Synchrotron-radiation study of phase transitions in phosphorus at high pressures and temperatures

Results are reported of an investigation using synchrotron radiation into the effects of temperatures up to 1173 K on pressure-induced phase transitions in phosphorus. A cubic type multi-anvil press was employed and a diffraction pattern in an energy-dispersive mode was taken for a period of time, typically 200 s, without suffering from a deterioration of the sample material. The pressure of the first transition, orthorhombic–rhombohedral (As-type), decreases with increasing temperature at a rate of 2.3 MPa K⁻¹ and the As-type structure is stable at a pressure as low as 2.6 GPa at a temperature of 1073 K. The volume discontinuity at the transition, ΔV, is 10% at room temperature and remains almost unchanged with increasing temperature. The axial ratio c/a, when the rhombohedral structure is referred to the hexagonal system, changes mostly with pressure but only slightly with temperature, approaching √6 = 2.45 on going to the second transition, rhombohedral–simple cubic. The pressure of this transition, in contrast to the first one, is independent of temperature but ΔV at this transition, 3.7%, continuously decreases with increasing temperature.