Transport, electronic, and structural properties of nanocrystalline CuAlO2delafossites

Abstract

This work reports on the effect of grain size on the electrical, thermal, and structural properties of CuAlO 2 samples obtained by solid-state reaction combined with ball milling. Electrical characterization made in microcrystalline and nanocrystalline samples shows that the electrical conductivity decreases several orders of magnitude for the nanocrystalline samples, and, in addition, there is a large discrepancy between the activation energies associated to thermoelectric power E S . The study of the Cu K-edge x-ray absorption spectra of the CuAlO 2 samples shows that the local structure around Cu is preserved after the sintering process, indicating that the observed behavior of the electrical conductivity is of intrinsic origin. Complex conductivity measurements as a function of frequency allow us to discard grain-boundaries effects on the electrical transport. Thus, the changes in σ (T ) and S(T ) are interpreted in terms of charge localization in the framework of small polarons. This is in agreement with the analysis of the near-edge region of the absorption spectra, which indicates that sintering favors the Cu-O hybridization. As a consequence, oxygen atoms progressively lose their capability of trapping holes, and the electrical conductivity is also enhanced.

Changing active sites in Cu–CHA catalysts: deNOx selectivity as a function of the preparation method

“…Recent Cu K-edge XAFS studies performed on CuAlO 2 suggest the presence of two nearest neighbor oxygens at a distance of 1.86 Å, followed by six Cu at 2.87 Å and six aluminums at 3.28 Å, in agreement with previous single crystal diffraction studies per- formed on the same [46]. The CuAlO 2 environment thus obtained is listed in Table 7 for comparison.…”

“…4p transition most commonly seen in Cu + systems. Linear Cu + systems such as Cu 2 O or CuAlO 2 , are known to have an intense peak at $8982 eV owing to a transition from a 1s orbital to a double degenerate 4p xy orbital [46]. On the contrary, in a 4-coordinate tetrahedral geometry, all p orbitals are closer to degeneracy, with a slight increase in their energy as compared to a doubly degenerate p xy orbital.…”

Metallic magnetism and change of conductivity in the nano to bulk transition of cobalt ferrite

“…4. 47 The ac conductivity of the system can be explained on the basis of the expression 41 r ac ¼ r 1 ðTÞ þ r 2 ðx; TÞ where r 1 (T) is due to band conduction and is related to dc conductivity which is temperature dependent. r 2 (x, T) represents hopping conduction which occurs among the octahedral B-sites and it is both temperature and frequency dependent.…”

“…Significant changes in the activation energy (550.7 meV to 485.2 meV) upon size change are found as in the case of ac conductivity studies. 40,47 …”

p‐Type Optoelectronic and Transparent Conducting Oxide Properties of Delafossite CuAl1/2Fe1/2O2

“…The linear slope for E s was separated into two regions, denoted E s1 for temperatures ranging from 300 to 500 K and E s2 for temperatures ranging from 500 to 1000 K. The E s1 and E s2 values were 0.014 and 0.044 eV, respectively. The results were smaller than value of CuAlO 2 , which was 0.153 eV for temperatures ranging from 300 to 700 K, and closely to value of CuFeO 2 , which was 0.028 eV for temperatures ranging from 300 to 950 K. In addition, the value of activation energy ( E σ ) was 0.075 eV. This value was lower than that of CuAlO 2 , which was 0.2 eV and higher than that of CuFeO 2 , which was 0.049 eV.…”