High temperature single crystal neutron diffraction analysis of theapatite-type oxide-ion conductors
Tokyo Institute of Technology Kotaro Fujii, Masatomo Yashima
1. Introduction

Oxide-ion conductors attract many interests because of their potential use for oxygen
separation membranes and cathodes of solid-oxide fuel cells (SOFCs). At the present time,oxide-ionic conducting ceramics are used at high temperature (800-1000°C) because therequired oxide-ion conductivity is achieved at such high temperatures. Therefore, adevelopment of high oxide-ion conductors at low or intermediate temperature (less than800 °C) are strongly required. Apatite-type lanthanum silicates are good candidates for theelectrolyte of intermediate temperature solid-state fuel cells (IT-SOFCs) because of their highoxide-ion conductivity at low temperature. Although understanding the mechanism of theoxide-ion conduction in the apatite-type lanthanum silicates is necessary for furtherdevelopment on the oxide-ion conductors, it has not been clearly understood. In order to resolve
this issue, precise and accurate crystal structure information is necessary. Most of previously
reported works were discussed using powder diffraction method and/or theoretical calculation,
which are not sufficient to discuss the details. A single crystal neutron diffraction technique ismost powerful method to reveal precise and accurate crystal structure including detail
information on the oxygen atoms (atomic position, atomic displacement parameters, disorder,and occupancy factors) which is important to understand the mechanism of the oxide-ionconduction. From our previous experiment at Senju diffractometer of J-PARC, we haverevealed the crystal structures of La9.33Si6O26 and a La-rich material La9.33+δSi6-δO26 at roomtemperature. In this proposal, we aim to conduct high-temperature single-crystal neutron
diffraction experiments for La9.33+δSi6-δO26.
2. Experiment
The single crystal of La9.33+δSi6-δO26 with size of ca. 1 ×1 × 1 mm3 is mounted on a Ta pin andused for the high-temperature measurement. The measurement was carried out at 1000 °C
using a vacuum furnace. We planned to take the diffraction data at two different φ orientation.
However, during the measurement of the second φ orientation set data, the measurement was stopped
because of the beam dump. We have to take the data again in other beam time.
3. Results

Fig. 1 shows an example of the diffraction image of the single crystal of La9.33+δSi6-δO26 at
1000 °C. The collected event data were converted into the time slicing data. The data reduction
was carried out using the Stargazer package. The reflection data file was analyzed using theJana2006. The room temperature structure was used as the initial model. Now we are doing
least square refinements for the obtained data. Because there are several structure models,and many refinement conditions, we need time to make clear conclusion of this analysis. At thepresent, we confirmed that the crystal structure of La9.33+δSi6-δO26 at 1000 °C belongs to thespace group P61/m, which is same with that of the room temperature.
1Fig. 1: An example of the diffraction image of the single crystal of the apatite-type lanthanum silicate.

4. Conclusion
The data analysis is now in progress and the result may obtain later. The data quality seems fine,
but we need to take the data again because we met the beam dump during the measurements.

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