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

The crystallographic structure of quasi-1D lithium purple bronze was investigated using neutron powder diffraction at temperatures \textit{T} in the range 5 K $< T < $ 295 K. Lattice parameters, atomic positions, and occupation numbers are reported. At room temperature, it has a monoclinic symmetry with space group \emph{P}2$_1$/\emph{m}, lattice parameters \textit{a} = 12.750(1) \AA, \textit{b} = 5.524(1) \AA, \textit{c} = 9.491(2) \AA, and $\beta$ = 90.593(1)$^{\circ}$. The stoichiometry was determined through chemical analysis \textit{and} refinement of the NPD data to be Li$_{0.924}$Mo$_6$O$_{17.6}$. The bond-valence-sum method was applied to calculate the valence of each Mo ion as a function of \textit{T}, which allows discussion of the mechanism by which charge is transferred between the double 1D conducting chains.