Influence of octahedral tilting on the microwave dielectric properties of A[sub 3]LaNb[sub 3]O[sub 12] hexagonal perovskites (A=Ba, Sr)

Rietveld refinement of room temperature (RT) neutron diffraction (ND) data reveals 12R-type hexagonal perovskites Ba3LaNb3O12 (BLN) and Sr3LaNb3O12 (SLN) to adopt space group R with tilted NbO6 octahedra. The presence of an octahedral tilt transition (Ttilt) at 465 K in BLN from R to Rm is proposed from a combination of high temperature ND data and fixed frequency permittivity measurements. Ttilt is estimated to be much higher at ∼ 720 K for SLN. The large difference in the RT temperature coefficient of the resonant frequency (τf), −100 ppm/K for BLN compared to −5 ppm/K for SLN, is attributed to the closer proximity of Ttilt to RT for BLN. τf in these 12R-type hexagonal perovskites can therefore be tuned by controlling the tolerance factor and therefore Ttilt in a manner similar to that used for many Ba- and Sr-based 3C-type ABO3 perovskites.