Effect of fluctuations on electron and phonon processes and thermodynamic parameters of Ag2Te and Ag2Se in the region of phase transition

Temperature dependences of electrical conductivity σ, thermoelectric power α, results of differential thermal analysis ΔT _y , thermal conductivity χ, temperature conductivity κ, and heat capacity C _p were studied in Ag₂Te and Ag₂Se semiconductors in the region of the phase transition. Two extrema are observed in the temperature dependence χ(T): a maximum in the region of the α′ → β′ transition and a minimum in the region of the β′ → β transition; these extrema are caused by the similar dependence C _p (T). It is shown that the α → α′ and β′ → β transitions are displacement transitions, while the α′ → β′ transition is of reconstruction type. It is established that the disorder parameter η in silver chalcogenides is highly smeared in the region of the phase transition; therefore, disordering of phases at the point of the phase transition is incomplete: 73, 62, and 48% in Ag₂Te, Ag₂Se, and Ag₂S, respectively. The minimum volumes V _ph for new phases are calculated; it is shown that the value of V _ph in displacement transitions is larger than in the reconstruction-type transitions.     

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.     

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.     

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.     

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.     

Role of intrinsic defects in splitting the energy spectra of charge carriers in Ag2Te

The energy gap and its temperature coefficient are calculated depending on the concentration of defects in silver telluride with a Te and Ag excess. On the basis of the obtained data, the correlation between the charge-carrier energy spectrum and the silver-telluride defectness is analyzed. It is established that the correlation is a reflection of more general fundamental relations between the energy structure and the defect concentration in Ag₂Te caused by ions and vacancies of silver atoms in the sublattice.     

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.     

AgIn2/Ag2In transformations in an In-49Sn/Ag soldered joint under thermal aging

The interfacial reaction between liquid In-49Sn solders and Ag substrates results in the formation of a thicker Ag₂In intermetallic compound accompanied with the development of a thin AgIn₂ layer. Through further aging of the In-49Sn/Ag soldered specimens at various temperatures ranging from room to 100°C, solid/solid trnasitions between Ag₂In and AgIn₂ intermetallic compounds can be observed. When the temperature drops below 75°C, Ag₂In will react with the In-49Sn solder to form the dominant AgIn₂ phase. Conversely, AgIn₂ is consumed at a higher temperature (e.g., 100°C) when reacting with the Ag substrate to create a now dominant Ag₂In phase. Lastly, the different mechanical, electrical, magnetic, and corrosion behaviors of both intermetallic compounds are respectively made known through direct measurements of the material properties of the individual Ag₂In and AgIn₂ bulk samples.     

High Thermoelectric Performance in Sulfide‐Type Argyrodites Compound Ag8Sn(S1−xSex)6 Enabled by Ultralow Lattice Thermal Conductivity and Extended Cubic Phase Regime

Argyrodites with a general chemical formula of A₈BC₆ are known for complex phase transitions, ultralow lattice thermal conductivity, and mixed electronic and ionic conduction. The coexistence of ionic conduction and promising thermoelectric performance have recently been reported in selenide and telluride argyrodites, but scarcely in sulfide argyrodites. Here, the thermoelectric properties of Ag₈Sn(S_1?xSe_x)₆ are reported. Specifically, Ag₈SnS₆ exhibits intrinsically ultralow lattice thermal conductivities of 0.61–0.31 W m^?1 K^?1 over the whole temperature range from 32 to 773 K due to distorted local crystal structure, relatively weak chemical bonding, rattler‐like Ag atoms, low‐lying optical modes, and dynamic disorder of Ag ions at high temperatures. Se doping shifts the orthorhombic–cubic phase transition from 457 K at x = 0 to 430 K at x = 0.10, thereby expanding the temperature range of the thermoelectrically favored cubic phase. A figure of merit zT value ≈ 0.80 is achieved at 773 K in Ag₈Sn(S_1?xSe_x)₆ (x = 0.03), the highest zT value reported in sulfide argyrodites. These results fill a knowledge gap of the thermoelectric study of argyrodites and contribute to a comprehensive understanding of the chemical bonding, lattice dynamics, and thermal transport of argyrodites.     

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.     

Synthesis of CdIn2Se4 and Cu0.5Ag1.5InSe3 Compounds via Chemical and Solid-State Methods

CdIn₂Se₄ and Cu_0.5Ag_1.5InSe₃ are high-performance thermoelectric materials. In this study, both CdIn₂Se₄ and Cu_0.5Ag_1.5InSe₃ powders were synthesized using a microwave and solution method followed by annealing in nitrogen atmosphere. CdIn₂Se₄ was synthesized by two routes. First, CdSe was prepared using a microwave method. Second, In metal was prepared using a solution method. The prepared metals were annealed in nitrogen atmosphere. From the x-ray diffraction (XRD) results, CdIn₂Se₄ was observed as the main phase with CdSe and In₂O₃ as contaminant phases. The synthesis of Cu_0.5Ag_1.5InSe₃ was also divided into two steps. First, CuAg and Se were prepared using a microwave method. Second, In metal was prepared using a solution method. The prepared metals were annealed in nitrogen atmosphere. From the XRD results, Cu_0.5Ag_1.5InSe₃ was observed as the main phase with Cu_0.5?x Ag_1.5?y In _x+y Se and Se as contaminant phases.     

Controlled Synthesis of Ag2S,Ag2Se,and Ag Nanofibers by Using a General Sacrificial Template and Their Application in Electronic Device Fabrication

Nanofiber bundles of Ag₂S, Ag₂Se, and Ag have been successfully synthesized by making use of Ag₂C₂O₄ template nanofiber bundles, utilizing both anion‐exchange and redox reactions. The obtained bundles were polycrystalline nanofibers composed of nanoparticles in which the precursor morphology was well‐preserved, indicating that Ag₂C₂O₄ nanofiber bundles acted as a general sacrificial template for the synthesis of silver‐based semiconductor and metal nanofibers. Dispersing media and transforming reactants were found to be key factors influencing the chemical transformation in the system. In particular, separate single‐crystalline Ag nanofibers were obtained via a nontemplate route when ascorbic acid was used as a relatively weak reductant. An electrical transfer and switching device was built with the obtained Ag₂S and Ag nanofiber bundles, utilizing the unique ion‐conductor nature of Ag₂S and revealing their potential applications in electronics.     

Power factor improvement of delafossite CuAlO2 by liquid-phase sintering with Ag2O addition

Thermoelectric delafossite (CuAlO₂)_1−x(Ag₂O)_x with 0<x<0.06 is prepared at three different sintering temperatures, 1323 K, 1373 K, and 1473 K. The samples are obtained from a mixture of CuO, Al₂O₃, and additive Ag₂O powders. The mixture is ground and then pressed with uniaxial pressure into pellets. Differential scanning calorimetry and X-ray diffraction spectroscopy show that the sintering temperature to synthesize delafossite CuAlO₂ with Ag₂O addition is below 1473 K. X-ray diffraction patterns show the major phase of delafossite 3R-CuAlO₂ along with a trace amount of 2H-CuAlO₂. A small amount of the Ag phase is present in the samples depending on the amount of Ag₂O addition and sintering temperature. Energy dispersive spectroscopy and backscattered electron image analyses show that the Ag phase is segregated around the grain boundary. Liquid-phase sintering is used to explain the growth mechanism. The CuAlO₂ samples with Ag₂O addition obviously exhibit enhanced bulk density, grain size, and electrical conductivity. The highest power factor (PF), obtained by CuAlO₂ with 2 at% of Ag₂O sintered at 1373 K, is 8.23×10⁻⁵ W/(m K²) at 873 K. Hence, our findings show an improvement for delafossite CuAlO₂ by Ag₂O addition.     

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.     

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.     

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.     

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.     

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.     

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.     

Chemical Stability of (Ag,Cu)2Se: a Historical Overview

Recent work on Cu_2?x Se has caused strong interest in this material due to its high reported peak zT (1.5) and the reduction of thermal conductivity through the mechanism of liquid-like suppression of heat capacity. In the 1960s, 3M patented Cu_1.97Ag_0.03Se as “TPM-217.” Over the following decade it was tested and developed by the 3M Corporation, at the National Aeronautics and Space Administration (NASA) Jet Propulsion Laboratory, Teledyne Energy Systems, and the General Atomics Corporation for use as a next-generation thermoelectric material. During these tests, extreme problems with material loss through Se vaporization and chemical reactions between the material and the device contacts were found. These problems were especially severe while operating under conditions of high $ iL/A. $ iL / A . As a result, the material system was abandoned. The results of these reports are discussed. A simple test of degradation of Cu₂Se under conditions of applied current and thermal gradient was performed and showed results compatible with the work done by General Atomics.