Neutron and X-ray diffraction experiments revealed that ferromagnetic (\mathrm{NaV}{6} \mathrm{O}{11}\left(T_{\mathrm{C}}=64.2 \mathrm{~K}\right)) undergoes two-step second-order structural phase transitions upon cooling: hexagonal ( (\mathrm{P6}{3} /) (m m c) \rightarrow) hexagonal (\left(P 6{3} m c\right) \rightarrow) orthorhombic ( (C m c 2_{1}) ). Its structure at (200 \mathrm{~K}) was determined by single-crystal X-ray analysis: hexagonal, (P 6_{3} m c, a=5.7049(2) \AA, c=13.0796(8) \AA), and (Z=2). The (\mathrm{P6}{3} / \mathrm{mmc} \rightarrow \mathrm{P6}{3} m c) transition is believed to accompany a magnetic transition at (245 \mathrm{~K} . \mathrm{V}(1)) octahedra, which construct an edgesharing kagomΓ©-type network, form a trimer on the (P 6_{3} / m m c \rightarrow) (\mathrm{P6}{3} m c) transition. V(2) octahedra, which form face-sharing dimers, separate into two nonequivalent types on the hexagonal-hexagonal transition. The (P 6{3} m c \rightarrow C m c 2_{1}) transition occurs at a temperature between 35 and (40 \mathrm{~K}), which is lower than (T_{\mathrm{c}} . \mathrm{V}(1)) octahedra split into two nonequivalent types on the (\mathrm{P6}{3} m c \rightarrow C m c 2{1}) transition. The transitions are compared with those in (\mathrm{V}{2} \mathrm{O}{3}, \mathrm{~V}{2} \mathrm{O}{4}), and (\mathrm{Li}{\mathrm{x}} \mathrm{VO}{2}). The crystal structure of (\mathrm{NaV}{6} \mathrm{O}{11}) lacks the center of symmetry below (T_{\mathrm{C}}), which allows a canted-spin arrangement for the ferromagnetic state if the Dzyaloshinski-Moriya interaction exists. No structural phase transition was observed for (\mathrm{SrV}{6} \mathrm{O}{11}) (\left(T_{\mathrm{C}}=70 \mathrm{~K}\right)) between 22 and (373 \mathrm{~K} . \odot 1994) Academic Press, Inc.