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We use a first-principles calculation and small-angle neutron scattering (SANS) to investigate the mechanism and the nanosize products of the sol-gel reaction with diphenylsilanediol (DPD) and 3-methacryloxypropyltrimethoxysilane (MEMO) precursors in synthesizing a hybrid waveguide material. It is predicted that switching between a DPD hydroxyl and a MEMO methoxy with a reaction rate of 6.8 x 10(-6) s(-1) at 300 K is the fastest process for the first reaction step, thus generating diphenylmethoxysilanol (DPM) and 3-methacryloxypropyldimethoxysilanol (MEDO) as products. However, we determine that this reaction pathway could be modified by the presence of the H2O released from a catalyst such as Ba(OH)2.H2O. Next, switching between the DPM hydroxyl and the MEDO methoxy is followed to generate diphenyldimethoxysilane (DPDM) and 3-methacryloxypropylmethoxysilanediol (MEMDO). However, condensation between a MEMDO hydroxyl and a DPDM methoxy is found to be most favorable for the third reaction step, which generates the DPDM-MEMDO dimer and CH3OH molecule as products. In a similar fashion, a DPDM methoxy of the DPDM-MEMDO dimer can condense with a MEMDO hydroxyl of the second DPDM-MEMDO dimer to increase the chain, but its reaction rate of 2.8 x 10(-11) s(-1) is predicted to be about 5 times smaller than that between a DPDM methoxy and a MEMDO hydroxyl. This implies that the reaction rate for the larger nanostructures becomes smaller. Additionally, our SANS measurements determine that the final products from our sol-gel reaction are on the nanometer scale, at sizes from 1.76 to 2.36 nm.

We describe the synthesis and structural characterization of new aliovalent cation-substituted n = 3 Aurivillius phases of the formulas, Bi 2 SrNaNb 2 TaO 12 (I), Bi 2 Sr 2 Nb 2 ZrO 12 (II), Bi 2 Sr 2 Nb 2.5 Fe 0.5 O 12 (III) and Bi 2 Sr 2 Nb 2.67 Zn 0.33 O 12 (IV). Energy dispersive X-ray (EDX) investigation of the chemical compositions showed that while cation-stoichiometric materials are formed for I and II, single-phase materials are obtained only for the compositions given for III and IV suggesting that the compositions tend to be oxygen-stoichiometric. The results show that aliovalent cation-substituted n = 3 Aurivillius phases similar to Bi 2 Sr 2 Nb 2 M IV O 12 (M = Ti, Mn) exist for several metal cations, viz., Ta V , Zr IV , Fe III and Zn II . Refinement of the crystal structures from the powder X-ray diffraction (XRD) data for I and III revealed that bismuth is essentially confined to the Bi 2 O 2 layers and the aliovalent cation shows a preference for the middle perovskite sheet. The present work is significant for two reasons: firstly, the presence of cations such as Sr 2+ , Na + (that do not contain lone pair s 2 electrons) in the perovskite slabs seems to render the structure centrosymmetric; secondly, a preferential/partial ordering of octahedral site cations in the perovskite slabs obtains in these materials, that seems to be dictated by a second order Jahn-Teller effect associated with d 0 cations.

An open cell configuration has been employed for the photoacoustic measurement of the thermal diffusivity of undoped Bi 2 Se 3 crystals and Bi 2 Se 3 crystals doped with various concentrations of Te. The amplitude of the photoacoustic signal obtained under heat transmission configuration as a function of chopping frequency is used to evaluate the numerical value of thermal diffusivity, α. Doped samples show a substantial reduction in the value of α compared to undoped samples. The variations in the thermal diffusivity of the doped samples are explained in terms of the phonon assisted heat transfer mechanism. It is seen that α is very sensitive to structural variations arising from doping. The experimentally observed results are correlated with X-ray diffraction studies.