A. Dziaugys

DIELECTRIC PROPERTIES OF Cu6PS5I SINGLE CRYSTALS

ABSTRACT The dielectric properties of Cu6PS5I crystals were investigated in broad frequency (20 Hz–1 GHz) and temperature (25 K–300 K) range. At high temperatures and low frequencies dielectric spectra are highly influenced by very high ionic conductivity. The temperature dependence of the static dielectric permittivity reveals just one phase transition at Tc1 = 156 K. The frequency spectra of conductivity, at low frequencies (20 Hz–1 MHz) obeys the Almond-West power law. The activation energy of conductivity changes at 194 K, which indicates the second phase transition of Cu6PS5I.

CONDUCTIVITY SPECTROSCOPY OF NEW AgInP2S6 CRYSTALS

ABSTRACT The dielectric properties of newly synthesized AgInP2S6 crystals were investigated in broad frequency (20 Hz–1 MHz) and temperature range (110 K–350 K). No anomaly in temperature dependence of complex dielectric permittivity indicating the polar phase transition can be detected down to the lowest temperatures. The dielectric properties of presented crystals are mainly caused by high conductivity. The frequency spectra of conductivity obey the Almond-West power law. The activation energy of conductivity was found to be E A /k = 7734 K. A suggestion is given that the electric conductivity in these crystals can be interpreted by ionic mechanism.

Valence fluctuations in Sn(Pb)2P2S6 ferroelectrics

The valence fluctuations which are related to the charge disproportionation of phosphorous ions P 4 + + P 4 + → P 3 + + P + 5 are the origin of ferroelectric and quantum paraelectric states in Sn(Pb)2P2S6 semiconductors. They involve recharging of SnPS3 or PbPS3 structural groups which can be represented as half-filled sites in the crystal lattice. Temperature–pressure phase diagram for Sn2P2S6 compound and temperature-composition phase diagram for (PbySn1–y)2P2S6 mixed crystals, which include tricritical points and where a temperature of phase transitions decrease to 0 K, together with the data about some softening of low energy optic phonons and rise of dielectric susceptibility at cooling in quantum paraelectric state of Pb2P2S6 are analyzed by GGA electron and phonon calculations and compared with electronic correlations models. The anharmonic quantum oscillators model is developed for description of phase diagrams and temperature dependence of dielectric susceptibility.

Chemical Bonding and Polarons in Sn2P2S(Se)6 Ferroelectrics

For ferroelectric-semiconductor Sn2P2Se6 the low temperature dielectric relaxation was found to be similar to that observed in Sn2P2S6 analog. This relaxation behavior was explained as an evidence of small polaron dynamics in the lattices with different covalence of bonds between atoms at presence of tin and sulfur (selenium) vacancies. The calculations in LDA approach of the electron band structure and the energy levels of intrinsic defects together with analysis of the Raman spectra and photoluminescence temperature behavior give evidences of polaron dynamics joining with donor-acceptor compensation processes.

Anisotropy effects in thick layered CuInP2S6 and CuInP2Se6 crystals

Results of broadband dielectric investigations of thick layered CuInP2S6 and CuInP2Se6 crystals in direction parallel and perpendicular to the layers are reported. No dielectric anomaly indicates spontaneous polarization appearance in the direction parallel to the layers in CuInP2S6 and CuInP2Se6 crystals. In this direction the dielectric dispersion is mostly caused by Maxwell-Wagner relaxation and ionic conductivity. The typical for ferroelectric phase transition dielectric anomaly is observed only in the direction perpendicular to the layers in both crystals. The activation energy of DC conductivity is much higher in the direction perpendicular to the layers, which confirms ionic nature of electrical conductivity in both crystals.

Phase transitions in CuBiP2Se6 crystals

Investigation results of dielectric (20 Hz–1 MHz) properties of layered CuBiP2Se6 crystals are presented. The temperature dependence of the static dielectric permittivity reveals the first-order “displacive” antiferroelectric phase transition at T c = 136 K. In the paraelectric phase, at low frequencies, dielectric spectra are highly influenced by the high ionic conductivity with the activation energy of 2473 K (0.21 eV). In the antiferroelectric phase the electrical conductivity and its activation energy (531.1 K (0.045 eV)) are considerably smaller. At low temperatures, the temperature behaviour of the distribution of relaxation times reveals complex freezing phenomena. A part of long relaxation time distribution is strongly affected by external direct current (DC) electric field and it is obviously caused by antiferroelectric domain dynamics.

Broadband dielectric investigations of indium rich CuInP2S6 layered crystals

Abstract Results on dielectric properties of layered indium rich CuIn1+δP2S6 crystals are presented. At low frequencies and high temperatures, the dielectric spectra are strongly influenced by the high ionic conductivity with the activation energy corresponding to 6815 K (0.587 eV). The high-frequency part of the dielectric spectra is determined by the relaxational soft mode. The critical slowing down and Debye-type dispersion show an order–disorder type of the ferrielectric phase transition at 330 K, which is substantially higher than for pure CuInP2S6. The high ferrielectric phase transition temperature suggests an idea that a frustrated ordering should appears in indium sublattice at low temperatures. Indeed, at low temperatures the complex dielectric permittivity exhibits behaviour typical for dipolar glasses.

Phase Transitions in Layered Semiconductor - Ferroelectrics

Andrius Dziaugys1, Juras Banys1, Vytautas Samulionis1, Jan Macutkevic2, Yulian Vysochanskii3, Vladimir Shvartsman4 and Wolfgang Kleemann5 1Department of Radiophysics, Faculty of Physics, Vilnius University, 2600 Vilnius 2Center for Physical Sciences and Technology, A. Gostauto 11, 2600 Vilnius 3Institute of Solid State Physics and Chemistry, Uzhgorod University, Uzhgorod 88000 4Institute for Materials Science, Duisburg-Essen University, 45141 Essen 5Faculty of physics, Duisburg-Essen University, 47048 Duisburg 1,2Lithuania 3Ukraine 4,5Germany

Dielectric Investigations of Layered Mn2P2S6 and Cu0.52Mn1.74P2S6 Single Crystals

The dielectric properties of Mn2P2S6 and Cu0.52Mn1.74P2S6 crystals were investigated in broad frequency (20 Hz – 1 MHz) and temperature range (100 K – 470 K). No anomaly in temperature dependence of complex dielectric permittivity indicating the polar phase transition can be detected in measured temperature range. The dielectric properties of presented crystals are mainly caused by high conductivity. The frequency spectra of conductivity obey the Almond-West power law. The activation energy of conductivity was found to be EA/k = 0.845 eV (Mn2P2S6) and EA/k = 0.81 eV (Cu0.52Mn1.74P2S6). A suggestion is given that the electric conductivity in these crystals can be interpreted by electronic mechanism.

Piezoelectric and Ultrasonic Studies of New Lamellar Crystals of CuInP2S6 Type

The ultrasonic and piezoelectric investigations were performed in CuInP2S6 family layered crystals, where Cu, In, S were substituted by Ag, Bi and Se. We have shown that in CuBiP2Se6 crystals the anomalies of piezoelectric sensitivity and ultrasonic velocity appears in the vicinity of two phase transitions at 145 K and 220 K. In AgBiP2S6 layered crystals the only one anomaly of ultrasonic velocity and attenuation was observed at 220 K. In these crystals below room temperature the large ultrasonic attenuation and corresponding velocity dispersion were observed. Such ultrasonic behaviour was supposed to be determined by acoustoionic interaction due to large ionic conductivity of Ag+ cations.