Extended x-ray absorption fine structure study of incorporation of Bi and Pb atoms into the crystal structure of Ba 4.5
- PDF / 201,209 Bytes
- 6 Pages / 612 x 792 pts (letter) Page_size
- 90 Downloads / 175 Views
MATERIALS RESEARCH
Welcome
Comments
Help
Extended x-ray absorption fine structure study of incorporation of Bi and Pb atoms into the crystal structure of Ba4.5 Nd9 Ti18 O54 M. Valant, I. Arˇcon, D. Suvorov, and A. Kodre “Joˇzef Stefan” Institute, Jamova 39, University of Ljubljana, 1000 Ljubljana, Slovenia
T. Negas Trans-Tech Inc., Adamstown, Maryland 21702
R. Frahm Hamburger Synchrotronstrahlungslabor at Deutsches Elektronen Synchrotron DESY, Hamburg, Germany (Received 22 April 1996; accepted 14 October 1996)
In the extended x-ray absorption fine structure (EXAFS) study of the local environment of Bi31 and Pb21 ions incorporated in Ba4.5 Nd9 Ti18 O54 , actual sites of Bi- and Pb-incorporation are determined. Evidence is given that dopant ions are not distributed randomly on all theoretically possible sites; Bi31 selectively enters one out of three possible channels, corresponding to the sites x 0.9484, y 0.2500, z 0.2939, and/or x 0.0455, y 0.2500, z 0.6928 previously occupied by Nd31 , while Pb21 selectively enters site x 0.4940, y 0.2500, and z 0.4993 previously shared by Ba21 and Nd31 .
I. INTRODUCTION
Solid solution with the general formula Ba62x R812/3x Ti18 O54 (R La–Gd) are already widely recognized as important high permittivity microwave ceramics.1 To meet the circuit designers’ performance criteria, additional optimization and tailoring of microwave dielectric properties of the solid solutions must be achieved by composition modification. Among the variety of additives for adapting the microwave dielectric properties of Ba62x Nd812/3x Ti18 O54 , the most common dopants are Bi and Pb.2–6 To achieve the ultimate improvement of microwave dielectric properties, both the mechanism of incorporation of the additive in the crystal structure and the influence of the dopant content on the dielectric properties must be well known. Studies of the phase relations and the crystal structure of the undoped phase are the groundwork for these investigations. A single crystal x-ray diffraction study of Ba3.75 Pr9.5 Ti18 O54 was performed by Matveeva et al.7 They showed that the crystal structure was made up of corner-sharing perovskite-like (TiO6 ) octahedra, which are joined according to the tetragonal tungsten-bronze pattern. In the framework of linked TiO6 octahedra, pentagonal and rhombic channels exist that can be occupied either by Ba or rare earth ions. Refinements of other isostructural solid solutions,8,9 in general, confirm the results of Matveeva et al. However, a major dispute has arisen over the occupancies of the J. Mater. Res., Vol. 12, No. 3, Mar 1997
http://journals.cambridge.org
Downloaded: 14 Mar 2015
pentagonal and rhombic sites. Recently Rawn10 has taken into account superstructural reflections, which have already been observed but neglected previously. The refinement of the Ba5.19 Sm8.54 Ti18 O54 , using the doubled cell, corresponding to Pnma space group, shows the fractional coordinates and occupancies as listed in Table I. According to that refinement10 the pentagona
Data Loading...