Synthesis and crystal-chemical characterization of spinels belonging to the MgAl2O4-CoAl2O4 series

Corso di Laurea in Georisorse Università degli Studi di Roma “La Sapienza”

Anno Accademico 2009-2010.

Laureanda: Veronica D'Ippolito (dopo aver conseguito il dottorato del 29°ciclo in Scienze della Terra presso l’Università degli "La Sapienza").
Relatore:Giovanni Battista Andreozzi, Dipartimento di Scienze della Terra-Università degli Studi di Roma “La Sapienza”.

Co-spinels are important compounds for many different uses as ceramic pigments, catalysts, and gemstones. Studies of cation distribution in these spinels are of considerable interest for a better understanding of relationships between structure and physical properties (such as colour).
Spinel single crystals having different compositions along the MgAl2O4-CoAl2O4 series were synthesized by flux growth method with Na2B4O7 as solvent.Low cooling rates (2°C/h) and linear temperature profiles were applied in the temperature interval 1200-800°C, and a continuous flow of CO2:H2 in the  ratio of 100/4 cm3/min was adopted to obtain a reducing atmosphere (with the oxygen fugacity ranging from 10-8 to 10-15 bars at 1200 and 800 °C, respectively). Selectedcrystals were analysed by single-crystal X-ray diffraction (CCD detector), electron microprobe and optical absorption spectroscopy. The crystals obtained were found to be chemically homogeneous, with the Co2+ content ranging from 0.06 to 1.00 atoms per formula unit, and varied in colour from light blue to intensely dark blue with increasing Co2+ content. The progressive substitution of Co2+ for Mg causes a large increase of tetrahedral bond length (from 1.919 to 1.944Å), which is responsible for the observed increase of both unit cell parameter (from8.079 to8.100 Å) and oxygen positional parameter (from0.2622to0.2635). Optimized structural formulae show that the cation inversion between tetrahedrally- and octahedrally-coordinated sites is directly correlated with equilibration temperature. At the constant temperature of 800 °C, the inversion degree decreases from 0.23 to 0.14 with increasing Co2+ content. With respect to Mg, Co2+ shows a higher preference for tetrahedral coordination, and its site preference for octahedral coordination (e.g., VICo2+/Cotot) increases from zero to 0.14 with increasing Co2+ content. The optical absorption spectra, measured between 4000 and 35000 cm-1, show a set of three bands at ~4300, 7200 and 17.000 cm-1 marking spin-allowed electronic d-d transitions of Co2+ in tetrahedral coordination.The absorption bands of Co3+ are absent, in agreement with the highly reducing conditions imposed during crystal growth. The observed transitions are due to Co2+ in tetrahedral coordination, and are the only responsible for the characteristic blue colour of cobalt spinels.

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