Structural mechanism of pyroelectricity in tourmaline

Pyroelectricity in tourmaline, known since antiquity, was ascribed by S. von Boguslawski to a charged, asymmetric, anharmonic oscillator based on the Einstein model of a crystal. His predicted values of the pyroelectric coefficient k were in good agreement with Ackermann's measurements in the range 20-400 K. We have tested Boguslawski's model by refining the structure, at 193 and 293 K, on a sphere of gem-quality elbaite. The pyroelectric effect is due primarily to the asymmetric anharmonic vibrations of O(1), the oxygen atom of point symmetry 3m which has a polar environment. Its centre of gravity moves 0.005 Å from 193 to 293 K. It is the only atom with a displacement well above experimental uncertainty. Its large thermal parameters, which are ten times their standard deviation at both temperatures, clearly invalidate the assumption of an ellipsoidal thermal movement. This probably holds for Na and 0(2), which also have abnormally large temperature factors, but show no significant displacement.