Neutron diffraction study of quasi-one-dimensional lithium purple bronze: Possible mechanism for dimensional crossover

Abstract

The crystallographic structure of quasi-one-dimensional lithium purple bronze (LPB) was investigated using neutron powder diffraction (NPD) at temperatures $T$ in the range 5 $<T<$ 295 K. It has a monoclinic symmetry with space group $P$2${}_{1}$/$m$, the lattice parameters at 295 K are $a$ $=$ 12.7530(4) \AA{}, $b$ $=$ 5.5239(1) \AA{}, $c$ $=$ 9.4909(2) \AA{}, and $\ensuremath{\beta}$ $=$ 90.588(2)${}^{\ensuremath{\circ}}$. The sample stoichiometry was determined, through chemical analysis and refinement of the NPD data, to be Li${}_{0.924}$Mo${}_{6}$O${}_{17}$. The linear thermal expansion of the lattice parameters agrees with previously reported high-resolution dilatometry measurements. The bond-valence-sum method was applied to calculate the valence of each Mo ion as a function of $T$, which allows discussion of the mechanism by which charge is transferred between the double one-dimensional conducting chains.