Centrosymmetric to noncentrosymmetric structural transformation of new quaternary selenides induced by isolated dimeric [Sn 2 Se 4 ] units: from Ba 8 Ga 2 Sn 7 Se 18 to Ba 10 Ga 2 Sn 9 Se 22 †

A noncentrosymmetric structure is a prerequisite of many important properties, such as ferroelectric, piezoelectric and nonlinear optical properties. The design synthesis of a noncentrosymmetric compound is highly desired but it is a great challenge. Herein, two new quaternary selenides, Ba 8 Ga 2 Sn 7 Se 18 , 1 and Ba 10 Ga 2 Sn 9 Se 22 , 2 , have been discovered by solid-state reactions. Compound 1 crystallizes in the centrosymmetric space group Pnma (no. 62) with a ¼ 12.466 (9) ˚ A, b ¼ 9.358 (6) ˚ A, c ¼ 18.14 (2) ˚ A, V ¼ 2116 (3) ˚ A 3 , Z ¼ 2; and 2 crystallizes in the noncentrosymmetric space group Cmc 2 1 (no. 36) with a ¼ 9.384 (2) ˚ A, b ¼ 44.834 (6) ˚ A, c ¼ 12.416 (2) ˚ A, V ¼ 5224 (2) ˚ A 3 , Z ¼ 4. The major motif of both 1 and 2 is the [Ga 2 Sn 7 Se 18 ] N 16 (cid:2) ladder chain built by alternately jointed GaSe 4 tetrahedra and SnSe 4 tetragonal pyramids, and to each of these polyhedra, an accessory (SnSe 3 pyramid or Sn 2 Se 4 dimer) is attached. Interestingly, the noncentrosymmetric structure of 2 is the combination of centrosymmetric 1 and the imbedded isolated dimeric [Sn 2 Se 4 ] with a C 2v polar symmetry that separates every two such ladder chains. 1 and 2 directly illustrate that a simple combination reaction can drive the centrosymmetric to noncentrosymmetric structural transformation.

Herein, two new selenides, centrosymmetric Ba 8 Ga 2 Sn 7 Se 18 , 1 and noncentrosymmetric Ba 10 Ga 2 Sn 9 Se 22 , 2, are discovered.For the rst time, 1 and 2 illustrate an interesting and direct structure transformation from centrosymmetric to noncentrosymmetric.The structure of 2 can be regarded as a composition of structure of 1 with an asymmetric matrix of the isolated dimeric [Sn 2 Se 4 ].Syntheses, crystal structures, electronic structures and DFT studies are also reported.

Synthesis
The reagents were used as obtained: Ba rod (Alfa Aesar China (Tianjin) Co., Ltd., 99.9%), Ga shot, Sn grain, and Se powder (Sinopharm Chemical Reagent Co., Ltd., 99.999%).And one single piece of the barium rod was used as the starting reactant.The binary compounds BaSe and SnSe were synthesized at 850 C by elemental mixture in sealed silica tubes under vacuum of 10 À2 Pa.][32][33][34] Ba 8 Ga 2 Sn 7 Se 18 , 1.The black block-shaped crystals of 1 were rst discovered by a reaction of elements of Ba, Ga, Sn, and Se in a 7 : 2 : 3 : 14 molar ratio with a total weight of 400 mg.The reactants were loaded into a graphite crucible, and sealed in an evacuated silica tube under 10 À2 Pa atmosphere, and then heated to 980 C within 50 h and kept there for 48 h, then cooled to 400 C at a rate of 5 C h À1 .
Many efforts revealed that such stoichiometric reactions could not produce pure phase.Instead, black crystals of 1, and Ba 6 Sn 6 Se 13 (ref.35) as a secondary phase together with few red crystals of Ba 5 Ga 2 Se 8 (ref.36) were produced.We considered the reason might be the insufficient contact/mixing between Ga and the other reactants.Therefore, binary BaSe, Ga 2 Se 3 and SnSe were used instead.The mixture of BaSe, Ga 2 Se 3 and SnSe in a molar ratio of 8 : 1 : 7 with a total weight of about 500 mg was ground carefully and pressed into a pellet.Then the pellet was put into a graphite crucible in an evacuated silica jacket, and subsequently heated to 750 C in 34 h, kept for 80 h, and then cooled to 300 C within 100 h before the furnace was turned off.The homogenous 1 was produced according to the XRD pattern as shown in ESI Fig. S1a.† Ba 10 Ga 2 Sn 9 Se 22 , 2. The black block-shaped crystals of 2 were initially obtained from a reaction of Ba, Ga, Sn, and Se elements in a 13 : 2 : 10 : 26 ratio with a total weight of 300 mg.And pure phase 2 was obtained by the reaction of cold-pressed pellet of the well ground mixture of binary BaSe, Ga 2 Se 3 and SnSe in a 10 : 1 : 9 ratio with a total weight of about 500 mg.The heating prole was heating to 750 C in 50 h, dwelling for 80 h, then cooling to 300 C at a rate of 4 C h À1 .The XRD pattern showed the homogeneity of the product (ESI Fig. S1b †).The crystals of 2 were relatively moisture sensitive.

Single-crystal X-ray crystallography
The single crystal diffraction data were collected on a Mercury CCD equipped with graphite-monochromated Mo Ka radiation (l ¼ 0.71073 Å) at 293 K.The data were corrected for Lorentz and polarization factors.And the structures were solved by the direct methods and rened by the full-matrix least-squares tting on F 2 using the SHELX-97. 37he single-crystal X-ray diffraction data revealed two sets of new unit cell parameters of a ¼ 12.5 Å, b ¼ 9.4 Å, c ¼ 18.1 Å, a ¼ b ¼ g ¼ 90 for 1; and a ¼ 9.4 Å, b ¼ 44.8 Å, c ¼ 12.4 Å, a ¼ b ¼ g ¼ 90 for 2 indicating the orthorhombic crystal system.Subsequently, the structure of 1 was solved in the suggested space group of Pnma (62) and rened with R 1 ¼ 9.49% and wR 2 ¼ 31.10%.However, an abnormal large atom displacement parameter of Sn(1) (U eq. ¼ 0.093) was seen, such a value was about 3-4 times as those of other Sn sites (Sn(2): 0.031, Sn(3): 0.021, Sn(4): 0.027).Therefore, the occupancy of Sn(1) was allowed to rene freely.The renement resulted in a reasonable temperature factor of 0.024 and an occupancy of 45.297% for Sn(1), and better R values (R 1 ¼ 0.0408 and wR 2 ¼ 0.0928).The PLATON 38 checking found no missing or higher symmetry elements exiting in 1.The partial occupancies of Sn were also found in compound BaAu 2 SnS 4 . 39To keep the charge balance, the occupancy of Sn(1) was xed to 0.5.Consequently, the nal R values were converged to R 1 ¼ 0.0415 and wR 2 ¼ 0.920 (Table 1, ESI Table S1a †).The nal charge-balanced formula was established as (Ba 2+ ) 8 (Ga 3+ ) 2 (Sn 2+ ) 7 (Se 2À ) 18 .Meanwhile, all the subgroups of Pnma were checking.For instance, similar renement of 1 in Pna2 1 generated higher R values of R 1 ¼ 0.0429 and wR 2 ¼ 0.1275.And the subsequent checking with PLATON suggested the space group Pnma.Other subgroups of Pnma, such as Pmn2 1 , Pmc2 1 , and P2 1 2 1 2 1 , gave similar results, and none of them generated reasonable bond lengths and atomic displacement parameters.Therefore, the structure solved in Pnma (62) space group was sound.

Elemental analyses
The semiquantitative energy dispersive X-ray spectra (EDX, Oxford INCA) were measured on a eld emission scanning electron microscope (FESEM, JSM6700F).The quantitative inductively coupled plasma (ICP) emission spectra were recorded with the aid of an Ultima-2 inductively coupled plasma emission spectrometer (ICP-OES).The EDX results conrmed the presence of Ba, Ga, Sn, and Se.And the results of EDX and ICP indicated the ratios of Ba : Ga : Sn : Se in the approximate molar ratio of 8 : 2 : 7 : 18 for 1 and 10 : 2 : 9 : 22 for 2 (ESI Fig. S2, Tables S4, and S5 †), which were in accordance with the single crystal diffraction renements.

Powder X-ray diffraction
The XRD patterns were collected on a Rigaku MiniFlex II diffractometer by using Cu Ka radiation.The scanning range was 10-70 in 2q with a step size of 0.02 .As shown in ESI Fig. S1, † the experimental XRD patterns of 1 and 2 were in good agreement with the simulated ones based on the single crystal data.

Thermal analyses
Thermogravimetric (TG) analyses of compound 1 and 2 were measured on a NETZSCH STA 449F3 simultaneous analyzer under a constant ow of N 2 .The samples were enclosed in an Al 2 O 3 crucible, heated from room temperature to 1200 C at a rate of 20 C min À1 .

Infrared and UV-Vis-near-IR diffuse reectance spectra
The UV-Vis-near-IR diffuse reectance spectra were measured at room temperature using a Perkin-Elmer Lambda 950 UV-Vis spectrophotometer equipped with an integrating sphere attachment and BaSO 4 as a reference.The absorption spectra were calculated from the reection spectra according to the Kubelka-Munk function: a/S ¼ (1 À R) 2 /2R, where a was the absorption coefficient, S was the scattering coefficient, and R was the reectance. 40The IR data were measured by a Nicolet Magana 750 FT-IR spectrophotometer in the range of 2.5-25 mm for 2. Powder sample of 2 was ground with KBr and pressed into a transparent pellet for the measurement.

Powder second harmonic generation (SHG) measurements
The SHG response of compound 2 was performed on a modied Kurtz-NLO system using 1064 nm and 2050 nm laser radiation. 41The particle size of the sieved sample was 30-46 mm.Single crystal of AgGaS 2 (2 cm Â 2 cm Â 2 cm single crystals from Anhui Institute of Optics and Fine Mechanics Chinese Academy of Sciences) was crushed and then sieved into similar particle size, and this was used as a reference.

Computational section
Utilizing density functional theory (DFT) implemented in the Vienna ab initio simulation package code, 42 the electronic structures were investigated.Projector augmented wave (PAW) method 43 were used for the ionic cores and the generalized gradient approximation (GGA) were used for the exchangecorrelation potential, in which the Perdew-Burke-Ernzerhof (PBE) type 44 exchange-correlation was adopted.Ba 5s 2 5p 6 6s 2 , Ga 4s 2 4p 1 , Sn 5s 2 5p 2 , and Se 4s 2 4p 4 were treated as valence electrons.The reciprocal space was sampled with 0.05 ÅÀ1 spacing in the Monkhorst-Pack scheme for the structure optimization, and denser k-point grids less than 0.02 ÅÀ1 spacing were adopted for the property calculation.A mesh cutoff energy of 500 eV was used to determine the self-consistent charge density.All geometries were relaxed until the Hellmann-Feynman force on atoms was less than 0.01 eV ÅÀ1 and the total energy change was less than 1.0 Â 10 À5 eV.The calculation models of 1 and 2 were shown in ESI Fig. S3 and S4.† The structure of the isostructural BaSnS 2 (ref.45) was adopted to simulate the BaSnSe 2 structure in the energy calculation of the combination reaction (Ba 8 Ga 2 Sn 7 Se 18 (1) + 2BaSnSe 2 ¼ Ba 10 -Ga 2 Sn 9 Se 22 (2)).
As detailed above, the packing sequence of 2 (.AA c At A 21 .along the b direction) differs from that of 1 (.AA a .along the c direction), giving rise to enlargement about two times of b ¼ 44.834 (6) Å in 2 with respect to c ¼ 18.14 (2) Å in 1, because that the embedding of the [Sn 2 Se 4 ] dimers separate every two ladder chains.These two structures directly describe that a centrosymmetric structure (1) is transformed to a noncentrosymmetric structure (2) by the introduction of the second building unit.These substantially support the commonly applied strategy of "diversity of the asymmetric unit would enhance the difficulty of spontaneously producing a mirror plane or inversion center during the packing process, an approach involving two or more types of asymmetric building units in a structure would have a high possibility to form a noncentrosymmetric structure". 15any other systems are worth exploring to see if the similar phenomenon exists.

Thermal analyses
The thermogravimetric (TG) curves of compounds 1 and 2 are shown in ESI Fig. S10a and b. † The data results indicate that both of the compounds are stable up to 800 C under a N 2 atmosphere.In order to further check the thermal stability of the two compounds, pure 1 and 2 were sealed in silica tubes under vacuum and annealed for 3 h at 850 C respectively.Aer such a treatment, the X-ray powder diffraction patterns of two compounds showed obvious impurities, which indicated the decomposition of two occurred at around 800 C (Fig. S10c and d in the ESI †).

UV-Vis-near-IR and IR spectra
The optical band gaps of 1 and 2 are approximately 1.65 and 1.67 eV, respectively, which are in agreement with their black color (Fig. 3).These values are similar to that of Ba 6 Sn 6 Se 13 (1.52 eV), 35 in which Sn cations have the mixed oxidation states of 2+ and 4+, but smaller than those of Ba 4 Ga 4 SnSe 12 (2.16eV), 28 Ba 6 Ga 2 SnSe 11 (1.99 eV), 28 and BaGa 2 SnSe 6 (1.95 eV), 29 in which the oxidation state of Sn is 4+.Thus, in Ba/Ga/Sn/Se system, compounds containing Sn 2+ or mixed valences (Sn 2+ and Sn 4+ ) have smaller band gaps than those merely containing Sn 4+ .The IR spectra and UV-Vis-near-IR spectra of 2 show the transparent

Second harmonic generation (SHG) property
The SHG of 2 was measured with the 1064 nm and 2050 nm laser radiation as the fundamental wavelength.Unfortunately, no obvious SHG signal was observed.The possible reason might be related to the poor crystallinity of this compound, longer fundamental wavelength should be tried, however, wavelength longer than 2050 nm is not available at present.

Electronic structures
The electronic band structures of 1 and 2 (Fig. 4) reveal the indirect band gaps of 0.83 and 0.88 eV, respectively.Those are smaller than the experimental values (1.65 and 1.67 eV) because of the inaccurate description of the eigenvalues of the electronic states for GGA.The total and partial densities of states (DOS and PDOS) of 1 and 2 are given in Fig. 5. Take 1 as an example, the valence band (VB) between À9 and À5 eV contains Ga-4s and Sn-5s states with minor Se-4p and Se-4s states.The VB from À5 to À2 eV is mainly composed of Ga-4p, Sn-5p, and Se-4p states.
The VB top predominantly originates from Se-4p, and minor Ga-4p, Sn-5s, and Sn-5p states.Above the Fermi level, the bottom of the conduction band (CB) is dominated by Sn-5p, and Se-4p states mixed with a small amount of Ga-4s, Ga-4p, and Sn-5s states.The CB ranging from 2 to 5 eV is composed of Ba-5d, Ga-4p, Sn-5p, and Se-4p states.Thus, the band gap absorption is mainly ascribed to the charge transitions from Se-4p to Sn-5p states.By comparing the PDOS of Sn

Energies calculation
The formation energies with respect to the isolated atoms of compounds 1, 2 and BaSnSe 2 were calculated by the following equation: where E f is the formation energy and E t is the total energy of a compound.Compounds 1 and 2 have the following chemical formula relationship:    This journal is © The Royal Society of Chemistry 2017 slightly energetically favorable.Hence, the transformation from 1 to noncentrosymmetric 2 is feasible.

Conclusion
In summary, two new selenides with their own structure types, centrosymmetric Ba 8 Ga 2 Sn 7 Se 18 , 1 and noncentrosymmetric Ba 10 Ga 2 Sn 9 Se 22 , 2, have been discovered by solid-state reactions.Both structures feature 1D ladder chain built by alternatively jointed GaSe 4 tetrahedron and SnSe 4 tetragonal pyramid.To each of these polyhedra, an accessory, SnSe 3 pyramid or Sn 2 Se 4 dimer, is attached.Interestingly, the structure of 2 is the combination of that of 1 and the isolated [Sn 2 Se 4 ] dimer with a C 2v symmetry.The embedding of the asymmetric dimeric [Sn 2 Se 4 ] in 2 vanishes the operations of n, 2 1 , and the center of symmetry that exist in 1.
Signicantly, 1 and 2 directly describe for the rst time that a centrosymmetric-to-noncentrosymmetric structure transformation is caused by a simple combination reaction.This work supports the commonly applied strategy of "involving two or more types of asymmetric building units in a structure would have a high possibility to form a noncentrosymmetric structure". 15This insight will shed useful light on the future exploration of new NCS materials.

Fig. 1
Fig. 1 The packing of the ladder chain in (a) Ba 8 Ga 2 Sn 7 Se 18 , 1 with a sequence of .AA a .; and (b) Ba 10 Ga 2 Sn 9 Se 22 , 2 with a sequence of .AA c A t A 21 ..; every two ladder chains in 2 are separated by isolated [Sn 2 Se 4 ] units (dark blue).The asymmetric unit of the ladder chain is marked by atom numbers.Each ladder chain is combined by Chain 1 (orange) and Chain 2 (blue) via sharing of common Se atoms.(c) The [Sn 2 Se 4 ] units in 2 viewed from a direction, and 2 1 screw axis at (1/4, 1/4, z) is visualized as a light blue line.
6(ref.29)) near the Fermi surface, we nd that the Sn-5s state of Sn 4+ has no obvious peak at the VB top, while for Sn 2+ , an obvious contribution of Sn-5s state appears at the VB top, which narrows the band gap.As a result, compounds containing Sn 2+ or mixed valences (Sn 2+ and Sn 4+ ) have smaller band gaps than those containing Sn 4+ .This is in good accordance with the experimental results (1.65 and 1.67 eV for 1 and 2; 2.16, 1.99, and 1.95 eV for Ba 4