Inorganic Chemistry

An investigation of electrical conductivity and defect structure in the fluorite related system Bi 3 + x Nb 0.8 W 0.2 O 7.1 + 3x/2 (0.0≤x≤2.0) using a.c. impedance spectroscopy and powder neutron diffraction is presented. All compositions exhibit incommensurate ordering of the cubic fluorite sublattice, with only the x=2.0 composition showing evidence for an order-disorder transition above 650°C. Non-linear behaviour in the thermal expansion of the cubic lattice parameter and the Arrhenius plot of total conductivity are discussed in terms of the defect structure.

A detailed investigation using X-ray and neutron powder diffraction as well as ac impedance spectroscopy in the substituted bismuth oxide system Bi 7 Nb 2 − 2x Y 2x O 15.5 − 2x is presented. Combined refinement of variable temperature X-ray and neutron powder diffraction data reveal both compositional and temperature dependencies of the oxide ion distribution in this system. Oxide ions are found to be distributed over three crystallographically distinct sites, with one site exclusively associated with the dopant cations Nb 5+ and Y 3+ leading to a distorted octahedral geometry for these ions. Changes in the oxide ion distribution are correlated with a non-linear thermal expansion as well as non-Arrhenius behaviour of total conductivity at high temperatures.

In this work, p-type nanoscale “soft superlattices” consisting of multilayer stacks of 25 nm Sb2Te3 on 25 nm (Bi0.2Sb0.8)2Te3 were fabricated by nanoalloying. With this technique, nanoscale layers of the elements Bi, Sb, and Te are deposited by sputtering onto a Si/SiO2 substrate and subsequently annealed to induce interdiffusion and a solid-state reaction to form the final superlattices. Different combinations of annealing temperatures were used in the annealing process. The in-plane electronic properties (Seebeck coefficient, electrical conductivity, charge carrier concentration, and carrier mobility) of these soft superlattices were examined. The cross-plane thermal conductivity was determined using time-domain thermal reflectance (TDTR). Secondary-ion mass spectrometry (SIMS) depth profiles reveal that the nanostructured thin films exhibit high stability against thermal interdiffusion during the annealing process. X-ray patterns of the samples display very strong texture with preferred c-orientation of the crystallites after the heat treatment. Scanning electron microscopy (SEM) cross-section images of the films show distinctly polycrystalline structure with increasing grain size for higher annealing temperatures, as confirmed by x-ray diffraction (XRD) analysis. Very high power factors exceeding 40 μW/cm K2, similar to values for bulk single crystals with comparable compositions, are observed for the soft superlattices. The nanostructure appears to be stable up to 300°C. For a sample annealed at 150°C, a thermal conductivity as low as 0.45 W/mK was determined. Based on different assumptions concerning the degree of anisotropy of the transport properties, a cross-plane figure of merit ZT of 0.6 to 1.9 can be estimated for the thin films annealed at 300°C.

Thin films of Bi 2 Te 3 and Sb 2 Te 3 were synthesized by the nanoalloying approach, which has recently been proven to yield V-VI compounds with good thermoelectric properties and has several advantages over ''conventional'' growth on hot substrates. Firstly, repeating layers of the elements Bi, Sb and Te with a thickness in the range between 0.2 nm and 2.4 nm were deposited on BaF 2 (111) substrates in an MBE system at room temperature with different deposition patterns for different samples, i.e. in bilayer and quintuple stacks, with different starting layer thicknesses and different Te contents. Subsequently, the element layer stacks were annealed in order to induce crystallization and compound formation of Bi 2 Te 3 and Sb 2 Te 3 thin films. The annealed thin films were characterized using X-ray diffractometry (XRD), scanning electron microscopy (SEM), and energy dispersive spectroscopy (EDS). The transport properties, i.e. electrical conductivities, carrier concentrations, carrier mobilities, Seebeck coefficients and thermal conductivities, were determined at room temperature for several sets of starting layer thicknesses and deposition patterns depending on the Te content. The texture was found to be strongly influenced by the starting thicknesses of the elemental layers in the deposition pattern. Results of temperature dependent measurements of the Seebeck coefficient and electrical conductivity on one sample of nanoalloyed Bi 2 Te 3 and Sb 2 Te 3 together with results from temperature dependent in situ XRD investigations are presented.

The thermal behavior of the oxide ion-conducting solid electrolyte Bi 4 YbO 7.5 was investigated using a combination of variable temperature X-ray and neutron powder diffraction, thermal analysis (DTA and TGA), and ac impedance spectroscopy. The title compound shows a fluorite-type structure throughout the measured temperature range (20−850°C), with a phase separation at ca. 600°C into a cubic δ-type phase and an orthorhombic phase of assumed stoichiometry Bi 17 Yb 7 O 36 . This type of transition is a relatively common feature in bismuth oxide-based systems and can limit their practical application. Here, the transition was carefully studied using isothermal measurements, which showed that it is accompanied by changes in oxide-ion stoichiometry, as well as significant disorder in the oxide ion sublattice in the δ-type phase. These results correlate with the observed electrical behavior. Analysis of the total neutron scattering through reverse Monte Carlo (RMC) modeling reveals details of the coordination environments for both cations. The oxide-ion vacancy distribution seems to be consistent with a favoring of ⟨100⟩ vacancy pairs, although ⟨110⟩ vacancy pairs exhibit the highest frequency as they have the maximum likelihood. A vacancy ordering model based on three vacancies per cell is presented.