Table of contents

In this letter we report the synthesis of nanostructures of molybdenum trioxides by directly oxidizing a spiral coil of molybdenum, at ambient atmosphere, by passing a current through the coil. The advantage of this approach is that the temperature of the substrate is low (normally below 200°C), and that the nanostructures to be formed could be chosen, via controlling the current or, equivalently, the temperature of the coil. We show that, by adjusting the current through the coil, α-MoO₃ lamellas with a thickness of ∼20–50 nm, and β-MoO₃ spheres of diameters down to nanometre scale can be synthesized at ambient atmosphere. These nanostructures exhibit a large optical band gap of ∼3.05 eV, and room-temperature photoluminescence at ∼395 nm. This study provides a simple, controllable way of fabricating metal oxide nanostructures of interest.

An algorithmic method is presented for determining all domain configurations and their symmetries in domain average engineered structures. This method is applied to PZN–PT single crystals by determining the domain configurations of domain states which arise in a phase transition between G = mm and F = 3m.

In this review, we will describe calculations using time-dependent density-functional theory (DFT) combined with pseudopotentials to determine excited state properties of matter. While a computational framework for ground state properties of condensed matter is well established, calculations for excited state properties are at a more formative stage. Time-dependent DFT represents an important advance by providing an explicit treatment of relevant correlation effects for electronic excitations. As such, it offers an ab initio formalism for excited states that avoids many of the drawbacks associated with empirical or semi-empirical methods. We will illustrate applications of time-dependent DFT to a variety of systems ranging from molecules and atomic clusters to quantum dots, which contain several hundred atoms.

Photoinduced phenomena are of general interest for new materials. Recently, photoinduced molecular desorption of oxygen has been reported in carbon nanotubes. Here we present, using thermopower measurements, that carbon nanotubes when exposed simultaneously to UV light and oxygen exhibit photoinduced oxidation of the nanotubes. At least two plausible mechanisms for the experimentally observed photoinduced oxidation are proposed: (i) a lower energy barrier for the adsorption of photo-generated singlet oxygen, or (ii) due to the presence of defects in carbon nanotubes that may facilitate the formation of locally electron-deficient and electron-rich regions on the nanotubes which facilitate the adsorption of oxygen molecules on the nanotubes.

The formation of CuZrHfTi bulk metallic glasses (BMGs) and the crystallization of the typical Cu₆₀Zr₂₀Hf₁₀Ti₁₀ BMG under ambient conditions and high pressure have been investigated by differential scanning calorimetry, x-ray diffraction (XRD) and in situ synchrotron radiation XRD. The effects of high pressure on crystallization and formation of the CuZrHfTi alloy are discussed. The Kauzmann temperature, T_K, where the entropy of the undercooled liquid equals that of the crystal, is also determined to be 724 K. The T_K is compared with the experimentally observed rate-dependent glass transition, T_g. The kinetic study of the crystallization shows that the Cu-based BMG has much larger activation energies obtained using Kissinger analysis and is markedly different in crystallization kinetic behaviour compared to that of other BMGs with better glass forming abilities. An apparent correlation between crystallization temperature and activation energy is found in various metallic glasses. The correlation is discussed and connected with the thermal stability of metallic glasses.

Structural phase transitions in the pyridinium iodide [C₅NH₆]^+.I⁻ and bromide [C₅NH₆]^+.Br⁻ crystals were studied by calorimetry, volumetric dilatometry and dielectric methods. The pressure dependences of the transition temperatures were determined by dielectric constant and differential thermal analysis measurements under hydrostatic pressures of up to 1 GPa. Analysis of the experimental data indicated that the p–T phase diagrams of simple pyridinium salts could include tri-critical points in the pressure range above 1 GPa. The effect of external hydrostatic pressure is correlated with the chemical pressure that appears due to the change in the anionic radius. A new model of the phase transition is proposed that includes antiferroelectric order and six-fold disorder of the pyridinium cations, respectively, in the low- and high-temperature phases.

The electronic energy band structure, density of states (DOS) and charge density contour of KNbO₃ in the paraelectric cubic phase have been studied using the full-potential linearized augmented plane wave method within the generalized gradient approximation for exchange and correlation. The band structure shows an indirect (R–Γ) band gap. From the DOS analysis as well as charge density studies, we find that the bonding between K and NbO₃ is mainly ionic while that between Nb and O is covalent. We have also reported results on the pressure variation of the energy gap of this compound and found that the band gap increases with increasing pressure. In order to understand the optical properties of the perovskite, the real and imaginary parts of the dielectric function, reflectivity, absorption coefficient, optical conductivity, electron energy-loss function, refractive index and extinction coefficient were calculated. The general profiles of the optical spectra were analysed and origins of the structures discussed.

Based on first-principles pseudopotential total energy calculations, the effects of Al substitution in Ti₃SiC₂, yielding chemical formula Ti₃Si_0.75Al_0.25C₂, were studied by examining the equilibrium properties and electronic structure. The lattice configurations, cohesive energy, atomic bonding properties and bulk modulus, as well as the electronic band structure, were investigated. Firstly, we obtained the equilibrium crystal parameters that included lattice constants, internal atomic coordinates and bond lengths. Then, the degrees of Ti–Si, Ti–C and Ti–Al covalent bondings in Ti₃Si_0.75Al_0.25C₂ were illustrated and compared with the corresponding values in Ti₃SiC₂ and Ti₃AlC₂ compounds. Furthermore, the magnitude and anisotropy of electrical conductivity were discussed by investigating the density of states and band structure. The bulk modulus of Ti₃Si_0.75Al_0.25C₂ yielded a value close to that of Ti₃AlC₂, which indicated the high bulk modulus had been preserved. In addition, we predicted better oxidation resistance of Ti₃Si_0.75Al_0.25C₂ than that of Ti₃SiC₂, and this was recently verified by an isothermal oxidation experiment of Al-cemented Ti₃SiC₂-based ceramic.

A discussion of the investigation of electronic localization at the thermodynamical limit is given; we show that the usual methods involve practical difficulties, especially for quasicrystals. A method based on the scaling of bands of a supercrystal is proposed. Then, the nature of the localization is related to the stability of the renormalization group around the fixed points of a trace map. For quasiperiodic systems, the scaling exponents are obtained using modified Lyapunov exponents and a Thouless formula. The case of the Fibonacci chain is considered as an example, and analytical expressions for the scaling exponents are found.

We explore, at T = 0, the magnetic properties of the S = 1/2 antiferromagnetic distorted diamond chain described by the Hamiltonian = ∑ _{j = 1}^N/3{J ₁ (S_3j−1 ·S_3j +S_3j ·S_3j+1)+J₂S_3j+1 ·S_3j+2 +J₃(S_3j−2 ·S_3j +S_3j ·S_3j+2)}−H ∑ _{l = 1}^NS_l^zwith J₁, J₂, J₃ ≥ 0, which models well A₃Cu₃(PO₄)₄ with A = Ca, Sr, Bi₄Cu₃V₂O₁₄ and azurite Cu₃(OH)₂(CO₃)₂. We employ the physical consideration, degenerate perturbation theory, level spectroscopy analysis of the numerical diagonalization data obtained by the Lanczos method and also the density matrix renormalization group (DMRG) method. We investigate the mechanisms of the magnetization plateaux at M = M_s/3 and (2/3)M_s, and also show the precise phase diagrams of the (J₂/J₁,J₃/J₁) plane concerned with these magnetization plateaux, where M = ∑ _{l = 1}^NS_l^z and M_s is the saturation magnetization. We also calculate the magnetization curves and the magnetization phase diagrams by means of the DMRG method.

We report measurements and analysis of the initial magnetic susceptibility and specific heat in polycrystalline Dy₃Ni and of magnetization processes in Dy₃Ni single crystals. The measured magnetic ordering temperature is T_N = 38 K and the spin-reorientation-transition and spin-freezing temperatures are T_t ∼ 23–26 K and T_sf = 53 K, respectively. The field-induced magnetic transitions occur at 5.9, 8.3 and 6.4 T along the a, b and c axes, respectively, and strong magnetic anisotropy persists up to 13 T. The energy scales of the exchange interaction and field-induced magnetic structural deformation are comparable.

The crystal structure and elastic and magnetic properties of the La_0.88MnO_x (2.82 ≤ x ≤ 2.96) series as a function of oxygen content have been investigated. The crystal structure of the samples has been found to be orthorhombic at x < 2.91 and monoclinic at x ≥ 2.91 depending on the oxygen concentration. Both crystal structure and elastic properties indicate the presence of static (x < 2.87) or dynamic (x ≥ 2.87) Jahn–Teller distortions under the whole range of doping. The strong correlation between the type of orbital state and magnetic properties of the samples is revealed. It is shown that compounds with an antiferromagnetic component have features of orbital ordering while ferromagnetic samples are orbitally disordered. The magnetic phase diagram of the La_0.88MnO_x (2.82 ≤ x ≤ 2.96) system is proposed on the basis of results obtained.

Dielectric properties of the relaxor ferroelectric ceramics PLZT 8/65/35 and 9.5/65/35 were studied in the broad frequency range of 100 Hz–1 THz at low temperatures below the freezing temperature. Nearly frequency-independent dielectric losses were observed up to 1 GHz on cooling down to 10 K. Their magnitude decreases exponentially with temperature, but remains remarkable high down to 10 K. A Landau-type thermodynamic model based on the perovskite structure near the morphotropic phase boundary is proposed for calculating the energy barriers for polarization reversal near the polar cluster boundaries and explaining the broad distribution function of relaxation times, which fits the observed frequency dependences of permittivity and losses below 1 GHz. High dielectric losses in the submillimetre region were explained by shear wave emission of vibrating polar cluster walls in an ac electric field and by piezoelectric resonances on polar clusters.

High-resolution neutron powder diffraction data were collected from a Pb(Zr_0.54Ti_0.46)O₃ powder sample. Data were analysed using Rietveld refinement. The diffraction patterns showed the presence of two 'coexisting' phases, which had the rhombohedral R3c and monoclinic Cm symmetries at 4 K. A phase transformation between the R3c and Cm phases occurred as a function of temperature. The Bragg peaks of the Cm phase showed strongly reflection-indices-dependent line broadening due to the spatial variation of the Zr/Ti ratio, δx. This was modelled using a microstrain broadening form developed by Stephens (1999 J. Appl. Crystallogr.32 281). Constant microstrain surfaces were plotted for each phase at different temperatures and the microscopic changes responsible for the observed changes were identified. The phase of Cm symmetry is reproducibly absent in the limit of δx = 0.

The structural, electronic and optical properties of tetragonal nonlinear optical (NLO) crystals, AgGa(S_xSe_1−x)₂ (x = 0.0, 0.25, 0.5, 0.75, and 1.0), were investigated theoretically and experimentally. The results obtained indicated that the electronic bandgaps, optical properties and bulk moduli of these compounds were linearly dependent on the substitution concentration of cations. From partial density of state analysis, it was found that the electronic states near the band edges of AgGa(S_xSe_1−x)₂ were a simple proportional mixture of the atomic orbitals of sulfur and selenium. A cell-volume effect was proposed as the major cause of the linear dependence of material properties on the substitution concentration. It was calculated that the second-order NLO susceptibilities were scaled with the cubic power of bandgap, although a minor deviation existed. This deviation arose from the optical transition moment products.

Due to an error in the comparison of magnetic and nuclear Bragg intensities, the magnitude of the ordered moment was underestimated by a factor square root 2. The correct value is mu = 4.72(7) mu_B. This value is close to the expected full Mn²⁺ moment of 5 mu_B and in accordance with the ordered moment observed in many other triangular manganese compounds. In the present case of a classical spin, S = 5/2, no moment reduction due to the partial frustration is expected, in contrast to the statement on page 670.

A second error is located on page 667. The first point in the list of variables should read: Q = 0.3111(5), corresponding to a rotation angle of alpha = ± 112.0(2)° between bc sheets separated by a/2, where the ± sign relates to the two different networks.

The basic conclusions of the article are unaffected. We thank N Stuesser for pointing out the errors.