Melting of bcc crystal Ta without the Lindemann criterion

Understanding of melting is deeply rooted in the Lindemann criterion which predicts that the transition occurs when the mean vibrational atomic displacement reaches a universal value. The criterion also finds its way in atomic description of kinetics of various structural phase transitions involving liquid and amorphous phases. Here we show using atomistic modeling in bcc crystal tantalum that neither the universal displacement exists nor melting occurs at the anticipated value from the Lindemann criterion. Instead, before and at melting a series of strongly correlated atomic diffusional motions are set in with the atomic displacement far more complicated than that predicted by Lindemann based on independent atomic vibrations. The displacement leads to formation of new extended atomic configurations composed of lattice chains and loops of Ta atoms still residing on the crystal lattice. It is the proliferation of these lattice chains that leads to melting.