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

The Freundlich model of adsorption for calculation of specific surface areaS

The specific surface area of solids and the surface area occupied by the active phase (metal or oxide) on a support are parameters of the utmost importance in adsorption and catalysis. For the determination of the former, the BET equation is universally established. For the evaluation of the latter the works of selective chemisorption, initiated by Emmett and Brunauer (I ), for metals and by Bridges et al. (2) and Weller et al. (3), for oxides have come to represent important contributions. Some of the classical models of adsorption have also been used for evaluation of specific surfaces (Langmuir equation) (4) and the dispersion of supported metals or oxides (Freundlich equation) (5). Both of them are applicable to physisorption as well as chemisorption processes

Secondary structure of sea anemone cytolysins in soluble and membrane bound form by infrared spectroscopy.

Attenuated total reflection (ATR) Fourier transform infrared spectroscopy (FTIR) was used to investigate the secondary structure of two pore-forming cytolysins from the sea anemone Stichodactyla helianthus and their interaction with lipid membranes. Frequency component analysis of the amide I' band indicated that these peptides are composed predominantly of beta structure, comprising 44-50% beta-sheet, 18-20% beta-turn, 12-15% alpha-helix, and 19-22% random coil. Upon interaction with lipid membranes a slight increase in the alpha-helical and beta-sheet structures was observed with a concomitant decrease of the unordered structure. Polarisation experiments indicated that both toxins had some disordering effect on the lipid layers. The dichroic ratio of the alpha-helical component of the membrane-bound toxin was 3.0-3.3, indicating that this element was oriented with an angle of 38 degrees-42 degrees with respect to the normal to the plane of the crystal surface, thus resulting almost parallel to the mean direction of the lipid chains.Journal ArticleResearch Support, Non-U.S. Gov'tSCOPUS: ar.jinfo:eu-repo/semantics/publishe

Aprendizaje colaborativo en las asignaturas de fisica y fisicoquimica farmacéutica

Este proyecto de innovación docente tiene como objetivo el aprendizaje colaborativo en las asignaturas obligatorias de Física Aplicada a Farmacia y Fisicoquímica Farmacéutica, de primer y tercer semestre del grado en Farmacia, con el diseño y creación de recursos educativos en abierto, en los que el alumno sea el centro del aprendizaje. En estos últimos cursos en las enseñanzas de Grado de las asignaturas que impartimos en el departamento, hemos observado la falta de motivación y el desinterés de los estudiantes por su propio aprendizaje. Con este proyecto nos propusimos crear un entorno colaborativo que permitiera la integración de los estudiantes en comunidades de aprendizaje y procesos organizativos. Durante el desarrollo del proyecto decidimos que sería interesante ampliar este reto a la asignatura optativa de noveno semestre, Sistemas de Liberación de Fármacos, que se impartía por primera vez y en la que el número de alumnos era muy inferior. El primer día de clase se informó a los estudiantes de la necesidad de presentar los trabajos de clase como parte de la evaluación continua con una contribución del 10% en la calificación final de la asignatura. Se solicitó que, de modo voluntario, formaran equipos de trabajo para elegir los temas y se informó de la selección de los mismos para su publicación en la página web, así como, se dio la posibilidad de participar en las X Jornadas Complutenses, IX Congreso Nacional Investigación Alumnos Pregraduados en CC. de la Salud y XIV Congreso CC. Veterinarias y Biomédicas. El resultado es accesible en la siguiente dirección: http://www.ucm.es/quimicafisica2/trabajos-de-fisica-aplicada-a-farmacia-y-fisicoquimica-farmaceutic