Temperature of the superconducting transition in the TiV alloy at pressures up to 60.8 GPa

The dependence of the temperature of the transition to the superconducting state T _c on the pressure up to 60.8 GPa is measured for the TiV alloy. The dependence T _c (P) is increasing except for an anomaly in the form of a local minimum near P = 10 GPa. At the maximum pressure of 60.8 GPa, the superconducting transition temperature T _c reaches 18.2 K. The obtained curve T _c (P) is compared with the known dependences for pure vanadium, for which T _c (P) increases to 17.2 K at P = 120 GPa, and for pure niobium and the ZrNb alloy, for which the dependences T _c (P) also have anomalies in the form of local maxima at pressures of 5–10 GPa.     

Structural and magnetic phase transitions in Pr<Subscript>0.7</Subscript>Ca<Subscript>0.3</Subscript>MnO<Subscript>3</Subscript> at high pressures

The crystal structure and Raman spectra of Pr_0.7Ca_0.3MnO₃ manganite at high pressures of up to 30 GPa and the magnetic structure at pressures of up to 1 GPa have been studied. A structural phase transition from the orthorhombic phase of the Pnma symmetry to the high-pressure orthorhombic phase of the Imma symmetry has been observed at P ∼ 15 GPa and room temperature. Anomalies of the pressure dependences of the bending and stretching vibrational modes have been observed in the region of the phase transition. A magnetic phase transition from the initial ferromagnetic ground state (T _C = 120 K) to the A-type antiferromagnetic state (T _N = 140 K) takes place at a relatively low pressure of P = 1 GPa in the low-temperature region. The structural mechanisms of the change of the character of the magnetic ordering have been discussed.     

Effect of external pressure on Tc of as‐grown and thermally treated superconducting Rbx Fe2–ySe2 single crystals

We report on the effect of external pressure on the superconducting transition temperature (T_c) of as‐grown and thermally treated single crystals of superconducting iron chalcogenide Rb_0.85Fe_1.9Se₂. The superconducting transition temperature of 27.1 K at ambient pressure for the as‐grown sample was found to increase up to 33.2 K for the sample annealed for 3 h at 215 °C in vacuum. An increase of T_c up to 28.2 K was observed for the as‐grown sample at a pressure of 0.83 GPa. For all the studied crystals, annealed in the temperature range between 215 °C and 290 °C, the external pressure seems to decrease the superconducting transition temperature and a negative pressure coefficient of T_c was observed. (© 2012 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

In situ high-pressure X-ray diffraction experiments and ab initio calculations of Co2P

In situ high-pressure experiments of Co2P are carried out by means of angle dispersive X-ray diffraction with diamond anvil cell technique. No phase transition is observed in the present pressure range up to 15 GPa at room temperature, even at high temperature and 15 GPa. Results of compression for Co2P are well presented by the second-order Birch-Murnaghan equation of state with V0 = 130.99(2)3 (1=0.1 nm) and K0 = 160(3) GPa. Axial compressibilities are described by compressional modulus of the axis: Ka = 123(2) GPa, Kb = 167(8) GPa and Kc = 220(7) GPa. Theoretical calculations further support the experimental results and indicate that C23-type Co2P is stable at high pressure compared with the C22-type phase.     

Pressure-induced superconductivity in tetragonal NdBa2Cu3O6+x

The appearance of superconductivity and relaxation of the transition temperature to its equilibrium value T _c≈30 K over the course of five days have been observed in a tetragonal sample of NdBa₂Cu₃O_6.67 under 1 GPa pressure. The time dependence T _c(t) correlates with a decrease of the room-temperature electrical resistance R(t). The superconducting phase disappears 1.3 h after the pressure is removed. This behavior is explained by a redistribution of charge as a result of pressure-induced oxygen reordering in the CuO_x planes. A large effect of oxygen ordering on the transition temperature under pressure has been observed near the metal-insulator transition (the largest of all those measured in the 1-2-3 system), along with a nonlinear temperature dependence of T _c. Fiz. Tverd. Tela (St. Petersburg) 39, 1328–1334 (August 1997)     

Dissociation and dissociative phase transition in dense hydrogen

A simple physical model is proposed for dissociating dense fluid hydrogen. We propose that free dissociated atoms interact via quantum electron-electron exchange analogously to the interaction in the liquid-metal phase of alkali metals. The density dependence of a hydrogen atom’s binding energy in such a quasi-liquid is calculated. It is shown that the transition from the molecular fluid to liquid hydrogen is a first-order phase transition. The critical parameters of the transition are determined: P _c = 72 GPa, T _c = 10500 K, and ρ _c = 0.5 g/cm³. The possibility of the metastable existence of atomic liquid hydrogen in a dissociated molecular fluid under decreased pressure is established.     

Pressure-induced amorphization of Gd2(MoO4)3: A high pressure Raman investigation

High pressure Raman spectroscopic studies on Gd₂(MoO₄)₃(GMO) have been carried out at ambient temperature in the diamond cell to 10 GPa hydrostatic pressure. These experiments have revealed pressure-induced phase transitions in GMO near 2 GPa and 6.0 GPa. The first transition is from Pba2(β′) phase to another undetermined crystalline phase, designated as phase II, and the second transition is to an amorphized state. On releasing pressure there is a partial reversion to the crystalline state. The Raman data indicate that the amorphization is due to disordering of the MoO₄ tetrahedral units. Further, it is inferred from the nature of the Raman bands in the amorphized material that the Mo-O bond lengths and bond angles have a range of values, instead of a few set values. The results of the present study as well as previous high pressure-high temperature quenching experiments strongly support that pressure-induced amorphization in GMO is a consequence of the kinetically impededβ toα phase transition. The system in frustration becomes disordered. The rare earth trimolybdates crystallizing in theβ′ structure are all expected to undergo similar pressure-induced amorphization.     

Pressure induced phase transformations and band structure of different high pressure phases in tellurium

We report here high-pressure x-ray diffraction (XRD) studies on tellurium (Te) at room temperature up to 40 GPa in the diamond anvil cell (DAC). The XRD measurements clearly indicate a sequence of pressure-induced phase transitions with increasing pressure. The data obtained in the pressure range 1 bar to 40 GPa fit five different crystalline phases out of Te: hexagonal Te (I) → monoclinic Te(II) → orthorhombic Te (III) → Β-Po-type Te(IV) → body-centered-cubic Te(V) at 4, 6.2, 11 and 27 GPa, respectively. The volume changes across these transitions are 10%, 1.5%, 0.3% and 0.5%, respectively. Self consistent electronic band structure calculations both for ambient and high pressure phases have been carried out using the tight binding linear muffin tin orbital (TB-LMTO) method within the atomic-sphere approximation (ASA). Reported here apart from the energy band calculations are the density of states (DOS), Fermi energy (E _f) at various high-pressure phases. Our calculations show that the ambient pressure hexagonal phase has a band gap of 0.42 eV whereas high-pressure phases are found to be metallic. We also found that the pressure induced semiconducting to metallic transition occurs at about 4 GPa which corresponds to the hexagonal phase to monoclinic phase transition. Equation of state and bulk modulus of different high-pressure phases have also been discussed.     

Superconductivity in the Zr-D system under pressure

The superconducting transition temperature T _c of the ZrD_0.48 alloy is measured in the pressure range up to 41.5 GPa. The measurements are carried out in a high-pressure chamber with diamond anvils by the inductometric method. It is found that T _c(P) increases to 3.1 K at a pressure below 30 GPa, exhibits a sharp increase up to 8 K near 30 GPa, and then smoothly decreases to ～6.5 K at 41.5 GPa. A similar dependence T _c(P) is obtained for pure Zr. The similarity of the T _c(P) curves suggests that the dependence T _c(P) observed for ZrD_0.48 is due to the presence of ω-phase in this alloy at pressures P<30 GPa and the ω-β transition at P≈30 GPa, which leads to the establishment of new ratios between the phases in the Zr-D system. In the pressure range studied, no indications are observed for new superconducting phases similar to the phases of intermediate composition in the Ti-H(D) system, which are formed by the hydrogen transfer from tetrahedral to octahedral interstitials.     

Frustrated exchange interactions formation at low temperatures and high hydrostatic pressures in La<Subscript>0.70</Subscript>Sr<Subscript>0.30</Subscript>Mn<Subscript>O2.85</Subscript>

The magnetic and thermal properties of the anion-deficient La_0.70Sr_0.30MnO_2.85 manganite are investigated in wide temperature (4–350 K) range, including under hydrostatic pressure (0–1.1 GPa). Throughout the pressure range investigated, the sample is spin glass with diffused phase transition into paramagnetic state. It is established, that spin glass state is a consequence of exchange interaction frustration of the ferromagnetic clusters embeded into antiferromagnetic clusters. The magnetic moment freezing temperature T _f of ferromagnetic clusters increases under pressure, freezing temperature dependence on pressure is characterized by derivative value ∼4.5 K/GPa, while the magnetic ordering T _MO temperature dependence is characterized by derivative value ∼13 K/GPa. The volume fraction of sample having ferromagnetic state is V _fer ∼ 13% and it increases under a pressure of 1.1 GPa by ΔV _fer ≈ 6%. Intensification of ferromagnetic properties of the anion-deficient La_0.70Sr_0.30MnO_2.85 manganite under hydrostatic pressure is a consequence of oxygen vacancies redistribution and unit cell parameters decrease. The most likely mechanism of frustrated exchange interactions formation is discussed.     

低温高压及高温高压对La80Al20非晶态结构及超导性的影响

用X射线衍射照相、X射线衍射仪及透射电子显微镜分析了用液相淬火技术制备的La₈₀Al₂₀合金样品,结果表明样品是非晶态结构。差热分析(DTA)结果表明,非晶态样品的晶化温度T_cr为280℃左右,玻璃温度T_g为242℃左右。非晶态样品低温高压(流体静压法)试验结果表明:压力从0至4.77kbar,T_c从3.87K提高到4.18K。高压下,超导前的剩余电阻与常压下相比较有明显下降。非晶态样品经高温高压 关键词：     

Phase diagram and equations of state of BaSO4

Abstract

The phase diagram and equations of state of BaSO₄, were determined up to 29 GPa and 1000 K in a resistance-heating type diamond anvil cell. At room temperature, barite is the stable form of BaSO₄ which undergoes a reversible phase transition at 10 GPa. The high-pressure form is tentatively determined to be triclinic. At high temperature, a similar phase transition takes place in BaSO₄, but at a pressure higher than that at room temperature. Our results indicate that the phase boundary of the two polymorphs in BasO₄ has a positive slope (dT/dP) of 90 K/GPa. The equations of state for both barite and its high-pressure phase are reported.     

High-pressure effect on the ferroelectric-paraelectric transition in PbTiO<Subscript>3</Subscript>

The crystal structure of lead titanate PbTiO₃ was investigated by energy dispersive X-ray diffraction at high pressures up to 4 GPa in a temperature range of 300–950 K. At the ambient conditions, the PbTiO₃ structure is tetragonal with the space group P4mm (ferroelectric phase). A structural phase transition into the cubic phase with a space group Pm[`3]mPm\bar 3m is observed at T = 747 K. It was found that the phase transition temperature decreases upon applying the high pressure with the coefficient dT _C /dP = -65 K/GPa. Dependences of parameters and volume of the unit cell on the pressure and temperature was found, and the bulk modulus and thermal expansion coefficients for the tetragonal and cubic phases of lead titanate have been calculated.     

Structural phase transition in Bi2Te3 under high pressure

Structural change in Bi₂Te₃ under high pressure up to 16.6 GPa has been studied by powder x-ray diffraction. We observed two times of phase transitions at room temperature at the pressures of 8 and 14 GPa, respectively. According to our preliminary result on electrical resistance, it is reasonable to suppose that superconducting transition with T _c =2.8 K at the pressures of 10.2 GPa is observed in phase II. On the other hand, we found anomalies of the pressure dependences of lattice parameters and volume at around 2 GPa, which probably means the change in electrical structure on the Fermi surface.     

Structural transitions in ice samples at low temperatures and pressures

The structure of ice samples formed in the decay of a water impurity gel at temperatures above 4 K and atmospheric pressure has been examined. The X-ray diffraction analysis indicates that three phases coexist in the initial sample at temperatures of 85–110 K. These phases are amorphous ice occupying up to 30% of the sample volume, cubic-phase ice I _c metastable at low pressures (∼60%), and normal hexagonal ice I _h (≤6%). The characteristic sizes of crystals of the cubic and hexagonal phases are about 6 and 30 nm, respectively. The amorphous phase at annealing above 110 K is gradually transformed to the crystalline phase both cubic and hexagonal. This transition is accompanied by two processes, including a fast increase in the sizes of cubicphase nanocrystals and the partial transition of the cubic phase I _c to the hexagonal one I _h. Hexagonal ice I _h prevails in the bulk of the sample above 200 K.     

FeTe as a candidate material for new iron-based superconductor

Tetragonal FeSe is a superconductor with a transition temperature T_c of 8 K and shows a huge enhancement of T_c with applying pressure. Tetragonal FeTe has a structure very analogous to superconducting FeSe, but does not show superconducting transition. We investigated the pressure effect of resistivity on FeTe. The resistivity at room temperature decreased with increasing pressure. An anomaly in resistivity around 80 K shifted towards a lower temperature with increasing pressure.     

Transport properties of NdBa2Cu3O6+x ceramics at the edge of the superconducting region upon a decrease in carrier density as a result of oxygen disordering in the Cu-O x planes

An initially nonsuperconducting ceramic sample with the composition NdBa₂Cu₃O_6+x is brought, by means of pressure and quenching, to a state with a high carrier density and a superconducting transition, after which it is returned to the initial state by gradual annealing in several steps. The evolution of the magnetoresistance of the sample showed that even in the most resistive state realized in the experiment the superconducting interaction influences the resistance of the sample at fields all the way up to 5–6 T. In an 8 T field the change in resistance in this state in the temperature interval from 0.4 K to 20 K is described well by a logarithmic law ΔR∝ logT. Pis’ma Zh. éksp. Teor. Fiz. 65, No. 6, 475–480 (25 March 1997)     

Effect of substitution of Ce on superconducting properties of Bi1.7Pb0.3Sr2Ca2−x Ce x Cu3O10+δ system

We have investigated the superconducting properties of the Bi_1.7 Pb_0.3Sr₂Ca_2−xCe _x Cu₃O_10+δ system with x=0.00, 0.02, 0.04, 0.08 and 0.1 by X-ray diffraction and magnetic susceptibility. The substitution of Ce for Ca has been found to drastically change the superconducting properties of the system. X-ray diffraction studies on these compounds indicate decrease in the c-parameter with increased substitution of Ce at Ca site and volume fraction of high T _c (2 : 2 : 2 : 3) phase decreases and low T _c phase increases. The magnetic susceptibility of this compound shows that the diamagnetic on set superconducting transition temperature (onset) varies from 109 K to 51 K for x=0.00, 0.02, 0.04, 0.08 and 0.1. These results suggest the possible existence of Ce in a tetravalent state rather than a trivalent state in this system; that is, Ca²⁺ → Ce⁴⁺ replacement changes the hole carrier concentration. Hole filling is the cause of lowering T _c of the system.     

High-pressure phase transition of carbon disulfide

The high-pressure phase transition of CS₂ was studied by combing ab initio molecular dynamics with total energy calculations. At 300 K the pieces of polymer structure were found to appear at 10 GPa in the molecular dynamics run, and further the CS₄ tetrahedral structure to appear at about 20 GPa. The phase transition was then studied in the structure of Cmca, α-quartz and β-quartz by using the first-principle total energy calculation method. A phase transition from Cmca to β-quartz was found at 10.6 GPa. The calculated lattice constants of β-quartz at atmospheric pressure are a=5.44 and c/a=1.138 with B₀=95 GPa. The calculation has also indicated that CS₂ decomposes at 20 GPa and below 1000 K.     

Effect of hydrostatic pressure on the superconductivity in fluorinated Hg<Subscript>0.8</Subscript>Ba<Subscript>2</Subscript>Ca<Subscript>2</Subscript>Cu<Subscript>3.2</Subscript>O<Subscript>8+δ</Subscript>

The effect of hydrostatic pressure on the superconducting transition temperature was measured for the Hg-1223 phase of a fluorinated mercury cuprate high-temperature superconductor with T_c(optim)=38 K. The value of the T_c derivative with respect to pressure was found to be rather high (11.0 K/GPa); at P=1.5 GPa, T_c=153.5 K. The results obtained are discussed in connection with works on the synthesis of such samples.