Possibilities of neutron powder diffraction for study of magnetic ferroelectrics with perovskite structure

S. A. Ivanov¹, S.- G. Eriksson², R. Tellgren³, H. Rundlöf³

¹Karpov' Institute of Physical Chemistry, Moscow, Russia

²Department of Inorganic Chemistry, University of Gothenburg, Sweden

³Institute of Chemistry, Uppsala University, Uppsala, Sweden

The interesting and frequently unexpected combination of physical properties of complex metal oxide with perovskite structure is very sensitive to even small structural changes and an understanding of the crystal chemistry of these compounds is crucial in the development of novel materials with an unusual combination of dielectric and magnetic properties. Although many perovskites have been investigated in recent years, structural information about A(B_2/3C_1/3)O₃ materials remains scarce and from a structural point of view still many points remain unclear.

In this study, we have paid attention to 1:2 ordered perovskites, and will report the temperature evolution of their nuclear and magnetic structures. NPD studies have been carried out to study the magnetically ordered ferroelectric materials with perovskite-related structure A₃Fe₂BO₉ (A=Pb,Sr,Ca,Ba; B=W,Te), prepared by a solid state route. This class of so-called ferroelectric magnets exhibits both electric-dipole and magnetic ordering within a definite temperature range.

NPD patterns have been collected at the Swedish Research Reactor R2 (Studsvik) at different temperatures in the range 10 to 1000 K. The high-temperature data were used to determine the structure in the nonpolar and nonmagnetic states, the exact stoichiometry and the cation site order. The data were analyzed with the FULLPROF software in order to characterize the temperature-induced phase transformations, and to determine the nuclear and magnetic structures. By replacing Pb by Sr, Ba or Ca the different types of lattice distortions of the initial cubic perovskite cell was found (tetragonal for Sr, hexagonal for Ba and monoclinic for Ca compounds). The size of the involved A-type cation is directly related to different types of structural distortions and B-site order/disorder. The structural peculiarities of distorted phases are discussed and possible mechanisms of phase transitions are presented.

It was found that for the Pb compound the Fe and W(Te) cations are randomly distributed over the B sites while in the case of Sr and Ca compounds a partial crystallographic order was established. T_C values appear to depend mainly on the nature of the A-cation (ion size and electronic configuration) and degree of cation ordering of Fe/W(Te).

The ordering of magnetic moments of Fe in these systems are analyzed. And all the compounds show G-type magnetic structure. Magnetic properties would be more sensitive to the content of W cations in comparison with Te ones. The correlation between ferroic distortions and the magnetic ordering is also briefly discussed.

Effect of oxygen nonstoichiometry on structural and magnetic properties was studied in the Ba compound. It was found that both the lattice distortion and physical properties are strongly influenced by the oxygen content.

The financial support for this research from the Swedish Strategic Foundation (SSF) within the frames of the program “Complex oxides for advanced applications” and Russian Foundation for Basic Research is gratefully acknowledged.