  Vanadium dioxide (VO2) undergoes a phase transition at a temperature of 340 K
between an insulating monoclinic M1 phase and a conducting rutile phase.
Accurate measurements of possible anisotropy of the electronic properties and
phonon features of VO2 in the insulating monoclinic M1 and metallic rutile
phases are a prerequisite for understanding the phase transition in this
correlated system. Recently, it has become possible to grow single domain
untwinned VO2 microcrystals which makes it possible to investigate the true
anisotropy of VO2. We performed polarized transmission infrared
micro-spectroscopy on these untwinned microcrystals in the spectral range
between 200 cm-1 and 6000 cm-1 and have obtained the anisotropic phonon
parameters and low frequency electronic properties in the insulating monoclinic
M1 and metallic rutile phases. We have also performed ab initio GGA+U total
energy calculations of phonon frequencies for both phases. We find our
measurements and calculations to be in good agreement.
