I hope you are still around. I too am VERY interested in this topic - FeO4 or K2FeO5. I have (many years ago) experience with synthesizing perbromate using F2 gas per E. Appleton's method for my masters research project. I did a one-time attempt to make FeO4 using the same method starting with K2FeO4 (ferrate VI) in concentrated KOH. After fluorinating the purple solution for a few minutes, it turned white, fizzed and bubbled for a few minutes, then slowly turned a yellowish-brown. Don't know if I made it, then it decomposed in the caustic solution. Like you have never be in a position to try this again or attempt any other procedure. Please respond if you are around. Thanks.