Topotactic transformation of gadolinite‐ to melilite‐type structure revisited

Abstract Topotactic reactions can lead to the formation of new phases, which cannot be obtained via classical synthesis methods. The high‐temperature topotactic transformation of gadolinite‐type compounds, homilite Ca 2 Fe 2+ B 2 Si 2 O 8 O 2 and datolite Ca 2 □B 2 Si 2 O 8 (OH) 2, into a melilite‐type compound, okayamalite Ca 2 B 2 SiO 7, was studied by in situ high‐temperature single‐crystal X‐ray diffraction and ex situ Raman spectroscopy. Furthermore, the density functional theory was used to calculate the electronic structures and Raman spectra of datolite, homilite, and okayamalite. At low temperatures (250–500°C), homilite undergoes the Fe oxidation process, whereas at high temperatures, both homilite and datolite transform into okayamalite (750 and 780°C, respectively). The transformation of datolite into okayamalite is a conversion of a single crystal to a polycrystalline aggregate, while the transformation of homilite into okayamalite is a transformation of a single crystal to a single crystal. Datolite derived from okayamalite is similar to natural okayamalite, whereas homilite derived from okayamalite shows Fe 3+ for Si/B substitution. The existence of tetrahedrally coordinated Fe 3+ leads to a stabilizing effect, resulting in the formation of a single crystal of okayamalite. In addition, the incorporation of iron into the structure of okaymalite leads to a change in the thermal behavior of the compound (α 11 = 22 and 15×10 −6 °C –1 for datolite‐derived and homilite‐derived okayamalites, respectively). However, the thermal expansion of okayamalites is almost isotropic, which is typical for melilite‐type compounds. datolite, high‐temperature, homilite, okayamalite, topotactic reaction