Raman fingerprint of chromate, aluminate and ferrite spinels

Synthetic and natural spinel single crystals having compositions closely approaching spinel end‐members ZnCr 2 O 4 , MgCr 2 O 4 , FeCr 2 O 4 , ZnAl 2 O 4 , MgAl 2 O 4 , CoAl 2 O 4 , FeAl 2 O 4 , MnAl 2 O 4 , MgFe 2 O 4 , and FeFe 2 O 4 were investigated by Raman spectroscopy in the 100–900 cm −1 range using both the red 632.8 nm line of a He‐Ne laser and the blue 473.1 nm 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 A 1 g mode at around 680 cm −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: F 2 g (1) at 202 cm −1 for MnAl 2 O 4 , E g at 408 cm −1 for MgAl 2 O 4 , F 2 g (2) at 516 cm −1 for CoAl 2 O 4 , F 2 g (3) at 661 cm −1 for ZnAl 2 O 4 , and A 1 g at 748 cm −1 for FeAl 2 O 4 . Noteworthy, analyzing the A 1 g , F 2 g (3), and, in particular, the E g 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.