Anisotropic thermal expansion of layeredMoO3crystals

A structural study of orthorhombic molybdenum trioxide ${\mathrm{MoO}}_{3}$ with a layered structure has been performed using synchrotron-radiation powder diffraction from 110 to 1000 K. ${\mathrm{MoO}}_{3}$ exhibits an anisotropic thermal expansion. With increasing temperature, the lattice expands remarkably along the a axis of the stacking direction, while it expands a little along the c axis but contracts slightly along the b axis. Based on the quasiharmonic approximation, the temperature variations in the lattice constants and thermal expansion coefficient are analyzed and they are reproduced quantitatively using three stretching mode frequencies for $\mathrm{Mo}---\mathrm{O}$ bonds, 1000, 820, and 670 ${\mathrm{cm}}^{\mathrm{\ensuremath{-}}1}$, as well as two low frequencies, 250 and 100 ${\mathrm{cm}}^{\mathrm{\ensuremath{-}}1}$. The large thermal expansion along the a axis is attributed to the coexistence of high frequency vibration due to the covalent $\mathrm{Mo}---\mathrm{O}$ bond and high compressibility due to the van der Waals gaps in ${\mathrm{MoO}}_{3}.$