Magnetic and New Optical Properties in the UV–visible Range of the Egyptian Blue Pigment Cuprorivaite CaCuSi₄O₁₀

Cuprorivaite CaCuSi₄O₁₀, the so-called Egyptian blue pigment, exhibits a characteristic and well-known near-infrared luminescence upon excitation in Cu²⁺ d–d transitions. Here, we show that upon excitation in the UV, this near-infrared emission is considerably amplified and is accompanied by three different UV–visible emission bands. These four UV-excited emissions are attributed to self-trapped excitons (emission at 2.8 eV), to excitons trapped at Cu sites (emission at 1.4 eV), and to trapped exciton emission where the hole is only trapped at Cu sites (emissions at 2.65 and 3.27 eV). Despite the high Cu content of cuprorivaite, electron magnetic resonance reveals only a weak antiferromagnetic interaction (about 0.08 meV), which averages out the hyperfine interaction of the electron spin with nuclear spins of ⁶³Cu and ⁶⁵Cu nuclei.