ISSN:0370-1972    

DOI:10.1002/pssb.2221910125

出版商:WILEY‐VCH Verlag    

年代:1995

数据来源: WILEY

ISSN:0370-1972    

DOI:10.1002/pssb.2221910126

出版商:WILEY‐VCH Verlag    

年代:1995

数据来源: WILEY

ISSN:0370-1972    

DOI:10.1002/pssb.2221910102

出版商:WILEY‐VCH Verlag    

年代:1995

数据来源: WILEY

摘要:

AbstractPolycrystalline materials of the composition Sr5−x−yCaxEuy(PO4)3Cl with 0 ≦x≦ 4.99 and 0 ≦y≦ 0.1 crystallize in the hexagonal crystal system. The space group is P63/m. The alkaline earth ions occupy two different crystallographical sites. In addition to X‐ray diffraction measurements, the optical properties are studied at room temperature. Moreover the emission spectra are measured at liquid nitrogen temperature. Refinement of the structure parameters is carried out using the Rietveld method. A varying calcium portion influences the lattice constantsa0andc0and therefore the luminescence of the samples. The luminescence arises from transitions between 4f65d and 4f7configurations o

ISSN:0370-1972    

DOI:10.1002/pssb.2221910103

出版商:WILEY‐VCH Verlag    

年代:1995

数据来源: WILEY

摘要:

AbstractIt is well‐known that the diffusion coefficients of the Cu+cation in the NaCl and KCl lattices exceed by three or four orders of magnitude the corresponding self‐diffusion coefficients in the intrinsic temperature regions. This fast diffusion of the Cu+has been explained in many papers as an interstitial diffusion although the optical spectra do not confirm the existence of interstitial Cu+. In this paper a new mechanism for fast diffusion is proposed. The model assumes that the equilibrium positions of the cationic impurities are noncentral and that the diffusion proceeds by hopping across the potential barrier along the nonlinear paths with the highest probability. The main result shows that the off‐center position considerably enhances the diffusion. Theoretical diffusion coefficients are obtained by modelling the potential barrier. Changes of the configuration entropy and the vibration spectra due to the presence of the noncentral impurity are included in the model. The numerical results of the diffusion coefficient computed in this way are in agreement with the experimental data and it is shown that if the impurity is placed close to the central site the due diffusion coefficient is close to that for the cationic self‐diffusion. The Li+cation case is dealt with as the case of the Cu

ISSN:0370-1972    

DOI:10.1002/pssb.2221910104

出版商:WILEY‐VCH Verlag    

年代:1995

数据来源: WILEY

摘要:

AbstractDiffusion of nickel in the intermetallic compound Ni3Al is measured in the temperature range from 1004 to 1422 K by applying two different techniques for the determination of the concentration profiles. The diffusion of the radioactive63Ni tracer in Ni3Al polycrystals (75.2 at% Ni) is analyzed in the temperature region from 1223 to 1422 K by the conventional serial sectioning technique using a precision parallel grinding device and an improved technique of detecting the low‐energy β‐decays of63Ni with high efficiency by liquid scintillation counting. In the low‐temperature region (1004 to 1259 K) the concentration profiles are determined by secondary ion mass spectrometry (SIMS) using the highly enriched stable64Ni tracer and Ni3Al single crystals (75.9 at% Ni). The SIMS technique is considered as particularly suitable for profile detection, owing to its potential of measuring also low diffusion coefficients with high accuracy. The temperature dependence of the diffusion coefficient DNivfollows a perfect linear Arrhenius relation in the investigated temperature range for Ni3Al. The frequency factorD0= (3.59 −1.50+2.59) × 10−4m2s−1and the activation enthalpyQ= (303.0 ± 5.3) kj mol−1correspond to a vacancy mechanism. An enhanced diffusivity at lower temperatures, which was previously reported in the literature and ascribed to the presence of constitutional vacancies, is not observed. It is concluded that Ni self‐diffusion in Ni3Al only occurs via thermal vacancies. This result is in agreement with recent positron annihilation studies by Badura and Schaefer where, independent of the composition, no constitutional vacancies are detected i

ISSN:0370-1972    

DOI:10.1002/pssb.2221910105

出版商:WILEY‐VCH Verlag    

年代:1995

数据来源: WILEY

摘要:

AbstractDiffusion in solids with random activation energy of individual jumps is analyzed. The experimentally observed relation between the activation energies of diffusion for metal glasses and for the corresponding crystals is explained under reasonable assumptions. The activation energy is shown to be not the most probable buth rather close to the minimum allowed (by the percolation mechanism) activation energy of individual jumps. For an interstitial solute the activation energy parametrically depends on the mean solute concentration. The expression for the diffusion coefficient is obtained. The short‐range diffusion is also considere

ISSN:0370-1972    

DOI:10.1002/pssb.2221910106

出版商:WILEY‐VCH Verlag    

年代:1995

数据来源: WILEY

摘要:

AbstractComputer simulation is used to study the dependence of the velocity of a dislocation (kink) on the applied motive force and temperature in the Frenkel‐Kontorova model (discrete sine‐Gordon system). The applied forces used exceed the Peierls‐Nabarro barrier. Heating of the atomic chain by the environment is simulated by introducing random thermal forces and frictional forces acting on individual atoms of the system into the Frenkel‐Kontorova equations. It is found that though in this model the velocity of a dislocation depends on temperature, this dependence is very weak. In the range of dislocation velocities studied, smaller velocities monotonically rise with growing temperature, while larger velocities depend on temperature nonmonotonically, first decreasing and then increasing with rising temp

ISSN:0370-1972    

DOI:10.1002/pssb.2221910107

出版商:WILEY‐VCH Verlag    

年代:1995

数据来源: WILEY

摘要:

AbstractThe ground state of a relativistic positronium (Ps) atom moving in the planar channeling regime in a Si crystal is investigated within the continuum potential approximation. The strong dependence of the internal structure of the Ps atom and its spatial distribution inside the crystal is found to be a function of the total energy of Ps. The squeezing of the atom and destroying of the spherical symmetry of the Ps ground state in a crystal with increasing Ps total energy is demonstrated. A brief discussion of applications and extensions is given.

ISSN:0370-1972    

DOI:10.1002/pssb.2221910108

出版商:WILEY‐VCH Verlag    

年代:1995

数据来源: WILEY

摘要:

AbstractA simple force constant model is applied to the vibrations of crystalline CCl2F2at low temperatures, and good agreement with recent spectroscopic results is obtained. The calculations suggest that two weak Raman peaks observed in the lattice region should be re‐assigned as fundamentals. Principal intermolecular interactions are identified, and it is shown that the intramolecular force constants change only slightly from their gas phase value

ISSN:0370-1972    

DOI:10.1002/pssb.2221910109

出版商:WILEY‐VCH Verlag    

年代:1995

数据来源: WILEY