High-pressure X-ray diffraction study of potassium chromate: pressure-induced orientational disorder and its implications for amorphization in A(2)BX(4) compounds

The high-pressure behaviour of potassium chromate, K2CTO4, has been investi gated by angle-dispersive X-ray diffraction in a diamond-anvil cell. K2CrO4 , a pseudoscheelite salt, was found to be highly compressible with a bulk m odulus B-0 = 26(2) GPa, with B-0(1) = 6.0(5) GPa; however, no pressure-indu ced phase transition was observed up to 52 GPa. The pressure dependences of the cell constant ratios, a/b and (a + b)/c, were found to be the reverse of those for AX(2) compounds under compression, due to the differences in t he nature of the constituent ions. A gradual decrease in the relative inten sities of several diffraction peaks was observed, beginning above 16 GPa. O n comparison with calculated diffraction patterns, the decrease in intensit y under compression was shown to be consistent with the disordering of the chromate ion tetrahedra. Since the tetrahedra are small compared with the c avity in which they are located, pressure-induced orientational disordering can occur initially without disruption of the metal sub-lattice. Pressure- induced disordering of the BX4 tetrahedra is the first step in the amorphiz ation of A(2)BX(4) compounds. (C) 1999 Elsevier Science Ltd. All rights res erved.