Charge transfer in mixed-valence solids. Part 9. Preparation, characterization, and optical spectroscopy of the mixed-valence mineral voltaite [aluminium pentairon(II) tri-iron(III) dipotassium dodecasulphate 18-hydrate] and its solid solutions with cadmium(II)

Abstract

Crystals of voltaite, K₂Fe^II₅Fe^III₃Al[SO₄]₁₂·18H₂O, and solid solutions K₂Fe^II_xCd^II_5–xFe^III₃Al[SO₄]_l2·18H₂O, have been prepared and characterized by chemical analysis and X-ray diffraction. The solid solutions are isostructural [O_h⁸(Fd3c)], a varying only from 27.25 (x= 5) to 27.49 Å(x= 0). Diffuse-reflectance spectra have been recorded at room temperature (5 000–50 000 cm^–1) and single-crystal transmission spectra at temperatures down to 4.2 K (5 000–25 000 cm^–1). From the spectrum of cadmium voltaite, iron(III) ligand-field transitions are found at 14 000 (⁶A_1g→⁴T_1g), 19 000 (⁶A_1g→⁴T_2g), and 23 500 cm^–1(⁶A_1g→⁴A_1g,⁴E_g). In the mixed-valence crystals a broad absorption band centred at 16 500 cm^–1 is the intervalence charge-transfer transition. Its half-width and integrated band area vary only slightly from room temperature to 4.2 K. The linear absorption constant of the intervalence transition in pure voltaite, extrapolated from the concentration dependence of the absorption constant in the crystals diluted with Cd^II, is 150 ± 3 cm^–1. The iron(II) ligand-field transition appears as a shoulder at 10 000 cm^–1. From the oscillator strength of the intervalence transition, the valence delocalization constant is estimated as 0.005 7. This value is compared with those of mixed-valence silicates and metal complexes.