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DOI: 10.1039/C6RA22000B (Paper) RSC Adv., 2016, 6, 99475-99481

Na9Sb(Ge2Q6)2 (Q = S, Se): two new antimony(III) quaternary chalcogenides with ethane-like [Ge2Q6]6− ligands†

Xiaowen Wu, Yi Hu, Hui Pan and Zhi Su*
College of Chemistry and Chemical Engineering, Xinjiang Normal University, Urumqi 830054, China. E-mail: suzhixj@sina.com

Received 2nd September 2016 , Accepted 13th October 2016

First published on 13th October 2016

Abstract

Two new antimony(III) quaternary chalcogenides, Na9Sb(Ge2S6)2 and Na9Sb(Ge2Se6)2, were successfully synthesized in vacuum-sealed silica tubes. They crystallize in different space groups: C2/m for Na9Sb(Ge2S6)2 and C2/c for Na9Sb(Ge2Se6)2. In comparison with their structures, similar ethane-like [Ge2Q6]6− ligands and infinite [(Na/Sb)Q4]n chains exist in both of them, but note that two interesting [Na2Sb2(Ge2S6)2]8− and [NaS6] layers are discovered in the structure of Na9Sb(Ge2S6)2, rather than a 3D framework structure for Na9Sb(Ge2Se6)2. Moreover, overall investigation of the quaternary chalcogenides containing M–M bonds (M = Si, Ge) indicates that the orientation of the M–M bonds has a close relation with the number ratio of sodium/another cation and cation radius in the crystal structure, which may provide a feasible way to predict the orientations of the M–M bonds in the layers for new compounds. The UV-vis-NIR diffuse reflectance and IR spectra of Na9Sb(Ge2S6)2 are measured and show that it can be expected to be a promising IR optical window material with a wide IR transmission range from 0.48–22 μm. Raman spectra further verify the IR absorption edges and the presence of Ge–Ge bonds for the title compounds.

Introduction

Recently, countless functional materials have been discovered including the metal oxides, halides and chalcogenides, which have greatly promoted the development of structural chemistry.1–6 Among them, metal chalcogenides have received widespread investigative attention because of their interesting structural features, as well as their fascinating physicochemical properties as potential candidates for nonlinear optics, electro-optics, optical storage, ferroelectrics, and thermoelectric energy conversion.7–15 Many of the chalcogenides containing the group 14 element – germanium (Ge) have shown rich crystal structures since the Ge atoms can be easily connected with different chalcogen atoms to form various building units, including the typical [GeQ4]4−,16 corner-shared [Ge2Q7]6−,17 edge-shared [Ge2Q6]4−,18 [Ge4Q10]4− clusters,19 and ∞[GeQ3]n chains (Q = S, Se).20 Moreover, a special structural motif – ethane-like [Ge2Q6]6− dumbbell ligand with the unusual Ge–Ge bond has been also discovered in several known compounds, such as K6Ge2S6,21 Na4MgGe2Se6,22 Na8Pb2(Ge2S6)2,23 Na9Sm(Ge2Se6)2,24 Na8Eu2(Ge2S6)2,25 and Na8Sn2(Ge2S6)2,23 etc. Note that some of the above compounds have shown the good potential as frequency-conversion materials in the infrared (IR) region, magnetic materials, or fluorescent materials.22–25 Recently, another element – antimony (Sb, III) has been also frequently introduced into crystal structures and can bridge a variety of different metal centers into extended structures because of its flexible coordination modes.26–31 For that reason, exploring the new chalcogenides by the incorporation of alkali-metal, antimony, germanium and chalcogen elements as the research system can be viewed as a feasible approach. However, to our best knowledge, this system has not been studied systematically and no related compounds have been reported so far. In this work, we have focused our efforts on the quaternary Na/Sb/Ge/Q (Q = S, Se) system and fortunately synthesized two new quaternary compounds: Na9Sb(Ge2S6)2 and Na9Sb(Ge2Se6)2. They crystallize in the different space groups (C2/m for Na9Sb(Ge2S6)2 and C2/c for Na9Sb(Ge2Se6)2) and show interesting structural features. Raman, IR, and diffuse reflection spectroscopic characterization are also systematically studied. Moreover, we have also systematically investigated the changing characters for the orientations of M–M (M = Si, Ge) bonds in a series of known quaternary chalcogenides.

Experimental

Synthesis and crystal growth

All of the initial chemicals were purchased and used without further purification. As for the easily oxidized Na element, all the preparation process was completed in a glovebox with Ar atmosphere.
Na9Sb(Ge2S6)2. A mixture of Na, Sb, Ge, and S with the stoichiometric ratio of 9[thin space (1/6-em)]:[thin space (1/6-em)]1[thin space (1/6-em)]:[thin space (1/6-em)]4[thin space (1/6-em)]:[thin space (1/6-em)]12 was loaded into a sealed silica tube evacuated to 10−3 Pa. The temperature is firstly heated to 600 °C in 30 h, left at this temperature for 72 h, then cooled to 300 °C at a rate of 3 °C h−1, and finally cooled rapidly to room temperature by switching off the furnace. After repeatedly washed with N,N-dimethylformamide (DMF), many yellow crystals were found and stable in air for several months.

Based on the ground crystals of Na9Sb(Ge2S6)2, its powder X-ray diffraction (XRD) data were carried out with a Bruker D2 PHASER diffractometer equipped with a diffracted beam monochromator set for Cu Kα radiation (λ = 1.5418 Å). The operating 2θ angle ranges from 10 to 70°. The experimental powder XRD pattern was found to be in agreement with the calculated one with the single crystal data (Fig. 1).


image file: c6ra22000b-f1.tif
Fig. 1 Powder XRD pattern of Na9Sb(Ge2S6)2.
Na9Sb(Ge2Se6)2. Initially, the reaction temperature curve was similar to that of Na9Sb(Ge2S6)2, and many deep-yellow and a few of red crystals finally existed in the silica tube. After the analysis with single crystal X-ray diffraction, it was revealed that the major products were the reported Na8Ge4Se10 crystals20 with deep-yellow color and only a few of red crystals are the target products – Na9Sb(Ge2Se6)2. In addition, through adjusting the ratio of reactants, reaction temperatures or cooling rates, the productivity of Na9Sb(Ge2Se6)2 is still relatively low, ∼10%. Owing to the lower yield and small particle size, we failed to pick enough crystals for PXRD and other spectral measurements.

Structural determination

High-quality single crystals of title compounds were chosen for data collection by SMART APEX II Single-Crystal Diffractometer using monochromatic Mo Kα radiation (λ = 0.71073 Å) at 296 K. Using the SHELXTL software package, the crystal structures were solved with direct methods and refined by the full-matrix least-squares on F2.32 Multi-scan absorption correction was used with the program XPREP.33 As for the Na9Sb(Ge2S6)2, during the refinement of its structure, the formula Na3SbGe2S6 with R1 = 15.61% was given by the first routine refinement, but the site of Sb atom shows relatively large anisotropy parameter (0.063 Å2) compared to the other atoms. When using the 100% Na atom replaced with Sb atom, the atomic anisotropy parameter was still abnormal (<0). Thus, we attempted to set the Na and Sb atoms into the one site and fortunately obtained the small R1 and suitable anisotropy parameter with the ratio of Na1[thin space (1/6-em)]:[thin space (1/6-em)]Sb1 (0.76[thin space (1/6-em)]:[thin space (1/6-em)]0.24) under the random refinement. To achieve the balanced formula, we have defined the occupancy of Na1[thin space (1/6-em)]:[thin space (1/6-em)]Sb1 to be 3[thin space (1/6-em)]:[thin space (1/6-em)]1 with the good R1 = 1.84%. Thus, the final formula was obtained to be Na9Sb(Ge2S6)2. In addition, with the similar refine process of Na9Sb(Ge2S6)2, Na9Sb(Ge2Se6)2 was also determined as the final formula. With the help of PLATON, all the structures were checked for missing symmetry elements and no other symmetries were found. Crystal data and structure refinement details are presented in Table 1. The final positional parameters and the isotropic displacement parameters are given in Table 2, and selected interatomic distances and angels are listed in Table 3.
Table 1 Crystal data and structure refinement for title compounds
a R1 = F0 − Fc/F0 and wR2 = [w(F02 − Fc2)2/wF04]1/2 for F02 > 2σ(F02).
Empirical formula Na9Sb(Ge2S6)2 Na9Sb(Ge2Se6)2
Formula weight 1003.74 1566.54
Crystal system Monoclinic Monoclinic
Space group C2/m C2/c
Unit cell dimensions a = 7.5857(7) Å a = 8.950(10) Å
b = 11.5743(11) Å b = 24.33(3) Å
c = 6.8175(11) Å c = 7.066(8) Å
β = 106.587(3)° β = 122.103(11)°
Z, V 1, 573.66(12) Å3 2, 1303(3) Å3
Density (calculated) 2.905 g cm−3 3.992 g cm−3
Completeness to theta = 24.98 96.1% 99.2%
Goodness-of-fit on F2 1.079 1.010
Final R indices [F02 > 2σ(F02)]a R1 = 0.0184 R1 = 0.0534
wR2 = 0.0421 wR2 = 0.1208
R indices (all data)a R1 = 0.0212 R1 = 0.1193
wR2 = 0.0429 wR2 = 0.1462
Largest diff. peak and hole 0.534/−0.445 e Å−3 1.168/−1.658 e Å−3

Table 2 Positional coordinates and equivalent isotropic displacement parameters for Na9Sb(Ge2S6)2 and Na9Sb(Ge2Se6)2
Site x y z Ueq (Å2) Occupancy
Na9Sb(Ge2S6)2
Ge1 0.1646(1) 0.5 0.5562(1) 0.012(1) 1
Sb1 0 0.1841(1) 0.5000 0.019(1) 0.25
Na1 0 0.1841(1) 0.5000 0.019(1) 0.75
S1 0.2530(1) 0.3419(1) 0.7433(1) 0.024(1) 1
S2 0.2655(2) 0.5 0.2835(2) 0.025(1) 1
Na2 0.5 0.5 0 0.029(1) 1
Na3 0 0.3416(2) 0 0.032(1) 1
[thin space (1/6-em)]
Na9Sb(Ge2Se6)2
Ge1 0.15863(1) 0.12484(6) 0.35440(1) 0.016(6) 1
Se1 0.25542(1) 0.12544(1) −0.26558(1) 0.028(5) 1
Se2 0.24537(1) 0.04475(1) 0.25382(1) 0.024(9) 1
Se3 0.25181(1) 0.20396(1) 0.25340(1) 0.025(6) 1
Sb1 0 −0.0419(2) 0.25 0.042(6) 0.25
Na1 0 −0.0419(2) 0.25 0.042(6) 0.75
Sb2 0.5 0.21542(1) 0.75 0.018(8) 0.25
Na2 0.5 0.21542(1) 0.75 0.018(8) 0.75
Na3 0.5 0.1231(4) 0.25 0.035(1) 1
Na4 0 0.1995(4) −0.25 0.031(3) 1
Na5 0.5 0.0384(4) 0.75 0.017(2) 1

Table 3 Selected bond lengths (Å) and angles (deg) for Na9Sb(Ge2S6)2 and Na9Sb(Ge2Se6)2
Na9Sb(Ge2S6)2 Na9Sb(Ge2Se6)2
Ge1–S2 2.2032(12) Ge1–Se1 2.343(3)
Ge1–S1 2.2218(8) Ge1–Se2 2.343(3)
Ge1–S1 2.2218(8) Ge1–Se3 2.354(3)
Ge1–Ge1 2.3940(8) Ge1–Ge1 2.406(4)
(Na/Sb)1–S1 2.8201(9) × 2 (Na/Sb)1–Se1 3.105(5) × 2
(Na/Sb)1–S1 2.8504(9) × 2 (Na/Sb)1–Se2 3.033(5) × 2
(Na/Sb)1–S2 2.8955(9) × 2 (Na/Sb)1–Se2 3.032(3) × 2
Na2–S1 2.8373(8) × 2 (Na/Sb)2–Se1 3.056(5) × 2
Na2–S1 2.8373(8) × 2 (Na/Sb)2–Se3 2.978(4) × 2
Na2–S2 2.9774(12) × 2 (Na/Sb)2–Se3 3.007(3) × 2
S2–Ge1–S1 111.32(3) Se1–Ge1–Se2 111.87(11)
S2–Ge1–S1 111.32(3) Se1–Ge1–Se3 111.57(12)
S1–Ge1–S1 110.91(4) Se2–Ge1–Se3 111.11(10)
S2–Ge1–Ge1 108.15(4) Se1–Ge1–Ge1 107.45(8)
S1–Ge1–Ge1 107.47(3) Se2–Ge1–Ge1 106.70(7)
S1–Ge1–Ge1 107.47(3) Se3–Ge1–Ge1 107.87(7)

UV-vis-NIR diffuse-reflectance spectroscopy

With a Shimadzu SolidSpec-3700DUV spectrophotometer, optical diffuse-reflectance data were collected in the wavelength range from 190 to 2600 nm at room temperature.

Infrared spectroscopy

Initially, the sample is mixed thoroughly with dried KBr and then pressed into a wafer with a hydraulic machine. Infrared spectral data were recorded on a Shimadzu IRAffinity-1 Fourier transform infrared spectrometer in the 400–4000 cm−1.

Raman spectroscopy

Raman spectral data were recorded on a LABRAM HR Evolution spectrometer equipped with a CCD detector using a diode laser radiation at 532 nm. Picked crystals were placed on a glass slide and measured by the power of 60 mW and beam diameter of 35 μm at an integration time of 15 s.

Results and discussion

Crystal structure

Na9Sb(Ge2S6)2. Na9Sb(Ge2S6)2 crystallizes in the monoclinic space group C2/m (no. 12) with the unit cell parameters of a = 7.5857(7) Å, b = 11.5743(11) Å, c = 6.8175(11) Å, β = 106.587(3)°, and Z = 1. Seen from its asymmetric unit, two Na atoms, one (Na/Sb = 0.25[thin space (1/6-em)]:[thin space (1/6-em)]0.75) atom, one Ge atom, and two S atoms exist. In its structure (Fig. 2a), the Na atoms are linked with six S atoms to form NaS6 octahedra with the Na–S bond distances range from 2.837(3) to 3.035(0) Å. The Na/Sb atoms are also connected with six S atoms to make up (Na/Sb)S6 octahedra with the d(Na/Sb–S) = 2.820(2) to 2.895(5) Å, which is slightly smaller than d(Na–S) owing to the smaller ionic radius for Sb than Na ions. In addition, the Ge atoms are not bonded with four S atoms to form typical GeS4 tetrahedra, but to form the special ethane-like [Ge2S6]6− units with the d(Ge–Ge) = 2.394(0) Å that are similar to those of related quaternary compounds, such as Na8Pb2(Ge2S6)2 [d(Ge–Ge) = 2.395 Å],23 Na8Sn2(Ge2S6)2 [d(Ge–Ge) = 2.393 Å],23 and Na8Eu2(Ge2S6)2 [d(Ge–Ge) = 2.394(6) Å].25 The (Na/Sb)S6 octahedra are firstly linked with each other by sharing edges to form the infinite [(Na/Sb)S4]n chains, then these chains are further linked with the isolated ethane-like [Ge2S6]6− to form a two-dimensional (2D) [Na2Sb2(Ge2S6)2]8− layer structure that is located at the ab plane (Fig. 2b). Note that these [(Na/Sb)S4]n chains do not exist in isolation and further connect with each other by sharing edges to form a 2D layer (Fig. 2c). In addition, the Ge–Ge bonds are parallel to the plane of the layer and the [Ge2S6]6− units show interesting staggered arrangements in the layer, which is similar to that in the Na4MgM2Se6 (M = Si, Ge),22 but are different from those of other related quaternary compounds in which their Si–Si or Ge–Ge bonds are located in opposite directions (almost perpendicular to the plane of the layer).23,25 All the Na atoms fill in the interlayer spaces and further connect with each other by sharing edges to make up a layer structure, and then link together with [(Na/Sb)2(Ge2S6)2]8− layers to form a 3D framework (Fig. 2a).
image file: c6ra22000b-f2.tif
Fig. 2 (a) Crystal structure of Na9Sb(Ge2S6)2; (b) A 2D [Na2Sb2(Ge2S6)2]8− layer in the ab plane; (c) A 2D [(Na/Sb)S4]n layer in the ab plane.
Na9Sb(Ge2Se6)2. Na9Sb(Ge2Se6)2 crystallizes in the monoclinic space group C2/c (no. 15) with the unit cell parameters of a = 8.950(10) Å, b = 24.33(3) Å, c = 7.066(8) Å, β = 122.103(11)°, and Z = 2. In the structure of Na9Sb(Ge2Se6)2, all the Na atoms are connected with six Se atoms to form the distorted NaSe6 octahedra with d(Na–Se) = 2.958 (8)–3.241 (4) Å. Among them, Na4Se6 (or Na5Se6) octahedra connect with themselves by sharing common edges to form the infinite (NaSe4)n chains, respectively. Note that the [Na4Se4]n chains are extended along the a axis, whereas the [Na5Se4]n chains are approximatively extended along the c axis (Fig. 3b). Then, the [Na4Se4]n and [Na5Se4]n chains further linked together to form the framework structure with Na3Se6 octahedra located at the channels (Fig. 3c). In addition, [(Na/Sb)1Se4]n and [(Na/Sb)2Se4]n chains are also shown similar structural features as the (NaSe4)n chains and linked together to form the framework structure (Fig. 3a). The whole structure of Na9Sb(Ge2Se6)2 can be viewed as the combination of countless infinite ∞[(Na/Sb)Se4]n (chains 1 and 2) and ∞[NaSe4]n (chains 3 and 4) chains along the different directions and isolated [Ge2Se6]6− units (Fig. 3d). In view of the previously reported quaternary compounds containing M–M bonds, all of them have the 2D layer in their structures, this 3D framework structural feature for Na9Sb(Ge2Se6)2 can be viewed as the first discovered example in the related quaternary compounds.
image file: c6ra22000b-f3.tif
Fig. 3 Crystal structure of Na9Sb(Ge2Se6)2. (Na/Sb)1: turquoise; (Na/Sb)2: blue; Na3: green; Na4: pink; Na5: gray.

Seen from the asymmetric unit of Na9Sb(Ge2Se6)2, it consists of three unique Na atoms, one Ge atom, three Se atoms, and other two sites occupied by disordered Na (75%) and Sb (25%) atoms, which is obviously different with that in the unit cell of Na9Sb(Ge2S6)2 (Fig. 4). Moreover, the doubling Z value (number of molecules in a unit cell, Z = 2) in Na9Sb(Ge2Se6)2 is also discovered compared with that (Z = 1) in Na9Sb(Ge2S6)2. Thus, based on the above reasons, different arrangement of cations in two compounds lead to the doubling cell parameter along the b axis in Na9Sb(Ge2Se6)2 with that in Na9Sb(Ge2S6)2.


image file: c6ra22000b-f4.tif
Fig. 4 The Na9Sb(Ge2S6)2 structure viewed down the c axis is compared to the Na9Sb(Ge2Se6)2 structure viewed down the c axis to accentuate the unit cell doubling that results from the arrangement of cations (note that the unit cell edges are marked by the red lines).

We have also chosen another mode to describe their structures of title compounds in terms of close packing on anion layers. For the structure of Na9Sb(Ge2S6)2, it can be approximatively viewed as the close packing of S2− anion layers along the c axis with cations located at the interlayers. Two successive layers are set to be A and B, and these layers are stacked in ABAB… modes along the c axis. Note that the filling cations between the A–B and B–A layers along the c axis are different. Seen from the Fig. 5a, (Na/Sb)1 and Ge26+ atoms are filled between the A–B layers, whereas the B–A layers are fully occupied by the Na2 and Na3 cations. The Ge–Ge bonds are perpendicular to the S2− anion layers and bridge the A–B layers together. As for the structure of Na9Sb(Ge2Se6)2 (Fig. 5b), it can be also viewed as the close packing of Se2− anion layers, but its packing direction is along the a axis, which is different with that of Na9Sb(Ge2S6)2 (along the c axis). We have also defined two successive layer as A and B, and these layers are stacked as ABAB… modes along the a axis. Be distinguished from the structure of Na9Sb(Ge2S6)2, all the cations are filled in the spaces between the A–B or B–A layers for Na9Sb(Ge2Se6)2. Moreover, the Ge26+ atoms are linked with six Se atoms to form the [Ge2Se6]6− units with the d(Ge–Ge) = 2.405(5) Å that is slightly larger than that (2.394(0) Å) in Na9Sb(Ge2S6)2. In addition, be different with that in Na9Sb(Ge2S6)2, the Ge–Ge bonds in Na9Sb(Ge2Se6)2 can bridge both of A–B and B–A layers together.


image file: c6ra22000b-f5.tif
Fig. 5 (a) Structure of Na9Sb(Ge2S6)2 viewed along the a-axis showing the stacking of ABAB… modes of anion layers with cations; (b) structure of Na9Sb(Ge2Se6)2 viewed along the c-axis also showing the filling of ABAB… modes of anion layers with cations.
Structural comparison. As for the title compounds, although they have the same stoichiometry, they crystallize in the different space groups (C2/m vs. C2/c) while the S atoms are replaced with Se atoms in crystal structure. In comparison with their structures, some of obviously different structural differences can be found, such as: (i) different distinct sites in the asymmetric unit and Z values of Na9Sb(Ge2S6)2 (7 and Z = 1) vs. Na9Sb(Ge2Se6)2 (9 and Z = 2); (ii) in Na9Sb(Ge2S6)2, the NaS6 units connect with each other to form the 2D layer structure, whereas NaSe6 units link together to make up a 3D framework structure in Na9Sb(Ge2Se6)2; (iii) [NaSe4]n chains are extended along the different directions in Na9Sb(Ge2Se6)2, which is different with the [NaS4]n chains only located at the ab plane in Na9Sb(Ge2S6)2; (iv) (Na/Sb)S6 and [Ge2S6]6− units connect together to form a [Na2Sb2(Ge2S6)2]8− layer located at ab plane in Na9Sb(Ge2S6)2, whereas a 3D framework structure composed of (Na/Sb)Se6 and [Ge2Se6]6− units exist in Na9Sb(Ge2Se6)2; (v) in Na9Sb(Ge2S6)2, Na atoms are located at the interlayer spaces, but in Na9Sb(Ge2Se6)2 that the Na atoms exist the channels formed by 3D framework structure. Moreover, previous researches also show that crystal structures can be changed with the different Q atoms, related examples include Ba2GeS4 (Pnma)35 vs. Ba2GeSe4 (P21/m),36 Ba2SiS4 (Pnma)37 vs. Ba2SiSe4 (P21/m),38 BaGa4S7 (Pmn21)39 vs. BaGa4Se7 (Pc),40 Na2BaSnS4 (Ī42d)14 vs. Na2BaSnSe4 (R3c),14 and BaAg2SnS4 (I222)41 vs. BaAg2SnSe4 (Ī42m).42 Therefore, the observation in this work further confirms that the slight change of anion size would result in different structure features, and future structure prediction should be devoted considerable attentions to the different chalcogen atoms.

Moreover, to our knowledge, known quaternary compounds containing [M2Q6]6− ligands (M = Si, Ge; Q = S, Se) have three types of chemical formula: Na8A2(M2Q6)2 (A = divalent ions), Na9B(M2Q6)2 (B = trivalent ions), and Na4MgM2Se6 (M = Si, Ge; Q = S, Se). Note that the orientations of the M–M bonds in these compounds have two forms (parallel or perpendicular to the ab plane). As for Na8A2(M2Q6)2 (A = divalent ions) compounds, their orientations of the M–M bonds are perpendicular to the ab plane, whereas the orientations of the M–M bonds in Na4MgM2Se6 (M = Si, Ge) are parallel to the ab plane. In addition, in the Na9B(M2Q6)2 (B = trivalent ions) compounds, the orientations of the M–M bonds in most of them are parallel to the ab plane, excepted for the Na9Sm(Si2Se6)2 that its orientation of the M–M bond is perpendicular to the ab plane. Note that Na9Sb(Ge2Se6)2 only has the 3D framework structure, thus, this compound is not included for comparison in this work. To explain the different orientations of the M–M bonds in quaternary compounds, we have systematically studied the related parameters in these compounds, such as the cation number ratio, cationic radius ratio (A2+/Na+ or B3+/Na+), length of [M2Q6]6− along the a axis, and length of [M2Q6]6− along the c axis, and the results are shown in Table 4. From this table, some of interesting results are found as follow: (i) compared with the cationic radius ratios, the orientations of the M–M bonds in the compounds with small cationic radius ratio are parallel to the ab plane, or in other words, introducing the cations with small ionic radius (such as Sb3+, Mg2+) into crystal structure is conducive to make the M–M bonds lie parallel to the ab plane; (ii) compared with the lengths of [M2Q6]6− along the a or c axis, while the lengths of [M2Q6]6− along the a axis are smaller than that along the c axis, the orientations of the M–M bonds in these compounds are perpendicular to the ab plane, whereas they are parallel to the ab plane. Note that only the length of [Si2Se6]6− along the a axis (4.58 Å) is smaller than that along the c axis (4.86 Å) in Na9Sm(Si2Se6)2 compared with other Na9B(M2Q6)2 compounds, so its orientation of the M–M bond is perpendicular to the ab plane. Based on the above analysis, it can be indicated that the smaller radius of cations (such as Mg2+, Sb3+) can spare more space to make the M–M bonds parallel to the ab plane, but the larger cations (such as Pb2+, Sn2+, Eu2+) may compress the space to make the M–M bonds perpendicular to the ab plane. This result may give us a feasible way to predict the orientations of the M–M bonds in the layers for new compounds.

Table 4 The comparison of related parameters in a series of quaternary compounds with [M2Q6]6− dimers
Na8A2(M2Q6)2 (A = divalent ions)
Compounds Na/A A2+/Na+ Length of [M2Q6]6− along the a axis Length of [M2Q6]6− along the c axis M–M direction Reference
Na8Pb2(Ge2S6)2 4 1.17 4.51 4.92 Perpendicular Ref. 23
Na8Sn2(Ge2S6)2 4 1.44 4.40 4.80 Perpendicular Ref. 23
Na8Eu2(Ge2S6)2 4 0.93 4.52 5.05 Perpendicular Ref. 25
Na8Eu2(Si2S6)2 4 0.93 4.39 4.94 Perpendicular Ref. 25
Na8Pb2(Si2Se6)2 4 1.17 4.49 4.92 Perpendicular Ref. 23
Na8Eu2(Si2Se6)2 4 0.93 4.48 4.91 Perpendicular Ref. 24
Na8Eu2(Ge2Se6)2 4 0.93 4.64 5.00 Perpendicular Ref. 34
Na4MgSi2Se6 4 0.71 5.28 4.28 Parallel Ref. 22
Na4MgGe2Se6 4 0.71 5.42 4.38 Parallel Ref. 22
Na9B(M2Q6)2 (B = trivalent ions)
Compounds Na/B B3+/Na+ Length of [M2Q6]6− along the a axis Length of [M2Q6]6− along the c axis M–M direction Reference
Na9Sm(Si2Se6)2 9 0.95 4.58 4.86 Perpendicular Ref. 24
Na9Sm(Ge2Se6)2 9 0.95 5.19 4.45 Parallel Ref. 24
Na9La(Ge2Se6)2 9 0.96 5.13 4.44 Parallel Ref. 24
Na9Sb(Ge2S6)2 9 0.75 5.31 4.33 Parallel This work

Optical properties. The UV-vis-NIR diffuse reflectance spectral data (Fig. 6) of Na9Sb(Ge2S6)2 are obtained and show that its optical band gap is about 2.58 eV (∼480 nm) and consistent with the yellow color of the crystal. The IR spectrum indicates that Na9Sb(Ge2S6)2 has wide optical transmission ranges up to the 458 cm−1 (∼22 μm), which agrees well with the Raman result (442 cm−1, ∼22 μm). Thus, this compound exhibits the wide transmission range from 0.48 to 22 μm that covers the important band ranges (3–5 and 8–12 μm) as the atmosphere transparent windows, which indicates that it has great potential as IR optical material. Hand-picked crystals of title compounds were used to measure the Raman data (Fig. 7). The absorption peaks at 414, 402 cm−1 for Na9Sb(Ge2S6)2 and 309 cm−1 for Na9Sb(Ge2Se6)2 can be assigned to the Ge–S or Ge–Se vibration, respectively.22–25 The other absorption peaks at 287 and 260 cm−1 for Na9Sb(Ge2S6)2 or 238 and 213 cm−1 for Na9Sb(Ge2Se6)2 attributed to the characteristic absorptions of the Ge–Ge modes,43,44 respectively, which are also similar to the results of known compounds Na8M2(Ge2S6)2 (M = Sn, Pb)23 and Na4MgGe2Se6.22
image file: c6ra22000b-f6.tif
Fig. 6 (a) Absorption spectrum of Na9Sb(Ge2S6)2. The inset diagram is the experimental bandgap; (b) IR spectrum of Na9Sb(Ge2S6)2.

image file: c6ra22000b-f7.tif
Fig. 7 Raman spectra of title compounds. (I) Na9Sb(Ge2S6)2; (II) Na9Sb(Ge2Se6)2.

Conclusions

In this work, we have successfully prepared two new antimony quaternary compounds with special ethane-like [Ge2Q6]6− ligands, Na9Sb(Ge2S6)2 and Na9Sb(Ge2Se6)2. Na9Sb(Ge2S6)2 crystallizes in the C2/m space group with Z = 1, whereas Na9Sb(Ge2Se6)2 belongs to the different space group (C2/c) with Z = 2. In the structure of Na9Sb(Ge2S6)2, it has two-type layer structures, such as [Na2Sb2(Ge2S6)2]8− and [NaS6] layer structures, which is different to the 3D framework structure in Na9Sb(Ge2Se6)2. Moreover, through the detail comparison in the on the quaternary compounds containing [M2Q6]6− ligands (M = Si, Ge; Q = S, Se), it can be concluded that the orientations of the M–M bonds are closely related to the cations number ratio and cationic radius ratios. The UV-vis-NIR diffuse reflectance, IR, and Raman spectra of Na9Sb(Ge2S6)2 are systematically studied and the results show that this compound has a wide IR transmission range from 0.48–22 μm and can be expected as the potential optical material in the IR region.

Acknowledgements

This work was supported by the doctoral research start-up fund of Xinjiang Normal University (No. XJNUBS1226).

Notes and references

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Footnote

† CCDC 1483616 and 1483617 for Na9Sb(Ge2S6)2 and Na9Sb(Ge2Se6)2, respectively. For crystallographic data in CIF or other electronic format see DOI: 10.1039/c6ra22000b
This journal is © The Royal Society of Chemistry 2016
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