Revealing the Origin and History of Lead-White Pigments by Their Photoluminescence Properties

The lead white pigment, composed of two main mineral phases cerussite PbCO₃ and hydrocerussite 2PbCO₃·Pb­(OH)₂, has been used in paintings since the Antiquity. The study of historical sources revealed that a large variety of lead white qualities were proposed, depending on the degree of sophistication of the pigment synthesis. Investigation of photoluminescence of the two constitutive mineral phases gave insight into the origin of the visible emission of these materials and emphasized the influence of structural defects on their photoluminescence properties. These effects were observed by combining emission and excitation spectra in two-dimensional representations. For each excitation wavelength, between 250 and 400 nm (4.9–3.1 eV), luminescence spectra were collected between 400 and 800 nm (3.1–1.5 eV). Two types of emission-excitation bands were identified: an emission excited in the optical bandgap of the compounds (about 5 eV), which depends on the constitutive phase (2.8 eV in cerussite and 2.1 eV in hydrocerussite), and broad emission bands in the same energy range excited below the optical gap, which are sensitive to the synthesis method and the nature of postsynthesis treatments. It is proposed that this sensitivity of photoluminescence properties of lead-white pigments could be used as fingerprints of their origin and history

Storage of Visible Light for Long-Lasting Phosphorescence in Chromium-Doped Zinc Gallate

ZnGa₂O₄:Cr³⁺ presents near-infrared long-lasting phosphorescence (LLP) suitable for in vivo bioimaging. It is a bright LLP material showing a main thermally stimulated luminescence (TSL) peak around 318 K. The TSL peak can be excited virtually by all visible wavelengths from 1.8 eV (680 nm) via d–d excitation of Cr³⁺ to above ZnGa₂O₄ band gap (4.5 eV–275 nm). The mechanism of LLP induced by visible light excitation is entirely localized around Cr_N2 ion that is a Cr³⁺ ion with an antisite defect as first cationic neighbor. The charging process involves trapping of an electron–hole pair at antisite defects of opposite charges, one of them being first cationic neighbor to Cr_N2. We propose that the driving force for charge separation in the excited states of chromium is the local electric field created by the neighboring pair of antisite defects. The cluster of defects formed by Cr_N2 ion and the complementary antisite defects is therefore able to store visible light. This unique property enables repeated excitation of LLP through living tissues in ZnGa₂O₄:Cr³⁺ biomarkers used for in vivo imaging. Upon excitation of ZnGa₂O₄:Cr³⁺ above 3.1 eV, LLP efficiency is amplified by band-assistance because of the position of Cr³⁺⁴T₁ (⁴F) state inside ZnGa₂O₄ conduction band. Additional TSL peaks emitted by all types of Cr³⁺ including defect-free Cr_R then appear at low temperature, showing that shallower trapping at defects located far away from Cr³⁺ occurs through band excitation