Epitaxial Growth of ϵ-Fe₂O₃ on Mullite Found through Studies on a Traditional Japanese Stoneware

In the course of our study on a Japanese traditional stoneware called “Bizen”, we have found that ϵ-Fe 2 O 3, which has been classically known but recently limelighted for its magnetic potentiality, crystallizes epitaxially on needlelike crystals of mullite, 3(Al,Fe) 2 O 3 ·2SiO 2 . This interesting reaction occurred when a pellet of Bizen clay powder was covered with rice straw and then heated to 1250 °C in a stream of nitrogen containing 1−2 vol % of oxygen. It is known that mullite forms from the clay on heating and that the rice straw provides potassium to induce partial melting in the pellet surface to a depth of ∼50 µm. The ϵ-Fe 2 O 3 -on-mullite particles formed in this molten region, changing their morphology with the oxygen partial pressure. Square-columnar single-crystalline particles with prismatic ends grew to ∼0.1 × 0.1 × 0.5 µm 3 at N 2 /O 2 = 99/1, while dendritic finlike crystals of ∼0.3 × 0.1 × 0.8 µm 3, which was then the biggest size ever reported, grew at N 2 /O 2 = 98/2. The relative ϵ-Fe 2 O 3 -to-mullite orientation also changed with the oxygen partial pressure. This discovery suggests morphological controllability of this metastable but magnetically interesting iron oxide as a function of the oxygen partial pressure.