Ba₃M^IIITiM^VO₉ (M^III = Fe, Ga, Y, Lu; M^V = Nb, Ta, Sb) perovskite oxides: Synthesis, structure and dielectric properties

We describe the synthesis, structures and dielectric properties of new perovskite oxides of the formula, Ba₃M^IIITiM ^VO₉, for M^III = Fe, Ga, Y, Lu and M^V = Nb, Ta, Sb. While M^V = Nb and Ta oxides adopt disordered/partially ordered 3C perovskite structures where M^III/Ti/M^V metal-oxygen octahedra are corner-connected, the M^V = Sb oxides show a distinct preference for the 6H structure, where Sb^V/Ti^IV metal-oxygen octahedra share a common face forming (Sb,Ti)O₉ dimers that are corner-connected to the M^IIIO₆ octahedra. The preference of antimony oxides (Sb^V:4d¹⁰) for the 6H structure - which arises from a special Sb^V-O chemical bonding that tends to avoid linear Sb-O-Sb linkages unlike Nb^V/Ta ^V:d⁰ atoms which prefer ∼180° Nb/Ta-O-Nb/Ta linkages - is consistent with the crystal chemistry of M^V-O oxides in general. The dielectric properties reveal a significant difference among M ^III members. All the oxides with the 3C structure excepting those with M^III = Fe show a normal low loss dielectric behaviour with ε = 20-60 in the temperature range 50-400 °C; the M^III = Fe members with this structure (M^V = Nb, Ta) display a relaxor-like ferroelectric behaviour with large ε values at frequencies ≤1 MHz (50-500 °C).