Pressure-induced electronic and structural transformations in bulk GeSe2 glass

The pressure dependence of the electrical resistivity of bulk GeSe₂ glass shows a semiconductor-to-metal transition at 7 GPa pressure. The high pressure phase is examined using the x-ray diffractometer and is found to be crystalline, with a face-centred cubic structure havinga = 4·06 A. The electrical conductivity has also been studied as a function of temperature at various pressures.     

Low temperature magnetoresistance in La1.32Sr1.68Mn2O7 layered manganite under hydrostatic pressure

The La_1.32Sr_1.68Mn₂O₇ layered manganite system has been studied by the low temperature electrical resistance and magnetoresistance under hydrostatic pressure up to 25 kbar. We have observe both, a Curie temperature (T_C) and a metal-insulator transition (T_MI) at 118 K in the ambient pressure. The applied pressure shifts the T_MI to higher temperature values and induces a second metal-insulator transition (T²_MI) at 90 K, in the temperature dependence of resistivity measurements. Also, the pressure suppresses the peak resistance abruptly at T_C. When an external field of 5 T is applied, we have observed a large negative magnetoresistance of 300% at the transition temperature and a 128% at 4.5 K. However, the increased pressure decreases the magnetoresistance ratio gradually. When the pressure reaches its maximum available value of 25 kbar, the magnetoresistance ratio decreases at a rate of 1.3%/kbar. From our experimental results, the decrease of magnetoresistance ratio with pressure is explained by the pressure induced canted spin state which is not favor for the spin polarized intergrain tunneling in layered manganites.     

Pressure dependence of the superconducting transition temperature of a (Mo0.6Ru0.4)86B14 metallic glass

We have studied the pressure dependence of the superconducting transition temperature of amorphous (Mo_0.6Ru_0.4)₈₆B₁₄ for hydrostatic pressures up to P ～ 9 kbar. The transition temperature T_c decreases with pressure at a rate dT_c/dP=-(9±1) mK kbar^-1. We estimate the Grüneisen parameter and the volume dependence of the electron-phonon coupling constant.     

High pressure X-ray diffraction study of CdAl2Se4 and Raman study of AAl2Se4 (A=Hg, Zn) and CdAl2X4 (X=Se, S)

We report the results of an X-ray diffraction study of CdAl₂Se₄ and of Raman studies of HgAl₂Se₄ and ZnAl₂Se₄ at room temperature, and of CdAl₂S₄ and CdAl₂Se₄ at 80 K at high pressure. The ambient pressure phase of CdAl₂Se₄ is stable up to a pressure of 9.1 GPa above which a phase transition to a disordered rock salt phase is observed. A fit of the volume pressure data to a Birch-Murnaghan type equation of state yields a bulk modulus of 52.1 GPa. The relative volume change at the phase transition at ∼9 GPa is about 10%. The analysis of the Raman data of HgAl₂Se₄ and ZnAl₂Se₄ reveals a general trend observed for different defect chalcopyrite materials. The line widths of the Raman peaks change at intermediate pressures between 4 and 6 GPa as an indication of the pressure induced two stage order-disorder transition observed in these materials. In addition, we include results of a low temperature Raman study of CdAl₂S₄ and CdAl₂Se₄, which shows a very weak temperature dependence of the Raman-active phonon modes.     

Raman spectra of CrO2 and MoO2 single Crystals

The Raman spectra of single crystals of CrO₂ and MoO₂ have been studied from 77°K to 500°K for a comparison with the heavily damped phonon features in metallic VO₂. The spectrum of MoO₂ is very similar to that of monoclinic VO₂ whereas CrO₂ exhibits phonon spectra similar to rutile with no appreciable change at the ferromagnetic c curie temperature. The results are discussed with respect to the various models proposed for the semiconductor-to-metal transition in VO₂.     

Anomalous temperature dependence of phonon modes in Bi2Ti4O11 and modified Bi2Ti4O11

We have observed the anomalous temperature dependence of the phonon modes in the Raman spectra of Bi₂Ti₄O₁₁ and the modified Bi₂Ti₄O₁₁ with the dopant of PbO. The square of the lowest mode frequency in both substances decreases linearly with increasing temperature, while the damping constant increases reaching a peak value at the transition temperature. The mode frequency, however, does not tend to zero but stays finite and the damping constant decreases above the transition temperature. The mode is Raman-active both above and below the transition temperature.     

Optical properties of V6O13 single crystals in the metallic and the semiconducting phase

The optical reflection of V₆O₁₃ single crystals has been studied between 0.06 and 3 eV at temperatures both below and above the semiconductor-to-metal transition temperature (T_t?150K). In addition to the metallic reflection, present only at T>T_t, discrete resonances are observed in both phases: four with polarization E//b and three with E⊥b.The results of a dispersion analysis using classical-oscillator and Drude terms are presented. The resonances above 2 eV are attributed to oxygen 2p - vanadium 3d interband transitions. The maxima at the lower energies are interpreted as transitions between vanadium 3d subbands.     

Pressure dependence of the superconducting transition temperature of T5T9 metallic glasses (T5Nb,Ta; T9Rh,Ir)

The pressure dependence of the superconducting transition temperature, T_c, is determined for eight metallic glasses in the series (Nb_1?xTa_x)₅₅T₄₅, where T Rh, Ir. From these data we estimate the Grüneisen parameter, γ_G, and the volume dependence of the electron-phonon coupling constant, λ. Pressure is observed to affect T_c through both γ_G and λ.     

Electrical transport and high pressure studies on bulk Ge20Te80 glass

The electrical resistivity of bulk Ge₂₀Te₈₀ has been measured as a function of pressure and temperature. At 5 GPa, an amorphous semiconductor-to-crystalline metal transition has been observed. The sample recovered from the high pressure cell, after the application of 7 GPa, has a face-centred cubic structure with a lattice constant of 6·42 A. In crystalline sample, the semiconductor-to-metal transition occurs at 7 GPa. The thermoelectric power has also been measured for glassy samples in the temperature range 300–240 K.     

Low-temperature electrical properties of V2Ga5

Electrical and magnetic measurements are reported for V₂Ga₅ crystals. A transition to the superconducting state was observed at 4.2 K. The temperature dependence of the critical fields and the Ginzburg-Landau-parameter ? were also determined. Information is also given about the temperature dependence of the ideal electrical resistivity between 10 and 300 K.     

流体静压力下快离子导体Rb4Cu16I7Cl13离子电导的研究

在流体静压力为0.5—11.6kbar,温度-80—80℃范围内,测量了Rb₄Cu₁₆I₇Cl₁₃多晶粉末“松散”样品和“致密”样品的离子电导。“松散”样品电导与压力的关系表明,在4.0—5.0kbar附近,电导存在极大值;“致密”样品电导随压力单调下降。在一定压力下,“致密”样品电导率随温度变化的趋势与常压结果相同,压力对Rb₄Cu₁₆I₇Cl_关键词：     

金属间化合物DyMn2Ge2的自发磁相变和场诱导的磁相变

在10—800K的温度范围内用X射线衍射方法测量了DyMn₂Ge₂化合物的晶格常数与温度的变化关系,观察到高温时DyMn₂Ge₂由顺磁状态到反铁磁状态的自发磁相变伴随着晶格常数a的负的磁弹性异常现象.在4.2K—200K的温度范围内测量了DyMn₂Ge₂的交流磁化率.在交换相互作用的分子场模型近似下,从理论上分析讨论了DyMn₂Ge₂的低温自发磁相变和场诱导的磁相变.计算了DyMn₂Ge₂单晶的磁化强度与温度的变化关系以及不同温度下外磁场沿晶轴c方向时的磁化曲线.理论分析和计算结果表明,温度低于33K时在DyMn₂Ge₂中观察到的场诱导的一级磁相变为由亚铁磁状态(Fi)到中间态(IS)相变. 关键词： 稀土-过渡族金属间化合物 磁结构 磁相变     

Magnetic properties of the bilayer manganese oxide (Pr0.6La0.4)1.2Sr1.8Mn2O7 under pressure

We investigated the effect of pressure on the magnetic properties of a single crystal of the bilayer manganese oxide (Pr_0.6La_0.4)_1.2Sr_1.8Mn₂O₇ by means of DC magnetization measurements under pressure. The ferromagnetic transition, which is accompanied by a metal-insulator transition, is highly sensitive to pressure. The pressure causes a structural variation, which affects the magnetic properties. We discuss the pressure dependence of the 3d electronic state of the Mn ion in this system.     

Raman spectra of TiO2-II,TiO2-III,SnO2, and GeO2 at high pressure

Raman spectra of the orthorhombic (II) and high pressure (III) phases of titanium dioxide at pressures to 372 kbar and effects of temperature and hydrostatic pressure on Raman spectra of the tetagonal cassiterite-like phases of TiO₂, GeO₂ and SnO₂ are described. At room temperature, the TiO₂ II–III transition is sluggish, and metastable coexistence was observed from 200 to 300 kbar. The Raman spectra of TiO₂-III imply that its primitive cell contains at least four formula units; however, the structure could not be established from the Raman spectra and available powder X-ray diffraction patterns.The temperature and pressure dependences of the spectrum of the tetragonal MO₂ phases together with bulk moduli and thermal expansion data were used to evaluate the pure-volume and pure-temperature contributions to the isobaric temperature dependence of the Raman frequencies. Large anharmonicities in TiO₂ are attributed to hybridization of the oxygen p states with the d states of the Ti ion. GeO₂, where p-electron bonding is involved, is much less Enharmonic.     

Study of magnetotransport in double-layered La1.4Ca1.6Mn2O7 manganite: Presence of nano-ferromagnetic domains in paramagnetic matrix

Double-layered manganite La_1.4Ca_1.6Mn₂O₇ has been synthesized using the solid-state reaction method. It had a metal-to-insulator transition at temperature T_M1≈127 K. The temperature dependence of ac susceptibility showed a broad ferromagnetic transition. The two-dimensional (2D)-ferromagnetic ordering temperature (T_C2) was observed as ≈245 K. The temperature dependence of its low-field magnetoresistance has been studied. The low-field magnetoresistance of double-layered manganite, in the temperature regions between T_M1 and T_C2, has been found to follow 1/T⁵. The observed behaviour of temperature dependence of resistivity and low-field magnetoresistance has been explained in terms of two-phase model where ferromagnetic domains exist in the matrix of paramagnetic regions in which spin-dependent tunneling of charge carriers occurs between the ferromagnetic correlated regions. Based on the two-phase model, the dimension of these ferromagnetic domains inside the paramagnetic matrix has been estimated as ∼12 Å.     

Pressure dependence of the rotational ground state tunnel splitting in Ni(NH3)6I2

Inelastic neutron scattering techniques have been used to determine the rotational tunnel splitting of the librational ground state of ammonia molecules in Ni(NH₃)₆I₂ as a function of hydrostatic pressure up to 5.6 kbar at 8 K. The resulting exponential dependence of the tunnel splitting on pressure was interpreted with the help of a concurrent measurement of the compressibility to suggest a dependence on the interatomic distance as r^?9 of the orientational potential. The phase transition temperature at 5.6 kbar was determined to be 27 K compared with 19.9 K at atmospheric pressure.     

Raman scattering of LiGaO2

The Raman spectrum of LiGaO₂ is reported together with a preliminary identification of the optical modes and their temperature dependence between 6 and 700 K. The observed temperature dependence of the lines gave no indication for second order processes. However, at low temperatures additional lines have been observed indicating a phase transition at approximately 65 K.     

Hyperfine characterization of Bi1.9Te0.1SrNb1.9Hf0.1O9

The Aurivillius type oxide Bi_1.9Te_0.1SrNb_1.9Hf_0.1O₉ has been studied by Perturbed Angular Correlations spectroscopy using ¹⁸¹Ta probes. The spin precession curves were measured from room temperature up to 873 K. Two sites are occupied by probes and the temperature dependence of both indicates a continuous phase transition at about 625 K. One site is ordered while the other is disordered. This situation is analyzed in terms of simple models already applied to perovskites. The transition temperature of the solid solution Bi_2−xTe_xSrNb_2−xHf_xO₉ (with 0≤x≤0.5) shows a strong dependence on composition.     

Electrical and magnetic properties of pseudo-one-dimensional calcium iridium oxide Ca5Ir3O12

The electrical and magnetic properties of one-dimensional calcium iridium oxide Ca₅Ir₃O₁₂ are investigated. A weak ferromagnetic transition has been found at 7.5 K through magnetic susceptibility measurements. At the same temperature, a λ-type specific heat anomaly has been observed. The effective magnetic moments in the paramagnetic temperature range and the magnetic entropy due to the magnetic transition indicates that the tetravalent and pentavalent Ir ions exist in the ratio of 1:2. Another λ-type anomaly has been observed at 105 K in the temperature dependence of the specific heat. The electrical conductivity shows one-dimensional Mott variable-range hopping conduction behavior.     

Ferroelastic phase transition and anomalous elastic and thermal properties of betaine borate (CH3)3NCH2COO·H3BO3

The temperature and pressure derivatives of the elastic constants of orthorhombic betaine borate, (CH₃)₃NCH₂COO·H₃BO₃, have been determined by measuring temperature and stress induced shifts of resonance frequencies of thick plates at ca. 15 MHz in the range between 140 and 300 K and 0 and 3 kbar. The elastic ‘shear’ resistance c₄₄ exhibits a value as low as 0.0492×10¹⁰Nm^-2at 293 K. With decreasing temperature c₄₄ approaches zero at ca. 142.5 K, indicating an acoustic soft mode behaviour connected with a ferroelastic phase transition. The softening of c₄₄ is described in a good approximation by c₄₄(T)_p=0 =alogT/T₀ with a=0.0663×10¹⁰Nm^-2 and T₀ = 139.5 K. Further, c₄₄ decreases with increasing pressure according to the linear relation c₄₄(p)_{T=293 K} = 0.0492?0.184×10^-4p (p in bar, c₄₄ in 10¹⁰ Nm^-2). All other elastic constants show a quite normal temperature and pressure dependence. At 293 K the transition is induced by a pressure of 2.65 kbar. The transition temperature T_c depends linearly on pressure according to T_c = 142.5+0.0568 p (pinbar, T_cinK). Passing through the transition no discontinuous change of the lattice constants is observed. The three principal coefficients of thermal expansion and the pressure derivatives of the dielectric constants exhibit discontinuities at the transition. The transition is of strongly second order.