Chemical H insertion into a low microtwinned EMD at temperatures close to 0 °C

Abstract

Hydrogen has been inserted chemically by a new method at temperatures close to 0 °C into a low microtwinned EMD made by a suspension bath process. 33 Compounds covering the range 0 ≤ s ≤ 0.85 where s is the value in MnO_1.97H_s were prepared. XRD patterns were line rich and showed that the insertion took place in three distinct stages; homogeneously between s = 0 and s = 0.28, then heterogeneously to s = 0.66 and finally homogeneously again for s ≥ 0.66. In the first homogeneous region the intergrowth structure was hinged ramsdellite–hinged pyrolusite: the inserted H was invisible to FTIR and must have been present as a sort of proton gas. The structural feature associated with this phenomenon was the near rectangular shape of the 2 × 1 tunnels of the preponderant hinged ramsdellite structure. The microtwinning was sufficient to restrict apical expansion of the ramsdellite octahedra to less than 2% and to prevent significant change of hinging angle during the first homogeneous region. In the second homogeneous region of H-insertion the intergrowth structure was groutite–hinged manganite. FTIR revealed that inserted H was present as O–H⋯O bonds in both parts of the intergrowth. The associated structural feature was the near parallelogram shape of the 2 × 1 tunnels of the preponderant groutite structure. In the central region, 0.28 ≤ s ≤ 0.66, H-insertion caused transformation of the end hinged ramsdellite–hinged pyrolusite structure (s = 0.28) into the initial groutite–hinged manganite structure (s = 0.66). Demicrotwinning must have accompanied the substantial expansion of apical distance and the major reduction in hinging angle required for this transformation. Throughout the heterogeneous region only the two phases represented by s = 0.28 and s = 0.66 were present.