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Title: Pressure-induced Structural Transitions and Phase Decomposition in Cd 2Nb 2O 7 Pyrochlore

Abstract

The structural transitions of the pyrochlore, Cd{sub 2}Nb{sub 2}O{sub 7}, at pressures up to 32.5 GPa have been investigated by in situ Raman scattering and angle-dispersive x-ray diffraction (ADXRD) methods. The x-ray diffraction results reveal that small amounts ({approx}7%) of metallic cadmium form by chemical decomposition at pressures greater than 4 GPa. Both Raman and XRD results indicate that a pressure-induced structural distortion from pyrochlore to defect fluorite occurs in pyrochlore Cd{sub 2}Nb{sub 2}O{sub 7} at pressures of 12-14 GPa. Subsequently, a new high-pressure phase formed and the phase transition was complete at {approx}27 GPa. The high-pressure phase is either monoclinic or orthorhombic and transforms to either the pyrochlore (or defect fluorite) structure or the amorphous state when quenched to ambient conditions. Energy dispersive spectroscopy (EDS) analysis and high-resolution transmission electron microscopy (HRTEM) observation of the quenched sample confirmed the Cd loss and resulting mixture of ordered pyrochlore, defect fluorite, high-pressure phase, as well as amorphous domains.

Authors:
 [1];  [1];  [1];  [1];  [1];  [2];  [3];  [4]
  1. University of Michigan
  2. ORNL
  3. National Synchrotron Light Source (NSLS)
  4. Florida International University, Miami
Publication Date:
Research Org.:
Oak Ridge National Lab. (ORNL), Oak Ridge, TN (United States)
Sponsoring Org.:
Work for Others (WFO)
OSTI Identifier:
979582
DOE Contract Number:
DE-AC05-00OR22725
Resource Type:
Journal Article
Resource Relation:
Journal Name: Physical Review B; Journal Volume: 74; Journal Issue: 17
Country of Publication:
United States
Language:
English
Subject:
71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSICS; AMORPHOUS STATE; CADMIUM; DEFECTS; FLUORITE; MIXTURES; PYROCHLORE; SCATTERING; SPECTROSCOPY; TRANSMISSION ELECTRON MICROSCOPY; X-RAY DIFFRACTION

Citation Formats

Zhang, F. X., Lian, Jie, Becker, U., Ewing, Rodney C., Wang, L. M., Boatner, Lynn A, Hu, Jingzhu, and Saxena, S. Pressure-induced Structural Transitions and Phase Decomposition in Cd2Nb2O7 Pyrochlore. United States: N. p., 2006. Web. doi:10.1103/PhysRevB.74.174116.
Zhang, F. X., Lian, Jie, Becker, U., Ewing, Rodney C., Wang, L. M., Boatner, Lynn A, Hu, Jingzhu, & Saxena, S. Pressure-induced Structural Transitions and Phase Decomposition in Cd2Nb2O7 Pyrochlore. United States. doi:10.1103/PhysRevB.74.174116.
Zhang, F. X., Lian, Jie, Becker, U., Ewing, Rodney C., Wang, L. M., Boatner, Lynn A, Hu, Jingzhu, and Saxena, S. Sun . "Pressure-induced Structural Transitions and Phase Decomposition in Cd2Nb2O7 Pyrochlore". United States. doi:10.1103/PhysRevB.74.174116.
@article{osti_979582,
title = {Pressure-induced Structural Transitions and Phase Decomposition in Cd2Nb2O7 Pyrochlore},
author = {Zhang, F. X. and Lian, Jie and Becker, U. and Ewing, Rodney C. and Wang, L. M. and Boatner, Lynn A and Hu, Jingzhu and Saxena, S.},
abstractNote = {The structural transitions of the pyrochlore, Cd{sub 2}Nb{sub 2}O{sub 7}, at pressures up to 32.5 GPa have been investigated by in situ Raman scattering and angle-dispersive x-ray diffraction (ADXRD) methods. The x-ray diffraction results reveal that small amounts ({approx}7%) of metallic cadmium form by chemical decomposition at pressures greater than 4 GPa. Both Raman and XRD results indicate that a pressure-induced structural distortion from pyrochlore to defect fluorite occurs in pyrochlore Cd{sub 2}Nb{sub 2}O{sub 7} at pressures of 12-14 GPa. Subsequently, a new high-pressure phase formed and the phase transition was complete at {approx}27 GPa. The high-pressure phase is either monoclinic or orthorhombic and transforms to either the pyrochlore (or defect fluorite) structure or the amorphous state when quenched to ambient conditions. Energy dispersive spectroscopy (EDS) analysis and high-resolution transmission electron microscopy (HRTEM) observation of the quenched sample confirmed the Cd loss and resulting mixture of ordered pyrochlore, defect fluorite, high-pressure phase, as well as amorphous domains.},
doi = {10.1103/PhysRevB.74.174116},
journal = {Physical Review B},
number = 17,
volume = 74,
place = {United States},
year = {Sun Jan 01 00:00:00 EST 2006},
month = {Sun Jan 01 00:00:00 EST 2006}
}
  • The structural transitions of the pyrochlore, Cd{sub 2}Nb{sub 2}O{sub 7}, at pressures up to 32.5 GPa have been investigated by in situ Raman scattering and angle-dispersive x-ray diffraction (ADXRD) methods. The x-ray diffraction results reveal that small amounts ({approx}7%) of metallic cadmium form by chemical decomposition at pressures greater than 4 GPa. Both Raman and XRD results indicate that a pressure-induced structural distortion from pyrochlore to defect fluorite occurs in pyrochlore Cd{sub 2}Nb{sub 2}O{sub 7} at pressures of 12-14 GPa. Subsequently, a new high-pressure phase formed and the phase transition was complete at {approx}27 GPa. The high-pressure phase is eithermore » monoclinic or orthorhombic and transforms to either the pyrochlore (or defect fluorite) structure or the amorphous state when quenched to ambient conditions. Energy dispersive spectroscopy (EDS) analysis and high-resolution transmission electron microscopy (HRTEM) observation of the quenched sample confirmed the Cd loss and resulting mixture of ordered pyrochlore, defect fluorite, high-pressure phase, as well as amorphous domains.« less
  • Cited by 1
  • The effects of hydrostatic pressure on the dielectric response of single crystal Cd2Nb2O7 (CNO) were investigated over the temperature range of 80-450 K. The main pressure effects are (1) large suppression of the magnitude of the dielectric constant {var_epsilon}{prime} over most of the temperature range and (2) very weak pressure dependence of the ferroelastic and ferroelectric (FE) transition temperatures and of the relaxational response near 200 K, as reported earlier for ceramic CNO. In view of result (2) the emphasis of the present paper is on the dielectric response above the transition region, i.e., in the high temperature paraelectric phase.more » The {var_epsilon}{prime}(T) response in this phase can be explained by the T dependence of the uncoupled soft FE mode frequency associated with the NbO6 octahedra, and the large decrease in {var_epsilon}{prime} with pressure follows from the expected increase of this frequency which can be estimated from the dielectric data. In this phase {var_epsilon}{prime} is very well represented by a Curie-Weiss law {var_epsilon}{prime}=C/(T-T{sub 0}), where C=1.16x10{sup 5} K and T{sub 0}=174 K at 1 bar. Remarkably, the logarithmic pressure derivatives of C and T0 are found to be essentially identical to those of BaTiO3 implicating the crucial role of the BO6 octahedra in the soft mode character of the pyrochlore (CNO) structure, as in the case of the ABO3 perovskite structure. Pressure had a relatively weak influence on the various overlapping dielectric relaxations below 200 K, but the results revealed a dipolar relaxation in the 350-400 K region. It is suggested that this relaxation is associated with the motion of a defect complex involving the oxygen vacancy, a common feature in the related ABO3 oxides.« less