Design and operation of a variable high-temperature oxygen partial-pressure probe device for solid-state NMR

Abstract

For most oxide systems, the stoichiometry at elevated temperatures is sensitive to the external oxygen partial pressure. Nuclear magnetic resonance is a powerful technique for studying the dynamics of defects caused by the nonstoichiometry in these solids, since NMR provides microscopic information about the system ( 1). For example, jump rates can be determined from the temperature dependence of nuclear spin-lattice relaxation (NSR) rates (Zeeman NSR rate 1 / T, ; rotating-frame NSR rate 1 / T,,) using standard nuclear spin relaxation theories (2). Also, it may be possible to measure the chemical diffusion of these systems. If the defects responsible for nonstoichiometry cause an additional NSR rate ( 3)) the change of the NSR rate at a constant temperature after a stepwise PO2 change would indicate chemical diffusion. To perform NMR diffusion studies on oxide systems the following capabilities are necessary: the operating temperature must be as high as possible, while the probe’s outer temperature must be maintained close to room temperature. This allows the probe to be used in a standard wide-bore superconducting NMR magnet. Further, the sample area must be isolated from the heater wires to allow the use of corrosive gas mixtures, which provide a wide range of oxygen partial pressures. Reduced oxygen partial pressures down to 1O-6 atm are attainable with argon-oxygen gas mixtures, and oxygen partial pressures between lo-* and lo-‘* atm are attained with carbon monoxide-carbon dioxide gas mixtures. In the latter case, the PO2 is determined by the equilibrium of the chemical reaction 102 + CO P CO2 (2). Here, we report the technical details of an NMR probe system which has the capabilities described above and has been successfully used for performing NMR measurements at temperatures between 150 and 1500 IS in r-utile ( TiOzmX) at different oxygen partial pressures (3). (The actual temperature range is limited by the physical properties of r-utile.) Many of the components used in this system are commercially available; only the oxygen partial-pressure measuring instruments ( ZrOz cells) and the NMR probe itself are homebuilt.