Structure-microwave dielectric property relations in Sr and Ca titanates

Tuning the temperature coefficient of resonant frequency (TCF) in microwave dielectrics has been attributed to two main mechanisms: (i) dilution of the average ionic polarisability and (ii) the onset of an octahedral tilt transition above room temperature. The contributions of these mechanisms have been examined using ceramics in the Srn+1TinO3n+1, (SrxCa1-x)3Ti2O7 and SrxCa1-xTiO3 series. In the Srn+1TinO3n+1 series, dielectric constant (εr) and TCF are directly proportional over a broad range of values, 100 - 37 and 800 -140MK-1, n=4 and 1, respectively. No structural phase transitions occur on cooling from the prototype symmetry and the mechanism of tuning TCF is attributed solely to dilution of the average ionic polarisability as the SrO:SrTiO3 ratio (εr 14.5:290) increases. The dilution effect can be quantified by the gradient of the curve which shows a decrease of ∼9-10MK-1 in TCF for each single point reduction in εr. Substituting Ca for Sr in the (SrxCa1-x)3Ti2O7 series resulted in an 80% reduction in TCF from +320 to +50MK-1, but only 20% in dielectric constant (57 to 46). Dilution was therefore eliminated as the main tuning mechanism. However, Ca substitution onto the A-site of Sr based RP phases induces a structural phase transition involving rotations of the O-octahedra. The behaviour of TCε and therefore TCF as a function of tolerance factor was similar in the (SrxCa1-x)3Ti2O7 series to that observed for Ba and Sr based complex perovskites. It was therefore concluded that tilting of the octahedra dominated tuning of TCF. The domain variance associated with structural phase transitions in the solid (SrxCa1-x)TiO3 solid solution series has been investigated using electron diffraction. Only electron diffraction patterns from end member compositions conformed to those simulated using other workers' X-ray diffraction and neutron diffraction refinement data. For compositions where x=0.8, a tetragonal (I4/mcm) antiphase tilt system (a deg a deg c-) is predicted. However, comparisons of simulated with experimental single domain patterns suggested that this composition has more than one axis tilted in antiphase. (author)