Molecular structure of Mg–Al, Mn–Al and Zn–Al halotrichites-type double sulfates—An infrared spectroscopic study

Near infrared (NIR), X-ray diffraction (XRD) and infrared (IR) spectroscopy have been applied to halotrichites of the formula MgAl2(SO4)4·22H2O, MnAl2(SO4)4·22H2O and ZnAl2(SO4)4·22H2O. Comparison of the halotrichites in different spectral regions has shown that the incorporation of a divalent transition metal into the halotrichite structure causes a shift in OH stretching band positions to lower wavenumbers. Therefore, an increase of the hydrogen bond strength of the bonded water is observed for divalent cations with a larger molecular mass. XRD has confirmed the formation of halotrichite for all three samples and characteristic peaks of halotrichite have been identified for each halotrichite-type compound. It has been observed that Mg–Al and Mn–Al halotrichite are very similar in structure, while Zn–Al showed several differences particularly in the NIR spectra. This work has shown that compounds with halotrichite structures can be synthesised and characterised by infrared and NIR spectroscopy.

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► Near infrared (NIR), X-ray diffraction (XRD) and infrared (IR) spectroscopy have been applied to halotrichites of the formula MgAl2(SO4)4·22H2O, MnAl2(SO4)4·22H2O and ZnAl2(SO4)4·22H2O.
► Comparison of the halotrichites in different spectral regions has shown that the incorporation of a divalent transition metal into the halotrichite structure causes a shift in OH stretching band positions to lower wavenumbers.
► Therefore, an increase of the hydrogen bond strength of the bonded water is observed for divalent cations with a larger molecular mass.
► This work has shown that compounds with halotrichite structures can be synthesised and characterised by infrared and NIR spectroscopy.