Spherulitic branching in the crystallization of liquid selenium

Liquid selenium is a spherulite-forming liquid. In a previous study [G. Ryschenkow and G. Faivre, J. Crystal Growth 87 (1988) 221], several spherulitic modes of crystallization have been observed to coexist, at a given undercooling of the liquid, with the growth of single crystals. The spherulitic modes were thought to be basically due to a mechanism inducing a regular polygonization of crystal during growth (small-angle branching). We present a morphological investigation of the spherulites of selenium, by optical and electron microscopy, which substantiate this conjecture. At medium undercooling of the liquid, the small-angle branching periodically triggers a homoepitaxial large-angle branching. This gives rise to nonringed spherulites. At higher undercooling the spherulites are ringed, which we attribute to the ineffectiveness of the large-angle branching. The existence of several spherulitic modes signifies that several stable regimes of the small-angle branching exist.