A TOPOLOGICAL TRANSITION OF B1-KOH-TLI-B2 TYPE AGCL UNDER HIGH-PRESSURE

A new B1-B2 transition with two intermediate phases was observed in AgCl in the pressure and temperature range up to 17.5 GPa and 500 degrees C by the X-ray powder diffraction method in situ. One intermediate phase has the KOH type structure, which is stable in the pressure range between about 8 and 12 GPa. The other has the TlI type structure which is stable up to 17 GPa at room temperature. The TlI type phase transforms to the B2 type phase at 17 GPa and 200 degrees C. We demonstrate that these four structures can be described systematically using a common monoclinic cell, and the cell parameters change continuously from the B1 to B2 structures. A topological mechanism by a simple deformation is proposed to explain these transformations, in which the [110] and [1(1) over bar0$] directions of the B1 structure correspond to the [100] and [0(1) over bar1$] of the B2 structure, respectively.