金红石高温高压相变的Raman光谱特征

以Ar作压力介质,在准静水压力条件下,利用激光加热DAC技术和显微Raman光谱原位测试技术,在0～35 GPa压力范围开展金红石的高温高压相变研究。在室温条件下,金红石结构TiO₂于13.4 GPa开始转变成斜锆石相,于21 GPa时转变完全,并直到35 GPa时斜锆石相稳定存在。在压力分别为29.4和35.0 GPa时,用YAG激光器发出的波长为1.064 μm的红外激光束扫描加热样品,TiO₂斜锆石高压相转变成另一Pbca结构高压相。卸压时,Pbca相于26.3 GPa时转变成斜锆石相。斜锆石相转变成Pbca相需要加热才能发生,而卸压时却在较小的压力区间即迅速转变完全,两相转变压力边界在28 GPa左右。进一步卸压,斜锆石相直到11 GPa仍稳定,在7.6 GPa时斜锆石相与α-PbO2相两相共存,5 GPa时完全转变成α-PbO₂相,并直到常压该相以亚稳定态存在。     

Thermoelectric properties of high-pressure silicon phases

The thermo emf in Czochralski-grown silicon single crystals (Cz-Si) was experimentally studied in a range of pressures up to 20 GPa. The pressure dependences revealed phase transitions in the metallic phase of silicon, which passed from tetragonal to orthorhombic and then to hexagonal lattice. The high-pressure silicon phases, as well as the metallic high-pressure phases in A_NB_8?N semiconductors, possess conductivity of the hole type. As the pressure decreases, the emf behavior reveals transitions to the metastable phases Si-XII and Si-III. Preliminary thermobaric treatment of the samples at a pressure of up to 1.5 GPa and a temperature of T=50–650°C influences the thermoelectric properties of Cz-Si at high pressures.     

Pressure-induced phase transition in tysonite LaF3

We have studied the high-pressure phase stability of LaF₃ using full-potential linear augmented plane wave method. We have shown that experimentally observed orthorhombic phase is less stable compared to the theoretically predicted tetragonal structure above 25 GPa pressure. The structural transition is mainly due to the steric repulsion of ions and electrons to higher pressures.     

High Pressure and High Temperature Studies on Manganese Oxides

High pressure and high temperature quench experiments on f -MnO 2 , Mn 2 O 3 and sol gel derived manganese oxides have been carried out to identify any new phases to which the materials may transform under high pressure and high temperature conditions. Results of ESR, DTA and TGA investigations on sol gel derived manganese oxide have shown it to be hausmannite Mn 3 O 4 , instead of n -Mn 2 O 3 as reported earlier in the literature. The sol gel derived manganese oxide transforms to n -Mn 2 O 3 when heated above 700°C. Sol gel derived Mn 3 O 4 , when quenched from 5 GPa and temperature range 800-1200°C, gives a mixture of Mn 3 O 4 (hausmannite) and a phase having CaMn 2 O 4 (marokite)-type structure. f -MnO 2 undergoes partial amorphization when pressure-quenched from 8 GPa at room temperature. The high pressure and high temperature quench experiments up to 5 GPa and 700°C showed that the decomposition temperature of f -MnO 2 increases with pressure. The new phase reported by Liu (1976) from diamond-anvil cell (DAC) experiments on pyrolusite MnO 2 is identified to be a low-density polymorph f -MnO 2 . This unusual result of formation of low-density f -MnO 2 , having an open structure at high pressure and high temperature, is probably due to quenching of a non-equilibrium phase in Liu's (1976) laser-heated DAC experiment.     

Structural transition of BaF2 nanocrystals under high pressure

The structural transition of BaF₂ nanocrystals is studied by in situ high pressure synchrotron radiation X-ray diffraction measurements up to about 21.2 GPa at ambient temperature. Two phase transformations were observed at 5.8 and 14.4 GPa, and the two high pressure phases are identified as orthorhombic (Pnma) phase and hexagonal (P6₃/mmc) phase by Rietveld refinement. Upon decompression, the α-PbCl₂-type metastable phase is retained when the pressure is released. Two phase transformations of the BaF₂ nanocrystals are higher than that in bulk BaF₂. It is proposed that the size effects are found to influence the BaF₂ nanocrystals high-pressure behaviors and the surface energy plays a significant role in the structural stability.     

Pressure-induced phase transition in ThO2 and PuO2

Abstract

Thorium and plutonium dioxides were studied under pressure by the energy dispersive X-ray diffraction method. A double conical slit assembly was used to collect simultaneously the diffracted radiation at five and seven degrees.

ThO₂ undergoes a phase transformation at 40 GPa. The high-pressure phase remains stable up to 55 GPa, the highest pressure reached in the experiment. For PuO₂, a structural transformation occurs near 39 GPa. The observed high-pressure phases of ThO₂ and PuO₂ exhibit similar diffraction spectra. Like for some other fluorite type compounds, the ThO₂ and PuO₂ high-pressure phase has been indexed in the PbCl₂-type structure. The bulk modulus has been calculated as B₀= 262 GPa with a pressure derivative of B₀' = 6.7 for ThO₂ and as B₀ = 379 GPa with B₀' = 2.4 for PuO₂. The volume decrease at the transition is 12% for PuO₂ and 8% for ThO₂.     

Fabrication and analysis of the structural phase transition of ZrO2 nanoparticles using modified facile sol–gel route

The synthesis of zirconia nanoparticles is achieved through a modified facile sol–gel route. The as-prepared gel is analyzed thermally using TGA and DTA techniques to spot the crystallization process of zirconia nanoparticles. The prepared gel is then annealed at different temperatures and the structure was found to change between tetragonal and monoclinic crystal systems. The first stable tetragonal phase is achieved after annealing for 2?h at 400°C. The annealed powders between 600°C and 800°C demonstrate mixed tetragonal/monoclinic phases. Annealing at 1000°C and higher temperatures up to 1200°C resulted in pure monoclinic phase. Cubic phase was not detected within the annealing temperature range in this study. The elemental analysis of the annealed powder confirmed the formation of zirconia nanoparticles with the chemical formula ZrO₂. The FTIR spectra of the annealed samples introduced a variation in the vibrational bands especially around the phase transition temperature. HR-TEM images reported the formation of nano-zirconia crystals with apparently large particle sizes. The optical energy gap of zirconia nanoparticles is investigated and determined.     

铌酸钾钠基无铅压电陶瓷的X射线衍射与相变分析

分析了斜方相、四方相铌酸钾钠基无铅压电陶瓷材料的结构和X射线衍射图谱的特点. 对于铌酸钾钠基压电材料斜方相结构, 从构成晶胞的一个单斜原胞进行分析, 计算出X射线衍射谱上每个衍射角附近的衍射峰数目和相对强度. 提出了2θ在20°—60°范围内根据(1 0 2)衍射峰(52°附近)和(1 2 1)衍射峰(57°附近)劈裂的数目区分斜方和四方相的新方法. 对于多晶陶瓷粉末, 可以更简便的由22°(或45°)附近前后峰的相对高低来判断斜方、四方相. 关键词： 铌酸钾钠 无铅压电陶瓷 X射线衍射 相变     

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.     

Raman Spectroscopy of Nanopolycrystalline Diamond Produced from Shungite at 15 GPa and 1600°C

JETP Letters - It has been demonstrated that the high-pressure (15 GPa) treatment of shungite samples (Karelia, Russia) at a temperature of 1600°C for 2 h transforms them to diamond. The...     

A new high-pressure phase of ZnSe with B9-type structure

High-pressure and high-temperature behavior of ZnSe was investigated by energy dispersive X-ray diffraction method up to 14 GPa and 800°C. A new high-pressure phase with B9 (HgS)-type structure is found near the B3-B1 phase boundary at room temperature, as predicted by an ab-initio calculation. The property and observed pressure region of the B9-type phase are in good agreement with the ab-initio calculation. At high-temperature condition above 300°C, only the direct transitions are observed between the B3 and B1 phases. The B3-B1 phase boundary is also determined to be P (GPa)=12.21−0.0039T (°C) for the temperature range between 300 and 800°C.     

Electrotransport and magnetic properties of Cr-GaSb phases synthesized under high pressure

The electrotransport and magnetic properties of new phases in the Cr-GaSb system were studied. The samples were prepared by high-pressure (P=6-8 GPa), high-temperature treatment and identified by x-ray diffraction and scanning electron microscopy. One of the CrGa(2)Sb(2) phases with an orthorhombic structure Iba2 has a combination of ferromagnetic and semiconductor properties and is potentially promising for spintronic applications. Another high-temperature phase is paramagnetic and identified as tetragonal I4/mcm.     

First principles study of structural,magnetic and electronic properties of half-metallic CrO<Subscript>2</Subscript> under pressure

A first-principles tight-binding linear muffin tin orbital (TB-LMTO) method within the local-density approximation is used to calculate the total energy, lattice parameter, bulk modulus, magnetic moment, density of states and energy band structures of half-metallic CrO₂ at ambient as well as at high pressure. The magnetic and structural stabilities are determined from the total energy calculations. From the present study we predict a magnetic transition from ferromagnetic (FM) state to a non-magnetic (NM) state at 65 GPa, which is of second order in nature. We also observe from our calculations that CrO₂ is more stable in tetragonal phase (rutile-type) at ambient conditions and undergoes a transition to an orthorhombic structure (CaCl₂-type) at 9.6 GPa, which is in good agreement with the experimental results. We predict a second structural phase transition from CaCl₂- to fluorite-type structure at 89.6 GPa with a volume collapse of 7.3%, which is yet to be confirmed experimentally. Interestingly, CrO₂ shows half metallicity under ambient conditions. After the first structural phase transition from tetragonal to orthorhombic, half metallicity has been retained in CrO₂ and it vanishes at a pressure of 41.6 GPa. Ferromagnetism is quenched at a pressure of 65 GPa.     

In situ high-pressure X-ray diffraction experiments and ab initio calculations of Co2P

In situ high-pressure experiments of Co2P are carried out by means of angle dispersive X-ray diffraction with diamond anvil cell technique. No phase transition is observed in the present pressure range up to 15 GPa at room temperature, even at high temperature and 15 GPa. Results of compression for Co2P are well presented by the second-order Birch-Murnaghan equation of state with V0 = 130.99(2)3 (1=0.1 nm) and K0 = 160(3) GPa. Axial compressibilities are described by compressional modulus of the axis: Ka = 123(2) GPa, Kb = 167(8) GPa and Kc = 220(7) GPa. Theoretical calculations further support the experimental results and indicate that C23-type Co2P is stable at high pressure compared with the C22-type phase.     

斜锆石（ZrO2）高温高压相变的Raman光谱研究

 以Ar作压力介质，在0~23 GPa压力范围内，利用金刚石压腔装置（DAC）和激光加温技术，采用显微拉曼光谱进行原位测试，对处于准静水压力条件下的斜锆石开展高温高压相变研究。研究结果表明：室温下斜锆石ZrO₂于3.4 GPa时开始发生相变，到10.4 GPa时其明显转变成一个空间群为Pbca的斜方相。此新相随着压力升高，直到15.3 GPa，仍稳定存在。通过研究，首次获得了Pbca相的拉曼谱图。随后在15.3 GPa压力下进行了激光加温后淬火，结果发现，加热前的Pbca相又转变成了空间群为Pnam的PbCl₂结构类型的高压相，该相直到实验最高压力23 GPa仍稳定存在。     

First-principles characterisation of the pressure-dependent elastic anisotropy of SnO2 polymorphs

Using DFT calculations, this study investigates the pressure-dependent variations of elastic anisotropy in the following SnO₂ phases: rutile-type (tetragonal; P4₂/mnm), CaCl₂-type (orthorhombic; Pnnm)-, α-PbO₂-type (orthorhombic; Pbcn)- and fluorite-type (cubic; Fm-3m). Experimentally, these polymorphs undergo sequential structural transitions from rutile-type → CaCl₂-type → α-PbO₂-type → fluorite-type with increasing pressure at 11.35, 14.69 and 58.22 GPa, respectively. We estimate the shear anisotropy (A₁ and A₃) on {1?0?0} and {0?0?1} crystallographic planes of the tetragonal phase and (A₁, A₂ and A₃) on {1?0?0}, {0?1?0} and {0?0?1} crystallographic planes of the orthorhombic phases. The rutile-type phase shows strongest shear anisotropy on the {0?0?1} planes (A₂ > 4.8), and the degree of anisotropy increases nonlinearly with pressure. In contrast, the anisotropy is almost absent on the {1?0?0} planes (ie A₁ ~ 1) irrespective of the pressure. The CaCl₂-type phase exhibits similar shear anisotropy behaviour preferentially on {0?0?1} (A₃ > 5), while A₁ and A₂ remain close to 1. The α-PbO₂-type phase shows strikingly different elastic anisotropy characterised by a reversal in anisotropy (A₃ > 1 to < 1) with increasing pressure at a threshold value of 38 GPa. We provide electronic density of states and atomic configuration to account for this pressure-dependent reversal in shear anisotropy. Our study also analyses the directional Young’s moduli for the tetragonal and orthorhombic phases as a function of pressure. Finally, we estimate the band gaps of these four SnO₂ phases as a function of pressure which are in agreement with the previous results.     

紫外拉曼光谱研究钇掺杂的氧化锆体系表面相变

本文用紫外拉曼 ,近可见拉曼光谱和XRD检测了不同焙烧温度下Y2 O3-ZrO2 的相变过程。紫外拉曼光谱对样品表面相变极其灵敏 ,而近可见拉曼光谱和XRD提供的主要是体相和表面的混合信息。在紫外拉曼谱图中 ,只观察到单斜相的谱峰 ,没有明显的四方晶相的信号 ,这表明样品的表面主要是单斜晶相。然而 ,XRD和近可见拉曼光谱的结果显示Y2 O3-ZrO2 体相是四方晶相。焙烧温度超过 40 0°时 ,紫外拉曼 ,近可见拉曼和XRD晶之间明显不同的结果表明Y2 O3-ZrO2 在表面区四方相很易转变为单斜相 ,体相中的四方相由于钇的添加而稳定存在。根据紫外拉曼和XRD结果 ,当升高温度时 ,在样品的表面形成一单斜相层 ,体相钇稳定的四方相 ,且由于Y2 O3的存在抑制了单斜相进一步向体相发展。     

A synchrotron study of high-pressure transformations in TiH0·74

Abstract

The crystal structure of the TiH_0·74 alloy was studied by the energy dispersive X-ray diffraction technique in the pressure range to 30·5 GPa at temperatures to 630 K. A phase transformation to the (η + ω) two-phase state was found to occur above 7 GPa at room temperature, then (η+ω)-TiH_0·74 remained stable up to P=30·5 GPa. Another phase transformation resulting in a single-phase state, ζ-TiH_0·74, was found to occur upon heating (η+ω)-TiH_0·74 above T ? 560 K. Both high-pressure phases, η and ζ, were indexed on the basis of the tetragonal sublattices of the Ti atoms with nearly the same specific volumes. It is assumed from the relation of the specific volumes that the hydrogen atoms occupy the tetrahedral interstices in the ζ-phase and the octahedral interstices in the η-phase.     

Investigation of phase transformations in Cu1.8S-Cu2S thin films by kinematic electron diffraction

The phase transformations of the compounds Cu_1.8S-Cu₂S have been investigated. All the changes in the sample due to a thermal treatment up to 300°C were continuously photographed by the kinematic electron diffraction method. It was shown that, depending on experiment conditions, the low-temperature ordered phase of digenite Cu_Z-XS transforms completely or partly into the hexagonal modification of chalcocite Cu₂S, which becomes ordered in its low-temperature orthorhombic modification. At temperatures from room to 100°C the orthorhombic chalcocite coexists with the low-temperature digenite, in a range from 100°C to 300°C the high-temperature hexagonal chalcocite coexists with the low-temperature digenite and at 300°C the sample contains a mixture of the hexagonal chalcocite and the f.c.c.-phase of digenite. Djurleite and other phases with intermediate compositions are not observed in these experiments.