The under-pressure behaviour of mechanical,electronic and optical properties of calcium titanate and its ground state thermoelectric response

Abstract

In this study, the elastic, electronic, optical and thermoelectric properties of CaTiO₃ perovskite oxide have been investigated using first-principles calculations. The generalised gradient approximation (GGA) has been employed for evaluating structural and elastic properties, while the modified Becke Johnson functional is used for studying the optical response of this compound. In addition to ground state physical properties, we also investigate the effects of pressure (0, 30, 60, 90 and 120 GPa) on the electronic structure of CaTiO₃. The application of pressure from 0 to 90 GPa shows that the indirect band gap (Γ-M) of CaTiO₃ increases with increasing pressure and at 120 GPa it spontaneously decreases transforming cubic CaTiO₃ to a direct (Γ-Γ) band gap material. The complex dielectric function and some optical parameters are also investigated under the application of pressures. All the calculated optical properties have been found to exhibit a shift to the higher energies with the increase of applied pressure suggesting potential optoelectronic device applications of CaTiO₃. The thermoelectric properties of CaTiO₃ have been computed at 0 GPa in terms of electrical conductivity, thermal conductivity and Seebeck coefficient.     

Fe deep level optical spectroscopy in InP

The optical cross-section σ_n⁰(hv) and σ_p⁰(hv) associated with the (Fe³⁺ ? Fe²⁺) deep level have been measured by Deep Level Optical Spectroscopy in n-type Fe doped samples of InP. Optical transitions are interpreted as transitions from the Fe²⁺ ground state to the Γ and L point minima of the conduction band for σ_n⁰(hv) and from the valence band to the ground and excited state for Fe²⁺ for σ_p(hv). A theoretical model which accounts for the main features of the experimental data is proposed.     

Thermoelectric properties of n-Bi2Te3 − x − y Se x S y solid solutions under high pressure

The thermoelectric properties of n-Bi₂Te_{3 ? x ? y} Se _x S _y solid solutions with atomic substitutions in the tellurium sublattice (x = 0.27, 0.3, y = 0, and x = y = 0.09) have been studied under a pressure to 8 GPa. It has been found that the Seebeck coefficient and the resistance decrease with increasing P, and power factor χ increases in all compositions and becomes maximal at pressures of 2–4 GPa. It has been shown that the power factor χ, which is proportional to the product of the effective mass of the density of states m/m m/m ₀ and the charge carrier mobility μ₀ in the form (m/m ₀)^3/2μ₀, increases with increasing pressure mainly due to the increase in the mobility and also depends on the solid solution composition. In the composition with substitution Te → Se + S (x = y = 0.09), the peculiarity of the dependence of m/m ₀ on P in the pressure range corresponding to maximal values of the power factor can be explained by the existence of an electronic topological transition. The increase in the power factor under pressure in n-type Bi₂Te_{3 ? x ? y} Se _x S _y solid solutions combined with similar data for p-type Bi_{2 ? x} Sb _x Te₃ solid solutions obtained earlier, including the estimations of possible changes in the thermal conductivity with increasing pressure, give grounds to design thermoelements with improved value of the thermoelectric figure-of-merit, which can be 50–70% at pressures of 2–4 GPa.     

Preparation and thermoelectric power of SnBi4Se7

Polycrystalline samples of Bi₂Se₃ and a stoichiometric ternary compound in the quasi-binary system SnSe–Bi₂Se₃ have been prepared and characterized by X-ray powder diffraction analysis. At room temperature the carrier concentration values are n?=?1.1?×?10¹⁹?cm^?3 for Bi₂Se₃ and n?=?0.53?×?10¹⁹?cm^?3 for SnBi₄Se₇. The thermoelectric power has been measured over the temperature range 90–420?K. The thermoelectric power of Bi₂Se₃ is higher than that for SnBi₄Se₇, which shows that the Sn impurity has an acceptor character. Therefore, doping Bi₂Se₃ with tin atoms does not improve thermoelectric properties of this material, due to decrease the value of the power factor σS ². Transport properties of the studied polycrystalline samples are characterized by a mixed transport mechanism of free carriers. It is necessary to add more than one Sn atom to the Bi₂Se₃ compound in order to suppress the electron concentration by one electron. Such behaviour of the dopant is explained by the formation of various structural defects. Besides the dominant substitutional defect, Sn_Bi, tin atoms also form uncharged defects, corresponding to seven-layer lamellae of the composition Se–Bi–Se–Sn–Se–Bi–Se which corresponds to the structure of the SnBi₂Se₄.     

Localization of conduction electrons in the ferromagnetic clusters AuFe

The optical-conductivity spectra of concentrated solutions Au_{1 ? x} Fe _x with x = 17 and 22 at % have been measured in a frequency range of (10–33) × 10³ cm^?1 at room temperature. The results are analyzed together with previous optical data obtained for compounds with x = 4–12 at %. It is found that the magnetic contributions σ_magn = σ_AuFe ? σ_Au to dc and low-frequency(10 cm^?1) conductivities for an Fe concentration below 4 at % are almost equal, while the low-frequency magnetic contribution for larger concentrations is significantly larger than the dc one. An absorption band at frequencies of 1000–3000 cm^?1 has been found for samples with concentrations x = 6–22 at %. The observed phenomena are attributed to the localization of electrons inside clusters containing ferromagnetically ordered iron ions.     

AgGaSe2 : A highly photoconductive material

High resistivity single crystals of AgGaSe₂ were grown by the horizontal Bridgman technique. The near band edge photoconductivity of the grown crystal at room temperature was found to be up to 2×10⁴ times higher than the dark conductivity, under the illumination of 10⁻³ W/cm². The photoconductivity spectrum consists primarily of three peaks, which are attributed to the transitions from Γ₇(A), Γ₆(B) and Γ₇(C) states of valence band to the conduction band Γ₆. The crystal field splitting and the spin-orbit splitting were determined from these peak energy positions of the photoconductivity spectrum.     

DFT investigation on the electronic and thermoelectric properties of ternary semiconductor AgBiS2 for energy conversion application

Via the FP-APW+lo method, we have performed a systematic theoretical study of the structural, electronic and thermoelectric properties of β-AgBiS₂ compound. The estimated structural properties such as cell parameters a and c, c/a ratio and internal parameters are in reasonable agreement with the earlier measured one. From band structure calculations we have found that β-AgBiS₂ is semiconductor with a band gap of 1.23 eV using the TB-mBJ approximation. In addition, the analysis of the total and partial DOS shows a considerable hybridization between Ag ‘d’ states and S ‘p’, Bi ‘s’ states indicating that both Ag-S and Bi-S have covalent character. The main thermoelectric properties such as electrical conductivity, thermo-power, electronic thermal conductivity, power factor and figure of merit are calculated and discussed. We observed that ZT increases when temperature is augmented and reached its maximum of 0.95 and 0.85 at 2 × 10¹⁹ cm⁻³ for p and n-type doping, respectively. Thus, β-AgBiS₂ compound has interesting thermoelectric properties in both p and n-type doping.     

GeI2 monolayer: a model thermoelectric material from 300 to 600 K

ABSTRACT

In this work, the electronic structure, optical properties and thermoelectric properties of the GeI₂ monolayer are calculated by the first principles with the Boltzmann transport equation. The monolayer is calculated as an indirect band gap semiconductor with an indirect band gap of a value 2.19?eV. This GeI₂ monolayer is good for absorbing low-energy photons, and it is insensitive to high-energy photons. The material is stable at temperatures up to 600?K, so we calculated the thermal conductivity (K_L), Seebeck coefficient (S), power factor (PF) and thermoelectric figure of merit (ZT) of the GeI₂ monolayer at various carrier concentrations from 300 to 600?K. Due to the lower group velocity, the GeI₂ monolayer has a lower thermal conductivity of 0.48?W/m?K at 300K. In P-type doping, the power factor can up to 0.11?mW/m?K², and its ZT value is 4.04 at 600?K of the GeI₂ monolayer, indicating that the GeI₂ monolayer is a potential thermoelectric material.     

Electrical conduction and thermoelectric properties of perovskite-type BaBi1−xSbxO3

To elucidate the thermoelectric properties at high temperatures, the electrical conductivity and Seebeck coefficient were measured at temperatures between 423 K and 973 K for perovskite-type ceramics of BaBi_1?xSb_xO₃ solid solutions with x=0.0–0.5. All the ceramics exhibit p-type semiconducting behaviors and electrical conduction is attributed to hopping of small polaronic holes localized on the pentavalent cations. Substitution of Bi with Sb causes the electrical conductivity σ and cell volume to decrease, but the Seebeck coefficient S to increase, suggesting that the Sb atoms are doped as Sb⁵⁺ and replace Bi⁵⁺, reducing 6s holes conduction from Bi⁵⁺(6s⁰) to Bi³⁺ (6s²). The thermoelectric power factor S ²σ has values of 6×10^?8–3×10^?5 W m^?1 K^?2 in the measured temperature range, and is maximized for an Sb-undoped BaBiO_3?δ, but decreases upon Sb doping due to the decreased σ values.     

Investigation of the pressure dependence of band gaps for silver halides within a first-principles method

The pressure dependence of the direct and indirect band gaps in rocksalt silver halides has been studied using the full-potential linearized augmented plane wave method within the generalized gradient approximation for the exchange-correlation potential. It is found that indirect band gaps (L→X and L→Γ) exhibit different responses to application of pressure. Similar trends are found for the indirect band gaps of AgCl and AgBr while the trend in AgI (L→Γ) band gap is different. In all the compounds, the effect of pressure on the direct band gaps (Γ→Γ, X→X and L→L) show qualitatively similar results. The fundamental indirect band gap (L→Γ) pressure coefficients are −4.19 meV (GPa)⁻¹ and −3.81 meV (GPa)⁻¹ for AgCl and AgBr while for AgI (L→X) it is −61.50 meV (GPa)⁻¹. The band gap pressure coefficient as well as the volume deformation potential for the various band gaps of the compounds have also been investigated.     

Specific features of the damage nucleation stage under severe loading of copper

The nucleation and evolution of damage in annealed coarsely crystalline M1-type copper subjected to fast loading to a pressure P ～ 32 GPa, followed by the action of tensile stresses σ _p with an intensity of ≈?2.0 GPa for a time t ≈ 0.3–1.5 μs, have been investigated numerically and experimentally. It has been shown that, at a specific combination of amplitude-time characteristics of the tensile stress pulse, damage localization in some cases at t < 1 μs has been observed in zones (～10–14 mm in size) alternating with “dead” zones (～3–5 mm in size) containing no visible damages. Pores are connected by “yield streamlets.” The existing multistage models of fracture kinetics have neither explained nor predicted the formation of a “band” damage structure or the presence of “yield streamlets” in specimens.     

Transport properties of MgB2 single crystals doped with electrons and holes

The thermoelectric power of C, Mn, C:Li, and Al:Li substituted MgB₂ single crystals has been investigated in the temperature range 10-300 K. Both the in-plane (S_ab) and the out-of-plane (S_c) thermopowers are positive for the non-substituted crystal and both S_ab and S_c change a sign for crystals doped with electrons where C is substituted for B in the amount larger than 5 at%. When Li is substituted for Mg, the π band rather than the σ band is doped with holes and the doping effects are much more subtle. The anisotropy ratio of the non-substituted crystal S_ab/S_c≈3 and this ratio is strongly reduced by the substitution of C. Isovalent magnetic Mn ions, which substitute for Mg with a drastic reduction of T_c, do not influence the values and temperature dependences of both S_ab and S_c.     

Jahn-Teller band transition in La3S4 and La3Se4

La₃S₄ and La₃Se₄ undergo a cubic to tetragonal phase transformation at a temperature of 103 and 70 K respectively, the c/a ratio is 0.984 for La₃S₄ and 0.987 for La₃Se₄. In these compounds the conduction electron Fermi energy happens to be close to a band structure anomaly which drives the phase transition. We find some indication, that the anomaly might be of ?-type.     

Fluorescence and absorption spectra of 9, 10-diazaphenanthrene solutions at 77°K

The S₁(n, √^*) ? S₀ fluorescence and absorption spectra of 9, 10-diazaphenanthrene solutions in hydrocarbons possess at 77°K a quasilinear structure (Shpolskii effect). In the frozen n-hexane matrix (c = 10^-4 M) the fluorescence spectrum displays a triplet structure (22 181, 22 169 and 22 149 cm^-1 for the O-O transition). The spectra of 9, 10-diazaphenanthrene solution are shifted towards the red compared with those of the single crystal (～ 900 cm^-1). A good agreement between the calculated and experimental values of this shift is obtained. A strong concentration effect on the structure of the spectra is discussed. A quasilinear structure is observed also in the second absorption band.     

Inversion of conductivity type in Bi2Te3−xSx crystals

Transport parameters and optical properties of Bi₂Te_3?xS_x single-crystals with x=0–0.18 were studied. With increasing sulphur content the concentration of free current carriers decreases up to x=0.12, due to the interaction of S^x_Te defects with antisite defects Bi^′_Te, and then the p-type conductivity changes to the n-type. The optical gap of Bi₂Te_3?xS_x crystals increases with increasing S content. The obtained results led to the preparation of Bi₂Te₃-Bi₂Te_3?xS_xp–n junction by the heat treatment of p-type Bi₂Te₃ in S vapours.     

LaBiTe3: An unusual thermoelectric material

Using first‐principles calculations and semi‐classical Boltzmann transport theory, the thermoelectric properties of LaBiTe₃ are studied. The band gap and, hence, the thermoelectric response are found to be easily tailored by application of strain. Independent of the temperature, the figure of merit turns out to be maximal at a doping of about 1.6 × 10²¹ cm^–3. At room temperature we obtain values of 0.4 and 0.5 for unstrained and moderately strained LaBiTe₃, which increases to 1.1 and 1.3 at 800 K. A large spin splitting is observed in the conduction band at the T point. Therefore, LaBiTe₃ merges characteristics that are interesting for thermoelectric as well as spintronic devices. (© 2014 WILEY‐VCH Verlag GmbH &Co. KGaA, Weinheim)     

The electronic structure and optical and magnetooptical properties of the CaCo<Subscript>2</Subscript> compound synthesized at a high pressure: Experiment and theory

The optical properties (the real ε₁ and imaginary ε₂ permittivity parts, optical conductivity σ, and reflectivity R) of the new ferromagnetic compound CaCo₂ in the Laves cubic phase (C15) synthesized at a pressure of 8.0 GPa were studied over the spectral range ?ω = 0.2–9 eV. The field and spectral (?ω = 0.5–4.2 eV) dependences of the equatorial Kerr effect were determined. The electronic structure and optical characteristics of CaCo₂ were calculated using the electron density functional theory by the linearized augmented-plane-wave method. The main band structure parameters of the compound were determined. The experimental and theoretical σ(ω) and R(ω) dependences were in satisfactory agreement with each other. The formation of the main absorption bands was found to be caused by the (p,d → d,p)-type electronic transitions related to the cobalt and calcium atoms. The exchange splitting of the 3d band of CaCo₂ was estimated, 2Δ_exc ～ (1–1.3) eV.     

An analysis of the thermoelectric efficiency of n-(Bi,Sb)2(Te,Se,S)3 solid solutions within an isotropic scattering model

A study is reported on the thermoelectric properties of n-type solid solutions Bi₂Te_3?y Se_y (y=0.12, 0.3, 0.36), Bi_2?x Sb_xTe_3?y Se_y (x=0.08, 0.12; y=0.24, 0.36), and Bi₂Te_3?z S_z (z=0.12, 0.21) as functions of carrier concentration within the 80-to 300-K range. It has been established that the highest thermoelectric efficiency Z is observed in the Bi₂Te_3?y Se_y (y=0.3) solid solution containing excess Te at optimum carrier concentrations (0.35×10¹⁹ cm^?3) and at temperatures from 80 to 250 K. The increase in Z in the Bi₂Te_3?y Se_y solid solution compared with Bi_2?x Sb_xTe_3?y Se_y and Bi₂Te_3?z S_z is accounted for by the high mobility μ₀, an increase in the effective mass m/m ₀ with decreasing temperature, the low lattice heat conductivity κ_L, and the weak anisotropy of the constant-energy surface in a model assuming isotropic carrier scattering.     

Ab initio study of thermoelectric properties of Cu<Subscript>3</Subscript>PSe<Subscript>4</Subscript> and Cu<Subscript>3</Subscript>PS<Subscript>4</Subscript>: alternative materials for thermoelectric applications

This letter discusses the thermoelectric properties of Cu₃PSe₄ and Cu₃PS₄ compounds, using the Ab initio calculations. These compounds are predicted to be good thermoelectric materials thanks to the nature of their band edge states. Seebeck coefficient of Cu₃PSe₄ exhibits a maximum value of 1256 µV/K at roopm temperature, whereas it is 2389 µV/K for Cu₃PS₄. Furthermore, the electrical conductivity is significantly enhanced with doping level while the electronic thermal conductivity is weakly increased. Besides, the factor of merit of these compounds shows a value around the unity only at low doping levels. Hence, this predicts that these compounds may present excellent thermoelectric properties, therefore they could be considered as alternatives for thermoelectric applications.     

Disorder-induced Raman scattering of In1−xGaxP in the transverse acoustic phonon region

Raman scattering measurements have been performed on In_1?xGa_xP (0.62?x?1) over the entire frequency range of first and second order scattering. Besides the already known disorder activated band between the LO(Γ) and TO(Γ) modes a new disorder activated band is found in the region of transverse acoustic phonons around 87 cm^-1. The position of the new band shifts only slightly with composition while its strength and line-shape change.