Effect of the Ca ²⁺ ‐substitution on the Microwave Dielectric Properties of M ₃ (PO ₄ ) ₂ (M = Ba, Sr)

ABSTRACT We investigated structures, sintering behaviors, and microwave dielectric properties of (A (1‐ x ) Ca x ) 3 (PO 4 ) 2 (A═Ba, Sr; 0≤ x ≤0.03). XRD and BSEM investigations confirmed the formation of limited solid solutions, accompanied by cell volume contraction as the Ca 2+ ‐doping concentration increases. A trace amount of (Sr,Ca) 2 P 2 O 7 secondary phase appears in the x = 0.03 composition of (Sr (1‐ x ) Ca x ) 3 (PO 4 ) 2 . The under‐bonded P–O bond valences increase, while those of the over‐bonded A‐O bonds decrease with increasing the Ca 2+ ‐doping concentration for both solid solutions. The Ca 2+ ‐doping induces a phase transition accompanied by volume expansion around 1200°C for the (Ba (1‐ x) Ca x ) 3 (PO 4 ) 2 , leading to a decrease in relative density with increasing doping concentration when the sintering temperature is ≥ 1200°C/2 h. The (Sr (1‐ x ) Ca x ) 3 (PO 4 ) 2 and (Ba (1‐ x) Ca x ) 3 (PO 4 ) 2 sintered at 1250°C/2 h have relative dielectric permittivities ( ε r ) of ∼13.5 and ∼11.8, respectively, changing little with Ca 2+ ‐doping concentration. The Q × f value decreases for the (Ba (1‐ x) Ca x ) 3 (PO 4 ) 2 , while increases for the (Sr (1‐ x ) Ca x ) 3 (PO 4 ) 2 solid solutions with increasing the Ca 2+ ‐doping level. The temperature coefficient of resonant frequency τ f of the (Ba (1‐ x) Ca x ) 3 (PO 4 ) 2 could be tuned to a near‐zero value of ∼1 ppm/°C coupled with a high Q × f value of ∼ 60 000 GHz at the x = 0.025 composition after sintering at 1250 C/2 h.