Structural phase transitions in the Ag2Nb4O11 – Na2Nb4O11 solid solution

The phase transitions between various structural modifications of the natrotantite-structured system xAg2Nb4O11 – (1-x)Na2Nb4O11 have been investigated and a phase diagram constructed as a function of temperature and composition. This shows three separate phase transition types: (1) paraelectric – ferroelectric, (2) rhombohedral – monoclinic and (3) a phase transition within the ferroelectric rhombohedral zone between space groups R3c and R3. The parent structure for the entire series has space group R-3c. Compositions with x > 0.75 are rhombohedral at all temperatures whereas compositions with x < 0.75 are all monoclinic at room temperature and below. At x = 0.75, rhombohedral and monoclinic phases coexist with the phase boundary below room temperature being virtually temperature-independent. The ferroelectric phase boundary extends into the monoclinic phase field. No evidence was found for the R3–R3c phase boundary extending into the monoclinic phase field and it is concluded that a triple point is formed

Synthesis, structure and dielectric properties of a new family of phases, ABC3O11 : A = La, Pr, Nd, Sm, Gd; B = Zr, Hf; C = Ta, Nb

Eight new phases with the general formula of ABC3O11 with different rare earth, (Zr, Hf), (Nb, Ta) combinations, have been prepared by solid-state reactions at a temperature range of 1200–1500 °C. The new phases—LaHfTa, LaHfNb, LaZrNb, PrHfTa, NdHfTa, NdHfNb, SmHfTa and GdHfTa—are characterised by X-ray and neutron diffraction data at room temperature and variable frequency impedance measurements. They are isostructural with LaZrTa3O11 which consist of alternating single layers of UO7 pentagonal bipyramids and octahedra as shown by Rietveld refinement of X-ray and neutron powder diffraction data. Lattice parameters decrease with decreasing size of rare earth element substitution at A-site, and of all, Gd is the smallest rare earth that formed LaZrTa3O11 analogues. Detailed attempts of attained and unattainable LaZrTa3O11 analogues with different temperatures are included in this paper. All phases are highly insulating with temperature-independent bulk permittivities in the range 17 to 50; LaHfNb demonstrates the highest permittivity. Arrhenius plot shows that the activation energies are in the range 0.8 to 1.94 eV

Characterization of vagal pathways mediating gastric accommodation reflex in rats

Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/65599/1/j.1469-7793.1997.479be.x.pd