R. Jakubas

Phase transitions in alkylammonium halogenoantimonates and bismuthates

Abstract A review of the results of studies on the structure, phase transitions and physical properties of alkylammonium halogenoantimonates (III) and bismuthates (III) is presented. The crystals of these compounds are characterized by a variety of anionic structures and show a number of phase transitions including those to polar phases. We concentrate here mainly on dielectric and pyroelectric properties, spontaneous polarization and DSC studies. The mechanism of the phase transitions is discussed based on the results of X-ray, dielectric relaxation, PMR, IR and R spectra studies gathered so far. A particular role is played by a reorientational motion of the alkylammonium cations.

Structure and phase transition in (CH3NH3)3Bi2Br9. A novel improper ferroelectrics

Abstract The crystals of (CH3NH3)3Bi2Br9 have been grown which at room temperature are isomorphous with Cs3Bi2Br9 and (CH3NH3)3Sb2Br9. The space group is P3m1, a = 0.807, c = 1.008 nm, V = 0.5685 nm3, Z = 1. The crystals reveal, when cooling, three phase transitions at 188 ± 0.5, 140 ± 1, and 101.5 ± 0.5 K. The high temperature phase (I) behaves as a plastic one with respect to a freedom of rotation of methylammonium cations. The two high temperature transitions I II and II III can be related, similarly to (CH3NH3)3Sb2Br9, to a freezing of reorientation of those cations. At 101.5 K a transition to the improper ferroelectric phase IV was discovered. The features of this phase are non-typical and the nature of the transition is unknown.

A new ferroelectric compound: (CH3NH3)5Bi2Br11

Abstract The crystals of (CH3NH3)5Bi2Br11 have been found to undergo a ferroparaelectric phase transition of the second-order type at 312.1 K. ec,max reaches 5.5 × 103 at Tc and spontaneous polarization 7 × 10−3 Cm−2 at 285 K. The X-ray diffraction study shows that the crystals belong to the orthorhombic symmetry and consist of methylammonium cations and Bi2Br11−5 anions formed by two BiBr6 octahedra linked via one bromine bridge.

Ferroelectric phase transition in tris (dimethylammonium) nonachlorodiantimonate (III), [NH2(CH3)2]3Sb2Cl9

Abstract Measurements of the electric permittivity and the spontaneous polarization revealed a para- to ferro-electric second order phase transition in tris (dimethylammonium) nonachlorodiantimonate at 242 K. The preliminary X-ray diffraction studies showed that, the paraelectric phase has monoclinic symmetry (space group P 2 1 a with a = 14.074(2), b = 9.048(2), c = 9.692(3) A , β = 95.56(2)°, Z = 2).

Ferroelectric properties of (C5H5NH)5Bi2Br11

A new pyridinium compound, (C5H5NH)5Bi2Br11, has been synthesized. The x-ray diffraction studies indicate that its structure is built up of four independent pyridinium cations and isolated Bi2Br115− groups. At room temperature it crystallizes in the monoclinic space group P21/n. Two phase transitions of second- and first-order type are detected by the differential scanning calorimetry measurements at 118 and 405/403 K (on heating–cooling), respectively. The corresponding anomalies in the linear thermal expansion at 118 and 405 K are observed along three crystallographic directions. The dielectric investigations reveal dielectric absorption and dispersion in the radio-frequency region. The experimental results were analyzed on the basis of dielectric formula which represents the sum of two independent relaxators. The pyroelectric measurements reveal that (C5H5NH)5Bi2Br11 becomes ferroelectric below 118 K and is characterized by the appearance of spontaneous polarization of the order of 3⋅10−3 C/m2 along th...

A ferroelectric inorganic–organic hybrid based on NLO-phore stilbazolium

A ferroelectric inorganic–organic hybrid based on in situ substituted stilbazolium cation (TAMS2+ = trimethylamino-N-methyl stilbazolium) has been obtained. Single-crystal X-ray structure analysis demonstrates that the NLO-phore organic component of the complex is embedded into the inorganic polymeric [Bi2Cl8]2− framework and shows a polar arrangement with the cooperation of the nonsymmetric inorganic component. The measurement of ferroelectric and nonlinear optical properties show typical ferroelectricity and second harmonic generation (SHG) responses as well as a significant improvement of the laser damage threshold and thermal stability of relative organic material.

Novel organic–inorganic hybrid ferroelectric: bis(imidazolium) pentachloroantimonate(III), (C3N2H5)2SbCl5

Electric permittivity, reaching ca. 4800 units at Tc, of the first haloantimonate(III) ferroelectric with R2MX5 composition characterized by unprecedented SbCl5 chains of cis-connected octahedra results from the mixed ‘order–disorder’ and ‘displacive’ mechanism of the phase transition.

Structure and ferroelectric properties of (C 3 N 2 H 5 ) 5 (Bi 2 Br 11 )

Pentakis (imidazolium) undecachlorodibismuthate(III) (C{sub 3}N{sub 2}H{sub 5}){sub 5}Bi{sub 2}Cl{sub 11}, has been synthesized and studied by means of the x-ray, differential scanning calorimetry (DSC), and dielectric methods over a wide temperature range. The room temperature crystal structure has been determined as monoclinic, space group P2{sub 1}/n with a=13.616 A, b=14.684 A, c=9.045 A, and {beta}=96.85 deg. Two solid-solid phase transitions: P4n2{yields}{sup 360K}P2{sub 1}/n (I{yields}II), and P2{sub 1}/n{yields}{sup 166K}P2{sub 1} (II{yields}III), discontinuous and continuous in nature, respectively, have been revealed. The dielectric and pyroelectric measurements allow us to characterize the lowest temperature phase (III) as ferroelectric with the Curie point at 166 K and saturated spontaneous polarization value of the order of 6x10{sup -3} C m{sup -2} along the b axis. The ferroelectric phase transition mechanism is governed by the dynamics of imidazolium cations. The room temperature, paraelectric phase (II) demonstrates additionally the ferroelastic properties.

Structure of pentakis(methylammonium) undecachlorodibismuthate(III), [NH3(CH3)]5Bi2Cl11, at 130 K and mechanism of the phase transitions

Pentakis(methylammonium) undecachlorodibismuthate(III), [NH 3 (CH 3 )] 5 Bi 2 Cl 11 , PMACB, at 130 K has the monoclinic structure (MLT). In the low-temperature phase, all the methylammonium cations are ordered (the C-N axes are fixed) and hydrogen bonded by weak N-H...Cl interactions, with distances between 3.04 and 3.42 A. In comparison to the orthorhombic room-temperature phase (ORT), in the MLT phase one can observe only a change in the torsion angle of bioctahedra units without any essential distortion of the BiCl 6 octahedra. The temperature evolution of the monoclinic angle showed that PMACB displays a continuous and subtle orthorhombic→monoclinic (Pca2 1 →P2 1 ) transition of ca 250±10 K. The change in volume of the unit cell around 170 K corroborated the presence of a structural anomaly. It was suggested that the 170 K anomaly is not accompanied by a change in the space group. Analysis of the disordering of the methylammonium cations between 349 and 130 K indicates the leading role played by the methylammonium cations (12) and (22) in the low-temperature transitions at 250 and 170 K. The crystal data of the low-temperature modifications at 130 K are: M r , = 968.3, monoclinic, P2 1 , a = 12.880 (4), b = 13.916 (3), c = 15.059 (1) A, γ = 90.13°, Z = 4, V = 2699 (1) A 3 , D x = 2,383 Mg m -3 , λ(Mo Kα) = 0.7107 A, μ = 140. 878 cm −1 , R = 0.031 for 5931 observed reflections [I > 3σ(I)]

Structure and mechanism of ferroelectric phase transition in tris (dimethylammonium)-nonachlorodiantimonate (III)

Abstract The X-ray diffraction studies were carried out for the crystals of [NH2(CH3)2]3Sb2Cl9 at 293 K in paraelectric phase. The crystals are monoclinic, space group P21/a, with the unit cell parameters: a = 14.045(4), b = 9.018(3), c = 9.670(2) A, β = 95.47(2)° and Z = 2. The SbCl6 distorted octahedra form infinite polyanion layers via chlorine bridges. The cavities in a twelve-membered (‒Sb‒Cl‒)6 rings are occupied by disordered dimethylammonium cations. The IR spectra and dielectric relaxation studies confirm the mechanism of para-ferroelectric phase transition in which the main role play the reorientations of dimethylammonium cations around 180° and their freezing below Tc = 242 K.