Home Physical Sciences Oxide meets silicide – synthesis and single-crystal structure of Ca21SrSi24O2
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Oxide meets silicide – synthesis and single-crystal structure of Ca21SrSi24O2

  • Olaf Reckeweg EMAIL logo and Francis J. DiSalvo
Published/Copyright: March 30, 2017
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Zeitschrift für Naturforschung B
From the journal Zeitschrift für Naturforschung B Volume 72 Issue 4

Abstract

A few black, rectangular thin plates of Ca21SrSi24O2 were obtained by serendipity in a solid-state reaction of calcium metal, strontium chloride and silicon powder at 1200 K for 2 days designed to produce ‘Ca2SrCl2[Si3]’. The title compound forms next to some CaSi and some remaining educts. Ca21SrSi24O2 crystallizes in the monoclinic space group C2/m (no. 12) with unit cell parameters of a=1895.2(2), b=450.63(5) and c=1397.33(18) pm and β=112.008(7)° (Z=1). The title compound shows planar, eight-membered, kinked Si8 chains with Si–Si distances between 241.4 and 245.0 pm indicating bonding interactions and kinked ‘rope ladders’ connecting the chains with interatomic Si–Si distances in the range 268.1–274.7 pm. Embedded in between these silicon substructures are columns of oxygen centered, apex sharing [(Ca1−x Srx)6/2O] octahedra and calcium ions.

Keywords: calcium; oxide; silicide; strontium; structure elucidation

1 Introduction

Triatomic moieties containing light atoms of the second period such as in Ca3Cl2[C3] [1] or in AE3Cl2[CBN] (AE=Ca [2] and AE=Sr [3]) are known for quite some time, but replacing boron, carbon and nitrogen in alkaline earth metal-halide matrices with silicon is less common. Attempts to synthesize ‘Ca3Cl2[Si3]’, ‘Sr3Cl2[Si3]’ or ‘Ca2SrCl2[Si3]’ were not successful – except for the latter attempt. The synthesis and the structural characterization of the only identified positive result, namely, Ca21SrSi24O2, is presented here.

2 Experimental section

2.1 Synthesis

All manipulations were performed in a glove box under purified argon unless otherwise stated. The educts were combined to result in a nominal composition of ‘Ca2SrCl2[Si3]’ and further mixed with an excess of alkaline earth metal and alkaline earth metal halide, which were supposed to serve as a reactive flux. Therefore, 81 mg (2 mmol) Ca (99.99%, distilled, dendritic pieces, Aldrich, St. Louis, MO, USA), 160 mg (2 mmol) SrCl2 (≥99.99%, anhydrous powder, Aldrich, St. Louis, MO, USA) and 40 mg (1.4 mmol) Si (99.999% powder, ~325 mesh, Strem, Newburyport, MA, USA) were arc-welded into a clean Ta container. The metal container was sealed into an evacuated silica tube. The tube was placed upright in a box furnace and heated to 930°C within 10 h. After a 2-day reaction time, the furnace was switched off and allowed to cool to room temperature. The product contained a few air and moisture sensitive black platelets of Ca21SrSi24O2 next to the educts and some CaSi. The source of the incorporated oxygen is most probably SrCl2 because in some reactions with this educt Sr4OCl6 formed even without additional oxygen sources such as CaO or SrO.

The title compound was not observed with nominal starting compositions such as ‘Ca3Cl2[Si3]’, ‘Sr3Cl2[Si3]’, ‘Ca22Si24O2‘ or ‘Sr22Si24O2‘ (with or without reactive flux or CaO or SrO, respectively, as oxygen source).

2.2 Crystallographic studies

Samples of the reaction mixture were removed from the glove box in polybutene oil (Aldrich, St. Louis, MO, USA, Mn~320, isobutylene>90%) for single-crystal selection under a polarization microscope, mounted in a drop of polybutene sustained in a plastic loop, and placed onto the goniometer in a cold stream of nitrogen [T=173(2) K], which froze the polybutene oil, thus keeping the crystal stationary and protected from oxygen and moisture in the air. Intensity data were collected with a Bruker X8 Apex II diffractometer equipped with a 4 K CCD detector and graphite-monochromatized MoKα radiation (λ=71.073 pm). The intensity data were handled with the program package [4] that came with the diffractometer. An empirical absorption correction was applied using Sadabs [5]. The intensity data were evaluated, and the input files for solving and refining the crystal structure were prepared by Xprep [4]. The program Shelxs-97 [6, 7] found only in the centric space group C2/m the calcium and silicon positions with the help of Direct Methods techniques. The oxygen position was apparent from the positions of the highest electron density on the difference Fourier map resulting from the first refinement cycles by full-matrix least-squares techniques on F2 in Shelxl-97 [8, 9]. For the Ca6 position, the displacement parameters nearly turned to zero and quite bad R values (wR2≈0.15) showed. Therefore, a mixed Ca2+/Sr2+ occupancy for this position was introduced and refined but constrained to full occupancy. The obtained site occupations were fixed for the final refinement cycles. Additional crystallographic details are described in Table 1. Atomic coordinates and equivalent isotropic displacement coefficients are shown in Table 2. Table 3 displays some selected average bond lengths and their respective range.

Table 1:

Summary of the single-crystal X-ray diffraction structure determination data of Ca21SrSi24O2.

CompoundCa21SrSi24O2
Mr1635.46
Crystal colorMetallic black
Crystal shapeRectangular plate
Crystal size, mm30.06×0.03×0.01
Crystal systemMonoclinic
Space group (no.); ZC2/m (12); 1
Lattice parameters: a; b; c, pm β, deg1895.2(2); 450.63(5); 1397.33(18); 112.008(7)
V, Å31106.4(2)
Dcalcd, g cm−32.46
F(000), e−810
μ, mm−14.3
DiffractometerBruker X8 Apex II with a 4 K CCD
Radiation; λ, pm; monochromatorMoKα ; 71.073; graphite
Scan mode; T, Kϕ and ω scans; 203(2)
Ranges, 2θmax, deg; h, k, l56.5; – 24→ 21, –6→ 6, –17→ 18
Data correctionLP, Sadabs [5]
Transmission: min; max0.640; 0.746
Reflections: measured; unique6083; 1537
Unique reflections with Fo>4σ(Fo)969
Rint; Rσ0.0605; 0.0703
Refined parameters75
R1a; wR2b; GoFc (all refl.)0.0919; 0.0875; 1.042
Factors x; y (weighting scheme)b0.0162; 11.572
Max. shift, esd, last refinement cycle<0.00005
∆ρfin (max, min), e− Å−31.14 (52 pm to Ca2), –1.16 (69 pm to O)
CSD number432276

aR1=Σ ||Fo|–|Fc||/Σ |Fo|; bwR2=[Σw(Fo2–Fc2)2 / Σ(wFo2)2]1/2; w=1/[σ2(Fo2)+(xP)2+yP], where P=[(Fo2)+2Fc2]/3 and x and y are constants adjusted by the program; cGoF(S)=[Σw(Fo2–Fc2)2 / (n–p)]1/2, with n being the number of reflections and p being the number of refined parameters.

Table 2:

Atomic coordinates, anisotropica and equivalent isotropicb displacement parameters (Uij/pm2) of Ca21SrSi24O2. U23=U12=0 due to site symmetries.

Atomx/ay/bz/cU11U22U33U13Ueq
Ca14i0.29520(9)00.12277(11)66(7)57(7)53(7)29(6)57(4)
Ca24i0.14573(9)00.21808(11)81(7)60(8)84(8)44(6)71(3)
Ca34i0.42965(8)½0.06050(11)59(9)55(8)63(8)38(6)54(3)
Ca44i0.35866(8)00.40250(10)51(7)31(7)19(7)16(5)33(3)
Ca52c½½½111(11)62(11)101(11)33(9)93(5)
Ca6/Sr (3/4:1/4)4i0.48269(7)00.68272(10)78(6)77(6)176(7)41(5)112(3)
Si14i0.04602(12)½0.08923(16)71(10)84(1)93(11)27(8)84(5)
Si24i0.17031(12)½0.07610(16)55(10)84(11)87(11)27(8)75(5)
Si34i0.26237(12)½0.25116(16)79(11)83(11)80(10)22(8)83(5)
Si44i0.39207(12)½0.26014(16)75(10)82(11)83(11)28(8)80(4)
Si54i0.14205(13)00.43105(17)111(11)120(11)111(11)51(9)111(5)
Si64i0.27971(14)00.58374(19)195(13)193(13)193(13)76(10)186(5)
O2d½0½296(53)163(49)415(59)173(46)280(21)

aThe anisotropic displacement factor takes the form: Uij=exp[–2π2(h2a*2U11+k2b*2U22+l2c*2U33+2klb*c*U23+2hla*c*U13+2hka*b*U12)]; bUeq is defined as a third of the orthogonalized Uij tensors.

Table 3:

Average of atomic distances (range in parentheses) for different oxides and silicides compared to those in Ca21SrSi24O2. All metal-silicon distances smaller than 340 pm were used.

CompoundRange of distancesAverage distanceRef.
CaOCa–O240.5240.5[10]
SrOSr–O258.0258.0[10]
Ca2SiCa–Si297.2–326.4309[11]
Si–Si––
CaSiCa–Si310.8–322.6314.5[12]
Si–Si247.1247.1
Ca5Si3Ca–Si308.9–321.6312.6[13]
Si–Si242.0242.0
CaSi2Ca–Si301.2–306.0302.8[14]
(R3̅m)Si–Si244.0–244.8244.4
CaSi2Ca–Si307.8–324.6314.5[14]
(I41/amd)Si–Si233.9–238.1236.7
Ca21SrSi24O2Ca–Si300.3–324.2315.0This work
ChainSi–Si241.4–245.0242.7
LadderSi–Si268.1–274.3271.2
Chain-LadderSi–Si270.9270.9
Ca/Sr–O225.3–269.3248.5

All distances are given in picometers.

Further details of the crystal structure investigation may be obtained from FIZ Karlsruhe, 76344 Eggenstein-Leopoldshafen, Germany (Fax: +49-7247-808-666; E-mail: crysdata@fiz-karlsruhe.de), on quoting the deposition number CSD-432276 for Ca21SrSi24O2.

3 Results and discussion

3.1 Crystal structure

The crystal structure of Ca21SrSi24O2 shows both ionic and metallic characteristics. Basically, ionic columns of oxygen centered, apex-sharing [(Ca/Sr)6/2O] octahedra parallel to the crystallographic b axis (Fig. 1) are embedded in a network of silicon moieties. One of these are kinked, non-helical Si8 chains located in the (ac) plane (Fig. 2a) with Si–Si distances between d=241.4 and 245.0 pm, indicating bonding interactions in accordance with other calcium silicides (Table 3). The other silicon substructure runs also parallel to the crystallographic b axis consisting of kinked ‘rope ladders’ (Fig. 2b) connecting the chains with interatomic Si–Si distances in the range between d=268.1 and 274.7 pm (Fig. 2b). These distances indicate relatively weak bonding interactions between the silicon moieties, but Ca2+ ions are embedded in between these structural features holding them together by Coulomb interactions (Fig. 3). The ionic [(Ca/Sr)6/2O] chains of octahedra are quite rigid in their coordination requirements and therefore in the atomic distances exhibited (Table 3), which causes the Si–Si distances to be stretched compared to the expected Si–Si covalent bonding distances in many silicides and as observed for the Si8 fragment in this compound (Table 3).

Fig. 1: Apex-sharing [(Ca,Sr)6/2O] octahedra parallel to the crystallographic b axis are shown. Ca positions are displayed as white cross-hatched circles, shared Ca/Sr positions as gray hatched circles and oxygen atoms as red hatched circles.
Fig. 1:

Apex-sharing [(Ca,Sr)6/2O] octahedra parallel to the crystallographic b axis are shown. Ca positions are displayed as white cross-hatched circles, shared Ca/Sr positions as gray hatched circles and oxygen atoms as red hatched circles.

Fig. 2: Structural motifs of the silicon substructures. (a) The eight-membered kinked silicon chain. (b) The kinked rope ladder Si substructure. Si8 atoms are shown as black spheres, silicon atoms belonging to the rope ladder motif are displayed as magenta colored spheres. Bonding interactions in between the Si8 unit are shown as black lines, and magenta lines indicate the shortest distances within the rope ladder and neon-green lines the closest distances between the Si8 moieties and the rope ladder units.
Fig. 2:

Structural motifs of the silicon substructures. (a) The eight-membered kinked silicon chain. (b) The kinked rope ladder Si substructure. Si8 atoms are shown as black spheres, silicon atoms belonging to the rope ladder motif are displayed as magenta colored spheres. Bonding interactions in between the Si8 unit are shown as black lines, and magenta lines indicate the shortest distances within the rope ladder and neon-green lines the closest distances between the Si8 moieties and the rope ladder units.

Fig. 3: Nonperspective view along the crystallographic b axis of the unit cell of Ca21SrSi24O2. The same color code as in Figs. 1 and 2 is applied; hatched white octahedra indicate [(Ca,Sr)6/2O] chains parallel to the crystallographic b axis.
Fig. 3:

Nonperspective view along the crystallographic b axis of the unit cell of Ca21SrSi24O2. The same color code as in Figs. 1 and 2 is applied; hatched white octahedra indicate [(Ca,Sr)6/2O] chains parallel to the crystallographic b axis.

If one assumes all of these Si–Si contacts as bonding interactions, the ionic formula according to the Zintl-Klemm concept would read as (AE2+)11(Si12−Si2−Si3−Si43− Si5−Si6)2(O2−) leading to an unbalanced charge distribution deviating from electroneutrality. Because the Si–Si distances are not always in the usual bonding range (especially for the rope ladder motif), an explanation for this deviation, the low yield and the black color indicating a metallic compound might be found. The low yields and the resulting lack of measurements of other physical properties do not allow further conclusions. Nevertheless, the volume per formula unit of Ca21SrSi24O2 obtained from our X-ray data agrees quite well within less than 2% with the calculated volume per formula unit summed up from the appropriate binaries (Table 4).

Table 4:

Volume per formula unit (given in Å3) of Ca21SrSi24O2 compared with the calculated volume per formula unit summed up from the experimentally determined volumes per formula unit of the binaries according to ΣVBin=V(CaO)+V(SrO)+16V(CaSi)+4V(CaSi2).

VExpΣVBin
CaO [10]27.8–
SrO [10]34.4–
CaSi [12]48.4–
CaSi2 [14]62.0–
Ca21SrSi24O21106.41085.1

4 Conclusions

A mixed intermetallic-ionic compound has been synthesized by serendipity and structurally characterized. Ca21SrSi24O2 shows a layered silicon network with large gaps filled by Ca2+ ions and columns of oxygen centered, apex-sharing [(Ca/Sr)6/2O] octahedra. The experimentally determined volume per formula unit for the title compound agrees well with the appropriate sum of the experimentally determined volumes per formula units of the binary compounds. Bond lengths are in the expected range except for the Si–Si distances connecting the different structural motifs consisting of silicon. The stability of the alkaline earth oxides likely prevents higher yields, because before these components can be incorporated on a large scale to form Ca21SrSi24O2 or related compounds, the silicon network breaks down and thermodynamically more stable binaries are formed.

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Received: 2017-1-20
Accepted: 2017-2-3
Published Online: 2017-3-30
Published in Print: 2017-4-1

©2017 Walter de Gruyter GmbH, Berlin/Boston

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https://doi.org/10.1515/znb-2017-0022
Keywords for this article
calcium; oxide; silicide; strontium; structure elucidation

Articles in the same Issue

  1. Frontmatter
  2. In this Issue
  3. Lanthanide(III) complexes with μ-SnSe4 and μ-Sn2Se6 linkers: solvothermal syntheses and properties of new Ln(III) selenidostannates decorated with linear polyamine
  4. A green approach for an efficient preparation of 2,4-diamino-6-aryl-5-pyrimidinecarbonitriles using a TiO2–SiO2 nanocomposite catalyst under solvent-free conditions
  5. New chalcones and thiopyrimidine analogues derived from mefenamic acid: microwave-assisted synthesis, anti-HIV activity and cytotoxicity as antileukemic agents
  6. Two copper(II) coordination polymers constructed by bis(4-(1H-imidazol-1-yl)phenyl)methanone and dicarboxylate ligands
  7. Synthesis of functionalised fluorinated pyridine derivatives by site-selective Suzuki-Miyaura cross-coupling reactions of halogenated pyridines
  8. Synthesis, crystal structure and biological activities of a Ag(I) complex based on the V-shaped ligand 1,3-bis(1-benzylbenzimidazol-2-yl)-2-thiapropane
  9. Ternary rhombohedral Laves phases RE2Rh3Ga (RE = Y, La–Nd, Sm, Gd–Er)
  10. Bixbyite-type phases in the system Ta-Zr-O-N
  11. Note
  12. Oxide meets silicide – synthesis and single-crystal structure of Ca21SrSi24O2
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