Phase stability and structural temperature dependence in sodium niobate: A high resolution powder neutron diffraction study

We report investigation of structural phase transitions in technologically important material sodium niobate as a function of temperature on heating over 300-1075 K. Our high resolution powder neutron diffraction data show variety of structural phase transitions ranging from non-polar antiferrodistortive to ferroelectric and antiferroelectric in nature. Discontinuous jump in lattice parameters is found only at 633 K that indicates that the transition of orthorhombic antiferroelectric P (space group Pbcm) to R (space group Pbnm) phase is first order in nature, while other successive phase transitions are of second order. New superlattice reflections appear at 680 K (R phase) and 770 K (S phase) that could be indexed using an intermediate long-period modulated orthorhombic structure whose lattice parameter along direction is 3 and 6 times that of the CaTiO3-like Pbnm structure respectively. The correlation of superlattice reflections with the phonon instability is discussed. The critical exponent ({\beta}) for the second order tetragonal to cubic phase transition at 950 K, corresponds to a value {\beta}$\approx 1/3$, as obtained from the temperature variation of order parameters (tilt angle and intensity of superlattice reflections). It is argued that this exponent is due to a second order phase transition close to a tricritical point. Based on our detailed temperature dependent neutron diffraction studies, the phase diagram of sodium niobate is presented that resolves existing ambiguities in the literature.Comment: 21 Pages, 8 Figure

Characterization and sintering of alumina-supported palladium catalysts

A comparative study of methods (H, CO and 0 chemisorption,0 and H titrations and some physical methods) for metal particle size determination in several Pd/Al0 catalysts (metal contents from 0.45to5.26%)was carried out. The results obtained strongly suggest that the H chemisorption in controlled conditions is the most accurate method. Proportions of bridged PdO and PdCO species were determined. It is proposed that the sinterization of these catalysts(in H, high vacuum and H0) between 723 and 973 K takes place by atomic migration. The rate of sintering increased in the series H< highvacuum<H0.This is due to the increased ability of atoms for surface migration caused by the presence of hydroxyl groups