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Abstract: By examining the nucleation and growth phenomena for HgI2 crystals in physical vapor transport ampoules with a variation of temperatures of the source powder side T sou and of the crystal growth side T cry both are near its solid–solid phase transition temperature , our experimental results reveal that under : (1) the solid (α or β) phase structure of the as-grown HgI2 crystal is surprisingly governed by T sou; (2) the temperature range of existence for the metastable β-HgI2 phase in crystal growth side is about 17°C below T c; and (3) by turning a rotating motor to act as mechanical perturbations, this temperature range of existence for β-phase now reduces to about 11°C below T c. To explain these phenomena, especially for the metastable β-phase formed at , a model based on an existence of the vaporized HgI2 clusters in vapor transport region with specific nanostructure dominated by T sou is proposed. [Copyright &y& Elsevier]