Anisotropic transport and structure of single-crystal molybdenum bronze, Li$_{0.33}$MoO$_{3}$

We present transport measurements (resistivity, thermopower, thermal conductivity) on single crystals of the quasi-one-dimensional (Q1D), small-gap semiconductor$^a$ Li$_{0.33}$MoO$_{3}$ in the temperature range 150-500 K. The Q1D character of this material is reflected in $T=300$~K resistivity ratios, $\rho_c$:$\rho_a$:$\rho_{b^*}\simeq$~1:20:180, and extreme anisotropy in the Seebeck coefficient within the $a-c$ planes, $S_c-S_a\simeq 250\ \mu$V/K. A weak structural anomaly near $T_s=355$~K (0.001\AA\ expansions along $c^*$ and $b^*$ directions, comparable contraction along $a^*$) is identified in the temperature-dependent lattice constants from x-ray diffraction, and is coincident with changes in the transport coefficients. Analysis of the transport data at $T>T_s$ shows that an intrinsic semiconductor model can be applied to explain transport along the most conducting $c$ axis, but along $a$ and $b^*$ the transport is better described by a non-adiabatic, small-polaron picture. \vskip .1in \noindent $^a$ B. T. Collins {\it et al.}, J. Sol. St. Chem. {\bf 76}, 319 (1988).