Thermo Optic Coefficients and Electronic Polarizabilities of Tungsten Bronzes Using Ellipsometry

Abstract

The values of dn/dT for thermally evaporated WO 3 thin films were found to be negative either in the heating cycle (295-373 K) or in the cooling cycle (100-300 K) and were found to be of order of 10 -5 K -1 . On the other hand thermo optic coefficients (TOCs: dn/dT and dk/dT values) of tungsten bronzes (H + , Li + , Na + ) with different concentrations were found to be positive and negative, respectively in both the heating and cooling cycles and they were found to be in the order of 10 -4 K -1 . Heating cycles show hysteresis loops of n and k for tungsten bronzes, which are good for electrochromic devices. On heating the bronzes at temperature higher than 400 K, there might be a reduction in the porosity but an irreversible disordering of hydrogen, lithium or sodium atoms could occur because of anomalous dispersions produced in the optical data. For cooling cycles, the calculated TOCs were again of the order of 10 -4 but there was an increase in dn/dT and a decrease in dk/dT values due to more amorphousness built up in the bronzes during the cooling cycles. This change in the values of TOCs for tungsten bronzes created small decrease in electronic polarizabilities (α e ) which were calculated in the range from 8.003 to 8.02 × 10 -24 cm 3 in the cooling cycles.