I suggest that you get a copy of "The properties of liquids and gases" by Poling, Prausnitz and O'Connell from your library. Chapter 11 of this classic text describes how to calculate the diffusion coefficients of small molecules, with a section on electrolytes.
You don't give a composition, but the limiting value of the mutual diffusion coefficient at infinite dilution can be calculated from the Nernst-Hartley relation and limiting ionic molar conductivities. (See Robinson and Stokes, Electrolyte Solutions, Butterworths, 1965, eqn 11.5 & Appendix 6.1). For a 1:1 salt,
D =(RT/F^2)(2(lambdaNa+)(lambdaTcO4-)/((lambdaNa+)+(lambdaTcO4-))
Dilute concentrations can be approximated by multiplying by (1+dlny+-/dlnc) if the activity coefficients are available (unlikely).
lambda(Na+) is 50.1 Scm^2mol^-1 at 25C, that for TcO4- is unlikely to have been measured, but it is probably somewhere around 50 to 70. Compare ClO4-, 67.4, IO4-, 54.6, ReO4-, 55.
Modeling of mutual diffusion coefficients is only helpful - to a point - for very dilute solutions and simple ions/molecules. For this pertechnetate I'm quite sure that solute-solvent and solute-solute interactions easily overcome the results of the available models. Of course educated guesses could be done on previous results but experimenting always returned different, sometimes surprising, results. If you're interested in pursue the experimental way please feel free to call me and discuss it ! Best regards.
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