On the occurrence of b.c.c.–h.c.p. structural phase transition in Mo under pressure

Abstract

To resolve the controversy on the occurrence of a b.c.c.–h.c.p. transition in shock as well as static measurements in Mo, we have done more accurate total energy electronic structure calculations. Our results show that this phasetransition should occur at much higher pressures than suggested by earlier calculations.     

两片型f.θ透镜设计

设计一种两片型ｆ·θ透镜，结构简单，畸变误差小于０．０８６％；并简述设计这种简单透镜的方法和应考虑的问题。     

Frequency response of loudspeakers by measuring theinduced e.m.f

1IntroductionTheperformanceofadirectradiatorloudspeakerisdirectIyrelatedtothediaphragmdis-placement.AccuratemeasurementoftIlefreqllenc,responseoftheloudspeakerbydifferentmethodsreqlliresananechoicchamberandacalibratednlicrophone.AI1echoicchambersoper-atingdowntorequiredlowfrequenciesareveryexpensive,a11dopenspacesareinconvenient.ThemethodsuggestedbyK.ele[l]isbasedonthefactthat,withsomerestrictions,thefarfieldfrequencyresponseofthespeakerissimilartothatofitsnearfieldresponse.Sincethereflected…     

Ratchet potential in d.c. SQUID’s and reduced two-junction interferometer models

For finite values of the parameter and in the presence of small structural asymmetry and inhomogeneity of the junctions parameters, the two-junction interferometer model can be described by means of a single non-linear differential equation in such a way that a ratchet potential for SQUIDs can be deduced. The resulting dynamical equation, derived for first-order corrections with respect to a symmetric and homogeneous model with =0, presents an additional second-harmonic term and a cosine term in the expression for the effective current-phase relation. For opportune values of the perturbation parameters a ratchet potential with pre-definite characteristics can be obtained.     

Antiproton-proton interactions at 5·7 Gev/c I. Nonannihilation channels

We present an analysis of nonannihilation channels in four-pronged interactions at 5·68 GeV/c. The transverse momenta and azimuthal angles distributions of nucléons and -mesons are compared with phase space. No essential influence of production on azimuthal correlation has been found. The seen difference of azimuthal angle distributions for like and unlike charged pions in annihilations appears to be more pronounced for small transverse momenta and for small transverse masses. For 10971 events of the reaction ¯pp¯pp^–+ we present cross sections for production and differential cross sections. The reaction is dominated by the strong quasi-two-body channel with cross section (1·66 ±±0·10) mb. We study in detail the production mechanism of this channel in terms of the absorption model.Dedicated to Academician Václav Votruba on the occasion of his seventieth birthday.The authors are indebted to the CERN-Prague collaboration on ¯pp interactions at 5·7 GeV/c for experimental data used in this paper and to Dr. M. Novak for valuable comments.     

Direct observation of the f–c hybridization in the ordered uranium films on W(110)

A key issue in metallic uranium and its related actinide compounds is the character of the f electrons, whether it is localized or itinerant.Here we grew well ordered uranium films on a W(110) substrate.The surface topography was investigated by scanning tunneling microscopy.The Fermi surface and band structure of the grown films were studied by angle-resolved photoemission spectroscopy.Large spectral weight can be observed around the Fermi level, which mainly comes from the f states.Additionally, we provided direct evidence that the f bands hybridize with the conduction bands in the uranium ordered films, which is different from previously reported mechanism of the direct f–f interaction.We propose that the above two mechanisms both exist in this system by manifesting themselves in different momentum spaces.Our results give a comprehensive study of the ordered uranium films and may throw new light on the study of the 5 f-electron character and physical properties of metallic uranium and other related actinide materials.     

关于Eu2+的f—f跃迁

文章综述了Eu2+在某些基质化合物中的f-f跃迁.在考察文献数据基础上,总结出Eu2+在氟化物(或氧化物)中产生f-f跃迁的某些规律性,并根据电负性、配位数及离子半径等提出一种判断Eu2+产生尖峰结构光谱的经验方法.在缺少结晶学数据情况下,该方法有可能用来预测Eu2+产生线状峰的新基质化合物.镧系离子的电子结构早已为人们所熟知.三价铕离子(Eu3+),有六个自旋平行的4f电子[1、2、3].根据洪德规则,Eu3+很容易变成Eu2+,4f壳层呈现半充满状态(4f7),获得与Gd3+具有相同的电子构型,从而趋于稳定.     

Gezielte 15N-Anreicherung von organischen Düngemitteln für Versuchszwecke 1. Mitt. Gewinnung von 15N-markierten Futtermitteln

Zur Herstellung ¹⁵N-markierter organischer Dünger tierischer Herkunft ist vorab die Produktion ¹⁵N-angereicherter Futtermittel erforderlich. Unter Verwendung von ¹⁵NH₁ ¹⁵NO₃ mit einer relativen ¹⁵N-Häufigkeit von 50 Atom-% als Düngemittel werden Rüben, Weidelgrasheu sowie Haferkorn und -stroh als eine komplette Tagesralion für einen Fütterungsversuch mit einer Milchkuh auf der Grundlage vorher notwendiger Kalkulationen erzeugt. Im Ergebnis von ¹⁵N-Verdünnung und N-Schwund stehen von den eingesetzten 105,5 g ¹⁵N im Düngemittel noch 44,0 g ¹⁵N im Futter für den Tierversuch und die Kot-Harn-Gewinnung zur Verfügung.     

Mean-square atomic displacements in f.c.c. crystals

The mean-square displacements of volume atoms, surface atoms and atoms near a vacancy are calculated for f.c.c. metals and rare gas solids, using a simple series expansion of the Wallis formula. We employ a model with rotationally invariant bond bending forces, unlike the de Launay model used by Masri and Dobrzynski. For atoms near a vacancy and for rare gas solids this method has been applied for the first time. The calculations are done to the R⁴ term in the expansion. The inclusion of the R⁴ term together with temperature-dependent elastic constants give results which are in better agreement with experiment. For rare gas solids Lindemann ratios calculated by this method agree closely with the values obtained from elaborate computations as well as those deduced from entropy data. The present calculation also supports the view that the melting of solids is initiated at the crystal surfaces and in the neighbourhood of vacancies.     

Sputtering induced topography development on f.c.c. metals

The generation and development of surface features by ion bombardment induced sputtering erosion of f.c.c. metals is reviewed. Studies with polycrystalline substrates reveal the plethora of individual features, both etch pits and cones and the repetitive features, such as ripple trains, which form differently on different crystallite surfaces. Studies with well defined single crystals clarify the origins of such features and the relative independence of their habit on ion species and substrate material.     

Theoretical evaluation of debye-waller factors for f.c.c. metals

Summary The temperature dependence of the Debye-Waller factor exponents for five f.c.c. metals—copper, silver, aluminum, nickel and gold—has been carried out under the modified axially symmetric model of Shukla and Closs. The theoretical computations are compared with the experimental one ofY θ_M and ≪u ²≫, namely the Debye-Waller factor temperature parameter, the effective X-ray characteristic temperature and the root mean square displacement. Reasonable agreement has been obtained between the theory and the experiment.

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Raassunto Si è calcolata la dipendenza dalla temperature degli esponenti del fattore di Debye e Waller per cinque metalli f.c.c.—rame, argento, alluminio, nickesl ed oro—secondo il modello modificato a simmetria assiale di Shukla e Closs. I calcoli teorici sono confrontati con i dati sperimentali suY,430-1 ed ≪u ²≫, cioè il parametro di temperature del fattore di Debye e Waller, la temperatura caratteristica efficace dei raggi X e lo spostamento della radice qudratica media. Si è ottenuto un ragionevole accordo tra teoria ed esperimenti.

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Реуме На основе модыфицированной аксиалтно симметричной модели Шукла и Клосса вЫчисляется темчературная зависимость фактора Дебая-Уоллера для пяти гранецентрированных металлов—меди, серебра, алюминия, нилеля и золота. Теогетичеслие вЫчисления сравниваются с зкспериментальнымн данными для У, θ_M и <u ²>, т.е. для температурного парамепра фактора Дебая-Уоллера, зффективной характеристической температуры и среднего квадрапа смешения. Получается, разумное согласие между теорией и зкспериментом.

     

Electroabsorption of excitons in f.c.c. TlBr and TlCl

The exciton binding energy, band gap, and static dialectric constant were obtained from electromodulation measurements on the face-centered cubic (NaCl structure) phase of TlBr. An unusual electroabsorption structure has been observed in TlBr and TlCl for features which have no established assignment.     

Strengthening at nanoscaled coherent twin boundary in f.c.c. metals

This paper analyses slip transfer at the boundary of nanoscaled growth twins in face-centred cubic (f.c.c.) metals for strengthening mechanism. The required stress for slip transfer, i.e. inter-twin flow stress, is obtained in a simple expression in terms of stacking fault energy and/or twin boundary (TB) energy, constriction energy and activation volume. For nanotwinned Al, Cu and Ni, inter-twin flow stress versus twin thickness remarkably shows Hall–Petch relationship. The Hall–Petch slope is rationalized for various reactions of screw and non-screw dislocations at the TB. Additionally, strengthening at the boundary of nanoscaled deformation twins in f.c.c. metals is analysed by evaluating required twinning stress. At small nanograin size, the prediction of deformation twin growth stress shows inverse grain-size effect on twinning, in agreement with recent experimental finding.     

Hall effect studies in ytterbium through the f.c.c.-h.c.p. transformation

The Hall coefficient (R_H) of polycrystalline Yb showing the f.c.c.-h.c.p. transformation has been determined at 20 koe in the range 1.8–420°K. Both the structural and the associated magnetic transformation would separately be expected to modify R_H, but the evidence suggests that the observed behaviour through the transition arises primarily from the effects of the change in structure. A more detailed interpretation is presently not possible, however, because of the lack of knowledge of the band structure of h.c.p. Yb.     

Relative stability of f.c.c. and b.c.c. structures for model systems at high temperatures

Summary Free-energy, entropy and volume differences between face-centered and body-centered cubic structures have been evaluated for model rare gas and alkali metal crystals by using the method of overlapping distributions. Stable phases are predicted in agreement with the behaviour of real materials in the regions of validity of classical mechanics and in agreement with the results of previous dynamical-simulation studies of crystal nucleation from the melt and of polymorphic transformations. The existence of a stable b.c.c. phase at high pressure and temperatures is predicted in this way for Lennard-Jones solids, while no high-pressure f.c.c. phase is expected for model Rb and Cs systems. We also show the possibility of making calculations of free-energy barriers to displacive crystalline transformations along a prescribed trajectory in configuration space. Work supported by the U.S. Department of Energy, Istituto per la Ricerca Scientifica e Tecnologica, Trento, and Gruppo Nazionale di Struttura della Materia del C.N.R., Italy.