On the thermal properties of Ag2Te and Ag2Se in the region of the phase transition

A setup is developed for measuring the thermal properties of Ag₂Te and Ag₂Se near the main phase transition and in the region of it in high vacuum. The samples are of stoichiometric composition with an excess of Ag, Te, and Se. Differential thermal analysis is performed and the temperature differences across and along the samples are measured by the pulsed light method. Using data on the thermal diffusivity a(T), the temperature dependence of the thermal conductivity K = apc is determined, where p is the density and c is the specific heat. The measurement technique used allows us to establish that the main phase transitions are accompanied by intense heat release. New phase transitions before and after the main structural phase transition with heat absorption are found. According to the DTA data, in the Ag₂Te sample with an excess of Ag (0.25 at.%, cn = 1.2 × 10¹⁹ cm^?3) the phase transition at 365 K is also accompanied by heat absorption. Thus, it is established that all of the phase transitions in Ag₂Te and Ag₂Se are first-order.     

Electronic properties of liquid Tl2Te, Tl2Se, Ag2Te, Cu2Te, and Cu2Se alloys

The dependences of the electrical conductivity σ(T) and thermoelectric power S(T) of Cu₂Te, Cu₂Se, Ag₂Te, Tl₂Te, and Tl₂Se liquid semiconductors were experimentally studied at high temperatures and pressures (up to 25 MPa). The melts were shown to exhibit a minima in the dependences σ(T) and a maximum in S(T) at the stoichiometric composition M₂X (where M=Tl, Ag, Cu; X=Te, Se). The results were interpreted on the basis of the Mott model with sp-d hybridization, taking into account the metal d state position with respect to the Fermi level.     

Electron-phonon processes in semiconductors at low temperatures

The thermal conductivity and thermopower are simultaneously studied in the semiconductors HgSe, HgTe, InP, GaAs, Cd _x H_{1 ? x} Te, and Ag₂S in which the effect of electron drag by phonons was detected at low temperatures T. It is found that x = 1–3 in the dependences K _ph ∝ T ^?x . It is shown that the underestimated values of x are caused by phonon scattering at certain defects. It is also found that the maxima of K _ph(T) and α_ph(T) coincide in all studied crystals, except for Ag₂S. For the first time, it is found that the strong effect α_ph ∝ T ^?3 is observed in Ag₂S, the α_ph maximum is at T = 16 K, and the K _ph maximum is at 27 K. The results obtained are in agreement with Callaway and Herring theory. The effect of magnetic field on the drag thermopower α_ph(T) in n-Cd _x H_1?x Te (in which the strong dependence α_ph ∝ T ^?3 and the strong effect of the magnetic field on it are detected) is considered for the first time. The results are compared with the Askerov theory. The force parameter A _ph(?) of the drag effect and its dependence on magnetic field are determined.     

Charge transport in silver chalcogenides in the region of phase transition

Data on the σ(T), R(T), and U(T) dependences in Ag₂Te, Ag₂Se, and Ag₂S in the region of the phase transition are analyzed. It is found that the phase transition in Ag₂Te is accompanied by a decrease in the electron concentration and this transition in Ag₂Se is accompanied by an increase in this concentration. The concentration of intrinsic charge carriers in Ag₂Te decreases by a factor of 4 as a result of the phase transition and increases by a factor of 2 in Ag₂Se. The effect of variation in the energy-band parameters in the region of phase transition on the electron mobility is considered. It is established that, in Ag₂Te and Ag₂S, electrons are scattered by optical phonons in the region of the phase transition, while electrons are scattered by acoustic phonons in the α and β phases. It is assumed that the anomalously large increase in σ and U in Ag₂S as a result of the phase transition is caused by an increase in the concentration n and a simultaneous decrease in σ _g and m _n ^* by a factor of about 2.     

Energy spectrum of charge carriers in Ag2Te

Conductivity σ(T) and Hall constant R(B, T) are studied for Ag₂Te with excess 0.1% of Te. The change in the R sign from (?) to (+) is found in dependences R(B) at various temperatures. In the temperature dependences of R in a range of 1–3 kG, two extrema are found, namely, minimum at T ～ 60 and maximum at T ≈ 80 K, and at B ≥ 5 kG, the double change in sign of R from (?) to (+) and from (+) to (?) is found. Temperatures of sign inversion for R depend on the magnetic field. At B = 15 kG, the sign of R varies from (?) to (+) at T ≈ 38 K, and from (+) to (?) at T ～ 70 K. It is found approximately in the region of the change in the sign of R(T), the concentration n(T) and electrical conductivity pass through the minimum. It is established that the minima of n(T) and σ(T), extrema in R(T), and sign inversion for R(T) from (?) to (+) as well as the overestimated temperature dependence n∞T ⁴ are caused by localization of conduction electrons at acceptor levels entering the conduction band of Ag₂Te. The values of parameters of electrons (n, μ _n ) and holes (p, μ _p ) at the points of the change in the sign of R(T) from (?) to (+) and from (+) to (?) are determined.     

Energy spectrum of charge carriers in Ag<Subscript>2</Subscript>Te

On the basis of investigations of the temperature and concentration dependences of kinetic coefficients (the Hall coefficientR, the electrical conductivity σ, and thermopower α₀) in n-type Ag₂Te, it is established that Ag atoms in Ag₂Te create the shallow donor levels located at a distance of (0.002?7 × 10^?5 T) eV from the bottom of the conduction band. It is shown that silver telluride has n-type conductivity starting with the deficiency of Ag ≥ 0.01 at % in the stoichiometric composition, and it is practically impossible to achieve the stoichiometric composition in Ag₂Te.     

Phase-Transformation-Induced Giant Deformation in Thermoelectric Ag2Se Semiconductor

In most semiconducting metal chalcogenides, a large deformation is usually accompanied by a phase transformation, while the deformation mechanism remains largely unexplored. Herein, a phase-transformation-induced deformation in Ag₂Se is investigated by in situ transmission electron microscopy, and a new ordered high-temperature phase (named as α ′-Ag₂Se) is identified. The Se Se bonds are folded when the Ag⁺-ion vacancies are ordered and become stretched when these vacancies are disordered. Such a stretch/fold of the Se Se bonds enables a fast and large deformation occurring during the phase transition. Meanwhile, the different Se Se bonding states in α-, α ′-, β-Ag₂Se phases lead to the formation of a large number of nanoslabs and the high concentration of dislocations at the interface, which flexibly accommodate the strain caused by the phase transformation. This study reveals the atomic mechanism of the deformation in Ag₂Se inorganic semiconductors during the phase transition, which also provides inspiration for understanding the phase transition process in other functional materials.     

Hysteresis in Ag<Subscript>2</Subscript>Te near and within the phase transition region

Electrical conductivity, the Hall coefficient, and thermoelectric power were studied and differential thermal analysis was carried out in Ag₂Te crystals near and within the range of phase transitions, in the directions of heating and cooling. A large hysteresis loop was observed. The results are discussed in terms of the theory of smeared phase transitions. Agreement between experimental data and theory is achieved in a second approximation of the inclusion function L₂(T) and its derivative with respect to temperature, dL₂/dT.     

Plasma resonance in Pb1 − x Ag x Te alloys

From the data of reflectance measurements for Pb_{1 ? x} Ag _x Te (x = 0.001–0.007) alloy single crystals, the hole concentration and conductivity are calculated as functions of the Ag content. It is established that, for all of the samples, the plasma minimum is in the infra-red region and shifts to shorter wavelengths, as the Ag content is increased. In this case, the hole concentration and conductivity increase.     

Effect of a phase transition on the electron energy spectrum in Ag<Subscript>2</Subscript>S

Temperature dependences of electrical conductivity σ, Hall coefficient R, and thermopower α₀ in Ag₂S are reported. It is established that at T ≈ 435 ± 5 K, all kinetic parameters vary drastically, which is associated with a change in parameters of the conduction band. It is shown that the dispersion law of electron energy in β-Ag₂S corresponds to the Kane model.     

Synthesis and Characterization of Crystalline Ag2Se Nanowires Through a Template‐Engaged Reaction at Room Temperature

Single‐crystalline Ag₂Se nanowires have been successfully synthesized through a template‐engaged topotactic reaction in which nanowires of trigonal selenium were transformed into Ag₂Se by reacting with aqueous AgNO₃ solutions at room temperature (RT). An interesting size‐dependent transition between two crystal structures has also been observed for this newly synthesized one‐dimensional system: The Ag₂Se nanowires adopted a tetragonal structure when their diameters were less than ∼40 nm; an orthorhombic structure was found to be more favorable as the diameter of these nanowires was increased beyond 40 nm. Since this reaction can be carried out at ambient pressure and temperature, it should be straightforward to scale up the entire process for the high‐volume production of Ag₂Se nanowires with well‐controlled sizes and crystal structures. These highly uniform nanowires of single‐crystalline Ag₂Se are potentially useful as photosensitizers, superionic conductors, magnetoresistive compounds, or thermoelectric materials. This work also represents the first demonstration of a template‐engaged process capable of generating single‐crystalline nanowires from the solution‐phase and at RT.     

Electrical and thermoelectric properties of <Emphasis Type="Italic">p</Emphasis>-Ag<Subscript>2</Subscript>Te in the β phase

The conductivity σ, Hall coefficient R, and thermoelectric power α₀ of p-Ag₂Te were studied in the temperature range of 300–550 K. Inconsistency between the signs of R and α₀ was observed at 420–550 K. These results are interpreted within the two-phase model with spherical constant energy surfaces. It is established that the inconsistency between the R and α₀ signs is due to the emergence of the scattering mechanisms with the parameters r_0ac, r₀₀, and r_0d an increase of about 50% in the ratio of the effective electron and hole masses as a result of the transition α → β.     

Special features of photoelectric properties of p-CdxHg1−x Te crystals at low temperatures: The effects of the freezing-out of holes and elastic stress

The temperature and pressure dependences of photoconductivity, the photoelectromagnetic effect, dark electrical conductivity, and the Hall coefficient in p-Cd_xHg_1?x Te samples with x≈0.20–0.22 were measured at low temperatures. It is shown that recombination transitions in the temperature region of T<30–40 K may be interpreted in the context of the two-level Shockley-Read model with allowance made for the freezing-out of majority charge carriers (holes). Furthermore, the second recombination center, which manifests itself only in the aforementioned temperature region, is an acceptor of a non-Coulomb type with an ionization energy of about 10–15 meV.     

Electronic Properties as a Function of Ag/Sb Ratio in Ag1?y Pb18Sb1+z Te20 Compounds

In this study efforts have been made to optimize the electronic properties such as the electrical conductivity and Seebeck coefficient of Ag_{1? y} Pb₁₈Sb_{1+ z} Te₂₀ (lead-antimony-silver-tellurium, LAST-18) compounds by systematically varying the Ag and Sb compositions with constant Pb/Te ratio. It was found that increasing the content of Sb relative to Ag raised the charge carrier density (n) and thereby the electrical conductivity and power factor. The results indicate that, for deficient Ag, the excess trivalent Sb atoms occupy divalent Pb sites in the unit cell, increasing the value of n in the system. It was established that the Seebeck coefficient decreases with increasing n, indicating a dominant acoustic phonon scattering mechanism in the current alloys. The results demonstrate that the interaction between Ag and Sb atoms plays a major role in determining the electronic properties in the current Ag_1?y Pb₁₈Sb_1+z Te₂₀ compounds.     

High Thermoelectric Performance in PbTe Due to Large Nanoscale Ag2Te Precipitates and La Doping

Thermoelectrics are being rapidly developed for waste heat recovery applications, particularly in automobiles, to reduce carbon emissions. PbTe‐based materials with small (<20 nm) nanoscale features have been previously shown to have high thermoelectric figure‐of‐merit, zT, largely arising from low lattice thermal conductivity particularly at low temperatures. Separating the various phonon scattering mechanisms and the electronic contribution to the thermal conductivity is a serious challenge to understanding, and further optimizing, these nanocomposites. Here we show that relatively large nanometer‐scale (50–200 nm) Ag₂Te precipitates in PbTe can be controlled according to the equilibrium phase diagram and these materials show intrinsic semiconductor behavior with high electrical resistivity, enabling direct measurement of the phonon thermal conductivity. This study provides direct evidence that even large nanometer‐scale microstructures reduce thermal conductivity below that of a macro‐scale composite of saturated alloys with Kapitza‐type interfacial thermal resistance at the same overall composition. Carrier concentration control is achieved with lanthanum doping, enabling independent control of the electronic properties and microstructure. These materials exhibit lattice thermal conductivity which approaches the theoretical minimum above ～650 K, even lower than that found with small nanoparticles. Optimally La‐doped n‐type PbTe‐Ag₂Te nanocomposites exhibit zT > 1.5 at 775 K.     

Thermoelectric figure of merit of Ag<Subscript>2</Subscript>Se with Ag and Se excess

In the temperature range of 100–300 K, the electric (σ) and thermoelectric (α₀) properties of Ag₂Se with an excess of Ag as high as ～0.1 at. % and Se as high as ～1.0 at. %, respectively, are investigated. From the data on σ, α₀, and χ_tot (thermal conductivities), the thermoelectric power α ₀ ² σ and the figure of merit Z are calculated. It is found that α ₀ ² σ and Z attain the peak values at room temperature and the electron concentration n ≈ 6.5 × 10¹⁸ cm^?3.     

Optical and ac conductivity investigations on Sn doped Ag2S thin films under the structural transition framework

This work deals with further optical and electrical investigations on Sn content in Ag₂S sprayed thin films and the beneficial effect of using this type of doping in addition to the results recently reported by R. Boughalmi, A. Boukhachem, I. Gaeid, K. Boubaker, M. Bouhafs, M. Amlouk, Mater. Sci. Semicond. Process. 16 (2013) 1584. The refractive index and extinction coefficient values in terms of Sn content were deduced from transmittance and reflectance data. Moreover, study of dielectric constants has been conducted; the dispersion parameters and high- frequency dielectric constant are determined. Finally, the electrical conductivity and conduction mechanism of these films are studied using an impedance spectroscopy technique in the frequency range 5 Hz–13 MHz at various temperatures (100–230 °C). Besides, the temperature dependence of ac conductivity measurements has been analyzed under the structural transition framework from β- to α-Ag₂S phases.     

Determination of Thermodynamic Parameters in the Cu<Subscript>1.95</Subscript>Ni<Subscript>0.05</Subscript>S Phase-Transition Regions

X-ray diffraction and differential thermal analysis data obtained in the Cu_1.95Ni_0.05S phase-transition region are analyzed. It is established that the low-temperature rhombic α phase in Cu_1.95Ni_0.05S transforms to the hexagonal β phase at temperatures of 370–390 K and to the cubic γ phase at temperatures of 740–765 K according to the scheme \(\alpha \to \mathop {\alpha + \beta }\limits_{370 - 390K} \to \mathop {\alpha + \gamma }\limits_{740 - 765K} \to \gamma \). It is determined (using the temperature dependence of differential thermal analysis) that the transition α → β is accompanied by heat absorption while the transition β → γ is accompanied by heat release. It is found that both transitions are allowed and belong to the reconstructive type. Both transitions are found to occur in a fluctuation volume of ~10^–20 cm³ at temperature rates of 0.11 and 0.08 K^–1. It is demonstrated that the transition α → γ is accompanied by alternation of the structures passing through the intermediate β phase, which is incommensurate with respect to the α and γ phases.     

Analysis of the magnetic properties of (La<Subscript>1 − <Emphasis Type="Italic">x</Emphasis></Subscript>Sr<Subscript><Emphasis Type="Italic">x</Emphasis></Subscript>)<Subscript>0.93</Subscript>MnO<Subscript>3</Subscript> manganites at high temperatures

For the first time, the Faraday method is used to measure the temperature dependence of paramagnetic susceptibility χ(T) of (La_{1 ? x} Sr _x )_0.93MnO₃ (x = 0.2, 0.25, or 0.3) manganites in the temperature interval 60–850°C. It is demonstrated that the dependences have two kinks and three linear sections. The kink of curve χ^?1(T) is related to polymorphic transformations (Q′ → Q* and Q* → R) that take place in the crystal lattices of the samples. The main magnetic characteristics of the samples are determined with the least-squares processing of curve χ^?1. Is is demonstrated that dependence χ^?1(T) obeys the Curie-Weiss law. The energy state of the magnetoactive manganese atom in the Q′-and Q*-phase samples is close to the energy state of a free Mn²⁺ ion. In the R phase, this state is close to the state of a free Mn³⁺ ion.     

The influence of the magnetic field on the effect of drag of electrons by phonons in <Emphasis Type="Italic">n</Emphasis>-Cd<Subscript><Emphasis Type="Italic">x</Emphasis></Subscript>Hg<Subscript>1 − <Emphasis Type="Italic">x</Emphasis></Subscript>Te

Thermopower in n-Cd_0.2Hg_0.8Te (6–100 K) is studied. A large effect of drag of the charge carriers by phonons α_ph is found. The influence of the magnetic field H on the drag thermopower is considered. It is established that the magnetic field exerts the effect mainly on the electron component of α_ph. The data are interpreted in the context of the theory taking into account the effect of H on thermopower α_ph, in which parameter A(ɛ) proportional to the static force of the drag effect is introduced. By the experimental data α_ph(T, H), T, and H dependences A(ɛ) are determined. It is shown that, as H increases, A(ɛ) sharply decreases. This explains a decrease in α_ph in the magnetic field, power index k in dependence α_ph ∝ T ^−κ, and narrowing the region of manifestation of the drag effect. It is established that at classically high fields, the drag effect in n-Cd_0.2Hg_0.8Te does not vanish.