oxide, as is best described as a random close-packing of cations [4]. In ferrites with spine1 structure and close-packed anion sublattice, the inter- stices of which ...
Reactivity of Solids, I (1989) 29-42 Elsevier Science Publishers B.V., Amsterdam
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in The Netherlands
Structure of amorphous ferrites of lanthanum, dysprosium, and bismuth
A.I. Rykov, Yu.T. Pavlukhin, NJ. Syrotina and V.V. Boldyrev Institute of Solid State Chemistry, Derzhavina 18, 630091 (U.S.S.R.) (Received
Sib. Branch Acad. Sci. USSR,
May 23rd, 1988; accepted
October
llth,
Novosibirsk
91,
1988)
Abstract Mechanical activation of the lanthanum, dysprosium, and bismuth morphotropic series of ferrites with perovskite structure leads to the formation of amorphous materials. Mossbauer spectra indicate lowering of the Fe3+ coordination number on amorphization. X-ray diffraction reveals the changes in the short-range order of the arrangement of the heavy cations with respect to the starting crystalline samples, as well as structural differences between the amorphous ferrites. The structure of the amorphous LaFeO, can be described as quasicrystalline with trigonal-prismatic La3+ configuration. Structural differences between the amorphous samples are due to distortions in the perovskite cells of the original crystal namely: an ionic misfit in DyFeO,, and covalency of the chemical bonds in BiFeO,.
Introduction The crystalline structure of some inorganic compounds can be effectively modified by mechanical activation (MA). This produces substances in a metastable, disordered state, and showing unusual physico-chemical properties [1,2]. We have shown that impulse MA of a complex oxide system with close-packed sublattice results in an amorphous phase in the material treated [3,4]. MA of some ferrite-spinels [3,5-71 leads to the formation of oxides with properties characteristic of amorphous magnetics usually obtained by the rapid quenching of the melt [f&9]. In a number of cases MA changes the short-range order of atom distribution throughout the solid. Thus, in amorphous Bi,O, obtained by MA, the short-range order does not correspond to the initial crystalline phase of this oxide, as is best described as a random close-packing of cations [4]. In ferrites with spine1 structure and close-packed anion sublattice, the interstices of which are occupied by small cations, one third of the cations change 0168-7336/89/$03.50
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coordination from one of tetrahedral to one of octahedral [5-7,101. Since it is the short-range order that determines many of the physical properties of materials, it is understandable that MA produces solids with new, often unusual, characteristics. In continuation of our studies, we present here an investigation of the disorder resulting from the MA ferrites of the perovskite ABO, type where A is a cation with a relatively large radius (La3+, Dy3+, Bi3+) and B is the Fe3+ cation. Our objective was to gain an insight as to the structure of the amorphous systems obtained by MA and to explain the changes in their properties induced by this treatment. Techniques sensitive to short-range order, such as X-ray diffraction (calculation of radial functions of the distribution of atoms) and Miissbauer spectroscopy (determination of the cation coordination) are used to investigate the type of disorder and to elucidate the nature of the amo~hous magnetism and the ferroelectricity.
Experimental
The lanthanum and dysprosium orthoferrites, and bismuth ferrite were obtained by standard ceramic technology from Fe203, La,O,, Bi,O, and Dy,(CO,), [l&20]. Amorphous ferrites of lanthanum and dysprosium were obtained by thermal decomposition in air of respective hexacyanoferrates M3+Fe~CN)~ (M3’ = La3+, Dy3+), by a previously described procedure
WI. Mechanical activation was carried out in a centrifugal planetary mill, EI-2 x balls and rotational speeds of the drums of 600 r/mm, or 800 r/min.
Sample activation
150, with ceramic 700 r/mm
X-Ray diffraction Measurements were carried out by use of DRON-3 diffractometer with graphite-monochromatized MO radiation (X,. = 0.7107 A). The experimental set-up and the calculation procedure for determination of radial functions of distribution (RFD) have been described in detail in [4].
Spectra were obtained with a JAGRS-4 spectrometer. chemical shift was determined with respect to a-Fe.
The
value
of
31
Results and discussion The structure of perovskite is ideally of cubic symmetry Pm3m. A unit cell includes one formula unit. Ion coordinates are usually chosen in such a way that the A-cation is in the centre of the cubic cell and B-cations are at its corners. In this case oxygen ions occupy the centres of the cube edges, forming a cubooctahedron surrounding the central cation A (coordination number 12). Oxygen ions and the large cations A form together a closepacked structure in which all anion holes are octahedral and are occupied by the small B cations. Only a small number of the ABO, perovskite-like complex oxides, such as SrTiO,, crystallize in an ideal cubic symmetry. Most show distortions that reduce the overall symmetry to orthorhombic or rhombohedral. Structural distortions of three different origins can be distinguished in perovskites [11,12]. First, the lowering of the symmetry can be caused by a poor fit between the size of the cation and the spatial dimensions of the structural site. The simplicity and high symmetry of the ideal cell of perovskite is stable for a wide range of ionic sizes [13]: 0.41R,
