Raman fingerprint of chromate, aluminate and ferrite spinels

Synthetic and natural spinel single crystals having compositions closely approaching spinel end-members ZnCr2O4, MgCr2O4, FeCr2O4, ZnAl2O4, MgAl2O4, CoAl2O4, FeAl2O4, MnAl2O4, MgFe2O4, and FeFe2O4 were investigated by Raman spectroscopy in the 100-900cm-1 range using both the red 632.8nm line of a He-Ne laser and the blue 473.1nm line of a solid-state Nd:YAG laser. Each end-member exhibits a Raman fingerprint with at least one peculiar peak in terms of Raman shift and relative intensity. Chromates and ferrites exhibit the most intense A1g mode at around 680cm-1, at lower wavenumbers than in the aluminates, in agreement with the heavier atomic mass of Cr and Fe with respect to Al. For aluminate spinels, the most intense and diagnostic peaks in the spectrum are as follows: F2g(1) at 202cm-1 for MnAl2O4, Eg at 408cm-1 for MgAl2O4, F2g(2) at 516cm-1 for CoAl2O4, F2g(3) at 661cm-1 for ZnAl2O4, and A1g at 748cm-1 for FeAl2O4. Noteworthy, analyzing the A1g, F2g(3), and, in particular, the Eg peak positions, it is possible to establish which subgroup a spinel belongs to; moreover, a careful inspection of both position and relative intensity of the same peaks allows the determination of the end-member type.