Metal-insulator transition and charge-density wave in ${\mathrm{Fe}}_{0.25}$${\mathrm{Nb}}_{0.75}$${\mathrm{Se}}_3$

The compound ${\mathrm{Fe}}_x{\mathrm{Nb}}_{1-x}{\mathrm{Se}}_3$, which has been prepared in the form of single crystals and powders, only forms in a narrow range of stoichiometry near $x=\frac{1}{4}$. The crystal structure of Nb${\mathrm{Se}}_3$ 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 ${\mathrm{Fe}}_{0.25}$ ${\mathrm{Nb}}_{0.75}$ ${\mathrm{Se}}_3$ 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 Nb${\mathrm{Se}}_3$. At temperatures below 19 K, the resistance rise is reasonable well described by the expression $\rho =C\exp {\left(\frac{T_0}{T}\right)}^{\frac{1}{4}}$, 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 ∼ 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 Nb${\mathrm{Se}}_3$ 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.