Crystal Structure of the Disordered Non-Centrosymmetric Compound Fe 0.43 Mo 2.56 SbO 9.5

: Single crystals of Fe 0.43 Mo 2.56 SbO 9.5 were obtained by hydrothermal techniques at 230 ◦ C. The crystal structure was determined from single crystal X-ray di ﬀ raction data. The compound crystallizes in the non-centrosymmetric space group Pc with unit cell parameters a = 4.0003(2) Å, b = 7.3355(3) Å, c = 12.6985(6) Å, β = 90 ◦ . The crystal structure comprises ﬁve crystallographically independent M atoms and one Sb 3 + atom, M atoms are of two kinds of partially occupied sites Mo 6 + and Fe 3 + . The building blocks consist of [SbO 3 O 0.5 O 0.5 E] octahedra (E = lone electron pair) and [(Mo / Fe)O 6 ] octahedra. The M = (Mo, Fe) and O atoms are arranged in a distorted hexagonal 2D-net, not the Sb atoms. The distortion of the net and consequently the symmetry reduction results mainly from the location of the Sb atoms. Disorder manifests itself as a splitting of the metal sites and as a consequent shortening of the Mo–Fe distances. Six (Mo / Fe)O 6 octahedra are connected to form a pseudohexagonal channel. The Sb 3 + atom is displaced from the pseudo -six-fold axis.


Introduction
The search for crystallographically non-centrosymmetric (NCS) materials is of current interest and of great importance due to the fact that such compounds may show interesting physical properties e.g., non-linear second harmonic generation, ferroelectricity etc. [1][2][3][4][5][6][7][8]. Involving cations having stereochemically active lone pairs e.g., Se 4+ , Te 4+ , Sb 3+ , As 3+ and also d 10 transition metal cations that are susceptible to second order Jahn-Teller distortion in the search for new compounds enhance the possibilities to form NCS materials [9,10].
There are comparatively few quaternary compounds that have been investigated so far compared to the number of ternary compounds, especially in the M-Co-Sb-O (M= Mo, Ag, Na, Sr, Ba) system [13,[20][21][22]. Some of these compounds exhibit interesting magnetic properties showing e.g., spin-glass transitions at low temperatures. Examples include the layered Ag 3 Co 2 SbO 6 and Sr 2 CoSbO 6 and Sr 2 CoSbO 5.63 (semiconductors) Na 3 Co 2 SbO 6 phases [13], the distorted Sr 3 CoSb 2 O 9 (insulator pervoskites [20] or the Ba 3 CoSb 2 O 9 and Ba 2 Co 1.4 Sb 0.6 O 6−y phases [22]. However, there is only one such phase found in the [M-Fe-Sb-O] system; e.g., FeSb 2−x Pb x O 4 (x = 0.2-0.7) [23]. The replacement of Sb 3+ by Pb 2+ induces oxidation of Fe 2+ to Fe 3+ . The substitution by Pb 2+ leads to C-type antiferromagnetic orbital crystal structure compared to A-type antiferromagnetic orbital crystal structure for FeSb 2 O 4 . An example of disordered crystal structure in the [Fe-Mo-O] phase is observed e.g., Fe 2 (MoO 4 ) 3 . The disorder is mainly due to the Mo atom [24,25].
Compounds in the [M-Sb-O] and [M-Fe-Sb-O] systems have mainly been synthesized by solid state reactions that involve heating the constituents in a silica ampoule. The temperature varies from 500 • C to 1600 • C for different systems. In the present study we instead utilize a hydrothermal synthesis technique to grow single crystals of a new quaternary phase with the composition Fe 0.43 Mo 2.56 SbO 9.5 . The crystals show diffuse scattering in addition to the Bragg reflections. After careful examination of many crystals by single crystal X-ray diffraction we concluded that the diffuse scattering, hence disorder, is intrinsic for this compound. The evidence is even more pronounced from the fact of lowering to non-centrosymmetric monoclinic symmetry with six-fold twinning instead of the higher hexagonal symmetry. Unfortunately it was not possible to synthesize phase-pure material for further characterization of the physical properties.

Materials and Methods
A mixture of FeF 2 :MoO 3 :Sb 2 O 3 =1:5:2 in 2 mL deionized water plus a few droplets of HF were sealed in an 18 mL teflon lined steel autoclave and heated to 230 • C at a rate of 1.6 • C/min. The plateau temperature was maintained for four days and thereafter the temperature was lowered to 30 • C with a rate 1.6 • C/min. The following starting chemicals were used: Sb 2 O 3 (99.97%, Sigma-Aldrich, St. Louis, MO, USA), FeF 2 (99.8%, Sigma-Aldrich), and MoO 3 (99.5%, Sigma-Aldrich). The hydrothermal synthesis yielded green single crystals of Fe 0.43 Mo 2.56 SbO 9.5 that were washed using water and ethanol followed by drying at room temperature. An unidentified [Sb-Mo-O] phase was also found from the EDS analysis.
Single crystal X-ray diffraction data were collected using a Bruker D8 Venture diffractometer equipped with a PHOTON 100 detector. Data integration, including correction for oblique incidence, was performed with the software CrysAlis RED [26]. Absorption correction was applied with the computer program SADABS [27]. The crystal structure was solved using the program Superflip [28] and refined by using the program JANA2006 [29]. Fe, Mo, and Sb atoms were refined with anisotropic temperature parameters and oxygens were refined isotropically; the crystal refinement data is summarized in Table 1.
Chemical compositions were obtained by EDS using a Hitachi M3000 tabletop scanning electron microscope and a JEOL JSB-7000F. The content of heavy elements was found to be 10.6 at% Fe, 61.3at% Mo, and 28.1at% Sb, see Supplementary Materials. The expected values are 10at% Fe, 50at% Mo and 40at% Sb. An unidentified [Sb-Mo-O] phase was also found as byproduct with the composition 66at% Sb and 33 at% Mo respectively.

Crystal Structure
The new compound Fe 0.43 Mo 2.56 SbO 9.5 crystallizes in the monoclinic non-centrosymmetric space group Pc with unit cell parameters a=4.0003(2) Å, b=7.3355(3) Å, c=12.6985(6) Å, β=90 • . The refined β is 90.005 (3) which is within three times standard uncertainty. This is why we fixed the β to 90 • , which showed convergence and good fit to data using space group Pc (monoclinic b-axis) compared to the possible space group in the higher symmetry of the orthorhombic cell. All crystallographic parameters from the refinement are summarized in Table 1 (0.04) and the overall occupancy is also close to 1. Mo3 (0.86) also has a higher occupancy than Fe3 (0.14) and together the occupancy is 1, see Table 2. There are ten crystallographically independent oxygen atoms that have been refined isotropically and the O (10) atom is half occupied in a disordered manner.
The Sb atom is coordinated with five oxygen atoms to complete the distorted square pyramid  (7) Å, see Table 3. The Sb-O distances are comparable with the distances found in cubic Sb 2 O 3 and monoclinic Sb 3 O 4 F [30,31]. Bond distances up to 2.76 Å for Sb-O are to be considered to belong to the primary coordination sphere according to the operative definition by Brown [32].

Discussion
The diffraction data suggests that a hexagonal crystal structure with space group P6/mmm would be a possible solution, but attempts to solve the crystals structure in this space group results in a very poor fit of the data. The solution is however sufficiently good to show the semblance of a hexagonal tungsten bronze type arrangement, the metal indicating a puckering of the layer. This, again, is a common structural response to the presence of a lone-pair element distorting the planar crystal structure and disrupting the mirror plane perpendicular to c.
There is no prescribed order for the modeling of this, but it is natural to first lower the symmetry from hexagonal P6/mmm to orthorhombic Cmmm and then to consider symmetry lowering compatible with the absence of a mirror plane perpendicular to c. This produces seven possible direct subgroups: Pmmn, Pman, Pbmn, Pban, Cmm2, C2/m11, C12/m1. The best result was found for Pbnm, but split positions caused by the symmetry remained and the symmetry was subsequently lowered through Pb2n to Pb11. No significant improvement was found on reducing the symmetry to P1. To control the veracity of this result, each of the other six maximal subgroups were taken as starting points for symmetry reduction, but these attempts verified the original result. The crystal structure in the orthorhombic group Pban could be reduced through the path Pban-Pb2n-Pb11 to yield the same result as before, as could Cmm2 via Cmm2-Cm11-Pb11 and Cmm2-Pba2-Pb11, but for the other orthorhombic starting points, the symmetry had to be reduced to P1 to yield a solution comparable to that in Pb11. The monoclinic group C2/m11 likewise could be reduced via C2/m11-Cm11-Pb11 and C2/m11-P2/b11-Pb11 to yield the same solution while C12/m1 yielded only the P1 solution.
The orientation of Mo*/Fe*atoms in the (bc) plane is shown in Figure 4. The cations Mo1*/Fe1*, Mo2*/Fe2*, and Mo3/Fe3 forms a pseudo-hexagonal channel, presented by a dotted line, see Figure 4. The distances in between the heavy atoms of the pseudo-hexagon vary from 3.616 Å to 3.837 Å and the angle varies from 114.55 • to 125.69 • . The antimony and oxygen atoms reside in the pseudo hexagons made up of six Mo*/Fe* and six O atoms. Inside the pseudohexagon there are two four-membered rings made of Mo*/Fe*, Sb and O atoms. Disorder manifests itself as a splitting of the metal sites on Mo*/Fe* in the pseudohexagons, which is why the symmetry is monoclinic instead of hexagonal. Sb is shifted away from the center of the pseudo 3-fold rotation axis, located in the pseudohexagonal channels. There is no prescribed order for the modeling of this, but it is natural to first lower the symmetry from hexagonal P6/mmm to orthorhombic Cmmm and then to consider symmetry lowering compatible with the absence of a mirror plane perpendicular to c. This produces seven possible direct subgroups: Pmmn, Pman, Pbmn, Pban, Cmm2, C2/m11, C12/m1. The best result was found for Pbnm, but split positions caused by the symmetry remained and the symmetry was subsequently lowered through Pb2n to Pb11. No significant improvement was found on reducing the symmetry to P1. To control the veracity of this result, each of the other six maximal subgroups were taken as starting points for symmetry reduction, but these attempts verified the original result. The crystal structure in the orthorhombic group Pban could be reduced through the path Pban-Pb2n-Pb11 to yield the same result as before, as could Cmm2 via Cmm2-Cm11-Pb11 and Cmm2-Pba2-Pb11, but for the other orthorhombic starting points, the symmetry had to be reduced to P1 to yield a solution comparable to that in Pb11. The monoclinic group C2/m11 likewise could be reduced via C2/m11-Cm11-Pb11 and C2/m11-P2/b11-Pb11 to yield the same solution while C12/m1 yielded only the P1 solution.
The orientation of Mo*/Fe*atoms in the (bc) plane is shown in Figure 4. The cations Mo1*/Fe1*, Mo2*/Fe2*, and Mo3/Fe3 forms a pseudo-hexagonal channel, presented by a dotted line, see Figure 4. The distances in between the heavy atoms of the pseudo-hexagon vary from 3.616 Å to 3.837 Å and the angle varies from 114.55° to 125.69°. The antimony and oxygen atoms reside in the pseudo hexagons made up of six Mo*/Fe* and six O atoms. Inside the pseudohexagon there are two four-membered rings made of Mo*/Fe*, Sb and O atoms. Disorder manifests itself as a splitting of the metal sites on Mo*/Fe* in the pseudohexagons, which is why the symmetry is monoclinic instead of hexagonal. Sb is shifted away from the center of the pseudo 3-fold rotation axis, located in the pseudohexagonal channels.

Conclusions
The new compound Fe0.43Mo2.56SbO9.5 crystallizes in the monoclinic non-centrosymmetric space group Pc with unit cell parameters a = 4.0003(2) Å, b = 7.3355(3) Å, c = 12.6985(6) Å, β = 90°. Five partially occupied sites with Mo 6+ /Fe 3+ atoms and one Sb 3+ atoms are present as crystallographically independent atoms. Several (Mo*/Fe*) sites sit at very short distances due to disorder in the crystal structure as evident from diffuse scattering. The lowering of symmetry confirms distortion in the crystal structure. The Sb atoms possess distorted octahedra of [SbO4E] units with two half occupied oxygen atoms. The Mo*/Fe* forms distorted octahedral units of [(Mo*/Fe*)O6]. Disorder manifests itself as a splitting of the metal sites on Mo*/Fe* in the pseudohexagonal channel, which is why lowering the symmetry to monoclinic from hexagonal is found. Sb is located in the pseudohexagonal channels and is shifted away from the center of the pseudo six-fold rotation axis6.

Conclusions
The new compound Fe 0.43 Mo 2.56 SbO 9.5 crystallizes in the monoclinic non-centrosymmetric space group Pc with unit cell parameters a = 4.0003(2) Å, b = 7.3355(3) Å, c = 12.6985(6) Å, β = 90 • . Five partially occupied sites with Mo 6+ /Fe 3+ atoms and one Sb 3+ atoms are present as crystallographically independent atoms. Several (Mo*/Fe*) sites sit at very short distances due to disorder in the crystal structure as evident from diffuse scattering. The lowering of symmetry confirms distortion in the crystal structure. The Sb atoms possess distorted octahedra of [SbO 4  Disorder manifests itself as a splitting of the metal sites on Mo*/Fe* in the pseudohexagonal channel, which is why lowering the symmetry to monoclinic from hexagonal is found. Sb is located in the pseudohexagonal channels and is shifted away from the center of the pseudo six-fold rotation axis6.

E] units with
Supplementary Materials: The following are available online at http://www.mdpi.com/2073-4352/9/1/40/s1, Table S1: Qualitative EDS analyses on single crystals were performed in a scanning electron microscope (JEOL-7000F) showing that all the intended elements are present in Fe 0.43 Mo 2.56 SbO 9.5 . Average composition is calculated considering several points in a crystal. Table S2: (a) Structural parameters of the cations and (b) anions in Fe 0.43 Mo 2.56 SbO 9.5 (Occ. = Occupation factor).Details of crystallographic file is deposited to CCDC (CCDC 1889226).
Author Contributions: S.I.A. planned the synthesis, characterization by EDX, data collection by X-ray diffraction, and preparation of the manuscript. S.L. solved the structure and helped in the discussion. M.J. participated in all the discussion with S.I.A. and S.L.

Funding:
The work was carried out with financial support from Stiftelsen Olle Engkvist Byggmästare and the Swedish Research Council.