Previously unknown phenomena, scale, and kinetic effects are revealed by introducing the finite width Δξ of the particle-exterior interface as the additional scale parameter and thermally activated melting in the phase field approach. In addition to traditional continuous barrierless premelting and melting for Δξ= 0, barrierless hysteretic jumplike premelting (melting) and thermally activated premelting (melting) via critical nucleus are revealed. A very rich temperature θ−Δξ transformation diagram is found, which includes various barrierless and thermally activated transformations between solid, melt, and surface melt, and complex hysteretic behavior under various temperature and Δξ trajectories. Bistable states (i.e., spontaneous thermally activated switching between two states) between solid and melt or surface melt are found for Al particles. Strong dependence of the melting temperature (which, in contrast to previous approaches, is defined for thermally activated premelting and melting) for nanoparticles of various radii on Δξ is found. Results are in good agreement with experiments for Al for Δξ=0.8–1.2nm. They open an unexplored direction of controlling surface melting and melting or solidification by controlling the width of the external surface and utilizing predicted phenomena. They also can be expanded for other phase transformations (e.g., amorphization, solid-solid diffusionless, diffusive, and electromagnetic transformations) and phenomena, imbedded particles, and mechanical effects.