Correlation between structures and microwave dielectric properties of Ba3.75Nd9.5-x Sm x Ti17.5(Cr1/2Nb1/2)0.5O54 ceramics

Highlights: • The near zero τ_f (+1.3 ppm/°C) for Ba_3.75Nd_5.5Sm₄Ti_17.5(Cr_1/2Nb_1/2)_0.5O₅₄ were obtained with single tungsten bronze phase. • The oxygen octahedron became more tilted with increase of Sm³⁺. • The blue shift of A_g(1) modes increased with increase of Sm³⁺. • The Q×f value increased with decrease of FWHM of A_g(1) and internal strain η. In this study, the influences of substitution of Sm³⁺ for Nd³⁺ on the crystal structure, Raman spectroscopy and microwave dielectric properties in Ba_3.75Nd_9.5Ti_17.5(Cr_1/2Nb_1/2)_0.5O₅₄ (BNTCN) ceramics were investigated. The XRD results showed that all Ba_3.75Nd_9.5-xSm_xTi_17.5(Cr_1/2Nb_1/2)_0.5O₅₄ (BNTCN-Sx) samples were crystallized as the orthorhombic tungsten-bronze type like structure. The unit cell volume of samples decreased as x increased, and the blue shift of Raman spectral peaks (A_g(1) and B_g) also confirmed this result. Moreover, the increase of tilt angles indicated that more and more oxygen octahedra became more tilted, consequently leading to the decrease of temperature coefficient of resonant frequency (τ_f) with increase of x value. The packing fraction and Q×f value of BNTCN-Sx ceramics reached the maximum value, whereas the FWHM of A_g(1) and internal strain η reached the minimum value at x = 3. The low internal strain resulted from stability of the crystal structure, which was conducive to improvement of Q×f value in the BNTCN-Sx ceramics. At last the BNTCN-Sx ceramics sintered at 1380 °C for 4 h exhibited excellent microwave dielectric properties: ε_r = 83.6, Q×f = 11,597 GHz, and τ_f = +1.3 ppm/°C when x = 4.