Abstract: Monophase crystalline mixed borates having the formula: M(II).sub.1-x Eu(II).sub.x M(III).sub.p Eu(III).sub.q Tb(III).sub.r B.sub.9 O.sub.16 wherein M(II) is at least one bivalent metal selected among barium, strontium, lead and calcium, it being understood that, in a given mixed borate, the lead and the calcium, together represent not more than 20% by moles based on the total number of moles of metals M(II); M(III) is a metal selected among lanthanum, gadolinium, yttrium, serium, lutetium and bismuth; x is a number higher or equal to zero and smaller than or equal to 0.2; p, q and r is different from zero; and the sum p+q+r equals 1; preparation method and luminescent composition containing them.