Energetics of Hydrogen States in SrZrO<SUB>3</SUB>

Abstract

The optimized geometries and formation energies of hydrogen in the perovskite-type oxide, SrZrO 3 , have been calculated using the density functional theory under the generalized gradient approximation (GGA). The stable hydrogen sites in the oxide are examined with special interests. It is shown that the hydrogen is tightly bound to an oxygen ion and located slightly outside the ZrO 6 octahedron in SrZrO 3 . The O-H bond orientates towards the second-nearest-neighbor oxygen ion, which induces large local displacements of adjacent oxygen ions. For example, the nearest-neighbor oxygen ions tend to approach the hydrogen. The addition of an acceptor dopant, yttrium, modifies such local distortion of oxygen ions around the hydrogen as to increase protonic conductivity. In addition, the calculated hydrogen formation energy shows that the hydrogen behaves as a shallow donor in SrZrO 3 . The fundamental principles of doping and atmosphere control in proton conducting oxides are also given in view of energetics.

Local Electronic Structure and Protonic Conductivity in Perovskite-Type Oxide, SrZrO<SUB>3</SUB>

“…[28][29][30] For convenience of the present discussion, the results in the undoped, Y-doped and Al-doped SrZrO 3 are reproduced in Figs It is apparent from Fig. 12 that the Y-O bond order is smaller than the Zr-O bond order, but the Al-O bond order is larger.…”

“…In our recent study, such local geometries around hydrogen and acceptor ion have been investigated by the first-principles density functional calculations. [28][29][30] By comparing the present study with the previous study, we can discuss the mechanism of protonic conduction in a qualitative way. As a result, some guiding principles for materials design will be obtained through the comparison.…”

“…While these two positions are possible in the crystal, explanation will be focused mainly on the H ð2Þ case, because the proton position in SrZrO 3 calculated by the psudopotential method is close to the H ð2Þ site. [28][29][30] Also, as shown in Fig. 2(c), M is surrounded by six oxygen ions, O ðiÞ (i ¼ 1{6).…”

“…The 27) and also by Kreuer et al 7) In addition, we have simulated local geometries around acceptor dopant ion and proton in SrZrO 3 using the plane-wave psudopotential method. [28][29][30] It is found that the introduction of proton and/ or acceptor dopant (e.g., Y, Al) into SrZrO 3 induces a large local lattice distortion.…”

Local Geometry and Energetics of Hydrogen in Orthorhombic SrZrO<SUB>3</SUB>

“…By comparing the optimized geometry around hydrogen in between the orthorhombic SrZrO 3 and the cubic SrZrO 3 , 29) it is found that the O-H bond length decreases from 0.1055 nm in the cubic phase to about 0.099 nm in the orthorhombic phase, whereas the Zr-H distance increases from 0.2187 nm in the cubic phase to about 0.23 nm in the orthorhombic phase. The orientation angle of O-H bond, increases from 12.9 in the cubic phase to about 40 in the orthorhombic phase.…”