Thermal hysteresis and memory effects in TlGaSe2 crystal with incommensurate phase

Temperature dependencies of dielectric permittivity of TlGaSe₂ have been measured under various thermal cycles. Peculiarities of anomalies in temperature dependencies of dielectric permittivity corresponding to structural phase transitions at 108 and 115?K are discussed. The coexistence of two different incommensurate structures in TlGaSe₂ was proposed. The phase transitions at 108 and 115?K are considered as commensurate lock-in transitions. As a result a new model of the structural phase transitions in TlGaSe₂ has been suggested.     

Structural phase transitions in Fe3+ -doped ferroelectric TlGaSe2 crystal

This paper presents the results of dielectric constant and Electron Paramagnetic Resonance (EPR) investigations of Fe³⁺-doped TlGaSe₂ single crystals in the temperature range of 15–300 K. The influence of Fe impurities on dielectric properties and phase transitions of TlGaSe₂ crystal has been studied. The results were considered in comparison with earlier observed results from pure TlGaSe₂ compounds. We observed the considerable decrease of the dielectric constant as well as the change of the shape of the temperature dependence of the dielectric constant in doped crystals. Some certain significant changes of EPR spectra, which are associated with a strong splitting and appearance of additional resonance lines, were observed at the temperatures below 110 K. Such transformations are considered as the result of non-equivalent displacements of different groups of Tl atoms during the structural phase transitions.     

Phase transitions and metastable states in TlGaSe2

The results of measurements of the dielectric constant of TlGaSe₂ in temperature range of successive phase transitions are presented. An anomaly in the temperature dependence of the real part of dielectric constant in TlGaSe₂ has been observed at about 242?K in addition to anomalies at 115, 108, and also near 65?K as reported in previous publications. The presence of temperature hysteresis effects in temperature interval between 115 and 242?K allowed making a conclusion about possible existence of an incommensurate phase in the mentioned temperature range. A model of succession of the structural phase transitions in TlGaSe₂ has been suggested.     

Electret states and the phase transition in a surface layer of the TlGaSe2 ferroelectric semiconductor

The electret polarization is investigated in the TlGaSe₂ ferroelectric semiconductor. It is proved for the first time that stable internal electric fields associated with residual electret polarization are induced in crystals of the TlGaSe₂ ferroelectric semiconductor at temperatures T < 200 K. It is experimentally established that the peak of the pyroelectric current measured in the vicinity of the phase transition to the ferroelectric polar phase depends substantially on the temperature at which the external electric field is switched off when the TlGaSe₂ ferroelectric crystal under investigation is preliminarily cooled from room temperature. The results obtained are discussed in the framework of a model according to which internal electret fields are induced by charges localized at different levels in the bulk and on the surface of the TlGaSe₂ ferroelectric crystal. These fields drastically change at temperatures in a narrow range near 135 K. The inference is made that a phase transition occurs in the surface layer of the TlGaSe₂ crystal at a temperature close to ～135 K.     

The aging effect within incommensurate phase in TlGaSe2

The effects of physical aging on the dielectric constant of TlGaSe₂ layered ferroelectric crystal were investigated. Isothermal aging processes were done within incommensurate phase in TlGaSe₂ ferroelectric single crystals when slow cooling was interrupted for a time at a constant temperature. The dielectric constant exhibited an unusual behaviour: it decreased exponentially with time. The results can be interpreted taking into account the interaction of the structure of the incommensurate phase with crystal fields of defects randomly distributed in the crystal structure. The measurements show that there are two relaxation mechanisms associated with two relaxation sites in mentioned temperature interval. This means that two different types of incommensurate phases occur in mentioned temperature interval.     

Layered character of dielectric function in TlGaSe2 at phase transition from exciton spectroscopy

It is shown that within the temperature region corresponding to paraphasia—ferroelectric phase transition dielectric constant of layered crystals TlGaSe₂ and TlInS₂ can be considered as consisting of two slabs with different dielectric constants ϵ₁, ϵ₂ and thickness d₁ and d₂ (d₁+d₂=c, c is the lattice vector projection in the direction normal to layer). So, the dielectric anomaly and spontaneous polarization occurring at phase transition takes place only in one of the slabs. This model is confirmed by some experimental results, such as dielectric function anisotropy and spectroscopy of excitons at phase transition temperatures.     

Anomalous behaviour of the Urbach edge and phase transitions in TlGaSe2

The long-wavelength tail of the optical absorption in TlGaSe₂, crystals at α=30−150cm^-1 is shown to obey Urbach's rule in the temperature range 4.2–294°K. The anomalous behaviour of the parameters of this rule suggests the presence of two phase transitions in TlGaSe₂ at 246°K and 101°K beside the known phase transitions at 120°K and 107°K. The presence of phase transitions in TlGaSe₂ at 246°K and 101°K is also confirmed by means of the heat capacity measurement.     

The non-isotropic character of electric and dielectric properties of ammonium zinc chloride crystal

The dielectric constant, ε, and the d.c. conductivity, σ, were measured along the a-, b- and c-axes of (NH₄)₂ZnCl₄ (AZC) crystal in the 300-450 K temperature range. Crystals of AZC grown from aqueous solutions containing excess of ZnCl₂ were used. The value of the dielectric permittivity of AZC is extremely small compared to other ferroelectric crystals. Pronounced broad or step-like peaks at the phase transition temperatures were detected along the a- and b-axes, while ε along the c-axis is temperature independent up to the end of the measuring range. Reciprocal of the dielectric permittivity in the range of the commensurate to incommensurate phase transition obeys a relation similar to the Curie-Weiss law that is valid for second order ferroelectric/paraelectric phase transitions. The constants of the proposed relationship applied to the cooling run are given. The J-E characteristics along the three crystallographic axes were measured in the normal, incommensurate, commensurate and antiferroelectric phases. Hence, the type of conduction mechanism has been estimated. Parameters of Poole-Frenkel and Richardson-Schottky types of conduction mechanism have been determined. The effect of applied electric field on the conductivity measurement was also tested. Conductivity anomalies with different character were observed at the phase transition temperatures. The lnσ−1000/T dependence revealed thermal activation energy of conduction along the a-, b- and c-axes with different values in different phases of AZC.     

Effect of external fields on the memory effect of the incommensurate phase in the ferroelectric-semiconductor TlGaSe<Subscript>2</Subscript>

The effect of external fields (dc electric field, light illumination) on the memory effect of the incommensurate phase in the ferroelectric-semiconductor TlGaSe₂ is studied using the measured dielectric constant. The results obtained are discussed. It is shown for the first time that the effect of external fields on the anomaly related to the memory effect in TlGaSe₂ can be reduced to the following universal empirical rule: when a sample is held for many hours at a constant temperature T ₀ in the temperature range of the incommensurate phase in a dc electric field, the deflection amplitude in the low-temperature part of the anomaly in the temperature dependence of the relative change in the dielectric constant Δ?/? increases (the deflection in the high-temperature part of the Δ?/? anomaly disappears) as compared to this segment in the dependence obtained during isothermal annealing of this sample at the same temperature without an electric field. The crystal remembers its thermal history at a temperature that is several kelvins higher than T ₀. Light illumination increases the deflection amplitude in the high-temperature part of the Δ?/?(T) anomaly and shifts the temperature at which the crystal remembers its thermal history toward lower temperatures with respect to T ₀.     

Low temperature phase transitions and magnetic structure of PrB6

Specific heat, thermal expansion and electrical resistivity measurements on PrB₆ single crystals show that there are two low temperature phase transitions at 6.9 K and 4.2 K, respectively, the latter temperature varying somewhat among different crystals. Neutron diffraction measurements were made on both single and polycrystalline samples of PrB₆. The neutron data indicate a spontaneous incommensurate magnetic ordering at 6.9 K with Q = (0.23, 0.23, 0.5) 2π/a_o. At 4.2 K a commensurate magnetic phase is seen with Q = (0.25, 0.25, 0.5) 2π/a_o coexisting with the incommensurate phase At 1.74 K, only the commensurate phase remains. A model is proposed for the commensurate antiferromagnetic structure and a profile analysis based on that model yields a magnetic moment of 1.77 Bohr magnetons per praseodymium ion at 1.74 K.     

The Urbach tails and optical absorption in layered semiconductor TlGaSe<Subscript>2</Subscript> and TlGaS<Subscript>2</Subscript> single crystals

TlGaSe₂ and TlGaS₂ single crystals were grown by the modified Bridgman-Stockbarger method. We report the result of an experimental study of the optical absorption of TlGaSe₂ and TlGaS₂ crystals. The absorption measurements were performed in steps of 10 K. The direct and indirect band gaps for TlGaSe₂ and TlGaS₂ samples were calculated as a function of temperature. The phonon energies in TlGaSe₂ and TlGaS₂ crystals were calculated as (39±4) and (9±4) meV at 240 K, respectively. At 10 K, direct and indirect band gaps were found as 2.294 and 2.148 eV for TlGaSe₂, 2.547 and 2.521 eV for TlGaS₂ crystals, respectively. The abrupt changes were observed in the direct and indirect band gaps in the some temperature ranges. These changes were interpreted as phase transformation temperatures. The steepness parameters and Urbach energy for TlGaSe₂ and TlGaS₂ samples increased with increasing sample temperature in the range (10–320) K.     

Magnetodielectric effects in Co-implanted TlInS2 and TlGaSe2 crystals

The results of investigations of dielectric and magnetodielectric properties of ternary layered TlInS₂ and TlGaSe₂ ferroelectric crystals implanted with 40 keV Co⁺ ions at the fluency of 1.0 × 10¹⁷ ion/cm² are presented. The temperature dependences of the dielectric susceptibility of Co-implanted samples showed that the formation of metal nanoparticulate composite layer in the near-surface irradiated region as a result of high-fluency Co implantation causes considerable shifts of well-known successive structural phase transition points to high temperatures in heating regime. It has been revealed that the application of the magnetic field in the direction perpendicular to implanted surface results in shifting of the phase transition points to low temperature region. The observed peculiarities are considered as magnetocapacitance (magnetodielectric) effects, which appeared as a result of magnetoelectric lock-in interaction between domains of ferroelectric and ferromagnetic substances of the composite structure.     

Colossal magnetostriction effect in HoMn2O5

We have investigated the detailed field and temperature dependence of the dielectric constant, electric polarization, magnetization and magnetostriction in orthorhombic HoMn₂O₅ single crystals. HoMn₂O₅ displays incommensurate antiferromagnetic ordering below 39 K, becoming commensurate on further cooling. The commensurate-incommensurate transition takes place at low temperatures. The inherent magnetic frustration in this material is lifted by a small lattice distortion, primarily involving shifts of the Mn³⁺ cations and giving rise to a canted antiferroelectric phase. Colossal magnetostriction effect was observed and a novel phase transition diagram was build.     

Anisotropy of the hopping conductivity in TlGaSe2 single crystals

The temperature dependences of the conductivities parallel and perpendicular to the layers in layered TlGaSe₂ single crystals are investigated in the temperature range from 10 K to 293 K. It is shown that hopping conduction with a variable hopping length among localized states near the Fermi level takes place in TlGaSe₂ single crystals in the low-temperature range, both along and across the layers. Hopping conduction along the layers begins to prevail over conduction in an allowed band only at very low temperatures (10–30 K), whereas hopping conduction across the layers is observed at fairly high temperatures (T?210 K) and spans a broader temperature range. The density of states near the Fermi level is determined, N _F=1.3×10¹⁹eV·cm³)^?1, along with the energy scatter of these states J=0.011 eV and the hopping lengths at various temperatures. The hopping length R along the layers of TlGaSe₂ single crystals increases from 130 Å to 170 Å as the temperature is lowered from 30 K to 10 K. The temperature dependence of the degree of anisotropy of the conductivity of TlGaSe₂ single crystals is investigated.     

Studies of the electrical properties of nearly perfect K2ZnCl4 single crystals as measured during commensurate incommensurate-paraelectric phase transitions

Electrical conductivity and dielectric measurements were carried in the temperature range covering the commensurate ferroelectric-incommensurate-paraelectric normal phases (300-600 K) for the three main crystallographic axes of K₂ZnCl₄ single crystals. The values of activation energies in the three phases were calculated and discussed. A thermal hysteresis of about 12 K is observed which deduce the presence of first order transition for the lock-in ferroelectric transition at T_c=404 K. Conductivity anomalies were observed in both ferroelectric and paraelectric phases. The conduction mechanism was discussed. The suggested occurrence of discommensuration in K₂ZnCl₄ crystals upon the lock-in transition in contrast with conductivity and dielectric results explains the anomalous behavior for the b-axis measurements. The orientation of these discommensuration was discussed on a view of projection in the three standard crystallographic directions.     

μSR investigation of the magnetic ordering in EuAs3

We have investigated the magnetic ordering and the incommensurate-commensurate phase transition in EuAs₃ by zero-field (ZF) and longitudinal-field μSR. In the commensurate phase, stable at temperatures below T_L=10.3 K, the ZF muon signal exhibits oscillations corresponding to four muon precession frequencies the lowest of which behaves anomalously. The muon signal shows no oscillation but exponential decay in the incommensurate phase stable in temperature range from T_L≈10.3 K up to T_N≈11 K. The temperature dependence of the fitted relaxation rate shows divergence-like behaviour at the ordering temperature T_N≈11 K and also at the lock-in transition T_L≈10.3 K. The results are in qualitative agreement with those previously obtained by neutron and X-ray magnetic scattering investigations except for the anomalous temperature dependence of the lowest frequency in the commensurate phase. We propose a model for this anomalous behaviour.     

Magnetoelectric behavior in the complex CaMn7O12 perovskite

We report a magnetoelectric effect in the double perovskite CaMn₇O₁₂, that shows a complex magnetic behavior below 90 K with two magnetic phases coexisting (one ferrimagnetic and the other modulated). A second magnetic transition, associated with changes in the magnetic modulation and magnetic ordering coherence lengths of the two magnetic phases occurs at 50 K (T_N2). A detailed structural characterization of this compound, that we have carried out by means of high-resolution X-ray powder diffraction, reveals an anisotropic thermal expansion of its lattice parameters at 50 K (T_N2). In addition, our study of the complex permittivity of this sample as a function of temperature, frequency and magnetic field shows very interesting results below 90 K and specially below 50 K: the dielectric constant ε′_r that was decreasing continuously on cooling experiences an upturn, and even more, on application of a magnetic field it shows a moderate magnetoelectric response. We attribute such dielectric behavior to the formation of electric dipoles by magnetostriction in this charge and spin ordered system, that are sensible to the presence of an external magnetic field.     

Properties of the secondary order parameter in Rb2ZnCl4

The dielectric constants and the spontaneous polarization of Rb₂ZnCl₄-crystals have been investigated in order to characterize the nature of the transitions at 303, 195 and 74.5 K. The dielectric anomalies around 303 K hint at a critical exponent β = 0.36 ± 0.03 in the incommensurate phase. Close to 195 K the commensurate state can be induced by electric fields in agreement with an appropriate Clausius-Clapeyron relation. The polarization measurements further show that Rb₂ZnCl₄ has a (second) ferroelectric transformation at 74.5 K.     

Study of the ferroelectric phase transition of TlGaSe2 by dielectric,calorimetric, infrared and X-ray diffraction measurements

The real part of the dielectric constant , the heat capacityc _p, the infrared reflectivity, and the X-ray diffraction of TlGaSe₂ have been measured in the temperature range from 12 K (30K) to 300 K. Both andc _p show two anomalies at about 110 K and 120 K. A study of the hysteresis loop as well as an investigation of the dielectric dispersion in the microwave region show that the phase below 110 K is ferroelectric. The crystal structure remains nearly unchanged in the course of the phase transition. The loss of the symmetry (C2/cCc) results from small positional shifts of the T1 atoms in the ab plane accompanied by a discontinuity in the axial ratios. We suggest, that the ferroelectricity is caused by the stereochemically active electron lone pair configuration of the Tl⁺ ion. Thus TlGaSe₂ may provide the first example for ferroelectricity caused by this mechanism.     

Defects forming the optical absorption edge in TlGaSe2 layered crystal

In this work, we present the results of optical experiments designed to investigate the changes in optical absorption spectra of TlGaSe₂ ferroelectric-semiconductor with incommensurate (INC) phase in experimental conditions where crystal is kept several hours within the INC-phase (the regime of so called “memory” effect). The fundamental absorption of TlGaSe₂, experimentally investigated by optical transmission measurements performed in the temperature range 15–300 K. An extraordinary modification of the optical absorption edge in the range of Urbach's tail is discovered as a result of the annealing within the INC-phase. The role of native defects forming the band edge in the observed phenomena in TlGaSe₂ is discussed.