2ND-ORDER RUTILE-TYPE TO CACL2-TYPE PHASE-TRANSITION IN BETA-MNO2 AT HIGH-PRESSURE

MnO2 was studied in a diamond anvil cell by angle-dispersive X-ray dif fraction up to 46 GPa using both an imaging plate and film. A phase tr ansition from the tetragonal rutile-type phase to an orthorhombic phas e was observed at below 1 GPa. The absence of discontinuity in the lat tice constants and the zero volume change are indicative of a second o rder transition and the pressure dependence of the spontaneous strain indicates a transition pressure of 0.3 GPa. Rietveld refinements of th e structure at high pressure confirmed that the orthorhombic phase has a CaCl2-type structure, space group Pnnm, a = 4.437(3), b = 4.312(3), c = 2.862(2) Angstrom, x(O) = 0.339(1), y(O) = 0.278(2) with Z = 2 at 7.3 GPa. The angle of rotation of the MnO, octahedra about their two- fold axes parallel to c was found to increase from 3 degrees at 1.0 GP a to 12 degrees at 9.4 GPa. The p-V data for the orthorhombic phase we re fitted using a Birch-Murnaghan equation of state yielding a bulk mo dulus of 328(18) GPa with a first pressure derivative of 4(2). The cha nges observed at this ferroelastic transition in MnO2 present a model for the corresponding phase transition in stishovite-SiO2 at high pres sure.