Abstract No. Barn0393 Pressure-Induced Cation Migration and Volume Expansion in Defect Pyrochlore Oxides

No. Barn0393 Pressure-Induced Cation Migration and Volume Expansion in Defect Pyrochlore Oxides P. W. Barnes (Ohio State Univ., Chemistry), P. M. Woodward (Ohio State Univ., Chemistry), Y. Lee (BNL, Physics), T. Vogt (BNL, Physics), J. A. Hriljac (Univ. Birmingham, Chemical Sciences) Beamline(s): X7A Introduction: Microporous materials, such as zeolites [1,2], undergo pressure-induced volume expansion (PIE) under increasing pressure. Previously, NH4NbWO6 (a defect pyrochlore) was shown to undergo pressureinduced expansion in hydrostatic media containing water [3]. The PIE observed in NH4NbWO6 was attributed to forceful insertion of water molecules into the pyrochlore structure. Due to the experimental setup used (energy dispersive x-ray diffraction), details of PIE in NH4NbWO6 could not be determined. By examining a series of different ANbWO6 pyrochlores using monochromatic synchrotron x-ray powder diffraction, the details and mechanism of PIE in these compounds was explored. Methods and Materials: The structural and compositional evolution of four members of the ANbWO6 (A= NH4, H, Rb, K) defect pyrochlore family have been studied as a function of pressure up to 7 GPa, using a diamond anvil cell and monochromatic synchrotron X-ray powder diffraction. Pressures exerted by the diamond anvil cell on the sample were measured using laser fluorescence spectroscopy. Results: In response to increasing hydrostatic pressure, NH4NbWO6 and RbNbWO6 both initially contract, but then undergo a fairly abrupt increase in their unit cell volume above a characteristic threshold pressure. In agreement with previous studies [3], NH4NbWO6 exhibits a 5.8% increase in the cubic unit cell edge once the pressure exceeds ~3.4 GPa. The increase in volume exhibited by RbNbWO6 (figure 1) first observed in this experiment is larger (~7.5%) than seen in the ammonium compound but is less abrupt, beginning near 3.0 GPa. Rietveld refinements reveal that the reversible expansion is driven by insertion of water into the structural channels, which interpenetrate the NbWO6 octahedral corner sharing framework. The insertion of extra water is accompanied by displacement of the NH4 or Rb ions to a smaller site in the channel structure, which triggers the pressure-induced expansion of the pyrochlore framework. Similar behavior is not observed for KNbWO6•H2O or HNbWO6•H2O, both of which contract in response to increasing pressure. For these smaller monovalent cations, pressure-induced volume expansion does not occur because the hydrated state, and subsequent cation shift, are already stable at ambient conditions. Acknowledgments: Research carried out in part at the NSLS at BNL is supported by the U.S. DoE, Division of Materials Sciences and Division of Chemical Sciences (DE-Ac02-98CH10886 for the X7A beamline). We acknowledge the Geophysical Lab of the Carnegie Institute for allowing us the use of the ruby laser system associated with the beamline X17C. References: [1]. I. A. Belitsky, B. A. Fursenko, S. P. Gabuda, O. V. Kholdeev, Yu. V. Seryotkin, “Structural Transformations in Natrolite and Edingtonite,” Physics and Chemistry of Minerals, 18, 497-505 (1992). [2]. Y. Lee, T. Vogt, J. A. Hriljac, J. B. Parise, G. Artioli, “Pressure-Induced Volume Expansion of Zeolites in the Natrolite Family,” Journal of the American Chemical Society, 124, 5466-5475 (2002). [3]. C. A. Perottoni, J. A. H. da Jornada, “Pressure Induced Water Expansion in the Defect Pyrochlore NH4NbWO6,” Physical Review Letters, 78, 2991-2994 (1997). 12 12.5 13 13.5 2θ () sc al ed in te ns ity 0.62 GPa 1.44 GPa 2.12 GPa 2.79 GPa 3.47 GPa 3.99 GPa 4.67 GPa 5.15 GPa (311) (222) 1