Synthesis and properties of thin film polymorphs of molybdenum trioxide

Thin film polymorphs of molybdenum trioxide have been synthesized by reactive r.f. sputtering in an atmosphere of argon plus oxygen. Films deposited on thermally insulated substrates are polycrystalline and exhibit preferred orientation. Depending on the oxygen partial pressure PO2 during sputtering, the films crystallize in either the thermodynamically stable α-MoO3 form (PO2 ≈ 50%) (unique two-dimensional layered structure) or the newly discovered, metastable monoclinic β-MoO3 phase (PO2 ≈ 10%) (three-dimensional ReO3-related structure). Metastable β films convert thermally to the α phase and the transformation appears topotactic. In contrast, films deposited on cooled substrates are amorphous. A correlation between the particular phase formed and adatom mobility is noted.