An unusual second-order-like phase transition in the perovskite-type layered compound ${\mathrm{Cs}}_2$${\mathrm{CdCl}}_4$ has been observed by Raman studies of the solid. The phase transition took place over a range of temperatures from 435 °C to about 460 °C. At any temperature within this temperature range Raman bands characteristic of the different cadmium coordination in each of the separate phases were detected simultaneously with relative intensities that were dependent on the temperature. Analysis of the Raman spectra indicated that the structure of ${\mathrm{Cs}}_2$${\mathrm{CdCl}}_4$ transformed from the ${\mathrm{K}}_2$${\mathrm{NiF}}_4$-type layered-perovskite-like low-temperature phase II (octahedral cadmium coordination) to the ${\mathrm{K}}_2$${\mathrm{SO}}_4$-type-structure high-temperature phase I (tetrahedral cadmium coordination). Relative-intensity measurements of bands characteristic of each cadmium coordination as a function of temperature indicated an equilibrium distribution. The enthalpy and entropy associated with the change of coordination were of the order of those usually observed on fusion. These large values are consistent with the major structural rearrangement from a network octahedral structure to discrete tetrahedral coordination. Although local order around the cadmium ion increased in the high-temperature phase the overall order of the lattice was considerably reduced because of the orientational disorder of the spherical, discrete ${\mathrm{CdCl}}_4^{2\mathrm{-}}$. Similar studies have been performed for solids of composition ${\mathrm{Cs}}_2$${\mathrm{CdCl}}_4$⋅0.5CsCl, ${\mathrm{Cs}}_2$${\mathrm{CdCl}}_4$⋅CsCl, and ${\mathrm{Cs}}_2$${\mathrm{CdCl}}_4$⋅2CsCl to follow the formation of ${\mathrm{Cs}}_3$${\mathrm{CdCl}}_5$ which is stable above 395 °C. The presence of the peak due to ${\mathrm{CdCl}}_4^{2\mathrm{-}}$ at temperatures as low as 365 °C signaled the onset of the reaction to ${\mathrm{Cs}}_3$${\mathrm{CdCl}}_5$.

The evolution of x-ray photoelectron and ultraviolet spectra and their modification at the phase transformation B2-B19(B19’) of equiatomic TiM and quasibinary Ti(Ni,M) compounds, where M stands for Fe, Co, Ni, Pd, Pt, Au, and Cu, are presented. Investigation of x-ray photoemission spectroscopy includes valence bands, satellites, and core-level energy studies. As the atomic number of M increases (both within the same period and along the group), its d states become more localized, the maximum and the center of gravity of the d bands shift towards the bottom of the valence band, and the M contribution to the density of states (DOS) at the Fermi level, N(${\mathit{E}}_{\mathit{F}}$), degrades. The DOS localization is accompanied by a spatial localization of the M d electrons resulting in weakening of the d-d covalent bonds between the alloy components and, thus, destabilizing the B2 phase. While the contribution of M d electrons to the DOS at ${\mathit{E}}_{\mathit{F}}$ decreases, the Ti d electron portion increases to such an extent that, e.g., in TiPd and TiAu, the M contribution to N(${\mathit{E}}_{\mathit{F}}$) is almost negligible. The increase in core-level energy of all partners in compounds with respect to the elemental state is discussed in terms of covalency. The M d band localization leads to an increase of many-body interactions and, as a consequence, we managed to observe the Pd satellites without resonance enhancement. The B2-B19’ phase transition is accompanied by modification of the DOS which affects the Ti d states in general. Having analyzed the electronic structure we introduce parameters controlling the B2 phase stability and suggest a phenomenological formula for the martensitic-transformation temperature which satisfactorily describes the experimental behavior.

We have conducted extensive high-pressure Raman-scattering studies in order to determine the effects of structure on the vibrational properties of layered aluminosilicate materials. The basal oxygen torsional mode is found to shift up with pressure at a rate that agrees reasonably with that expected from a simple van der Waals model. We identify a mode at 50 ${\mathrm{cm}}^{\mathrm{-}1}$ in margarite as the interlayer shear mode. The Raman band near 700 ${\mathrm{cm}}^{\mathrm{-}1}$ in all clays is linked to an oxygen-layer breathing mode. The pressure coefficient of the hydroxyl stretching mode near 3600 ${\mathrm{cm}}^{\mathrm{-}1}$ is found to have a structure-dependent sign; it is positive for trioctahedral structures and negative for dioctahedral silicates. This is found to be associated with hydroxyl orientation and Coulombic repulsion between the hydroxyl proton and the interlayer cation.

The modulation wave vector q=δ(T)${\mathbf{c}}^{\mathrm{*}}$ of deuterated betaine calcium chloride dihydrate (D-BCCD) was measured as a function of the temperature for certain dc electric fields, using elastic neutron scattering. The results obtained are compared with the behavior of the dielectric constant of electric-field-biased D-BCCD. The temperature range of stability of the polar phases with δ=2/7, 4) / 15 ,2/9, 2) / 11 increases with a dc electric field along b. The results are discussed qualitatively, in terms of Landau theory.

The dynamical properties of β-${\mathrm{Si}}_3$${\mathrm{N}}_4$ (${\mathit{C}}_{6\mathit{h}}^2$) have been investigated within the framework of a simple phenomenological model, based on a short-range pair potential. In developing the model, we have discussed some structural peculiarities of the lattice, which argue for the possibility of its pressure-induced destabilization, caused by interatomic bond tensions arising as a response to external influence. Thus, the nature of the instability can be attributed to a purely mechanical effect whose description does not require any ad hoc model parameters. The simulation of the hydrostatically compressed lattice has demonstrated the vanishing of a low-frequency mode inside the Brillouin zone (k=0,0,0.36) at a pressure of 72 kbar, thus predicting a second-order phase transition to a structure with incommensurate modulation along the z direction. Since the destabilization of β-${\mathrm{Si}}_3$${\mathrm{N}}_4$ has never been observed, in contrast to the α (${\mathit{C}}_{3\mathit{v}}^4$)→β (${\mathit{C}}_{6\mathit{h}}^2$) conversion, this theoretical result has been considered in detail, including aspects of its reliability and of the possibility of the β-α structural transformation proceeding via an intermediate incommensurate phase.

${\mathrm{YFe}}_{12\mathrm{-}\mathit{x}}$${\mathrm{Mo}}_{\mathit{x}}$ (x=1 and 3) and full-charged nitrides were prepared. Neutron-powder-diffraction experiments were performed at 10 K and room temperature on both the host compounds and their nitrides. The Rietveld technique was used for data analysis. Mo was found to reside mainly on the 8i site in the tetragonal ${\mathrm{ThMn}}_{12}$ structure (space group I4/mmm), with a small fraction on the 8f site. The nitrides retain the crystallographic symmetry of the host compounds. Nitrogen atoms are located on the 2b site with nearly 100% occupancy. The nitrogenation-induced modification of the rare-earth environment and in turn the magnetocrystalline anisotropy properties are discussed. The refined moments are comparable with those obtained by magnetization measurement.

${\mathrm{Pb}}_3$(${\mathrm{PO}}_4$${)}_2$ has two types of domain walls, the W wall and the W’ wall. However, both types of domain walls in ${\mathrm{Pb}}_3$(${\mathrm{PO}}_4$${)}_2$ belong to the W wall, because they both can be represented as crystallographically prominent planes of fixed indices. We will call them the ${\mathit{W}}_{\mathit{m}}$ wall and the ${\mathit{W}}_{\mathit{b}}$ wall instead of the W wall and the W’ wall, respectively. The z axis of the prototypic phase is not parallel but a little inclined to the normal of (100). Local reorientations in a narrow temperature range near ${\mathit{T}}_{\mathit{c}}$ cause superpositions of small inclined z axes above 180 °C, which give a small birefringence at the conoscope image.

${\mathrm{RuO}}_2$ was studied up to 40.2 GPa by angle-dispersive x-ray diffraction. Two phase transitions were observed, the first from the rutile-structured ambient phase to an orthorhombic phase below 8.0 GPa with cell parameters a=4.615(17), b=4.230(13), and c=3.112(4) Å, Z=2 at 11.3 GPa. The second transition at just below 13 GPa was to a fluorite-type structure with a=4.727(4) Å, Z=4 at 40.3 GPa. The first transition appeared to be second order producing a structure that is related to the ${\mathrm{CaCl}}_2$ type. Several reflexions were found to be common to all three phases, indicating that the rutile-to-fluorite phase transformation occurs via a diffusionless mechanism dominated by oxygen displacement in the xy plane of rutile. Using group theory, we propose possible diffusionless pathways for the transformation from the initial rutile structure, via the observed, orthorhombic, ${\mathrm{CaCl}}_2$-like intermediate, to the fluorite structure, based on a series of group-subgroup relationships. Ruthenium dioxide under ambient conditions is isostructural with stishovite and hence provides a model for the high-pressure behavior of ${\mathrm{SiO}}_2$.

Optical transitions between 5f states of tetravalent californium ion doped (1 metal-atom %) into ${\mathrm{CeF}}_4$ exhibit unusually large inhomogeneous broadening. The nature of the inhomogeneous broadening in this system has been studied by using fluorescence line narrowing and excitation line narrowing (ELN). It is shown that the energy distributions of different electronic states of ${\mathrm{Cf}}^{4+}$ in this system are correlated. In the ELN experiments, reduced excitation linewidth was obtained when selectively monitoring fluorescence emission. A linear relation was observed between the excitation energies of crystal-field states of the ${\mathrm{G}}_{4\prime }^5$ manifold and the fluorescence wavelength monitored across the inhomogeneous profile of a ${\mathrm{G}}_{6\prime }^5$–${\mathrm{F}}_{6\prime }^7$ transition. Analysis of these results by means of a microscopic theory proposed by Laird and Skinner [J. Chem. Phys. 90, 3880 (1989)] has provided insights into the structural properties of this disordered system.

Single crystals of phenothiazine undergo a displacive order-disorder phase transition near 250 K. The microscopic mechanism of this transition is not clear owing to the complexity of a disordered lattice and the lack of direct mode-anharmonicity data. Unstable optical modes have been observed in phenothiazine and are regarded as coupled, by symmetry, to an acoustic mode that becomes overdamped through the phase transition. The latter is considered to have the primary role in the phase transition. This premise is in doubt, however, based on the optical-mode anharmonicity directly observed by uniaxial-piezomodulation-Raman-spectroscopy results presented here. Anharmonic responses are observed in the low-temperature spectra at frequencies that correspond to room-temperature Raman values and thus support persistent metastable, room-temperature, lattice structures that are intergrown within the low-temperature phase. This result is indicative of a first-order phase change. A 22-${\mathrm{cm}}^{\mathrm{-}1}$ mode at room temperature displays a signficantly anharmonic response that may correspond to the primary order parameter or to coupling with an overdamped infrared active mode.

Analysis of asymmetric line shapes of the synchrotron-generated x rays diffracted at 12 K from ca. 85% deuterated single-crystal copper ammonium Tutton salt, (${\mathrm{ND}}_4$${)}_2$Cu(${\mathrm{OD}}_2$${)}_6$(${\mathrm{SO}}_4$${)}_2$, shows a breakdown in translational order that we interpret as regions of two monoclinic phases segregated on a scale of 100–200 unit cells in fixed relative orientation. The separation and orientation can be understood as the best epitactic fit between crystals differing very slightly in Jahn-Teller distortion at the Cu site. Previous studies have shown two distinct much larger Jahn-Teller distortions at the Cu site to be close in free energy. At 180 K more perfect order is regained, and the process is reversible. This indicates that the phase separation is a property of the macroscopically uniform single crystal, and is not caused by the crystal existing in two large as-grown parts differing in deuteration. At higher temperatures, to 320 K, linewidths again increase, but then with no asymmetry. The decrease in domain size may be connected with an approaching order-disorder transition, which is also reflected in changes in the cell constants and in the ammonium-ion librational motions.

We report the observation of two resonance channels in the 2p resonant photoemission of ferromagnetic nickel using circularly polarized soft x rays. By identifying the local 3${\mathit{d}}^8$ and 3${\mathit{d}}^7$ configurations in the valence-band photoemission spectra, we are able to assign structures in 2p core-level-absorption and magnetic-circular-dichroism spectra to many-body 2${\mathit{p}}^5$3${\mathit{d}}^{10}$ and 2${\mathit{p}}^5$3${\mathit{d}}^9$ configurations as well as to a critical point in the band structure. The Ni data provide a critical test for theoretical models of x-ray absorption and magnetic-circular-dichroism spectroscopy of transition-metal magnetic materials. This work also demonstrates the unique possibilities provided by circularly polarized light for separating the Auger process from the resonant-photoemission process and unraveling the electronic structure.

The structural features and magnetic properties of the intermetallic compounds α-${\mathrm{Co}}_2$Ge (low-temperature phase, LTP) and β-${\mathrm{Co}}_2$Ge (high-temperature phase, HTP) upon mechanical milling were investigated by x-ray diffraction, differential scanning calorimetry (DSC), and magnetic measurements. It turns out that starting from both the ordered LTP with ${\mathrm{Co}}_2$Si-type orthorhombic structure and the ordered HTP with ${\mathrm{Ni}}_2$In-type hexagonal structure mechanical milling generates atomic disorder during the early stage of milling and transforms the materials to an amorphous state after long-time milling. Both the LTP and the HTP are ferromagnetic at lower temperatures with Curie temperatures of 46.4 K and 6 K, respectively. The magnetic moment per Co atom at 4.2 K is 0.113${\mathrm{\mu }}_{\mathit{B}}$ in the LTP and 0.103${\mathrm{\mu }}_{\mathit{B}}$ in the HTP. The average magnetization at 4.2 K and the Curie temperatures of the LTP and the HTP continuously increase with increasing milling time in the early stage of milling. During the intermediate stage of milling, a discontinuous decrease in magnetic-ordering temperature (a mixture of two magnetic phases) is observed in the LTP, which strongly indicates the formation of the amorphous phase. In DSC scans the exothermic heat effects are evident, which correspond to the atomic reordering process of the disordered compounds and to the crystallization of the amorphous phase and the subsequent growth of nanometer-scale crystallites. All physical parameters measured in the present investigation tend to become constant after prolonged periods of milling. The good agremeent of all the experimental results obtained by different techniques proves that by mechanical milling well-defined metastable states in ${\mathrm{Co}}_2$Ge are generated. The amorphous phase as the final state shows spin-glass behavior: a transition at 43 K from the paramagnetic state to the spin-glass state is clearly observed upon cooling from room temperature to liquid-helium temperature.

The scalar-electric-potential distribution set up by an ion traveling in the vicinity of a plane solid-vacuum interface, that is, the surface-wake potential, is investigated with the specular-reflection model to describe the response of the surface and with the random-phase approximation for the dielectric function of the bulk material. This permits us to address the study of the low-velocity surface wake: the static potential is found to have a dip at the position of the ion; that dip is shifted towards the direction opposite to the velocity vector for velocities smaller than the threshold of creation of plasmons (≊1.3${\mathit{v}}_{\mathit{F}}$). Extensive numerical calculations are presented for an ion both inside and outside aluminum. Comparison to the results obtained with the plasmon-pole dielectric function indicates excellent agreement for velocities larger than ≊1.3${\mathit{v}}_{\mathit{F}}$. On the other side, the possibility of surface-wake riding is suggested, by analogy with bulk-wake riding postulated in the past. In it, the electron would be bound in the first trough of the surface-wake potential set up when the ion describes a grazing trajectory. The main feature introduced by the surface with respect to the bulk consists of allowing the use of ions of higher charge, reducing in this way the relative importance of the electron self-energy, and in addition, giving rise to larger binding energies. When the ion beam is directed along a special direction of an oriented crystal surface, the mechanism of resonant coherent excitation could provide a way for experimentally detecting this phenomenon through the emission of the bound electron with well-defined energy and around a preferential direction.

The problem of scattering of high-energy electrons by solids is formulated on the basis of a kinetic equation (KE) for the one-particle density matrix. This equation provides a general treatment of spatial and temporal coherence of electrons and takes account of both elastic and inelastic scattering for arbitrary geometry of diffraction. Formulating the KE, we prove that if the energy of an electorn is sufficiently high (E∼100 keV), the problem of multiple elastic and inelastic scattering by a solid is entirely determined by two universal functions, namely, the Coulomb potential averaged over the motion of the crystal particles and the mixed dynamic form factor of inelastic excitations, which is related to the time-dependent correlation function of the positions of electrons and nuclei in a solid. We show that in all the diffraction experiments the scattering cross section contains information about both these functions and discuss the possibility of their separate determination. The KE method generalizes previous theoretical approaches to the description of multiple inelastic scattering of high-energy electrons, and in the case of single inelastic interactions the solution of the KE reduces to the distorted-wave approximation. As an illustration, we consider an application of the KE to the problem of multiple scattering of high-energy electrons by collective electronic excitations of a crystal. Numerical solution of the KE is shown to be consistent with experimental observations, and evidence is found for the existence of a mechanism of damping of coherence by small-angle inelastic scattering of high-energy electrons by a crystal.

Shake-up lines associated with the ${\mathrm{Na}}^{+}$ [1s2p]3p atomiclike configurations in NaCl and several Na compounds have been identified in x-ray photoelectron spectra using commercial Al and Mg Kα sources. Two clearly separated peaks in a ratio of about 1:2 split by 4.4 eV and with a binding energy of 56.6 and 61.0 eV greater than that of the 1s peak are observed. The possibility to perform a similar shake-up spectroscopy on heavier third-period elements for atomic and condensed-matter studies is addressed.

We have investigated classical waves propagating in periodic binary composites. For electromagnetic waves the network topology, in which the low-velocity material forms a continuous network, is more favorable for the appearance of gaps. In contrast, for scalar and elastic waves the cermet topology, in which the low-velocity material consists of isolated inclusions surrounded by the high-velocity host material, is more favorable.

In order to demonstrate experimentally that the competition between the antiferroelectric and ferroelectric interactions stabilizing Sm${\mathit{C}}_{\mathit{A}}^{\mathrm{*}}$ and Sm${\mathit{C}}^{\mathrm{*}}$ (the pairing energy of the transverse dipole moments in adjacent smectic layers and the packing entropy resulting from the excluded volume effect) produces a variety of antiferroelectric and ferroelectric subphases in the temperature region between antiferroelectric Sm${\mathit{C}}_{\mathit{A}}^{\mathrm{*}}$ and ferroelectric Sm${\mathit{C}}^{\mathrm{*}}$, we have tried to observe the subphases in several binary mixtures of chiral smectic liquid crystals. The subphases were identified by the electric field dependence of cono- scopic figures and the apparent tilt angle as a function of field strength. Systematically changing the mixing ratio of two properly chosen compounds and studying the stability of each subphase, we have substantiated that the subphases between Sm${\mathit{C}}_{\mathit{A}}^{\mathrm{*}}$ and Sm${\mathit{C}}^{\mathrm{*}}$ form a Devil’s staircase, Sm${\mathit{C}}_{\mathit{A}}^{\mathrm{*}}$(${\mathit{q}}_{\mathit{T}}$). Here, ${\mathit{q}}_{\mathit{T}}$ is an irreducible rational number, which specifies a fraction of the ferroelectric ordering in the antiferroelectric ordering. We have also discussed the relationship of Sm${\mathit{C}}_{\mathit{A}}^{\mathrm{*}}$(${\mathit{q}}_{\mathit{T}}$) to the other staircase, Sm${\mathit{C}}_{\mathrm{\alpha }}^{\mathrm{*}}$(${\mathit{q}}_{\mathit{T}}$), that was proposed to exist as a result of the rather macroscopic, electrostatic interaction among two-dimensional spontaneous polarizations in smectic layers.

In this paper the segregation of block copolymer deuterated polystyrene 2-vinylpyridine micelles in a thin film homopolymer polystyrene matrix, observed by cross-sectional transmission-electron microscopy, is presented. The experimental results together with a theoretical analysis demonstrate that the micelles observed must have been formed already in the solution from which the films were prepared by spin coating. Annealing at temperatures above the glass-transition temperatures leads to preferential segregation of the micelles to existing interfaces. Furthermore at sufficiently high concentration, micelles are found to form clusters due to their mutual attraction. It is demonstrated that existing theories compare favorably with these experimental results.

We have studied the structural, dynamical, and electronic properties of liquid ${\mathrm{Li}}_{12}$${\mathrm{Si}}_7$ by means of first-principles molecular-dynamics simulations. We find that the Si atoms give rise to a covalently bonded network that can be described as originating from interconnected short chains and stars. The elements of the crystal structure that survive in the liquid state are the preferential twofold Si coordination and the angle between Si bonds. The structure factor is in overall agreement with neutron-scattering data, except for a slight nonuniform shift of peak positions. Electronic-structure calculations show that the Fermi energy is in a minimum of the electronic density of states. The resulting dc conductivity is in good agreement with the resistivity measurements by Meyer et al. Our results appear consistent with the extended Zintl principle recently proposed by Hafner and Jank.