Relationship between Oxygen Content and Crystal Structure of W Films Electrodeposited in Molten CsF-CsCl-WO₃

The formation of W film with different crystal structures, α-W and β-W, has been reported in molten salt electroplating; however, the mechanism has not been sufficiently explored in previous studies. Here, the electrochemical behavior of W ions and the relationship between the oxygen content and crystal structure of electrodeposited W films were investigated in CsF-CsCl eutectic melts after adding 2.0 mol% of WO₃ at 773-973 K. The oxygen content in W films electrodeposited at various temperatures was analyzed using an inert gas fusion infrared absorption method. The oxygen content in the W films gradually decreased from 6.65 to 0.19 at% as the bath temperature increased from 773 to 973 K, and the crystal phase changed from β-W to α-W, as confirmed by X-ray diffractometry. Then, vacuum annealing treatment was performed on the β-W films electrodeposited at 823 K. Mirror-like α-W films were obtained after annealing at 973 K for 3 h; the oxygen content in the films decreased from 1.82 to 0.41 at%. This result shows that mirror-like α-W films are obtained by a two-step method of electrodeposition and vacuum annealing.