Iron-Doped Magnesium Aluminosilicate Transparent Glass-Ceramics. Mg-Petalite or Spinel Nanocrystals? Competing Mechanisms

Transparent glass-ceramics based on Fe2+:Mg-petalite and/or Fe2+:MgAl2O4 nanocrystals were obtained from the initial glass by single and two-stage heat-treatments at temperatures from 800 to 1000 °C. ZrTiO4 and spinel crystallized during the DSC scan up to 1000 °C. Spinel nanocrystals 9-12 nm in size also appeared during single and two-stage heat-treatments at temperatures of 850 - 1000 °C. Mg-petalite crystallites ~30 nm in size evolved in the narrow temperature range from 850 to 900 °C during single-stage holding periods. A maximum fraction of Mg-petalite crystallized at 850 °C. Once formed, Mg-petalite is preserved upon further heating and holding even at 1000 °C for 6 h. Mg-petalite and spinel transformed into sapphirine and highly siliceous residual glass during heating at 1100 °C. Competing crystallization mechanisms are discussed. In materials with a weakly developed liquid-liquid phase-separated structure, crystallization of Mg-petalite from the magnesium aluminosilicate glass predominates, and spinel becomes an additional phase. Spinel crystallizes as the main phase from glasses with the developed liquid-liquid phase-separated structure. Its crystallization is accompanied by the formation of highly siliceous glass, from which Mg-petalite crystallization is impossible. Intense absorption band with maximum at ~1.9 μm due to fourfold coordinated Fe2+ ions in spinel nanocrystals is used as a spectral indicator of spinel formation. Glass-ceramics are relevant for the development of saturable absorbers intended for lasers operating at 1.6-2.4 μm.