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Z. Naturforsch. 2017; 72(11)b: 847–853

Dominik Stoiber, Matej Bobnar, Peter Höhn and Rainer Niewa* alkaline earth tetrelides of the type

Li2AeTt (Ae = Ca, Ba, Tt = Si, Ge, Sn, Pb): synthesis, structures and physical properties https://doi.org/10.1515/znb-2017-0103 Heusler-type (MnCu2Al) structure with occupancy of octa- Received June 28, 2017; accepted July 20, 2017 hedral holes by Li and mixed Li/Mg occupancy in tetrahe- dral holes of the fcc packing of Tt. More recently, Li MgSi Abstract: Single of the compounds Li AeTt 2 2 was reported [3, 4] with ordered lithium and (Ae = Ca, Ba, Tt = Si, Ge, Sn, Pb) were grown in reactive arrangements leading to lower crystallographic symmetry. lithium melts in sealed ampoules from an equi- One model comprises a 2 × 2 × 2 superstructure, the second molar ratio of the alkaline earth and the respective a more complicated order. The compounds Li CaSn and 4 element. All compounds, with the exception of 2 Li CaPb also exhibit a Heusler-type , but Li CaSn and Li CaPb, are isotypic and crystallize in an 2 2 2 in contrast to the Mg containing phases with an ordered orthorhombic unit cell (space group Pmmn, no. 59). The arrangement of fully occupied sites with Li in tetrahedral crystal structure can be characterized as superimposed and Ca in octahedral coordination by Tt, due to the larger corrugated networks of Li Tt connected by or 2 difference in ionic radii compared to the magnesium com- within the third dimension. Li CaSn and 2 pounds. A similar trend is observed, for example, in α- Li CaPb crystallize in the cubic space group Fm3m̅ (no. 2 LiMgN (disordered) [5], β-LiMgN (ordered) [5] and LiCaN 225) in a Heusler-type (MnCu Al) structure. According to 2 (ordered) [6], although in fluorite-type arrangements. Of and electric resistivity measure- the analogous tetrel compounds with other alkaline earth ments, the compounds Li BaGe, Li BaSn, and Li BaPb rep- 2 2 2 only Li BaSi [7] has been known for about half resent diamagnetic activated semiconductors. 2 a century, while to the best of our knowledge no other

Keywords: crystal structure determination; Heusler element combinations have been reported. Li2KAs [8] was phases; semiconductors; tetrelides. reported to have the same moisture sensitivity and

metallic luster as the new compounds Li2CaTt (Tt = Si, Ge) and Li BaTt (Tt = Ge, Sn, Pb) reported in this work, which Dedicated to: Professor Dietrich Gudat on the occasion of his 60th 2 birthday. are all isotypic to Li2BaSi.

2 Experimental 1 Introduction The syntheses of the title compounds were carried out in

Ternary compounds Li2MgTt (Tt = Si, Ge, Sn, Pb) [1, 2] are tantalum ampoules by reacting equimolar amounts of the known and the containing compound has been and the in an excess studied in the course of the ever increasing demand for of lithium, which served as reactant and . Single crys- new materials with emphasis on storage [3] and tals of up to 4 mm length (Fig. 1) were obtained by heating lithium conductivity [4] for usage as electrode mate- the reaction mixture to 800°C, subsequent slow cooling to rial. All above compounds were reported to crystallize in a 400°C with 2 K h−1 and followed by cooling to room tem- perature. Dissolution of the excess lithium was carried out by extraction with liquid . An alternative *Corresponding author: Rainer Niewa, Institut für Anorganische route using the same starting materials but employing Chemie, Universität Stuttgart, Pfaffenwaldring 55, 70569 Stuttgart, HCTAF (high temperature centrifugation aided filtration, , Tel: +49(0)711/685-64217, −1 Tmax = 750°C, ΔT = 1 K h , Tcent = 300°C) [9] led to single E-mail: [email protected] crystals of similar size in single- samples for all Ba Dominik Stoiber: Institut für Anorganische Chemie, Universität Stuttgart, Pfaffenwaldring 55, 70569 Stuttgart, Germany phases. Phase-pure single crystalline samples can gen- Matej Bobnar and Peter Höhn: Max-Planck-Institut für Chemische erally be obtained according to powder X-ray diffraction Physik fester Stoffe, Nöthnitzer Str. 40, 01187 Dresden, Germany (Fig. 2).

848 D. Stoiber et al.: Li2AeTt: synthesis, crystal structures and physical properties

X-Shape program followed by a crystal structure refine- ment with the Shelx software [10]. Further details of the crystal structure investiga- tions may be obtained from FIZ Karlsruhe, 76344 Eggen- stein-Leopoldshafen, Germany (fax: +49-7247-808-666; e-mail: [email protected]) on quoting the depo- sition numbers CSD-433265, CSD-433266, CSD-433267, CSD-433268, CSD-433269, CSD-433270, CSD-433271 and CSD-433272. The resistivities of polycrystalline agglomerates of 3 3 Li2BaSn (1.2 × 4.2 × 1.3 mm ) and Li2BaPb (0.6 × 0.8 × 1 mm ) were measured with a home-built resistivity equipment, which allows for the samples to be handled in an inert atmosphere. The samples were contacted using Ag paint and 25 μm thick Pt wires.

The magnetic susceptibilities of Li2BaGe, Li2BaSn, and Fig. 1: Needle-like single crystals of Li2BaGe with mm-scale below. Li2BaPb were measured on a SQUID magnetometer (MPMS-

XL7, Quantum Design) in the external fields of μ0H = 3.5 and 7 T using gel capsules as containers for the samples. Powder X-ray diffraction measurements were carried out at room temperature on a Stoe Stadi-p diffractometer with MoKα1 radiation and a Ge(111) monochromator using the STOE sample holders and geometry. The data collec- 3 Results and discussion tion was performed with a Mythen 1 K detector in a 2θ range of 5–50°. X-ray diffraction intensity data of single 3.1 Crystal structure description crystals sealed in glass capillaries were collected at room temperature on a Bruker κ-CCD single crystal diffracto­ The isotypes Li2AeTt (Ae = Ca, Tt = Si, Ge, and Ae = Ba, meter using MoKα1 radiation with a maximum 2ϑ of 55°. Tt = Si, Ge, Sn, Pb) crystallize in the orthorhombic space

A numerical absorption correction was applied using the group Pmmn (no. 59), while Li2CaSn and Li2CaPb realize

Fig. 2: Measured (blue) and simulated (red) powder patterns of a ground single crystalline sample of Li2BaSi.

D. Stoiber et al.: Li2AeTt: synthesis, crystal structures and physical properties 849

via edges into two-dimensional slabs, in which the group 4 element occupies the extreme positions of the corruga- tion (Fig. 4, middle). The slabs are superimposed along the c-axis. The Ca and Ba atoms are located in between the slabs and coordinated by the group 4 element in form of a distorted tetrahedron. As a result, the group 4 element is surrounded by six lithium in one hemisphere and Scheme 1: Overview of the different crystal structures of com- four barium ions in the other, resulting in a ten-fold coor- pounds Li AeTt (Ae = Mg, Ca, Ba, Tt = Si, Ge, Sn, Pb). 2 dination (Fig. 4, right). Remarkably, comparing the unit cell parameters of the a cubic Heusler-type arrangement in Fm3m̅ (no. 225). two silicon or compounds (Tables 1 and 2), the A graphical representation of this situation in relation b and c axes remain almost constant and only the a axis is to the respective magnesium compounds can be found significantly altered. The two-dimensional lithium-tetrel in Scheme 1. Upon changing the alkaline earth metal to network is rigid along the b axis but, due to the sinusoidal a different composition appears, which we corrugation, flexible along the a axis. The shorter wave- will report in an independent contribution. Crystallo- length of the corrugated net to a larger amplitude, graphic data of the title compounds are summarized in which should result in an increase of the c axis. However, Tables 1 and 2, atomic positions in Tables 3 and 4. The this is compensated by the smaller of calcium orthorhombic Li2BaSi-type compounds can be obtained in compared to barium. Fig. 3 illustrates the situation: the single-phase as needle-like crystals with length of several angle α shows that the corrugated network of lithium and mm (see Figs. 1 and 2). The crystal structures of these the tetrel in the crystal structure is merely compressed/ compounds are composed of slabs of a corrugated two- expanded along the a axis for the barium (α = 89.74°) com- dimensional network formed by lithium and the group pared to the calcium (α = 73.79°) compound. 4 element. Calcium or barium, respectively, are incorpo- Upon comparison of the unit cell parameters of the rated between these slabs (Fig. 3). Lithium is triangularly silicon with the germanium compound and the with coordinated by the group 4 element and located slightly the compound an only conspicuously minor change out of plane (Fig. 4, left). These triangles are connected strikes the eye. This indicates that the elements that differ

Table 1: Selected crystal structure data and results from structure determination of Li2CaTt (Tt = Si, Ge, Sn, Pb).

Compound Li2CaSi Li2CaGe Li2CaSn Li2CaPb Crystal system Orthorhombic Cubic Space group Pmmn Fm3m̅ Unit cell parameters a/Å 5.7495(3) 5.7743(4) 6.9352(4) 6.9835(4) b/Å 4.6236(3) 4.6498(3) a a c/Å 6.3432(4) 6.3683(4) a a V/Å3 168.62(2) 170.98(2) 333.56(3) 340.58(3) Z 2 4 −3 Dx/g · cm 1.62 2.46 3.44 5.09 Absorption correction Numerical μ/mm−1 1.9 10.1 8.9 50.8 F(000)/e− 80 116 304 432

2θmax/deg 55.70 55.55 54.56 54.15 h, k, l range −7:7, −6:6, −8:8 −6:5, −7:7, −8:8 −8:8, −8:8, −8:8 −8:8, −8:7, −8:8 Refl. measured 2614 2542 708 725 Refl. unique 246 249 35 35

Rint 0.0404 0.0415 0.0398 0.0589 Extinction coefficient 0.06(1) 0.036(4) 0.058(4) 0.020(3) Refined parameters 16 16 5 5 Goof 1.170 1.155 1.262 1.263 R1, wR2 (all data) 0.0144, 0.0302 0.0132, 0.0257 0.0083, 0.0201 0.0140, 0.0343 Largest peak, hole difference map/e− Å−3 0.22, −0.24 0.31, −0.31 0.21, −0.30 0.64, −1.51

850 D. Stoiber et al.: Li2AeTt: synthesis, crystal structures and physical properties

Table 2: Selected crystal structure data and results from structure determination of Li2BaTt (Tt = Si, Ge, Sn, Pb).

Compound Li2BaSi Li2BaGe Li2BaSn Li2BaPb

Crystal system Orthorhombic Space group Pmmn Unit cell parameters a/Å 6.7403(3) 6.7655(3) 7.2028(6) 7.1696(5) b/Å 4.6816(1) 4.7141(2) 4.9291(4) 4.9652(3) c/Å 6.2649(2) 6.3042(3) 6.3352(5) 6.4515(4) V/Å3 197.69(1) 201.06(2) 224.92(3) 229.66(3) Z 2 −3 Dx/g · cm 3.01 3.70 3.99 5.18 Absorption correction Numerical μ/mm−1 10.1 16.9 14.0 45.0 F(000)/e− 152 188 224 288

2θmax/deg 54.83 54.92 54.98 55.01 h, k, l range −8:8, −5:5, −7:8 −8:8, −5:6, −8:7 −9:9, −6:6, −8:7 −9:9, −6:5, −8:7 Refl. measured 4175 4150 4086 4239 Refl. unique 273 279 312 317

Rint 0.0322 0.0454 0.0472 0.0804 Extinction coefficient 0.081(3) 0.022(2) 0.012(1) 0.0025(7) Refined parameters 16 16 16 16 Goof 1.192 1.252 1.205 1.159 R1, wR2 (all data) 0.0111, 0.0266 0.0202, 0.0386 0.0200, 0.0438 0.0246, 0.0573 Largest peak, hole difference map/e− Å−3 0.58, −0.42 0.97, −0.72 1.47, −0.65 1.97, −1.69

Table 3: Wyckoff positions, fractional atomic coordinates and dis- Table 4: Wyckoff positions, fractional atomic coordinates and placement parameters of Li AeTt (Ae = Ca, Ba, Tt = Si, Ge, Sn, Pb). 2 displacement parameters of Li2CaSn and Li2CaPb.

Atom Site x y z Ueq Site x y z Ueq

Li2BaSi Li2CaSn Li 4f 0.0481(9) ¼ 0.4080(7) 0.024(1) Li 8c ¼ ¼ ¼ 0.023(3) Ba 2a ¼ ¼ 0.90188(4) 0.0206(2) Ca 4b ½ 0 0 0.0240(5) Si 2b ¼ ¾ 0.3048(2) 0.0179(2) Sn 4a 0 0 0 0.0150(4)

Li2CaPb Li2BaGe Li 4f 0.046(1) ¼ 0.408(1) 0.025(2) Li 8c ¼ ¼ ¼ 0.05(1) Ca 4b ½ 0 0 0.036(2) Ba 2a ¼ ¼ 0.90196(7) 0.02202) Pb 4a 0 0 0 0.0175(8) Ge 2b ¼ ¾ 0.3026(1) 0.0191(2)

Li2BaSn Li 4f 0.048(1) ¼ 0.397(1) 0.031(3) Ba 2a ¼ ¼ 0.88276(7) 0.0257(2) from one compound to the other possess very similar ionic Sn 2b ¼ ¾ 0.30342(7) 0.0222(2) radii. This observation is also supported by the distances Li BaPb 2 between the tetrel and lithium or alkaline earth element, Li 4f 0.043(3) ¼ 0.400(3) 0.029(4) respectively (Table 5), and is a result of the reduction of Ba 2a ¼ ¼ 0.8868(1) 0.0247(3) Pb 2b ¼ ¾ 0.29796(8) 0.0228(2) atomic radii due to the incorporation of the d group from silicon to germanium and f group elements from Li2CaSi Li 4f 0.0251(4) ¼ 0.3993(4) 0.0236(5) tin to lead in combination with the enhanced relativistic Ca 2a ¼ ¼ 0.93255(5) 0.0139(2) contraction of the latter. Similar observations were made Si 2b ¼ ¾ 0.25935(7) 0.0127(2) earlier, for example, for compounds Ca2Tt (Tt = Si, Ge, Sn, Li2CaGe Pb) [11, 12]. Li 4f 0.0241(6) ¼ 0.3993(5) 0.0232(6) The magnesium compounds Li2MgTt (Tt = Si, Ge, Ca 2a ¼ ¼ 0.93239(8) 0.0141(2) Sn, Pb) crystallize in the cubic space group Fm3̅m (no. Ge 2b ¼ ¾ 0.25815(3) 0.0129(1) 225) in a Heusler-type (MnCu2Al) structure. One Wyckoff

D. Stoiber et al.: Li2AeTt: synthesis, crystal structures and physical properties 851

The crystal structure and thus the atomic arrange-

ment of compounds Li2AeTt (Scheme 1) changes from a 6 + 4-fold coordination (Fig. 4, right) of the tetrel in the orthorhombic structures (Ae = Ba, Tt = Si, Ge, Sn, Pb

and Li2CaSi, Li2CaGe) to a 8 + 6 fold coordination (Fig. 5, c α α right) in the Heusler type phases (Ae = Mg, Tt = Si, Ge, Sn,

Pb and Li2CaSn, Li2CaPb). This transition appears as a a result of the different ratios r(Ae2+)/r(Tt4−) of the respec- tive ionic radii. The tetrel atoms are coordinated by eight Fig. 3: View of the crystal structure of Li BaSi (left) and Li CaSi 2 2 lithium and six magnesium atoms, while the coordina- (right) along the b axis. Red: lithium, gray: group 4 element, orange: barium or calcium, respectively. The angle α indicates a measure for tion number drops to 10, six lithium and only four barium + + the degree of corrugation of the network of lithium and the tetrel, atoms, upon going from small Mg2 to large Ba2 . In the which is compressed/expanded along the a axis for the barium as calcium compounds the large tetrel atoms Sn and Pb can compared to the calcium compounds. α equals to 89.74° and 73.79° support the higher coordination number, whereas Si and for the barium and the calcium compound, respectively. Ge are too small for the coordination by 6 calcium and 8 lithium atoms and thus the compounds exhibit the 4 + 6 fold coordination. position is reported to be equally occupied by lithium and magnesium in a disordered arrangement for the Ge to Pb [1, 2] compounds, whereas Li2MgSi [4] was found ordered more recently and exhibits a 2 × 2 × 2 superstruc- 3.2 Electrical and magnetic properties ture. The title compounds Li2CaSn and Li2CaPb, however, crystallize in an ordered Heusler-type arrangement The electrical resistivity of polycrystalline agglomerates of

(Fig. 5, left). Each element fully occupies one Wyckoff Li2BaSn and Li2BaPb is shown in Fig. 6 indicating semi- position (Table 4) such that the smaller lithium has a conducting behavior for Li2BaPb, whereas the Li2BaSn shorter distance to the tetrel compared to the relatively sample may be considered as a heavily doped degener- larger calcium (Table 5). ated semiconductor with a small band-gap (~100 meV).

Fig. 4: Crystal structure details of the Li2BaSi-type orthorhombic compounds Li2AeTt. Left: trigonal planar coordination of lithium (red) by the group four element (gray), with lithium located out of plane. Middle: Li2Tt network forming the sinusoidal slabs that can be seen in Fig. 3 Right: 10-fold coordination of the tetrel by four alkaline earth metal atoms (orange) and six lithium atoms.

Table 5: Selected distances Li–Tt and Ae–Tt (in Å) for compounds Li2AeTt (Ae = Ca, Ba, Tt = Si, Ge, Sn, Pb).

Compound Li2CaTt Li2BaTt d(Li–Si) 2.681(2) 2× 2.794(1) 4× 2.697(5) 2× 2.784(3) 4× d(Li–Ge) 2.695(3) 2× 2.813(2) 4× 2.712(9) 2× 2.810(5) 4× d(Li–Sn) 3.0030(2) 8× 2.867(9) 2× 2.922(5) 4× d(Li–Pb) 3.0239(2) 8× 2.86(2) 2× 2.97(1) 4× d(Ae–Si) 3.1051(4) 2× 3.1219(3) 2× 3.4425(9) 2× 3.6103(4) 2× d(Ae–Ge) 3.1159(4) 2× 3.1318(3) 2× 3.4546(7) 2× 3.6202(3) 2× d(Ae–Sn) 3.4676(2) 6× 3.6299(5) 2× 3.7896(3) 2× d(Ae–Pb) 3.4918(2) 6× 3.6332(8) 2× 3.7777(4) 2×

852 D. Stoiber et al.: Li2AeTt: synthesis, crystal structures and physical properties

The magnetic susceptibilities of Li2BaGe, Li2BaSn,

and Li2BaPb obtained at 7 T are shown in Fig. 7 The small upturns of the susceptibility at the lowest temperatures are probably due to minor paramagnetic impurities. No saturated magnetization indicating ferromagnetic impurities was observed. The extrapolated values of the susceptibilities of the diamagnetic samples at T = 0 K −5 −5 −5 −1 are χ0 = −1.6 × 10 , −6.7 × 10 , and −0.9 × 10 emu mol , respectively.

Fig. 5: Left: heusler-type unit cell of compounds Li2CaSn and

Li2CaPb. Right: coordination of the tetrel (gray) by eight lithium (red) and six calcium (orange) atoms. 4 Summary

The crystal structures of lithium alkaline earth metal

tetrelides of the type Li2AeTt (Ae = Ca, Ba, Tt = Si, Ge, Sn, Pb) are presented. The compounds can be obtained as single-phase single crystalline samples from reactive lithium melts and either subsequent dissolution of excess lithium in liquid ammonia or high temperature centrifu- gation aided filtration. Smaller alkaline earth metal or larger tetrel components trigger the formation of Heusler type-related arrangements, larger alkaline earth metals in combination with the smaller tetrels rather lead to the

Li2BaSi structure type characterized by smaller coordina- tion numbers than the cubic Heusler type. Similarities in ionic radii of the silicide and germanide anions and of the stannide and plumbide anions result from the d and/or f block incorporation within this row of elements

Fig. 6: Electrical resistivity measurements of polycrystalline in combination with the enhanced relativistic contrac- tion of lead. Electric resistivity measurements evidenced agglomerates of Li2BaSn and Li2BaPb. that the compounds may be considered as heavily doped degenerated semiconductors whereas magnetic suscepti- bility measurements showed the diamagnetic behavior of the samples.

Acknowledgements: We would like to thank Dr. Falk ­Lissner and Dr. Sabine Strobel for the collection of the ­single crystal diffraction data.

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D. Stoiber et al.: Li2AeTt: synthesis, crystal structures and physical properties 853

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