Single crystal and its application

The invention concerns a quadratic single crystal of composition Z(H,D)2MO4, wherein: Z is an element or a group of elements, or a mixture of elements and/or of groups of elements selected among the group K, N(H,D)4, Rb, Ce; M is an element selected among the group P, As; and (H,D) is hydrogen and/or deuterium comprising a substantially large dimension parallelepiped zone, in particular whereof the dimension of each of the edges of the surfaces AC1, AC2, AC3, is not less than 200 mm, in particular not less than 500 mm, obtained by crystalline growth in solution from a substantially parallelepiped single crystal germ (2, 22), whereof the edges of the surfaces have dimension AG1, AG2, AG3. The invention is characterised in that at least the dimension of one edge, AG1, of the germ is not lessthan one tenth, preferably one quarter of the dimension of an edge of the surfaces of the single crystal and at least another dimension of the germ, AG3, is not more than one fifth, preferably than one tenth of the greatest