Sound velocity measurements of BaCeO₃ and BaCe_0.85Y_0.15O_2.925 perovskites and effect of oxygen vacancies on the elasticity

Jianzhong Zhang^1,2, Baosheng Li¹, Jennifer Kung¹, Donald J. Weidner¹, Yusheng Zhao², Alex Navrotsky³

¹Mineral Physics Institute, SUNY at Stony Brook

²Los Alamos Neutron Science Center, Los Alamos National Laboratory

³University of California at Davis

jzhang@lanl.gov

NSLS-X17B (MAP)
"It is generally accepted that the Earth's lower mantle is dominated by MgSiO₃-rich minerals with a perovskite structure. Minor elements (such as Ca, Fe, Al), however, may have significant effects on the stability and properties of this perovskite phase"¹. Point defects, for example, may be present in the Mg-rich perovskites containing ferric Fe and Al, a case that has not been established but may play a key role in the understanding of fundamental mantle processes. In contrast to the silicate perovskite, the defect chemistry of ceramic A₂+B₄+O₃ perovskites have been extensively studied. Trivalent rare-earth elements (M = Y, Yb, Nd, Gd) have been found to substitute tetravalent ions, resulting a doped series A₂+(B_4+1-xM_x)O_3-0.5x, in which the diminished positive charges is compensated by oxygen vacancies. These materials have been considered to be useful analogs for mantle perovskite². Furthermore, the point defects (cation vacancy) have been demonstrated to have a significant influence on the elastic properties³. The primary goal of this study is to explore the effect of oxygen vacancies on the elastic bulk (Ks) and shear (G) modulus in these perovskites.

Fully-densed, polycrystalline specimen of BaCeO₃ and BaCe_0.85Y_0.15O_2.925 perovskites, 2.0 mm diameter and 1.0-1.4 mm length, were studied using ultrasonic interferometry at ambient and high pressures, in conjunction with synchrotron x-ray diffraction. The experiment was performed in a DIA-type, cubic anvil apparatus (SAM85) installed at the superwiggler beamline X17B1 at NSLS in Brookhaven National Laboratory. A dual mode Lithium Niobate transducer (10 degree Y-cut, 30 MHz for S wave and 50 MHz for P wave) mounted at the back of the WC anvil enabled us to collect travel time data for both P and S waves in a single experiment. A glass buffer rod was inserted into the boron epoxy cell assembly between the WC anvil and the sample with gold foils (2 micron thickness) placed at the interface to enhance the mechanical coupling. In all experiments, the sample pressure was determined using Decker pressure scale from the X-ray diffraction data for NaCl.

Compressional and shear wave velocities were collected at ambient conditions and at high pressure and temperature up to 9 GPa and 773 K. Preliminary data reduction yielded similar values of ambient bulk and shear moduli for BaCeO₃ (K₀ = 101-103 GPa; G = 48-49 GPa) and BaCe_0.85Y_0.15O_2.925 (K₀ = 102-106 GPa; G = 46-47 GPa) perovskite. At ambient temperature and at pressures below 2.5 GPa, both BaCeO₃ and BaCe_0.85Y_0.15O_2.925 perovskites show small but positive pressure derivatives in the shear modulus, as expected for most polycrystalline materials. At higher pressures, however, the shear-wave travel times in both perovskites increase with increasing pressure, resulting a negative dependence on pressure of the shear modulus between 2.5 and 9 GPa. This behavior is regarded to be highly unusual as the ultrasonic measurements were carried out on the polycrystalline specimen. This shear-mode softening is likely to be associated with the phase transition previously observed at 18-22 GPa⁴; further work at higher pressures is needed to interpret the present observations.

This work is jointly supported by the NSF-funded Consortium for Materials Properties Research in Earth Sciences and by the Department of Energy.

References:

¹B.J. Wood and D.C. Rubie, Science 273, 1522 (1996); J. Zhang and D.J. Weidner, Science, 284, 782 (1999).

²A. Navrotsky, Science 284, 1788 (1999).

³J. Zhang, Phys. Rev. Lett. 84, 507 (2000).

⁴S. Loridant and G. Lucazeau, J. Ram. Spectr., 30, 485 (1999)."