Theory of order–disorder phase transitions of B-cations in AB′1/2 B′′1/2O3 perovskites

Perovskite-like oxides AB′1/2 B′′1/2O3 with two different cations in the B-sublattice may experience cation order–disorder phase transitions. In many cases the degree of cation ordering can be varied by suitable synthesis conditions or subsequent sample treatment, which has a fundamental impact on the physical properties of such compounds. Therefore, understanding the mechanism of cation order–disorder phase transition and estimation of the phase transition temperature is of paramount importance for tuning of properties of such double perovskites. In this work, based on the earlier proposed cation–anion elastic bonds model, a theory of order–disorder phase transitions of B-cations in AB′1/2 B′′1/2O3 perovskites is presented, which allows reliable estimation of the phase transition temperatures and of the reduced lattice constants of such double perovskites.

Computer modeling of Ba(2)RE(3+)NbO(6) (RE(3+) = rare-earth and Y) compounds

In this work we have performed atomistic simulations in Ba(2)RE(3+)NbO(6) (RE(3+) = La, Ce, Nd, Pr, Pm, Sm, Eu, Gd, Tb, Dy, Y, Ho, Er, Tm, Yb and Lu) compounds in order to predict their physical properties and behavior under lanthanide substitutions. The potential model adopted describes very well the structural and dielectric properties of these materials. The dependence of the tolerance factor on their physical properties was investigated and the results indicate that the lattice energy, sound velocities and bulk modulus do not show morphotropic phase boundaries between the three phases in which these compounds crystallize. These observables have a linear dependence on the tolerance factor. Only the elastic constant shows morphotropic phase boundaries.

Synthesis and structural characterisation of the new K2NiF4-type phases, A2In0.5Sb0.5O4(A = Sr, Ba)

In this paper the synthesis and structural characterisation of two new K2NiF4-type phases, Ba2In(0.5)Sb(0.5)O4 and Sr2In(0.5)Sb(0.5)O4, are reported. These are the first examples of K2NiF4 compounds of general formula A2MIII(0.5)M'(V)(0.5)O4 with both 3+ and 5+ cations in the octahedral sites. Ba2In(0.5)Sb(0.5)O4 is shown to have a tetragonal cell [space group I4/mmm, a= 4.1651(1), c= 13.299(1) A] with an essentially disordered arrangement of In and Sb. In the case of Sr2In(0.5)Sb(0.5)O4, however, ordering of In and Sb is observed leading to an expanded unit cell [Pmcb, a= 5.7592(1), b= 5.7740(1), c= 12.543(1) A]. The results therefore show that varying the size of the alkaline earth cation has a pronounced effect on the ordering of In and Sb within the structure.