An ordered low‐temperature phase of barium nitro­prusside trihydrate studied by neutron diffraction

Crystals of barium penta­cyano­nitro­syl­ferrate trihydrate (barium nitro­prusside trihydrate), BaFe(CN)₅(NO)]·3H₂O, have been studied by neutron diffraction in order to ex­amine the structural behaviour of the compound in the 20–120 K temperature range and to determine the structure at 105 K. The results show the existence of a new crystal phase of the compound at 80 K (with a duplicated a parameter), which still exists at 20 K. The crystal structure at 105 K shows a rearrangement of the water mol­ecules, which results in an ordered structure with P1 symmetry. Two of the four independent nitro­prusside cations are rotated by 4.5° around the 100] direction.