Determination of the Hydrogen Positions in the Novel Barium Boroarsenate Ba[B 2 As 2 O 8 (OH) 2 ] by Combined Single Crystal X‐ray and Powder Neutron Investigations

The boroarsenate Ba[B 2 As 2 O 8 (OH) 2 ] was obtained by the reaction of NH 4 H 2 AsO 4 , B(OH) 3 and BaBr 2 · 2H 2 O in the melt. Ba[B 2 As 2 O 8 (OH) 2 ] was obtained as thin colorless needles, together with spherical crystals of BAsO 4 as by‐product, grown on a pellet of Ba[BAsO 5 ]. The products could be separated mechanically. For neutron scattering experiments a sample was prepared with 11 B(OH) 3 as a starting material. The crystal structure of Ba[B 2 As 2 O 8 (OH) 2 ] was determined by single‐crystal X‐ray diffraction and exhibits a layer structure with an unprecedented layer topology. The exact positions of the hydrogen atoms were determined using combined single‐crystal X‐ray and powder neutron diffraction investigations. Ba[B 2 As 2 O 8 (OH) 2 ] was further characterized by IR spectroscopy and EDX analysis.