THE POST-COTUNNITE PHASE IN BACL2, BABR2 AND BAI2 UNDER HIGH-PRESSURE

Angle-dispersive powder diffraction patterns have been obtained as a function of pressure in a diamond-anvil cell for BaCl2, BaBr2 and BaI2 (P-max=40 GPa). These cotunnite-structure compounds (Pnam, Z=4) at ambient conditions transform to a common monoclinic (P112(1)/a, Z=8) high-pressure structure between 5 and 15 GPa at room temperature. The low-pressure cotunnite cell is doubled along a and slightly distorted, gamma = 87-89 degrees; the coordination number has increased from 9 to 10. This monoclinic structure is a distortion of the Co2Si type (Pnam, 2=4) found in intermetallic AB(2) compounds with the same coordination number; it is the same as that already found for PbCl2 and SnCl2. Simulations of the diffraction patterns of the three barium dihalides with the same atomic positions give excellent agreement with the observed diffraction patterns although the atomic diffusion factors vary greatly. A new structure type is thus added to the structures of the AX(2) compounds and this is now the final structure that has been reached in the high-pressure sequence of phase transformations in these materials.