The invention refers to new chemical compounds belonging to the known class of REE mixed ligand complexes, namely, to the compound of di-(nitrate)acetylacetonatobis (1,10-fenantrolin)lanthanide (III) of the general formula [Ln(NO3)2Acac(Phen)2]*H20, where Ln is TbxEr1-x, Acac is acetylacetonate-ion, Phen is 1,10-fenantrolin in the form suitable for application as a luminescent additive to ink for the hidden marking the valuables to provide protection against forgery. Such additive has three protective features suitable for automatic control independently of each other. The first protective feature is the ability to luminescence in green light under the impact of UV-radiation. The second protective feature is the ability to luminescence in green light under the impact of X-rays. The third protective feature is the Thermo-luminescence, which means the ability to accumulate for indefinite time span the energy of UV- and X-rays with subsequent green light emission at heating. It allows univocal identification of valuables and improves the protection efficiency.