Complex rare-earth titanates with the perovskite structure: Rietveld studies and dielectric properties

Abstract

Several new compositions of the type (Ln_2/5Ba_2/5Ca_1/5)(Cd2/5Ti3/5)O₃ and (Ln_1/3Ba_1/3Ca_1/3)(Cd_1/3Ti_2/3)O₃ (Ln = La, Nd, Sm, Gd, Dy) pertaining to the perovskite structure have been investigated. We have also tried to substitute the cadmium site with zinc. The oxides are found to crystallize in the disordered cubic structure (by powder X-ray diffraction studies) with lattice parameters of 4.0360(6) and 3.9996(6) Å for Ln = La and Nd, respectively. Rietveld refinement of powder X-ray data of La---Cd-based oxide gave conclusive evidence in favor of the cubic perovskite structure with refined composition (La_0.361(7)Ba_0.690(7)Ca_0.17(2))(Cd_0.114(3)Ti_0.886(7))O_3.096(24) (a=4.0411(26) Å, Pm3̅m , Rp=10.40, Rwp=13.82) for the oxide with the loaded composition (Ln_2/5Ba_2/5Ca_1/5)(Cd_2/5Ti_3/5)O₃. Note that the composition obtained by Rietveld refinement shows lower cadmium content than the nominal compositions. The refined compositions for the oxide with the nominal composition (Ln_1/3Ba_1/3Ca_1/3)(Cd_1/3Ti_2/3)O₃ was (La_0.36(3)Ba_0.62(3)Ca_0.12(1))(Cd_0.146(8)Ti_0.854(8))O_2.77(9) (a=4.03041(12) Å; wRp=0.0590, Rp=0.0341). In case of the oxide with nominal composition (La_2/5Ba_2/5Ca_1/5)(Cd_(1.5/5)Zn_0.5/5Ti_3/5)O₃ the composition obtained after Rietveld studies was (La_0.4Ba_0.4Ca_0.2)(Cd_0.072(5)Zn_0.128(5)Ti_0.572(5))O_2.58(2) (a=3.9875(18) Å, space group Pm3m; R(F²)=0.0323, Rp=0.0823, Rwp=0.0965). The dielectric constant for the oxides with nominal composition (Ln_2/5Ba_2/5Ca_1/5)(Cd_2/5Ti_3/5)O₃ lie in the range of 35 to 50 with reasonably low loss of 0.0017 to 0.0021. The dielectric constant of the oxides with nominal composition (Ln_1/3Ba_1/3Ca_1/3)(Cd_1/3Ti_2/3)O₃ show higher values in the range of 57 to 80 and slightly higher loss of ~0.006-0.007. These oxides also show reasonable stability of the dielectric constant with temperature.