. . "wherein X1 is V, Nb, Ta, Cr, W, Ga, Ce and/or La; X2 is Li, Na, K, Rb, Cs, Cu, Ag, Au, Pd and/or Pt; X3 is Sn, Pb, Sb, Bi, Te, Fe, Co and/or Ni; X4 is Si, Al, Ti and/or Zr; a is 0 to 2; d is 0 to 2, with the proviso that the sum of a and d is at least 0.20; b is 0 to 1.5, c is 0 to 10, with the proviso that the sum of b and c is at least 0.1; e is 0 to 0.5, f is 0 to 0.5, g is 0 to 20 and h is a n" . .