Crystallographic and electron diffraction data for sapphire (α‐Al2O3) are presented which enable ready and unique identification of TEM diffraction patterns and facilitate image analysis. Crystallographic data is presented in the form of stereographic projections and in figures listing angles between planes and angles between zones. Electron diffraction data consist of computer‐simulated diffraction patterns and tables of extinction distances calculated using atomic and ionic electron scattering factors.
Sintered (1475 °C/2 h) pellets of Ba(Zn1/3Ta2/3)O3 were annealed at temperatures ranging from 1550 to 1625 °C/15 h and subsequently quenched into water. The degree of B-site order between the Zn and Ta cations was investigated using x-ray diffraction (XRD) and transmission electron microscopy (TEM). Samples quenched from 1575 to 1600 °C exhibit the highest degree of order by XRD. Dark-field TEM images from these samples revealed 1:2, Zn:Ta, trigonal, ordered domains approximately 0.4 μm diam near grain boundaries but 100 nm in the grain interior. Samples annealed and quenched from 1625 °C exhibited no ordered superlattice reflections by XRD. However, diffuse scatter along 〈111〉 directions and fcc rather than trigonal ordered reflections were observed in electron diffraction patterns. Disordered samples were subsequently annealed at 1500 °C/15 h (below the order–disorder phase transition ∼1625 °C) and ±13{hkl} superlattice reflections reappeared in XRD patterns demonstrating the reversibility of the phase transition.
Tuning the temperature coefficient of resonant frequency (τf) in microwave dielectrics has been attributed to two main mechanisms: (i) dilution of the average ionic polarizability; (ii) the onset of an octahedral tilt transition above room temperature. The contributions of each mechanism have been isolated using ceramics in the Srn+1TinO3n+1, SrxCa1−x)3Ti2O7, and (SrxCa1−x)TiO3 series. In the Srn+1TinO3n+1 series, relative permittivity (εr) and τf are linearly proportional over a broad range of values, 100–37 and 800–140 ppm/°C, n = 4 and 1, respectively. No structural phase transitions occur on cooling from the prototype symmetry, and the mechanism of tuning is attributed solely to dilution of the average ionic polarizability as the SrO:SrTiO3 ratio increases. Exchanging Ca for Sr in the (SrxCa1−x)3Ti2O7 series resulted in an 80% reduction in the magnitude of τf from +320 to +50 ppm/°C but only 21% in permittivity (58 to 46). The effect was nonlinear and attributed primarily to the onset of a phase transition involving rotations of the octahedra on cooling. Superlattice reflections associated with the octahedral tilt transition have been identified.
An electron diffraction study performed on thin sections of Ca x Sr 1−x TiO 3 ceramics with compositions x = 0.2 and 0.5 has revealed diffraction patterns that are inconsistent with currently accepted space group symmetries. Here, the data are presented and alternative models suggested. It is proposed that Ca x Sr 1−x TiO 3 has the space group C2/m at x = 0.2 and the space group P2 1 /m across the range 0.2 < x < 0.6. The sequence of phases across the solid solution is therefore proposed to be Pm 3m ⇒ I 4/mcm ⇒ C2/m ⇒ P2 1 /m ⇒ Pnma x = 0 x < 0.2 x = 0.2 0.2 < x < 0.6 x > 0.6.
Ba6–3xNd8+2xTi18O54 (BNT) is a useful dielectric resonator at microwave frequencies. Recent work has shown that the major secondary phase in this system can be indexed as Nd4Ti9O24 (JCPDS 33–943). The present study has evealed that when half of the stoichiometric Ba+2 is replaced with Ca+2 or Sr+2, a third, tilted perovskite-structured phase, NdTiO3, is also present. It forms as coherent intragranular regions within BNT grains and always contains some of the divalent dopant species. The orientational relationship between NdTiO3 and BNT has been established.
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