Neutral to Ionic Pressure-Induced Phase Transition in the Mixed Stack Charge-Transfer Crystal ClMePD-DMeDCNQI

In the present work we report a spectroscopic investigation of the pressure-induced neutral to ionic (NI) phase transition in the mixed stack charge-transfer crystal 2-chloro-5-methyl-p-phenylenediamine 2,5-dimethyl-dicyanoquinonediimine, ClMePD-DMeDCNQI. The pressure induced phase change is studied by calculating, from spectroscopic data, the degree of ionicity, 𝜌 , which undergoes a continuous change as a function of pressure, reaching a value of about 0.65 at 3 GPa. The NI crossing reveals unique properties with respect to previously studied cases on similar compounds.     

Temperature Evolution of the Crystal Structure of AgNbO3

X-ray powder diffraction patterns of the AgNbO₃ have been obtained in a wide temperature range: from room temperature up to 850 K. The type of distortion and the multiplicity of the pseudo-cubic perovskite cell has been determined from the splitting of the main diffraction lines and from analysis of the superlattice peaks. The scheme of oxygen octahedra tilting and the sequence of the distorted pseudocubic cell is: <artwork name="GPHT21121eu1">     

Photo-Induced Phase Transition to a Striped Polaron Crystal in Cuprates

Here we report the evidence of a disorder-to-order photo-induced phase transition (PIPT) under X-ray (h x = 12.4 keV) illumination in oxygen doped La 2 CuO 4 + i . The commensurate striped polaron crystalline phase is induced by the irradiation of X-rays in a cuprate perovskite at hole doping i ¨ 1/8 and high Cu-O micro-strain, k >7%. The nucleation of long range crystalline phases of holes on the oxygen 2 p x,y orbitals in the CuO 2 plane associated with pseudo Jahn Teller local lattice distortions (LLD) is favored by mobile counter-ions (interstitial oxygen ions in the La 2 O 2 layer) in the temperature range 200-350 K. The PIPT appears beyond a threshold dose of 10 17 photons/cm 2 at 220 K and a power law of the polaron crystal growth is observed as a function of the X-ray illumination dose.     

Femtosecond X-ray Studies of Photo-induced Structural Phase Transitions

We summarize recent advancements in the application of femtosecond X-ray scattering to experimental studies of photo-induced, structural phase transitions. Dynamic measurements of atomic rearrangement during solid-liquid and solid-solid transitions, as measured by ultrafast X-ray diffraction using plasma sources of line radiation are reported.     

Low-Temperature Phase Transitions in the Trigonal Modification of Cs 3 Bi 2 Br 9 and Cs 3 Sb 2 I 9

The trigonal (P-3 m1) modification of Cs 3 Bi 2 Br 9 and Cs 3 Sb 2 I 9 have been studied using NQR, X-ray single crystal and powder pattern methods. Moreover, the heat capacity was measured in a wide temperature interval: 4-300 K. In Cs 3 Bi 2 Br 9 a second-order phase transition was found at T C = 96 K. The low-temperature phase is monoclinic (C12/c1), with the unit cell doubled along the [001] direction. Cs 3 Sb 2 I 9 has a sequence of phase transitions at T C = 85 K, T i = 78 K and T L = 72.1 K. The monoclinic structure below 85 K is isomorphic with the low-temperature structure of Cs 3 Bi 2 Br 9 . According to calorimetric data the lock-in transition at 72.1 K is discontinuous.     

Solid and Liquid AgI at High Pressure and High Temperature: A X-ray Absorption Spectroscopy Study

We report about recent X-ray absorption spectroscopy (XAS) measurements on solid and liquid AgI under high pressure. The structural behaviour of AgI has been investigated to pressures P～4.3 GPa at room temperature and to P～1.8 GPa at 1100 K. The high temperature/high pressure conditions have been obtained by means of a large-volume press of the Paris-Edinburgh type, coupled with a 10 mm boron/epoxy biconical gasket. The absorption spectra have been collected in transmission mode, both at the K-edge of Ag and I, and the samples have been characterized in situ by energy scanning X-ray diffraction at fixed angles. Our XAS results for solid AgI are compatible with previous X-ray diffraction measurements. For liquid AgI, we observe a slight change in the intensity and a shift in the frequency of the XAS oscillations with respect to what obtained in the case of the ambient pressure liquid.     

焙烧高岭土的相变研究

对湿化学方法处理过的高岭土作不同温度焙烧得不同样品，对不同样品进行XRD研究表明，高岭土随焙烧温度的不同发生一系列相应。用DTA，TG，IR对其相变进行了印证，得到了一致的结果。     

Phase transitions and equations of state at multimegabar pressures

Abstract

Review of phase transitions and equations of state at multimegabar pressures (100–300 GPa) is presented. Energy dispersive x-ray diffraction techniques in conjunction with synchrotron radiation sources are used. Besides several transition metals, Pt to 282 GPa, Re to 251 GPa, W to 209 GPa, and Fe to 255 GPa, the special focus is on Group IVA elements and isoelectronic III-V compounds. At high pressure, the isoelectronic materials are isostructural and exhibit similar equation of state.     

Smectic A-C-A Liquid Crystal Reentrance: A Photon Transmission Study

Ultraviolet/visible photon transmission measurements were applied to study liquid crystal phase transitions in the binary BOPDOB-BOPOOB mixture. A sequence exhibiting a new reentrant, namely isotropic-nematic-smectic A-C-A as temperature is lowered, is identified. For the smectic A-C transition, the critical exponent g is seen to cross over from the mean-field 0.501 - 0.008 to the helium-like 0.336 - 0.012 as the transition is approached. The reentrant C-A transition is first-order, signalling a tricritical point in the phase diagram.     

Phase transitions and electrical characteristics of tungsten substituted barium titanate

In the present work, phase transitions and electrical characteristics of tungsten substituted barium titanate are reported. Structural analysis of the prepared samples was carried out to confirm the formation of the compounds in proper phase. Scanning electron micrographs of the compounds have been used to investigate the grain morphology of the material. The dielectric properties of the samples have been studied as a function of temperature and frequency. The variation of dielectric constant (ε_r) with temperature shows a diffuse kind of ferro–paraelectric phase transition. The diffusitivity of the phase transitions has been calculated. Complex impedance spectroscopy analysis was carried out to investigate the electrical characteristics of the specimen.     

Pulsed-ESR Study of Spin Solitons as a Probe of Neutral-to-Ionic Phase Transition of TTF-CA Single Crystal

Ionic phase of charge-transfer crystal composed of tetrathiafulvalene (TTF) and p -chloranil (CA) has been studied by two-pulse electron-spin echo (ESE) at 4 K. By measuring the angular dependence of the field-swept ESE spectrum, at least three kinds of paramagnetic centers have been resolved, which are presumably ascribed to spin solitons formed at domain boundary. With varying the time separation between the microwave pulses, envelope modulations due to anisotropic hyperfine interaction have been detected. By analyzing the modulation, we have firstly obtained ENDOR spectrum of the spin solitons. Response of the ESE intensity for the optical excitation has been also examined.     

光栅-液晶复合结构太赫兹移相器

随着太赫兹技术及其应用的快速发展,各类太赫兹控制器件需求也随之增加,作为太赫兹系统重要器件之一,太赫兹波移相器成为当前研究热点.已有移相器存在着尺寸较大、结构复杂、相移量较小等问题,为克服上述缺陷,提出一种光栅-液晶复合结构太赫兹移相器,该器件结构为石英、石墨烯、液晶盒、光栅结构、石墨烯和石英组成.通过改变石墨烯电极上...     

稀土离子光谱探针的研究:EuS4N配合物在低温下的可逆相变

从配合物EuS4N低温荧光光谱的研究，可得到温度变化时配合物结构变化多方面的信息。从光谱转变的温度可知道结构变化对应的温度在约160K，对^5D0→^7F0跃迁谱峰的分析可推知结构变化前后Eu^3 配位结构的情况。从发光强度与温度变化的速度、方向的关系可推测结构变化时的热效应，并通过对样品温度的直接测量进行了验证。对配合物发生这种结构变化的可能原因进行了分析。研究表明，Eu^3 离子光谱探针在低温晶体结构的研究中具有某些特殊的优点，可作为单晶X-射线结构分析技术有益的补充。     

Dynamics of the Incommensurate Phase of the Fully Deuterated Diacetylene 2,4-Hexadiynylene Bis( p -Toluene Sulfonate)

Inelastic coherent neutron and Raman scattering is used to make a dynamical study of the disubstituted and fully deuterated diacetylene 2,4-hexadiynylene bis( p -toluenesulfonate) ( p TS-D) in a partially polymerized state ( x = 1.5%). The aim of this study is to investigate the dynamics of the incommensurate phase and to make a comparison with the dynamics of the same monocrystal in the high-temperature phase [Even et al. , Phys. Rev ., B52 , 7142.] and the dynamics of the pure monomer p TS-D crystal [Bertault et al ., Adv. Mat. for Optics and Electronics , 6 , 317.]. The phason and amplitudon modes are observed as very damped phonon-like excitations. The effect of polymerization on the critical dynamics in p TS-D is discussed.     

利用拉曼光谱对短链烷烃的相变过程及旋转相的研究

利用激光拉曼光谱研究了二十六烷的凝结相变过程。在冷却过程中,二十六烷经历了从液相→旋转相→固态单斜晶相的相变过程。主要研究了烷烃分子碳链上CH₂弯曲振动、CH₂剪式振动、C—C伸展振动和CH₃平面摇摆等振动模式的变化过程,包括这些模的频率、半宽和强度变化等,研究在相变过程中不同振动模式的的演变顺序和过程,反映各种振动模式与分子结构的相关性,并了解反式构象和歪曲构象在相变过程中的变化,从而更深入的理解烷烃等非晶体物质的相变过程。此外,通过分析800～1 500 cm-1波段各个拉曼波峰在降温过程中的变化过程,得到了二十六烷的旋转相的温度区间,证明可以利用拉曼光谱来观察烷烃相变过程中出现的旋转相。     

PcNi-VO2复合膜相变光谱特性研究

采用射频磁控溅射的方法在蓝宝石衬底上沉积了高品质的VO2薄膜,并在其上旋涂了酞菁镍[C8H17O]8PcNi薄膜.通过XRD研究了VO2薄膜的微结构.利用红外光谱仪观察了PcNi/VO2复合膜相变前后光学性质的改变,发现PcNi膜在1.5～5.5 μm对VO2膜有增透作用,PcNi/VO2复合膜相对于VO2膜的热色性和相变温度都没有改变.预期PcNi/VO2膜比单一PcNi膜和VO2膜的光限幅能力会更强.     

层状铁电体Ba_(1-x)Bi_(2+x)Ti_xNb_(2-x)O_9的结构相变研究

本文应用拉曼光谱方法研究了Ba1-xBi2+xTixNb2-xO9的结构相变。与x≠0的情况相比,x=0的情况BaBi2Bn2O9表现出了明确不同的拉曼光谱。前者有与Aurivillius层状铁电体家族的铁电相相似的拉曼光谱。因此认为BaBi2Nb2O9发生了铁电—非铁电相变。此外,用内模方法对层次铁电体Ba1-xBi2+xTixNb2-xO9的拉曼振动模式进行了初步的指认     

Structural Phase Transition and the Dielectric Permittivity of the Model Lipid Bilayer [(CH2)12(NH3)2]CuCl4

Structural phase transitions in the lipid-like bilayer material [(CH₂)₁₂(NH₃)₂]CuCl₄ have been observed using differential thermal scanning. The compound shows an irreversible thermochromic transition at ? 465 K and three reversible transitions at T ₁ = 433 ± 4 K and T ₂ = 411 ± 2 K and T ₃ = 358 K. The transition at 350 K is ascribed to chain melting. The other two correspond to crystalline phase transformation.

Phase (IV) T₃ = 358 ± 2K Phase (III) T₂ = 411 ± 2K Phase (II) T₁ = 433 ± 4K Phase (I)

Dielectric permittivity is studied as a function of temperature in the range 300-440 K and frequency, range (60 Hz-100 kHz). It confirms the observed transitions. The dielectric permittivity reflects rotational and conformational transitions for the compound. The variation of the real part of the conductivity with temperature is thermally activated in the temperature range above 350 K, with frequency-dependent activation energy, the values of activation energy lie in the range of ionic hopping. The dependence of the conductivity on frequency follows the universal power law σ = σ₀ + A(T) ω ^{s ( T )} with 0<s<1. Comparison of this material with other members of the series is discussed     

A New Spin-Crossover-Complex Phase Generated by Photo-Induced Phase Transition

We clarified that photo-induced phase is quite different in structure from the thermally-induced phase by resonant Raman and infrared absorption spectroscopy in the spin-crossover complex, [Fe-(2-pic) 3 ]Cl 2 EtOH. In the photo-induced phase we observed a number of additional Raman lines which are not observed in the thermally-induced phases. The infrared absorption spectrum of the photo-induced phase has different structures from those of thermally-induced phases. The additional Raman lines are assigned to infrared-active vibrational modes that are strongly prohibited by selection rules in the thermally-induced phase. These results indicate that a dramatic symmetry lowering should take place in the photo-induced phase.     

Structure and ferroelectric properties of BaBi3.8M0.2(Ti3.8Nb0.2)O15 (M=Mg, Ca, Sr and Ba) ceramics

Bismuth layer structured compounds BaBi_3.8M_0.2(Ti_3.8Nb_0.2)O₁₅, with M=Mg, Ca, Sr and Ba, were synthesized through a modified chemical route. Phase and structure of the compounds were analyzed by X-ray diffraction. The structure of Mg²⁺ based compound was orthorhombic, while it was tetragonal for the other three compositions. Experimental results indicate that Mg²⁺ prefers to substitute in the Bi₂O₂ layer. The room temperature permittivity was maximum for Ba²⁺ based compound (∼432). The piezoelectric coefficient was significantly enhanced in Mg²⁺ based compound. The Curie temperature was found to decrease gradually in the order Mg>Ca>Sr>Ba based compound. There was a marked improvement in the relaxor behavior and remnant polarization for Ba²⁺ based compound. The structural changes and dielectric, ferroelectric and piezoelectric properties of all the compounds was discussed and co-related.