Effect of Bi substitution on the structural, optical, electrical, and microwave dielectric properties of Sm_1-xBi_xTaO₄ ceramics

LnTaO₄ and LnNbO₄ compounds exhibit comparable crystal structures and chemical characteristics. However, their microwave dielectric behaviors differ markedly, which highlights the need to clarify the origin of these discrepancies. In this work, SmTaO₄ was selected as a model system to investigate the effects of Bi3+ doping on its crystal structure and microwave dielectric properties. Sm_1-xBi_xTaO₄ (0 ≤ x ≤ 0.04) solid solutions were prepared by solid-state reaction and sintered to more than 98.5 % relative density at 1600 °C for x = 0 and 1550 °C for 0 < x ≤ 0.04. XRD, Rietveld refinement, and EDS mapping confirmed the successful incorporation of Bi3+. The microwave dielectric properties of samples exhibit high Q × f, which decreases from 2.03 × 105 to 9.12 × 104 GHz as x increases from 0 to 0.04. The reduction in Q × f with increasing Bi content is attributed to enhanced lattice anharmonicity, grain boundary loss, and conduction loss. The τ_f of Sm_1-xBi_xTaO₄ shifts slightly from −76.32 to −70.48 ppm °C−1 as x increases from 0 to 0.04. The fundamental difference in the effect of Bi3+ doping on τ_f between LnTaO₄ and LnNbO₄ originates from the higher lattice rigidity of the LnTaO₄ crystal structure, which limits the solubility of Bi3+ and results in only minor modifications impact on the crystal structure and phonon behavior.