The study of photoluminescence band-tail-localized excitons in ${\mathrm{ZrS}}_3$ and in its solid solution ${\mathrm{Zr}}_{1\mathrm{-}\mathit{x}}$${\mathrm{Hf}}_{\mathit{x}}$${\mathrm{S}}_3$ confirms the existence of two polytypes for these one-dimensional materials. Fitting of the spectra, with a model proposed by Ouadjaout and Marfaing, associates a density of states proportional to exp[-(ɛ/${\mathrm{\varepsilon }}_0$${)}^{3/2}$] and implies both defect and compositional fluctuations. Gap energy, parallel and perpendicular to the chain direction, has been calculated in ${\mathrm{Zr}}_{1\mathrm{-}\mathit{x}}$${\mathrm{Hf}}_{\mathit{x}}$${\mathrm{S}}_3$, and its variation with composition x is found to follow a quadratic law. © 1996 The American Physical Society.

• Received 22 December 1995

DOI:https://doi.org/10.1103/PhysRevB.53.9852