< Tc, where T denotes the temperature. Limited spectral data is available in the literature for luminous radiation,

This paper presents optical data for VO? films prepared by evaporation and annealing post- treatment. Their near-infrared solar transmittance can be strongly temperature-dependent, which makes them interesting for use on thermochromic smart windows. Thin surface coatings can be used for obtaining energy efficiency of windows.1 Different types of coating are of interest depending on the climate. In a hot climate one wants to decrease the throughput of solar energy while maintaining the luminous transmittance, which can be accomplished by a "solar control" coating with significant transmittance at 0.4 2) films give prospects for the development of thermochromic smart windows. Several transition metal oxides show an abrupt semiconductor-metal transition when a transition temperature TC is exceeded.5"7 This is illustrated for different vanadium oxide crystals in Fig. 1, which summarizes data taken from Refs. 8-10. VOa is of particular interest since TC lies at 68°C, i.e., in a convenient range for technical applications. The metallic high-temperature phase is tetragonal (rutile; space group P42/mnm) character­ ized by chains of equidistant V atoms along the tetragonal c axis. The semiconducting low temperature phase is monoclinic (space group P2i/c); the V-V distances alternate between a larger and a smaller value. The V-V pairs are tilted slightly off the monoclinic a axis. Theoretical models6'7*'11 13 for the semiconductor-metal crossover usually revolve around a Mott-Hubbard-like transition7'12 (caused, essentially, by the off-axis displacement of the V atoms) or electron trapping in homopolar bonds6'l1(associated with the pairing of the V atoms). The structural transformation is manifested in a large change of many physical properties. Figure 1 illustrates this for the case of the electrical conductivity, which is seen to in­ crease by several orders of magnitude when TC is surpassed. The magnitude of the change, and also its hysteresis, depends on sample preparation, as one may infer from the shown VOa data. In the present work, we are primarily interested in a corresponding modification of the optical properties at T < TC, where T denotes the temperature. Limited spectral data is available in the literature for luminous radiation,11*" near infrared solar radia­