Ellipsometric Investigations of Electronic Polarizability and Thermo-optic Coefficients of ZxMoO3 (Z = H+, Li+) Bronzes

Abstract

We have measured mass densities, thermo-optic coefficients, and electronic polarizability of MoO3 thin films and of HxMoO3 and LixMoO3 bronze thin films over the temperature range from 100 K to 373 K using ellipsometry. The mass densities of MoO3 and molybdenum bronzes with different concentrations (x) changed from 3.76 g/cm3 to 2.45 g/cm3 during the heating and cooling cycles. Thermo-optic coefficients (TOCs) or dn/dT and dk/dT values of the molybdenum bronzes were found to have anomalous dispersion in the heating cycles over the range 295–373 K. On the other hand, the values of dn/dT for MoO3 thin films and for ZxMoO3 bronzes were found to be negative and positive, respectively over the temperature range 100–295 K and were on the order of 10−4 K−1. The respective values of dk/dT were found to be positive and negative and were on the order of 10−4 K−1 in the same temperature range. A little increase in the values of TOCs for the molybdenum bronzes is due to small increase in electronic polarizability, which was calculated in the range from 8.20 to 8.22 × 10−24 cm3 over the temperature range 100–373 K. All these small values are due to creation of more amorphousness in the MoO3 structure during the cooling cycles. The n and k data and TOCs values of molybdenum bronzes are explained using analytical model. The reported ellipsometric data is also interpreted in terms of small-polarons and bipolarons using configurational coordinate model.