1. Simulation of Oxygen Ion Ordering as a Result of Temperature Change
Andrew Wilson & Ashley Predith
Spring
May 13, 2002

2. Brownmillerite SrFeO2.5 Solid oxide fuel cell cathode material.
At stoichiometry, SrFeO3. Vacancies align in chains in every other octahedra.
Solid oxide fuel cell cathode material.
Vacancies experimentally ‘disorder’ at 850 °C.

3. Goal
Model the oxygen vacancy order-disorder and calculate the temperature at which the vacancies disorder in SrFeO2.5.

4. Method: Parallel Monte Carlo
Create a lattice of spins, and calculate energy.
In each MC step, calculate wi for each spin and sum: wMC = wi.

5. Parallel Monte Carlo, continued…
Pick a random number to fall on the line, and flip the spin associated with the wi. Recalculate energy.
Iterate over MC steps until equilibrium.
Repeat for each temperature.

6. Description of Code C++ programming language Lattice class
Message passing
Sets up supernode structure
Keeps track of crystal energy
Updates temperature, generates rand

7. Description of Code, continued…
Material class
Defines atomic orientation
Calculates energy when flipping spin
Computation time between serial and parallel is about the same. Speed up is due to the structure being entirely in distributed memory instead of writing it to disk.

8. Discussion of Results
Our MC results on SrFeO2.5 system Anticipated results.

9. Problems and Solutions
Problem: Equilibrium energy does not change when temperature changes. The energy expression for our system is not accurate.
Solution: Add in terms to the energy expression to account for second, third, and fourth nearest neighbors. Determine J1, J2, J3, and J4 from a cluster expansion of many calculated structures with the same parent structure.