Structural phase transitions in the ferromagnetic vanadium oxide NaV₆O₁₁

Neutron and X-ray diffraction experiments revealed that ferromagnetic NaV₆O₁₁ (Tc = 64.2 K) undergoes two-step second-order structural phase transitions upon cooling: hexagonal (P6₃/mmc) → hexagonal (P6₃mc) → orthorhombic (Cmc2₁). Its structure at 200 K was determined by single-crystal X-ray analysis: hexagonal, P6₃mc, a = 5.7049(2) Å, c = 13.0796(8) Å, and Z = 2. The P 6₃/mmc → P6₃mc transition is believed to accompany a magnetic transition at 245 K. V(1) octahedra, which construct an edge-sharing kagomé-type network, form a trimer on the P6₃/mmc → P 6₃mc transition. V(2) octahedra, which form face-sharing dimers, separate into two nonequivalent types on the hexagonal-hexagonal transition. The P6₃mc → Cmc 2₁ transition occurs at a temperature between 35 and 40 K, which is lower than T_c. V(1) octahedra split into two nonequivalent types on the P6₃mc → Cmc 2₁ transition. The transitions are compared with those in V₂O₃, V₂O₄, and Li_xVO₂. The crystal structure of NaV₆O₁₁ lacks the center of symmetry below T_c, which allows a canted-spin arrangement for the ferromagnetic state if the Dzyaloshinski-Moriya interaction exists. No structural phase transition was observed for SrV₆O₁₁ (T_c = 70 K) between 22 and 373 K.