Polar Bonds Self-Ordering and Negative Thermal Expansion

Abstract

Thermal expansion coefficient, reflecting the peculiarities of crystal inter-atomic bonds, in most of crystals increases with temperature rise. The negative value of this coefficient, seen in the polar crystals in a certain temperature interval, correlates to the configurational entropy, which in opposed to vibrational entropy increases as pressure grows. Negative coefficient of thermal expansion testifies to dynamic self-ordering of polar bonds in crystals, and is seen not only in the piezoelectrics, but also in the semiconductors (like Si) that might be due to ordering of the virtual hexagonal polar phase.