X-ray reflectometry measurements have been made on Langmuir-Blodgett (LB) films of an amine-substituted tertiary octyl calix[8]arene and of alternating layers of acid and amine-substituted tertiary octyl calix[8]arene as well on LB films of alternating layers acid/amine-substituted tertiary butyl calix[4]arenes. The total film thickness and the bilayer spacing were obtained for each sample from its reflectivity profile R(Q) vs. Q. Simulations of the reflectivity profiles were made with a programme based on Parratt's recursive relations, using models of their multilayer structures. The "pinched loop" molecular conformation which, rather surprisingly, describes the layer structure in an acid substituted tert-octyl calix[8]arene, also describes the structures in the amine-substituted and the alternate layer acid/amine-substituted calix[8]arenes, with the exception that the amine head-groups are larger and their interpenetration is prevented. In contrast, the smaller calix bowl in the acid/amine-substituted calix[4]arene molecules defines their conformation more rigidly. The layer structure of the alternate layer acid/amine-substituted calix[4]arene sample is therefore well described by the repetition of the classical bowl and chain configuration. (C) 2013 Elsevier B.V. All rights reserved.

An electro-optical activity has been recently reported for hybrid nanocomposite thin films where inorganic silicon carbide nanocrystals (ncSiC) are incorporated into polymer matrices. The role of the interface SiC polymer is suggested as the origin of the observed second order nonlinear optical susceptibility in the hybrid materials based on poly-(methylmethacrylate) (PMMA) or poly-(N-vinylcarbazole) matrices. In this work, we report an analysis of the electro-optical response of this hybrid system as a function of the ncSiC content and surface state in order to precise the interface effect in the observed phenomenon. Two specific ncSiC samples with similar morphology and different surface states are incorporated in the PMMA matrix. The effective Pockels parameters of the corresponding hybrid nanocomposites have been estimated up to 7.59 +/- 0.74 pm/V (1 wt. % of ncSiC in the matrix). The interfacial region ncSiC polymer is found to play the main role in the observed effect. Particularly, the electronic defects on the ncSiC nanocrystal surface modify the interfacial electrical interactions between the two components. The results are interpreted and discussed on the basis of the strong influence of these active centers in the interfacial region at the nanoscale, which are found to monitor the local hyperpolarizabilities and the macroscopic nonlinear optical susceptibilities. This approach allows us to complete the description and understanding of the electro-optical response in the hybrid SiC/polymer systems.

The vibrational properties of silicon carbide nanoparticles (np-SiC) were investigated as function of the nanocrystal size (5-25 nm) and the features of their outermost surfaces. Raman experiments and numerical methods were conjugated to characterize the signatures from the active SiC normal modes and the vibrational density of states (VDOS). The Raman spectra of the nanopowders were marked by VDOS signals which correlate with the SiC amorphous fractions favoured by the high specific surfaces of the nanoparticles and their surface reconstruction. Quantitative interpretation of the experimental VDOS features, IR absorption and Raman scattering properties in nanosized SiC were carried out by means of numerical methods developed on SiC clusters with suitable structures and sizes.

Experiments performed by x-ray reflectivity, grazing incidence small angle x-ray scattering (GISAXS), and transmission electron microscopy (TEM) on a cosputtered nanocermet thin film of Pt-Al2O3 are presented. It is shown that the morphology of such a heterogeneous material can be well interpreted by combining the information obtained from the three techniques. In particular, the layering of metal nanoparticles in the immediate vicinity of the substrate is clearly evidenced. GISAXS results are interpreted via a model which yields spherical nanoparticles of diameter 2R=3.1 nm, separated on the average by a distance of 5.8 nm. The evidence for the layering of particles close to the substrate is deduced from the analysis of the specular reflectivity and probed directly by TEM.

Metal-organic multilayers of manganese stearate prepared by Langmuir-Blodgett (LB) technique on hydrophobic Si substrate were studied by grazing incidence X-ray scattering techniques. Grazing incidence X-ray diffraction measurements show distorted hexagonal in-plane structure of the molecules. Reflectivity measurements show that the LB films consist of two types of blocks having slightly different bilayer separation, but well arranged. The reasons for which these LB films present such imperfection are discussed. (C) 2000 Elsevier Science B.V. All rights reserved.

A full investigation of structural instabilities in the mixed perovskite crystal Rb0.68K0.32CaF3 was performed by optical birefringence, X-ray diffraction and Raman scattering. A sequence for the phase transitions, according to the Glazer formalism, is proposed in agreement with that occurring in both the pure crystals RbCaF3 and KCaF3. Raman spectra are interpreted using group theory analysis and discussed in the framework of a classical Landau behaviour. With this work, the unusual phase transition mechanism observed for the perovskite KCaF3 appears accessible to experiment and is then studied in detail.

It is shown here that the observation of the phenomenon of like small angle scattering of X-rays in very thin heterogeneous films, can be made comparatively easily by using grazing angle reflectometry of X-rays. The feasibility was achieved with co-sputtered thin films of approximately 600 Å thickness, made up by crystalline platinum clusters embedded in an amorphous alumina matrix. The experimental reflectivity profiles are simulated by the intensity superposition of two components: (i) the specular part caused by the usual interference phenomenon between the partial waves reflected from the air-film and film-substrate interfaces, and (ii) the like-small angle scattering part due to diffraction by platinum clusters. It is found that the shape of such clusters is spherical characterized by mean values of diameter \(\) and inter-cluster distance \(\) of the order 29 Å and 45 Å respectively with standard deviations \(\) and \(\) of the order of 3 Å. Such an observation of both the interference and diffraction phenomena indicates that the thin granular film exhibits both its continuous and heterogeneous aspects together.

The nuclear and magnetic structures of the layered compound (that undergoes structural and magnetic phase transitions at and respectively) are investigated by neutron powder diffraction over a large temperature range (673-2 K). At 673, 543 and 413 K, it is deduced that exhibits the ideal high symmetry structure (aristotype) with Bmmb space group; there is no evidence for the phase transition observed by Hidaka et al at 563 K. The diffraction patterns collected below (at 300, 170 K) are consistent with the Pcmn space group and the phase transition mainly arises from octahedron rotation around the [001] axis. The magnetic structure is reinvestigated in the framework of the new low temperature space group. It is deduced that the moments are aligned normally to the sheets with antiferromagnetic coupling in the planes. It seems that only the coexistence of two phases can describe the magnetic interactions between the planes.

(samarium fluoride) exhibits orthorhombic symmetry (space group Pnma; type) at room temperature and rhombohedral symmetry (space group type) at high temperature. In this paper we describe a study of the structural phase transition mechanism occurring in this compound performed by means of differential thermal analysis, x-ray diffraction and Raman scattering. The transition temperature was determined to be and the first-order character of the orthorhombic-rhombohedral phase transition was confirmed. The crystal structure of was refined in the two phases using a Rietveld powder diffraction method. The correlation between the coordination polyhedra appropriate to the two structure types has been established and a proposed transition mechanism is described in this paper. Moreover, a full vibrational investigation of both phases of is presented together with a group theory analysis.

X-ray diffraction studies on solution-grown LiKSO4 have been performed between 70 K and 300 K. The lattice parameters have been obtained as a function of the temperature. The hexagonal to trigonal phase transition at 205 K is seen by a slight increase of the c lattice parameter, while the ferroelastic transition at 182 K is accompanied by the appearance of an important spontaneous elastic deformation. The deformation and the domain pattern are analysed via a 'squeezing' model of the hexagonal lattice which leads to a monoclinic lattice below T-c = 182 K.

The room-temperature Raman spectra of KFeF4 (phase II) were measured under high pressures up to 3.5 GPa. The pressure dependence of the frequencies exhibits strong changes at 1.2 GPa and minor changes at 2.2 Gpa. The former is attributed to a martensitic phase transition leading to the so-called TlAlF4-type structure. A hysteresis of about 0.15 GPa is observed. A strong hardening of two modes under pressure suggests that the phase change is achieved because of octahedra rotations which weaken the clamping between successive octahedra sheets. The singularity at 2.2 Gpa presumably arises from a quasi-reversible phase transition in the TlAlF4-type structural arrangement. The vibrational frequencies are discussed in comparison with those obtained in isomorphous compounds.

In the framework of the study of structural phase transitions in fluoroperovskites AM${\mathrm{F}}_{3}$, sodium manganese fluoride ${\mathrm{NaMnF}}_{3}$ is especially interesting because it is a rare distorted perovskite that can be obtained easily as a monodomain sample in an orthorhombic low-temperature phase. X-ray experiments were performed to confirm this quasimonodomain character. Raman spectra of ${\mathrm{NaMnF}}_{3}$ were recorded and interpreted between 40 and 573 K. From group-theory analysis it was shown that the orthorhombic-cubic high-temperature phase transition which occurs in ${\mathrm{NaMnF}}_{3}$ can be imputed to the condensation of vibrational modes located at the \ensuremath{\Gamma}, R, M, and X points of the first cubic Brillouin zone. Assuming that the Raman modes which are not responsible for the transition are temperature independent and with help from compatibility diagrams, frequencies of high-symmetry zone boundary modes in the high-temperature phase (cubic symmetry) were deduced only from experimental data obtained in the orthorhombic phase. Consequently these data were used to adjust the parameters of a rigid-ion model in the ideal cubic phase. The calculated phonon spectrum and the phonon density of states were deduced. The calculated cubic elastic constants ${\mathit{C}}_{11}$, ${\mathit{C}}_{12}$, ${\mathit{C}}_{44}$ are consistent with the elastic constants measured by Brillouin scattering in the quasicubic approximation. Moreover, the reported Raman study provides evidence for the occurrence of a two-magnon scattering due to antiferromagnetic character below ${\mathit{T}}_{\mathit{N}}$ =60 K with persistence of a broad line above the N\'eel temperature.

KFeF4 is a well characterized planar antiferromagnet with S=5/2 and is an excellent example of a two-dimensional Heisenberg antiferromagnet. Using neutron-scattering techniques, a study of both the static and dynamic critical behaviour above the Neel temperature, TN=136.75+or-0.35 K, has been undertaken. It is found that in the region above 1.04TN, the static correlation length, the structure-factor peak intensity and the width of the dynamic scattering are all qualitatively described by the theories developed for Heisenberg antiferromagnets. The data have been compared with both the theory for a quantum Heisenberg antiferromagnet. As developed for La2CuO4, and also the model for a classical Heisenberg antiferromagnet. Both these theories describe the data equally well. Below 1.04TN the system is expected to behave like a two-dimensional Ising system.

The magnetic properties of the two-dimensional antiferromagnet KFeF4 have been studied by inelastic neutron-scattering. The spin- wave dispersion curve is measured in the (h h l) plane at 15K. The existence of a gap (0.65 THz) is confirmed. The dispersion along the [0 0 ξ] direction, normal to FeF6 sheets is smaller than the experimental accuracy; on the other hand a strong dispersion is observed along the [ξ ξ 0] direction. The dispersion curves are well described by a simple spin-wave theory (HE = 211 T, HA = 1.3 T, |J| = 9.8 cm-1, α = 6.2 × 10-3).

We report the observation of one-magnon and two-magnon Raman scattering in the two-dimensional antiferromagnet KFeF4. The magnetic exchange and the anisotropy field values are derived from the experimental data. The exchange interaction slightly but significantly differs from the measured one using inelastic neutron scattering. The high value of the anisotropy field, compared to the predicted one, is confirmed.

The layered compound KFeF4 that exhibits a structural phase transition at Tc approximately=380 K from Bmmb symmetry (phase I) to Pcmn symmetry (phase II) is studied as a function of temperature by polarized Raman scattering. All the Raman lines are attributed on the basis of (i) symmetry analysis of the normal modes of vibration in the aristotype phase (phase I) and the compatibilities relations with the symmetries in phase II; (ii) comparison with RbFeF4 in which the phonon spectrum can be calculated. The phase transition is clearly associated with a soft mode but a residual Raman signal is still observed above Tc; this is attributed to the existence of a disordered phase. Two additional modes exhibit a soft behaviour connected with a (virtual) martensitic transformation.

The structure of K2FeF5 is reinvestigated: space groupPbcn,Z = 16, a = 7.4059(4), b = 12.8771(9), c = 20.4282(13)Å(previous proposition: S.G.Pn21a withc, a, b permutation).R = 0.036 (Rw = 0.040) for 1663 reflections and 149 parameters. All essential features of the previous description are maintained.