Effect of hydrostatic pressure on modulated structures in thiourea

Abstract

The phase diagram of thiourea as a function of temperature and hydrostatic pressure is investigated by means of neutron diffraction. Accurate satellite-position measurements reveal the existence of commensurate phases corresponding to the rational values $\frac{1}{3}$, $\frac{1}{7}$, and $\frac{1}{9}$ for the modulation wave vector. The results are discussed qualitatively in terms of ordinary Landau theory and in the light of Aubry's "devil's-staircase" approach.