Electrical resistivity, Seebeck coefficient and Hall coefficient of pure thallium were measured at temperatures from 100 to 550 K, i.e. almost to the melting temperature of this metal. The history of the experiemental investigations of the transport properties of Tl is shortly reviewed. This is a first publication of the experimental data on the Seebeck coefficient and the Hall coefficient of Tl in such a broad temperature range.

Electrical resistivity, Seebeck coefficient and Hall coefficient of pure thallium were measured at temperatures from 100 K to 550 K, i.e. almost to the melting temperature of this metal. The history of the experiemental investigations of the transport properties of Tl is shortly reviewed. This is a first publication of the experimental data on the Seebeck coefficient and the Hall coefficient of Tl in such a broad temperature range.

In this work, we study thermoelectric properties of GeTei-based alloys, doped with bismuth, with partial substitution of lead for germanium: Ge0.86Pb0.1Bi0.04Te. The aim of the study is to explore the possibility of increasing the thermoelectric efficiency of a compound by combining optimal doping and isovalent substitution to improve the electronic properties with a simultaneous decrease of the lattice thermal conductivity. We studied alloy samples prepared in two different research laboratories using similar, but not completely identical procedures. It is shown that the electronic (thermoelectric power and electrical conductivity) properties of the samples of the two groups are in good agreement with each other. The properties of alloys depend on the thermal history of the samples due to the presence at temperatures of 600–800 K of a phase transition from a low-temperature rhombohedral to a high-temperature cubic structural modification and missibility gap in GeTe–PbTe quasibinary system below 870 K. The thermoelectric figure of merit of alloys reaches a maximum value of 1.5 at a temperature of about 750 K.

Recently, there has been renewed interest in thermoelectric materials based on germanium telluride, which demonstrate high efficiency in mid-temperature range. This paper discusses the theoretical description of the phonon spectrum and lattice thermal conductivity in GeTe using ab initio methods. Using these methods, the temperature dependence of the lattice thermal conductivity in the rhombohedral phase was calculated and effects of scattering by point defects and nanostructuring were estimated. The modification of the phonon spectrum upon the transition to the high-temperature cubic phase is investigated. The calculated temperature dependences of the lattice thermal conductivity are compared with the available experimental data on GeTe and its solid solutions.

Lattice thermal conductivity of solid solutions based on cobalt monosilicide and monogermanide was studied. Electrical and thermal conductivities of CoSi and solid solutions of CoGexSi1-x (x=0.03 and 0.07) were experimentally measured in the temperature range of 80-350 K. Phonon spectra and thermal conductivity in CoGexSi1-x solid solutions were calculated using ab initio lattice dynamics. The results of calculations are in good agreement with the results of measurements obtained in the present work and with the literature data. It was shown that already at a content of 10-15 at.% of Ge, the lattice thermal conductivity decreases by about a factor of 2 compared with pure cobalt monosilicide, and at a germanium content of about 70 at.%, the decrease can reach 3.5 times. Keywords: lattice thermal conductivity, thermoelectrics, first principle lattice dynamics.

In this work, we study the properties of GeTe -based alloys, doped with bismuth, with partial substitution of lead for germanium: Ge0.86Pb0.1Bi0.04Te. The aim of the study is to explore the possibility of increasing the thermoelectric efficiency of a compound by combining optimal doping and isovalent substitution to improve the electronic properties with a simultaneous decrease of the lattice thermal conductivity. We studied alloy samples prepared in two different research laboratories using similar, but not completely identical procedures. It is shown that the electronic (thermoelectric power and electrical conductivity) properties of the samples of the two groups are in good agreement with each other. The properties of alloys depend on the thermal history of the samples due to the presence at temperatures of 600–800 K of a phase transition from a low-temperature rhombohedral to a high-temperature cubic structural modification. The thermoelectric figure of merit of alloys reaches a maximum value of 1.5 at a temperature of about 750 K.

Lattice thermal conductivity of solid solutions based on cobalt monosilicide and monogermanide was studied. Electrical and thermal conductivities of CoSi and solid solutions of CoGe(x)Si(1-x) (x=0.03 and 0.07) were experimentally measured in the temperature range of 80-350K. Phonon spectra and thermal conductivity in CoGe(x)Si(1-x) solid solutions were calculated using ab initio lattice dynamics. The results of calculations are in good agreement with the results of measurements obtained in the present work and with the literature data. It was shown that already at a content of 10–15 at.% of Ge, the lattice thermal conductivity decreases by about a factor of 2 compared with pure cobalt monosilicide, and at a germanium content of about 70 at.%, the decrease can reach 3.5 times.

The results of theoretical calculations show the possibility of increasing the thermoelectric figure of merit in solid solutions of Mg 2 Si–Mg 2 Sn using nanoinclusions (NEAT approach). The thermoelectric properties of the Mg 2 Si 0.2 Sn 0.8 solid solution, doped with antimony to obtain the optimal concentration of free electrons, with the addition of SiO 2 and TiO 2 impurities as additional scattering centers were studied. During the preparation of solid solution samples by hot pressing, nanoparticles of silicon dioxide and titanium dioxide of 15–200 nm in size at a concentration of 0.5–1 vol % were added. The coefficients of thermopower, electrical conductivity, and thermal conductivity were measured on these samples in the temperature range from 300 to 800 K. An insignificant decrease in thermal conductivity was shown for samples containing additional impurities. In this case, an increase in the thermoelectric figure of merit was not observed due to a decrease in the mobility of majority carriers.

The results of theoretical calculations show the possibility of increasing the thermoelectric figure of merit in solid solutions of Mg2Si–Mg2Sn using nanoinclusions (NEAT approach). The thermoelectric properties of the Mg2Si0.2Sn0.8 solid solution, doped with antimony to obtain the optimal concentration of free electrons, with the addition of SiO2 and TiO2 impurities as additional scattering centers were studied. During the preparation of solid solution samples by hot pressing, nanoparticles of silicon dioxide and titanium dioxide of 15–200 nm in size at a concentration of 0.5–1 vol % were added. The coefficients of thermopower, electrical conductivity, and thermal conductivity were measured on these samples in the temperature range from 300 to 800 K. An insignificant decrease in thermal conductivity was shown for samples containing additional impurities. In this case, an increase in the thermoelectric figure of merit was not observed due to a decrease in the mobility of majority carriers.

We study the thermoelectric and galvanomagnetic properties of cobalt monosilicide (CoSi) and its solid solutions with FeSi and NiSi. Recent CoSi band structure calculations, confirmed by ARPES measurements, revealed several differences of the electronic structure from the previous standard two-band model for semi-metallic compounds. The discovered features of the CoSi band structure require modifications of previously used models for description of material transport properties. We investigate the temperature dependences of the Seebeck coefficient, electrical resistivity, and Hall coefficient in the temperature range from 100 to 800 K for CoSi and for its solid solutions with FeSi and NiSi. The ab initio calculation of the band structure and transport coefficient were carried out using the Quantum Espresso software package. The results of the study showed that the main features of the thermoelectric and galvanomagnetic properties of CoSi and its solid solutions with FeSi and NiSi at high temperatures can be adequately described using the ab initio calculated band structure, taking into account the energy dependence of the relaxation time.

Recently, there has been renewed interest in thermoelectric materials based on germanium telluride, which demonstrate high efficiency in mid-temperature range. This paper discusses the theoretical description of the phonon spectrum and lattice thermal conductivity in GeTe using ab initio methods. Using these methods, the temperature dependence of the lattice thermal conductivity in the rhombohedral phase was calculated and effects of scattering by point defects and nanostructuring were estimated. The modification of the phonon spectrum upon the transition to the high-temperature cubic phase is investigated. The calculated temperature dependences of the lattice thermal conductivity are compared with the available experimental data on GeTe and its solid solutions.

In this study, we demonstrate that introducing of rare-earth elements, La or Pr, into the Bi-O charge reservoir layer of BiCuSeO leads to an increase of both, the charge carrier concentration and the effective mass. Although the charge carrier mobility slightly decreases upon Bi$^{3+}$ to $R^{3+}$ substitution, the electronic transport properties are significantly improved in a broad temperature range from 100 K to 800 K. In particular, the electrical resistivity decreases by two times, while the Seebeck coefficient drops from 323 ${\mu}$V K$^{-1}$ to 238 ${\mu}$V K$^{-1}$ at 800 K. Thus, a power factor of nearly 3 ${\mu}$W cm$^{-1}$ K$^{-2}$ is achieved for Bi$_{0.92}$La$_{0.08}$CuSeO sample at 800 K. Meanwhile, a noticeable decrease of the lattice thermal conductivity is observed for the doped samples, which can be attributed to the enhanced point defect scattering mostly originated from atomic mass fluctuations between $R$ and Bi. Ultimately, a maximum $zT$ value of nearly 0.34 at 800 K is obtained for the Bi$_{0.92}$La$_{0.08}$CuSeO sample, which is ~30% higher than that of pristine BiCuSeO.

In this study, we demonstrate that introducing of less electronegative R-elements (R = La or Pr) into the Bi–O charge reservoir layer of BiCuSeO enhances the chemical bond ionicity and leads to an increase of both, the charge carrier concentration and the effective mass. Although the charge carrier mobility decreases upon Bi3+ to R3+ substitution, the electronic transport properties are significantly improved in a broad temperature range from 100 K to 800 K. In particular, the electrical resistivity decreases by two times, while the Seebeck coefficient drops from 323 μV K–1 to 238 μV K–1 at 800 K. Thus, a power factor of nearly 3 μW cm–1 K–2 is achieved for Bi0.92La0.08CuSeO sample at 800 K. Meanwhile, a noticeable decrease of the lattice thermal conductivity is observed for the doped samples, which can be attributed to the enhanced point defect scattering mostly originated from atomic mass fluctuations between R and Bi. Ultimately, a maximum zT value of nearly 0.34 at 800 K is obtained for the Bi0.92La0.08CuSeO sample, which is ~30% higher than that of pristine BiCuSeO. Introduction The oxyselenides family MCuSeO, where M = trivalent cation such as Bi or rare-earth elements, was reported for the first time in 1993 by Boris A. Popovkin group.1,2 The related compounds (so-called 1111 phases) crystallize in the tetragonal layered ZrCuSiAs structure type with P4/nmm space group and two formula units per cell. The crystal structure is composed of alternately stacked along the c axis insulating (M2O2) layers with ionic bonds and conducting (Cu2Se2) layers with covalent bonds, as shown in Fig. 1a.1,3,4 Generally, the RCuSeO (R = rare-earth) are wide gap p-type semiconductors (Eg ~3 eV), with the valence band maximum (VBM) and the conduction band minimum (CBM) at the Γ point of the Brillouin zone.5–8 In both cases, RCuSeO and BiCuSeO, the VBM is composed of the hybridized Cu 3d and Se 4p orbitals.9 BiCuSeO is also p-type semiconductor, but with a narrow bandgap of about 0.8 eV. Such evolution in the band structure originates from the presence of Bi 6p orbitals at the bottom of the conduction band.5,10,11 To date, BiCuSeO oxyselenides have attracted considerable attention and were intensively studied as promising Pb-free families of thermoelectric materials owing to their intrinsically low thermal conductivity and relatively high Seebeck coefficient. The remarkable thermoelectric performance was reported for Ba-doped BiCuSeO with the dimensionless figure of merit zT value ~1.4 at 923 K.12,13 Here zT is defined as zT = S2 σ T κ–1, where S, σ, T and κ are the Seebeck coefficient, electrical conductivity, absolute temperature, and total thermal conductivity, respectively.14,15 The charge carrier concentration in BiCuSeO can be increased using partial substitution of Bi3+ by divalent or monovalent ions inducing extra holes. However, the intrinsically low charge carrier mobility, which is usually below ~20 cm2 V–1 s–1 for pristine BiCuSeO, is deteriorated to less than 2 cm2 V–1 s–1 by heavy doping with alkali or alkaline earth elements, which greatly hampers the further possible electrical transport properties improvement.9 Thus, another strategy, such as band structure engineering along with tuning of chemical bonding is utilized to decouple the trade-off between the charge carrier concentration and their mobility.16–19 The charge carries in BiCuSeO are predominantly scattered by acoustic phonons and thus their mobility is closely related to the effective mass and the carrierphonon coupling.20,21 At the same time, the effective mass, m*, is largely determined by the ionicity (covalence) of the corresponding chemical bond. Since the electronegativity reflects the tendency of an atom to attract an electron density, the increase in bond ionicity will decrease the overlapping of the shared electron densities leading to increased effective mass. Furthermore, partial substitution of Bi by less electronegative atoms can lead to more electrons being released into the system, which may contribute to the enhanced charge carrier concentration. Besides, the Cu vacancies play an essential role in the BiCuSeO electrical transport and are believed to be the origin of the p-type conduction in the nominally undoped BiCuSeO.9,10,22 Indeed, it was reported that the copper vacancies seem to be thermodynamically favored due to their low formation energy23–26 relative to other types of intrinsic defects, which results in unintentional doping of nominally undoped MCuSeO based compounds due to the presence of a faint amount of copper vacancies5,11,27 according to Kröger-Vink notation:28 X 0 Cu Cu surface Cu Cu . ' V h → + + (1) Thus, the charge carrier concentration and the related electrical conductivity values strongly depend on the precise chemical composition, especially on the presence a National University of Science and Technology MISIS, 119049 Moscow, Russia b Ioffe Institute, 194021 St. Petersburg, Russia * E-mail: novitskiy@misis.ru c The University of New South Wales, 2052 Sydney, Australia d Sobolev Institute of Geology and Mineralogy, 630090 Novosibirsk, Russia e L.N. Gumilyov Eurasian National University, 010000 Nur-Sultan, Kazakhstan ‡ Present address: Luleå University of Technology, 97187 Luleå, Sweden † Electronic Supplementary Information (ESI) available PREPRINT Novitskii et al. April 21, 2021 | 2 of vacancies, and can vary from ~0.1 Ω–1 cm–1 to more than 1 Ω–1 cm–1 at room temperature and exhibit degenerate as well as nondegenerate behavior with temperature.10,11,23,27,29–32 Here we demonstrate experimentally that the carrier concentration and effective mass can be noticeably increased by doping with less electronegative rare-earth elements (La or Pr) at the Bi site in (Bi2O2) layers. Consequently, a maximum zT of ~0.34 at 800 K can be achieved for the Bi0.92La0.08CuSeO sample, ~30% enhancement as compared with that of the pristine sample. The experimental study was combined with ab initio density functional theory (DFT) calculations in order to clarify the effect of band structure evolution with doping on transport properties. Experimental details The starting chemicals for the synthesis of Bi1– xRxCuSeO (R = La or Pr, x = 0 – 0.08) were the fine commercial powders of Bi2O3 (99.5%, Reachem), La2O3 (99.99%, Rare Metallic), Pr2O3 (99.99%, Shin-Etsu Chemical) and Bi (≥ 99.95%, Component Reaktiv), Se (99.90%, Reachem), Cu (≥ 99.5%, Rushim). The powders were weighed, according to the stoichiometric ratio, and then ball milled (BM) in an argon atmosphere at 400 rpm for 8 hours (planetary micro mill Pulverisette 7 premium line, Fritsch, Germany). The obtained mixture was coldpressed into pellets and sealed in an evacuated to 10–3 Torr silica tubes. The tubes were heated to 573 K with the rate of 5 K min–1 and held for 8 hours, then cooled to room temperature. The resulting specimens were crushed, ground, and ball-milled again in argon at 400 rpm for 4 hours, cold-pressed, and resealed in another evacuated quartz tubes. The samples were annealed at 973 K for 12 hours. The pellets were ball milled one more time in argon at 300 rpm for 8 hours. For ball milling processing, the zirconium oxide vials with a volume of 45 ml and balls with a diameter of 5 mm were used. In order to obtain bulk samples, the powders were densified by spark plasma sintering (Labox 650, Sinter-Land, Japan) at 973 K for 5 minutes under uniaxial pressure of 50 MPa. The cylindrical specimens of 12.7 mm in diameter and 10 mm high were annealed at 973 K for 6 hours in an argon atmosphere. For comparative analysis, one pristine BiCuSeO sample was prepared without using the ball milling during powder preparation, with hand-grinding instead. More details on the experimental procedure can be found in the electronic supporting information file (ESI). The synthesized samples were characterized by a range of techniques, including X-ray fluorescence (XRF), powder X-ray diffraction (XRD), scanning electron microscopy (SEM), energy-dispersive X-ray spectroscopy (EDS), Brunauere-Emmette-Teller (BET) surface area analysis, and various physical measurements, such as the electrical and thermal conductivities, Seebeck coefficient and Hall constant measurements. The effective average particle size of the synthesized powders was calculated as D = 6d–1 A–1 (where d is the density) from the low-temperature adsorption isotherm measurements of the specific surface area, A, using a Nova 1200e analyzer (Quantachrome Instruments, USA). The XRF spectra were taken with a ZSK Primus II spectrometer (Rigaku, Japan). X-ray diffraction data were collected over the angular range 20 ≤ 2θ (deg) ≤ 80, on a Miniflex 600-2 diffractometer (Rigaku, Japan), using CoKα radiation (λ = 1.7903 Å) at room temperature. Rietveld refinement was performed using PDXL software (Rigaku, Japan). SEM and EDS were performed using Vega 3SB scanning electron microscope (Tescan, Czech Republic) in conjunction with an EDS detector (x-act, Oxford Instruments, UK). Temperature dependencies of the electrical resistivity and the Seebeck coefficient were simultaneously measured with high resolution (temperature increment ~1 K) by the standard 4-probe and differential methods, respectively, under He atmosphere using a homemade system.33 The uncertainty of the Seebeck coefficient and the electrical resistivity measurements is 5% ± 0.5 μV/K and 2%, respectively. Thermal conductivity, κ, was determined from thermal Figure 1. (a) Crystal structure of the BiCuSeO and (b) XRD patterns of the Bi1–xRxCuSeO (R = La or Pr, x = 0 – 0.08). †Pristine BiCuSeO sample prepared without ball milling. PREPRINT Novitskii et al. April 21, 2021 | 3 diffusivity measurements using relationship κ = χ·Cp·d, where χ is the thermal diffusivity coefficient, Cp is the specific heat capacity. The relative bulk density was measured by the Archimedes method. Temperature dependencies of the thermal diffusivity were measured by the laser flash diffusivity metho

In this work, we study the Hall coefficient and the conductivity of cobalt monosilide CoSi, as well as Co1−xFexSi and Co1−xNixCo alloys with contents up to 8 at% of iron and up to 5 at% of nickel. The temperature dependences of the Hall coefficient and of the conductivity were measured in the temperature range of 77−800K. The theoretical interpretation of the experimental dependencies is based on two different models of the electronic structure of the compound: a simple 2-band semimetallic structure with small overlap of isotropic parabolic bands; and ab initio electronic structure, containing near Fermi energy topological features with multiply degenerate intersections of bands.

Nanocomposite thermoelectrics based on Bi 0.45 Sb 1.55 Te 2.985 solid solution of p -type conductivity are fabricated by the hot pressing of nanopowders of this solid solution with the addition of SiO 2 microparticles. Investigations of the thermoelectric properties show that the thermoelectric power of the nanocomposites increases in a wide temperature range of 80–420 K, while the thermal conductivity considerably decreases at 80–320 K, which, despite a decrease in the electrical conductivity, leads to an increase in the thermoelectric efficiency in the nanostructured material without the SiO 2 addition by almost 50% (at 300 K). When adding SiO 2 , the efficiency decreases. The initial thermoelectric fabricated without nanostructuring, in which the maximal thermoelectric figure of merit ZT = 1 at 390 K, is most efficient at temperatures above 350 K.

The transport properties of p -type oxyselenides with the chemical composition Bi 1 – x La x CuSeO ( x = 0.02, 0.04, 0.06) are investigated. An analysis of the temperature dependences of the material resistivity and charge-carrier concentration and mobility show that the substitution of La 3+ for Bi 3+ ions increases the carrier concentration, presumably, due to the generation of holes as a result of the formation of bismuth vacancies with an increase in the degree of substitution.

The twofold decrease in hole concentration of all PbTe samples at 77–450 K and the same effect achieved at 77 K and heavy doping only due to introduction in PbTe of tin small addition, supplemented by observation in indicated range of temperature hysteresis in hole concentration, are the grounds for developing an other approach to a study of the energy spectrum in A(IV)B(VI) materials, an alternative two-band model. It is based on the common for these materials phenomenon — compensation of current carriers as a reaction on electroactive doping. The appearance of compensation is associated with initial (at temperatures below 77 K) two-charge process — participation in a single doping act of the pair of electrons due to their mutual attraction. With a rise in temperature, as with introduction of tin, the situation is changed. The study of all aspects of transformation is a task for further research.