  The high-pressure behaviour of PbS was investigated by angular dispersive
X-ray powder diffraction up to pressures of 6.8 GPa. Experiments were
accompanied by first principles calculations at the density functional theory
level. By combining both methods reliable data for the elastic properties of
rock-salt type alpha- and high-pressure beta-PbS could be obtained. beta-PbS
could be determined to crystallise in the CrB-type (B33), with space group
Cmcm. The reversible ferro-elastic alpha/beta transition is of first order. It
is accompanied by a large volume discontinuity of about 5% and a coexistence
region of the two phases. A gliding mechanism of {001} bilayers along one of
the cubic <110>-directions governs the phase transition which can be described
in terms of group/subgroup relationships via a common subgroup, despite its
reconstructive character. The quadrupling of the primitive unit cell indicates
a wave vector (0,0,pi/a) on the Delta-line of the Brillouin zone.
