Diffuse X-ray scattering and nanoclusters in the Hg2Br2 model ferroelastics

Diffuse maxima are revealed and studied for the first time in the x-ray scattering patterns of Hg₂Br₂ single crystals. These maxima originate from clusters of the orthorhombic phase due to the phase transition in the paraphase tetragonal matrix. The nucleation and growth of clusters are initiated by spatiotemporal fluctuations of the order parameter corresponding to the soft transverse acoustic mode at X points of the Brillouin zone boundary. Information on the behavior of the susceptibility and correlation length with variations in temperature and on the shape and anisotropy of the clusters is obtained, and the critical exponents are determined.     

Influence of a strontium-enriched intergranular cluster network on the electrical conductivity of In2O3-SrO ceramics

A metastable hexagonal R-phase is revealed in polycrystalline In₂O₃-SrO samples, which has the form of a network made up of mesoscopic clusters (60–180 Å in size). The clusters arise from strontium-enriched regions near grain boundaries in the main cubic structure of indium oxide. It is shown that annealing in oxygen at T _a ? 300°C saturates dangling bonds between the R-phase and the matrix and makes the system metastable. This state shows up in the presence of (i) solitary diffuse maxima from the R-phase imposed on Debye lines from the main phase in the X-ray diffraction pattern and (ii) the electron cyclotron resonance (ECR) line with g = 1.875. In addition, the sample in this state acquires a high resistivity (ρ ～ 10⁶ Ω cm). Relaxation at T ? 300°C after annealing at T _a > 300°C disrupts bonds between the strontium-enriched clusters of the R-phase and the indium oxide matrix. This causes spatial separation of the clusters, disruption of their coherent bonds with the matrix structure, and escape of excess oxygen from the sample along grain boundaries. As a result, a new stable state forms, which is characterized by (i) a series of diffuse maxima from the R-phase imposed on lines assigned to the main phase, (ii) the presence of the ECR line with g = 2 with the line with g = 1.875 retained, and (iii) the transition of the sample to a low-resistivity state (ρ ～ 100 Ω cm).     

Thermopower and electrical resistivity of La0.875Sr0.125MnO3 single crystals under hydrostatic pressure

The thermopower α and the electrical resistivity ρ of La_1?x Sr_xMnO₃ single crystals with a strontium content x=0.125, which corresponds to the stoichiometric composition of the new charge-ordered (CO) phase, are measured in the temperature range 77–300 K at pressures up to 12 kbar. The dependence α(T) exhibits two maxima. The first low-temperature maximum is associated with the formation of the charge-ordered phase. The second high-temperature maximum is attributed to the O → O′ structural transition between the orthorhombic phases and the formation of ferromagnetic clusters. It is found that the phase transition observed at P>9.2 kbar is accompanied by a substantial shift of both maxima toward the low-temperature range.     

Clusters in the model ferroelastic Hg2Cl2

The critical fluctuations induced by a ferroelastic phase transition and corresponding to the soft TA mode at the X point at Brillouin zone boundary have been studied in the model crystal Hg₂Cl₂ within a broad temperature and wave-vector range. Anisotropic diffuse x-ray scattering maxima associated with order-parameter fluctuations and nucleation of low-temperature orthorhombic clusters in the high-temperature tetragonal matrix have been found to exist at these points. New information has been obtained on the temperature dependence of the susceptibility and correlation length, cluster shape and anisotropy, and the critical exponents. Fiz. Tverd. Tela (St. Petersburg) 41, 148–152 (January 1999)     

Effect of ionizing radiation on the dielectric characteristics of TlInS2 and TlGaS2 single crystals

The dependences of the permittivity and electrical conductivity of TlInS₂ and TlGaS₂ single crystals on the temperature and electron beam irradiation dose have been studied. It has been established that, as the electron irradiation dose increases, the electrical conductivity σ significantly increases, whereas the permittivity ? decreases over the entire temperature range covered (80–320 K). It has been shown that anomalies in the form of maxima in the temperature dependences σ(T) and ?(T) are observed in the regions characteristic of phase transitions in TlInS₂. Irradiation of the TlInS₂ and TlGaS₂ crystals with electrons to doses of 10¹⁵ and 10¹⁶ cm^?2 does not affect their phase transition temperatures. The dispersion curves of the permittivity ? of the TlGaS₂ crystal have been constructed.     

Electronic photodissociation spectra of the Arn-C4H2 (n=1-4) weakly bound cationic complexes

Electronic spectra of a series of weakly bound clusters consisting of argon (Ar_n, n=1-4) bound to the butadiyne cation, C₄H₂⁺, have been recorded in the visible range from 440 to 520 nm by photodissociation. The C₄H₂⁺ fragment signal was recorded as a function of the laser wavelength during excitation of the A←X electronic transition. The observed transitions were assigned to the band origin of the cationic complexes and to vibronic bands involving excitation of the ν₃ and ν₇ vibrational modes of the C₄H₂⁺ moiety, as well as combination bands of these modes. Comparison of the photodissociation spectra of the various clusters reveals a small blue shift, 25 cm⁻¹ of the band maxima relative to the corresponding transitions reported from gas phase spectra of the bare C₄H₂⁺ cation. The magnitude of the blue shift of each band increases with successive Ar solvation up to n=3. Furthermore, each band becomes increasingly broadened towards the red with the addition of Ar atoms due to an increasing number of unresolved transitions involving excited intermolecular modes.     

Magic numbers of sodium clusters

The cohesive energy E_c of crystalline-like b.c.c. and f.c.c. sodium clusters is calculated as a function of the number of atoms in the cluster using the density functional formalism. The maxima of E_c define the theoretical magic numbers for crystalline clusters. A comparison with theoretical results using the jellium background model and with experimental magic numbers obtained by Knight and co-workers suggests that sodium clusters prepared in those expeiments are in a disordered liquid-like or amorphous state.     

A neutron polarization analysis study of Ce2Fe17 in its paramagnetic phase

Spin-flip (paramagnetic) scattering and neutron depolarization studies were performed on Ce₂Fe₁₇ in its paramagnetic phase on the Dhruva neutron polarization analysis spectrometer. The absence of normalQ dependence of the scattered spin flip intensity shows that Ce₂Fe₁₇ is not a normal paramagnetic and there exist superparamagnetic clusters of sufficiently large dimensions (～100Å). The observed neutron depolarization gives an indication of the dynamics of these Ce₂Fe₁₇ superparamagnetic clusters.     

E-T phase diagrams of a solid solution of the multicomponent PbZn1/3Nb2/3O3-PbMg1/3Nb2/3O3-PbNi1/3Nb2/3O3-PbTiO3 system near the morphotropic phase boundary

The specific features of the dielectric and pyroelectric responses of a solid solution in the barium-doped multicomponent yPbZn_1/3Nb_2/3O₃-mPbMg_1/3Nb_2/3O₃-nPbNi_1/3Nb_2/3O₃-xPbTiO₃ system of the composition y = 0.0982, m = 0.4541, n = 0.1477, and x = 0.3 near the morphotropic phase boundary have been investigated. It has been assumed that the maxima revealed in the dependences of the reversible permittivity on the electric field strength with both the forward and backward changes in the field (E ₌) are associated with the induced phase transition. Based on the experimental results, the E-T phase diagrams are constructed in the temperature range from ?100 to 150°C for different temperature-field regimes: (i) variation in the electric field E ₌ at a fixed temperature of the sample and (ii) variation in the temperature of the sample at a constant value of E ₌. It has been found that there is a singular point in the E-T phase diagram and that, in the vicinity of this point, the dielectric and pyroelectric responses of the studied ceramics exhibit specific features.     

Density-functional theory study of structures,stabilities, and electronic properties of the Cu2-doped silicon clusters: Comparison with pure silicon clusters

Equilibrium geometric structures, stabilities, and electronic properties of Si_nCu₂ (n=1–8) clusters and pure silicon Si_n (n=3–10) clusters are investigated systematically by exchange-correlation density functional (B3LYP). The optimized geometries show that the most stable isomers have 3D structure for n=2, 4–8, and Cu-substituted Si_n+2 clusters is dominating growth pattern for the Si_nCu₂ clusters. The calculated averaged binding energies, fragmentation energies, second-order difference of energies, and the HOMO–LUMO gaps show that Si₂Cu₂ and Si₅Cu₂ clusters have enhanced relative stabilities and chemical stability than their neighboring clusters. Electronic properties of Si_nCu₂ (n=1–8) clusters are studied by calculating the natural population analysis and electrostatic potential, where the results show that the two copper atoms always possess positive charge and positive potential surround them. In addition, the VIP, VEA and the chemical hardness (η) are also analyzed and compared.     

Influence of defects on the phase transition in squaric acid (H2C4O4)

We have investigated the influence of defects induced by chromium and deuterium doping on the phase transition in solid squaric acid (H₂C₄O₄) by high resolution ¹³C NMR. Deuterium doping alone is observed to increase the phase transition temperature T_c linearly with the ²H concentration, whereas chromium defects, destroy the local order and lead to clusters of the high temperature phase. Correspondingly the critical temperature decreases with increasing chromium doping and the critical temperature region is smeared out.     

EXAFS study of the local Gd environment in the magnetic superconductor (La1−xGdx)Os2

The near-neighbor environment of Gd in the pseudobinary compound La_1?xGd_xOs₂ has been investigated by studying the EXAFS on the Gd LIII, La LII, and Os LIII edges. For Gd concentrations of x = 0.07 and x = 0.14, we find that the Gd predominantly substitutes on the La sites in the LaOs₂ lattice rather than forming local Gd metal or GdOs₂ clusters. These results are discussed in view of the fact that La_1?xGd_xOs₂ shows spin-glass-like properties (x ≤ 0.15) in the superconducting phase.     

Structural transformation in Mg2NiH4

X-Ray diffraction measurements show that on heating Mg₂NiH₄ in a 1 atm pressure H₂ atmosphere, above ～250°C it transforms into a cubic structure, metal atoms in CaF₂ arrangement, a = 6.525 Å. It is concluded that the H atoms are in tetrahedral clusters, and that the structure is only weakly ionic. This conclusion is also supported by NMR measurements. The 20°C structure of Mg₂NiH₄ is shown to be describable primarily as a slight monoclinic distortion of the cubic unit cell; a = 6.594 Å, b = 6.412 Å, c = 6.490 Å and β = 93.1°. However, weak small angle lines show that a longer range order exists and that the true unit cell, which we have not determined, must be very large. To what extent the cubic phase should be considered a high temperature and/or low concentration (Mg₂NiH_4??) phase is not resolved.     

The phase transition in VO2

The electronic d-states in VO₂ are investigated by a semi-empirical MO LCAO method. The calculations of clusters for the crystal structures above and below the phase transition point show that the energy gap is mainly formed by the crystal distortions. The energies calculated agree well with the ESCA experimental estimations and explain the large gap values obtained from the optical data.     

Disintegration of Ar N,Kr N,and (N2) N clusters during collisions with highly vibrationally excited SF6 molecules

It is shown that, when highly vibrationally excited SF₆ molecules (with vibrational energy E _vib ≥ 0.5–2 eV) collide with weakly bound van der Waals Ar _N , Kr _N , or (N₂) _N (N ≤ 30–40 atoms in a cluster) clusters in intersecting molecular and cluster beams, the molecules are trapped by the clusters, the clusters then undergo full disintegration, and the trapped molecules become free. The method of studying this process and the results obtained are described. The possibilities of application of this method for selective doping of clusters by molecules, laser separation of isotopes, and selective transportation of molecules to a surface are discussed.     

Measurement of nanoparticle temperature in a (CO2) N cluster beam using SF6 molecules as tiny probe thermometers

A temperature measurement technique using SF₆ molecules as tiny probe thermometers is described, and results are presented, for large (CO₂) _N van der Waals clusters (with N ≥ 10²) in a cluster beam. The SF₆ molecules captured by (CO₂) _N clusters in crossed cluster and molecular beams sublimate (evaporate) after a certain time, carrying information about the cluster velocity and internal temperature. Experiments are performed using detection of these molecules with an uncooled pyroelectric detector and infrared multiphoton excitation. The multiphoton absorption spectra of molecules sublimating from clusters are compared with the IR multiphoton absorption spectra of SF₆ in the incoming beam. As a result, the nanoparticle temperature in the (CO₂) _N cluster beam is estimated as T _cl < 150 K. Time-of-flight measurements using a pyroelectric detector and a pulsed CO₂ laser are performed to determine the velocity (kinetic energy) of SF₆ molecules sublimating from clusters, and the cluster temperature is found to be T _cl = 105 ± 15 K. The effects of various factors on the results of nanoparticle temperature measurements are analyzed. The potential use of the proposed technique for vibrational cooling of molecules to low temperatures is discussed.     

Phase transitions in the CN<Subscript>x</Subscript>-TiN system attendant on the variation of the bias voltage during ion-stimulated growth

The structure of amorphous CN_x-TiN films grown by ion-stimulated deposition at a bias voltage U = 200–500 V is studied by X-ray diffraction. As the bias voltage increases in the range U = 300–360 V, the CN_x-TiN films are shown to undergo a phase transition in the amorphous phase having different order scales (20–50 Å): this transition is related to an increase in the content of the fraction of medium-cell (4 Å) carbon clusters as compared to the fractions of clusters with large (8 Å) and small (2 Å) cells. Under these conditions, 80–150 Å crystalline clusters undergo the phase transition from the Ti₂C(N) carbide into graphite (C _g) and diamond (C_d); the last two phases are represented by 100-Å clusters.     

Photoemission investigations on nanostructured TiO2 grown by cluster assembling

Nanostructured titanium dioxide (ns-TiO₂) films were grown by supersonic cluster beam deposition method. Transmission electron microscopy demonstrated that films are mainly composed by TiO₂ nanocrystals embedded in an amorphous TiO₂ phase while their electronic structure was studied by photoemission spectroscopy. The cluster assembled ns-TiO₂ films are expected to exhibit several structural and chemical defects owing to the large surface to volume ratio of the deposited clusters. Ultraviolet photoemission spectra (hv = 50 eV) from the valence band unveil the presence of a restrained amount of surface Ti 3d defect states in the band gap, whereas Ti 2p core level X-ray photoelectron (hv = 630 eV) spectra do not manifestly disclose these defects.     

Molecular trajectory algorithm for random walks on percolation systems at criticality in two and three dimensions

Random walk simulations based on a molecular trajectory algorithm are performed on critical percolation clusters. The analysis of corrections to scaling is carried out. It has been found that the fractal dimension of the random walk on the incipient infinite cluster is d_w=2.873±0.008 in two dimensions and 3.78 ± 0.02 in three dimensions. If instead the diffusion is averaged over all clusters at the threshold not subject to the infinite restriction, the corresponding critical exponent k is found to be k=0.3307±0.0014 for two-dimensional space and 0.199 ± 0.002 for three-dimensional space. Moreover, in our simulations the asymptotic behaviors of local critical exponents are reached much earlier than in other numerical methods.     

A theoretical study of photoelectron spectra of NbO2, MoO2 and RuO2 by cluster models

DV-Xα calculations have been applied to various small clusters of rutile-family dioxides (NbO₂, MoO₂ RuO₂). It appears that by taking into consideration the potential due to the atomic charges, the density, the ionization cross sections of the energy levels, and by summing the density of states (DOS) of the two different clusters representing surface structures, computations on even small clusters provide information which compares well with the experimental XPS spectra.