Electrical conductivity and amorphization of Sc2(WO4)3 at high pressures and temperatures

Impedance spectroscopy measurements and synchrotron X-ray diffraction studies of Sc₂(WO₄)₃ at 400°C have been carried out as a function of pressure up to 4.4 GPa. Ionic conductivity shows normal decrease with increase in pressure up to 2.9 GPa, but then increases at higher pressures. The XRD results show that Sc₂(WO₄)₃ undergoes pressure-induced amorphization at pressures coincident with the reversal in conductivity behavior. The loss of crystal structure at high pressure is consistent with growing evidence of pressure-induced amorphization in negative thermal expansion materials, such as Sc₂(WO₄)₃. The increase in conductivity in the amorphized state is interpreted as the result of an increase in structural entropy and a concomitant reduction of energy barriers for ionic transport.     

The measurement of saturated rock electrical conductivity at lower crustal temperatures and high pressures

Abstract

A method has been developed to measure the electrical conductivity of electrolyte saturated rocks under simulated lower crustal temperatures and pressures. The method uses a metal sleeved sample, and a guard-ring electrode system has been used to minimise leakage currents through the sleeve. Seals were developed to mate the metallic and ceramic parts of the cell.     

Measurements of saturated basic and acidic rock electrical conductivities at lower crustal temperatures and high pressures

Abstract

Laboratory measurements of the electrical conductivity of brine saturated acidic and metabasic rocks were done at confining pressures up to 0.2 GPa, high pore pressures and temperatures of 900°C. Acidic rocks showed conductivities insufficient to explain the lower crustal high conductivity layer. Basic rocks, however, showed conductivities consistent with the high conductivity layer at those temperatures that the layer is thought to possess.     

Ionic to electronic dominant conductivity in Al2(WO4)3 at high pressure and high temperature

Electrical conduction and crystal structure of Al₂(WO₄)₃ at 400 °C have been studied as a function of pressure up to 5.5 GPa using impedance methods and synchrotron radiation X-ray diffraction, respectively. AC impedance spectroscopy and DC polarization measurements reveal an ionic to electronic dominant transition in electrical conductivity at a pressure as low as 0.9 GPa. Conductivity increases with pressure and reaches a maximum at 4.0 GPa, where the conductivity value is 5 orders of magnitude greater than the 1 atm value. Upon decompression, the conductivity retains the maximum value until the sample is cooled at 0.5 GPa. The high pressure-temperature X-ray diffraction results show that the lattice parameters decrease as pressure increases and the crystal structure undergoes an orthorhombic to tetragonal-like transformation at a pressure ∼3.0 GPa. The change of conduction mechanism from ionic to electronic may be explained by means of pressure-induced valence change of W⁶⁺→W⁵⁺, which results in electron transfer between W⁵⁺-W⁶⁺ sites at high pressure.     

Electrical conductivity and defect structure of Mg-doped Cr2O3

The electrical conductivity of Cr₂O₃ nominally doped with 2 mol% MgO has been studied by the four point a.c. technique as a function of the oxygen activity (O₂ + Ar, CO + CO₂ and H₂ + H₂O) in the temperature range 400–1200 °C. It is concluded that Cr₂O₃ doped with MgO is an extrinsic conductor and that the dissolved Mg-dopant is compensated by the formation of electron holes at near atmospheric oxygen pressures and by oxygen vacancies (or possibly interstitial chromium ions) at highly reduced oxygen activities (in CO + CO₂ and H₂ + H₂O gas mixtures). In H₂ + H₂O mixtures Mg-doped chromia also dissolves hydrogen as protons and significantly affects the defect structure and electrical conductivity. The defect structure of the oxide under various conditions is discussed.     

In situ X-ray observation of phase transformation in Fe2O3 at high pressures and high temperatures

A laser-heated sample in a diamond anvil cell and synchrotron X-ray radiation was used to carry out structural characterization of the phase transformation of Fe₂O₃ at high pressures (30-96 GPa) and high temperature. The Rh₂O₃(II) (or orthorhombic perovskite) structure transforms to a new phase, which exhibits X-ray diffraction data that are indicative of a CaIrO₃-type structure. The CaIrO₃-type structure exhibited an orthorhombic symmetry (space group: Cmcm) that was stable at temperatures of 1200-2800 K and pressure of 96 GPa (the highest pressure used). Unambiguous assignment of such a structure requires experimental evidence for the presence of two Fe species. Based on the equation of state of gold, the phase boundary of the CaIrO₃-type phase transformation was P (GPa)=59+0.0022×(T−1200) (K).     

Compact multianvil device for in situ studies at high pressures and temperatures

We describe a recently developed device for in situ studies at pressures up to 25 GPa and temperatures up to 2300 K. The system consists of a 450 ton V7 Paris-Edinburgh press combined with a Stony Brook ‘T-cup’ multianvil stage. Such a compact large-volume set-up has a total mass of 100 kg only and can be readily used on most synchrotron radiation facilities. The optimization of the set-up by off-line tests is detailed, and we present some X-ray diffraction results which demonstrate the potential of the technique.     

The band gap and electrical resistivity of FeS2-pyrite at high pressures

The indirect energy gap and electrical resistivity of FeS₂-pyrite have been measured at high pressures and 300 K using optical absorption spectroscopy and electrical conductivity measurements. Absorption spectra extend to ∼28 GPa, while resistivity is determined to ∼34 GPa. The band gap of FeS₂ is indirect throughout this pressure range and decreases linearly with pressure at a rate of −1.13(9)×10⁻² eV/GPa. If this linear trend continues, FeS₂ is expected to metallize at a pressure of 80(±8) GPa. The logarithm of resistivity also linearly decreases with pressure to 14 GPa with a slope of −0.101(±0.001)/GPa. However, between 14 and 34 GPa, the logarithm of resistivity is nearly constant, with a slope of −0.011(±0.003)/GPa. The measured resistivity of pyrite may be generated predominantly by extrinsic effects.     

Experimental measurement of the electrical conductivity of pyroxenite at high temperature and high pressure under different oxygen fugacities

Electrical conductivities of pyroxenite were measured between frequencies of 10^?1 and 10⁶ Hz in a multi-anvil pressure apparatus using different solid buffers (Ni+NiO, Fe+Fe₃O₄, Fe+FeO and Mo+MoO₂) to stabilize the partial pressure of oxygen. The temperature ranged from 1073 to 1423 K (800 to 1200 °C) and the pressure from 1.0 to 4.0 GPa. We observe that: (1) the electrical conductivity (σ) of pyroxenite depends on frequency; (2) σ tends to increase with rising temperature (T), and Log σ and 1/T obey a linear Arrhenius relationship; (3) under control of the buffer Fe+Fe₃O₄, σ tends to decrease with rising pressure, nevertheless the activation enthalpy tends to increase. For the first time we have obtained values for the activation energy and activation bulk volume of the main charge carriers, which are (1.60±0.07) eV and (0.05±0.03) cm³/mol, respectively; (4) for a given pressure and temperature, σ tends to rise with increased oxygen fugacity, whereas the activation enthalpy and preexponential factor tend to decrease; and (5) the behaviour of the electrical conductivity at high temperature and high pressure can be reasonably interpreted by assuming that small polarons provide the dominant conduction mechanism in the pyroxenite samples.     

Electrical conductivity in TlI-TiO2 composite solid electrolyte

Composite materials of formula (1−x)TlI−xTiO₂, x=0-0.7, have been prepared and studied by X-ray diffraction, differential scanning calorimetry, scanning electron microscopy and electrical conductivity. The materials were found to be binary phase systems with titania particles distributed between the grains of thallium iodide. The electrical conductivity got enhanced in the composition range x=0.1-0.5 and then decreased with further increase in the titania content. The behaviour is explained in terms of disordering phenomena at the interface regions and space-charge layers formed in the bulk grains of thallium iodide. Moreover, the increased content of titania in the system leads to the disappearance of order-disorder (β-α) phase transition in thallium iodide, which is usually observed in the pure compound. This behaviour was explained by the stabilizing effect of β-phase at high temperatures and at higher contents of titania. X-ray diffractograms do not show any indication to the presence of α-phase at ambient temperature, i.e. the phase could not be stabilized in the investigated system.     

Structural study of ND4I at high pressures and low temperatures

Abstract

Structure, positional, and thermal parameters of ND₄I were studied at high pressures up to 90 kbar and low temperatures down to 10 K using time-of-flight neutron diffraction. The phase transition from a disordered CsCI-type cubic phase ND₄I(II) into a recently discovered high pressure phase ND₄I(V) was observed at P = 80(5) kbar. Surprisingly, the structure of the high pressure phase V was found to bear a strong resemblance to that of the ambient pressure, low-temperature phase III - tetragonal structure with an antiparallel ordering of ammonium ions, space group P4/nmm. The critical value of the deuterium positional parameter corresponding to the II-V transition is close to the one for the phase transition between the disordered and ordered CsCl-type cubic phases II and IV in other ammonium halides.     

Ionic conductivity of the perovskites NaMgF3, KMgF3, KMgK3 and KZnF3 at high temperatures

We have carried out a study of the ionic conductivity of NaMgF₃, KMgF₃ and KZnF₃ up to temperatures close to the melting point. Our results, in contrast to previous reports in the literature, show no abnormal ionic conductivity at high temperatures. Care in interpretation of results is required because of surface electronic conduction.     

Hopping type of conduction in (Na0.5Bi0.5)ZrO3 ceramic

Polycrystalline sample with (Na_0.5Bi_0.5)ZrO₃ (NBZ) stoichiometry was prepared using a high-temperature solid-state reaction technique. X-ray diffraction (XRD) analyses indicate the formation of a single-phase perovskite-type orthorhombic structure. AC impedance plot is used as tool to analyse the electrical behaviour of the sample as a function of temperature at different frequency. The AC impedance studies revealed the presence of grain boundary effect and evidence of a negative temperature coefficient of resistance (NTCR) character. Pseudo Cole-Cole and complex electric modulus analyses indicated non-Debye-type dielectric relaxation. The AC conductivity obeys the universal power law. The pair approximation type correlated barrier hopping (CBH) model explains the universal behaviour of the s exponent. The apparent activation energy to the conduction process and minimum hopping distance are discussed.     

Electrical studies on bulk Se96Sn4 semiconducting glass before and after gamma irradiation

Bulk Se₉₆Sn₄ chalcogenide glass was prepared by melt quenching technique and irradiated by different doses of 4, 8, 12, 24 and 33 kGy using ⁶⁰Co gamma emitter. I-V characteristics were obtained for this glass, before and after gamma irradiation, in the temperature range 200-300 K. Ohmic behavior was observed at low electric fields (≤1×10⁴ V/m), while at higher fields, a deviation from ohmic towards non-ohmic behavior was observed. The plots of ln(I/V) vs. V were found to be straight lines and the slopes of these lines decrease linearly with temperature indicating the presence of SCLC. In the temperature range of measurements, the dependence of DC conductivity on temperature at low electric field shows two types of conduction channels, one in high temperature range 270-300 K and the other at low temperature range 200-270 K. Analysis of the experimental data shows that the conductivity at room temperature decreases with increase in irradiation dose. This is attributed to rupturing of SnSe_4/2 structural units, upon irradiation, and rebuilt of Se atoms between Se chains. This redistribution of bonds, induced by gamma irradiation, is responsible for the corresponding increase in the activation energy. The obtained values of the activation energy indicate that the conduction occurs due to thermally assisted charge carriers movement in the band tail of localized states. However, in the low temperature range, results obtained from Mott’s variable range hopping (VRH) model reveal that the density of localized states has its maximum value at a gamma dose of 12 kGy, while the disorder parameter T_o, hopping distance R_hop and hopping energy W have their minimum value at this particular dose.     

Electrical and thermal conductivity of soft solder at low temperatures

The electrical resistivity of soft solder (Pb_0.28Sn_0.72) has been measured in the temperature range 4.2 K to 300 K. The ‘alloy’ becomes electrically superconducting at a temperature of 6.9 K. Above this, in the entire temperature range, the resistivity could be described, apart from the residual resistivity, by the weighted average of the resistivities of the individual constituents which are derived from the Bloch-Grüneisen relation. The results are in accordance with the phase diagram, which shows a co-existence of two phases in almost the entire range of concentration of the Pb-Sn binary system. It has been shown that the thermal conductivity data on soft solder as well as on Pb_0.7Sn_0.3, both taken from literature, could be interpreted on the same basis, below and above the ‘superconducting transition temperature’. Recent results on other Pb-Sn systems are discussed in the light of this interpretation.     

Thermoelectric properties of Bi0.9Sb0.1 prepared by high pressure

The Bi_0.9Sb_0.1 powders were prepared by mechanical alloying and then pressed under 6 GPa at different pressing temperatures. X-ray diffraction spectra showed that the single phase was formed. The nanostructure of grain was observed by bright-field imaging. Electrical conductivity, Seebeck coefficient, and thermal conductivity had been investigated in the temperature range of 80-300 K. The absolute Seebeck coefficient value of 120.3 μV/K was measured at 130 K. The figure-of-merit reached a maximum value of 0.90×10⁻³ K⁻¹ at 140 K.     

Phase transition of ZnS at high pressures and temperatures

The high-pressure behaviour of zinc sulphide, ZnS, has been investigated, using an in situ X-ray powder diffraction technique in a diamond anvil cell, at pressures and temperatures up to 35 GPa and 1000 K, respectively. The pressure-induced phase transition from a zincblende (B3) to a rocksalt (B1) structure was observed. This transition occurred at 13.4 GPa and at room temperature, and a negative dependence on temperature for this transition was confirmed. The transition boundary was determined to be P (GPa) = 14.4 ? 0.0033 × T (K).     

Electrical properties of (PBS)0·59TiS2 crystals at high pressures up to 20GPa

Abstract

The measurements of thermoelectric power S and resistance p at high pressure synthetic diamond anvils cell were performed for (PbS)0·59TiS2 and TiS2 crystals. The phase transition was found at P?;2GPa accompanied by descend of ρ and |S| for (PbS)o·59TiS2. This transition is connected with structural change of PbS fragment from pseudocubic cell to orthorombic one and as consequence, with change of the electron concentration in Tis2-layers. From the electronic structure calculations for TiS2, the semiconductor-metal transition occurs at pressure P ≥ 4 GPa. Experimentally at this pressure range the decrease of ρ(P) was observed for (PbS)0·59TiS2 crystals.     

Na2CO3的高压拉曼光谱研究

碳酸盐是碳在地球内部的重要载体之一,其在地幔高温高压条件下的晶体化学是理解地球深部碳的赋存状态和循环过程的关键,而结构稳定性和相变是晶体化学最基本的研究内容。碳酸钠（Na₂CO₃）是一种常见的碱性碳酸盐矿物,在产自地幔过渡带-下地幔的金刚石中已发现含钠的碳酸盐矿物包裹体,这成为碳酸钠能够俯冲进入地幔深部的直接矿物学证据。前人利用拉曼光谱技术研究了Na₂CO₃在常温常压下的晶格振动模式,但其在高压下的稳定性和结构变化却鲜有报道。利用金刚石压腔装置结合先进的共聚焦拉曼光谱技术,以硅油作为传压介质,在准静水压力条件下,在0.001～27.53 GPa压力区间对Na₂CO₃粉末在600~1 200 cm-1波段的振动特征进行了细致地分析。本次实验重点分析了[CO₃]2-基团振动模式在升压和卸压过程中的行为。结果表明,在0.001～11.88 GPa压力范围内,[CO₃]2-基团对称伸缩振动γ₁（1 088.06和1 070.76 cm-1）、反对称伸缩振动γ₃（865.10和797.50 cm-1）和面内弯曲振动γ₄（720.10和696.71 cm-1）都出现了振动峰的分裂。随着压力增加,所有振动峰都向高频率漂移,半高宽也逐渐增加。在13.40 GPa时,Na₂CO₃发生结构相变,具体表现为690.08 cm-1处出现1条新的拉曼峰,并且随着压力升高该峰的强度逐渐增大。同时反对称伸缩振动峰γ₃以及面内弯曲振动峰γ₄的强度持续减弱,半高宽也继续变大。这些现象表明Na₂CO₃结构相变源于[CO₃]2-内部晶格变化。当压力卸载到4.18 GPa时,[CO₃]2-的振动模式与常温常压下的完全吻合,相变出现的新峰也已经消失,表明该相变是由[CO₃]2-基团畸变引起的并且具有可逆性。继续升压至27.53 GPa,拉曼光谱继续蓝移,Na₂CO₃的拉曼谱线再没有变化,说明高压相在这一压强范围内保持稳定。在整个加压过程中,反对称伸缩振动γ₃和面内弯曲振动γ₄处的拉曼峰出现强度减弱现象。同时也计算了各个峰频率对压力的依赖系数dγ/dP,结果显示[CO₃]2-基团内各个振动模式对压力的响应是不同的,这很可能与Ｃ-Ｏ键的键长有关。最后,对比发现,对称伸缩振动γ₁峰的强度比反对称伸缩振动γ₃和面内弯曲振动γ₄峰的强度大,并且[CO₃]2-基团对称伸缩振动γ₁受压力影响相对较小,可以用来区别不同种类的碳酸盐矿物。