Vanadium dioxide (VO2) undergoes a phase transition at a temperature of 340 Kbetween an insulating monoclinic M1 phase and a conducting rutile phase.Accurate measurements of possible anisotropy of the electronic properties andphonon features of VO2 in the insulating monoclinic M1 and metallic rutilephases are a prerequisite for understanding the phase transition in thiscorrelated system. Recently, it has become possible to grow single domainuntwinned VO2 microcrystals which makes it possible to investigate the trueanisotropy of VO2. We performed polarized transmission infraredmicro-spectroscopy on these untwinned microcrystals in the spectral rangebetween 200 cm-1 and 6000 cm-1 and have obtained the anisotropic phononparameters and low frequency electronic properties in the insulating monoclinicM1 and metallic rutile phases. We have also performed ab initio GGA+U totalenergy calculations of phonon frequencies for both phases. We find ourmeasurements and calculations to be in good agreement.
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