Crystal structure of La(Mg2/3M1/3)O-3 (M = Nb,Ta) microwave dielectric ceramics

Crystal structure of (Ba1-xLax)(Mg(1+x/3)Nb(2-x/3))O-3 (0.5 less than or eq ual to x less than or equal to 1.0) and La(Mg2/3Ta1/3)O-3 microwave dielect ric ceramics were analysed by the Rietveld refinement method using X-ray an d neutron powder diffraction data. In the composition range 0.5 less than o r equal to x less than or equal to 0.07, the 1:1 ordered cubic structure (< Fm(3)over bar m>) is maintained. The end members, La(Mg2/3Nb1/3)O-3 and La( Mg2/3Ta1/3)O-3, are most successfully refined using an orthorhombic structu re Pbnm (a approximate to 5.61, b approximate to 5.64, c approximate to 7.9 5 Angstrom) contrary to the expectations of earlier works which suggested a n 1:1 ordered rhombohedral or monoclinic structure. The structure is simila r to CaTiO3 having oxygen-octahedron tilting and La-displacements. The smal l ionic size of La+3 and hence a small tolerance factor(t approximate to 0. 92) is responsible for the orthorhombic distortion. No long-range order of B-site cations exists. (C) 1999 Elsevier Science B.V. All rights reserved.