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 13, 17, and 19 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.