Incorporation of Pb into the crystal structure of Ba^6-3xNd^8+2xTi¹⁸O⁵⁴ solid solution

Abstract

Investigations of a substitutional mechanism of Pb incorporation into the crystal structure of Ba_6−3xNd_8+2xTi1₈O₅₄ performed by x-ray diffraction analysis, scanning electron microscopy, and energy dispersive and wavelength dispersive x-ray spectroscopy revealed that Pb²⁺ substitutes for Ba²⁺ according to the formula (Ba_1−zPb_z)_6−3xNd_8+2xTi1₈O₅₄. The solid solubility limit for 0.5 = x = 0.6 compositions was determined to be at 0.35 ≤ z < 0.4 (nominal composition) which, according to measurements of PbO loss occurring during the heat treatment, gives 0.30 ≤ z < 0.35 (analyses of matrix phase). Increasing the Pb²⁺ concentration in (Ba_1−zPb_z)_4.5Nd₉Ti1₈O₅₄, results in tf decreasing from an initial positive value (80 ppm/K) to a negative value at the solid solubility limit (−25 ppm/K at z = 0.35). In the same concentration range the Q-value decreases from an initial 2000 to 1250 (z. = 0.35), measured at 4 GHz, while permittivity remains almost constant (k = 87 ± 1.5). After exceeding the solid solubility limit of Pb²⁺ in (Ba_1−zPb_z)_4.5Nd₉Ti1₈O₅₄ the appearance of secondary phases (Nd₄Ti₉O₂₄ and Pb-rich phase at grain boundaries) changes the trends of the microwave dielectric properties; permittivity decreases, Q-value remains almost constant, and T^f increases.