L. Sobczyk

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.

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.

Ferroelectric phase transition in [(CH3)3 NH]3 Sb2Cl9 (TMACA)

Abstract The ferroelectric first order phase transition at 367 K has been discovered in TMACA crystals, which is presumably related to a motion of trimethylammonium cations.

A new ferroelectric crystal: (CH3NH3)5Bi2Cl11

Abstract A new ferroelectric crystal from the family of alkylammonium halogenoantimonates and halogenobismuthates, namely (CH3NH3)5Bi2Cl11 (PMACB), was grown. It revealed a second order phase transition at 307 K with a high peak of the dielectric permittivity: ϵ ≃ 5.103 along the c-axis. X-ray diffraction, DSC and dielectric studies showed that PMACB is isomorphous (orthorhombic, space group Pca21) with the bromine analogue (CH3NH3)5Bi2Cl11 (PMABB).

Ferroelectricity and phase transitions in tris (dimethylammonium) nonabromodiantimonate (III)

The crystals of tris (dimethylammonium) nonabromodiantimonate (III) (DMABA) were grown which show two phase transitions at 164 and 228 K. Below 164 K the crystal possesses the ferroelectric properties. At room temperature the crystal is isomorphous with the chlorine derivative: the space group P21/a, a = 14.612(8), b = 9.228(4), c = 10.005(8) A, β = 94.91(6)°, Z = 2. The substantial differences between bromine and chlorine compounds result, most probably, from the fact that NH2 … Br hydrogen bonds are slightly weaker than NH2 … Cl bridges.

Isotopic effects in NH…N hydrogen bonds

Abstract The value of the isotopic ratio (ISR) v s (NH)/ v s (ND) has been determined for a number of crystalline compounds containing homoconjugated [NHN] + cations, including various salts of 1,8-bis(N,N-dimethylamino)naphthalene (DMAN). The results obtained, together with the literature data, enabled us to plot a correlation between the ISR and v s (NH) which appears to be similar to that found by Novak for OHO systems. For DMAN · HPF 6 the highest ISR value known so far (2.05) is reported. An empirical analytical form of the correlation is proposed.

Dielectric properties and phase transitions in (CH3NH3)3Sb2Br9

X-ray diffraction, dielectric and differential scanning calorimetry studies have been performed on (CH3NH3)3Sb2Br9 crystals. The first-order structural phase transitions at 168 and 131K have been revealed to be related most probably to a freezing of the reorientational motion of CH3NH3+ cations.

Phase transitions in (CH3NH3)3Bi2I9 (MAIB)

Abstract Crystals of tris (methylammonium) nonaiododibismuthate were grown which turned out to be isomorphous with Cs3Bi2I9 and (CH3NH3)3Sb2I9. Two phase transitions, at 223 (second order) and 142 K (first order), were revealed. The low temperature phase transition is characterized by a strong dielectric anomaly. The pyroelectric behaviour could indicate a ferrielectric ordering below 142 K.

Structure and IR spectroscopic behaviour of 1,8-bis-(dimethylamino)naphthalene 2,4-dinitroimidazolate

Abstract The crystal structure of 1,8-bis(dimethylamino)naphthalene (DMAN) 2,4-dinitroimidazolate, C 17 H 20 N 6 O 4 , has been determined by X-ray diffraction. The crystals are monoclinic, P2 1 / c , a = 13.426(4), b = 10.465(3), c = 15.915(4) » , β = 126.12(4)°, Z = 4. The structure was solved by direct methods, and refined to an R value of 0.033 for 2291 non-zero independent amplitudes. The [NN⋯N] + bridges of 2.606(3) » with ∠NHN = 160(3)° are characterized by an asymmetric proton density distribution. The IR protonic absorption is located in two regions at about 650 and 1950 cm −1 showing relatively small intensity. The isotopic ratio ν(NHN/ν(NDN) for the low frequency region is almost unity. It seems that hydrogen bonds in protonated DMAN are characterized by a flat asymmetric single minimum potential for the proton motion.