Photosensitivity of Sol-Gel Derived Aluminosilicate Planar Waveguides Doped with Ce3+ Ions

Aluminosilicate planar waveguides doped with Ce³⁺ ions have been prepared by combining a sol gel process and the dip-coating technique. Characterization of the samples has proven their good optical quality and their amorphous structure. For the first time to our knowledge, the photosensitivity of these materials has then been studied. The effect of hydrogen loading on the photosensitivity has been worked out. High fluence irradiation led to the photoablation of the film, but low fluence irradiation demonstrated a variation of the refractive index of n = 5.10^–4 for the hydrogenated Ce³⁺-doped waveguides. These results confirm the potentiality of this material for use in integrated optics. From a fundamental point of view, the versatility of the sol-gel process will allow the study of the effect of the chemical parameters on the photosensitivity of this material and thus a better understanding of the mechanisms of its photorefractivity.     

Vibration—rotation spectra of CO in liquid alkanes

Vibration—rotation infrared spectra have been obtained for CO dissolved in a variety of branched and normal alkanes. The normalized band profiles yield rotational correlation functions, correlation times and the torques acting on the CO molecule. These quantities and the band shape depend only on the alkane free volume, contrasting with the situation for DC1 solute where they showed a strong dependence on solvent molecular shape. The Gordon m-diffusion model for rotational diffusion gives good predictions for the correlation functions and a complete fit is obtained using the generalized m-diffusion model incorporating a small frequency-dependence of the effectiveness of a collision in changing the rotor angular momentum. This success of simple stochastic theory contrasts with its failure for DCl in these alkanes.     

Pulsed NMRON studies of insulating magnetic materials

The apparatus and instrumental techniques developed for pulsed NMRON studies of insulating ordered magnets are described. Experiments have been performed on two salts,⁵⁴Mn-MnCl₂·4H₂O and⁵⁴Mn-Mn(COOCH₃)₂·4H₂O, and rotation patterns, free induction decays and spin echoes have been obtained for both single quantum and double quantum transitions. The pulsed technique can produce signals larger than those obtained in CW NMRON, and is well suited for the measurement of the very short spin-lattice relaxation times that can occur in these systems.     

Nuclear orientation and nuclear spin-lattice relaxation in insulating magnetically ordered crystals

The mechanism of nuclear spin-lattice relaxation in insulating magnetically ordered crystals at low temperatures is discussed. In 3-dimensional systems the relaxation time T₁ is long, but in the lower dimensional systems it can be quite short making them possible candidates for hosts in on-line experiments. An experiment in 2-dimensional⁵⁴Mn−Mn (COOCH₃)₂·4H₂O is discussed with particular reference to the relatively short value of T₁ in low applied fields. A preliminary experiment in the 1-dimensional system⁵⁴Mn−(CH₃)₄NMnCl₃ (TMMC) is also described. Pulsed NMRON measurements in⁵⁴Mn−MnCl₂·4H₂O are outlined and the advant-systems with fast relaxation emphasized.     

Temperature-dependent a factor in thermal unimolecular reactions II. Exponential dependence

It is assumed that A_∞ in the limiting high-pressure unimolecular rate constant expression k_∞ = A_∞ exp(?E_a∞/kT) is given by A_∞ = A″_∞e^BkT. The test case is the decomposition butane → 2 ethyl, where a negative B reproduces the temperature dependence of A_∞ about as well as the previously considered power law A_∞ = A′_∞(kT)ⁿ. It is shown that the term e^BkT modifies significantly the high-temperature falloff of the general-pressure rate constant k_uni and its various derivatives, and admits of a fairly simple interpretation in terms of Benson's restricted rotor theory. On physical grounds, the exponential law appears more reasonable than the power law.     

A study of the induced magnetic hyperfine fields in Fe/Ag multilayers

Near perpendicular magnetic hyperfine fields in ^110mAg have been observed in Fe/Ag multilayers. These fields are studied by low temperature nuclear orientation (LTNO) in multilayers [Ag(x ML)/Fe(y ML)]₂₀, with (x,y) monolayer (ML) values of (2,10), (3,9), (5,10) and (3,18). The ^110mAg γ-ray anisotropy was measured as a function of applied magnetic field parallel to the multilayer. The average induced hyperfine field of Ag is significantly influenced by the quality of the multilayer as measured by X-ray diffraction. This revised version was published online in August 2006 with corrections to the Cover Date.     

On the Origin of the Non-Arrhenius Na-ion Conductivity in Na3OBr

The sodium-rich antiperovskites (NaRAPs) with composition Na₃OB (B=Br, Cl, I, BH₄, etc.) are a family of materials that has recently attracted great interest for application as solid electrolytes in sodium metal batteries. Non-Arrhenius ionic conductivities have been reported for these materials, the origin of which is poorly understood. In this work, we combined temperature-resolved bulk and local characterisation methods to gain an insight into the origin of this unusual behaviour using Na₃OBr as a model system. We first excluded crystallographic disorder on the anion sites as the cause of the change in activation energy; then identified the presence of a poorly crystalline impurities, not detectable by XRD, and elucidated their effect on ionic conductivity. These findings improve understanding of the processing-structure-properties relationships pertaining to NaRAPs and highlight the need to determine these relationships in other materials systems, which will accelerate the development of high-performance solid electrolytes.