Lecture I. Zverena
|21 april 2005||FWN-Building 5114.0004, Nijenborgh 4, 9747 AG, Groningen|
|Speaker:||Prof. I. Zverena|
|Affiliation:||St. Petersburg State University|
|Title:||Cationic ordering and stability of perovskite-type layered complex oxides|
|Date:||Thu Apr 21, 2005|
|Telephone:||+31 50 363 4419|
Layered oxides exhibit a wealth of physical and chemical properties but their technological applications for design of materials is restricted due to the instability of the crystal structure and possible degradation of the desired properties. In this report, we present a brief revue on general aspects (structural, thermodynamic and kinetic) of process of formation and the stability of Ruddlesden-Popper phases which exhibit layered type structure. As objects under investigation are K2NiF4 and Sr3Ti2O7 type which exemplify the 1/1 and 2/1 intergrowths of ABO3 perovskite (P) and AO rock salt (RS) type slabs.
The relationship between phase stability of complex compounds and the character of the distribution of rare-earth and alkali-earth cations with different charge over equivalent structure positions is considered for the case of perovskite-type layered LnCaAlO4 (Ln = Y, La, Nd, Gd, Ho, Er, Yb) and Ln2SrAl2O7 (Ln = La, Nd, Sm, Eu, Gd, Tb, Gy, Ho) oxides and their solid solutions including solutions of paramagnetic substitution.
The cationic ordering determines the limits of the stability of compounds and the area of the existence of solid solutions. The ordering decreases the fluctuation of compositions which could create local areas enriched in rare-earth cations in perovskite layers and could be a precursor of perovskite phase. Statistical distribution was found for thermal unstable oxides which decompose during annealing. Layered oxides with distribution close to statistical follow the same mechanism of decomposition in the process of melting. The dependence of the type of phase diagrams on ordering phenomena exists both for whole series of substituted solid solutions and for solutions with the limited solubility. The cationic substitution and the transformation from the statistical distribution to the ordering phenomena have consequence for the stabilization or the destabilization of the structure.
Finally, in so-called "disordered" structures a local or position ordering of cations of different charge may take place. The degree of ordering depends on the effective charge, size and spin configuration of the cations and can be modified by thermal annealing and cationic substitutions.
|Laatst gewijzigd:||22 oktober 2012 14:31|