Evolution of the spectral response of amorphous europium molybdate under annealing

The phase transformations occurring in amorphous europium molybdate Eu₂(MoO₄)₃ during annealing at atmospheric pressure are studied using optical spectroscopy and x-ray diffractometry. It is established that the metastable β phase is formed at a temperature of ～550°C, whereas the transition to the stable equilibrium α phase takes place at higher temperatures T ≥ 700°C. The spectral characteristics of the α phase, which differ substantially from those of the amorphous state and the β phase, are measured for the first time.     

Compression Behaviour of Ni77P23 Amorphous Alloy up to 30.5 GPa

The compression behaviour of Ni77P23 amorphous alloy is investigated at room temperature in a diamond-anvil cell instrument using in-situ high pressure energy dispersive x-ray diffraction with a synchrotron radiation source. The equation of state is determined by fitting the experimental data according to the Birch-Murnaghan equation. It is found that the structure of Ni77P23 amorphous alloy is stable under pressures up to 30.5 GPa. Within the pressure range from zero to the experimental one, the pressure-induced structural relaxation is reversible.     

Equation of state and high-pressure irreversible amorphization in Y3Fe5O12

The change of crystal structure in yttrium iron garnet Y₃Fe₅O₁₂ was studied at room temperature at high pressures up to ∼55 GPa by the x-ray diffraction technique in diamond anvil cells. At a pressure of about ∼50 GPa, a drastic change in the x-ray diffraction pattern was observed indicating the transition into an amorphouslike state. When the pressure was increased, the bulk modulus of YIG was found to be 193 ± 4 GPa. It was also found that the amorphous state was retained after decompression down to ambient pressure. From the shape of x-ray patterns in the “amorphous” phase, it was concluded that the local atomic structure consists of iron-oxygen FeO₆ octahedral complexes with disordered orientations of local axis and of randomly arranged others ion fragments with the overall Y₃Fe₅O₁₂ composition. For the amorphous phase, it was evaluated that the bulk modulus of FeO₆ octahedral complexes is about 260 GPa. The text was submitted by the authors in English.     

偏硼酸钡低温相晶体在高压下的电学性质与相变

 在金刚石压砧装置上，采用电容和电阻测量方法研究了偏硼酸钡低温相晶体（β-BaB₂O₄）在室温下和16 GPa内的电容、电阻与压力的关系。实验结果表明，它的电容在2.1、4.6、6.4、8、9.5、10.7 GPa左右都有一个突变。这说明β-BaB₂O₄内部的结构状态在这些压力下都发生了变化，可能发生了相变。还发现β-BaB₂O₄样品在较高压力下已发生了非晶化转变，而且是不可逆的，在卸压后被保留下来。这个非晶化转变的压力大约在11~12 GPa。     

Super-pressed and super-cooled (?) P(EO)20LiBETI

High pressure conductivity measurements have been carried out on P(EO)₂₀LiBETI from 295 K to 368 K. The decrease of electrical conductivity with pressure is larger in the partially crystalline phase (low temperature and low pressure or high temperature and high pressure) than in the fully amorphous phase (high temperature and low pressure). It is found that if the phase transition is approached from the crystalline phase (decreasing pressure), the pressure of the phase transition varies from 0 to 0.23 GPa as the temperature increases from 336 K to 358 K. The shift of the phase transition temperature with pressure is approximately the same as the shift of the glass transition temperature with pressure for pure PEO. This can be understood in terms of the defect diffusion model. If the material is above 336 K and is in the fully amorphous phase, after pressure is increased above the critical pressure, the material remains in the amorphous phase for extended periods of time before transforming to the partially crystalline phase. This is reminiscent of a super-pressed state but may be an indication of slow crystallization kinetics.     

Compressibility of Fe1.087Te: a high pressure X-ray diffraction study

Fe_1.087Te exhibits three phases in the pressure range from ambient to 16.6?GPa and becomes amorphous at higher pressures. All three phases have tetragonal symmetry. The low pressure T-phase is stable in the pressure range 0≤P<4.1?GPa and is found to be relatively soft having zero pressure bulk modulus B ₀=36(1)?GPa. The intermediate cT-phase is less compressible with B ₀=88(5)?GPa and stable in the pressure range 4.1≤P<10?GPa while a more compressible phase was observed between 10 and 16.6?GPa.     

静高压下有表面化学反应的非晶合金晶化研究

在静高压3—5GPa,510—660℃温度下,研究了在晶化过程中其表面与Al发生反应的非晶(Fe_0.99,Mo_0.01)₇₈Si₉B₁₃合金的晶化过程。发现在4GPa左右,510—660℃的温度范围内,非晶FMSB晶化为纳米α-Fe(Al)相,在其他压力下,晶化为α-Fe(Mo,Si),(Fe,Mo)₃B或Fe₂B相。利用简单固体模型对其晶化的热力学机制 关键词：     

Superconductivity of κ-(BEDT-TTF)2Cu[N(CN)2]I under pressure

The insulating state of κ-(BEDT-TTF)₂Cu[N(CN)₂]I salt appearing at ambient pressure at low temperatures is suppressed by hydrostatic pressure. The resistive measurements showed that the emerging metallic state reveals superconductivity in high-quality crystals. The superconducting state with the transition temperature of about 8 K is stable at pressures higher than 0.1 GPa.     

Amorphouslike diffraction pattern in solid metallic titanium

Amorphouslike diffraction patterns of solid elemental titanium have been detected under high pressure and high temperature using in situ energy-dispersive x-ray diffraction and a multianvil press. The onset pressure and the temperature of formation of amorphous titanium is found to be close to the alpha-beta-omega triple point in the P-T phase diagram. Amorphous Ti has been found to be thermally stable up to 1250 degrees C for at least 3 min at some pressures. By analyzing the conditions for producing amorphous elemental Zr and Ti, we observed a multi-phase-point amorphization phenomenon for preparing single-element bulk amorphous metals. The results reported may open a new way to preparing single-element bulk amorphous metals with a high thermal stability.     

Phase transformations in amorphous fullerite C60 under high pressure and high temperature

First phase transformations of amorphous fullerite C₆₀ at high temperatures (up to 1800 K) and high pressures (up to 8 GPa) have been investigated and compared with the previous studies on the crystalline fullerite. The study was conducted using neutron diffraction and Raman spectroscopy. The amorphous fullerite was obtained by ball-milling. We have shown that under thermobaric treatment no crystallization of amorphous fullerite into С₆₀ molecular modification is observed, and it transforms into amorphous-like or crystalline graphite. A kinetic diagram of phase transformation of amorphous fullerite in temperature–pressure coordinates was constructed for the first time. Unlike in crystalline fullerite, no crystalline polymerized phases were formed under thermobaric treatment on amorphous fullerite. We found that amorphous fullerite turned out to be less resistant to thermobaric treatment, and amorphous-like or crystalline graphite were formed at lower temperatures than in crystalline fullerite.     

Superconductivity in the Ti-D system under pressure

The pressure dependence of the superconducting transition temperature in TiD_0.74 has been measured up to 30 GPa in a diamond high-pressure chamber. It is found that the deuteride TiD_0.74 becomes a superconductor at pressures corresponding to the transition to the high-pressure ζ phase, with a transition temperature that increases from 4.17 to 4.43 K in the interval P=14–30 GPa. The value extrapolated to atmospheric pressure T _c (0)=4.0 K is significantly lower than the superconducting transition temperature (T _c =5.0 K) measured earlier in the metastable state obtained by quenching TiD_0.74 under pressure. It is assumed that the significant difference of the extrapolated value from the superconducting transition temperature in the metastable state after quenching under pressure is caused by a phase transition on the path from the stability region of the ζ phase under pressure to the region of the metastable state at atmospheric pressure. Fiz. Tverd. Tela (St. Petersburg) 40, 2153–2155 (December 1998)     

High pressure study of acetophenone azine

High pressure Raman spectra of acetophenone azine (APA) have been measured up to 17.7 GPa with a diamond anvil cell. Two crystalline-to-crystalline phase transformations are found at pressures about 3.6 and 5.8 GPa. A disappearance of external modes and the C–H vibration at pressures higher than 8.7 GPa suggests that the sample undergoes a phase transition to amorphous or orientationally disordered (plastic) state, and the amorphization was completed at about 12.1 GPa. The disordered state is unstable and, then, a polymerization transformation reaction occurs with a further pressure increase. After the pressure has been released, the polymerization state can remain at the ambient condition, indicating that the virgin crystalline state is not recovered. The results show that the phenomenon underlying the pressure induced phase transition of APA may involve profound changes in the coordination environments of the symmetric aromatic azine.     

Superfluid molecular hydrogen

In order to produce a supercooled liquid phase of molecular hydrogen that may possibly change at a sufficiently low temperature to a superfluid state, it is suggested to reduce the temperature of its equilibrium coexistence with the solid phase by means of developing different pressures in these phases through the use of linear mechanical pressure on the solid phase or of external electric field. The thermodynamic functions of hydrogen are calculated in both the stable and metastable regions; its phase diagram and the region of possible transition to a superfluid state are also found. The values of excess pressure on the solid phase and of external electric field intensity are estimated, which are necessary for the stabilization of this state.     

LiIO3-NaIO3赝二元系相图及其非晶态晶化动力学的研究

本文对LiIOO₃-NaIO₃赝二元系的相图,相变,非晶态的形成和稳定性,以及晶化的动力学过程等进行了仔细的研究,LiIO₃-NaIO₃赝二元系属共晶体系,共晶温度为325℃,共晶点成份为含50m/o LiIO₃,用共晶点附近成份的试样,在超过熔点150℃的情况下首次获得碘酸盐的非晶态,非晶态的存在降低了LiIO₃的表观相变温度,强X射线的辐照,各种空气湿度,细粒度等因素都加 关键词：     

C60高压X射线衍射研究

 采用同步辐射X光源和能量色散法对高纯C₆₀粉末样品进行高压原位X光衍射实验。由金刚石对顶压砧高压装置（DAC）产生高压,用已知状态方程的Pt粉末作内标,由Pt的衍射数据确定样品压力,最高压力达30 GPa。实验结果表明：室温常压下原始C₆₀样品为面心立方结构,晶格常数a=1.420 86 nm。高压下C₆₀的结构有所变化：从p=13.7 GPa开始,(311)线发生劈裂,形成低对称相;随着压力增加,衍射线逐渐变宽,强度逐渐变弱,压力超过25 GPa,衍射背底隆起,C₆₀开始转化成非晶相;在30 GPa左右,衍射线条完全消失,标志着向非晶相转化过程的完成。人们也对C₆₀样品不同压力的高压“淬火”相进行了X光衍射实验。采用非静水压的装样方式,最高压力达44 GPa,结果在30 GPa以上,C₆₀也转变为非晶相。最后我们对C₆₀晶体的压致非晶化现象进行了初步的讨论。     

Phase Transitions in a Mixture of Amorphous C<Subscript>60</Subscript> and C<Subscript>70</Subscript> Fullerene Phases at High Temperatures and Pressures

Phase transitions in two types of amorphous fullerene phases (C₆₀–C₇₀ (50/50) mixtures and an amorpous C₇₀ fullerene phase) are studied via neutron diffraction at pressures of 2–8 GPa and temperatures of 200–1100°C. Fullerenes are amorphized by grinding in a ball mill and sintered under quasi-hydrostatic pressure in a toroidal-type chamber. Diffraction studies are performed ex situ. It is shown that the amorphous phase of fullerenes retains its structure at temperatures of 200–500°C, and amorphous graphite is formed at 800–1100°C with a subsequent transition to crystalline graphite. This process is slow in a mixture of fullerenes, compared to C₇₀ fullerene. According to neutron diffraction data, the amorphous graphite formed from amorphous fullerene phases has anisotropy that is much weaker in a fullerene mixture.     

Pressure induced lattice instability and phase separation in the cuprates

High pressure structural studies using a synchrotron source and Raman measurements on various cuprates reveal several structural modifications. The data have shown strong deviations from the normal equation of state at characteristic pressures, hysteresis, and the appearance of additional peaks that can be attributed to a new phase. The combined data of synchrotron angle-dispersive experiments with the optical measurements indicate that at some critical pressures, at least for certain compounds, non-linear effects are observed together with phase separation that affect the distribution of the carriers and the transition temperature. The comparison of the data with those induced by an internal pressure by an atomic substitution indicates that the effect is related to the existence of carriers within the CuO₂ superconducting planes.     

Amorphous boron nitride at high pressure

The pressure-induced phase transformation in hexagonal boron nitrite and amorphous boron nitrite is studied using ab initio molecular dynamics simulations. The hexagonal-to-wurtzite phase transformation is successfully reproduced in the simulation with a transformation mechanism similar to one suggested in experiment. Amorphous boron nitrite, on the other hand, gradually transforms to a high-density amorphous phase with the application of pressure. This phase transformation is irreversible because a densified amorphous state having both sp³ and sp² bonds is recovered upon pressure release. The high-density amorphous state mainly consists of sp³ bonds and its local structure is quite similar to recently proposed intermediate boron nitrite phases, in particular tetragonal structure (P4₂/mnm), rather than the known the wurtzite or cubic boron nitrite due to the existence of four membered rings and edge sharing connectivity. On the basis of this finding we propose that amorphous boron nitrite might be best candidate as a starting structure to synthesize the intermediate phase(s) at high pressure and temperature (probably below 800 °C) conditions.     

Hydrogen solubility in amorphous Mg<Subscript>0.6</Subscript>SiO<Subscript>2.6</Subscript> at high pressure

The solubility of hydrogen in amorphous Mg_0.6SiO_2.6 at a temperature of 250°C and pressures up to 75 kbar is studied using a quenching technique. The molar ratio H₂/formula unit is found to nonlinearly increase with pressure from x = 0.12 at P = 10 kbar to x = 0.303 at P = 75 kbar. An investigation of the quenched samples by Raman spectroscopy demonstrated that hydrogen dissolves in amorphous Mg_0.6SiO_2.6 in the form of H₂ molecules. X-ray diffraction and Raman studies showed that the hydrogenation of the samples is likely to be accompanied by a phase transition in the amorphous lattice of Mg_0.6SiO_2.6 at P ≈ 52.5 kbar to a denser amorphous modification.     

Interaction between Intermetallic Compounds <Emphasis Type="Italic">R</Emphasis>Ni<Subscript>3</Subscript> (<Emphasis Type="Italic">R</Emphasis> = Gd,Dy) and Hydrogen at Low Temperatures

The hydride phases of the intermetallic compounds GdNi₃ and DyNi₃ are synthesized at room temperature and at 273 K under a hydrogen pressure of 1–30 bar. The phase composition of the obtained samples is established using the X-ray diffraction method and the crystal-lattice parameters of the hydride phase are determined. The crystal phases are synthesized at room temperature under a hydrogen pressure of about 1 bar. At 273 K and under a pressure of 30 bar, amorphous samples are formed. The desorption of hydrogen from amorphous hydrides at 573 K leads to the formation of well-crystallized samples of the initial intermetallides. The amorphous samples are formed due to the ordering of hydrogen atoms in the metallic matrix of the hydride at low temperatures.