The effects of double-site yttria-doping on the structures and dielectric properties of (Ca,Sr)z(Zr,Ti)O3−δ perovskite ceramics

Abstract

Double-site yttria-doped (Ca,Sr)_z(Zr,Ti)O_3−δ (CSZTY) ceramics were synthesized using the solid-state reaction method. The correlation among the crystal structure, microstructure and microwave dielectric properties was investigated. Their structures were characterized by X-ray diffraction (XRD), transmission electron microscopy (TEM) and Raman spectroscopy. CSZTY ceramics have a perovskite structure with coexisting orthorhombic and cubic phases; the main phase is the orthorhombic Pnma phase. Rietveld refinement, atomic-resolved high angle annular dark field (HAADF) STEM images and energy dispersive spectroscopy (EDS) for elemental mapping revealed that Y³⁺ cations occupy both A and B-sites in the perovskite structure (ABO₃). Upon increasing Y³⁺ cations distributed on A and B-sites in CSZT ceramics, the factor of tolerance t decreases and the tilting angle of BO₆ octahedra and the unit-cell volume increase. Superlattice reflections appearing along the [100]_c, [110]_c and [012]_c zone axes are observed in the selected area electron diffraction (SAED) patterns as a result of cell doubling caused by the octahedral tilt transitions. Raman spectra show that upon increasing the doping amount of yttria, before and after 450 cm⁻¹, the frequencies of some vibrational modes correlated with B ion movement in the BO₆ octahedron blue shift and red shift, respectively. Upon increasing the octahedral tilting angle, the polarizability of the ions produces corresponding changes, the dielectric constant ε_r decreases, the value of Q × f increases, and the temperature coefficient of dielectric permittivity τ_c increases while the resonant frequency temperature coefficient τ_f decreases. At x = 0.15, the CSZTY ceramics show optimized microwave dielectric properties: ε_r = 29.8, Q × f = 23 528 GHz, τ_c = 0.50 ppm °C⁻¹, and τ_f = −4.25 ppm °C⁻¹. This work provides a method to optimize the microwave dielectric properties of perovskite structure materials.