Piezochromism and anomalous near-infrared luminescence evolution of BaCuSi4O10 and BaCuSi2O6via pressure-induced phase transition

Abstract

The inorganic pigments BaCuSi₄O₁₀ (Han blue) and BaCuSi₂O₆ (Han purple) possess a bright hue in a square-plane [CuO₄]⁶⁻ chromophore, which also gives intense and broad emission in the near-infrared (NIR) region. In this paper, we utilize the pressure technique to explore the evolution of the color and the NIR luminescence from the crystal-field of the [CuO₄]⁶⁻ chromophore in BaCuSi₄O₁₀ and BaCuSi₂O₆. The absorption and NIR emission spectra of BaCuSi₄O₁₀ show sudden changes at the phase transition points (∼3 GPa and 15 GPa), where the square coordination of Cu²⁺ evolves into over-bonded and then tetrahedra, along with the space group alternation from P4/ncc to P42₁2 and finally to P2₁2₁2. In contrast, the NIR emission from BaCuSi₂O₆ weakens constantly upon compression up to 15 GPa, accompanied by piezochromism from blue to green, which aligns with the transformation of Cu²⁺ coordination from square-planar to tetrahedron. Our study contributes to the understanding of the behavior of these ancient pigments under extreme conditions and further offers insights into the regulation of photophysical properties through pressure-driven Cu²⁺ crystal fields.