The orientational glass instability and localization in (KCN) x (KBr)1−x and (KCN) x (NaCN)1−x

Substitutional disorder in mixed crystals with orientational and translational degrees of freedom leads to the appearance of random strain fields. The strains act as static scattering centers for dynamic modes. Near a ferroelastic phase transition where the crystal is very soft, internal friction processes dominate the dynamic restoring forces. The resulting nonergodic instability marks the onset of the orientational glass state, which is characterized by a freezing-in of orientational and translational modes without long range order. The method is based on the study of dynamic equations of mode-coupling type and was originally developed by Götze for the Anderson localization.Dedicated to Professor Harry Thomas on the occasion of his 60th birthday     

Dielectric properties of (KBr)1−x (KCN) x

The temperature dependence of the dielectric constant of (KBr)_1–x (KCN) _x was measured at frequencies of 87.5 Hz, 1, 10 and 100 kHz for concentrations ofx=0, 0.01, 0.04, 0.14, 0.25, and 0.50. Both the real and the imaginary part showed maxima at characteristic temperatures which depend on the measuring frequency and the concentration. The results are analogous to the behaviour of the magnetic susceptibility in spin glasses and support the existence of an orientational glass state.     

Inelastic and quasi-elastic light scattering in (NaCN)1−x(KCN)x quadrupolar glasses

The elastic constantsc ₁₁ andc ₄₄, and the attenuation of longitudinal phonons along (100)-direction in (NaCN)_1–x(KCN)_x mixed crystals have been determined by brillouin spectroscopy in the temperature range fromT=10K–300K. Minima inc ₄₄ (T) which determine the freezing tempratureT _F in these orientational glasses appear at lower temperatures compared to neutron scattering results. The coupling of molecular reorientations to longitudinal phonons in NaCN-rich smaples shows up in dispersion effects inc ₁₁ (T) and the appearence of a dynamical central peak nearT _F .U.A. 1119 is associated to the Centre National de la Recherche Scientifique     

Non-ergodic instability and the quadrupolar glass state in K(CN) x Br1−x and (NaCN) x (KCN)1−x

Random strain fields, due to substitutional disorder, couple to translational and orientational dynamic modes in plastic crystals. One distinguishes the cases of weak and strong random fields. In weak fields, the crystal comes close to a structural phase transition. Then elastic restoring forces are weak while internal friction dominates. The resulting non ergodic instability marks the onset of a structural glass state. The evolution of the glass state is discussed within a selfconsistent theory. The results of an extensive numerical study are compared with experiment. In presence of strong random fields no instability occurs, and orientational freezing is a continuous process.     

High-frequency dielectric study of the dynamics of (KBr)1−x (KCN) x mixed crystals

Dielectric measurements have been performed at 1 and 10 GHz in (KBr)_1–x (KCN) _x mixed crystals for cyanide concentrationsx=0.25 andx=0.50 in the temperature rangeT=1.7–150 K. The maxima of the real and imaginary parts of the dielectric constant falls into the Arrhenius straight lines obtained at lower frequencies. These dipolar lines cross over the so-called quadrupolar (acoustic) lines, in contradiction with theoretical predictions on the nature of the freezing. We also present a qualitative model which reinterprets the dipolar and quadrupolar freezings in terms of an individual and a collective processes.Associated with the Centre National de la Recherche Scientifique     

xPMnS-(1－x)PZN陶瓷的相变特性研究

研究了xPMnS-(1-x)PZN 四元系压电陶瓷的相变特征，分析了组成变化对材料相变特性的影响.结果表明，xPMnS-(1-x)PZN陶瓷具有弥散性相变特点，在相变过程中存在明显的介电热滞.电子结构计算结果表明，弥散相变的原因是由于不同B位原子与周围氧原子成键强度不同所致.当x值较大(或较小)时具有弛豫铁电体特征，相变弥散性较强；当组成位于x=0.4附近时具有正常-弛豫铁电体特征，相变的弥散性较弱. 关键词： xPMnS-(1－x)PZN 弥散相变 介电热滞     

三元合金Ga_(0.52)In_(0.48)P的喇曼散射谱

采用室温下微区Raman散射方法 ,观测到了GaInP2 的LO双模行为和禁戒的TO模 ,由于晶格有序导致晶体对称性从Td 降低为C3v,从而使禁戒的TO模变为Raman活性。在所有的样品中都观测到了由晶格无序激活的DALA模。在有序样品中 ,除观测到了二级Raman散射峰LO1+LO2 以外 ,还观测到了由超晶格效应所导致的FLA折叠模和LO模的分裂。对LO模峰谷比b/a的分析表明 ,随着晶格有序度的增加 ,b/a值减小。这是因为 :一方面主要是由于禁戒的TO模变为Raman活性所引起的 ,另一方面 ,还可能有LO1模和LO2 模分裂的贡献。在实验上 ,可以用b/a值或FLA的强度来表示样品的有序度。     

Theory of ferromagnetism in Ca(1-x)La(x)B(6)

Novel ferromagnetism in Ca(1-x)La(x)B(6) is studied in terms of the Ginzburg-Landau theory for excitonic-order parameters, taking into account symmetry of the wave functions. We found that the minima of the free energy break both inversion and time-reversal symmetries, while the product of these two remains preserved. This explains various novelties of the ferromagnetism and predicts a number of magnetic properties, including the magnetoelectric effect, which can be tested experimentally.     

First-principles calculation of alloy scattering in Ge(x)Si(1-x)

First-principles electronic structure methods are used to find the rates of intravalley and intervalley n-type carrier scattering due to alloy disorder in Si(1-x)Ge(x) alloys. The required alloy scattering matrix elements are calculated from the energy splitting of nearly degenerate Bloch states which arises when one average host atom is replaced by a Ge or Si atom in supercells containing up to 128 atoms. Scattering parameters for all relevant Delta and L intravalley and intervalley alloy scattering are calculated. Atomic relaxation is found to have a substantial effect on the scattering parameters. f-type intervalley scattering between Delta valleys is found to be comparable to other scattering channels. The n-type carrier mobility, calculated from the scattering rate using the Boltzmann transport equation in the relaxation time approximation, is in excellent agreement with experiments on bulk, unstrained alloys.     

Structure of the heavy electron compounds Ce(Cu x Al1-x )5 and Ce(Cu x Ga1-x )5, [0.6≦x≦0.8]

Neutron powder diffraction measurtements on the recently discovered heavy electron compounds Ce(Cu _x Al_1-x )₅ and Ce(Cu _x Ga_1-x )₅ show that Al or Ga atoms in these compounds locate exclusively on the 3g sites of space groupP6/mmm. Further Rietveld analysis shows that these atoms on the 3g sites are disordered.     

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