锗酸盐晶体压致非晶化的研究

高压拉曼散射研究表明．ＣｕＧｅＯ３，Ｌｉ２ＧｅＯ３和Ｌｉ６Ｇｅ２Ｏ７三种晶体分别在７，１２和１１ＧＰａ压力下转变为非晶。在高于起始转变压力以上一定范围压致非晶是可逆的，ＣｕＧｅＯ３，Ｌｉ２ＧｅＯ３和Ｌｉ６Ｇｅ２Ｏ７压致非晶的不可逆转变压力分别为１４．１，２０和２０ＧＰａ。压致非晶ＣｕＧｅＯ３的重新晶化温度在６００℃附近。锗酸及系列晶体的压致非晶化与它们的成份和结构有关，随着在这一系列晶体中Ｌｉ２Ｏ含量的增加，压致非晶化的压力趋于减小。     

Pressure dependence of ionic conductivity of hydrated and dehydrated zeolites A

Hydrated and dehydrated zeolites MA (where M=Li, Na and K) with LTA structure have been studied by impedance spectroscopy with scanning frequency from 1 Hz to 1 MHz at high pressure up to 4.5 GPa and high temperature up to 250 °C. Anomalous increase in electrical AC conductivity at about 1.5–2 GPa observed in hydrated zeolites is associated with changes in crystalline structure leading to the formation of high-diffusion state of cation and water stuffing of the channels.

In dehydrated zeolites, electrical conductivity is controlled by diffusion of cations (Li⁺, Na⁺ and K⁺), which is determined by cation sites and aluminosilicate ring windows. LiA and NaA zeolites show normal decrease of conductivity with pressure, whereas KA zeolite exhibits the anomalous dependence with considerable increase and then fast decrease of conductivity. The behaviour of KA zeolite is associated with nearly central location of cation site in 8-membered ring, different from that in LiA and NaA zeolites.     

Polyamorphism in silicon nanocrystals under pressure

Many works have been devoted to describing mechanisms of pressure-induced polyamorphism. This phenomenon is apparent in the phase transition between low- and high-density amorphous states (LDA and HDA) upon the application of pressure, resulting in substantial changes in the structure and physical properties of the amorphous state. The HDA–LDA transition in Si nanocrystals is observed when recording Raman spectra in situ during decompression at 6.68 GPa.     

Semiconductor-metal transition in selenium under shock compression

Phase transitions in selenium are studied by time-resolved measurements of the electrical conductivity under shock compression at a pressure of up to 32 GPa. The pressure dependence of the electrical conductivity (σ(P)) has two portions: a sharp increase at P < 21 GPa and a plateau at P > 21 GPa. The experimental data and the temperature estimates indicate that, at P < 21 GPa, selenium is in the semiconductor state. The energy gap of semiconducting selenium decreases substantially under compression. At P > 21 GPa, the electrical conductivity saturates at ～10⁴ Ω^?1 cm^?1. Such a high value of the electrical conductivity shows the effective semiconductor-metal transition taking place in shock-compressed selenium. Experiments with samples having different initial densities demonstrate the effect of temperature on the phase transition. For example, powdered selenium experiences the transition at a lower shock pressure than solid selenium. Comparison of the temperature estimates with the phase diagram of selenium shows that powdered selenium metallizes in a shock wave as a result of melting. The most plausible mechanism behind the shock-induced semiconductor-metal transition in solid selenium is melting or the transition in the solid phase. Under shock compression, the metallic phase arises without a noticeable time delay. After relief, the metallic phase persists for a time, delaying the reverse transition.     

Electrical conductivity and equations of states of β-rhombohedral boron in the megabar dynamic pressure range

The pressure dependence of the conductivity of boron under conditions of a stepwise shock compression of megabaric range is studied. With this purpose, the following problems have been solved. The conductivity of boron has been measured in the range of dynamic pressures, where boron has different high-pressure phases. The equations of state of β-rhombohedral and amorphous boron have been constructed in a megabaric pressure range. The thermodynamic states of boron in the conditions of these experiments are calculated, which, in combination with the measurement data, made it possible to determine the change in the boron conductivity in the conditions of strong stepwise shock compression at dynamic pressures to 110 GPa. The increase in the conductivity of polycrystalline boron at megabar pressures is interpreted as a result of a nonmetal–metal transition.

     

聚苯硫醚熔体的压致凝固行为

采用施加压力的方法将聚苯硫醚熔体凝固,凝固后获得的聚苯硫醚样品经过降温和卸压后在常温常压下回收. X射线衍射和差示扫描量热分析表明:约20 ms时间的快速压缩过程可以抑制熔体结晶,制备出非晶态聚苯硫醚块材,样品的表面及中心都是非晶态.非晶态聚苯硫醚的玻璃化转变温度和晶化温度分别为318和362 K.常压下的退火实验表明,非晶态聚苯硫醚在425 K等温结晶的产物为正交相晶型.压致凝固法中熔体的凝固不是靠温度变化,而是靠压力变化,样品表面和内部处在一致的温度下同时受压凝固,避免了热传导对非晶尺寸的影响,因此非常有利于获得结构均匀的大尺寸非晶态材料.     

Pressure-induced phase transitions for goethite investigated by Raman spectroscopy and electrical conductivity

We reported two pressure-induced phase transitions of goethite up to ～35?GPa using a diamond anvil cell in conjunction with ac impendence spectroscopy, Raman spectra at room temperature. The first pressure-induced phase transition at ～7.0?GPa is manifested in noticeable changes in six Raman-active modes, two obvious splitting phenomena for the modes and the variations in the slope of conductivity. The second phase transition at ～20?GPa was characterized by an obviously drop in electrical conductivity and the noticeable changes in the Raman-active modes. The variations in activation energy with increasing pressure were also discussed to reveal the electrical properties of goethite at high pressure.     

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.     

Simultaneous electrical and X-ray diffraction studies on neodymium metal to 152 GPa

Using designer diamond anvils and angle dispersive X-ray diffraction technique at a synchrotron source, we have performed simultaneous electrical and structural studies on neodymium metal to 152 GPa in a diamond anvil cell. Four-probe electrical resistance measurement shows a 38% decrease in the electrical resistivity, associated with the delocalization of the 4f-shell electrons, starting at 100 GPa up to a final pressure of 152 GPa. The continuous decrease in electrical resistivity is consistent with the observation of a gradual phase transition to α-U structure in this pressure range. The (1 1 1) diffraction peak of α-U structure first appears at 100 GPa and increases in intensity with increasing pressure to 152 GPa. This increase in intensity is attributed to an increasing volume fraction of α-U phase and an increase in structural y-parameter from 0.07 at 118 GPa to 0.095 at 152 GPa. In contrast to the abrupt pressure-induced f-electron transition seen in cerium and praseodymium, the continuous evolution of α-U structure and electrical resistivity in neodymium confirms the gradual nature of 4f delocalization process in this element.     

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。     

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.     

Pressure-induced superconductor-insulator transition in the spinel compound CuRh2S4

We performed resistivity measurements in CuRh2S4 under quasihydrostatic pressure of up to 8.0 GPa, and found a pressure-induced superconductor-insulator transition. Initially, with increasing pressure, the superconducting transition temperature T(c) increases from 4.7 K at ambient pressure to 6.4 K at 4.0 GPa, but decreases at higher pressures. With further compression, superconductivity in CuRh2S4 disappears abruptly at a critical pressure P(SI) between 5.0 and 5.6 GPa, when it becomes an insulator.     

High pressure X-ray diffraction study of SrMnO3 perovskite

Using a diamond anvil cell device and synchrotron radiation, the in-situ high-pressure structure of SrMnO3 has been investigated. At pressure up to 28.6 GPa, no pressure-induced phase transition is observed. The lattice parameters as a function of pressure is reported, and the relationship of the axial compression coefficients is β<,a>> β<,c>. The isothermal bulk modulus K<,298>=266(4) GPa is also obtained by fitting the pressure- volume data using the Murnaghan equation of state.     

Synchrotron X-ray diffraction of SiO2 to multimegabar pressures

Abstract

α-Quartz was compressed at room temperature in a diamond-anvil cell without a medium to maximum pressures of 31 to 213 GPa and was studied by energy-dispersive synchrotron X-ray diffraction. Broad peaks observed in a previous high-pressure diffraction study of silica glass are evident in the present study of quartz compression, providing in situ confirmation of pressure-induced amorphization above 21 GPa. The 21-GPa crystalline-crystalline (quartz 1–11) transformation previously observed on quasihydrostatic compression of quartz is found to also occur under the current nonhydrostatic conditions, at the identical pressure. With nonhydrostatic compression, however, new sharp diffraction lines are observed at this pressure. The measurements show the coexistence of at least one amorphous and two crystalline phases above 21 GPa and below 43 GPa. The two crystalline phases are identified as quartz II and a new, high-pressure silica phase. The high-pressure phases, both crystalline and amorphous, can be quenched to ambient conditions from a maximum pressure of 43 GPa. With compression above 43 GPa, the diffraction pattern from quartz II is lost and the second crystalline phase persists to above 200 GPa.     

High pressure X-ray diffraction study of CaMnO3 perovskite

Using a diamond anvil cell device and synchrotron radiation,the in-situ high-pressure structure of CaMnO3 has been investigated.In the pressure up to 36.5 GPa,no pressure-induced phase transition is observed.The pressure dependence on the lattice parameters of CaMnO3 is reported,and the relationship of the axial compression coefficients is βa >βc > βb.The isothermal bulk modulus K298=224(25)GPa is also obtained by fitting the pressure-volume data using the Murnaghan equation of state.     

Volume and structural study of Fe64Mn36 anti-ferromagnetic Invar alloy under high pressure

We have investigated the pressure variation of the volume and structure of an FCC Fe₆₄Mn₃₆ anti-ferromagnetic Invar alloy. The inclination of the pressure-volume (P-V) curve of the FCC structure becomes discontinuous at a pressure of 4 GPa. According to the bulk modulus at zero pressure estimated by the Birch-Murnaghan equation of state, the pressure between 4 and 10 GPa is 33 GPa larger than that at a pressure below 4 GPa. Considering previous experiments on magnetism at high pressure the Neel temperature at 4 GPa almost decreases to room temperature. These results suggest that the increase in the bulk modulus by 33 GPa can be attributed to the pressure-induced magnetic phase transition from anti-ferromagnetism to paramagnetism. Volume at zero pressure was estimated using the Birch-Murnaghan equation of state. The volume of FCC structure in the anti-ferromagnetic state was 1.17% larger than the volume in the paramagnetic state, namely, the spontaneous magnetostriction was 1.17%. Pressure-induced structural transition from FCC to HCP occurs with an increase in the pressure, especially at up to 5 GPa. The value of c/a is 1.62; this value almost corresponds to that of an ideal HCP structure. The bulk modulus of the HCP structure estimated by the Birch-Murnaghan equation of state is larger than that of the FCC structure, and the volume/atom ratio is smaller than that of the FCC structure.     

In-Situ High Pressure Raman Spectrum and Electrical Property of PbMoO4

In-situ high pressure Raman spectra and electrical conductivity measurements of scheelite-structure compound PbMoO4 are presented. The Raman spectrum of PbMoO4 is determined up to 26.5 GPa on a powdered sample in a diamond anvil cell （DAC） under nonhydrostatic conditions. The PbMoO4 gradully experiences the trans- formation from the crystal to amorphous between 9.2 and 12.5 GPa. The crystal to amorphous transition may be due to the mechanical deformation and the crystalographic transformation. Furthermore, the electrical conductivity of PbMoO4 is in situ measured accurately using a microcircuit fabricated on a DAC based on the van der Pauw method. The results show that the electrical conductivity of PbMoO4 increases with increases of pressure and temperature. At 26.5 GPa, the electrical conductivity value of PbMoO4 at 295K is 1.93 - 10-4 S/cm, while it raises by one order of magnitude at 430K and reached 3.33 - 10-3 S/cm. However, at 430K, compared with the electrical conductivity value of PbMoO4 at 26.5 GPa, it drops by about two order magnitude at 7.4 GPa and achieves 2.81 × 10^-5 S/cm. This indicates that the effect of pressure on the electrical conductivity of PbMoO4 is more obvious than that of temperature.     

一种新的恶二唑衍生物在金刚石对顶砧中的电阻率随压力和温度的变化关系

利用金刚石对顶砧测量了恶二唑衍生物微晶, 1,4-bis[(4-methyloxyphenyl)-1,3,4-oxadiazolyl]- 2,5-bisheptyloxyphenylene (OXD-2), 电阻随压力和温度的变化关系，并利用有限元分析方法计算了样品的电阻率。实验中，测量压力和温度达到了16 GPa和150℃。样品的电阻率随着温度的升高而降低，说明样品表现出半导体传导特性。在90-100 ℃之间，样品的电阻率有一明显的下降，说明这时发生了温度诱导的相变。随着压力的增加，样品的电阻率在6GPa左右达到最大值，此后随着压力的增加而下降。结合原位x光数据，在6GPa左右的电阻突变应该是由于样品在压力的诱导下发生了无序化的相变。     

Stability of Ferromagnetism in Fe, Co, and Ni Metals under High Pressure with GGA and GGA+U

The stability of the ferromagnetic state in Fe, Co, and Ni metals under high pressure is investigated using generalized gradient approximation (GGA) and GGA+U within the density functional theory (DFT). It is found that the ferromagnetic state under pressure is very different for Fe, Co, and Ni metals, and is closely associated with the crystal structure. In the case of Fe, a ferromagnetic bcc ground state is obtained at ambient pressure and a nonmagnetic hcp ground state is found at pressure around 12 and 115 GPa for GGA and GGA+U, respectively. For Co, the phase transition from a ferromagnetic hcp to a nonmagnetic fcc is found around 107 GPa for GGA. In contrast to Fe and Co, a ferromagnetic fcc state in Ni is maintained even at 200 GPa. The calculated results suggest that the suppression of ferromagnetism in Fe, Co, and Ni is due to pressure-induced decrease of the density of state at the Fermi level.     

Anomalous high-pressure Jahn-Teller behavior in CuWO4

High-pressure optical-absorption measurements performed in CuWO(4) up to 20 GPa provide experimental evidence of the persistence of the Jahn-Teller (JT) distortion in the whole pressure range both in the low-pressure triclinic and in the high-pressure monoclinic phase. The electron-lattice couplings associated with the e(g)(E?e) and t(2g)(T?e) orbitals of Cu(2+) in CuWO(4) are obtained from correlations between the JT distortion of the CuO(6) octahedron and the associated structure of Cu(2+) d-electronic levels. This distortion and its associated JT energy (E(JT)) decrease upon compression in both phases. However, both the distortion and associated E(JT) increase sharply at the phase-transition pressure (P(PT)=9.9 GPa), and we estimate that the JT distortion persists for a wide pressure range not being suppressed up to 37 GPa. These results shed light on the transition mechanism of multiferroic CuWO(4), suggesting that the pressure-induced structural phase transition is a way to minimize the distortive effects associated with the toughness of the JT distortion.