首页 | 本学科首页   官方微博 | 高级检索  
相似文献
 共查询到20条相似文献,搜索用时 46 毫秒
1.
The structural stability and electrical properties of A1B2-type MnB2 were studied based on high pressure angle- dispersive x-ray diffraction, in situ electrical resistivity measured in a diamond anvil cell (DAC) and first-principles calcu- lations under high pressure. The x-ray diffraction results show that the structure of A1B2-type MnB2 remains stable up to 42.6 GPa. From the equation of state of MnB2, we obtained a bulk modulus value of 169.9~3.7 GPa with a fixed pressure derivative of 4, which indicates that A1B2-type MnB2 is a hard and incompressible material. The electrical resistance un- dergoes a transition at about 19.3 GPa, which can be explained by a transition of manganese 3d electrons from localization to delocalization under high pressure.  相似文献   

2.
The electrical conductivity of powdered LiCr 0.35 Mn0.65O2 is measured under high pressure up to 26.22 GPa in the temperature range 300-413 K by using a diamond anvil cell. It is found that both conductivity and activation enthalpy change discontinuously at 5.36 GPa and 21.66 GPa. In the pressure range 1.10-5.36 GPa, pressure increases the activation enthalpy and reduces the carrier scattering, which finally leads to the conductivity increase. In the pressure ranges 6.32-21.66 GPa and 22.60-26.22 GPa, the activation enthalpy decreases with pressure increasing, which has a positive contribution to electrical conductivity increase. Two pressure-induced structural phase transitions are found by in-situ x-ray diffraction under high pressure, which results in the discontinuous changes of conductivity and activation enthalpy.  相似文献   

3.
The insulator-metal transition triggered by pressure in charge transfer insulator NiS2 is investigated by combining high-pressure electrical transport,synchrotron x-ray diffraction and Raman spectroscopy measurements up to40-50 GPa.Upon compression,we show that the metallization firstly appears in the low temperature region at~3.2 GPa and then extends to room temperature at~8.0 GPa.During the insulator-metal transition,the bond length of S-S dimer extracted from the synchrotron x-ray diffraction increases with pressure,which is supported by the observation of abnormal red-shift of the Raman modes between 3.2 and 7.1 GPa.Considering the decreasing bonding-antibonding splitting due to the expansion of S-S dimer,the charge gap between the S-ppπ* band and the upper Hubbard band of Ni-3 d eg state is remarkabl.y decreased.These results consistently indicate that the elongated S-S dimer plays a predominant role in the insulator-metal transition under high pressure,even though the p-d hybridization is enhanced simultaneously,in accordance with a scenario of charge-gap-controlled type.  相似文献   

4.
The structural phase transition, strength, and texture of vanadium have been studied under nonhydrostatic compression up to 70 GPa using an angle-dispersive radial x-ray diffraction technique in a 2-fold paranomic diamond anvil cell and up to 38 GPa using an angle-dispersive x-ray diffraction technique in a modified Mao–Bell diamond anvil cell at room temperature. We have confirmed a phase transition from body-centered cubic structure to rhombohedral structure at 27–32 GPa under nonhydrostatic compression. The radial x-ray diffraction data yields a bulk modulus K_0= 141(5) GPa and its pressure derivative K_0′= 5.4(7) for the bcc phase and K_0= 154(13) GPa with K_0′= 3.8(3) for the rhombohedral phase at ψ = 54.7°. The nonhydrostatic x-ray diffraction data of both bcc and rhombohedral phases yields a bulk modulus K_0= 188(5) GPa with K_0′= 2.1(3). Combined with the independent constraints on the high-pressure shear modulus, it is found that the vanadium sample can support a differential stress of ~1.6 GPa when it starts to yield with plastic deformation at ~36 GPa. A maximum differential stress as high as ~ 1.7 GPa can be supported by vanadium at the pressure of ~ 47 GPa.In addition, we have investigated the texture up to 70 GPa using the software package MAUD. It is convinced that the bodycentered cubic to rhombohedral phase transition and plastic deformation due to stress under high pressures are responsible for the development of texture.  相似文献   

5.
The electrical transport properties and structures of Y2 O3/ZrO2 solid solution have been studied under high pressure up to 23.2 GPa by means of in situ impedance spectroscopy and x-ray diffraction(XRD) measurements.In the impedance spectra, it can be found that the pressure-dependent resistance of Y2 O3/ZrO2 presents two different change trends before and after 13.3 GPa, but the crystal symmetry still remains stable in the cubic structure revealed by the XRD measurement and Rietveld refinement.The pressure dependence of the lattice constant and unit cell volume shows that the Y2 O3/ZrO2 solid solution undergoes an isostructural phase transition at 13.1 GPa, which is responsible for the abnormal change in resistance.By fitting the volume data with the Birch–Murnaghan equation of state, we found that the bulk modulus B0 of the Y2 O3/ZrO2 solid solution increases by 131.9% from 125.2 GPa to 290.3 GPa due to the pressure-induced isostructural phase transition.  相似文献   

6.
PtS_2, which is one of the group-10 transition metal dichalcogenides, attracts increasing attention due to its extraordinary properties under external modulations as predicted by theory, such as tunable bandgap and indirect-to-direct gap transition under strain; however, these properties have not been verified experimentally. Here we report the first experimental exploration of its optoelectronic properties under external pressure. We find that the photocurrent is weakly pressuredependent below 3 GPa but increases significantly in the pressure range of 3 GPa–4 GPa, with a maximum ~ 6 times higher than that at ambient pressure. X-ray diffraction data shows that no structural phase transition can be observed up to26.8 GPa, which indicates a stable lattice structure of PtS_2 under high pressure. This is further supported by our Raman measurements with an observation of linear blue-shifts of the two Raman-active modes to 6.4 GPa. The pressure-enhanced photocurrent is related to the indirect-to-direct/quasi-direct bandgap transition under pressure, resembling the gap behavior under compression strain as predicted theoretically.  相似文献   

7.
Silane(SiH_4) is a promising hydrogen-rich compound for pursing high temperature superconducting.Previous high pressure measurements of Raman,x-ray diffraction and theoretical studies on SiH_4 mainly focused on its polymorphic structures above 50 GPa,while the structure and the stability under lower pressure range are still unclear.Here we report an investigation of condensed SiH_4 by Brillouin scattering measurements at high temperature up to 407 K and high pressure up to 18 GPa.Brillouin scattering frequencies of fluid SiH_4 under compression are obtained under isothermal conditions of 300 K,359 K and 407 K.The SiH4 becomes unstable with increasing temperature and subsequently decomposes into silicon and H2.We find that compression at room temperature induces two velocity anomalies corresponding to a fluid-solid state transition and a phase Ⅳ to phase V transition,respectively.Brillouin scattering spectra has been a powerful tool to investigate the fruitful phases and structure transitions in the hydrogen-rich compound under extreme conditions.  相似文献   

8.
The structures and the phase transitions of ScH3 under high pressure are investigated using first-principles calculations. The calculated structural parameters at zero pressure agree well with the available experimental data. With increasing pressure, the transition sequence hcp (GdH3 -type)→ C2/m →fcc→hcp (YH3-type)→Cmcm of ScH3 is predicted first; the corresponding transition pressures at 0 K are 23 GPa, 25 GPa, 348 GPa, and 477 GPa, respectively. The C2/m symmetry structure is a possible candidate but not a good one as the intermediate state from hexagonal to cubic in ScH3 . On the other hand, via the analysis of the structures of hexagonal ScH2.9 , cubic ScH3 , and cubic ScH2 , we find that the repulsive interactions of H-H atoms must play an important role in the transition from hexagonal to cubic.  相似文献   

9.
The incomplete decomposition product of metastable hydrazine(N2H4)instead of the energetically favorable ammonia(NH3)upon decompression is one drawback in applications of energetic material oligomeric hydronitrogens.We explore the stability of hydrazine molecules in hydrazine hydrate(N2H4 H2O)under pressure in diamond anvil cells(DACs)combined with in situ Raman spectroscopy and synchrotron x-ray diffraction(XRD)measurements.The results show that one NH2 branch forms NH3 group by hydrogen bonds between hydrazine and water molecules after the sample crystallizes at 3.2 GPa.The strengthening hydrogen bonds cause the torsion of hydrazine molecules and further dominate a phase transition at 7.2 GPa.Surprisingly,the NN single bonds are strengthened with increasing pressure,which keeps the hydrazine molecules stable up to the ultimate pressure of 36 GPa.Furthermore,the main diffraction patterns show continuous shift to higher degrees in the whole pressure range while some weak lines disappear above 8.2 GPa.The present peak-indexing results of the diffraction patterns with Materials Studio show that the phase transition occurs in the same monoclinic crystal system.Upon decompression,all of the hydrazine molecules extract from hydrazine hydrate crystal at 2.3 GPa,which may provide a new way to purify hydrazine from hydrate.  相似文献   

10.
In-situ energy dispersive x-ray diffraction on ZnS nanocrystalline was carried out under high pressure by using a diamond anvil cell. Phase transition of wurtzite of 10nm ZnS to rocksalt occurred at 16.0GPa, which was higher than that of the bulk materials. The structures of ZnS nanocrystalline at different pressures were built by using materials studio and the bulk modulus, and the pressure derivative of ZnS nanocrystalline were derived by fitting the equation of Birch-Murnaghan. The resulting modulus was higher than that of the corresponding bulk material, which indicates that the nanomaterial has higher hardness than its bulk materials.  相似文献   

11.
The electronic structure and magnetic properties of new layered oxyselenide compounds La2O3Fe2Se2 and La2O3Co2Se2 are studied by first-principles calculations. Our results indicate that both compounds are Mott-insulators with orbital ordering. The ground states of both compounds are the checkerboard antiferromagnetic states, which are different from the iron pnictide superconductors, although their structures are similar to those of the Fe-As-based superconductors.  相似文献   

12.
Incommensurate modulations in Ba2TiSi2O8, Sr2TiSi2O8, and Ba2TiGe2O8 are compared based on their corresponding electron diffraction patterns. The dependence of the modulations on chemical composition provides a suitable model system for the investigation of incommensurations in framework structures using high-resolution transmission electron microscopy (HRTEM). A widening of quantitative HRTEM utilising the concept of rigid units is proposed allowing for a determination of atomistic displacements responsible for the formation of incommensurately modulated structures.  相似文献   

13.
Magnetometric and neutron diffraction studies of polycrystalline NdCo2GE2, ErCo2Ge2 and PrFe2Ge2 compounds were carried out in the temperature range between 4.2 and 300 K. All samples are antiferromagnetic with Néel temperature 26.5, ~ 4.2 and 13 K, respectively. The RECo2Ge2 compounds have collinear antiferromagnetic order of +?+? type. For PrFe2Ge2 a sinusoidal magnetic structure is observed. Magnetic moment is localized on RE atoms only and is equal to that of RE3+ free ion value. In ErCo2Ge2 the magnetic moment of Er atoms is perpendicular to the c-axis, whereas for remaining compounds it is parallel to the c-axis.  相似文献   

14.
本文用X射线和差热分析方法对BaO-Li2O-B2O3三元系中的两个截面:BaB2O4-Li2B2O4和BaB2O4-Li2O作了研究。在BaB2O4-Li2B2O4赝二元系中发现了一个新的化合物4BaB2O4·Li2B2O4。化合物在930±3℃由包晶反应形成,并与Li2B2O4形成共晶反应。共晶温度为797±3℃,共晶点组分为79mol%Li2B2O4。在BaB2O4-Li2O截面中也存在化合物4BaB2O4·Li2B2O4,其包晶反应温度从930±3℃随Li2O含量增加下降到908±3℃。在组分60mol%Li2O处形成另一个新的化合物2BaB2O4·3Li2O。该化合物在630±3℃也是由包晶反应形成,并与Li2O和Li2CO3分别形成共晶反应,共晶温度分别为400±3℃和612±3℃。在BaB2O4-Li2B2O4和BaB2O4-Li2O体系中都没有观察到固溶体。用计算机程序分别对化合物4BaB2O4·Li2B2O4和2BaB2O4·3Li2O的X射线粉末衍射图案进行了指标化,其结果:4BaB2O4·Li2B2O4的空间群为Pmma,a=13.033?,b=14.630?,c=4.247?,每个单胞包含两个化合式单位;2BaB2O4·3Li2O的空间群为Pmmm,a=4.814?,b=9.897?,c=11.523?,每个单胞也含有两个化合式单位。 关键词:  相似文献   

15.
A neutron diffraction study of polycrystalline PrCu2Si2 [1], PrCu2Ge2 [2], PrFe2Ge2 [3] and NdFe2Ge2 [4] intermetallics carried out at liquid helium temperature shows the presence of a collinear antiferromagnetic order below TN = 19 ± 1 K [1], TN = 16 ± 1 K [2], TN = 9 ± 1 K [3] and 13 ± 1 K [4]. Magnetic moment, parallel to the c-axis is localized on RE ions only. The magnetic structure of these compounds consists of ferromagnetic layers perpendicular to the c-axis coupled antiferromagnetically with sequence +-+- for PrCu2Si2 and PrCu2Ge2 and +--+ for PrFe2Ge2 and NdFe2Ge2. The RE moments amount close to the free ion values for Fe containing compounds but are smaller in those containing Cu suggesting a fairly strong influence of crystal field.  相似文献   

16.
Different compositions in the Lu2Si2O7-Sc2Si2O7 system have been synthesized following the ceramic method. All XRD patterns are compatible with the thortveitite structure (β-RE2Si2O7 polymorph). Unit cell parameters change linearly with composition, which indicates a complete solid solubility of Sc2Si2O7 in Lu2Si2O7. 29Si MAS NMR spectra show a decrease of the 29Si chemical shift with increasing Sc content. A correlation reported in the literature to predict 29Si chemical shifts in silicates is applied here to obtain the theoretical variation in 29Si chemical shift values in the system Lu2Si2O7-Sc2Si2O7 and the results compare favourably with the values obtained experimentally. The FWHM values of the 29Si MAS NMR curves indicate a random distribution of Lu and Sc in the structure of the intermediate members. Finally, the IR study of the system confirms the solubility of Sc2Si2O7 in Lu2Si2O7, showing the splitting of several modes in the intermediate members and a linear shift of the frequency on going from one end-member to the other.  相似文献   

17.
In this paper, we present the spectral results of Dy3+ and Pr3+ (1.0 mol%) ions doped Bi2O3-ZnF2-B2O3-Li2O-Na2O glasses. Measurements of X-ray diffraction (XRD), differential scanning calorimetry (DSC) profiles of these rare-earth ions doped glasses have been carried out. From the DSC thermograms, glass transition (Tg), crystallization (Tc) and melting (Tm) temperatures have been evaluated. The direct and indirect optical band gaps have been calculated based on the glasses UV absorption spectra. The emission spectrum of Dy3+:glass has shown two emission transitions 4F7/26H15/2 (482 nm) and 4F7/26H13/2 (576 nm) with an excitation at 390 nm wavelength and Pr3+:glass has shown a strong emission transition 1D23H4 (610 nm) with an excitation at 445 nm. Upon exposure to UV radiation, Dy3+ and Pr3+ glasses have shown bright yellow and reddish colors, respectively, from their surfaces.  相似文献   

18.
Electrical and electrochemical properties of the 70Li2S·(30 − x)P2S5·xP2S3 and the 70Li2S·(30 − x)P2S5·xP2O5 (mol%) glass-ceramics prepared by the mechanical milling technique were investigated. Glass-ceramics with 1 mol% P2S3 and 3 mol% P2O5 showed the highest conductivity of 5.4 × 10− 3 S cm− 1 and 4.6 × 10− 3 S cm− 1, respectively. Moreover, these glass-ceramics showed higher electrochemical stability than the 70Li2S·30P2S5 (mol%) glass-ceramic. From the XRD patterns of the obtained glass-ceramics, trivalent phosphorus and oxygen were incorporated into the Li7P3S11 crystal. We therefore presume that the Li7P3S11 analogous crystals, which were formed by incorporating trivalent phosphorus and oxygen into the Li7P3S11 crystal, improve the electrical and electrochemical properties of the glass-ceramics. An all-solid-state cell using the 70Li2S·29P2S5·1P2S3 (mol%) glass-ceramic as solid electrolyte operated under the high current density of 12.7 mA cm− 2 at the high temperature of 100 °C. The cell showed an excellent cyclability of over 700 cycles without capacity loss.  相似文献   

19.
Glass-ceramics have been derived from 4.5MgO(45−x)CaO34SiO216P2O50.5CaF2xFe2O3 (x=5, 10, 15, 20 wt%) glasses by heat treatment. Room temperature electron paramagnetic resonance (EPR) spectra and temperature-dependent magnetic susceptibility (χ) of the glass-ceramics have been obtained. The EPR absorption line centered at g≈4.3 disappeared at higher concentrations of iron oxide. The intensity and line width of the EPR absorption line centered at g≈2.1 increased as the iron oxide concentration was increased. Temperature-dependent magnetization of samples with low iron oxide content revealed ferrimagnetic as well as paramagnetic contributions. Information about the structural changes involving iron ions, their valence state and the type of magnetic interactions between the Fe ions as a function of composition was obtained using EPR and χ studies.  相似文献   

20.
The results of molecular beam Fourier transform microwave (FTMW) investigations of the van der Waals complexes of difluoromethane with 1,1-difluoroethene (DFE) and trifluoroethene (TFE) are reported. The rotational parameters of the complexes have been interpreted in terms of a Cs geometry with the two H or F atoms of CH2F2 lying out of the σv symmetry plane of the complexes. The complexes are bound by three hydrogen bonds, of which one is the stronger C-F?H-C type, and two are the weaker C-H?F?H-C or C-F?H?F-C types for DFM-DFE or DFM-TFE, respectively. The most notable difference in the two complexes is that the out of plane atoms are two hydrogens for DFM-DFE, but are two fluorines for DFM-TFE. Additional information on the structure and hydrogen bonding has been obtained from ab initio calculations.  相似文献   

设为首页 | 免责声明 | 关于勤云 | 加入收藏

Copyright©北京勤云科技发展有限公司  京ICP备09084417号