Exploring the Colour of 3d Transition‐Metal Ions in Trigonal Bipyramidal Coordination: Identification of Purple‐Blue (CoO5) and Beige‐Red (NiO5) Chromophores in LiMgBO3 Host

Abstract In an attempt to develop new coloured inorganic oxides, we have investigated the substitution of 3d transition‐metal ions in LiMgBO 3 host [where Mg II has a trigonal bipyramidal (TBP) oxygen coordination]. We find that single‐phase materials are formed for LiMg 1– x Co II x BO 3 (0 < x ≤ 1.0), LiMg 1– x Ni II x BO 3 (0 < x ≤ 0.1), LiMg 1– x Cu II x BO 3 (0 < x ≤ 0.1) and also Li 1– x Mg 1– x Fe III x BO 3 (0 < x ≤ 0.1) of which the Co II and Ni II derivatives are strongly coloured, purple‐blue and beige‐red, respectively, thus identifying TBP CoO 5 and NiO 5 as new chromophores for these colours.