Effect of synthesis route on optical properties of Cr:Al₂O₃ transparent ceramics sintered under high pressure

Transparent polycrystalline ceramics doped with active ions are suitable for many potential optical applications. Fabrication methods that utilize applied external pressure show promise as they allow reducing sintering temperatures and produce transparent non-cubic ceramics. However, not much attention has been given to the dissolution of doping elements while sintering under relatively high pressure and low temperature conditions. In this study, we employed high-pressure spark plasma sintering (HPSPS) for fabrication of Cr:Al₂O₃ ceramics and investigated the effect of two different synthesis routes on densification and optical properties. Namely, HPSPS of Cr-doped alumina powders prepared by directly doping with 0.05, 0.2, and 0.5 Cr at.% via co-precipitation and alumina mixed with similar mol.% of Cr₂O₃ nanoparticles towards doping via solid-state reactive sintering (so-called ‘synthesized’ and ‘mixed’, respectively). Remarkable differences in optical properties between samples obtained by each method were observed. The ceramics fabricated from synthesized powders exhibited pink shades of ruby, high transparency, and strong photoluminescence. In contrast, the mixed ceramics exhibited green color, low transparency, and weak photoluminescence. It was found that in the green samples, most of the Cr₂O₃ nanoparticles remain as undissolved nano-inclusions. These inclusions are detrimental to optical properties and cause green pigmentation of the alumina. Post-sintering heat treatment dissolved the inclusions and the added Cr³⁺ in alumina increased photoluminescence. Evidently, doping transparent alumina via reactive sintering is not viable for processes such as HPSPS. Nevertheless, rapid low temperature densification might enable the design of functional translucent ceramics with integrated second phase nanoparticles.