Channel structure in the new BiCoPO5. Comparison with BiNiPO5. Crystal structure, lone pair localisation and infrared characterisation

The new compound BiCoPO5 monoclinic, P2(1)/n, a = 7.2470(1) Angstrom, b = 1 1.2851(2) Angstrom, c = 5.2260(1) Angstrom and beta = 107.843(1)degrees, Z = 4, was synthesised and structurally characterised by powder X-ray diffrac tion and infrared spectroscopy. It is isostructural with bismuth nickel oxy phosphate BiNiOPO4. The crystal structure is built from a complex tridimens ional assembly of (Co/Ni)(2)O-10 dimers linked by PO4 groups, This forms la rge tunnels running along c which host Bi3+ cations. Smaller tunnels runnin g along a and crossing the latter were also evidenced. It is noteworthy tha t the original BiNiPO5 lattice is appreciably increased with Co2+ cations a s the transition metal. This feature is essentially pointed out by the Co-O longer distances. The Bi3+ cation is surrounded by a strongly distorted ox ygen octahedron. Reducing the Bi-O bonds to the three shortest, Bi environm ent can be considered as tetrahedral considering the BiO(3)Lp polyhedron, L p = 6s(2) lone pair (Lp). The lone pair localisation was performed from ele ctrostatic interactions and revealed Lp - Bi distance of 0.68 Angstrom and 0.58 Angstrom, Tor Co2+ and Ni2+ compounds respectively. The infrared spect ra of the two compounds are essentially the same and only show slight shift s of the significant bands.