. "t the required amount of such compounds as carbonate or oxide containing barium, magnesium and tantalum, in order to mainly get a Ba(Mg.sub.1/3 Ta.sub.2/3)O.sub.3 compound by calcining,(b) before or after calcination, adding phosphorus in an amount from 0.005 to 1.0% by weight in the form of phosphorus compounds to the powder,(c) calcining, and pressing the powder to obtain a green body of the des" . . . . .