Shear induced phase transition in PbO under high pressure

We have studied the structural behavior of lead monoxide (PbO) as a function of pressure via angular dispersive X-ray diffraction employing two different pressure transmitting media that were quasi-hydrostatic (N2) and non-hydrostatic (MgO), respectively. Besides litharge (α-PbO) and massicot (β-PbO), which are both stable at ambient pressure, there is an orthorhombic γ-PbO phase which appears upon application of pressure to α-PbO. We have found that the orthorhombic γ-PbO phase is favored by shear stress under non-hydrostatic conditions. α-PbO shows strong anisotropy in compressibility. The a-axis is rather incompressible with a linear stiffness coefficient of Ka0=540(30) GPa whereas the c-axis stiffness is Kc0=25(1) GPa. The bulk modulus of α-PbO is K0=23.1(3) GPa and its derivative K0′=7.0(3).