Article pubs.acs.org/JPCC
Oxygen Diffusion in Mayenite Markus Teusner,† Roger A. De Souza,*,† Holger Krause,‡ Stefan G. Ebbinghaus,‡ Badreddine Belghoul,† and Manfred Martin*,†,§ †
Institut für Physikalische Chemie, RWTH Aachen University, Aachen, Germany Institut für Chemie, Martin-Luther-University Halle-Wittenberg, Halle (Saale), Germany § JARA-Energy, Forschungszentrum Jülich and RWTH Aachen University, Germany ‡
ABSTRACT: Oxygen transport in mayenite single crystals was studied by means of 18O/16O isotope exchange experiments and time-of-flight secondary ion mass spectrometry (ToF-SIMS). Oxygen tracer diffusion coefficients D* and oxygen surface exchange coefficients k* were determined as a function of temperature, 773 ≤ T/K ≤ 1273, at an oxygen activity of aO2 = 0.1, and as a function of oxygen activity, 0.01 ≤ aO2 ≤ 0.9, at a temperature of T = 1123 K. Two diffusion processes were observed: a fast diffusion process that is attributed to the interstitialcy diffusion of free oxygen ions (O2−) and a slow diffusion process that is attributed to the interstitialcy diffusion of superoxide ions (O2−).
1. INTRODUCTION Mayenite is a calcium aluminum oxide, Ca12Al14O33, that adopts an unusual cage-like structure. The calcium and aluminum ions form, with 32 of the 33 oxygen ions, the cage framework. The remaining oxygen ion, known as the free oxygen ion or as the extra-framework ion, is distributed randomly in the cages (such that a sixth of the cages are occupied) and may be regarded as an interstitial defect. This free oxygen ion can be replaced by other anionic species. Jeevaratnam et al.1 were the first to show this, by exchanging O2− with Cl− or F−. Later work2−11 showed that O2−, O−, OH−, NH2−, CN−, NO2−, and even e− (to yield an electride) can replace the free O2− in the cages. At elevated temperatures, undoped mayenite exhibits high and exclusively ionic conductivity.12 The absolute magnitude of the ionic conductivity was reported to be only 1 order of magnitude less than that of the well-known oxygen-ion conducting electrolyte yttria-stabilized zirconia (YSZ). This high ionic conductivity, together with the facile replacement of the free oxygen ions, requires that these free oxygen ions are mobile. The mechanism of migration, according to theoretical calculations13,14 and high-temperature neutron diffraction data,5 is not a direct interstitial jump of O2− from one cage to another but an indirect interstitialcy process involving a framework oxygen ion. Oxygen tracer diffusion experiments on mayenite have been performed by Kilo et al.15 In their diffusion profiles they © 2015 American Chemical Society
observed two features, which they explained in terms of two different diffusion mechanisms: fast diffusion of the free oxygen ions by an interstitialcy (or interstitial) mechanism, and slow diffusion of the framework oxygen ions. The values of the tracer diffusion coefficients they determined for the fast and slow diffusion processes are, however, around 3 and 7 orders of magnitude smaller, respectively, than diffusion coefficients (converted with the Nernst−Einstein equation) from the conductivity data of Lacerda et al.12 and Lee et al.16 (both of which agree very well with one another). The activation enthalpy that Kilo et al. calculated from their data, surprisingly, is identical to values from the conductivity studies, 0.83 eV.12,16 A further discrepancy concerns the possible incorporation of oxygen (as O2−) or of water (as OH−), which occurs in air at temperatures T < 1100 K.8,11 These species may completely replace the free O2− within the cages, yet, despite the change in the type and number of charge carriers, conductivity data10,14 indicate a single activation enthalpy over the range 600 < T/K < 1600. The primary aims of this study are to resolve the discrepancies between literature reports5,12−16 concerning (i) the mechanisms of oxygen diffusion in mayenite; (ii) the Received: December 24, 2014 Revised: April 17, 2015 Published: April 17, 2015 9721
DOI: 10.1021/jp512863u J. Phys. Chem. C 2015, 119, 9721−9727
Article
The Journal of Physical Chemistry C
The isotopic fraction was calculated from the measured secondary ion intensities of the first burst i1(A), according to
absolute magnitude of the oxygen diffusion coefficients; and (iii) the nature of the diffusing/conducting species. To this end, we re-examine oxygen isotope diffusion in mayenite by means of 18O/16O exchange experiments and time-of-flight secondary ion mass spectrometry (ToF-SIMS).
n* =
i1(18O) i1(18O) + i1(16O)
(1)
This value was corrected for the isotope fraction of the annealing gas ng* and the measured background fraction nbg* according to
2. EXPERIMENTAL SECTION All experiments were performed on mayenite single crystals that were grown in a four-mirror optical floating zone furnace (FZ-Z-10000; Crystal System Corp.), equipped with 1500 W halogen lamps. Rods of the polycrystalline starting material (prepared by solid state synthesis from high purity α-Al2O3 and CaCO3)17 were pressed in a cold hydrostatic press (Riken Seiki; HPTS-E-2000-W) at 70 MPa and afterward sintered in air at 1573 K for 24 h. To avoid the formation of cracks and bubbles, the crystal growth was conducted in two steps. The first run was performed at a rather high growth speed of 5 mm h−1 to eliminate gas bubbles from the melt. Afterward the rod was remelted at a much slower growth rate of 0.2 mm h−1. During the crystal growth, seed and feed rods were counterrotated at a speed of 10 rpm. Both growth steps were carried out in an atmosphere composed of 2% oxygen and 98% nitrogen.17 The resulting samples are transparent, colorless boules. All 18O/16O isotope exchange experiments were performed18 on samples cut from one single crystal boule of 12 mm diameter. Samples of thickness 1 mm were polished on both sides to an r.m.s. roughness of ±20 nm (as determined by interference microscopy images of an area of 0.9 mm × 1.2 mm). Each sample was first equilibrated at the temperature and oxygen activity of interest in dry oxygen of normal isotopic abundance (99.996% purity) for a period of time at least 10 times that of the isotope exchange anneal. The sample was then quenched to room temperature, and the exchange rig was evacuated to a total pressure of 10−8 mbar. Subsequently the rig was refilled with 18O-enriched oxygen gas, and the sample was annealed at the same temperature and oxygen activity for a time t. Experiments as a function of temperature, 773 ≤ T/K ≤ 1273, were performed at an oxygen activity of aO2 = 0.1 (aO2 = pO2/p⊖, p⊖ = 1 bar). Experiments as a function of oxygen activity, 0.01 ≤ aO2 ≤ 0.9, were carried out (by varying the absolute pressure of oxygen) at a temperature of T = 1123 K. The temperature was measured with a Pt−Pt/Rh thermocouple placed directly next to the sample; the pressure was measured with an active capacitive manometer (type CMR 271, Pfeiffer Vacuum, Asslar, Germany). The diffusion temperatures and times were corrected for heating and cooling periods and are listed in Tables 1 and 2. The 18O isotopic fraction n*(x) was determined by ToFSIMS on a ToF-SIMS IV machine (ION-TOF GmbH, Muenster, Germany). The measurements were conducted with 25 keV Ga+ ions for ToF analysis (in burst mode19), 2 keV Cs+ ions for sputter etching of the sample, and
