ABSTRACT
The effects of Na+ substitution by Y3+ on the structural, microstructural, dielectric and electrical properties of Ba2Na(1-3x)YxNb5O15 compositions with (x = 0, 0.02 and 0.04) have been studied in detail. The solid solutions of different compositions were prepared by the solid state reaction route method and characterized by X-Ray Diffraction (XRD), Scanning Electron Microscopy (SEM), and Complex Impedance Spectroscopy (CIS) techniques. The XRD study confirmed that all prepared compositions have a single-phase orthorhombic tungsten bronze structure with space group Cmm2 at room temperature. The microstructural studies revealed a grain shape and size change in response to increasing Y3+ concentration. The dielectric properties of the obtained compositions are evaluated over a temperature range of 40-600 °C. The dielectric properties were improved for the Y2O3-substituted Ba2NaNb5O15 compound compared to the undoped Ba2NaNb5O15 compound. The non-Debye type relaxation mechanism is confirmed by the -Zâ³ versus Z' traces. The grain contribution was studied using an equivalent electrical circuit with a Resistor R, a Capacitor C, and a Constant-Phase Element CPE in parallel, in the absence of the grain boundary response and the electrode effect in the frequency range 10 Hz-1MHz. The experimental AC conductivity data were evaluated by using Jonscher's power law. The activation energies obtained from the relaxation and conduction processes, present two different regions as a function of temperature related to the two electrical processes for the prepared ceramics.
ABSTRACT
This paper presents a systematic study of the substitution effect by lanthanides (Ln3+ = Sm3+ and La3+) in the A-site of perovskite Ba(1-x)Ln2x/3(Ti0.99Zr0.01)O3 with a substitution rate equal to 3.4%. All samples were synthesized by the classical solid-state reaction route and characterized by X-ray diffraction and a complex impedance spectroscopy technique. The synthesized compounds exhibit single-phase perovskite structures without detectable secondary phases. The P4mm space group was verified by the Rietveld method from the X-ray diffraction data, with the tetragonal distortion decreasing with the increasing ionic radius of the lanthanides. SEM micrographs of all ceramics revealed high densification, low porosity and homogeneous distribution of grains of different sizes over the entire surface. The dielectric properties of non-doped and Sm3+ and La3+ doped Ba(1-x)Ln2x/3(Ti0.99Zr0.01)O3 compound are studied in the temperature range of 40-250 °C. The dielectric permittivity ε' increases and the ferroelectric-paraelectric phase transition temperature decreases when the lanthanides are inserted into the A-site of Ba(1-x)Ln2x/3(Ti0.99Zr0.01)O3 perovskite. The Nyquist plots indicate a non-Debye type relaxation process. Conductivity and electrical modulus plots as a function of frequency (10 to 106 Hz) include two electrical responses corresponding to grain and grain boundary effects for all ceramics studied.
