Rare‐Earth Hydroxometalates Ba[RE(OH)5] with RE=Tb, Dy, Ho

Abstract Colorless crystals of the new hydroxometalates Ba[ RE (OH) 5 ] with the rare‐earth elements RE =Tb, Dy, Ho were synthesized under ultra‐alkaline conditions in a KOH hydroflux at 200 °C. Single‐crystal X‐ray diffraction revealed that the three compounds crystallize isostructural in the monoclinic space group P 2 1 / n (no. 14). In the crystal structure, the rare‐earth cations are coordinated by the oxygen atoms of seven hydroxide anions, which define a distorted pentagonal bipyramid. These polyhedra share edges of their basal ring forming infinite chains that run parallel to the [010] direction. Hydrogen bonds connect the chains into layers parallel to the (101) plane. The Ba 2+ cations are located between these layers and surrounded by nine oxygen atoms. Ba[Dy(OH) 5 ] is paramagnetic and shows no luminescence under UV light. When heated in synthetic air or argon, water is released in well‐defined steps. Ba[Dy(OH) 5 ] decomposes via DyOOH to Dy 2 O 3 , which then reacts with the remaining Ba(OH) 2 to form BaDy 2 O 4 . Thus, the hydroxometalates can be used as carbon‐free precursors for oxides.