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Exploring structural changes and distortions in quaternary perovskites and defect pyrochlores using powder diffraction techniques

Barnes, Paris W
http://rave.ohiolink.edu/etdc/view?acc_num=osu1064346592

Abstract Details

2003, Doctor of Philosophy, Ohio State University, Chemistry.
The perovskite structure with 1:1 M-site cation ordering (or double perovskite; A 2 MM'X 6 ) is a well known and extensively studied structure type in solid state chemistry. The ideal double perovskite has cubic symmetry, but many are distorted from the ideal structure. Structural distortions seen in perovskites are caused by electronic factors (i.e., Jahn-Teller ions), M-cation displacement from the center of the MX 6 octahedra (i.e., cations with a stereoactive lone pair of electrons), and most commonly, octahedral tilting. Tilting of the octahedra within the perovskite structure leads to lowering of its symmetry. A major factor that influences the degree of octahedral tilting in a given compound is the nature of the cuboctahedral A-cation. Perovskites with Ba 2+ as the A-site cation typically exhibit cubic symmetry and those with Ca 2+ have orthorhombic or monoclinic symmetry. Changes (or lack thereof) in space group symmetry for A = Ba 2+ and Ca 2+ are often easily seen in the X-ray powder diffraction data (XRPD) of their respective compounds. Yet, compounds with A = Sr 2+ are prone to subtle octahedral tilting distortions that are not readily seen in XRPD, so many are mistakenly assigned to the incorrect space group. In this study, nine double perovskites with A = Sr 2+ (M 3+ = Al, Sc, Cr, Mn, Fe, Co, Ga, Y; M 5+ = Nb, Ta, Sb), were examined using Rietveld refinements of XRPD and neutron powder diffraction data (NPD) in order to appropriately discern their respective crystallographic symmetry. The approach taken for determining appropriate possible space groups, the reliability of peak splitting seen in the XRPD data for determining space group symmetry, the extent of M-site cation ordering, and the degree of octahedral tilting seen in this family of compounds will be discussed. The principles used to determine the appropriate space group symmetry for the nine compounds examined by NPD was applied to the XRPD data of sixteen additional double perovskites. The perovskite CaCu 3 Ti 4 O 12 has received significant attention recently due to its giant dielectric constant (ε = 1.0×10 4 ). Such high dielectric constants are seen in ferroelectric or relaxor materials, both of which show sharp maxima in ε as a function of temperature. This is not the case CaCu 3 Ti 4 O 12 , whose dielectric constant shows little temperature dependence between 100 and 600 K. Variable temperature studies of the crystal structure clearly showed that no distortion of the crystal structure occurs down to 35 K. This structural evidence has led to several possible mechanisms for the dielectric properties of this material. In this work, additional evidence supporting the extrinsic internal boundary layer capacitance mechanism is reported. NPD data for CaCu 3 Ti 4 O 12 and high resolution synchrotron XRPD for CdCu 3 Ti 4 O 12 is analyzed by Rietveld analysis in order to compare the structural behavior of these two compounds as a function of temperature. These results are compared to variable temperature stability and crystal structure calculations performed using the perovskite structure prediction software SPuDS. Similar calculations examining the octahedral tilting distortions in CaTiO 3 as a function of temperature were also completed and the results are compared to those obtained for ACu 3 Ti 4 O 12 (A 2+ = Ca, Cd). Additional variable temperature structure prediction calculations for a number of ACu 3 Ti 4 O 12 compositions (A 2+ = Ca, Cd; M 4+ = Ru, Ge, Mn) were completed to examine any trends seen in structural stability. The structural and compositional evolution of four members of the ANbWO6 (A= NH 4 + , H + , Rb + , K + ) defect pyrochlore family have been studied as a function of pressure up to 7 GPa, using a diamond anvil cell and monochromatic synchrotron x-ray powder diffraction. In response to increasing hydrostatic pressure, NH 4 NbWO 6 and RbNbWO6 both initially contract, but then undergo a fairly abrupt increase in their unit cell volume above a characteristic threshold pressure. NH 4 NbWO 6 exhibits a 5.8 percent increase in the cubic unit cell edge once the pressure exceeds 3.4 GPa, while the RbNbWO6 unit cell expansion is larger (7.5 percent) but less abrupt, beginning near 3.0 GPa. Rietveld refinements reveal that the reversible expansion is driven by insertion of water into the structural channels that interpenetrate the NbWO 6 - octahedral corner sharing framework. The insertion of extra water is accompanied by displacement of the NH4+ or Rb+ ions to a smaller site in the channel structure, which triggers the pressure-induced expansion of the pyrochlore framework. This mechanism explains the counterintuitive expansion of the pyrochlore framework in response to application of external pressure. It should be noted that the expansion exhibited by the pyrochlore framework must coincide with a decrease in the volume of the hydrostatic fluid so that the net volume of the system decreases with increasing pressure. Similar behavior is not observed for KNbWO 6 .H 2 O or HNbWO 6 .H 2
Patrick Woodward (Advisor)
255 p.
Chemistry, Inorganic
double perovskites; X-ray powder diffraction; neutron powder diffraction; calcium copper titanate; defect pyrochlore; pressure induced volume expansion (PIE)

Recommended Citations

Citations

  • Barnes, P. W. (2003). Exploring structural changes and distortions in quaternary perovskites and defect pyrochlores using powder diffraction techniques [Doctoral dissertation, Ohio State University]. OhioLINK Electronic Theses and Dissertations Center. http://rave.ohiolink.edu/etdc/view?acc_num=osu1064346592

    APA Style (7th edition)

  • Barnes, Paris. Exploring structural changes and distortions in quaternary perovskites and defect pyrochlores using powder diffraction techniques. 2003. Ohio State University, Doctoral dissertation. OhioLINK Electronic Theses and Dissertations Center, http://rave.ohiolink.edu/etdc/view?acc_num=osu1064346592.

    MLA Style (8th edition)

  • Barnes, Paris. "Exploring structural changes and distortions in quaternary perovskites and defect pyrochlores using powder diffraction techniques." Doctoral dissertation, Ohio State University, 2003. http://rave.ohiolink.edu/etdc/view?acc_num=osu1064346592

    Chicago Manual of Style (17th edition)

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