Phase transition in Pd_(40)Ni_(10)Cu_(30)P_(20) bulk metallic glass under HP & HT

Bulk metallic glass (BMG) can be regarded as a metallic solid with a frozen-in melt structure which lacks atomic periodicity. Generally, BMG is not in an internal equilib- rium state (IES) but may change to the IES when it obtains an appropriate energy to overcome the energy barrier. High pressure and temperature can be a driving force to cause various transitions in amorphous materials, such as a transition from the amor- phous to the crystalline[1―8] or from the crystalline to the amorp…     

Phase transition in Pd40Ni10Cu30P20 bulk metallic glass under HP & HT

The phase transitions in Pd₄₀Ni₁₀Cu₃₀P₂₀ bulk metallic glass (BMG) have been studied under high pressure and high temperature (HP & HT) by X-ray diffaction measurements with synchrotron radiation source. We found that the BMG underwent a phase transitions of amorphous-crystalline-amorphous at 10 GPa upon heating. The parallel experiments were carried out at 7 GPa, while we did not observe the amorphous-crystalline-amorphous transitions by increasing temperature. Quenching the melted BMG at 7 GPa, it was found that the phase crystallized from the melt differed from the primary phase crystallized from the starting amorphous solid upon heating suggesting there existed a distinct mechanism in two cases.     

Phase transition in Pd<Subscript>40</Subscript>Ni<Subscript>10</Subscript>Cu<Subscript>30</Subscript>P<Subscript>20</Subscript> bulk metallic glass under HP &; HT

The phase transitions in Pd₄₀Ni₁₀Cu₃₀P₂₀ bulk metallic glass (BMG) have been studied under high pressure and high temperature (HP & HT) by X-ray diffaction measurements with synchrotron radiation source. We found that the BMG underwent a phase transitions of amorphous-crystalline-amorphous at 10 GPa upon heating. The parallel experiments were carried out at 7 GPa, while we did not observe the amorphous-crystalline-amorphous transitions by increasing temperature. Quenching the melted BMG at 7 GPa, it was found that the phase crystallized from the melt differed from the primary phase crystallized from the starting amorphous solid upon heating suggesting there existed a distinct mechanism in two cases.     

高温高压下冲击波淬火的锆基大块金属玻璃的相演化过程研究

 采用同步辐射能散X射线衍射技术,研究了高温高压下利用冲击波淬火技术制备的Zr₄₁Ti₁₄Cu_12.5Ni₁₀Be_22.5大块金属玻璃的相演化过程。研究结果发现：在实验压力范围内,在不同压力下试样具有相同的初始析出相Zr₂Be₁₇,但是随后的相演化过程是不同的,根据应用压力的不同,试样的相演化过程可以分为3个不同的区域;另外,试样的晶化温度随着压力的增大而升高,但是在6.0 GPa存在一个突然的下降,在此压力点试样具有不同于其它压力点的相演化过程。相演化过程的不同和晶化温度的突然下降,可能归因于在不同压力下试样具有不同的原子构型。     

Photothermal-chemical synthesis of P-S-H ternary hydride at high pressures

The recent discovery of room temperature superconductivity (283 K) in carbonaceous sulfur hydride (C-S-H) has attracted much interest in ternary hydrogen rich materials. In this report, ternary hydride P-S-H was synthesized through a photothermal-chemical reaction from elemental sulfur (S), phosphorus (P) and molecular hydrogen (H₂) at high pressures and room temperature. Raman spectroscopy under pressure shows that H₂S and PH₃ compounds are synthesized after laser heating at 0.9 GPa, and a ternary van der Waals compound P-S-H is synthesized with further compression to 4.6 GPa. The P-S-H compound is probably a mixed alloy of PH₃ and (H₂S)₂H₂ with a guest-host structure similar to the C-S-H system. The ternary hydride can persist up to 35.6 GPa at least and shows two phase transitions at approximately 23.6 GPa and 32.8 GPa, respectively.     

Changes in the crystalline structure of chlorpropamide at high pressures and temperatures

The crystalline structure of chlorpropamide is studied by the X-ray diffraction method at high pressures up to 4.2 GPa and in the temperature range 300–450 K. At normal pressure and upon heating to its melting point T = 396 K no phase transitions are found in chlorpropamide. When the initial α form of chlorpropamide is recrystallized, the appearance of a polymorphic ε phase is observed. After recrystallization, the high pressure effect causes partial amorphization of chlorpropamide at pressures of P ～ 3 GPa. Baric and temperature coefficients are obtained for the α and ε forms of chlorpropamide.     

Phase transitions in cerium at high pressures (up to 15 GPa) and high temperatures

Phase transitions in cerium have been studied by the electrical resistance method in the 15-GPa pressure range at high temperatures. At pressures above 10 GPa, cerium represents a mixture of stable and metastable phases, the composition of this mixture being dependent on the trajectory in the P-T plane that leads to a given point. Transformations in both stable and metastable components of the mixture proceeding rather independently display a complicated picture of phase transitions. It was assumed that only the α (fcc) and α′ (α-U) phases are stable at pressures above the well-known γ-α transition, the other phases being metastable. The proposed bow-shaped equilibrium phase diagram includes an extremely wide hysteresis region, where stable and metastable phases can coexist. The fcc α phase alone survives upon heating above 50°C at 15 GPa.     

Compression behavior of Zr_(41)Ti_(14)Cu_(12.5)Ni_(10)Be_(22.5) bulk metallic glass up to 24 GPa

With the development of multicomponent Zr-based glass bulk metallic glasses(BMGs) with a larger geometry by a conventional casting process with a low coolingrate[1—4], high pressure, which is the simplest way to vary the interatomic distance ofsubstance and thus change its physical properties, has been employed as an importantmeans to investigate structural relaxation and the crystallization of the BMGs[5]. Relativeto crystalline, metallic glasses have excess free volume, which will annihil…     

Structural studies of the P-T phase diagram of sodium niobate

The crystal structure of sodium niobate (NaNbO₃) has been investigated by energy-dispersive X-ray diffraction at high pressures (up to 4.3 GPa) in the temperature range 300–1050 K. At normal conditions, NaNbO₃ has an orthorhombic structure with Pbcm symmetry (antiferroelectric P phase). Upon heating, sodium niobate undergoes a series of consecutive transitions between structural modulated phases P-R-S-T(1)-T(2)-U; these transitions manifest themselves as anomalies in the temperature dependences of the positions and widths of diffraction peaks. Application of high pressure leads to a decrease in the temperatures of the structural transitions to the R, S, T(1), T(2), and U phases with different baric coefficients. A phase diagram for sodium niobate has been build in the pressure range 0–4.3 GPa and the temperature range 300–1050 K. The dependences of the unit-cell parameters and volume on pressure and temperature have been obtained. The bulk modulus and the volume coefficients of thermal expansion have been calculated for different structural modulated phases of sodium niobate. A phase transition (presumably, from the antiferroelectric orthorhombic P phase to the ferroelectric rhombohedral N phase) has been observed at high pressure (P = 1.6 GPa) and room temperature.     

Raman spectroscopic and optical absorption study of the high pressure behaviour and polymorphism in KO2

Abstract

Raman scattering, visible absorption, and optical observation studies have been made on polycrystalline potassium superoxide (KO₂) in a diamond anvil cell as a function of pressure and temperature. Three new phases are observed. With increasing pressure at 298 K, KO₂ transforms from the well known modified CaC₂ structure (Phase II), to two new phases (VII, and VIII). The transformation from III to VII occurs at about 3.2GPa. Phase VII transforms to phase VIII at about 4.4GPa. However, in some samples phase VII does not occur and phase II transforms directly into phase VIII at about 4.2 GPa. These structural transformations are indicated by marked changes in the Raman spectrum. The transitions out of phase II are also marked by a discontinuous red shift in the optical absorption edge. From optical observations we have also determined the pressure and temperature dependence of the transitions from phase II to the high temperature cubic (B1) phase I as well as from the high pressure phases VII and VIII to a new nonbirefringent phase IX. This new phase IX has the cubic B2 (CsCl) structure as is shown by our recent X-ray synchrotron experiments.     

Electron Transport Property of CdTe under High Pressure and Moderate Temperature by In-Situ Resistivity Measurement in Diamond Anvil Cell

In situ resistivity measurement has been performed to investigate the electron transport property of powered CdTe under high pressure and moderate temperature in a designed diamond anvil cell. Several abnormal resistivity changes can be found at room temperature when the pressure increases from ambient to 33 GPa. The abnormal resistivity changes at about 3.8 GPa and 10 GPa are caused by the structural phase transitions to the rock-salt phase and to the Cmcm phase, respectively. The other abnormal resistivity changes at about 6.5 GPa, 15.5 GPa, 22.2 GPa and about 30 GPa never observed before are due to the electronic phase transitions of CdTe. The origin of the abnormal change occurred at about 6.5 GPa is discussed. The temperature dependence of the resistivity of CdTe shows its semiconducting behaviour at least before 11.3 GPa.     

Phase transitions in CsHSO4 up to 2.5 GPa: Impedance spectroscopy under pressure

Phase transitions in CsHSO₄ at pressures up to 2.5 GPa have been studied with the help of electrical impedance measurements. The phase boundaries have been identified with the help of calculated activation energies of electrical conductivity and dielectric relaxation time. The derived temperatures of phase transition from the low conductive phase II into super ionic phase I at pressure less than 1 GPa confirm the previous results of Ponyatovski? et al. (1985) [4] and Friesel et al. (1989) [27]. The phase diagram derived in this study for pressure larger than 1 GPa differs from the data of Ponyatovski? et al. (1985) [4]. The phase transitions IV-VI and VI-I occur at higher temperatures having significantly larger Clapeyron slope. The phase VII was not identified from heating cycle and appears only under cooling between phases I and VI. The phase VIII was detected at 2.5 GPa at T<350 K and only during heating.     

Structural relaxation of Zr_(41)Ti_(14)Cu_(12.5)Ni_(10)Be_(22.5) bulk metallic glass under high pressure

Zr41Ti14Cu12.5Ni10Be22.5bulk metallic glass (BMG) is annealed at 573 K under 3 GPaand its structural relaxation is investigated by X-ray diffraction, ultrasonic study, compression as well as sliding wear measurements. It is found that after the ZrTiCuNiBe BMG sample was annealed under high pressure, the mechanical properties were improved. Moreover, the BMG with relaxed structure exhibits markedly different acoustic properties. These results are attributed to the fact that relaxation under high-pressure results in a microstructural transformation in the BMG.     

Reconstructive structural phase transitions in dense Mg

The question raised recently about whether the high-pressure phase transitions of Mg follow a hexagonal close-packed (hcp) → body centered cubic (bcc) or hcp → double hexagonal close-packed (dhcp) → bcc sequence at room temperature is examined by the use of first principles density functional methods. Enthalpy calculations show that the bcc structure replaces the hcp structure to become the most stable structure near 48 GPa, whereas the dhcp structure is never the most stable structure in the pressure range of interest. The characterized phase-transition mechanisms indicate that the hcp → dhcp transition is also associated with a higher enthalpy barrier. At room temperature, the structural sequence hcp → bcc is therefore more energetically favorable for Mg. The same conclusion is also reached from the simulations of the phase transitions using metadynamics methods. At room temperature, the metadynamics simulations predict the onset of a hcp → bcc transition at 40 GPa and the transition becomes more prominent upon further compression. At high temperatures, the metadynamics simulations reveal a structural fluctuation among the hcp, dhcp, and bcc structures at 15 GPa. With increasing pressure, the structural evolution at high temperatures becomes more unambiguous and eventually settles to a bcc structure once sufficient pressure is applied.     

基于铰链式六面顶压机的二级6-8型大腔体静高压装置

报道了一种新型二级6-8型大腔体静高压装置.该装置是以国产DS6×800T铰链式六面顶压机为构架,在其六面体压腔中直接放入二级6-8模(球分割)增压装置以产生10GPa以上的压力,还实验了不同规格的预密封边和不同密度的叶蜡石对压力产生效率的影响,在室温下用BiⅠ-Ⅱ(2.55GPa),Ⅲ-Ⅴ(7.7GPa)和SnⅠ-Ⅱ(9.4GPa)在高压下的相变对14／8(8面体传压介质边长／8面体压腔边长)规格压腔进行了压力标定.实验结果表明,该系统可以在加载压力(油压)约为3×10⁶N(～42 关键词： 铰链式六面顶压机 6-8型球分割大腔体静高压装置 压力标定     

A high pressure Raman study of KTP and pressure induced phase transitions

Potassium titanate orthrophosphate KTiOPO₄ (KTP) has been studied by high pressure Raman technique to 17 GPa using a diamond cell. The Raman data reveal that two phase transitions occur in the system: one near 5.5 GPa and another near 10 GPa. The Lower-pressure transition is definitely first-order but appears to be driven by the phonon mode near 56 cm⁻¹, which exhibits marked softening. A mean field-like behavior is observed. It is hypothesized that this transition is likely to be from ferroelectric (FE) to an antiferroelectric (AF) phase. The 10 GPa transition may be due to AF-PE (paraelectric) transition driven by pressure from high temperature to room temperature. From the larger pressure responce of the PO₄ vibrational modes it is believed that the PO₄ polyhedral compression is larger than the TiO₆ polyhedral compression. leading to polyhedral tilt transitions.     

高压下ZnSe直流和交流电学性质的研究

采用高压原位测量技术在0–35 GPa压力范围内对ZnSe直流和交流电学性质进行了研究. 通过分析直流电学测量结果可知,在实验压力区间内ZnSe经历了由纤锌矿转变为朱砂相再转变为岩盐相的两次相结构转变. 分析温度与材料电阻率的变化关系表明ZnSe在高压下的相变为金属化相变,并通过交流阻抗谱的测量实验证实了这个结论. 进一步比较低压条件下晶粒和晶界电阻的变化,表明朱砂相结构的ZnSe更接近各向同性材料. 关键词： 高压 ZnSe 电学     

Zr-based bulk metallic glass as a cylinder material for high pressure apparatuses

Zirconium-based bulk metallic glass (Zr-based BMG) has outstanding properties as a cylinder material for piston-cylinder high pressure apparatuses and is especially useful for neutron scattering. The piston-cylinder consisting of a Zr-based BMG cylinder with outer/inner diameters of 8.8/2.5?mm sustains pressures up to 1.81?GPa and ruptured at 2.0?GPa, with pressure values determined by the superconducting temperature of lead. The neutron attenuation of Zr-based BMG is similar to that of TiZr null-scattering alloy and more transparent than that of CuBe alloy. No contamination of sharp Bragg reflections is observed in the neutron diffraction pattern for Zr-based BMG. The magnetic susceptibility of Zr-based BMG is similar to that of CuBe alloy; this leads to a potential application for measurements of magnetic properties under pressure.     

High pressure investigations of melamine

We have performed mid- and far-infrared (IR), Raman, and angular dispersive X-ray diffraction studies on melamine at high pressure up to 36 GPa. We have confirmed the presence of three phase transitions; the first between 1 and 2 GPa, the second between 7 and 9 GPa, and the third near 16 GPa. We observed a softening of the N–H symmetric and antisymmetric vibrations with pressure, suggesting that intermolecular hydrogen bonding increases as the intermolecular distance decreases similarly to what was observed in triamino-trinitrobenzene. The molecular decompression data from core intramolecular peaks of mid-IR and Raman indicate that melamine did not chemically decompose up to the highest investigated pressures but the sample suffered some irreversible amorphization. We have further clarified the lack of observation of any phase transitions in prior Raman and IR studies by examining the pressure dependence of other uninvestigated modes of vibration.     

Compression behavior of Zr<Subscript>41</Subscript>Ti<Subscript>14</Subscript>Cu<Subscript>12.5</Subscript>Ni<Subscript>10</Subscript>Be<Subscript>22.5</Subscript> bulk metallic glass up to 24 GPa

The compression of a Zr₄₁Ti₁₄Cu_12.5Ni₁₀Be_22.5 bulk metallic glass (BMG) is investigated at room temperature up to 24 GPa using in-situ high pressure energy dispersive X-ray diffraction with a synchrotron radiation source. The pressure-induced structural relaxation is exhibited. It is found that below about 8 GPa, the existence of excess free volume contributes to the rapid structural relaxation, which gives rise to the rapid volumetric change, and the structural relaxation results in the structural stiffness under higher pressure.