TECHNOLOGY FOCUS - INORGANIC CHEMISTRY - Preparation (Claimed): Preparation of (I) comprises either: reacting at least one inorganic salt (of halides, nitrates, sulfates or phosphates) of a metal belonging to the group of lanthanides, where the salt optionally being in its hydrate form, with at least two salts of an alkaline metal, or of an alkaline-earth metal, of two mono-anionic ligands belonging to the same chemical group, different from each other, at 10-45 degrees C for 10-60 minutes; or reacting at least one lanthanide compound of formula (Ln(A1)w(L1L2)) (IB) with at least one lanthanide compound of formula (Ln(B1)w(L1L2) (IC) and, optionally with at least one lanthanide compound of formula (Ln(C1)w(L1L2)) (ID), in the presence of at least one organic solvent e.g. dichloromethane, chloroform, diethylether, tetrahydrofuran, toluene and cyclohexane, at 10-45 degrees C for 10-60 minutes.w = 2 or 3. Preferred Definitions:    Ln = Eu (preferred), Tb, Sm, Er, Nd or Yb;    R1 = trifluoromethyl; either    x+y = 3; and    z = 0; or    x, y, z = 1;    R3 = thiophene, naphthyl, trifluoromethyl or phenanthrene;    R2 = hydrogen; and    L1L2 = substituted phenanthroline, preferably 3,4,7,8-tetramethyl-1,10-phenanthroline.    EXAMPLE - 4,4,4-Trifluoro-1-(2-thienyl)-1,3-butanedione (1.135 g) and ethyl ether (40 ml) were introduced into a 250 ml flask, and 4,4,4-trifluoro-1-(2-naphthyl)-1,3-butanedione (0.681 g) were added to the solution thus obtained. The whole was stirred, at 25 degrees C, until a clear light yellow-colored solution is obtained. Aqueous solution (76 ml) of 0.1N sodium hydroxide was added to this solution. The reaction mixture was vigorously stirred to facilitate the contact between the two liquid phases, organic and aqueous, for 15 minutes, at 25 degrees C. The reaction mixture was then transferred to a separating funnel and the organic and aqueous phases were separated. The organic phase was subjected to two washings with water (15 ml). The washing waters of the organic phase were added to the aqueous phase previously separated followed by a solution obtained by dissolving europium trichloride hexahydrate (1.213 g) in water (10 ml). A waxy light yellow-coloured solid was instantaneously formed and the whole was stirred, at 25 degrees C for 30 minutes. Methylene chloride (30 ml) was then added to obtain a rapid dissolution of the solid present. The mixture thus obtained was transferred to a separating funnel and the two phases obtained, organic and aqueous, were separated. The aqueous phase was further extracted with methylene chloride (10 ml). The overall organic phase (obtained by joining the organic phases produced) was subjected to two washings with water (10 ml), subsequently added with a solution of 3,4,7,8-tetramethyl-1,10-phenanthroline (0.569 g) in methylene chloride (10 ml) and finally left under stirring, at 25 degrees C for 16 hours. At the end, the solvent was evaporated at reduced pressure and the residual solid was dried at 50 degrees C, under vacuum (0.01 mmHg) for 16 hours. The yellow-colored raw solid obtained was purified by fractionated crystallization, slowly adding heptane to a solution of the raw solid in methylene chloride to obtain bis(4,4,4-trifluoro-1-(2-thienyl)-1,3-butanedione)mono(4,4,4-trifluoro-1-(2-naphthyl)-1,3-butanedione)phenyl europium complex (2.079 g) as a light yellow-colored crystalline solid.