The aim of this article is to establish the physical origin of hysteresis in solid state reactions. It had not been identified by the conventional science that remained limited by phenomenological modeling. The hysteresis revealed itself in detail as an essential component of the molecular mechanism of phase transitions in our studies of good transparent single crystals by optical microscopy and X-rays. The exclusive cause of hysteresis is nucleation lags and rooted in the nature of the

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... on far fro m the classical "random fluctuation" model. The nuclei are localized on crystal defects where nucleation lags are encoded. The hysteresis in structural, ferro magnetic and ferroelectric phase transitions, and upon magnetization and polarization results fro m the underlying ro le of structural rearrangements in these processes. Format ion of structural and magnetic hysteresis loops is analyzed in detail.