Incorporation of Pb into the crystal structure of Ba6-3xNd8+2xTi18O54 solid solution

Investigations of a substitutional mechanism of Pb incorporation into the c rystal structure of Ba6-3xNd8+2xTi18O54 performed by x-ray diffraction anal ysis, scanning electron microscopy, and energy dispersive and wavelength di spersive x-ray spectroscopy revealed that Pb2+ substitutes for Ba2+ accordi ng to the formula (Ba1-zPbz)(6-3x)Nd8+2xTi18O54. The solid solubility limit for 0.5 > x > 0.6 compositions was determined to be at 0.35 less than or e qual to z < 0.4 (nominal composition) which, according to measurements of P bO loss occurring during the heat treatment, gives 0.30 less than or equal to z < 0.35 (analyses of matrix phase). Increasing the Pb2+ concentration i n (Ba1-zPbz)(4.5)Nd9Ti18O54, results in tau(f) decreasing from an initial p ositive 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 decrea ses from an initial 2000 to 1250 (z = 0.35), measured at 4 GHz, while permi ttivity remains almost constant (kappa = 87 +/- 1.5). After exceeding the s olid solubility limit of Pb2+ in (Ba1-zPbz)(4.5)Nd9Ti18O54 the appearance o f secondary phases (Nd4Ti9O24 and Pb-rich phase at grain boundaries) change s the trends of the microwave dielectric properties; permittivity decreases , Q-value remains almost constant, and tau(f) increases.