Magnesia‐carbon (MgO‐C) bricks partially made of recycled magnesia (MgO) are examined in hardened and coked state using Raman and photoluminescence (PL) spectroscopy, focusing on the MgO grains compared with nominally pure MgO powder for reference. Raman and PL signals in the visible to near infrared range are distinguished and interpreted by varying the excitation wavelength between 532, 633, and 785 nm. Besides a broad PL band centered at 565 nm characteristic of the brick‐derived MgO, the origin of two sharp high‐intensity PL signals (699, 871 nm) surrounded by symmetrical sidebands is clarified. The 699 nm luminescence reveals the presence of trivalent chromium (Cr 3+ ) impurities in the MgO lattice of MgO‐C bricks as well as MgO powder, while the latter, V 2+ ‐related PL signal is only observed for the brick‐derived MgO. The nature of the symmetrical sidebands is investigated by temperature‐dependent spectroscopic measurements between 100 and 295 K. The temperature dependence of the intensity ratio between higher energy (anti‐Stokes) and lower energy (Stokes) sidebands confirms the phonon involvement in both the MgO:Cr 3+ and MgO:V 2+ signals. This study enhances the spectroscopic methods to apply to industrial refractory materials, while it reveals impurities on a defect level that is not accessible by any well‐established analysis.
Richter et al. (Thu,) studied this question.