Impact of ambient oxygen on the surface structure ofα-Cr2O3(0001)

Abstract

Surface x-ray diffraction has been employed to quantitatively assess the surface structure of ␣-Cr 2 O 3 ͑0001͒ as a function of oxygen partial pressure at room temperature. In ultrahigh vacuum, the surface is found to exhibit a partially occupied double layer of chromium atoms. At an oxygen partial pressure of 1 ϫ 10 −2 mbar, the surface is determined to be terminated by chromyl species ͑Crv O͒, clearly demonstrating that the presence of oxygen can significantly influence the structure of ␣-Cr 2 O 3 ͑0001͒.

Geometry of α-Cr2O3(0001) as a Function of H2O Partial Pressure

“…Numerical labeling of atoms is employed for identification purposes. Layer occupancies determined for Cr 2 O 3 –H 2 O UHV (current study) and Cr 2 O 3 –clean UHV 7 are indicated.…”

“…To begin the search for a structural solution, the clean surface structure (Cr 2 O 3 –clean UHV ) determined in recent quantitative LEED (LEED-IV) 15 and SXRD 7 studies was employed as a starting point. This surface exhibits a topmost partially occupied double layer of Cr atoms (Cr 0.31 –Cr 0.61 –O 2.4 – from LEED-IV, 15 and Cr 0.22 –Cr 0.31 –O 3 – from SXRD 7 ). Given that the current measurements were undertaken following exposure to ∼2000 L of H 2 O, terminations with surface Cr atoms bonded to one or more OH/H 2 O species were tested.…”

“…h, k , and l are the reciprocal lattice vectors, and are defined with reference to the real space (1 × 1) unit cell of the α-Cr 2 O 3 (0001) surface, described by lattice vectors ( a 1 , a 2 , a 3 ) which are parallel to the [100], [010], [001] directions, respectively. The magnitudes of these lattice vectors are a 1 = a 2 = a = 4.957 Å, and a 3 = c = 13.592 Å, 7 where a and c are the bulk lattice constants.…”

“…Selection of this reflection was based upon its sensitivity to such variation as a function of O 2 partial pressure. 7 On the basis of these measurements (see below), a further systematic series of X-ray reflections was acquired from α-Cr 2 O 3 (0001) at a H 2 O partial pressure of ∼30 mbar; henceforth this surface will be referred to as Cr 2 O 3 –H 2 O 30mbar . It should be noted that as 30 mbar of H 2 O is equivalent to ∼100% relative humidity with the substrate at room temperature, one would expect the surface to be submerged beneath multiple monolayers of H 2 O in this environment.…”

“…This work builds on a previous SXRD study examining the impact of O 2 on the surface structure of α-Cr 2 O 3 (0001). 7 …”