Monophase crystalline mixed borates having the formula: M(II)1-xEu(II)xM(III)pEu(III)qTb(III)rB9O16 wherein M(II) is at least one bivalent metal selected amongst 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 amongst 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 represent each a number which may vary from 0 to 1, it being understood that, for a given borate: one of the numbers x, q and r is different from zero; and the sum p + q + r equals 1; preparation method and luminescent composition containing them.