impurity peaks (<20% of strongest main peak). Four probe electrical resistivity measurements were carried out with a pellet that had been annealed at 650  C. The sample was found to be highly resistive with r > 20 MUcm. Polycrystalline single phase, Na15Ge8SnP, could not be prepared from a stoichiometric reaction of the elements; it likely precipitates from a nonstoichiometric melt at low temperature. 2.3. Crystallographic studies Samples of the reaction mixture were removed from the glovebox in polybutene oil for single-crystal selection. Black

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crystals were manually selected from the reaction mixture, mounted in a drop of polybutene oil sustained in a plastic loop, and placed onto the goniometer under a cold nitrogen stream that froze the polybutene oil thus keeping the crystals stationary and protected from oxygen and moisture in the air. Several crystals were selected to check crystal uniformity and the unit cell parameters for both compounds were determined. Finally we accepted the best quality crystals for data collection. Preliminary examination and data collection were performed on a Bruker X8 Apex II diffractometer equipped with 4K CCD detector and graphite-monochromatized Mo ˚ ). The initial tetragonal (Na12Ka radiation (l ¼ 0.7107 A Ge8Sn) and orthorhombic (Na15Ge8SnP) cell constants and orientation matrix were obtained by using APEX2 [35]. The program SAINT was used to integrate the data [36]. An empirical absorption correction was applied using SADABS [37]. The initial input files for solving the crystal structure were prepared by XPREP [36]. The distribution of normalized structure factor E-values (hE2  1i ¼ 0.781 for Na12Ge8Sn; 0.959 for Na15Ge8SnP) indicates that Na12Ge8Sn and Na15Ge8SnP should be noncentrosymmetric and centrosymmetric, respectively. The initial positions for all atoms were obtained by using direct methods of the SHELXS97 [38] and the structure was refined by full-matrix least-squares techniques with the use of the SHELXL97 [38] in the WinGX program package [39]. For Na12Ge8Sn, the final cycle of refinement performed on F2o with 6786 unique reflections afforded residuals wR2 ¼ 0.1213 and a conventional R index based on 5551 reflections having F2o > 2sðF2o Þ of 0.0481. The highest peak ˚ 3) and deepest hole (2.27 e/A ˚ 3) are located 0.83 A ˚ (5.10 e/A ˚ from Ge(1) and Ge(2), respectively, due to data and 0.67 A

Fig. 1. (a) A view of the crystal structure of Na12Ge8Sn along the c axis. (b) Anionic 1N ½Na3 ðGe4 Þ2 5 chain. (c) Cationic 1N ½Na9 Sn5þ chain. Na atoms are gray, Sn atoms are black, and Ge atoms are yellow-green.

528

Y. Dong et al. / Solid State Sciences 10 (2008) 525e532

Fig. 2. (a) Projection view of Na15Ge8SnP along the c axis. (b) Saw-like cationic 1N ½Na15 SnP8þ chains. Na atoms are gray, Sn atoms are black, P atoms are red, and Ge atoms are yellow-green.

truncation errors (2q z 62 ) and the empirical absorption correction that was used for this irregularly shaped crystal. For Na15Ge8SnP, the final cycle of refinement performed on F2o with 2842 unique reflections afforded residuals wR2 ¼ 0.0669 and a conventional R index based on 2023 reflections having ˚ 3) and deepF2o > 2sðF2o Þ of 0.0313. The highest peak (0.89 e/A 3 ˚ est hole (1.71 e/A ) are located near Na(2) and Sn, respectively, most likely due to data truncation errors (2q z 70 ). The atomic parameters were standardized by using the program STRUCTURE TIDY [40]. For both compounds, no additional symmetry was found using the ADDSYM [41] algorithm in the PLATON [42] program packages.

2.4. Electronic structure calculations GGA-PW91 [43,44] DFT based periodic calculations were carried out in a PAW [45,46] basis using VASP-4.6 package [47e50]. Plane wave cut off energy was fixed at 355 eV. All electronic iterations were converged within 105 eV and all the ionic relaxations were converged within 106 eV. The lattice parameters of the optimized geometry used to calculate band structure and density of states varied less than 1% from the experimental values (DFT optimized lattice parame˚ , b ¼ 14.22 A ˚ , c ¼ 22.01 A ˚ , a ¼ b ¼ g ¼ 90 ters: a ¼ 14.22 A ˚ ˚ , c ¼ 11.19 A ˚, for Na12Ge8Sn and a ¼ 17.64 A, b ¼ 12.98 A

Fig. 3. The coordination environments of each tetrahedral (Ge4)4 units. (a) Cluster A of Na12Ge8Sn. (b) Cluster B of Na12Ge8Sn. (c) Cluster of Na15Ge8SnP. Displacement ellipsoids are drawn at the 50% probability level.

Y. Dong et al. / Solid State Sciences 10 (2008) 525e532

a ¼ b ¼ g ¼ 90 for Na15Ge8SnP). Monkhorst Pack [51] grid of k-points (6  6  4 for Na12Ge8Sn; 4  6  6 for Na15Ge8SnP) was used to calculate the density of states. Wigner Seitz radii used to calculate projected density of states were ˚ for Na, 1.22 A ˚ for Ge, 1.57 A ˚ for Sn, and 1.23 A ˚ for P. 1.76 A 3. Results and discussion Attempts to grow single crystals of nitrides in the NaeGee SneN or NaeGeePeSneN systems from metallic alloy Na/ Sn reactive fluxes resulted in the formation of nitrogen gas and the growth of single crystals of new ternary and quaternary mixed anionic Zintl compounds, Na12Ge8Sn and Na15Ge8SnP. Crystallographic details, fractional atomic coordinates, and equivalent isotropic displacement parameters are given in Tables 1 and 2, respectively. Na12Ge8Sn crystallizes in a new structure type (tP168) in the noncentrosymmetric tetragonal space group P41212, with 12 independent positions for Na (8b), 8 for Ge (8b), and 2 for Sn (4a) atoms and 8 formula units per cell. The crystal structure consists of isolated Naþ cations, anionic (Ge4)4 tetrahedral clusters, and Sn4 anions. Fig. 1(a) shows the crystal structure of Na12Ge8Sn, which can be visualized as containing two kinds of chains ð1N ½Na9 Sn5þ and 1N ½Na3 ðGe4 Þ2 5 Þ running parallel to the c axis. In the anionic 1N ½Na3 ðGe4 Þ2 5 chain, the Na(1) atom is surrounded by four (Ge4)4 tetrahedral clusters and four Na (two Na(5) and two Na(10)) atoms in a highly distorted anti-rectangular prism (Fig. 1(b)). These Na(1) centered anti-rectangular prisms share opposite rectangular faces which are composed of two (Ge4)4 tetrahedral and two Na atoms and extend along the c axis with a 41 screw symmetry. In the cationic 1N ½Na9 Sn5þ chain, Sn(1) and Sn(2) are surrounded by 12 Na atoms in a highly distorted Na icosahedral geometry (Fig. 1(c)). These polyhedra share trigonal faces to make a one-dimensional chain along the c axis. Each Sn centered Na icosahedron is surrounded by 8 (Ge4)4 and 8 Na atoms, which come from four 5 1 neighbors, in a distorted anti-octagonal prism. N ½Na3 ðGe4 Þ2  Na15Ge8SnP adopts a new structure type (oC100) and crystallizes in the centrosymmetric orthorhombic space group Cmcm, with six independent positions for Na (two 16h, two 8g, one 8f, and one 4c), three for Ge (one 16h and two 8g), one for Sn (4a), and one for P (4c) atoms. The structure of Na15Ge8SnP is composed of cationic 1N ½Na15 SnP8þ chains extending along the c axis with anionic (Ge4)4 tetrahedral clusters between the chains (Fig. 2(a)). In the cationic 1N ½Na15 SnP8þ chain, Sn atoms are surrounded by 14 Na atoms in a bicapped hexagonal prism, the two additional Na atoms cap the hexagonal faces. The P atoms are bonded to nine Na atoms in a tricapped trigonal prism. The Sn centered Na bicapped hexagonal prisms share their rectangular faces to make main a onedimensional chain along the c axis. P centered Na tricapped trigonal prisms attach to the Sn centered Na bicapped hexagonal prism chain on alternate sides to form a ‘‘saw tooth’’ pattern. Those saw tooth cationic 1N ½Na15 SnP8þ chains mesh with each other along the bc plane (Fig. 2(b)). The eight (Ge4)4 clusters surround the Sn chains in an anti-trapezoidal

529

arrangement. As shown in Fig. 2(b), there are empty sites (A), which are surrounded by eight Na atoms in a distorted cubic geometry, between two 1N ½Na15 SnP8þ chains. Since it is unusual to have large unoccupied holes in solid state structures, we examined the site potential at (0, 1/2, 0) assuming the usual ionic charges on the constituent atoms using EUTEX [52]. The site potential is close to zero (þ3.9 V), as would be expected for an unoccupied site. Normally, the potential at an occupied site is 10 V times the ion charge [53]. The main structural features of both compounds are very similar (isolated Naþ cations, anionic (Ge4)4 tetrahedral clusters, as well as isolated Sn4 and P3 anions). The addition of anionic P3 atoms to Na12Ge8Sn and three more Naþ cations for charge balance logically leads to the composition Na15 Ge8SnP. In Zintl chemistry, isolated tetrahedral geometries are common structural moieties in the three bonded species Table 3 ˚ ] and angles [ ] for Na12Ge8Sn and Na15Ge8SnP Selected bond lengths [A Na12Ge8Sn Ge(1)eGe(2) Ge(1)eGe(3) Ge(1)eGe(4) Ge(2)eGe(3) Ge(2)eGe(4) Ge(3)eGe(4)

2.5537(15) 2.5457(13) 2.5858(12) 2.5404(15) 2.6254(12) 2.5613(13)

Ge(5)eGe(6) Ge(5)eGe(7) Ge(5)eGe(8) Ge(6)eGe(7) Ge(6)eGe(8) Ge(7)eGe(8)

2.5967(13) 2.5825(12) 2.6144(11) 2.5510(14) 2.5629(13) 2.5179(12)

Sn(1)eNa(2)  2 Sn(1)eNa(3)  2 Sn(1)eNa(4)  2 Sn(1)eNa(6)  2 Sn(1)eNa(8)  2 Sn(1)eNa(9)  2

3.285(4) 3.721(5) 3.259(4) 3.283(4) 3.149(4) 3.373(4)

Sn(2)eNa(4)  2 Sn(2)eNa(6)  2 Sn(2)eNa(7)  2 Sn(2)eNa(9)  2 Sn(2)eNa(11)  2 Sn(2)eNa(12)  2

3.308(4) 3.380(4) 3.221(4) 3.382(4) 3.423(5) 3.342(4)

Ge(2)eGe(1)eGe(3) Ge(2)eGe(1)eGe(4) Ge(3)eGe(1)eGe(4)

59.76(4) 61.43(4) 59.88(4)

Ge(6)eGe(5)eGe(7) Ge(6)eGe(5)eGe(8) Ge(7)eGe(5)eGe(8)

59.02(4) 58.92(3) 57.96(3)

Ge(1)eGe(2)eGe(3) Ge(1)eGe(2)eGe(4) Ge(3)eGe(2)eGe(4)

59.96(4) 59.88(3) 59.42(4)

Ge(5)eGe(6)eGe(7) Ge(5)eGe(6)eGe(8) Ge(7)eGe(6)eGe(8)

60.21(3) 60.89(3) 58.99(3)

Ge(1)eGe(3)eGe(2) Ge(1)eGe(3)eGe(4) Ge(2)eGe(3)eGe(4)

60.28(4) 60.84(4) 61.94(4)

Ge(5)eGe(7)eGe(6) Ge(5)eGe(7)eGe(8) Ge(6)eGe(7)eGe(8)

60.77(4) 61.66(3) 60.74(4)

Ge(1)eGe(4)eGe(3) Ge(1)eGe(4)eGe(2) Ge(2)eGe(4)eGe(3)

59.28(4) 58.68(4) 58.64(4)

Ge(5)eGe(8)eGe(6) Ge(5)eGe(8)eGe(7) Ge(6)eGe(8)eGe(7)

60.20(3) 60.39(3) 60.27(4)

Na15Ge8SnP Ge(1)eGe(1)#1 Ge(1)eGe(2) Ge(1)eGe(3)

2.5201(7) 2.5494(5) 2.5996(5)

Ge(2)eGe(1)#1 Ge(2)eGe(3) Ge(3)eGe(1)#1

2.5494(5) 2.6000(8) 2.5996(5)

SneNa(2)  4 SneNa(3)  4 SneNa(4)  4 SneNa(5)  2

3.7258(16) 3.6817(13) 3.4883(12) 3.2417(17)

PeNa(1)  4 PeNa(3)  2 PeNa(5)  2 PeNa(6)

2.8650(12) 2.913(2) 2.9898(17) 2.8770(25)

Ge(1)#1eGe(1)eGe(2) Ge(1)#1eGe(1)eGe(3) Ge(2)eGe(1)eGe(3)

60.379(10) 61.006(9) 60.647(18)

Ge(1)#1eGe(2)eGe(1) Ge(1)#1eGe(2)eGe(3) Ge(1)eGe(2)eGe(3)

59.242(19) 60.632(13) 60.632(13)

Ge(1)#1eGe(3)eGe(1) Ge(1)#1eGe(3)eGe(2) Ge(1)eGe(3)eGe(2)

57.988(19) 58.721(14) 58.721(14)

Symmetry transformations for Na15Ge8SnP: #1 x, y, z þ 1/2.

Y. Dong et al. / Solid State Sciences 10 (2008) 525e532

530

a

b

Na12Ge8Sn

Na15Ge8SnP

1.0 0.8 0.6

Energy (eV)

0.4 0.2 0.0 -0.2 -0.4 -0.6 -0.8 -1.0

G

Z

R

A

ZM

AX

RG

X

M

G

G

Z

T

R

ZX

U

TS

A

Y

RG

Z

R

Z

UY

S

X

G

T R

U G

G X

Y

X S

M

Fig. 4. The band structures of (a) Na12Ge8Sn and (b) Na15Ge8SnP. The dotted line represents the Fermi level. The locations of the special k-points used in the band structure plot are also shown.

(8  N rule; N ¼ 5) and tetrahedral clusters are the dominant feature in many structural types [17e20,26e28]. Fig. 3 shows the local coordination environments for each (Ge4)4 tetrahedral cluster of the title compounds. In Na12Ge8Sn, eight crystallographically independent germanium atoms form two distorted independent anionic (Ge4)4 tetrahedral clusters.

a

The first cluster (A) is composed of Ge(1), Ge(2), Ge(3), and Ge(4) and the second one (B) is defined by Ge(5), Ge(6), Ge(7), and Ge(8). Both have similar sizes and C1 symmetry. A and B clusters are enclosed by 16 and 15 Na atoms, respectively. In cluster A, four Na atoms locate above the faces of the (Ge4)4 tetrahedron, three are edge bridging and nine

b 12.0 10.0 Na Ge Sn

Na (s, p)

8.0

Energy (eV)

6.0 4.0 2.0

Sn (p)

Na Ge Sn P

Na (s, p)

Sn (p) Ge (p)

Ge (p)

0.0 P (p)

-2.0 -4.0 -6.0

Sn (s)

Sn (s) Ge (s)

-8.0 -10.0

Ge (s) P (s)

Na12Ge8Sn

Na15Ge8SnP

Fig. 5. The calculated total density of states (DOS) and partial densities of states (PDOS) for (a) Na12Ge8Sn and (b) Na15Ge8SnP. Fermi level is indicated by a dotted line.

Y. Dong et al. / Solid State Sciences 10 (2008) 525e532

4-

•• Ge

•• Ge

Ge

••

Ge

••

vertex capping. In cluster B, there is one face capping, six edge bridging, and eight vertex capping Na atoms. In Na15Ge8SnP, three crystallographically independent germanium atoms form one distorted (Ge4)4 tetrahedral cluster. This cluster has Cs symmetry in which the mirror plane contains Ge(2) and Ge(3) and bisects the Ge(1) and Ge(1) edge. This cluster is surrounded by 13 Na atoms, of which are 3 face capping, 4 edge bridging, and 6 vertex capping. Selected bond distances for all the compounds are listed in Table 3. No significant differences of the GeeGe distances occur in the anionic (Ge4)4 tetrahedral cluster units. The GeeGe ˚ to 2.6254(12) A ˚ for Na12 distances range from 2.5179(12) A ˚ ˚ Ge8Sn and from 2.5201(7) A to 2.6000(8) A for Na15Ge8SnP. These values are in good agreement with each other and (Ge4)4 containing compounds such as Na12Ge17 (2.50e ˚ ) [27] and NaGe (2.53e2.58 A ˚ ) [28]. The GeeGee 2.62 A 4 Ge angles of (Ge4) for each compound are close to those expected for regular tetrahedral geometry (see Table 3). The ˚ for Na12 shortest intercluster GeeGe distances are over 4.4 A ˚ Ge8Sn and 3.8 A for Na15Ge8SnP and these indicate that there is no significant GeeGe intercluster bonding interactions. The ˚ for Na12Ge8Sn and SneNa distances (3.149(4)e3.721(5) A ˚ 3.2417(17)e3.7258(16) A for Na15Ge8SnP) are very similar to the SneNa lengths reported in the binary compounds [21e24]. PeNa distances for Na15Ge8SnP range from ˚ to 2.9898(17) A ˚ . These values are comparable 2.8650(12) A ˚ ) [54] to those in the binary Na3P (2.8586(3)e2.9073(17) A ˚ ) [55]. SneSn disand ternary NaSrP (2.8984(7)e2.9579(7) A tances in the 1N ½Na9 Sn5þ for Na12Ge8Sn and 1N ½Na15 SnP8þ ˚ and 5.555(1) A ˚ , respectively. for Na15Ge8SnP are 5.466(4) A ˚ for Na12Ge8Sn and 5.0 A ˚ SneGe distances are over 4.5 A ˚. for Na15Ge8SnP and PeGe distances are also over 4.4 A These indicate that there are no significant SneSn, SneGe, and PeGe bonding interactions. Traditionally, most Zintl compounds have a closed shell electronic configuration and are semiconductors or insulators because electrons of electropositive elements (mainly alkali or alkaline-earth metals) totally transfer to electronegative elements (mainly group 13, 14, and 15). However, recent experimental and theoretical studies show that some Zintl compounds violate this expectation and display metallic behavior. In some cases, the s orbitals of the electropositive elements contribute significantly to the wave functions at the Fermi level [18,31,56,57]. Thus, in Zintl compounds, metallic behavior with the heavier main group elements may occur due to band overlap or by only partial transfer of electrons from cations to anions [58]. In order to understand the electronic structures for both compounds, we have performed band structure calculations. Due to limitations in DFT, band gaps are often underestimated [59]. Further, due to the large unit cells, we are limited to calculations at a small mesh of k-points. Fig. 4(a) and (b) show the band dispersion curves of the title compounds along special directions in the Brillouin zone. For Na12Ge8Sn, the Fermi level cuts through the band along GZ and also crosses bands along GX and MG. For Na15Ge8SnP, the Fermi level similarly crosses bands along GY and XG. The projected density of states (Fig. 5(a) and (b)) for both

531

Fig. 6. Schematic diagram of tetrahedral (Ge4)4 cluster with lone pairs pointing along the 3-fold axes of the tetrahedron.

compounds indicates a very large contribution of Na states (both of s and p character) between 11 eVand 4 eV. In this energy range, Ge, Sn, and P states are mostly of p orbital character. The s states of Ge, Sn and P lie at a much lower energy, between 6 eV and 10 eV. Just below the Fermi level (0.0 eV to 4.0 eV), the admixture of Na orbitals is also relatively large but p states of Ge, Sn and P dominate. The projected DOS diagrams indicate a week overlap of the filled valence band and the empty conduction band. However, as mentioned above, density function theory usually underestimates the magnitude of band gaps. Indeed, the DOS at Fermi energy is very small and the change in DOS is very steep around the Fermi level suggesting the beginning of a band gap. A gap is consistent with the measured high resistance of Na12Ge8Sn. Most Na states are above the Fermi energy and a formal charge can be assigned as þ1. A large part of the DOS for Sn and P are below the Fermi energy, thus one might think of them as Sn4 and P3, respectively. Ge atoms take up a tetrahedral structure shown in Fig. 6 (a charge of 1 for each Ge atom), with lone pairs pointing along the corners of the tetrahedron. One might view the compound as a phase where the anionic lattice has both covalent (Zintl type anions formed by (Ge4)4 tetrahedron) and ionic parts (formed by Sn4 and P3 ions). As a result, for both title compounds, three bounded Ge atoms can be assigned 1 charge for each isolated (Ge4)4 tetrahedral cluster and also isolated sodium, tin, and phosphorous can be assigned þ1, 4, and 3 to produce electron precise charge states: 4 þ 4 3 4 ([Naþ]12[Ge4 4 ]2[Sn ] and [Na ]15[Ge4 ]2[Sn ][P ]). 4. Conclusions Two mixed anionic Zintl compounds, Na12Ge8Sn and Na15Ge8SnP, have been obtained during attempts to prepare new nitrides and characterized by X-ray structure determination and electronic structure calculations. Although the basic structural components are common (isolated Naþ, (Ge4)4, Sn4, P3), the title compounds crystallize in new structure types (tP168 for Na12Ge8Sn and oC100 for Na15Ge8SnP, respectively). Moreover, these are the first examples of compounds in the NaeGeeSn ternary and NaeGeeSneP quaternary systems.

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