Z. Naturforsch. 2020; 75(1–2)b: 209–216 Ulrich Schwarz*, Rodrigo Castillo, Julia M. Hübner, Aron Wosylus, Yurii Prots, Matej Bobnar and Yuri Grin The untypical high-pressure Zintl phase SrGe6 https://doi.org/10.1515/znb-2019-0197 with a significantly reduced excess of electrons may be Received November 21, 2019; accepted November 28, 2019 realized by chemical substitution [6] or vacancy formation [7–9], e.g. in the modulated silicide CeSi [10], the devel- Abstract: The binary strontium germanide SrGe was 1.82 6 opment of defects is more typical for polyanions of ger- synthesized at high-pressure high-temperature condi- manium or tin. In these compounds, the electron excess tions of approximately 10 GPa and typically 1400 K before of the framework is reduced (or even completely compen- quenching to ambient conditions. At ambient pressure, sated) by the formation of defects, which are surrounded SrGe decomposes in a monotropic fashion at T = 680(10) 6 by atoms bearing free electron pairs [7–9]. The vacancies K into SrGe and Ge, indicating its metastable character. 2 are often randomly distributed [7], but a few commensu- Single-crystal X-ray diffraction data indicate that the rate superstructures have been reported, e.g. for Ba Ge □ compound SrGe adopts a new monoclinic structure type 8 43 3 6 [8] and M Sn □ (M = Rb, Cs [9]). Similarly, in samples comprising a unique three-dimensional framework of 8 44 2 SrGe with x ≈ 0.5 [7], the EuGa Ge -type crystal structure germanium atoms with unusual cages hosting the stron- 6−x 2 4 [11, 12] exhibits a significant number of vacancies in the tium cations. Quantum chemical analysis of the chemical network. In a first study, the arrangement of these voids bonding shows that the framework consists of three- and has been described by a disorder model involving one four- bonded germanium atoms yielding the precise elec- randomly half-occupied germanium position [7]. Later, an tron count Sr[(4bGe0] [(3b)Ge−] in accordance with the 4 2 independent investigation evidenced satellite reflections 8 − N rule and the Zintl concept. Conflicting with that, for composition x ≈ 0.45 [13], which unveil incommensu- a pseudo-gap in the electronic density of states appears rate modulation of the vacancy pattern. clearly below the Fermi level, and elaborate bonding anal- In the present concerted experimental and theoretical ysis reveals additional Sr–Ge interactions in the concave study, we report on the high-pressure, high-temperature coordination polyhedron of the strontium atoms. synthesis and the crystal structure of SrGe6 and the solid Keywords: germanium; high-pressure synthesis; stron- solution Sr(Ge1−xSix)6 (x = 0.41). The bonding properties of tium; Zintl phase. the binary compound are analyzed by quantum-chemical methods operating in positional space. Dedicated to: Professor Arndt Simon on the occasion of his 80th birthday. 2 Experimental 1 Introduction Preparation and sample handling except the high-pres- sure, high-temperature synthesis was performed in argon- A number of tetrel-rich atomic arrangements comprise filled glove boxes (MBraun, H2O < 0.1 ppm; O2 < 0.1 ppm) anionic frameworks formed by p-block elements. Silicon- in order to avoid contamination of the product. SrGe2 rich compounds MSi6 (M = Ca, Sr, Ba, Eu) [1–5] are charac- was prepared as a precursor by arc-melting of strontium- terized by four-bonded network atoms implying an excess germanium mixtures with an atomic ratio of 1:2. of electrons with respect to the 8–N rule and the Zintl- For the high-pressure synthesis of SrGe6, SrGe2 was Klemm concept. This surplus may be described by the mixed with germanium to yield a final ratio Sr:Ge of 1:6. 2+ 0 − charge balance M [(4b)Si6 ] × 2e . Although compositions For the quasi-binary phase Sr(Ge1−xSix)6, a mixture of com- position Sr:Ge:Si = 1:3:3 was prepared. The feed materials were placed in crucibles machined from hexagonal boron *Corresponding author: Ulrich Schwarz, Max-Planck-Institut nitride. High-pressure, high-temperature experiments für Chemische Physik fester Stoffe, Nöthnitzer Straße 40, 01187 were performed in an octrahedral multianvil press with Dresden, Germany, e-mail: [email protected] Rodrigo Castillo, Julia M. Hübner, Aron Wosylus, Yurii Prots, Matej MgO octrahedra of 14 mm edge length [14]. High tempera- Bobnar and Yuri Grin: Max-Planck-Institut für Chemische Physik tures were realized by resistive heating of graphite tubes. fester Stoffe, Nöthnitzer Straße 40, 01187 Dresden, Germany Pressure and temperature calibration had been performed Open Access. © 2020 Ulrich Schwarz et al., published by De Gruyter. This work is licensed under the Creative Commons Attribution 4.0 Public License. 210 U. Schwarz et al.: The untypical Zintl phase SrGe6 prior to the experiments by in situ monitoring of the resist- Table 1: Experimental details for the single-crystal X-ray diffraction ance changes of bismuth [15], and by performing cali- measurement at room temperature (293 K) using a Rigaku AFC7 α λ = bration heating runs with a thermocouple, respectively. diffractometer operated with MoK radiation ( 0.71073 Å) and a Mercury CCD detector as well as characteristic results of the least- Samples were subjected to pressures of typically 8(1)–10(1) squares refinements. GPa and 1100(100)–1600(150) K applying annealing times between 30 min and 5 h. Crystal data Phase identification was realized by powder X-ray Chemical formula Sr(Ge1−xSix)6 SrGe6 diffraction experiments (Huber image plate Guinier (x = 0.41) Space group P2 /m P2 /m camera G670) employing CuKα radiation, λ = 1.54056 Å. 1 1 1 a/Å 12.284(1) 12.501(3) Single crystal X-ray diffraction data was measured with b/Å 4.0377(2) 4.0628(8) a Rigaku AFC7 diffractometer operated with MoKα radia- c/Å 13.050(1) 13.232(4) tion (λ = 0.71073 Å) and a Mercury CCD detector. For crystal β/° 103.224(4) 103.51(1) structure refinement and crystallographic calculations, V/Å3 630.1(2) 653.5(5) −3 the program package WinCSD [16] was used. Calculated density/g cm 4.35 5.32 Absorption coefficient/cm−1 26.7 37.1 Differential Scanning Calorimetry (DSC) experiments Data collection were performed with closed alumina crucibles in argon Radiation; wavelength/Å MoKα; 0.71073 MoKα; 0.71073 atmosphere (Netzsch DSC 404c) at temperatures between Measured; unique 5109; 4214 4533; 3205 473 and 1223 K. For the observed effects, onset tempera- reflections tures are given in the text and in the figures. Requivalent 0.047 0.065 Reflections with Fo > 4 σ(Fo) 1641 1709 Ranges hkl −17 ≤ h ≤ 15 −16 ≤ h ≤ 16 −5 ≤ k ≤ 5 −4 ≤ k ≤ 5 −18 ≤ l ≤ 17 −17 ≤ l ≤ 16 3 Quantum chemical calculations RF; Rw 0.049; 0.051 0.076; 0.079 Goodness of fit 1.02 1.04 Electronic structure calculations and bonding analysis Number of refined 98 86 parameters were carried out with the TB-LMTO-ASA [17] program Residual density max; 1.3; −1.1 3.8; −2.7 package using the experimentally determined lattice min/e− Å−3 parameters and atomic coordinates (Tables 1 and 2). The Barth-Hedin exchange potential [18] was employed and CCDC 1970028 and 1970029 contain the supplementary crystallographic data for this paper. These data can be obtained the radial scalar-relativistic Dirac equation was solved to free of charge from The Cambridge Crystallographic Data Centre via get the partial waves. Although the calculation within the www.ccdc.cam.ac.uk/data_request/cif. atomic sphere approximation (ASA) already includes cor- rections for the neglect of interstitial regions and partial waves of higher order [19], the addition of empty spheres Table 2: Atomic coordinates and isotropic displacement parameters (in Å2) of SrGe . was necessary. The following radii of the atomic spheres 6 were applied: r(Sr1) = 1.939 Å, r(Sr2) = 2.270 Å, r(Ge1) = 1.415 Atom Wyckoff x/a y/b z/c B a = = = eq Å, r(Ge2) 1.402 Å, r(Ge3) 1.354 Å, r(Ge4) 1.367 site Å, r(Ge5) = 1.354 Å, r(Ge6) = 1.359 Å, r(Ge7) = 1.334 Å, Sr1 2e 0.0155(3) ¼ 0.2280(3) 2.4(1) r(Ge8) = 1.334 Å, r(Ge9) = 1.353 Å, r(Ge10) = 1.359 Å, Sr2 2e 0.4611(4) ¼ 0.7465(4) 2.8(1) = = r(Ge11) 1.440 Å, r(Ge12) 1.355 Å. For the self-consistent Ge1 2e 0.0415(4) ¼ 0.4723(4) 2.7(1) calculation, a basis set containing Sr(5s,4d) and Ge(4s,4p) Ge2 2e 0.5028(3) ¼ 0.4472(3) 1. 8(1) orbitals was selected with Sr(5p,4f) and Ge(4d) functions Ge3 2e 0.7037(4) ¼ 0.4586(3) 1.9(1) being downfolded. The electronic DOS was calculated Ge4 2e 0.5043(4) ¼ 0.0497(4) 3.4(1) using the mesh of 4 × 12 × 4 k-points. Ge5 2e 0.7640(4) ¼ 0.9653(3) 2.0(1) Ge6 2e 0.3323(4) ¼ 0.1199(3) 2.0(1) Analysis of the chemical bonding in positional space Ge7 2e 0.2164(4) ¼ 0.5996(3) 2.1(1) was performed by means of the electron localizability Ge8 2e 0.1943(4) ¼ 0.7810(3) 2.2(1) indicator (ELI) calculated in its ELI-D representation [20, Ge9 2e 0.7813(4) ¼ 0.6516(3) 2.1(1) 21] together with the electron density with a dedicated Ge10 2e 0.3245(4) ¼ 0.3095(3) 2.0(1) module implemented within the TB-LMTO-ASA program Ge11 2e 0.9580(4) ¼ 0.9490(3) 2.0(1) package.