具有本征低晶格热导率的硫化银快离子导体的热电性能

硫化银(Ag_2S)是一种典型的快离子导体材料,前期关于Ag_2S的研究主要集中在光电和生物等领域.最近的研究表明, a-Ag_2S具有和金属一样的良好延展性和变形能力.但是, Ag_2S的热电性能尚无公开报道.本工作合成了单相Ag_2S化合物,系统研究了其在300—600 K范围的物相变化、离子迁移特性和电热输运性质.研究发现, Ag_2S在300—600 K温度区间表现出半导体的电输运性质.由于单斜-体心立方相晶体结构转变, Ag_2S的离子电导率、载流子浓度、迁移率、电导率、泽贝克系数等性质在455 K前后出现急剧变化.在550 K, Ag_2S的功率因子最高可达5μW·cm~(–1)·K~(–2). Ag_2S在300—600 K温度区间均表现出本征的低晶格热导率(低于0.6 W·m~(–1)·K~(–1)). S亚晶格中随机分布的类液态Ag离子是导致b-Ag_2S体心立方相具有低晶格热导率的主要原因.在573 K, Ag_2S的热电优值可达0.55,与Ag_2Se, Ag_2Te, CuAgSe等已报道的Ag基快离子导体热电材料的性能相当.     

Lithium ion-poly (ethylene oxide) complexes. I. Effect of anion on conductivity

The ionic conductivities of a series of lithium salt-poly (ethylene oxide) complexes have been studied from ambient temperature to approximately 400 K. Plots of the variation of conductivity with temperature indicate a transition in behaviorr between 330–360 K. The activation energies in the high temperature regime vary from 0.16 eV (LiH₂PO₄) to 1.45 eV (LiNO₃) and in the low temperature regime from 0.86 eV (LiBF₄) to 2.47 eV (LiH₂PO₄). Conductivity values for the salts tested also exhibits wide variation. The lowest measured conductivity was 10^?9 S/cm and the highest was 10^?4 S/cm. The anion plays an important role in the total measured conductivity. Salts with oxygen-containing anions form complexes with an apparent higher conductivity when the sample is vacuum dried. This suggets hydrogen bonding with residual water in the polymer impeding anionic mobility. In one of the salt-complexes (LiBF₄) residual water and thermal history, were shown not to significantly affect conductivity.     

Proton conductivity and phase transition in potassium hydroxide monohydrate

The ionic conductivity of a proton conductor, namely, potassium hydroxide monohydrate, has been studied in the temperature range of 200–410 K. It has been established that the temperature dependence of the conductivity has the Arrhenius form with an activation enthalpy of ～0.4 eV. The pre-exponential factors for the intervals above and below room temperature differ by a factor of ～2.5. The anomalous temperature behavior observed in the range of 285–345 K indicates a phase transition with T _c ～ 295 K. The mechanism of proton transport has been discussed.     

Exchange interaction effects on the optical properties of LuMnO3

We have measured the optical conductivity of single crystal LuMnO3 from 10 to 45000 cm(-1) at temperatures between 4 and 300 K. A symmetry allowed on-site Mn d-d transition near 1.7 eV is observed to blueshift ( approximately 0.1 eV) in the antiferromagnetic state due to Mn-Mn superexchange interactions. Similar anomalies are observed in the temperature dependence of the TO phonon frequencies which arise from spin-phonon interaction. We find that the known anomaly in the temperature dependence of the quasistatic dielectric constant epsilon(0) below T(N) approximately 90 K is overwhelmingly dominated by the phonon contributions.     

Phase transitions in CuBiP2Se6 crystals

Investigation results of dielectric (20?Hz–1?MHz) properties of layered CuBiP₂Se₆ crystals are presented. The temperature dependence of the static dielectric permittivity reveals the first-order “displacive” antiferroelectric phase transition at T _c?=?136?K. In the paraelectric phase, at low frequencies, dielectric spectra are highly influenced by the high ionic conductivity with the activation energy of 2473?K (0.21?eV). In the antiferroelectric phase the electrical conductivity and its activation energy (531.1?K (0.045?eV)) are considerably smaller. At low temperatures, the temperature behaviour of the distribution of relaxation times reveals complex freezing phenomena. A part of long relaxation time distribution is strongly affected by external direct current (DC) electric field and it is obviously caused by antiferroelectric domain dynamics.     

Structure and transport properties of stephanite (Ag5SbS4) according to antimony nuclear quadrupole resonance

Silver sulfo-antimonide Ag₅SbS₄ (stephanite) has been studied by nuclear quadrupole resonance (NQR) spectroscopy on antimony nuclei. The temperature dependences of the spectroscopic and relaxation parameters have been examined in the range of 4.2?C395 K. A phase transition at 140 K and internal motions with an activation energy of 0.29 eV have been experimentally detected. The nature of the phase transition and diffusion of silver ions has been discussed in view of the reported data.     

Two-carrier conduction in Rb-TCNQ-II

Single crystal electrical resistivity and thermoelectric power (TEP) measurements as a function of temperature are repotted in Rb-TCNQ-II. Results suggest intrinsic conductivity by holes above 360 K with an activation energy of 0.44 eV. Below room temperature conductivity is extrinsic and is due to donors located 0.18 eV below the conduction band.     

The contribution of anion disorder to ionic conductivity on single crystals of β-PbF2

The effect of the different cooling processes on the disorder of flourine ions and ionic conductivity in β-PbF₂ has been studied by X-ray method and ionic conductivity measurements on single crystals below the transition temperature T_c. The spike-like diffuse scattering was observed along the <111>¹ directions around the Bragg reflections. The activation energies for the conduction process are 0.40 eV for the sample quenched from 970 K and 0.54 eV for the one from 720 K. The higher the quenching temperature is, the higher the conductivity and the lower the activation energy become. The dependence of conductivity on the different cooling processes is more evident in single crystals than in polycrystalline samples. The contribution of the different cooling processes to ionic conductivity can be quantitatively explained by the extent of ordering of mobile fluorine ions. Time dependence of ionic conductivity has not been observed.     

Cation mobility in the Cu6PS5X (X=Cl,Br, I) argyrodites

The electrical conductivity of Cu₆PS₅Cl shows purely Arrhenius behavior throughout the temperature range 170–450 K with no evidence of the 241 K thermodynamic phase transition previously reported. Cu₆PS₅Br exhibits two changes in activation energy. The first, at 251 K, apparently coincides with a previously determined thermodynamic transition. The second, at 321 K, is likely associated with a conduction transformation from ionic to electronic. The conductivity of Cu₆PS₅I is characterized by a cusp at a temperature of 194 K, far removed from a previously reported thermodynamic transition at 221 K. In addition, a broad and continuous change in activation energy appears to coincide with another previously reported phase transition at 270 K.     

Photoluminescence investigations on the nanoscale phase transition in Pb(Mg1/3Nb2/3)O3

The temperature dependence of the photoluminescence spectra of the relaxor ferroelectric Pb(Mg_1/3Nb_2/3)O₃ has been reported. The temperature behaviours of the 1.57, 1.67 and 1.73 eV bands indicate a phase transition at 110 K. This is attributed to a structural phase transition in the charged nanoshell. Analysis of the temperature dependence of 1.67 eV band intensity with a thermal quenching model indicated the existence of a phonon mode at 1153 cm⁻¹. This mode is identified in the Raman spectra measurement. The intensity of the 1.73 eV band showing an anomalous behaviour at 210 K is attributed to a transition from a crystalline phase to an amorphous phase in the charged nanoshell.     

DC conductivity and dielectric constant of cellulose nitrate detector

The temperature dependence of DC conductivity as well as the temperature and frequency dependence of dielectric constant of cellulose nitrate (CN) track detector was undertaken for better understanding of the track response of the detector. In the temperature range −100° to + 100°C, CN is found to be an insulating semiconductor with activation energies 0.04 eV, 0.78 eV and 1.24 eV. The transition of the activation processes takes place at (−10±1)°C and (60±1)°C. The high activation energy transition temperature coincides with the glass transition temperature of CN. The dielectric constant is found to increase with temperature and decrease with frequency. These results reveal the physical basis of the Ion Explosion Spike model of Fleischer et al. (1975). A possible explanation of the registration temperature effect is also given on the basis of these results.     

Phase transitions and electrical properties of CsH(SO4)0.76(SeO4)0.24 mixed crystals

The study by X-ray diffraction, calorimetry, vibrational and impedance spectroscopy of CsH(SO₄)_0.76(SeO₄)_0.24 new solid solution is presented. Crystals of this composition undergo two phase transitions at T = 333 and 408 K. The last one at 408 K is a superionic-protonic transition (SPT) related to a rapid [HS(Se)O₄⁻] reorientation and fast H⁺ diffusion. A sudden jump in the conductivity plot confirms the presence of this transition. Above 408 K, this high temperature phase is characterized by high electrical conductivity (7 × 10^t-3 Ω⁻ cm⁻¹) and low activation energy (E_a < 0.3 eV).     

Study of trapping and recombination centres in Tl2InGaTe4 chain crystals by dark electrical conductivity and photoconductivity measurements

Dark electrical conductivity and photoconductivity of Tl₂InGaTe₄ single crystals have been measured and analyzed in the temperature region 100–300 K. The dark electrical conductivity measurements revealed an intrinsic- or extrinsic-type of conductivity above or below 210 K, respectively. From intrinsic conductivity data analysis, the energy band gap of Tl₂InGaTe₄ crystals was determined as 0.85 eV. In the extrinsic region, the dark conductivity arises from a donor energy level located at 0.30 eV below the conduction band. The photocurrent increases with increasing illumination intensity. The recombination mechanism in the crystal changes as temperature decreases due to the effect of exponential trapping centres. Two trapping and/or recombination centres located at 89 and 27 meV were determined from the temperature dependence of the photocurrent, which decreased or increased with increasing temperature in the regions above or below 180 K, respectively.     

Ferroelectric phase transition in Ga2Te3 single crystals

Measurements of the electrical conductivity and Hall effect were carried out in a wide temperature range (200-500 K) for Ga₂Te₃ crystals. The crystals were grown in single crystalline form by making a modification of the travelling heater method technique. The measurements revealed unusual observations in the electric conductivity and Hall mobility indicating the presence of some type of phase transitions at about 430 K. So, ferroelectric behavior was examined for confirming the presence of second-order (ferroelectric) phase transition. An energy gap of 1.21 eV and depth of the impurity center of 0.11 eV were found.     

Thermal analysis, Raman spectroscopy and complex impedance analysis of Cu2+-doped KDP

Raman spectroscopy and differential thermal analysis (DTA) and thermogravimetric analysis have been carried out on Cu-doped KH₂PO₄ (Cu-KDP). X-ray diffraction powder data reveal that the structure of the KDP crystal does not change with the additive Cu²⁺ ion. DTA analysis and Raman study of Cu-KDP as a function of temperature reveal that this compound undergoes two phase transitions at about Ttr =453 and 473 K. The electrical conductivity measurements on polycrystalline pellet of Cu-KDP (5) are performed from room temperature (RT) up to 495 K. Only one phase transition is observed at 470 K. The activation energy in the migration is 0.42 eV in the temperature range from RT to 470 K. For temperature above 470 K, the activation energy of the superprotonic phase is 1.87 eV.     

Combined experimental and theoretical investigation of the premartensitic transition in Ni2MnGa

Ultraviolet-photoemission (UPS) measurements and supporting specific-heat, thermal-expansion, resistivity, and magnetic-moment measurements are reported for the magnetic shape-memory alloy Ni2MnGa over the temperature range 100T(PM) is due to the Ni d minority-spin electrons. Below T(M) this peak disappears, resulting in an enhanced density of states at energies around 0.8 eV. This enhancement reflects Ni d and Mn d electronic contributions to the majority-spin density of states.     

Temperature-dependent photoluminescence from bicrystalline ZnS (core/shell) nanocables

The experimental study of the temperature-dependent photoluminescence properties of bicrystalline ZnS core/shell nanocables under 10 K to 300 K was reported. The results show that there are two distinct peaks situated at the UV (about 372.6 nm) and green (about 500 nm) regions, respectively, and one blue-shoulder band (about 465 nm) superimposed between them for all spectra. The UV peak shows an obvious redshift with increasing temperature. The analyses of Gaussian-fitted the blue-shoulder band and the green bands from 10 K to 300 K reveal that all spectra can be well fitted by three Gaussian peaks: blue peak (B, about 2.67 eV), two green peaks (G1, about 2.47 eV and G2, about 2.42 eV). With increasing temperature, the blue band shows blueshift. The green bands show anomalous transition: green band G1 shows blueshift-redshift transition and green band G2 shows redshift-blueshift. The origins of these bands and their temperature-dependent shifts are explained based on defect levels and strong carrier localization effect at the defect levels in addition to band-gap shrinkage.     

Electronic transport mechanisms in tetraphenyleprophyrin thin films

Thermally-evaporated thin films of tetraphenylporphyrin, TPP, with thickness range from (175 to 735) nm had been prepared. Annealing temperatures ranging from (273–473) K do not influence the amorphous structure of these films. The influence of environmental conditions: film thickness, temperature and frequency on the electrical properties of TPP thin films had been reported. It was found that dc conductivity increases with increasing temperature and film thickness. The extrinsic conduction mechanism is operating in temperature range of (293–380) K with activation energy of 0.13 eV. The intrinsic one is in temperatures >380 K via phonon assisted hopping of small polaron with activation energy of 0.855 eV. The ac electrical conductivity and dielectric relaxation in the temperature range (293–473) K and in frequency range (0.1–100) kHz had been also studied. It had been shown that theoretical curves generated from correlated barrier hopping (CBH) model gives the best fitting with experimental results. Analysis of these results proved that conduction occurs at low temperatures (300–370) K by phonon assisted hopping between localized states and it is performed by single polaron hopping process at higher temperatures. The temperature and frequency dependence of both the real and imaginary parts of dielectric constant had been reported.     

On the behaviour of TSeF-TCNQ under pressure

We report b-axis electrical conductivity data for TSeF-TCNQ single crystals from 12 to 300 K under hydrostatic pressures up to 9 kbar. The single anomaly visible in the conductivity at 29 K and the low temperature conductivity gap rise under pressure at the same rate of ～ 6% kbar^-1. It has been found that the pressure dependence of the metal-insulator phase transition is qualitatively consistent with a mean field formulation of the Peierls transition. A Gruneisen constant of 0.64 for TSeF-TCNQ has been derived from this pressure study together with recent optical and compressibility investigations. The pressure dependence of the conductivity anisotropies at room temperature in TSeF-TCNQ and TTF-TCNQ are reported. The magnitude of the anisotropies in the two compounds are found to be essentially the same. The striking result, however, is that the anisotropies in both compounds are found to be independent of pressure up to 9 kbar.     

Jahn-teller dynamics in charge-ordered manganites from raman spectroscopy

The Raman spectra of the charge-ordered manganite Pr0.65Ca0.35MnO3 were studied as functions of temperature and excitation energy and compared to magnetic moment and electrical conductivity behaviors. Both the charge ordering ( T(co) approximately 225 K) and the antiferromagnetic transitions ( T(N) approximately 175 K) affect the spectral shape and intensity, indicating strong charge-lattice and spin-lattice couplings. Below T(co) a transition from dynamic Jahn-Teller distortions to a collective static distortion takes place. A change of the spectra is observed on increasing the excitation energy above 2.5 eV and it is attributed to a resonant polaron excitation.