Neutron powder diffraction and magnetic measurements on RbTil3, RbVI3, and CsVI3

CsVI₃ (a = 8.124(1) c = 6.774(1)Å,Z = 2, P6₃/mmc at 293 K) adopts the BaNiO₃ structure. Three-dimensional magnetic ordering takes place atT_c = 32(1)K. At 1.2 K the magnetic moment is 1.64(5) μ_B and it forms a 120° spin structure in the basal plane. RbVI₃ (a = 13.863(2) c = 6.807(1) Å,Z = 6, P6₃cmor P3¯c1 at 293 K) and RbTiI₃ (a = 14.024(3) Å,c = 6.796(2) Å,Z = 6, P6₃cm orP3¯c1 at 293 K) adopt a distorted BaNiO₃ structure, probably isostructural with KNiCl₃.T_c of RbVI₃ is 25(1) K. At 1.2 K, RbVI₃ has a spin structure similar to the one of CsVI₃ with a magnetic moment of 1.44(6) μ_B. RbTiI₃ shows no magnetic ordering at 4.2 K. It is shown that a deviation from the 120° structure is expected for compounds with a distorted BaNiO₃ structure such as RbVI₃. The cell dimensions of CsTiI₃ are reported.