Incorporation of Pb into the crystal structure of Ba 6-3x Nd 8+2x Ti 18 O 54 solid solution
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stigations of a substitutional mechanism of Pb incorporation into the crystal structure of Ba6−3xNd8+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 (Ba1−zPbz)6−3xNd8+2xTi18O54. The solid solubility limit for 0.5 > x > 0.6 compositions was determined to be at 0.35 艋 z < 0.4 (nominal composition) which, according to measurements of PbO loss occurring during the heat treatment, gives 0.30 艋 z < 0.35 (analyses of matrix phase). Increasing the Pb2+ concentration in (Ba1−zPbz)4.5Nd9Ti18O54, results in f 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 ( ⳱ 87 ± 1.5). After exceeding the solid solubility limit of Pb2+ in (Ba1−zPbz)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 f increases.
I. INTRODUCTION
In recent years a number of different microwave ceramic compositions have been developed and commercialized. In the permittivity (⬘) range from 30 to 40 a selection of microwave ceramics exists exhibiting Qvalues from 50,000 to as high as 200,000. Even though the permittivity range of 40 < ⬘ < 70 was initially not commercially interesting, the development of highquality microwave ceramics with permittivities in this range has subsequently attracted significant commercial attention. Microwave components with even higher permittivities (70 < ⬘ < 90) are almost exclusively manufactured from the Ba6−3xR8+2xTi18O54-based compositions (R ⳱ La to Gd). The crystal structure of Ba6−3xR8+2xTi18O54 solid solutions includes elements of tungsten bronze with channels extending in the short-axis direction, as shown in Fig. 1.1,2 Corner-sharing TiO6 octahedra form a network with three types of channels: pentagonal, rhombic, and triangular. Rare-earth ions occupy the rhombic channels, Ba ions completely fill the pentagonal channels (for x < 2⁄3), whereas the remaining Ba ions share the rhombic channels. The triangular channels are empty.3 The solid solubility region of the Nd analogue extends in the range 0 < x < 0.74 or 0.2 < x < 0.7.5 The coordination numbers for the cations cannot be unambiguously determined due to the irregular coordination sphere. For comparative J. Mater. Res., Vol. 15, No. 8, Aug 2000
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purposes, we decided on the basis of the bond lengths to use an average coordination number of eight for all the ions on the rhombic sites and ten for Ba2+ ions on the pentagonal sites.6 Dielectric properties of the Ba6−3xR8+2xTi18O54 solid solutions vary according to the rare-earth ion (R) and the composition
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