J. Mróz

Properties and structural phase transitions of (CH3NH3)3Sb2I9

Abstract The tris (methylammonium) nonaiododiantimonate (III) crystals were grown. The preliminary X-ray diffraction studies showed that at room temperature they are isomorphous with Cs3Sb2I9. The crystals are hexagonal, space group P63/mmc with the unit cell parameters; a = 8.543 (1), c = 21.520 (7) a, Z = 2, V = 1360 A3. The DSC studies revealed two structural phase transitions at T c1 = 147 K, and T c2 = 111 K. The dielectric studies showed only one anomaly at T c1 = 147 K. The high temperature phase behaves like a plastic one with respect to a freedom of rotational motions of methylammonium cations. The dielectric and pyroelectric properties seem to indicate antiferroelectric ordering below T c1.

Crystal structure and dielectric relaxation studies of the [N(CH3)3H]3Sb2(1−x)Bi2xCl9 mixed crystals

The influence on ferroelectric properties of a partial substitution of antimony by bismuth in the [N(CH3)3H]3Sb2Cl9 (TMACA) crystals are investigated by dielectric and x-ray diffraction methods. The dielectric relaxation in the [N(CH3)3H]3Sb2(1−x)Bi2xCl9 (TMACAB) mixed crystals (x=0, 0.07, 0.33, 0.38, and 1) is studied in the frequency range between 20 Hz and 1 GHz and temperature range between 170 and 375 K. The ferroelectric properties of mixed crystals are preserved for the Bi concentration less than 0.33. The increase of the Bi concentration distinctly influences the phase transition temperature but only weakly the dynamical properties of those crystals. The dielectric response of the ferroelectric TMACAB crystals is characterized by a presence of two relaxation modes. One of them behaves critically and the other one is only thermally activated. The disappearance of ferroelectric properties is connected with the essential change of crystal structure of mixed compounds. The anionic sublattice of nonfer...

Investigation of phase transitions in (CH3NH3)5Bi2Cl11 crystals by pyroelectric method

Abstract The pyroelectric properties of (CH3NH3)5Bi2Cl11 crystals were measured in the range 100–320 K. A ferropyroelectric type phase transition at 170 K was confirmed. Critical exponent β ≃ 0.5 was found in the region 0.5–12 K below Tc = 307 K.

Dielectric and pyroelectric properties of (CH3NH3)3Bi2Br9 crystals in the ferroelectric phase transition region

Abstract Dielectric and pyroelectric properties of isomorphous crystals (CH3NH3)3Me2Br9 (Me = Sb, Bi) in the low temperature region have been compared. The observed dielectric dispersion in the vicinity of ferroelectric phase transition of both crystals revealed the complex nature. The ferroelastic domain structure in all low temperature phases of these two salts exhibits considerable similarity.

Dielectric properties of (CH3NH3)5Bi2Br11 crystals under hydrostatic pressure

Abstract The dielectric properties of (CH3NH3)5Bi2Br11 single crystals under hydrostatic pressure within the 0–200 MPa were measured. The positive pressure coefficient dTc/dp = 58 deg GPa−1 close to ferro-paraelectric (II–I) phase transition was found. The pressure dependences confirmed the order-disorder mechanism of the (II–I) phase transition.

Dielectric and ferroelectric properties of the mixed crystals system (CH3NH3)5Bi2Cl11(1-x) Br11x

Abstract Dielectric and pyroelectric properties of the mixed crystals system, (CH3NH3)5Bi2Cl11x (0 < x < 1), were systematically investigated. Temperature dependence of ξ c in the vicinity of ferro-paraelectric phase transition was measured for the crystals with x = 0.91, 0.44, 0.27 0.05 in the frequency region 1 kHz-1 MHz. The substitution of bromine atoms by chlorine ones and vice versa, drastically reduces the magnitude of ξ c at Tc but does not influence on the temperature behaviour of the spontaneous polarization.

Specific heat overcritical behaviour in (CH3NH3)5Bi2Cl11 (MAPCB) crystals

The results of the study of thermal properties of MAPCB crystals in the temperature interval 90-330 K are presented. Specific heat anomalies at 160 K and 307 K are revealed and discussed. It is shown that both anomalies can be described quantitatively in the phenomenological model considering the low-temperature anomaly as an overcritical “trace” of the first order isomorphic phase transition.

Pyroelectric and dilatometric studies on [nh2(ch3)213sb2x9(x =cl, br)

Abstract Pyroelectric and dilatometric properties of isomorphous crystals; [NH2(CH3)2]3Sb2Cl9 (DMACA) and [NH2(CH3)2]3Sb2Br9 (DMABA) have been compared. A nontypical temperature behaviour of P, (T) in both dimethylammonium crystals was found. The paraelectric effect has been observed down to liquid helium temperature. The dilatometric measurements corroborate the continuous ferro-paraelectric phase transition both for DMACA and DMABA.

Influence of pyroelectric charges on the degree of polarization of TGS crystals

Abstract Thermodynamic aspects of the temperature dependent pyroelectric voltage were considered for high load resistances for ferroelectrics with second-order phase transitions. These considerations have been applied to an explanation of the experimentally obtained temperature hysteresis of a pyroelectric voltage in TGS crystals. It was proved that the electric field of a pyroelectric voltage can cause either polarization or depolarization of a crystal, according to the direction of temperature changes.

Dielectric and ferroelectric properties of the mixed crystals system (CH3NH3)5Bi2(1-x)Sb2xCl11

Dielectric and pyroelectric properties of the mixed crystals system, (CH3NH3)5Bi2(1 − x)Sb2xCl11 (0 < x < 0.25) were systematically investigated. Temperature dependencies of e′c in the vicinity of ferro-paraelectric phase transition were measured for the mixed crystals with x = 0.05, 0.07, 0.11, 0.13 and 0.25 in the frequency region 1 kHz–1 MHz. The substitution of bismuth atoms by antimony drastically reduces the magnitude of e′c and shifts the ferro-paraelectric phase transition towards higher temperatures. The dielectric dispersion of the complex electric permittivity, ec∗, in x = 0.05 crystals was studied in the frequency range from 30 to 1000 MHz. Around 321 K phase transition, two dielectric relaxators are postulated; a low-frequency one in the megahertz region showing a critical slowing down and a high-frequency one in the gigahertz region.