Order-disorder transition of the metal vacancies in the vanadium-sulfur system. I. An experimental study

The phase diagram of the VS-VS ₂ system was studied by high-temperature DTA and X-ray measurements. As a result, it has been confirmed that the vacancy-ordered V ₃S ₄-type structure transforms into the CdI ₂-type structure at T_{c 1} while the vacancy-ordered V ₅S ₈-type structure shows successive transformations on heating; V ₅S ₈ type limit_→^{T _c2}V₃S₄ type limit_→^{T _c1}Cdl₂ type. The mechanisms of the phase transitions are discussed from the viewpoint of structural relationships. The phase diagram of the VS-VS ₂ system is constructed in the temperature range up to 1200°C. In the VSe system similar results to those in the VS system have been obtained. The VSSe system has also been studied and the result supports the above-mentioned results on the VS and VSe systems.