Four new representatives of the natrochalcite structure type s.s. – A+Me2+2(X6+O4)2[(H2O)(OH)], monoclinic, space group C2/m, Z = 2 – with A+ = Na or K, Me2+ = Co or Ni, and X6+ = S or Se, were structurally characterised in Part I of this study. These compounds feature an extraordinary hydrogen bonding system, based on formal H3O−2 units with extremely strong intramolecular hydrogen bonds. In particular, the Na-sulphates of Co and Ni exhibit the shortest known oxygen donor–acceptor distances hitherto reported for minerals or mineral-type phases at ambient conditions (and with fully occupied donor/acceptor positions), i.e. 2.429 and 2.420 Å , respectively.
In the present part, the nature of these formal H3O2− units is investigated by powder- and polarised single-crystal FTIR spectroscopy. The single-crystal spectra are characterised by a strictly polarised, extremely broad and asymmetric absorption band, caused by the O–H stretching vibration of the strong intramolecular hydrogen bond. In the sulphates the maximum of this band is located around 1000 cm−1, shifted to slightly higher wavenumbers in the selenates, while the stretching modes of the longer terminal hydrogen bonds are found around 3400 cm−1. The additional occurrence of various bending modes, e.g. at ~1640 cm−1, strongly indicates the assignment of relevant absorption bands to distinct H2O and OH− groups. This also confirms the non-centrosymmetric configuration of the formal H3O2− unit and the presence of a split hydrogen position along the strong hydrogen bond, even at O···O distances as short as 2.42 Å .
In addition, polarised optical absorption spectra of the two sodium cobalt compounds are presented. The spectra are interpreted in terms of a pseudotetragonal crystal field, compressed along the short octahedral H(2)O–Co–OH(2) axis. The extracted crystal field and interelectronic repulsion parameters indicate the presence of comparatively strong crystal fields and a rather high ionic character of the Co–O bonds in these natrochalcite-type compounds. Due to the strong contribution of the close H2O/OH− ligands along the pseudotetragonally compressed axis, the spectroscopic tetragonal field compression exceeds the respective geometric compression of the CoO6 polyhedra, in agreement with the high position of water molecules in the spectrochemical series of ligands.