Incommensurate phase in lead monoxideα-PbO below 208 K

We report a low-temperature x-ray investigation of lead monoxide α-PbO. Our results reveal for the first time the existence of a modulation below $T_I$=208 K which breaks the tetragonal symmetry and leads to an average orthorhombic structure. The direction of the modulation vector was determined thanks to very precise powder-diffraction measurements, despite the very low orthorhombic distortion. This incommensurate ferroelastic phase possesses a modulation vector k=δ${\mathrm{b}}^{\mathrm{*}}$ which remains constant (δ=0.370) from $T_I$ down to 5 K. The absence of a lock-in transition as well as an abnormally low critical exponent β=0.24 shows the great similarity of this incommensurate phase with that of ${\mathrm{BaMnF}}_4$.