One-dimensional composite host-guest structure in BaVS3

A detailed synchrotron X-ray diffraction (XRD) study performed with a single crystal of BaVS3 (barium vanadium trisulfide) in the temperature range between 10 and 295 K is reported. Aside from the known tetragonal-orthorhombic (240 K) and orthorhombic-monoclinic (69 K) phase transitions, in the 130 < T <= 295 K range the overall structure can be viewed as a host-guest (H-G) composite. The BaS3 matrix is the host, while the V-chains form the guest. The two subsystems lock in at T-LOCK = 130 +/- 20 K. This temperature is marked by a symmetry change from orthorhombic to monoclinic. This results in the formation of twins, implying a structural phase transition identified here for the first time. From the refined structural data, it is possible to follow, starting already at 295 K downwards, the stepwise transformation of VS6 octahedra into VS5 tetragonal pyramids as the origin of the structure evolution. The new findings will yield a better understanding of the complex electronic phase diagram of BaVS3.