Raman spectra of TiO2-II,TiO2-III,SnO2, and GeO2 at high pressure

Raman spectra of the orthorhombic (II) and high pressure (III) phases of titanium dioxide at pressures to 372 kbar and effects of temperature and hydrostatic pressure on Raman spectra of the tetagonal cassiterite-like phases of TiO₂, GeO₂ and SnO₂ are described. At room temperature, the TiO₂ II–III transition is sluggish, and metastable coexistence was observed from 200 to 300 kbar. The Raman spectra of TiO₂-III imply that its primitive cell contains at least four formula units; however, the structure could not be established from the Raman spectra and available powder X-ray diffraction patterns.The temperature and pressure dependences of the spectrum of the tetragonal MO₂ phases together with bulk moduli and thermal expansion data were used to evaluate the pure-volume and pure-temperature contributions to the isobaric temperature dependence of the Raman frequencies. Large anharmonicities in TiO₂ are attributed to hybridization of the oxygen p states with the d states of the Ti ion. GeO₂, where p-electron bonding is involved, is much less Enharmonic.     

High pressure Raman spectroscopy of spinel-type ferrite ZnFe2O4

An in-situ Raman spectroscopic study was conducted to explore the pressure induced phase transformation of spinel-type ferrite ZnFe₂O₄. Results indicate that ferrite ZnFe₂O₄ initially transforms to an orthorhombic structure phase (CaFe₂O₄-polymorph) at a pressure of 24.6 GPa. Such a phase transformation is complete at 34.2 GPa, and continuously remains stable to the peak pressure of 61.9 GPa. The coexistence of the two phases over a wide range of pressure implies a sluggish mechanism upon the spinel-to-orthorhombic phase transition. Upon release of pressure, the high pressure ZnFe₂O₄ polymorph is quenchable at ambient conditions.     

High-pressure phase transitions of Mg2Ge and Mg2Sn: First-principles calculations

Phase transitions of the anti-fluorite compounds Mg₂Ge and Mg₂Sn under high pressure were investigated using the first-principles plane-wave method within the pseudopotential and generalized gradient approximations. The calculated results show that Mg₂Ge and Mg₂Sn undergo two first-order phase transitions at high pressure and the sequence of the pressure-induced phase transitions is from the anti-fluorite to the anti-cotunnite, and then to the Ni₂In-type structure. The high pressure behaviors of Mg₂Ge and Mg₂Sn are similar to Mg₂Si and the isostructural alkali-metal oxide Li₂O. Moreover, the electronic and optical properties of both the anti-fluorite and the high-pressure phases are presented.     

过渡金属化合物OsB2与OsO2低压缩性的第一性原理计算研究

利用基于密度泛函理论平面波赝势法的第一性原理计算, 研究了过渡金属化合物OsB₂和OsO₂的金红石相、黄铁矿相与萤石相三种结构在高压下的状态方程和结构特性以及OsO₂可能的高压相变.理论计算结果支持OsB₂与OsO₂的萤石相是潜在超低可压缩性的硬性材料.同时，也分析了它们的电子结构，力求理解大体变模量和高硬度的微观机制.结果表明，可以利用过渡金属高的价电子浓度，掺入硼、氧、碳、氮等轻的元素形成强的 关键词： 过渡金属化合物 密度泛函理论 低压缩性 高压     

(Fe0.1Co0.55Ni0.35)78Si8B14金属玻璃的晶化过程及压力的影响(Ⅰ)——晶化时的相析出过程

用X射线衍射法研究了(Fe_0.1Co_0.55Ni_0.35)₇₈Si₈B₁₄金属玻璃在常压下及20kbar高压下晶化过程中的析出相及析出过程。结果表明在上述压力下晶化过程都分成两个阶段,分别对应于初级晶化和共晶晶化。在常压下,初级晶化时析出fcc-Co晶体,而共晶晶化对应着Ni₃₁Si₁₂和(FeCoNi)₃(SiB)相的析出。随着回火温度的增高或时间的延长,(FeCoNi)₃(SiB)相逐渐转变为(FeCoNi)₂₃B₆相。20kbar高压下的晶化析出过程与常压下不同的是:提高了晶化温度,在共晶晶化阶段出现了Co₂B相。此外,压力还阻止(FeCoNi)₂₃B₆相的形成。从热力学和动力学的角度讨论了压力对金属玻璃晶化过程的影响。 关键词：     

High pressure Raman spectroscopic study of spinel MgCr2O4

An in situ Raman spectroscopic study was conducted to investigate the pressure induced phase transformation of MgCr₂O₄ spinel up to pressures of 76.4 GPa. Results indicate that MgCr₂O₄ spinel undergoes a phase transformation to the CaFe₂O₄ (or CaTi₂O₄) structure at 14.2 GPa, and this transition is complete at 30.1 GPa. The coexistence of two phases over a wide range of pressure implies a sluggish transition mechanism. No evidence was observed to support the pressure-induced dissociation of MgCr₂O₄ at 5.7-18.8 GPa, predicted by the theoretical simulation. This high pressure MgCr₂O₄ polymorphism remains stable upon release of pressure, but at ambient conditions, it transforms to the spinel phase.     

Microstructural, dielectric and spectroscopic properties of Li2O-Nb2O5-ZrO2-SiO2 glass system crystallized with V2O5

Li₂O-Nb₂O₅-ZrO₂-SiO₂ glasses mixed with different concentrations of V₂O₅ were crystallized. The samples were characterized by XRD, SEM and DTA techniques. The SEM pictures indicated that the samples contain well defined and randomly distributed crystal grains. The X-ray diffraction studies have revealed the presence of several crystalline phases in these samples. Optical absorption, ESR and photoluminescence spectral studies on these samples have indicated that a considerable proportion of vanadium ions do exist in V⁴⁺ state in addition to V⁵⁺ state and the redox ratio seems to be increasing with increase in the concentration of crystallizing agent V₂O₅. The infrared spectral studies have pointed out the existence of conventional SiO₄, ZrO₄, NbO₆, VO structural units in the glass ceramic network. The study of dielectric properties suggested a decrease in the insulating character of the glass ceramics with increase in the crystallizing agent. A.C. conductivity in the high temperature region seems to be connected mainly with the polarons involved in the process of transfer from V⁴⁺↔V⁵⁺ ions.     

Origin of anisotropy for the HDDR Nd13.5Fe79.5B7 magnetic powders

For the HDDR Nd_13.5Fe_79.5B₇ magnetic powders, effects of disproportionation time and hydrogen pressure on the anisotropy were studied during the slow desorption stage. Studies showed that shorter disproportionation times caused the magnetic powders displaying higher anisotropy. With increasing disproportionation times, the degree of crystallographic alignment decreased. This in turn caused a drop in remanence and anisotropic character. Longer disporportionation times have also been correlated to a change in disproportionated microstructure from lamella to columnar. XRD (X-Ray Diffraction) studies showed that except NdH₂,α-Fe and Fe₂B, no other phases were included in the disproportionation mixture. This elucidated that the strong anisotropy is only related to a lamella disproportionation microstructure, which corresponds to a short disproportionation times. The lamella disproportionation microstructure may remain or inherit the alignment of original Nd₂Fe₁₄B grain, and may also be related to the alignment of the newly formed Nd₂Fe₁₄B grain. Thus, the anisotropic formation mechanism of ternary magnetic powders accords with “anisotropy-mediating phase” model. If the disproportionation mixture were carried out an optimum hydrogen pressure treatment during the HDDR process, the degree of crystallographic alignment can be further enhanced.     

Features of the local structural disorder in Li2O-CaF2-P2O5 glass-ceramics with Cr2O3 as nucleating agent

Li₂O-CaF₂-P₂O₅ glasses mixed with different concentrations of Cr₂O₃ (ranging from 0 to 1.0 mol%) were crystallized. The samples were characterized by X-ray diffraction, scanning electron microscopy, energy dispersive spectroscopy (EDS), differential thermal analysis and conventional spectroscopic techniques. The X-ray diffraction and scanning electron microscopic studies reveal the presence of lithium phosphate, calcium phosphate and chromium phosphate (complexes of Cr³⁺, Cr⁵⁺ and Cr⁶⁺ ions) crystal phases. The study on DTA suggests that the crystallization is predominantly due to the surface crystallization when the concentration of nucleating agent Cr₂O₃ is around 0.8 mol%. The IR and Raman spectral studies of these samples indicate that the sample crystallized with 0.8 mol% Cr₂O₃ is more compact and possesses high rigidity due to the presence of chromium ions largely in tetrahedral positions.     

Synthesis and magnetic properties of hard/soft SrFe12O19/Ni0.7Zn0.3Fe2O4 nanocomposite magnets

Magnetic nanocomposite SrFe₁₂O₁₉/Ni_0.7Zn_0.3Fe₂O₄ powders with different weight fractions of the Ni_0.7Zn_0.3Fe₂O₄ soft ferrite were synthesized by a combination of the sol–gel self-propagation and glyoxilate precursor methods. The results of magnetic measurements revealed the higher Mr/Ms ratio for the nanocomposites than that for the single phase SrFe₁₂O₁₉ which proves the existence of the intergrain exchange coupling between hard and soft magnetic phases with the exchange spring behavior. The highest Mr/Ms ratio of 0.63 was obtained in the composite consisting of 80 wt% of SrFe₁₂O₁₉ and 20 wt% Ni_0.7Zn_0.3Fe₂O₄. The microstructural studies of this sample exhibited the average dimensions of hard and soft phases about 20 nm and 15 nm, respectively which are small enough for strong exchange coupling according to the theoretical studies. The variations of the reduced remanence (Mr/Ms) with increasing the weight fraction of the soft phase could be also explained by the role of the exchange and dipolar interactions in tuning the magnetic properties of the nanocomposites.     

Pressure-induced ferroelectricity in {N(CH3)4}2FeCl4

The hydrostatic pressure effect on the stability of phases I, II, III and IV in {N(CH₃)₄}₂FeCl₄ was studied by dielectric and DTA measurements in the temperature and pressure region of -40-30°C and 0–2 kbar. The pressure-induced new phase V found at applied pressures between 0.3 and 1.0 kbar was confirmed to be ferroelectric by the observation of D-E hysteresis loop. These results are compared with those of {N(CH₃)₄}₂XCl₄ (X: Co, Zn and Mn) in this paper.     

Vibrational study of phase transitions and conductivity mechanism in H3OUO2PO4.3 H2O (HUP)

Infrared and Raman spectra of polycrystalline H₃OUO₂PO₄.3 H₂O (HUP) have been envestigated at various temperatures between 50 K and 300 K. The most temperature sensitive bands corresponding to PO₄ and H₂O librations, U-OPO₃ stretching and OH stretching vibrations indicate four different phases of HUP and allow to propose a phase transition mechanism from a quasiliquid state of protonated species in R.T. phase to a fully ordered crystal below 100 K. Protonic conductivity mechanism of room and low temperature phases is discussed.     

Effect of pressure on the electrical resistivity of bulk Ge20Te80 glass

An irreversible pressure induced semiconductor-to-metal transition in bulk Ge₂₀Te₈₀ glass is observed at about 5 GPa pressure. The high pressure phase has a face centered cubic structure with a lattice constant 6.42 A° as deduced by X-ray diffraction studies on the pressure quenched samples. The temperature and pressure dependence of the electrical resistivity confirms the observed transition to be a semiconductor-to-metal transition. The temperature dependence of thermo electric power is also reported.     

Growth and thermal stability of epitaxial BiFeO3 thin films

We have investigated the oxygen pressure and the temperature dependence on BiFeO₃ thin films deposited on SrTiO₃ substrates by pulsed laser deposition. Reflection high energy electron diffraction (RHEED), atomic force microscopy (AFM) and X-ray diffraction measurements indicate that high-quality epitaxial thin films are obtained for and T=650 °C. Outside of this pressure-temperature window, parasitic peaks attributed to β-Bi₂O₃ appear. We find an increase of the out-of-plane lattice parameter with oxygen pressure that we ascribe to Bi-deficiency due to its high volatility at low pressure. Ex-situ anneals have been performed and results show that as-grown single-phase BiFeO₃ thin films degrade after annealing, whereas as-grown BiFeO₃ containing impurity phases evolve toward a single-phase structure. These experiments demonstrate that parasitic phases can stabilize compounds which are usually unstable in air at elevated temperatures.     

Pressure-induced structural phase transition in ReO3

We have investigated the pressure-induced structural phase transition in ReO₃ by neutron diffraction on a single crystal. We collected neutron diffraction intensities from the ambient and high pressure phases at P=7 kbar and refined the crystal structures. We have determined the stability of the high pressure phase as a function temperature down to T=2 K and have constructed the (P-T) phase diagram. The critical pressure is P_c=5.2 kbar at T=300 K and decreases almost linearly with decreasing temperature to become P_c=2.5 kbar at T=50 K. The phase transition is driven by the softening of the M₃ phonon mode. The high pressure phase is formed by the rigid rotation of almost undistorted ReO₆ octahedra and the Re-O-Re angle deviates from 180°. We do not see any evidence for the existence of the tetragonal (P4/mbm) intermediate pressure phase reported earlier.     

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.     

On the phase diagram of the [(CH3)4N]2ZnCl4−xBrx system

The [(CH₃)₄N]₂ZnCl_4?xBr_x system shows a complete miscibility with a common Pnma high temperature phase. The (x,T)-phase diagram (x given by [(CH₃)₄N]₂ZnCl_4?xBr_x) was determined by means of DSC- and X-ray measurements. In the vicinity of both end members an analogy to the (P,T)-phase diagrams of pure compounds is observed with an increasing x corresponding to an increase of pressure. In the intermediate part of the system other effects become important leading to large coexistence regions of different phases. This is evidently due to the simultaneous occupancy of anion tetrahedra by different numbers of Cl and Br atoms. The non-equivalent ZnX₄-tetrahedra of solid solutions do not suppress the occurence of the incommensurate phases. They exist between x = 0 to ?3 and are interrupted by a supposedly ferroelectric phase occuring up to x ?0.4. Around x ? 3 a triple point occurs the nature of which needs a closer elucidation.     

高压下BeP2N4结构相变和电子结构的第一性原理计算

 基于密度泛函理论的第一性原理方法,计算了硅铍石型和尖晶石型结构BeP₂N₄的总能量随体积的变化关系。利用Brich-Murnaghan状态方程,通过能量和体积拟合,得到了2种结构的体变模量及其对压强的一阶导数。在压力作用下,BeP₂N₄的相变是从硅铍石型结构（空间群R-3,No.148）转变到尖晶石型结构（空间群Fd-3m,No.227）,计算出的相变点与其它理论值符合得非常好。同时计算了BeP₂N₄的相对晶格常数a/a₀和相对体积V/V₀的压缩率,在低压下发现,尖晶石结构BeP₂N₄的压缩率接近金刚石,进一步计算了不同压力下的体弹模量B_H、剪切模量G_H、B_H/G_H和杨氏模量E。此外,对两种结构的BeP₂N₄的电子态密度和带隙随压强的变化关系进行了计算和分析。结果表明：在压力作用下,上价带顶向费米能级移动,并有一定的展宽。Be—N、P—N键缩短,电子转移增加,导致电荷发生重新分布。     

Magnetoelectric properties of particulate and bi-layer PMN-PT/CoFe2O4 composites

Our studies comprise electrical dielectric and magnetoelectric properties of CoFe₂O₄ (CFO) and Pb(Mg_1/3Nb_2/3)_0.67Ti_0.33O₃ [PMN-PT] magnetoelectric composites. The individual phases were prepared by conventional ceramic method. The particulate composites of ferrite and ferroelectric phases were prepared in ferroelectric rich region. Presence of both the phases in the composites was confirmed using X-ray diffraction techniques. The scanning electron microscopic images recorded in backscattered mode were used to study the microstructure of composites. Lattice constant, dielectric constant, electrical resistivity, ferroelectric, and magnetic properties of individual as well as particulate composites were studied. Further the bi-layer composites were made using the discs obtained from the powders of individual phases where hot press technique was employed to obtain disc of individual phases. CFO phase used in bi-layer composites was obtained using chemical co-precipitation technique. Magnetoelectric (ME) measurements were carried out on both, particulate and layered magnetoelectric composites. Comparison of ME signal obtained from particulate and layered composites revealed that the layered composites gives superior magnetoelectric signal. ME data obtained for layered composites show good agreement with the theoretical model.