An electron diffraction and bond valence sum study of the space group symmetries and structures of the photocatalytic 1:2 B site ordered A3CoNb2O9 perovskites (A=Ca2+, Sr2+, Ba2+)

A temperature-dependent structural investigation of electrical transitions in A3conb2o9 perovskites (A=Ca2+, Sr2+, Ba2+)

“…The room temperature P12 1 /c1 monoclinic (but very close to metrically cubic) Sr 3 structure to that of the Ca compound (i.e., a b À b À c + octahedrally rotated 1:2 perovskite). An unconstrained refinement for the pattern at 400 1C once more proved to be unstable, so the same rigid octahedral rotational constraints were used as for the Ca compound above.…”

“…Samples were synthesised by conventional solid-state synthesis (see Section 2.1 of [3]). Neutron powder diffraction data were collected on the samples using neutrons of wavelength ffi1.8846 Å , over a 2y range of 101o2yo1351 and step size of 0.051 on the High Resolution Powder Diffractometer (HRPD) at the High Flux Australian Reactor (HIFAR) operated by the Australian Nuclear Science and Technology Organisation (ANSTO).…”

“…In practice, the positions of the Co, Nb and O ions can be realistically modelled and the structure successfully refined assuming essentially rigid CoO 6 and NbO 6 octahedra along with a well-defined b À b À c + (in Glazer notation [5]) octahedral rotation scheme. The parameters to be refined are as follows: e 110 (describing the amplitude of octahedral rotation around the [1 1 0] p axis), e z (describing the amplitude of octahedral rotation around the [0 0 1] p axis), e o and e o 0 which are additional parameters describing the correlated expansion/contraction of the CoO 6 /NbO 6 octahedra (see Table 3 of [3] for more details on these parameters).…”

“…Note that this parameter was not included in Table 3 of [3] as it was considered a second-order effect and one that could be ignored if very small rotation angles were assumed. In practice, however, the Ca compound undergoes octahedral rotations that refine to approximately 111 around the [0 0 1] p axis and $161 around the [1 1 0] p axis.…”

“…Previous detailed structural studies [3] have shown that the room temperature structures of the A ¼ Ca and Sr compounds are monoclinic P12 1 /c1 (a m ¼ [1 1 2] p , b m ¼ [1 1 0] p , c m ¼ 3[1 1 0] p , subscript p for perovskite parent sub-lattice) perovskite-related superstructure phases whilst that of the A ¼ Ba compound is trigonal P3m1…”