Structural characterization, thermal, dielectric and vibrational properties of tris(allylammonium) hexabromoantimonate(III), (C 3 H 5 NH 3 ) 3 SbBr 6 I. Płowas ´ a , A. Białon ´ ska a , R. Jakubas a , G. Bator a, * , B. Zarychta b , J. Baran c a Faculty of Chemistry, University of Wrocław, Joliot-Curie 14, 50–383 Wrocław, Poland b Institute of Chemistry, University of Opole, Oleska 48, 45-951 Opole, Poland c Institute of Low Temperature and Structure Research, PAS, Okólna 2, 50-950 Wrocław, Poland article info Article history: Received 14 April 2010 In final form 13 July 2010 Available online 18 July 2010 Keywords: Hexabromoantimonate(III) Allylammonium Thermal properties Dielectric properties X-ray diffraction Infrared and Raman spectroscopy abstract The novel inorganic–organic hybrid material, allylammonium hexabromoantimonate(III), (C 3 H 5 NH 3 ) 3 - SbBr 6 , has been synthesized and its structure has been determined by means of the single-crystal X- ray diffraction studies at five temperatures (273, 248, 220, 170 and 100 K). At room temperature the com- pound crystallizes in the monoclinic space group, C2/m. Its crystal structure is composed of the discrete SbBr 3 6 anions and three non-equivalent allylammonium, (C 3 H 5 NH 3 ) + , cations. In (C 3 H 5 NH 3 ) 3 SbBr 6 three solid–solid structural phase transitions are detected: a continuous one at 260/256 K (on heating–cooling) from phase I to II, a discontinuous one at 227/208 K (II?III) and another discontinuous at 197/191 K (III?IV). The electric properties of the compound have been measured in a wide temperature region (150–300 K). Temperature-dependent vibrational properties in the frequency region 3500–500 cm 1 have been reviewed. Possible mechanisms of the phase transitions in (C 3 H 5 NH 3 ) 3 SbBr 6 are discussed on the basis of the presented results. Ó 2010 Elsevier B.V. All rights reserved. 1. Introduction Halogenoantimonates(III) and halogenobismuthates(III) of the general formula R a M b X 3b+a (where R denotes the organic cation, M–Sb(III) or Bi(III) and X–halogen atom: Cl, Br, I) cause much inter- est because of their unique properties and potential applications as a nonlinear polar and nonlinear optical materials [1–3]. Numerous structural studies show that these metal(III) halide complexes are characterized by a rich diversity of the anionic forms [4,5]. The an- ionic substructure, because of its large electric polarizability, was found to play an essential role in the generation of polar properties. It has turned out that the ferroelectric properties had some ten- dency to appear in the two-dimensional (2D) anionic sublayers encountered in the R 3 M 2 X 9 -type compounds. On the other hand, several synthesized ferroelectrics crystallize in R 5 M 2 X 11 composi- tion, where the discrete bioctahedral units, M 2 X 5 11 , constitute the anionic sublattice [6–11]. The role of the cations, their size and symmetry, is not fully recognized yet. It is known, however, that in the case of 2D ferroelectrics, characterized by the R 3 M 2 X 9 com- position, their cations, such as monomethyl-, dimethyl- or trime- thylammonium [12–14], are rather small. On the other hand, in the R 5 M 2 X 11 compounds the polar properties are found in either the small alkylammonium cations (methylammonium) or the aro- matic ones (pyridinium and imidazolium) [15–18]. The ferroelec- tricity of these subgroups of crystals is due to the reorientational motion of the organic cations possessing a permanent dipole mo- ment. Additionally, the interaction between the cationic and anionic substructures significantly affects the creation of ferroelec- tricity in these organic–inorganic hybrid materials. The R 5 M 2 X 11 - type compounds belong to the promising materials because their dielectric properties resemble to some extent those found in the well-known ferroelectric TGS-family (TGS – tri-glycine sulfate) [19]. The substituted aromatic organic cations embedded in the empty spaces of a halogenoantimonates(III) or halogenobismuth- ates(III) ionic matrices have not been properly covered in the stud- ies on the hybrid crystals. Nevertheless, some examples of compounds which experience structural phase transitions (SPTs) have been found. The pyridinium derivatives (C 5 H 5 NH)SbCl 4 [20] and (C 5 H 5 NH)SbBr 4 [21], undergo the order–disorder phase transi- tions connected with the dynamics of pyridine rings. In these salts the pyridinium cations perform free reorientation about the pseu- do-sixfold axis perpendicular to the ring plane. In turn, in the halogenoantimonates(III) containing in their structure substituted pyridinium cations, e.g., in (4  CH 3 C 5 H 4 NH) 2 SbBr 5 [22] or in (4  CH 3 C 5 H 4 NH)SbCl 4 [23], the cationic motion is more restricted in comparison to that in the unsubstituted pyridinium analogs. Thus, as a rule, in such a type of compounds their room tempera- ture phases are partially or completely ordered. 0301-0104/$ - see front matter Ó 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.chemphys.2010.07.012 * Corresponding author. E-mail addresses: gb@wchuwr.pl, gb@wchuwr.chem.uni.wroc.pl (G. Bator). Chemical Physics 375 (2010) 16–25 Contents lists available at ScienceDirect Chemical Physics journal homepage: www.elsevier.com/locate/chemphys