This paper demonstrates the potential of near-infrared (NIR) electronic spectroscopy in nondestructive monitoring of a chemical reaction of inorganic functional material. For this purpose NIR spectra in the 12 000-4000 cm<sup>−1</sup> region were measured for high reflective green-black (HRGB) pigments (Co<sub>0.5</sub>Mg<sub>0.5</sub>Fe<sub>0.5</sub>Al<sub>1.5</sub>O<sub>4</sub>) calcined at 1000, 1100, and 1200 °C and pigments with the same components as HRGB but calcined at different temperatures (500-900 °C) (hereafter, called “Pigments A”) . NIR spectra of their components such as Co<sub>3</sub>O<sub>4</sub>, MgO, Fe<sub>2</sub>O<sub>3</sub>, and Al<sub>2</sub>O<sub>3</sub> were also measured. The NIR spectra of Pigments A show two major broad bands. One arises from a <sup>4</sup><i>A</i><sub>2</sub>→<sup>4</sup><i>T</i><sub>1</sub> (<i>T<sub>h</sub></i>) d-d transition of Co(II) in the 9000-6000 cm<sup>−1</sup> region. The other band in the 12 000-9000 cm<sup>−1</sup> region is assigned to a foot of the charge-transfer (CT) band of Fe<sub>2</sub>O<sub>3</sub>. The Co(II) band contains three component bands that are characteristic of a spinel structure. A shoulder arising from (A<sub>1-<i>x</i></sub>B<i><sub>x</sub></i>)<sup>Th</sup>(A<i><sub>x</sub></i>B<sub>2-<i>x</i></sub>)<sup>Oh</sup>O<sub>4</sub> (A≡Co, Mg, B≡Fe, Al; inverse spinel structure) emerges near 5900 cm<sup>−1</sup> in the spectra of Pigments A calcined in the temperature range of 700-900 °C, indicating that the Pigments A calcined in this temperature range assume an inverse spinel structure. When the calcination temperature is above 1000 °C, the final product, HRGB, is produced. This is confirmed from the fact that HRGB shows peaks characteristic of a spinel structure that have different wavenumbers from those of the corresponding peaks of Pigments A. Wide-angle X-ray diffraction (WAXD) patterns were also measured for HRGB, Pigments A, and their components. Based on the NIR and WAXD data we investigated calcination-temperature-dependent crystal structural changes of the components. We also developed partial least squares (PLS) calibration models for the 9000-6000 cm<sup>−1</sup> region of the NIR spectra of HRGB and Pigments A. The score plot of latent variable (LV) 2 of the calibration model for calcination temperature demonstrates clearly the existence of an intermediate of the calcination reaction, which may be (A<sub>1-<i>x</i></sub>B<i><sub>x</sub></i>)<sup>Th</sup>(A<i><sub>x</sub></i>B<sub>2-<i>x</i></sub>)<sup>Oh</sup>O<sub>4</sub> (A≡Co, Mg, B≡Fe, Al).
YUSUKE MORISAWA, SATOSHI NOMURA, KAZUTOSHI SANADA, and YUKIHIRO OZAKI, "Monitoring of a Calcination Reaction of High Reflective Green-Black (HRGB) Pigments by Using Near-Infrared Electronic Spectroscopy: Calcination Temperature-Dependent Crystal Structural Changes of Their Components and Calibration of the Extent of the Reaction," Appl. Spectrosc. 66, 665-672 (2012)