Abstract: Bi_1-xHo_xVO₄ (0.1 ≤ x ≤ 0.9) ceramics were prepared via a solid-state reaction method, and all the ceramics could be well densified in the 920-980 °C range. The ceramics with 0.1 ≤ x < 0.4 were composed of both monoclinic scheelite (M) and tetragonal zircon (T) phases, and a single M phase could be obtained in the range of x ≥ 0.4. The measured ε_r decreased from 58.9 (x = 0.1) to 14.7 (x = 0.9), so do the calculated values (ε_{r(C - M)} = 34.3-12.1), and the main reason for ε_r > ε_{r(C - M)} was the rattling of Ho³⁺ in the dodecahedron. Two points with zero τ_f appeared in Bi_1-xHo_xVO₄ (0 ≤ x ≤ 1) ceramics, and the best microwave dielectric properties with ε_r = 16.6, Q × f = 18,400 GHz (f = 10.69 GHz), and τ_f = +3.29 ppm/°C were obtained in the Bi_0.2Ho_0.8VO₄ ceramic. The change in temperature coefficient of ionic polarizability (τ_αm) caused by the rattling effect of cations is the physical essence that affects τ_f. Therefore, the rattling effect can be used as an effective mechanism to regulate τ_f in low-ε_r materials. Furthermore, there was no chemical reaction between Bi_1-xHo_xVO₄ and Ag electrode, which indicates potential applications in low-temperature co-fired ceramic (LTCC) technology.

Key words: Bi_1-xHo_xVO₄, Near-zero τ_f;, Bond valence, Rattling effect, LTCC