Nanoquartz vs. macroquartz: a study of the α↔β phase transition

The first-order character of the α↔β transition in micron-size quartz is shown to be strongly reduced in natural nano-size quartz with no significant variation of the transition temperature. Three different experimental techniques (differential scanning calorimetry, X-ray diffraction and second-harmonic generation) are applied on natural nano-size quartz to evaluate the variation of the character of the transition. The excess entropy of nano-size quartz is described in the framework of Landau theory. The renormalization of the fourth-order term is explained to arise from the elastic 3D-clamping imposed on each grain by the surrounding nanocrystallites. The presence of hydrous species and inhomogeneous internal stress due to the growth process are considered as well in the interpretation. The anomalous increase in the linewidth of diffraction maxima at the transition temperature is related to surface relaxation phenomena.