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Observations of the polarisation-sensitive optical absorption in single-crystal proustite have been made over the temperature range from 82 to 460 K. By combining the data obtained from a number of samples of varying thickness the complete absorption spectrum has been found for absorption coefficients of up to approximately=2*104 cm-1. The experimental results show that the optical inter-band transitions are dominated by indirect processes up to photon energies of about 3 eV. The phonons involved in these transitions have been tentatively identified. Deviations from the simple indirect transition scheme at high absorption coefficients are attributed to additional indirect transitions to higher branch conduction band minima. A possible schematic band structure is suggested close to the Gamma point.

Measurements of the complex dielectric constant of single-crystal proustite over the frequency range 10-2-5*106 Hz have shown the presence of a temperature-dependent relaxation process with characteristic times of between 1*10-2 and 6.5*10-4 s. Three activation energies are observed in the resistance measurements which correlate with reported phase transitions in this material. The dielectric relaxation can be interpreted in terms of a combined model, in which the 'Universal' dielectric dispersion law describing the anomalous low-frequency behaviour is combined with a two-layer Maxwell-Wagner model to account for the relaxation processes. The limited dielectric properties of the material have been established.