The effect of magnetic structures on the electrical resistivity of thulium

The influence of the magnetic structures in thulium with the residual resistance ratio (RRR) of 140 on the electrical resistivity was determined from 4·2 K up to 70 K. The measurements indicated the Néel temperatureT _N = 57.5 K and the Curie temperature T_C = 32 K. Furthermore, a new transition was found at T_H = 49 K which did not appear in the neutron diffraction measurements.Dedicated to Dr. Svatopluk Krupika on the occasion of his 65th birthday.We would like to express our thanks to V. Sojeková and H. Bobulská for their assistance in the measurements.     

Effect of point defects on the behavior of the soft mode at a structural phase transition

We obtain, in the model of point defects, formulas for the temperature and concentration dependences of the damping of the soft mode frequency in the vibrational dynamics regime and corrections to the relaxation frequency and to the damping constant for the overdamped regime. We consider the case of unpolarized point defects in a random field model. It is shown that temperature anomalies of a specified magnitude and the number of contributions in the lower order in anharmonism near the structural phase transition temperature depend substantially on the dynamic behavior of the soft mode. The estimates show that the contributions caused by the defects can, under certain conditions, equal or even exceed fluctuational contributions. We discuss applications of these results.Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 8, pp. 39–44, August, 1989.The author wishes to thank A. P. Levanyuk for useful discussions of the work and criticisms.     

A systematic analysis of defects in ion-implanted silicon

A classification scheme for the different forms of implant-related damage which arise upon annealing consisting of five categories is presented. Category I damage is subthreshold damage or that which results prior to the formation of an amorphous layer. If the dose is increased sufficiently to result in the formation of an amorphous layer then the defects which form beyond the amorphous/crystalline (a/c) interface are classified as category II (end of range) damage. Category III defects are associated with the solid phase epitaxial growth of the amorphous layer. The most common forms of this damage are microtwins, hairpin dislocations and segregation related defects. It is possible to produce a buried amorphous layer upon implantation, If this occurs, then the defects which form when the two a/c interfaces meet are termed category IV (clamshell, zipper) defects. Finally, category V defects arise from exceeding the solid solubility of the implanted species in the substrate at the annealing temperature. These defects are most often precipitates or dislocation loops.In addition to presenting examples of this classification scheme, new results emphasizing category II, IV, and V defects will be presented. For category II defects, the source, dose and mass dependence as well as the influence of pre- and post-amorphization is discussed. The category IV defects which arise from buried amorphous layers in {100} oriented As implanted samples is presented. Half loop dislocations which arise during annealing of high dose As implants, are shown to originate in the category V defects and grow upon dissolution of As clusters and precipitates.     

Positron annihilation studies of vacancy-type defects

The annihilation radiation of low energy positrons gives information on the electronic and defect structure of solids. There are three conventionally measurable quantities: the positron lifetime, the angular correlation of 2 annihilation radiation and the Doppler-broadened annihilation line shape. In the presence of lattice defects the annihilation characteristics show considerable changes. This is due to positron trapping at defects like vacancies and their agglomerates, voids, dislocations and grain boundaries. The concentration of defects can be deduced from the ratio of trapped and free positrons.The annihilation characteristics are different for different defect configurations. Positrons reveal vacancy agglomeration and the lifetime of trapped positrons gives estimates on the size of microvoids in the range of 2–10 Å. Various examples on the study of equilibrium and non-equilibrium defects, radiation damage and defect annealing are presented. Special emphasis is given to vacancy recovery and vacancy-impurity interactions in electron and neutron irradiated bcc transition metals like Fe, Mo, Nb, Ta.     

Molecular dynamics in crystalline acetone studied by dielectric spectroscopy and neutron diffraction

Dielectric spectroscopy and neutron diffraction experiments in acetone have been performed in order to clarify the dynamical behavior and the structural changes associated to the unsolved thermal transition that takes place in the solid state around 130 K. The combination of dielectric experiments with neutron diffraction reveals the existence of a dielectric process in the stable crystalline phase of acetone. The evolution with temperature of the dielectric process, within the temperature range where the peak of the heat capacity was reported, supports that the transition is not of order–disorder type. The origin of the dielectric dispersion has been assigned to structural defects in the orthorhombic crystal phase of acetone.     

On the structural phase transition in rare earth elpasolites

The structural phase transition in two representatives of the rare earth elpasolite fluorides, Rb₂NaHoF₆ and Rb₂NaTmF₆, are studied in detail. The symmetry of the soft mode is determined from Raman spectra, X-ray and neutron powder diffraction data to be ₄ ⁺ . From the temperature dependence of the elastic constants of both compounds, estimates of various phase transition parameters are given, e.g. strain soft mode coupling constant and soft mode frequency. The softening of the elastic constants forT>T _c is explained by the strain coupling to the fluctuations of the soft mode coordinates. The energy of the soft phonon at the zone boundary is estimated from the data and compared with that of K₂ReCl₆. The phase transition mechanisms in rare earth elpasolites and hexahalometallates are discussed.     

An inhomogeneously distorted state of the crystal structure of a Zn<Subscript>0.95</Subscript>Fe<Subscript>0.05</Subscript>Se cubic crystal

The structural state of a bulk Zn_0.95Fe_0.05Se cubic crystal grown by the chemical transport method from the gas phase has been investigated using thermal neutron diffraction at room temperature. It has been found that the measured neutron diffraction patterns of the crystal, in addition to structural Bragg peaks, contain a clearly identified system of superstructure reflections with the wave vector k = (1/3 1/3 1/3)2π/a (where a is the parameter of the cubic unit cell), which is interpreted as a clear evidence of the incipient transition state preceding the concentration phase transformation fcc ? hcp. It has been shown that the resulting structural state includes an inhomogeneous microstrain field with the possible appearance of long-wavelength modulations based on the initial sphalerite structure.     

Microstructure changes in high-speed hot rolling of steel rods

Transmission electron microscopy (TEM), quantitative texture analysis by means of neutron diffraction (QTA) and X-ray diffraction profile analysis (XDPA) were used to study the microstructure changes occurring in high-speed hot rolling ( 1500 s^–1;T=1073 K) of high-alloy stainless steel X8CrTi17 (ferrite) and X8CrNiTi18.10 (austenite). The investigations indicate that at higher deformation rates recrystallization due to adiabatic overheating becomes an important process of microstructure formation.     

Slow magnetic order-order transition in the spin chain antiferromagnet Ca3Co2O6

Using powder neutron diffraction, we have discovered an unusual magnetic order-order transition in the Ising spin chain compound Ca3Co2O6. On lowering the temperature, an antiferromagnetic phase with a propagation vector k=(0.5,-0.5,1) emerges from a higher temperature spin density wave structure with k=(0,0,1.01). This transition occurs over an unprecedented time scale of several hours and is never complete.     

Static and dynamic properties of XY systems with extended defects in cubic anisotropic crystallines

Static and dynamic critical behavior ofXY systems in cubic anisotropic crystallines, with extended defects (or quenched nonmagnetic impurities) strongly correlated along _d -dimensional space and randomly distributed ind – _d dimensions, were studied. These extended defects make the systems coordinate anisotropic, resulting in unique critical behavior due to competition between the cubic anisotropy and the coordinate anisotropy. The systems were analyzed by an ^1/2 (4 – d) type of expansion with double expansion parameters based on a renormalization-group (RG) approach. Critical exponents were calculated near the second-order phase transition point and the behavior of the first-order transition was evaluated near the tricritical point.     

Effect of high pressure on the crystal structure of Sr<Subscript>1−<Emphasis Type="Italic">x</Emphasis></Subscript>La<Subscript>x</Subscript>CuO<Subscript>2</Subscript> compounds

The effect of high pressure on the tetragonal structure of infinite-layer compounds Sr_1?xLa_xCuO₂ (x=0.07, 0.13) and the orthorhombic structure of the SrCuO₂ compound is investigated using powder neutron diffraction. It is found that infinite-layer compounds are characterized by strong compression anisotropy. The mechanism of the phase transition from the orthorhombic phase to the tetragonal phase is discussed.     

From unstable Minkowski space to the inflationary universe

It is shown that Minkowski space is unstable in the context of semiclassical gravity. There exists a threshold mass, of the quantized matter field, which marks the dividing line between stable and unstable vacuum fluctuations of matter in flat space-time. The Minkowski vacuum gravitational-matter system undergoes a phase transition above this critical point, the new phase being a self-consistently generated de Sitter Euclidean cosmology. Its total energy is degenerate with respect to that of empty Minkowski space-time. It represents an appropriate candidate for the primeval configuration of an inflationarylike universe.This essay received the fourth award from the Gravity Research Foundation for the year 1983 [Ed.].     

Pruning-induced phase transition observed by a scattering method

In hyperbolic systems, transient chaos is associated with an underlying chaotic saddle in phase space. The structure of the chaotic saddle of a class of piecewise linear, area-preserving, two-dimensional maps with overall constant Lyapunov exponents has been observed by a scattering method. The free energy obtained in this way displays a phase transition at <0 in spite of the fact that no phase transition occurs in the free energy dedcued from the spectrum of Lyapunov exponents. This is possible because pruning introduces a second effective scaling exponent by creating, at each level of the approximation, particular small pieces in the incomplete Cantor set approximating the saddle. The second scaling arises for a subset of values of the control parameter that is dense in the parameter interval.     

Neutron polarization analysis on the multiferroic TbMn2O5

Polycrystalline TbMn₂O₅ was prepared by the standard solid-state reaction method and characterized by powder X-ray diffraction and magnetization to assure it is of single phase. Heat capacity measurements on the compound reveal an antiferromagnetic phase transition at 45 K. A broad peak below 6 K in the heat capacity measurements corresponds to the crossover transition of Tb³⁺ ordering. To confirm these magnetic orderings, neutron powder diffractions on TbMn₂O₅ with XYZ neutron polarization analysis were performed at the diffuse neutron scattering (DNS) spectrometer, FRJ-II, by using neutron wavelength of 4.8 Å in the temperature range of 1.8–250 K. Magnetic scattering was separated from nuclear coherent and spin incoherent scattering contributions. Long-range ordered magnetic peaks were observed below 39 K which is consistent with the heat capacity results. The drastic increasing intensities below 6 K indicate the ferromagnetic transition in Tb³⁺ orderings.     

Structural and property changes in poly (vinylidene fluoride–trifluoroethylene) 70/30 mol % copolymer induced by proton irradiation

Poly(vinylidene fluoride–trifluoroethylene) 70/30 mol% copolymer has been irradiated with 3 MeV protons at doses ranging from 43 to 200 Mrad. The effects of irradiation on the polarization hysteresis, dielectric properties, lattice spacing, phase transition behavior and electric-field-induced strain have been studied. The irradiated copolymer exhibits the characteristic behavior of a relaxor ferroelectric, including frequency dispersion of the dielectric constant, which follows the Vogel–Fulcher rule. These results indicate that the proton irradiation breaks up the coherent polarization domains in the copolymer into nano-sized regions, thereby converting the copolymer to a relaxor ferroelectric. X-ray diffraction measurements show that the nano-sized regions are in the non-polar phase. Since the lattice spacing of the non-polar phase is substantially different from that of the polar phase, the local phase transformation between these two phases induced by an external electric field gives rise to a large lattice strain and hence a giant electrostrictive response. PACS 77     

High pressure powder X-ray diffraction study of Cr2As and pressure-induced structural phase transition

Cr₂As, which is an intermetallic compound of Cu₂Sb-type with Strukturbericht designation C38 was studied under a pressure of up to 32.5 GPa at room temperature using in situ synchrotron X-ray powder diffraction with a diamond anvil cell. From the X-ray diffraction analysis, our results showed that the Cu₂Sb (C38)-type phase of Cr₂As undergoes a pressure-induced structural phase transition near 15.4 GPa. The high-pressure (HP) phase of Cr₂As is suggested as an orthorhombic structure. No additional structural phase transition was observed up to 32.5 GPa, and the initial low-pressure (LP) Cu₂Sb (C38)-type structure was recovered as the pressure was released, implying that the observed pressure-induced structural phase transformation was reversible. The pressure–volume data of Cr₂As was fitted to a second-order Birch–Murnaghan equation of state, which yielded a bulk modulus of $B_0=125\left(3\right)$ and 340 (12) GPa for the LP and the HP phases, respectively. Furthermore, the $a$ axis is more compressible than the $c$ axis for the LP phase of Cr₂As. The anisotropic compressibility of the studied crystal is discussed in terms of the crystallography stacking.     

Structural and magnetic inhomogeneities,phase transitions, 55Mn nuclear magnetic resonance,and magnetoresistive properties of La0.6 − x Nd x Sr0.3Mn1.1O3-δ ceramics

The structure, lattice imperfection, and properties of ceramic samples La_{0.6 ? x} Nd _x Sr_0.3Mn_1.1O_3-δ (x = 0–0.4) have been investigated using the X-ray diffraction, resistive, magnetic (χ_ac, ⁵⁵Mn NMR), magnetoresistive and microscopic methods. It has been shown that there is a satisfactory agreement between the concentration decrease in the lattice parameters a of the rhombohedral (x = 0, 0.1, 0.2) and cubic (x = 0.3, 0.4) perovskite structures and the average ionic radii $\bar R$ for the lattice containing anion vacancies, cation vacancies, and nanostructured clusters with Mn²⁺ ions in A-positions. With an increase in the neodymium concentration x, the vacancy-type imperfection increases, the cluster-type imperfection decreases, the temperatures of metal-semiconductor phase transition T _ms and ferromagnetic-paramagnetic phase transition T _C decrease, and the content of the ferromagnetic phase decreases. The anomalous hysteresis is associated with the appearance of unidirectional exchange anisotropy induced in a clustered perovskite structure consisting of a ferromagnetic matrix and a planar antiferromagnetic cluster coherently coupled with it. An analysis of the asymmetrically broadened ⁵⁵Mn NMR spectra has revealed a high-frequency electronic double exchange (Mn³⁺-O^2?-Mn⁴⁺) ? (Mn⁴⁺-O^2?-Mn³⁺) and an inhomogeneity of the magnetic and charge states of manganese due to the heterogeneous environment of the manganese ions by other ions and defects. The observed changes in the resonant frequency and width of the resonance curve are caused by changes in the ratio Mn³⁺/Mn⁴⁺ and magnetic inhomogeneity. An increase in the neodymium concentration x leads to a decrease in the ferromagnetic phase content determined from the dependences 4πNχ_ac(T) and the ⁵⁵Mn NMR curves. The phase diagram characterizes an interrelation between the composition, the imperfection of the structure, and the transport, magnetic, and magnetoresistive properties of lanthanum neodymium manganite perovskites. It has been found that there is a correlation between the imperfection, magnetic inhomogeneity, coercive force, and magnetoresistance effect exhibited by the perovskite structure.     

Controlled nucleation of point defects on a disclination line near a free surface during smectic-A-to-nematic directional melting

A disclination line populated with point defects that break the translational symmetry forms near a free nematic (N) interface in a confined geometry. The disclination line is, however, absent in the smectic-A phase (SmA). We use this fact to control the formation of point defect distributions on a disclination line by directional melting of the SmA phase in a temperature gradient. A threshold velocity ( v _th) exists below which a defect-free disclination line is formed. The frequency of nucleation of point defects increases steadily for v > v _th and exhibits a remarkable regularity. We derive an empirical scaling for v _th in terms of the experimental tuning parameters. We propose a simple model that allows to understand the formation of the point defects. Received 1 October 2002 / Published online: 15 April 2003 RID="a" ID="a"Permanent address: Departament de Quımica Fısica, Universitat de Barcelona, Martı i Franquès 1, Barcelona 08028, Spain; e-mail: jignes@qf.ub.es     

Localized vibrations ofH andD in Ta and their relation to theH(D) potential

The localized vibrations ofH(D) in and TaH(D) _x have been studied by high resolution inelastic neutron scattering. The frequencies of the fundamentals have been determined with very high accuracy. The corresponding line widths are extremely narrow in contrast to literature data. The slight broadening of the higher fundamental in the phase could be related to the orthorhombic distortion of this phase. For the first time higher harmonics of localizedH(D) vibrations in Ta could be observed. Their frequency shift relative to the harmonic value and the deviation of the fundamental frequencies from the harmonic isotope effect could consistently be described by one anharmonicity parameter for theH(D) potential.     

Phase transitions in BaCeO3: neutron diffraction and Raman studies

Polycrystalline samples and small single crystals of the perovskite BaCeO₃ were studied by neutron diffraction and Raman spectrometry between 300 and 1200 K. The controversy about the phase transitions originally deduced from our previous Raman study and those observed since by neutron diffraction by Knight has stimulated this work. Pretransitional effects which are detected by Raman much before long-range ordering takes place can partly explain the above disagreement. A continuous monitoring of the structural changes by neutron diffraction and by Raman spectroscopy including polarization analysis has allowed discussion of the transition mechanisms: The first transition Pnma–Imma takes place at 573 K and is of second order. Although some modes soften when the temperature is raised as in many of these perovskite compounds the transition is likely partly displacive partly order–disorder. The Raman modes which disappear transform in modes at the X point of the Brillouin zone of the Imma phase. The second transition Imma–R c takes place at 673 K and is first order. The last transition R c–Pm3m occurs above 1200 K and the transition temperature which can be deduced by extrapolation to zero Raman intensity is in good agreement with neutron results. This second order transition is progressive and begins at about 400 K, the intermediate R c structure appearing as an attempt for slowing down the structural evolution toward the cubic perovskite form.