— We have studied the phase diagram of the TlInSe 2 –TlTmSe 2 system and shown that it contains a congruently melting compound with the composition Tl 2 InTmSe 4 (1 : 1 ratio of the constituent selenides). At room temperature, TlInSe 2 dissolves 5 mol % TlTmSe 2 . According to X-ray diffraction data, Tl 2 InTmSe 4 crystallizes in tetragonal symmetry. The electrical conductivity, Hall coefficient, and thermal conductivity of the Tl 2 InTmSe 4 compound have been measured as functions of temperature. We have determined the conductivity type and band gap of Tl 2 InTmSe 4 crystals. It has been shown that charge carriers in the Tl 2 InTmSe 4 compound are scattered predominantly by longitudinal acoustic phonons.

We have synthesized samples based on the layered compound TlGaSe2 and containing thulium: (1–x)TlGaSe2 · xTm with x = 0.001, 0.005, 0.01, and 0.02. The polycrystalline samples have been used as charges for growing crystals with the corresponding compositions by the Bridgman method. The phase composition of the (1–x)TlGaSe2 · xTm samples has been determined by X-ray diffraction analysis. Their dielectric properties have been studied in ac electric fields at frequencies in the range f = 5 × 104 to 3.5 × 107 Hz. We have identified the relaxation character of the dielectric permittivity, the nature of the dielectric loss, and the hopping mechanism of charge transport in the (1–x)TlGaSe2 · xTm crystals. Our results demonstrate that increasing the thulium concentration in the crystals reduces the mean hop distance and time of charge carriers and increases the ac conductivity and the density of localized states near the Fermi level in the crystals.

Синтезированы образцы на основе слоистого TlGaSe2, содержащие тулий: (1-x)TlGaSe2 xTm, с х=0.001, 0.005, 0.01 и 0.02. Из полученных поликристаллических образцов методом Бриджмена выращены кристаллы соответствующих составов. Состав образцов (1-x)TlGaSe2 xTm определен методом рентгенофазового анализа. Изучены диэлектрические свойства образцов в переменных электрических полях с частотой f=5 104-3.5 107 Гц. Установлены релаксационный характер диэлектрической проницаемости, природа диэлектрических потерь, а также прыжковый механизм переноса заряда в образцах (1-x)TlGaSe2 xTm. Показано, что с увеличением концентрации тулия среднее расстояние и время прыжков носителей заряда уменьшаются, а проводимость на переменном токе и плотность локализованных состояний вблизи уровня Ферми увеличиваются.

Triple compounds with the formulas TlFeS2 and TlFeSe2 and magnetic properties are investigated via X-ray diffraction, differential-thermal, and thermogravimetric analyses. Their crystal structures are found. It is ascertained that they are described by monoclinic space group C2/m under normal conditions. The atomic coordinates and interatomic distances are determined. From differential-thermal and thermogravimetric analyses performed at temperatures of 30–900°C, it is revealed that stronger covalent bonds are implemented in TlFeS2 crystals than in TlFeSe2 compounds. The effects associated with energy exchange in the compounds under study are observed. The physical nature of the observed effects is discussed.

Методами рентгеновской дифракции, дифференциально-термического и термогравиметрического анализа исследованы тройные соединения составов TlFeS2 и TlFeSe2, обладающие магнитными свойст- вами. Определены их кристаллические структуры, и установлено, что в нормальных условиях они описываются моноклинной пространственной группой симметрии С2/m. Определены координаты атомов и межатомные расстояния. В результате дифференциально-термического и термогравимет- рического анализа в температурном диапазоне 30-900 °C установлено, что в кристаллах TlFeS2 по сравнению с TlFeSе2 реализуются более сильные ковалентные связи. Наблюдаются эффекты, связанные с обменом энергии в исследуемых соединениях. Обсуждается физическая сущность наблюдаемых эффектов.

The effect of silver ions (2 mol %) on the dielectric properties and electrical conductivity of TlGaS 2 single crystals grown by the Bridgman–Stockbarger method is investigated. The experimental results of studying the frequency dispersion of the dielectric coefficients of TlGaS 2 single crystals (2 mol % Ag) makes it possible to establish the nature of dielectric losses and the charge-transfer mechanism, to evaluate the density of states near the Fermi level, the spread of states, the average hopping time and length, and the concentration of deep traps responsible for ac conductivity. The Ag doping of the TlGaS 2 single crystals results in an increase in the density of states near the Fermi level and in a decrease in the average hopping time and length.

AbstractThe effect of silver ions (2 mol %) on the dielectric properties and electrical conductivity of TlGaS_2 single crystals grown by the Bridgman–Stockbarger method is investigated. The experimental results of studying the frequency dispersion of the dielectric coefficients of TlGaS_2 single crystals (2 mol % Ag) makes it possible to establish the nature of dielectric losses and the charge-transfer mechanism, to evaluate the density of states near the Fermi level, the spread of states, the average hopping time and length, and the concentration of deep traps responsible for ac conductivity. The Ag doping of the TlGaS_2 single crystals results in an increase in the density of states near the Fermi level and in a decrease in the average hopping time and length.

The single crystals of (TlGaS2)1–х(TlInSe2)х (х = 0...0.5) solid solutions have been grown up. The photoelectric, X-ray dosimetric, dielectric and optical characteristics of the (TlGaS2)1-х(TlInSe2)х solid solutions with various compositions have been determined. The maximum and spectral range of photosensitivity were found to redshift as x increases from 0 to 0.5. Both the photoand X-ray sensitivity of these solid solutions are higher than those of pure TlGaS2. The nature of dielectric losses and the hopping mechanism of charge transport in the (TlGaS2)1–х(TlInSe2)х solid solutions have been established from the experimental results on high-frequency dielectric measurements. The temperature dependences of exciton peak position for various compositions (x = 0...0.3) have been investigated within 77...180 K temperature interval. It has been ascertained that, with increasing x in (TlGaS2)1–х(TlInSe2)х solid solutions, the width of their forbidden gap decreases.

Проведены исследования влияния состава и температуры на электропроводность и диэлектрическую проницаемость системы Tl(GaS2)1-x(InSe2)x. Установлено, что с ростом температуры диэлектрическая проницаемость и удельная проводимость увеличиваются, а с ростом концентрации InSe2 изменяются по линейному закону, при этом проводимость уменьшается, а диэлектрическая проницаемость растет. Подтверждено наличие в исследованной системе двух рядов твердых растворов. Показано, что с ростом концентрации InSe2 температуры фазовых переходов соразмерная-несоразмерная фаза смещаются в область более низких температур. Работа выполнена при поддержке Белорусского фонда фундаментальных исследований (договор N Ф15-016 от 04 мая 2015 г.). DOI: 10.21883/FTT.2017.08.44745.22

The effect of composition and temperature on the electrical conductivity and the permittivity of the Tl(GaS2)1 – x (InSe2) x has been studied. It is found that the permittivity and the conductivity increase with temperature and vary linearly as the InSe2 concentration increases: in this case, the conductivity decreases and the permittivity increases. It is confirmed that the system under study contains two series of solid solutions. It is shown that the commensurate–incommensurate phase transition temperatures shift to lower values as the InSe2 concentration increases.

The unit-cell parameters of crystals obtained in the Tl(GaS 2 ) 1– x (InSe 2 ) x system are measured by X-ray diffraction. The relationship between these parameters and composition is determined. It is shown that, with growing x , the a , b , and c parameters increase and the β angle decreases. Two types of solid solutions are found in the Tl(GaS 2 ) 1– x (InSe 2 ) x system: one is based on compound TlGaS 2 with monoclinic structure and the other is based on TlInSe 2 with tetragonal structure.

The compounds TlInSe 2 and TlGaTe 2 have been prepared by direct elemental synthesis, and the (TlInSe 2 ) 0.2 (TlGaTe 2 ) 0.8 solid solution has been synthesized from them. Crystals of the (TlInSe 2 ) 0.2 (TlGaTe 2 ) 0.8 solid solution with a tetragonal structure have been grown, and their dc electrical conductivity and thermo-electric power have been measured in the temperature range 77–347 K. The results demonstrate that, at low temperatures, in the region of hopping conduction, the thermoelectric power of (TlInSe 2 ) 0.2 (TlGaTe 2 ) 0.8 is proportional to T . With increasing temperature, the thermoelectric power becomes inversely proportional to temperature when the conductivity begins to be dominated by charge carriers excited to the allowed band.

The results of high-frequency dielectric measurements on obtained TlIn1-xErxSe2 single crystals provided an opportunity to determine the mechanisms of dielectric losses and charge transport, and also to evaluate the density of states at the Fermi level; the average time of charge carrier hopping between localized states, average hopping distance, scattering of trap states near the Fermi level; concentration of deep traps.

The crystal structure of the semiconductors TlFeSe2 and TlFeS2 has been studied by mean of neutron diffraction at room temperature and high pressure up to 4.2 GPa and 5.2 GPa, respectively. The chain-like monoclinic crystal structure with space group C2/m remains unchanged in both compounds in the entire pressure range. The anisotropic lattice compression and the large difference in the bulk modulus and the compressibility coefficients of the structural parameters of the two compounds have been observed. The mechanism of the observed phenomena was discussed. The enhancement of the one-dimensional antiferromagnetic and metallic character was predicted.

The temperature dependences of electrical conductivity over localized states in the solubility region of mixed crystals of compositions (TlInSe2)1–x (TlGaTe2) x at x = 0, 0.1 0.2, 0.8, 0.9, and 1 are studied. It is established that hopping conductivity with a variable hop length over localized states near the Fermi level occurs in this system. The conductivity activation energy is determined; and the density of states in the vicinity of the Fermi level, their dispersion, localization radius, and average carrier hop distance for all compositions are estimated; and the concentration dependences of the calculated parameters are plotted.

The real and imaginary parts of the complex dielectric permittivity, the dielectric loss tangent, and ac conductivity of crystals of TlInSe2-TlGaTe2 solid solutions have been measured as functions of frequency in the range 50 kHz to 35 MHz, and the ΔG T 0 and heat capacity of the (TlInSe2)1 − x (TlGaTe2) x solid solutions have been determined as functions of temperature. The results demonstrate that, over the entire composition range, the measured physical properties of the solid solutions correlate with their composition.

The magnetic properties and heat capacity of the TlGdS 2 compound were studied. It was shown that the ferrimagnetic-paramagnetic phase transition occurs below 75 K. The X-ray structural and magnetic investigations of the compounds with the general formula Tl MeX 2 ( Me is 3 d metal; X is S, Se) were performed for the first time in [1].