Synthesis of ferroelectric perovskites through aqueous-solution techniques

The hydrolysis of niobates in aqueous solutions has been applied to the coprecipitation of PbNb2/3Mg1/303 leading to a low temperature synthesis of this ferroelectric relaxor ceramic. The effect of hydrolysis conditions - such as the concentration of bases and acids used, their rates of addition and the nature of the precursors salts - is described. The perovskite phase appears after heating at 350°C and is obtained as a pure phase at 800°C after heat treatment for 1 h. Relaxor ferroelectric ceramics with a high dielectric constant can be obtained by sintering at 1000°C The process has been also applied to the synthesis of other relaxor ceramic compositions such as PZN (PbNb2/3Zn1/303) and PFN (PbNb1/2Fe1/203) compounds

Optical properties of F centers in KCN crystal

Absorption, excitation and emission spectra of F centers in KCN were measured between 4.5-175 K. Due to the cubic to orthorhombic structural phase transition at 168 K, the F band splits into 3 components. Their properties in the antiferroelectric phase (T <83 K) are explained by an internal Stark effect due to the local electric field. The single emission band has a decay time of 21.5 ns at 4.5 K

Radio and thermoluminescence studies in CsI doped with F centers

Radioluminescence, thermoluminescence, and u.v. excitation measurements in CsI additively colored crystals showed a new luminescent component at 2.5 eV. This is attributed to the radiative recombination of Vk centers with F centers. Another new component at 2.72 eV was observed only under uv excitation of 5.14 eV. This is related to the radiative recombination of localized excitons at F+ center sites (&#945; band)

EPR optical detection of F centre pairs in alkali halides. - I : Pumping cycle kinetics and characteristics of the resonances

The EPR of F centres in the ground and excited states was optically detected in the following alkali halide crystals: NaCl, KF, KCl, KBr, KI, RbBr, and RbI. A decrease of the radiative quantum efficiency of the F centre luminescence was observed when microwave transitions were induced between the spin levels. The mechanism responsible for this effect was an electronic tunnelling through the crystal field potential; the electron in the relaxed excited state of an F centre (F~*) is transferred nonradiatively to another nearby F centre in its ground state (F0), and leads to the momentary formation of an ƒ¿ and an F\u27 centre. Such a process is a function of the total spin of the F~*-F0 pair. The role played by the paired centres was confirmed by measurements at different F centre concentration. Moreover, at high optical excitation pumping rates, the population of the intermediate complexes (F\u27-ƒ¿) is large enough to allow an estimation of the rate of the reverse process F\u27 + ƒ¿ \u27¨ F0 + F0

Magnetooptic properties of self-trapped excitons in cesium halides

A model for the electronic structure of self-trapped excitons in Cs halides is proposed. By applying it, reasonable interpretations were obtained for the magnetic circular polarization (MCP) of the 2 intrinsic emissions in CsI, and for the MCP and EPR in CsBr

Avalanche dynamics, surface roughening and self-organized criticality - experiments on a 3 dimensional pile of rice

We present a two-dimensional system which exhibits features of self-organized criticality. The avalanches which occur on the surface of a pile of rice are found to exhibit finite size scaling in their probability distribution. The critical exponents are $\tau$ = 1.21(2) for the avalanche size distribution and $D$ = 1.99(2) for the cut-off size. Furthermore the geometry of the avalanches is studied leading to a fractal dimension of the active sites of $d_B$ = 1.58(2). Using a set of scaling relations, we can calculate the roughness exponent $\alpha = D - d_B$ = 0.41(3) and the dynamic exponent $z = D(2 - \tau)$ = 1.56(8). This result is compared with that obtained from a power spectrum analysis of the surface roughness, which yields $\alpha$ = 0.42(3) and $z$ = 1.5(1) in excellent agreement with those obtained from the scaling relations.Comment: 7 pages, 8 figures, accepted for publication in PR

The Structural Colors of Photonic Glasses

The color of materials usually originates from a combination of wavelength-dependent absorption and scattering. Controlling the color without the use of absorbing dyes is of practical interest, not only because of undesired bleaching properties of dyes but also regarding minimization of environmental and health issues. Color control without dyes can be achieved by tuning the material's scattering properties in controlling size and spatial arrangement of scatterers. Herein, calibrated photonic glasses (PGs), which are isotropic materials made by random aggregation of nonabsorbing, monodisperse colloidal polystyrene spheres, are used to generate a wide spectral range of purely structural, angular-independent colors. Experimental reflectance spectra for different sized spheres compare well with a recent theoretical model, which establishes the latter as a tool for color mapping in PGs. It allows to determine the range of visible colors accessible in PGs as function of size, packing fraction, and refractive index of scatterers. It also predicts color saturation on top of the white reflectance as function of the sample's optical thickness. Blue, green, and red are obtained even with low index, while saturated green, cyan, yellow, and magenta can be reached in higher index PGs over several orders of magnitude of sample thickness.Deutsche Forschungsgemeinschaft. Grant Number: SFB1214 Swiss National Science Foundation. Grant Number: 200020M_162846 Alexander von Humboldt Foundatio