We describe the synthesis, structures and dielectric properties of new perovskite oxides of the formula, Ba3MIIITiMVO9, for MIII = Fe, Ga, Y, Lu and MV = Nb, Ta, Sb. While MV = Nb and Ta oxides adopt disordered/partially ordered 3C perovskite structures where MIII/Ti/MV metal-oxygen octahedra are corner-connected, the MV = Sb oxides show a distinct preference for the 6H structure, where SbV/TiIV metal-oxygen octahedra share a common face forming (Sb,Ti)O9 dimers that are corner-connected to the MIIIO6 octahedra. The preference of antimony oxides (SbV:4d10) for the 6H structure – which arises from a special SbV–O chemical bonding that tends to avoid linear Sb–O–Sb linkages unlike NbV/TaV:d0 atoms which prefer 180° Nb/Ta–O–Nb/Ta linkages – is consistent with the crystal chemistry of MV–O oxides in general. The dielectric properties reveal a significant difference among MIII members. All the oxides with the 3C structure excepting those with MIII = Fe show a normal low loss dielectric behaviour with ε = 20–60 in the temperature range 50–400 °C; the MIII = Fe members with this structure (MV = Nb, Ta) display a relaxor-like ferroelectric behaviour with large ε values at frequencies ≤1 MHz (50–500 °C).
Investigation of structure and dielectric properties of Ba3MIIITiMVO9 oxides brings out the role of various MIII and MV atoms for the formation of 3C and 6H perovskites and their dielectric properties.