Metal-insulator transition in hollandite vanadate, K₂V₈O₁₆

Abstract

In our research interest on titanium oxyphosphates, several compounds M II 0.5 TiO(PO 4) have been prepared with M II = Mg , Fe , Co , Ni , Cu , Zn. They all belong to the monoclinic P2 1 /c structural type determined "ab initio" for Ni 0.5 TiO(PO 4) from X-ray powder diffraction[1].Thermostructural study of Cu 0.5 TiO(PO 4) shows a phase transition (800°C during heating, 400°C during cooling). Single crystals of the phase have been obtained. Its cell presents a Jahn Teller deformation with Cu-O elongation in the (a,c) plane: a=7.5612(4) ; b=7.0919(4) ; c=7.4874(4)Å ; =122.25(6)° ; V=339.55(6)Å 3. The single crystal structure confirms and states more precisely the previous powder model (R 1 =0.023; wR 2 = 0.063). The phase has been obtained stabilised at room temperature in a powder mixture (38%)+ (62%) without impurities. X-ray diffraction pattern of this powder can be indexed in a like-type P2 1 /c monoclinic cell : a=7.1081(10) ; b=7.7384(12) ; c=7.3013(10) Å ; =119.28(1)° ; V=350.3(1)Å 3. As the-structure is not a good starting model for Rietveld refinement (divergence), an "ab-initio" structure determination has been done. The refined structure corresponds to a "rocking" of the Jahn Teller elongation from the (a,c) plane to the b direction.