Crystal Growth, Single‐Crystal Structure Refinement and Unusual Ligand‐Field Splittings of Lazulite‐Type Oxidephosphates MTi 2 O 2 (PO 4 ) 2 (M = Fe II , Co II , Ni II )

Single crystals of oxidephosphates MTi 2 O 2 (PO 4 ) 2 [M: Fe (dark red), Co (pinkish red), Ni (green)] with edge‐lengths up to 0.4 mm were grown by chemical vapour transport. FeTi 2 O 2 (PO 4 ) 2 and CoTi 2 O 2 (PO 4 ) 2 are isotypic to NiTi 2 O 2 (PO 4 ) 2 . The crystal structure of the latter was previously solved from powder data [FeTi 2 O 2 (PO 4 ) 2 (data for CoTi 2 O 2 (PO 4 ) 2 and NiTi 2 O 2 (PO 4 ) 2 in brackets): monoclinic, P 2 1 / c , Z = 2, a = 7.394(3) (7.381(6), 7.388(4)) Å, b = 7.396(2) (7.371(5), 7.334(10)) Å, c = 7.401(3) (7.366(6), 7.340(3)) Å, β = 120.20(3) (120.26(6), 120.12(4))°, R 1 = 0.0393 (0.0309, 0.0539) wR 2 = 0.1154 (0.0740, 0.1389), 2160 (1059, 1564) independent reflections, 75 (76, 77) variables]. The single‐crystal study allowed improved refinement using anisotropic displacement parameters, yielded lower standard deviations for the structural parameters and revealed a small amount of cation disordering. Twinning and cation disordering within the structures are rationalized by a detailed crystallographic classification of the MTi 2 O 2 (PO 4 ) 2 structure type in terms of group‐subgroup relations. The structure is characterized by a three‐dimensional network of [PO 4 ] tetrahedra and [M II Ti 2 O 12 ] groups formed by face‐sharing of [M II O 6 ] and [TiO 6 ] octahedra. Electronic absorption spectra of MTi 2 O 2 (PO 4 ) 2 in the UV/VIS/NIR region show rather large ligand‐field splittings for the strongly trigonally distorted chromophors [M II O 6 ] (M = Fe, Co, Ni) with interelectronic repulsion parameters beeing slightly smaller than in other phosphates. Interpretation of the spectra within the framework of the angular overlap model reveals a significant second‐sphere ligand field effect of Ti IV ions on the electronic levels of the Ni II and Co II .