Crystal chemistry of synthetic Co- and Ni-analogues of natrochalcite _ the shortest known hydrogen bonds among mineral-type compounds. Part II: Spectroscopic studies

摘要

Four new representatives of the natrochalcite structure type s.s. _ A~+Me~(2+)_2(X~6+0_4)_2[(H_20)(OH)], monoclinic, space group C2/m, Z = 2 _ with A~+ = Na or K, Me~(2+) = Co or Ni, and X~(6+) = S or Se, were structurally characterised in Part I of this study. These compounds feature an extraordinary hydrogen bonding system, based on formal H_3O_2~- units with extremely strong intramo_lecular 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 A, respectively. In the present part, the nature of these formal H_3O_2~- 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 0_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 H_2O and OH~_ groups. This also confirms the non-centrosymmetric configuration of the formal H_30_2~- unit and the presence of a split hydrogen position along the strong hydrogen bond, even at O__O distances as short as 2.42 A. 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 H_2O/0H~-ligands along the pseudotetragonally compressed axis, the spectroscopic tetragonal field compression exceeds the respective geometric compression of the Co0_6 polyhedra, in agreement with the high position of water molecules in the spectrochemical series of ligands.