Electrical conduction in non-metallic rare-earth solids

Schematic energy band diagrams for the genesis of charge carriers in non-metallic rare-earth solids have been presented. It has been shown that positions of 4f bands have significant effect on the genesis and nature of charge carriers, their conduction mechanism and magnitude of electrical conductivity () and Seebeck coefficient (S) of the solid. Relevant relations have been given for both and S in different situations. Experimental data on rare-earth sesquioxides (R₂O₃), rare-earth tungstates [R₂(WO₄)₃] and rare-earth molybdates [R₂(MoO₄)₃] in the intrinsic range have been explained as examples for the validity of energy band diagrams.     

Electrical conduction in narrow band trivalent transition metal tungstates

The measurements of electrical conductivity () and Seebeck coefficient (S) of trivalent transition metal tungstates M₂(WO₄)₃ with M = Cr, Fe and Ni have been reported in the temperature range of 450 to 1250 K. The results have been explained on the basis of a schematic energy band diagram appropriate for these solids. The estimated value of the energy band gap is slightly more than 3 eV.     

Electrical transport as a function of temperature in urea and cobalt (III)-urea complex

The temperature dependence of the electrical conductivity, and the voltage–current dependence of urea and its cobalt solid complex were determined. The values of the thermal activation energies indicate that the two compounds behave like semiconductors. The semiconducting properties of the ligand urea are due to the electron delocalization via -bond formation (C=NH,C=₂ ⁺) as well as the lone pair of electrons on the oxygen atoms of urea molecules. The high conductivity values of urea compared with those of its cobalt solid complex was suggested to be due to the ease of releasing the lone pair electrons on the oxygen atom as well as the hydrogen transfer during enol formation of urea molecules. The infrared, ultraviolet and visible absorption measurements were carried out to rationalize the mechanism of the conduction process in the investigated compounds.     

Electrical transport in light rare-earth molybdates

Rare-earth molybdates of the type R₂(MoO₄)₃ with R=La, Ce, Pr, Nd, Sm and Eu were prepared and characterized, and the electrical conductivity, and Seebeck coefficient, S in the temperature range 450–1200 K were measured. These molybdates are concluded to be insulating solids with a band gap which increases slowly going down the series from 2.30 eV for La molybdate to 3.20 eV for Eu molybdate. The plots of log and S versus T ^–1 show, in general, three linear regions with two break temperatures T ₁ and T ₂ occurring due to a change in the conduction mechanism. At higher temperatures the intrinsic conduction in these solids occurs via a band mechanism. The O^2– 2p and Mo⁶⁺ 4d orbitals form the valence and conduction bands, respectively. These bands are the main support of conduction in La, Sm and Eu molybdates; however, for Ce, Pr and Nd molybdates 4f ⁿ levels fall within the band gap and become very effective in electrical conduction. The main charge-carrying entities seem to be electrons in Ce, Pr and Nd molybdates and holes in La, Sm and Eu molybdates. On the basis of mobility calculations of charge carriers it is concluded that the charge carriers in these bands become polarons which are, in fact, the charge carrying entities. At lower temperatures electrical conduction is mainly extrinsic. Cerium molybdate shows a semiconductor-semimetal transition around 940 K.     

Charge transport in CaTiO3: I. Electrical conductivity

The present paper reports the electrical conductivity of polycrystalline undoped CaTiO₃ in the temperature range 973–1323 K under controlled oxygen partial pressure (10–10⁵ Pa). The electrical conductivity data are considered in terms of defect disorder and related semiconducting properties of CaTiO₃. The values of the p(O₂) exponent of electrical conductivity at high p(O₂), that vary between 1/4.3 and 1/6.2 at 973 and 1323 K, respectively, are considered in terms of theoretical defect disorder model of p-type CaTiO₃ and increasing effect of minority charge carriers (electrons) with temperature on p-type conduction. The activation energy of the electrical conductivity, assuming 125.3 kJ/mol at 10 Pa and 94.4 kJ/mol at 72 kPa, has been considered in terms of the formation of defect and their mobility. The band gap, determined from the minimum of electrical conductivity corresponding to the n–p transition is equal to 2.77 eV.     

Electrical conduction in light rare-earth tungstates

This paper reports the results of electrical conductivity (σ) and thermoelectric power (H) of light rare-earth tungstates in the temperature range 600–1200K. Holes are the dominant charge carriers over the whole studied temperature range for Nd, Sm and Gd tungstates. However, in the case of La, Ce and Pr tungstates, the conduction is dominated by electrons at lower temperatures, but above 950K in La, 800K in Ce and 950K in Pr the dominant charge carriers become holes. A sharp break and change in the log σ vs 1/T slope occurs in La, Ce and Pr tungstates around the same temperature at which the dominant charge carrier changes from electrons to holes. In the case of Nd, the conductivity anomaly occurs around 1020K without any change in the nature of the charge carrier. The data have been analysed using band theory.     

Dielectric relaxation and conduction in the system Ba1-xLaxSn1-xCrxO 3

Attempts have been made to synthesize the solid solution, Ba_1-xLa_xSn_1-zCr_xO ₃ for x 0.20 and study its electrical behavior. It has been found that a single phase solid solution forms for compositions up to x 0.10. The structure remains cubic. Grain size of the samples decreases with increasing x. Dielectric relaxation in these materials is attributed to reorientation of dipoles due to hopping of electrons among various oxidation states of Sn ions around oxygen vacancies. Using complex plane impedance analysis, it has been confirmed that dielectric relaxation and conduction occur by the same process.     

Electrical conductivity and defect structure of polycrystalline tin dioxide doped with antimony oxide

Electrical conductivity () of tin dioxide doped with antimony has been measured as functions of temperature and oxygen partial pressure (p0_2> ). Variation of electrical conductivity is explained by assuming that the antimony oxide forms a substitutional solid solution and doubly ionized oxygen vacancies are predominant defects. Above –10^–5 atm oxygen partial pressure antimony ions are present predominantly in the pentavalent state in tin dioxide lattice. However, it is converted to the trivalent state below this oxygen partial pressure accompanied by a sudden rise in conductivity.     

Some electrical transport studies on compounds of the Tl-Te system

Galvanomagnetic, thermoelectric and electrical conductivity measurements were made on single crystal specimens of Tl₂Te₃, TlTe and -phase in the temperature range from 77° K up to 500 to 700° K. All crystals were found to be of p-type conductivity. Measurements of the electrical conductivity versus composition have revealed that -phase represents solid solutions of Tl and Te in the compound with formula Tl₅Te₃. Also, it has been found that both TlTe and -phase samples exhibit a metallic behaviour in contrast to Tl₂Te₃, which has semiconducting properties. In the intrinsic conduction region a thermal energy gap of a 0.68 ± 0.03 eV was found, which was compared with that obtained from optical transmission measurements. The electrical properties of Tl-Te compounds are discussed on the basis of the general valence rule.     

Electrical transport studies on heavy rare-earth tungstates

This paper reports the measurement of thermoelectric-power (S) at different temperatures (800–1100 K) and electrical conductivity (σ) as a function of electric field strength, time, ac signal frequency and temperature (650–1200 K) for pressed pellets of heavy rare-earth tungstates (HRET) with a general formula RE₂(WO₄)₃ [where RE = Tb, Dy, Ho, Er, Tm and Yb]. These tungstates are typical insulating compounds with room-temperature σ value less than 10^?10 ohm^?1 cm^?1 and become semiconductors at elevated temperature with σ values of the order of 10^?5 ohm^?1 cm^?1 at 1200 K. The S vs T^?1 and log σ vs T^?1 plots are linear, but a change in the slope of straight lines occurs at a temperature (T_B) which lies between 900–1025 K for different tungstates. These break temperatures are the same for both S and σ plots. It has been found that HRET are mixed ionic-electronic conductors. Above T_B the electronic conduction dominates over the ionic conduction, but below T_B both become comparable. The electronic conduction above T_B is intrinsic with large polaron holes as the principal charge carriers; they conduct via a band mechanism. The energy band gap lies in the range 3.2 to 4.0 eV, and the charge carrier mobility in the range 3.8×10^?2 to 2.5 cm²/V-Sec for the different tungstates. Below T_B both electronic and ionic conduction are extrinsic. The electrons conduct via a thermally activated hopping mechanism with an activation energy lying in the range 0.87 to 1.30 eV, and the holes via a diffusion process with an activation energy lying in the range 0.88 to 1.23 eV for the different tungstates.     

TlInS2–TlCeS2Solid Solutions

TlInS₂–TlCeS₂solid solutions were characterized by x-ray diffraction and electrical measurements. The solubility limit of TlCeS₂in TlInS₂was found to be 8 mol %. The electrical conductivity and Hall coefficient of TlIn_{1 – x} Ce _x S₂(0 x 0.08) were measured from 300 to 1100 K. It was found that, as the Ce content of the solid solution increases, the band gap decreases, and the lattice parameters increase.     

Properties of TlInSe2–TlNdSe2 Solid Solutions

TlIn_{1 – x} Nd _x Se₂(0 < x 0.1) solid solutions were characterized by x-ray diffraction and temperature-dependent electrical conductivity, Hall effect, thermoelectric power, and heat capacity measurements. With increasing Nd content, the lattice parameters of the solid solutions increase almost linearly, and their band gap decreases.     

Electrical transport in heavy rare-earth vanadates

This paper reports measurements of electrical conductivity () and Seebeck coefficient (S) between 300 and 1250 K and differential thermal analysis (DTA) and thermogravimetric analysis (TGA) between 300 and 1200 K, together with X-ray diffraction studies of heavy rare-earth vanadates (RVO₄ with R=Tb, Dy, Ho, Er and Yb). All these vanadates have been found to have a tetragonal unit cell. The DTA study shows a flat dip in the temperature interval 1075 to 1300 K, indicating a possible structural phase transition of these compounds. Practically no weight loss has been observed in TGA from 300 to 1200 K in any of the vanadates. All RVO₄ are semiconducting materials with the room-temperature value lying in the range 10^–12 to 10^{–3 –1} m^–1, becoming of the order of 10^{–2 –1} m^–1 around 1000 K. The electrical conductivity of all vanadates exhibits an exponential increase in the temperature intervals 420 K toT ₁ andT ₁ toT ₂, with different values of the activation energy. A log againstT ^–1 plot shows a peak aroundT ₃ and drops to a minimum value aroundT ₄, before increasing again with temperature.T ₄ >T ₃ >T ₂ >T ₁ are different for different vanadates and these are termed break temperatures.T ₄ lies well within the temperature range of the DTA peak and can be termed the phase transition temperature. In the lower temperature interval the electrical conduction is essentially extrinsic. The localized charge carriers on defect centres conduct by a hopping mechanism. The defect centres are V⁴⁺ ions in all vanadates with R⁴⁺ centres in some of them. It is concluded that in the temperature intervalT ₁ <T <T ₂ the conduction mechanism is of the intrinsic band type, with oxygen 2p and vanadium 3d as the valence and conduction bands, respectively. Related parameters like the energy band gap and the mobilities of the charge carriers have also been evaluated. The low values of mobility suggest that large polarons with intermediate coupling are the charge carriers rather than bare electrons in the intrinsic region. All these vanadates tend to become metallic, but before this is achieved the phase change makes the conductivity smaller.     

Thermopower measurements on SiO x thin films

Thermopower and d.c. electrical conductivity measurements have been carried out between 125 and 625 K on SiO _x thin films, 130 nm thick, deposited on to Corning 7059 substratesin vacuo 1 mPa at 1.5 nm sec^–1. The thermopower, d.c. conductivity and their respective activation energies are fitted to a polynomial expression in 1/T. Below 400 K, the thermopower is negative, at 400 K the thermopower activation energy is approximately zero and the dominant current carriers are holes at the valence band edge, between 400 and 470 they are polaronic holes, between 470 and 590 K non-polaronic holes, and above 590 K electrons. Energy band diagrams are proposed for each temperature range studied.     

Gamma-ray dosimetric properties of molybdenum phosphate glasses

In the present work, a study of the temperature dependence of the d.c. electrical conductivity and conduction activation for a series of MoO₃-P₂O₃ glass systems has been carried out. The conductivity measurements of the unirradiated glass specimens proved to be mainly dependent on both temperature and transition metal ion content in the glass matrix. The results of the present investigation have shown that the conduction mechanism would be due to the electron exchange between the low and high valency states of the MoO₃ oxide (Mo" and Mo⁶⁺). The radiation-induced conductivity of the glass system studied, produced by gamma rays, has also been measured experimentally. The d.c. electrical conductivity has proved to be dose dependent, which showed a decrease with increasing -dose. The results reflect some evidence of the-ray dosimetric potential of the glass specimens studied.     

Electrical properties of vanadium pentoxide doped with lithium and sodium in the α-phase range

The electrical conductivity and Seebeck coefficients have been measured from room temperature to 500° C for polycrystalline V₂O₅ and V₂O₅ doped with lithium and sodium in the -phase range. The conductivity increases with doping and the energy of activation decreases. The Seebeck coefficient indicates that electrons are the majority carriers. The results have been discussed in terms of the two-level hopping model.     

Electrical transport in new La1−xNdxTlO3 (0 ⩽x⩽ 1) system

The electrical conductivity () and thermoelectric power (S) for the system La_1–x Nd _x TlO₃ (0 x 1.0) in the temperature range 295 to 805 K are reported. Both log andS as a function ofT have been found to be linear with a break in the slope around a specific temperature,T _B. The break temperature systematically decreases as we proceed from the systemx=0 tox=1.0. It has been concluded that conduction belowT _B is extrinsic and takes place owing to hopping of electrons localized on the defect centre Tl²⁺ to Tl³⁺ on normal sites. Conduction aboveT _B is the normal band type in all systems. The energy band gap has been evaluated in all cases and it has been found that it decreases systematically fromx=0 tox=1.0.     

Preparation, Structure, and Electrical Conductivity of La(Ga,Mn)O3 Ceramics

(La_0.90.1)(Ga_{1 – x} Mn _x )O _y ( = vacancy;x = 0, 0.2, 0.4, 0.6, 0.8, 1.0) perovskite solid solutions are prepared by solid-state reactions and characterized by x-ray diffraction, electron microscopy, and dielectric spectroscopy. The solid solutions have a rhombohedral structure over the entire composition range. Mn substitution leads to a transition from ionic hopping conduction, with a linear variation of log (T) with 1/T, to ionic–electronic conduction. The unit-cell volume and conductivity of the solid solutions are shown to correlate with their tolerance factor.     

Oxygen ion conduction in γ-Bi2O3 doped with Sb2O3

Electrical conduction in bcc-Bi₂O₃ doped with Sb₂O₃ was investigated by measuring electrical conductivity, as a function of temperature and oxygen partial pressure , and ionic transference number. The-Bi₂O₃ doped with 1 to 3 mol% Sb₂O₃ was stable up to 550° C and showed an oxygen ionic conduction in the region of 10⁵ to 10^–9 Pa. As the Sb₂O₃ content increased, ionic conductivity increased up to 2.5 mol % Sb₂O₃ (1.8×10^–3–1cm^–1 at 500° C) and then decreased. However, the activation energy for ionic conduction remained almost unchanged. It was proposed that the-Bi₂O₃ contains a lot of oxygen vacancies and incorporated Sb⁵⁺ ions at tetrahedral sites which affect the concentration of oxygen vacancy effective for conduction.     

Investigation of the gapless state induced by pressure in Hg1−x Cd x Te alloys

We report the results of an experimental study of the gapless state (GS) induced inp-type semimetallic alloys Hg _1–x Cd _x Te (0<x<0.15) by pressure. Galvanomagnetic effects in weak magnetic fields (2T<300 K) and the Shubnikov-de Haas effect have been investigated in the pressure interval 1p<15 kbar. Direct evidence for the existence inp-type semimetallic alloys of an impurity hole band overlapping with the conduction band by 3–4 me V is obtained. At liquid helium temperatures the Fermi level is located in the impurity band, so that two groups of carriers take part in transport effects: light electrons in the conduction band and heavy holes in the impurity band. The electron Fermi energyE _F is proved to be essentially constant during the transition to the GS. A linear dependence of the electron effective mass at the band edgem* (0) upon the gapE _g is obtained. A significant role of scattering of electrons into the impurity band at liquid helium temperatures is revealed.