Novel Method to Achieve Temperature-Stable Microwave Dielectric Ceramics: A Case in the Fergusonite-Structured NdNbO4 System

Microwave dielectric ceramics with permittivity (ε_r) ∼ 20 play an important role in massive multiple-input multiple-output (MIMO) technology in 5G. Although fergusonite-structured materials with low dielectric loss are good candidates for 5G application, tuning the temperature coefficient of resonant frequency (TCF) remains a problem. In the present work, smaller V⁵⁺ ions (r_V = 0.355 Å, with coordination number (CN) = 4) were substituted for Nb⁵⁺ (r_Nb = 0.48 Å with CN = 4) in the Nd(Nb_1-xV_x)O₄ ceramics, which, according to in situ X-ray diffraction data, lowered the fergusonite-to-scheelite phase transition (T_F-S) to 400 °C for x = 0.2. The thermal expansion coefficient (α_L) of the high-temperature scheelite phase was +11 ppm/°C, whereas for the low-temperature fergusonite phase, it was + 14 < α_L < + 15 ppm/°C. The abrupt change in α_L, the associated negative temperature coefficient of permittivity (τ_ε), and the minimum value of ε_r at T_F-S resulted in a near-zero TCF ∼ (+7.8 ppm/°C) for Nd(Nb_0.8V_0.2)O₄ (ε_r ∼ 18.6 and Qf ∼ 70,100 GHz). A method to design near-zero TCF compositions based on modulation of τ_ε and α_L at T_F-S is thus demonstrated that may also be extended to other fergusonite systems.