Rietveld refinement of room temperature (RT) neutron diffraction (ND) data reveals 12R -type hexagonal perovskites Ba3 LaNb3 O 12 (BLN) and Sr3 LaNb3 O12 (SLN) to adopt space group R 3- with tilted NbO6 octahedra. The presence of an octahedral tilt transition (Ttilt) at 465 K in BLN from R 3- to R 3- m 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 AB O3 perovskites.