Yu. E. Smirnov

Structure of Cobalt Hexacyanoferrate Films Synthesized from a Complex Electrolyte

Films of cobalt hexacyanoferrate (CHCF) are produced under potentiodynamic conditions (E= –0.25 to 0.6 V, v= 0.05 V s–1) from a 2 mM K3Fe(CN)6+ 2 mM Na3Co(NO2)6+ 0.5 M Na2SO4solution. According to scanning electron microscopy, the thick CHCF film initially obtained during the synthesis has a heavily porous structure unusual for films of hexacyanoferrates of transition metals, the size of its crystallites is 200–600 nm. A thinner film has a more compact structure and its crystallites are smaller. The X-ray diffraction pattern for a reduced film shows rhombohedral distortion of the fcc lattice with parameters a= 1.035 nm and α = 91.43°. The patterns for partially and completely oxidized films nicely fit an fcc crystalline lattice typical for hexacyanoferrates (ais 1.006 and 0.993 nm, respectively). The assumption that both hexacyanoferrate and cobalt ions are electroactive in this particular case is confirmed by the IR and X-ray photoelectron spectroscopy techniques.

Cation distribution and interatomic interactions in oxides with heterovalent isomorphism : X. Structure of the Ho2SrAl2O7 oxide at 100, 298, and 673 K

Crystallochemical data for the Ho2SrAl2O7 oxide at 100, 293, and 673 K were obtained by full-profile X-ray analysis. Analysis of the thermal expansion anisotropy showed that increased temperature gives rise to equalization of metal-oxygen bond lengths in all oxygen polyhedra at the expense of lengthening of some bonds and virtual invariability of others.

Cation Distribution and Interatomic Interactions in Oxides with Heterovalent Isomorphism: VI. Solid Solutions Nd2Sr1-xCaxAl2O7

The range of existence of Nd2Sr1-xCaxAl2O7 solid solutions (x ≤ 0.5) was established. The distribution of neodymium, strontium, and calcium cations over two structural sites was studied by full-profile X-ray analysis. Unlike La2Sr1-xCaxAl2O7 solid solutions, Ca2+ cations fill not only nine-vertex AO9 polyhedra, but also oxygen cubic octahedra AO12. Introduction of calcium results in disordered distribution of Nd3+ cations, destabilization of the layered structure, and decomposition of the solid solutions.

Cation distribution and interatomic interactions in oxides with heterovalent isomorphism: VII. CrNbO4 and LiFeO2 oxides

A high degree of local ordering of isomorphous cations, determined by a great difference in their effective charges, was found in stoichiometric CrNbO4 and LiFeO2 oxides. The character of the ordering (one-dimensional in CrNbO4, supposedly approaching the AlWO4 type, and two-dimensional in LiFeO2 with a symmetry decreased to tetragonal and approaching the CuAu type) depends on the anisotropy of cation distribution in the cation coordination spheres. Microdomains with an almost complete ordering inside one block of the crystallite mosaic may develop in LiFeO2.

Cation Distribution and Interatomic Interactions in Oxides with Heterovalent Isomorphism: V. Complex Aluminates LnCaAl3O7

A short-range ordering of lanthanide and calcium atoms was found in isomorphous La3+ and Ca2+ cations in stoichiometric oxides LnCaAl3O7 (Ln = La, Nd, Sm, Gd, Ho, Er). It takes place almost exclusively within the first cation coordination sphere. The cations form electrostatically linked mixed cation pairs Ln3+-Ca2+ in the nearest neighboring points of a common structural site. No spatial ordering of the pairs themselves is observed.

Cation Distribution and Interatomic Interactions in Oxides with Heterovalent Isomorphism: III. Complex Aluminates LnCaAlO4 (Ln = Y, La, Nd, Gd, Ho, Er, Yb)

The heterovalent isomorphism of Ln3++ and Ca2+ cations in stoichiometric LnCaAlO4 oxides was studied. The X-ray diffractometry of the intensity of Laue background modulations revealed a positive short-range order in Ln3+ and Ca2+ distribution over points of one structural site common for both ions, which results in formation of stable Ln3+-Ca2+ couples in adjacent nine-coordinate oxygen polyhedra. The degree of order and the interchange energy estimated in the quasichemical approximation depend both on the effective cationic charges and on the number of unpaired f electrons of the lanthanide atom.

Cation distribution and interatomic interactions in oxides with heterovalent isomorphism: XII. Gd2Sr1−x Ca x Al2O7 solid solutions

The area of existence of Gd2Sr1−xCaxAl2O7 solid solutions at x ≤ 0.5 was determined by the X-ray phase analysis. It was found by full-profile X-ray structural analysis that, in contrast to La2Sr1−xCaxAl2O7 solid solutions, the Ca2+ cations occupy not only AO9 nine-vertex fragments, but also AO12 oxygen cubooctahedra. Full ordering of Sr2+ cations in the perovskite layer is observed at the calcium content x 0.5.

Cation Distribution and Interatomic Interactions in Oxides with Heterovalent Isomorphism: IV.1 Temperature Dependence of Short-Order Parameters in the YCaAlO4 Oxide

The degree of local ordering of Y3+ and Ca2+ cations in the stoichiometric YCaAlO4 oxide was studied as a function of annealing temperature. The modulation of the Laue scattering background on the X-ray diffractograms of samples subjected to long-term calcination at 900, 110, 1300, and 1500°C was measured. The positive short-range ordering of Y3+ and Ca2+ decreases with increasing temperature. The temperature dependence of the degree of ordering is in a sufficiently good agreement with the results of calculations in the quasichemical approximation.