Metal-insulator transition and charge-density wave in Fe0.25Nb0.75Se3

The compound FexNb1-xSe3, which has been prepared in the form of single crystals and powders, only forms in a narrow range of stoichiometry near x=14. The crystal structure of NbSe3 is radically modified by the addition of iron and contains four chains of metal atoms per unit cell, rather than six, as in the pure material. The resistance of Fe0.25Nb0.75Se3 rises by nine orders of magnitude as the temperature is lowered from 120 to 2.8 K, although at room temperature the resistivity is comparable to pure NbSe3. At temperatures below 19 K, the resistance rise is reasonable well described by the expression Cexp(T0T)14, characteristic of a Mott or Anderson type of metal-insulator transition. X-ray studies show the formation of an incommensurate charge-density-wave superlattice below 1/4 140 K. This can enhance the metal-insulator transition and indicates that the Fermi-surface instability is an extremely dominant feature in compounds of the NbSe3 type. The absence of a superlattice at room temperature indicates that the iron is randomly substituted in either two or four of the Nb-atom chains.