Topotactic Growth of β‐Alumina on α‐Alumina

Thin foils of polycrystalline α‐alumina were reacted with a potassium‐rich vapor at ≤900°C. Potassium β‐alumina formed along α‐alumina grain boundaries and protruded from holes in the foils. Conventional transmission electron microscopy was used to analyze the α‐alumina/β‐alumina phase boundary for possible orientation relations.