(H2O)[V2IIIF6] and Pyr-VF3:: Hydrothermal synthesis, structure determination, and magnetic characterization of new fluorides with the pyrochlore type

Small green triangular crystals of a new tridimensional hydrated vanadium(III) fluoride have been hydrothermally prepared in a simple way by heating a mixture of V: HF : H2O in the molar ratio 1 : 1: 50 at 200degreesC for 3 days. Its structure was solved by X-ray diffraction in the cubic space group Fd (3) over barm (No. 227) with a = 10.4636(2) Angstrom. (H2O) [(V2F6)-F-III] presents the structure of the well-known pyrochlore type. The cavities contain water molecules which are lost by heating (T < 250degreesC) without any structural change. The thermal stability of this new form of VF3 (Pyr-VF3) with the pyrochlore structure is compared to that of the already known Pyr-FeF3 (1). The magnetic behavior of PyrVF(3) is described. (C) 2001 Elsevier Science.