Dopant site selectivity in BaCe0.85M0.15O3-δ by extended x-ray absorption fine structure

Rare earth doped BaCeO3 has been widely investigated as a proton conducting material. Trivalent dopants are generally assumed to fully occupy the Ce4+-site, and thereby introduce oxygen vacancies into the perovskite structure. Recent studies indicate the possibility of partial dopant incorporation onto the Ba2+-site concomitant with BaO evaporation, reducing the oxygen vacancy content. Because proton incorporation requires, as a first step, the generation of oxygen vacancies such dopant partitioning is detrimental to protonic conductivity. A quantitative Extended X-ray Absorption Fine Structure (EXAFS) study of BaCe0.85M0.15O3-δ (M=Yb,Gd) is presented here along with complementary x-ray powder diffraction and electron probe chemical analyses. The EXAFS results demonstrate that as much as 4.6% of the ytterbium and 7.2% of the gadolinium intended for incorporation onto the Ce site, in fact, resides on the Ba site. The results are in qualitative agreement with the diffraction and chemical analyses, which additionally show an even greater extent of Nd incorporation on the Ba site

Transport and Helfand moments in the Lennard-Jones fluid. II. Thermal Conductivity

The thermal conductivity is calculated with the Helfand-moment method in the Lennard-Jones fluid near the triple point. The Helfand moment of thermal conductivity is here derived for molecular dynamics with periodic boundary conditions. Thermal conductivity is given by a generalized Einstein relation with this Helfand moment. We compute thermal conductivity by this new method and compare it with our own values obtained by the standard Green-Kubo method. The agreement is excellent.Comment: Submitted to the Journal of Chemical Physic

Nanoscale alpha-structural domains in the phonon-glass thermoelectric material beta-Zn4Sb3

A study of the local atomic structure of the promising thermoelectric material beta-Zn4Sb3, using atomic pair distribution function (PDF) analysis of x-ray- and neutron-diffraction data, suggests that the material is nanostructured. The local structure of the beta phase closely resembles that of the low-temperature alpha phase. The alpha structure contains ordered zinc interstitial atoms which are not long range ordered in the beta phase. A rough estimate of the domain size from a visual inspection of the PDF is <~10 nm. It is probable that the nanoscale domains found in this study play an important role in the exceptionally low thermal conductivity of beta-Zn4Sb3

Defect chemistry and transport properties of BaxCe0.85M0.15O3-d

The site-incorporation mechanism of M3+ dopants into A2+B4+O3 perovskites controls the overall defect chemistry and thus their transport properties. For charge-balance reasons, incorporation onto the A2+-site would require the creation of negatively charged point defects (such as cation vacancies), whereas incorporation onto the B4+-site is accompanied by the generation of positively charged defects, typically oxygen vacancies. Oxygen-vacancy content, in turn, is relevant to proton-conducting oxides in which protons are introduced via the dissolution of hydroxyl ions at vacant oxygen sites. We propose here, on the basis of x-ray powder diffraction studies, electron microscopy, chemical analysis, thermal gravimetric analysis, and alternating current impedance spectroscopy, that nominally B-site doped barium cerate can exhibit dopant partitioning as a consequence of barium evaporation at elevated temperatures. Such partitioning and the presence of significant dopant concentrations on the A-site negatively impact proton conductivity. Specific materials examined are BaxCe0.85M0.15O3-d (x = 0.85 - 1.20; M = Nd, Gd, Yb). The compositional limits for the maximum A-site incorporation are experimentally determined to be: (Ba0.919Nd0.081)(Ce0.919Nd0.081)O3, (Ba0.974Gd0.026)(Ce0.872Gd0.128)O2.875, and Ba(Ce0.85Yb0.15)O2.925. As a consequence of the greater ability of larger cations to exist on the Ba site, the H2O adsorption and proton conductivities of large-cation doped barium cerates are lower than those of small-cation doped analogs

Transport and Helfand moments in the Lennard-Jones fluid. I. Shear viscosity

We propose a new method, the Helfand-moment method, to compute the shear viscosity by equilibrium molecular dynamics in periodic systems. In this method, the shear viscosity is written as an Einstein-like relation in terms of the variance of the so-called Helfand moment. This quantity, is modified in order to satisfy systems with periodic boundary conditions usually considered in molecular dynamics. We calculate the shear viscosity in the Lennard-Jones fluid near the triple point thanks to this new technique. We show that the results of the Helfand-moment method are in excellent agreement with the results of the standard Green-Kubo method.Comment: Submitted to the Journal of Chemical Physic

Engineering the Next Generation of Solid State Proton Conductors: Synthesis and Properties of Ba_(3−x)K_(x)H_(x)(PO_4)_2

A new series of compounds with general chemical formula Ba_(3−x)K_(x)H_(x)(PO_4)_2 has been successfully prepared. This particular stoichiometry was targeted as a candidate solid-state proton conductor because of its anticipated structural similarity to known M_(3)H(XO_4)_2 superprotonic conductors (M = Cs, Rb, NH4, K; X = Se, S) and to the known trigonal compound Ba_(3)(PO_4)_2. The materials were synthesized from aqueous solution using barium acetate, dipotassium hydrogen phosphate, and potassium hydroxide as starting materials. Through variations in the initial solution stoichiometry or the synthesis temperature, the final stoichiometry could be controlled from x ~ 0.5 to ~1. X-ray powder diffraction, energy dispersive spectroscopy chemical analysis, ^(1)H magic angle spinning (MAS) nuclear magnetic spectroscopy, and thermogravimetric analysis were all employed to establish potassium and proton incorporation. The diffraction data confirmed crystallization of a trigonal phase, and chemical analysis showed the (Ba+K):P ratio to be 3:2, consistent with the target stoichiometry. The conductivity of the Ba_(3−x)K_(x)H_(x)(PO_4)_2 materials, as measured by A.C. impedance spectroscopy, is about 3 orders of magnitude greater than the end-member Ba_(3)(PO_4)_2 material with only a slight dependence on x, however, it is substantially lower than that of typical superprotonic conductors and of the M_(3)H(XO_4)_2 materials in particular. The close proximity of Ba to the hydrogen bond site is proposed to explain this behavior. At 250 °C, the conductivity is 2.4 × 10^(−5) S/cm for the composition x = 0.80, which, when combined with the water insolubility and the relatively high thermal stability, may render Ba_(3−x)K_(x)H_(x)(PO_4)_2 an attractive alternative in selected electrochemical applications to known superprotonic conductors

Ionic and Electronic Conductivity of Nanostructured, Samaria-Doped Ceria

The ionic and electronic conductivities of samaria doped ceria electrolytes, Ce_(0.85)Sm_(0.15)O_(1.925−δ), with nanometric grain size have been evaluated. Nanostructured bulk specimens were obtained using a combination of high specific-surface-area starting materials and suitable sintering profiles under conventional, pressureless conditions. Bulk specimens with relatively high density (≥92% of theoretical density) and low medium grain size (as small as 33 nm) were achieved. Electrical A.C. impedance spectra were recorded over wide temperature (150 to 650°C) and oxygen partial pressure ranges (0.21 to 10^(−31) atm). Under all measurement conditions the total conductivity decreased monotonically with decreasing grain size. In both the electrolytic and mixed conducting regimes this behavior is attributed to the high number density of high resistance grain boundaries. The results suggest a possible variation in effective grain boundary width with grain size, as well as a possible variation in specific grain boundary resistance with decreasing oxygen partial pressure. No evidence appears for either enhanced reducibility or enhanced electronic conductivity upon nanostructuring

Lattice thermal conductivity of self-assembled PbTe-Sb_2Te_3 composites with nanometer lamellae

In the system of PbTe and Sb_2Te_3, a metastable compound Pb_2Sb_6Te_(11) appears by solidification processing. It has been reported that this compound is decomposed into the two immiscible thermoelectric materials forming nanosized lamellar structure by heat treatments. The fraction transformed and the inter-lamellar spacing was systematically investigated. In this work, the thermal conductivities and the electrical resistivities have been measured as functions of annealing time through the transformation and the coarsening processes to clarify the effect of the fraction transformed and the inter-lamellar spacing. The thermal conductivity of Pb_2Sb_6Te_(11) is lower than that after the decomposition. The lattice part of the thermal conductivity of PbTe/Sb_2Te_3 lamellar samples decreases with decreasing inter-lamellar spacing. This is considered to be due to the coarsening of the microstructure

Typology characterization of farmers in Africa RISING sites in Tanzania

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