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.     

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.     

Structure,thermal stability and electrical transport in coloured thallous tungstates

Different coloured thallous tungstates were prepared by solid state reaction between thallous carbonate and tungsten trioxide at higher temperatures. The structure and thermal stability of different thallous tungstates were determined by powder X-ray diffraction and thermogravimetric analysis, respectively. The electrical conductivity () and thermoelectric power (5) of pressed solid pellets of Tl₂WO₄ are reported in the temperature range 295 to 950 K. It is concluded that these materials have large numbers of oxygen ion vacancies and conduction occurs due to the electrons left by oxygen ions while escaping the solid. These electrons are trapped in the valence band and form centres analogous to a helium atom and electrical conductivity results when they enter the conduction band. The values of donor ionization energies have also been evaluated.     

Microscopic annealing process and its impact on superconductivity in T'-structure electron-doped copper oxides

High-transition-temperature superconductivity arises in copper oxides when holes or electrons are doped into the CuO(2) planes of their insulating parent compounds. Whereas hole doping quickly induces metallic behaviour and superconductivity in many cuprates, electron doping alone is insufficient in materials such as R(2)CuO(4) (R is Nd, Pr, La, Ce and so on), where it is necessary to anneal an as-grown sample in a low-oxygen environment to remove a tiny amount of oxygen in order to induce superconductivity. Here we show that the microscopic process of oxygen reduction repairs Cu deficiencies in the as-grown materials and creates oxygen vacancies in the stoichiometric CuO(2) planes, effectively reducing disorder and providing itinerant carriers for superconductivity. The resolution of this long-standing materials issue suggests that the fundamental mechanism for superconductivity is the same for electron- and hole-doped copper oxides.     

Promoting the La solution in 2:14:1-type compound:Resultant chemical deviation and microstructural nanoheterogeneity

RE₂Fe₁₄B-based(RE,rare earth)permanent magnets containing abundant and cheap La/Ce have attracted intense attention recently.In comparison with Ce that can fully replace Nd in the 2:14:1 lattice,La substitution for Nd has long been limited at a low level.Here we present that through doping La35 Ce65 alloy with the La/Ce ratio in natural mineral,stable 2:14:1 phase can be maintained at 1273 K within the entire substitution range of[(La₃₅Ce₆₅)、(Pr₂₀Nd₈₀)_1-x]_2.14Fe₁₄B(0.6≤x≤1.0,at.%),as verified by composition analysis,microstructural characterization and magnetic measurements.Interestingly,the promoted La solution in 2:14:1 phase induces two unique findings upon coexisting La-Ce-Pr-Nd:i)Compared to Ce that fits well with the nominal concentration,La deviates noticeably from the nominal one;ii)Nanoscale spinodal-decomposition-like phase separation is observed due to different solubilities of La-Ce-Pr-Nd elements in 2:14:1 phase.Above joint effects induce higher Curie temperature than estimation based on the rule of mixture,which delights the prospect of La₃₅Ce₆₅alloy in developing low-cost permanent materials.     

钙钛矿锰氧化物（La0.8Ln0.2）2/3Ca1/3MnO3（Ln=La，Ce，Pr，Nd，Sm，Eu）的低场电输运性能分析

用溶胶-凝胶法制备了系列掺杂（La0.8Ln0.2）2/3Ca1/3MnO3（Ln=La,Ce,Pr,Nd,Sm,Eu）纳米级晶体。对比在La被其它镧系元素部分替代后,引起的A位离子平均半径的改变和磁矩变化对电输运及磁阻性能的影响,实验分析表明,各个样品在同样大小磁场下的磁电阻存在较大的差别,样品的磁电阻随原子序数的增加而明显变大,由镧系收缩引起晶格畸变所产生的本征磁电阻占主导地位,随着外加磁场的增加,不同掺杂样品间的峰值电阻差异被弱化;各掺杂样品间的转变温度差与掺杂离子半径差和磁矩差密切相关,且离子半径的差异对转变温度的改变贡献更大。     

Development of cost-effective nanocrystalline multi-component(Ce,La,Y)-Fe-B permanent magnetic alloys containing no critical rare earth elements of Dy,Tb,Pr and Nd

Here we first report the fully abundant rare earth(RE)-based nanocrystalline multi-component(Ce,La,Y)-Fe-B alloys containing no critical RE elements of Nd,Pr,Dy,and Tb by melt-spinning technique.The roles of La and Y substitutions for Ce have been fully understood.La plays a positive role on both thermal stability and room-temperature(RT)magnetic properties.The enhanced coercivity Hcj by partial substitution of La is attributed to the increases of anisotropy field HA and the formation of continuously distributed grain boundaries resulting from the suppression of CeFe2 phase.Although Y substitution is not benefit for Hcj,both remanent polarization Jr and thermal stability have been effectively improved since Y2Fe14B shows relatively high saturation magnetization Ms and a positive temperature coefficient of HA over a certain temperature range.In addition,RE element segregation has been confirmed,La prefers to enter into the grain boundaries than Ce and Y prefers to remain in the 2∶14∶1 phase.Based on these understanding,a series of melt-spun(Ce,La,Y)-Fe-B alloys have been designed.A relatively good combination of magnetic properties with maximum energy product(BH)max=7.4 MGOe,Hcj=400 kA/m,and Jr=0.63 T has been obtained in[(Ce0.8La0.2)0.7Y0.3]17Fe78B6 alloy,together with high Curie temperature(Tc=488 K)and low temperature coefficients of remanence(α=-0.255％/K)and coercivity(β=-0.246％/K).     

Effect of addition of elements on two-glass-forming ability of Zr–Ce–Co–Cu immiscible alloys

The effects of Al element addition and partial substitution of Ce with La, Pr, and Nd on the two-glass-forming ability of the phase-separating Zr–Ce–Co–Cu alloy system have been studied. The distribution of the additions in the two coexistent immiscible liquids was analysed thermodynamically. The results indicate that Al almost equally distributes in the two liquids, whereas the elements La, Pr, and Nd are predominantly found in the Ce–Cu-rich liquid. X-ray diffraction analysis reveals that these additions obviously enhance the two-glass-forming ability of the coexisting liquids. This work presents a strategy for improving the two-glass-forming ability of immiscible alloys to obtain phase-separated bulk metallic glasses.     

Composition-dependent tunability of thermoelectric properties at low temperature for Pr-doped LPFCO double perovskite

The current research focuses on the synthesis, characterization, and low temperature thermoelectric characteristics along with optical bandgap analysis of La_2?xPr_xFeCoO₆ (x?=?0, 0.25, 0.50, 0.75, and 1) double perovskite. The sintered sample’s crystal structure, microstructural features, electrical properties, and thermal transport parameters were examined. The nanocrystalline single-phase material was confirmed after 6 h of sintering, and the crystallite size increases with increasing Pr concentration along with the occurrences of various oxidation states of La, Pr, Fe, Co, and O. The conductivity analysis confirms the presence of a closest neighbor hopping charge carrier conduction mechanism due to decreased bandgap in entire samples. For all the compositions (x), conductivity, power factor, and figure of merit were increased with increasing temperature and Pr-content, confirming the presence of larger charge carrier concentration along with decreased bandgap. Positive S values indicate the presence of p-type charge carriers in LPFCO double perovskite and intrinsic behavior was conserved with increasing Pr-doping concentration (x). The highest Figure of Merit (ZT?=??~?0.007) was observed for the LaPrFeCoO₆ compound at 300 K. The observed results of ZT and their narrow optical bandgap suggested that, at ambient temperature, the LaPrFeCoO₆ compound can be also a good choice for solar cells, sensors, bolometers, and other optoelectronic devices as well.

     

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.     

Gas-Phase Conversion of Oxide Phases of Lanthanides and Uranium to Water-Soluble Compounds

Radiochemistry - Gas-phase conversion to water-soluble compounds of individual rare-earth element (REE) (La, Ce, Pr, Nd, Sm, Eu, Dy, Ho, Er, Tm, Lu) oxides and oxide systems (U,Ce)Ox and (U,Nd)Ox...     

La0.5-xCexBa0.5CoO3化合物的磁性和输运特性

在La0.5Ba0.5CoO3中,系统研究了Ce对La的替代效应.Ce的掺入首先产生了电荷转移效应.材料高温磁化率测量表明,每个Ce原子向Co的3d壳层转移2.86个电子,结果随Ce掺入量增加,材料磁矩成线性下降.另外,随Ce含量增加,材料居里温度单调下降,这是由于稀土离子的尺寸效应.在所研究的温度范围内,所有材料的导电机理都属于极化子的变程跳跃导电.由于电荷转移效应,使材料电阻率随Ce掺入量增加而迅速加大.当La全部被Ce替代后,室温下材料的电阻率提高了五个数量级以上.     

Electrical transport in rare-earth orthochromites

The electrical conductivity (σ) and Seebeck coefficient (

) measurements on pressed pellets of La, Nd and Sm orthochromites are reported for the temperature range 300 to 1000K. These orthochromites exhibit extrinsic, p-type conduction in the whole temperature range. In both measurements, breaks define a transition temperature (T_B), which has been predicted to be due to some dielectric anomaly. Seebeck coefficient measurements show that thermal generation of charge carriers begins above T_B.     

Thermopower Peculiarities of Uniaxial-Strained Bismuth Nanowires

We present the results of the investigations of the thermopower and electrical resistance of Pyrex-coated Bi nanowires. Here we show that there is a critical thickness of wire below which structural defects can strongly influence the charge transport in Bi nanowires, fabricated by the melt spinning method. At low temperature the contribution of holes to the thermopower dominates the electron contribution. Thermoelectric parameters of Bi nanowires may be controlled by applying a thermal treatment and/or a uniaxial pressure. A fairly high value of thermoelectric power factor was found under strain in the temperature range of 80–300 K, where the dominant mechanism contributing to the thermopower is diffusive thermoelectric generation with electrons as the majority carrier.     

Electrical transport in light rare-earth vanadates

This paper reports electrical transport studies of rare-earth vanadates (RVO₄ with R = Ce, Pr, Nd, Sm, Eu and Gd), prepared by solid state reaction and characterized by X-ray diffraction studies. TGA study (300 to 1200 K) shows no weight loss; possible phase transitions in the range 1075 to 1300 K have been indicated by DTA studies. All these vanadates are typical semiconducting compounds with room temperature electrical conductivity (σ) lying between 10⁻⁴ and 10⁻²Ω⁻¹m⁻¹. Measurements of σ and the Seebeck coefficient (S) are reported in the temperature interval 400 to 1200 K. Two linear regions 400 to T ₁K and T ₁ to T ₂K have been obtained from the log σ against T ⁻¹ as well as the S against T ⁻¹ plots followed by a peak around T ₃ and minima around T ₄K. T ₄>T ₃>T ₂>T ₁, are different for different vanadates. It has been concluded that in the interval 400 to T ₁K, conduction is of the extrinsic hopping type with Ce⁴⁺ in CeVO₄, Pr⁴⁺ in PrVO₄ and V⁴⁺ in Nd to Gd vanadates as dominant defect centres. In the temperature interval T ₁, to T ₂K, the conduction has been shown to be of the intrinsic band type in all vanadates with polarons of intermediate coupling strength as the dominant charge carriers. Above T ₂ all vanadates tend to become metallic, but before this is achieved the phase change makes the conductivity smaller. T ₄ is close enough to the temperature corresponding to the DTA peak to be termed the phase transition temperature.     

Free carrier absorption in self-activated PbWO4 and Ce-doped Y3(Al0.25Ga0.75)3O12 and Gd3Al2Ga3O12 garnet scintillators

Nonequilibrium carrier dynamics in the scintillators prospective for fast timing in high energy physics and medical imaging applications was studied. The time-resolved free carrier absorption investigation was carried out to study the dynamics of nonequilibrium carriers in wide-band-gap scintillation materials: self-activated led tungstate (PbWO₄, PWO) ant two garnet crystals, GAGG:Ce and YAGG:Ce. It was shown that free electrons appear in the conduction band of PWO and YAGG:Ce crystals within a sub-picosecond time scale, while the free holes in GAGG:Ce appear due to delocalization from Gd³⁺ ground states to the valence band within a few picoseconds after short-pulse excitation. The influence of Gd ions on the nonequilibrium carrier dynamics is discussed on the base of comparison the results of the free carrier absorption in GAGG:Ce containing gadolinium and in YAGG without Gd in the host lattice.     

Lanthanide metals adsorbed in an open-cage Fullerene: first-principles calculations

In this paper, the properties of a 12-membered-ring open-cage fullerene, which encapsulate La, Ce, Pr, Nd, Pm, Sm, Eu or Gd, as a guest atom, were calculated using first-principle calculations. Calculations show that La etc. lanthanide metal atoms can be stably adsorbed on the openings of the fullerenes. The average adsorption energy is about -3.65 eV in which Pr and Nd have relatively large adsorption energy with -4.75 eV and -4.63 eV, respectively. The Gd is stably adsorbed on the side wall near bottom of the fullerene with adsorption energy of -3.05 eV. The PDOS of adsorbed lanthanides were analyzed, respectively. Magnetic property of adsorbed lanthanides was also discussed. From the calculation, it is proved that most of the adsorbed lanthanides keep good magnetic property. Finally, vertical ionization potential and vertical electron affinity of the complex compounds were researched. The results show that the possibility of donating electrons of the 12-membered-ring open-cage fullerene is significantly affected by the endohedral lanthanide metals. From our calculations, it is believed that the complex compounds could be promising candidates for medicine-carrier.     

Correlation between photorefractivity,photoconductivity, and dark conductivity in dye-doped,low molecular mass nematic liquid crystal cells

The photoconductivity and photorefractivity of dye-doped low molecular mass nematic liquid crystal homeotropic cells exhibit a threshold at an applied voltage of the order of 1 V (in our samples). At about the same voltage, the dark conductivity of the cells exhibits a change from a cubic current/voltage characteristic to a linear one. We explain these observations by postulating that the cell's conductivity is due to ionic charge carriers generated near the electrodes by some electrochemical processes activated by the injection of electrons and holes. Below the noted threshold, the dark current is limited by a residual ionic space–charge distribution, and grows as the cube of the applied voltage. In this regime, photogenerated charge carriers are screened and are prevented from forming the static space–charge field needed to initiate an orientational photorefractive response. Above that threshold, the residual ionic space–charge disappears, the cell is a weak ohmic electrolyte, and the photogenerated charges contribute to the photoconductivity and photorefractivity of the cell.     

Electronic transport in RuO2-based thick film resistors at low temperatures

The electrical conductivity of various RuO₂-based thick film paste resistors was investigated in the temperature range between 50 mK and 20K. It is shown that models based on variable range hopping and simple models based on tunnelling of charge carriers between conductive grains do not provide a satisfactory explanation for the temperature dependence of the electrical conductivity in these materials at very low temperatures. We suggest a new mechanism based on tunnelling of electrons through graded barriers between conductive particles.     

Static and Dynamic Properties of the Insulator–Metal Transition in Magnetic Semiconductors, Including the Perovskites

All magnetic semiconductors experience an insulator–metal transition with decreasing temperature near and below the magnetic ordering temperature. T_c, as long as the carrier concentration, whether electrons or holes, is low enough. For somewhat higher carrier concentrations, a resistivity anomaly ordinarily occurs near T_c, which cannot be explained in terms of scattering from the magnetic fluctuations alone. The thrust of this paper is to review the magnetotransport properties of various magnetic semiconductors and to present arguments that magnetic polarons, a many body state involving charge carriers and the localized magnetic spins in their immediate neighborhoods, are involved in the physical processes leading to the insulator–metal transition. Magnetic polarons have been observed directly by a variety of physical techniques including, for example, magnetic measurements, neutron diffraction, scanning tunneling microscopy, and, most recently, by noise measurements. This article will review these physical manifestations of the existence of magnetic polarons and relate them to the transition.