The excitons involved in this study are the longitudinal and the Γ_(5) and Γ_(6) free excitons, as well as the donor bound exciton (D_(0),X). The temperature dependence of the energy positions of the Γ_(5), Γ_(6), and (D_(0),X) excitons are well accounted for by the Varshni equation (Y. P. Varshni, Physica (Amsterdam) 34, 149 (1967)). In the same temperature range, the energy positions of the longitudinal excitons depart from the predictions of the Varshni equation. The separation between the longitudinal- and transverse-mode free excitons has been previously reported. One component of this separation is the polarizability, which has a temperature dependence. The longitudinal exciton therefore has a band-gap temperature dependence, predicted by the Varshni equation, as well as an additional dependence due to polarizability. This temperature dependence has been accounted for by the Varshni equation, plus an additional linear and a quadratic temperature dependent term.
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