Multianalytical Study of Historical Luminescent Lithopone for the Detection of Impurities and Trace Metal Ions

We have explored the performance of an integrated multianalytical approach to the analysis of a series of microsamples of historical lithopone (a coprecipitate of ZnS + -BaSO4) produced at the beginning of the 20th century, based on the combination of spectrally- and lifetime-resolved photoluminescence (PL) microscopy imaging and electron paramagnetic resonance (EPR) spectroscopy. Multispectral imaging of the PL emission from microsamples revealed the presence of different luminescence centers emitting in the visible spectrum, which we have hypothesized as trace Cu and Mn impurities unintentionally introduced into the ZnS crystal lattice during synthesis, which act as deep traps for electrons. Time-resolved PL imaging analyses highlighted the microsecond decay-kinetic behavior of the emission, confirming the trap state nature of the luminescence centers. EPR confirmed the presence of Cu and Mn, further providing information on the microenvironment of defects in the ZnS crystalline lattice related to specific paramagnetic ions. The multianalytical approach provides important insights into the historical synthesis of lithophone and will be useful for the rapid screening and mapping of impurities in complex semiconductor pigments and other artists' materials.