Structural Phase Transformations of ZnS Nanocrystalline Under High Pressure

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.     

Isostructural Phase Transition of TiN under High Pressure

In situ high-pressure energy dispersive x-ray diffraction experiments on polycrystalline powder TiN with NaCltype structure have been conducted with the pressure up to 30.1 GPa by using a diamond anvil cell instrument with synchrotron radiation at room temperature. The experimental results suggest that an isostructural phase transition might exist at about 7 GPa as revealed by the discontinuity of V/Vo with pressure.     

Phase Transition and EOS of Marmatite （Zn0.76Fe0.23S） up to 623K and 17 GPa

In situ energy dispersive x-ray diffraction for natural marmatite （Zn0.76Fe0.23S） is performed up to 17. 7 GPa and 623 K. It is fit, ted by the Birch-Murnaghan equation of state （EOS） that Ko and α0 for marmatite are 85（3）GPa and 0.79（16）＊10^-4 K^-1, respectively. Fe^2＋ isomorphic replacing to Zn^2＋ in natural crystal is responsible for high bulk modulus and thermal expansivity of marmatite. Temperature derivative of bulk modulus （OK/OT）p for marmatite is fitted to be -0.044（23） GPaK^-1. The unambiguous B3-B1 phase boundaries for marmatite are determined to be Pupper（GPa）= 15.50 - 0.016T（℃） and Plower （GPa）=9.94-0.012T（℃） at 300-623K.     

Structural Stability of CaCuMn6O12 under High Pressure and Low Temperature

In situ high pressure energy-dispersive x-ray synchrotron radiation diffraction and resistance experiments are carried out on CaCuMn6O12. Its crystal structure is stable in the measured pressure range. The equation of state of CaCuMn6O12 is obtained from the V/Vo - P relationship （V and Vo are the volumes at pressure P and at atmosphere）. The bulk modulus Bo is calculated based on the Birch-Murnaghan equation. Low temperature x-ray diffraction shows no phase transition occurring down to 160K.     

Formation of Co3O4 Nanotubes and the Magnetic Behaviour at Low Temperature

Tubular Co3O4 nanostructures were prepared from cobalt nanowires embedded in an anodic alumina template. The morphologies of nanowires/nanotubes were studied by transmission electron microscopy, and x-ray diffraction was used in the analysis of the nanostructures and phases. A possible formation mechanism of the process from nanowires to nanotubes is discussed. The vibrating sample magnetometer measurements show anomalous magnetic behaviour of the cobalt oxide nanotubes at low temperature.     

Atomic and Electronic Structures of Cd0.96Zn0.04Te（l10） Surface

X-Ray diffraction is used to analyse the lattice structure of Cdo.96Zno.o4 Te （CZT）, and the lattice constant is measured to be 0.647nm. The atomic structure of the clean CZT（110） surface obtained by Ar^＋ etching in vacuum is observed by low-energy electron diffraction, where no surface reconstruction is discovered. Angleresolved photoemission spectroscopy was used to characterize the surface state of the clean CZT （110） surface, by which we find a 1.5-eV-wide surface band with the peak at 0.9eV below the Fermi energy containing about 6.9 × 10^14 electrons/cm^2, approximately one electron per surface atom.     

Phase Transition and High Piezoactivity of Sb Doped （Na0.53K0.435Li0.035）Nb0.94Ta0.06 O3 Lead-Free Ceramics

Lead-free piezoelectric ceramics with the composition of （Na0.53K0.435Li0.035）Nb0.94Ta0.06 O3 （NKLNST） axe synthesized by a conventional solid-state sintering process. An MPB-like region between orthorhombic and pseudocubic phases is found in this system. The density, piezoelectric and dielectric properties axe enhanced greatly in this region. A composition （Na0.53K0.435Li0.035）（Nb0.94Ta0.06）O3 is found to have excellent electrical properties： d33 = 320pC/N, kp= 49% and kt =43%, as well as relatively low loss, tan δ=4.2%, and high relative density higher than 96%, which indicate that this ceramics is a promising lead-free piezoceramics replacing for lead zirconate titanate.     

High Solubility of Hetero-Valence Ion （Cu^2＋） for Reducing Phase Transition and Thermal Expansion of ZrV1.6P0.4O7

Large thermal expansion at room temperature and high phase transition temperature of ZrV2O7 limit its practical applications and are reduced by the high solubility of hetero-valence ion （Cu^2＋） on the basis of an equal-valence substitution of P^5＋ for V^5＋. The temperature-dependent Raman spectra show that Zr0.9CU0.1V1.6P0.4O6.9 maintains a normal parent cubic structure till 173 K and transforms to a 3 × 3 × 3 cubic superstructure below 173K. Temperature dependent x-ray diffraction patterns of Zr0.9CU0.1V1.6P0.4O6.9 show near zero and negative thermal expansion. High solubility of lower valence Cu^2＋ relates to an equal-valence substitution of smaller pH for P^5＋, which extends the bond angle of V（P）-O-V in ZrV1.6P0.4O7 close to 180°. The change of microstructure is considered to be responsible for reduced phase transition temperature and thermal expansion.     

Raman and X-Ray Investigation of Pyrope Garnet （Mg0.76Fe0.14Ca0.10）3Al2Si3O12 under High Pressure

The compressional behavlour of natural pyrope garnet is investigated by using angle-dlspersive synchrotron radiation x-ray diffraction and Raman spectroscopy in a diamond anvil cell at room temperature. The pressureinduced phase transition does not occur under given pressure. The equation of state of pyrope garnet is determined under pressure up to 25.3 GPa. The bulk modulus KTO is 199 GPa, with its first pressure derivative K′TO fixed to 4. The Raman spectra of pyrope garnet are studied. A new Raman peak nearly at 743 cm^-1 is observed in a bending vibration of the SiO4 tetrahedra frequency range at pressure of about 28 GPa. We suggest that the new Raman peak results from the lattice distortion of the SiO4 tetrahedra. All the Raman frequencies continuously increase with the increasing pressure. The average pressure derivative of the high frequency modes （650-1000 cm^-1） is larger than that of the low frequency （smaller than 650 cm^-1）. Based on these data, the mode Grǖneisen parameters for pyrope are obtained.     

顽火辉石的高温高压同步辐射研究

 采用同步辐射能量色散X射线衍射技术、激光加热技术和金刚石对顶砧（DAC）高压装置,在温度为2 000 K和压力为23 GPa的范围内,对采自地幔二辉橄榄岩中的顽火斜方辉石,进行了原位的高温高压能量色散X射线衍射（EDXRD）测量。实验结果表明：当压力为15.3 GPa、温度为1 600 K时（相当于地球内部410 km处的地震波不连续界面的温压环境）,顽火斜方辉石转变为橄榄石的β相——瓦兹利石（Wadsleyite）相;继续加温加压至2 000 K、23 GPa时（相当于地球内部670 km处的地震波不连续界面的温压环境）,顽火斜方辉石相变为钛铁矿（Ilmenite）结构和钙钛矿（Perovskite）结构的混和相。实验结果进一步证明,在地幔中存在的两个地震波不连续界面是由橄榄石、顽火斜方辉石等矿物的相变引起的。     

CaF2纳米晶粒结构相变行为的尺寸依赖性（英文）

利用原位高压同步辐射X射线衍射方法,对尺寸为11 nm的CaF₂纳米晶粒进行高压结构相变和压缩特性研究。当压力为12 GPa时,观察到由萤石结构向α-PbCl₂结构转变的一次相变,该相变压力点远高于体材料,但略低于粒径更小的CaF₂纳米晶体。相比体材料,纳米尺寸的CaF₂样品的体弹模量更大,说明其更难被压缩。当压力释放至常压时,11 nm的CaF₂纳米晶粒的α-PbCl₂型亚稳相结构被保留下来,相变不可逆。分析了影响11 nm CaF₂纳米晶粒独特高压行为的原因,判定尺寸效应为主要因素,该尺寸下较高的表面能导致结构稳定性增强和体积模量增加。     

Phase Transition and Phonon Spectrum of Zinc-Blende Structure ZnX （X = S,Se, Te）

Calculations have been performed to investigate the pressure-induced solid-solid phase transitions and the mechanical stability for three zinc-blende II-VI semiconductor compounds： ZnS, ZnSe, ZnTe by ab initio plane-wave pseudopotential density functional theory （DFT）. Using the generalized gradient approximation （GGA） for exchange and correlation in the scheme of Perdew-Wang 1991 （P Wgl ）, the ground state properties and equation of state are obtained, which are well consistent with the experimental data available and other calculations. On the basis of the forth-order Birch-Murnaghan equation of states, the transition pressures Pt are determined through the analysis of enthalpy variation with pressure. A linear-response approach is used to calculate the frequencies of the phonon dispersion. Finally, by the calculations of phonon frequencies, some thermodynamic properties such as the vibrational contribution to the Helmholtz free energy （F）, enthedpy （H）, entropy （S）, and the heat capacity （Cv ） are also successfully obtained.     

Relation between martensitic transformation temperature range and lattice distortion ratio of NiMnGaCoCu Heusler alloys

In order to study the relation between martensitic transformation temperature range AT （where AT is the difference between martensitic transformation start and finish temperature） and lattice distortion ratio （c/a） of martensitic transforma~ tion, a series of Ni46Mnz8_xGa22Co4Cux （x = 2-5） Heusler alloys is prepared by arc melting method. The vibration sample magnetometer （VSM） experiment results show that AT increases when x 〉 4 and decreases when x 〈 4 with x increasing, and the minimal AT （about 1 K） is found at x = 4. Ambient X-ray diffraction （XRD） results show that AT is proportional to c/a for non-modulated Ni46Mn28_xGa22Co4Cux （x = 2-5） martensites. The relation between AT and c/a is in agreement with the analysis result obtained from crystal lattice mismatch model. About 1000-ppm strain is found for the sample at x = 4 when heating temperature increases from 323 K to 324 K. These properties, which allow a modulation of AT and temperature-induced strain during martensitic transformation, suggest Ni46Mn24Ga22Co4Cu4 can be a promising actuator and sensor.     

Preparation and the physical properties of antiperovskite-type compounds Cd1_xInxNNi3（n〈x〈0.2）and Cd1-yCuyNNi3（0〈y〈0.2）

Two series of Cdl-xInxNNi3 （0 〈 x 〈 0.2） and Cd1_yCuyNNi3 （0 〈 y 〈 0.2） samples were prepared from CdO, In203, CuO, and nickel powders under NH3 atmosphere at 773 K. The structural and physical properties were investigated by means of X-ray powder diffraction temperature-dependent resistivity and magnetic measurements. X-ray powder diffraction results showed that the Cd1-xInxNNi3 and Cd1_yCuyNNi3 compounds have a typical antiperovskite structure, and the CdNNi3, Cd0.9In0.1NNi3, and Cd0.9Cu0.1NNia compounds show metallic temperature-dependent resistivity and exhibit a Fermi liquid behavior at low temperature. In contrast to the paramagnetism previously reported, the CdNNi3 sample exhibits very soft and weak ferromagnetism, and no superconductivity was found in the Cd1-xInxNNi3 and Cdl-yCuyNNi3 samples down to 2 K. Each sample exhibited very soft and weak ferromagnetism, and the temperature dependence of the magnetization of the Cd1-xInxNNi3 and Cd1_yCuyNNi3 samples can be well fitted to the combination of a Bloch term and a Curie-Weiss term.     

Synthesis,thermionic emission and magnetic properties of （NdxGdl-x）B6

Polycrystalline rare-earth hexaborides （NdxGdl-x）B6 （x = 0, 0.2, 0.6, 0.8, 1） were prepared by the reactive spark plasma sintering （SPS） method using mixed powder of GdH2, NdH2 and B. The effects of Nd doping on the crystal structure, the grain orientation, the thermionie emission and the magnetic properties of the hexaboride were investigated by X-ray diffraction, electron backscattered diffraction and magnetic measurements. It is found that all the samples sintered by the SPS method exhibit high densities （〉95%） and high values of Vickers hardness （2319 kg/mm2）. The values are much higher than those obtained in the traditional method. With the increase of Nd content,the thermionic emission current density increases from 11 to 16.30 A/cm2 and the magnetic phase transition temperature increases from 5.85 to 7.95 K. Thus, the SPS technique is a suitable method to synthesize the dense rare-earth hexaborides with excellent properties.     

Phase Transition and High Piezoactivity of Sr Doped Lead-Free （Na0.53K0.422Li0.048） （Nb0.89Sb0.06Wa0.05）O3 Ceramics

Lead-free piezoelectric ceramics （Na0.53K0.422Li0.048 ） （Nb0.89Sb0.06 Ta0.05 ）03 （NKLNST） ＋ x tool SrCO3 are prepared by conventional solid state sintering method. The specimens with pure perovskite structure show tetragonal phase at x 〈 0.01, and become pseudo-cubic phase at x 〉 0.02. A lattice parameter discontinuity is found in the specimens with 0.004 〈 x 〈 0.0075, along with a great improvement in piezoactivity. The 0.004 mol SrCO3 added NKLNST ceramics possesses outstanding performances of kp = 0.53, kt = 0.26, and d33=309 pC/N. Moreover, the Sr^2＋ modification inhibits the gra/n growth, decreases the Curie temperature, and induces a diffuse phase transition.     

Growth and optical properties of Cr~（3＋）-doped CdWO_4 single crystals

A high-quality Cr 3+:CdWO4 single crystal at a size of approximatelyΦ25×80 mm is grown using the Bridgman method with CdO,WO3,and Cr2O3 as raw materials and their molar ratio of 100:100:0.5.The temperature gradient of solid-liquid interface at growth is approximately 50?C/cm and the growth rate is 0.05 mm/h.The X-ray diffraction(XRD),absorption,excitation,and emission spectra of different parts of the as-grown and O2-annealed crystals are investigated.Two strong broad optical absorption bands of about 472 and 708 nm are observed,and they are associated with the transitions 4 A2→ 4 T1 and 4 A2→ 4 T2.The weak 4 T2→ 2 E transition(the R-line)at 632 nm is also observed.The crystal-field parameter Dq and the Racah parameters B and C are estimated to be 1 412.4,776.8,and 3 427.6 cm? 1,respectively,according to the absorption spectra and crystal-splitting theory.A broadband fluorescence at about 1 000 nm due to 4 T2→ 4 A2 transition is produced by exciting the samples at 675 nm.After being annealed in an O2 atmosphere,the crystals become more transparent,while the effective light absorption of Cr 3+ ions is evidently enhanced and the emission intensity is also strengthened due to the reduction of oxygen vacancies in the CdWO4 crystal after annealing.     

Polarized Micro-Raman Study of the Temperature-Induced Phase Transition In 0.67 Pb（Mg1/3Nb2/3）O3-0.33PbTiO3

Polarized Raman spectra of ferroelectric relaxor 0.67Pb（Mg1/3Nb2/3）O3-0.33PbTiO3 （0.67PMN-0.33PT） single crystal are systematically investigated in a wide temperature range from -196 to 600℃ by micro-Raman scattering technique. The results clearly reveal that there are two structural phase transitions in such composite ferroelectric relaxor： the rhombohedral-tetragonal （R- T） phase transition and the tetragonal-cubic （T- C） phase transition. The former occurs at about TR-T =34℃, corresponding to the vanishing of the soft A1 mode at 106cm^-1 recorded in the parallel polarization. The latter appears at about TT-C = 144℃, which can be verified with the vanishing of mode at 780cm^-1 measured in the crossed polarization.     

Crystal structures, phase relationships, and magnetic phase transitions of RsM4 compounds （R = rare earths, M = Si, Ge）

Our recent studies of the crystal structures, phase transitions, and magnetic properties of intermetallic compounds RsM4 （R = rare earths; M = Si, Ge） are reviewed briefly. First, crystal structures, phase relationships, and magnetic prop- erties of several 5：4 compounds, including Nd5 Si4-xGex, Pr5 Si4_xGex, Gds-xLaxGe4, La5 Si4, and Gd5 Sn4, are presented. In particular, the canted spin structures as well as the magnetic phase transitions in PrsSi2Ge2 and PrsGe4 investigated by neutron powder diffractions and small-angle neutron scattering are reviewed. Second, the crystal structures and magnetic properties of the most studied compounds Gds（Si,Ge）4 are summarized. The focus is on the parent compound GdsGe4, which is an amazing material exhibiting magnetic anisotropy, angular dependent spin-flop transition, metastable magnetic response, Griffiths-like phase, thermal effect under pulsed fields, antiferromagnetic and ferromagnetic resonances, pro- nounced effects of impurities, and high-field induced magnetic transitions.