SUBSTRUCTURE AND SUPERSTRUCTURE OF FOUR-LAYER CA-EXCHANGED BIRNESSITE

Abstract

Synthetic Ca-exchanged birnessite (CaBi) was studied by X-ray and selected-area elec- tron diffraction (XRD, SAED). The substructure of CaBi may be described with a four- layer monoclinic subcell with a 5 5.150, b 5 2.844, c 5 4c 95 28.16 A ˚, and b 5 90.38. Two different superstructures of CaBi were distinguished. CaBi type I has cell parameters A 5 3a 5 15.45, B 5 3b 5 8.472 A ˚. The stacking sequence in this unit cell may be described as defect-free OSOS, where successive layers are shifted relative to their pre- decessors by 0 (O) or b/2 (S) along the b axis. The complete description of stacking involves the structure of the layer itself, the structure of the interlayer, and the shift from this layer to the next one. CaBi type II can be described as a regular interstratification of AP 5 3 /2(a 2 b), BP 5 4b, g5 118.98 and AP 5 3 /2(a 1 b), BP 52 4b, g5 118.98 supercells that are connected by a mirror plane in projection on the a-b plane. Its stacking sequence is a random interstratification of OSOS (90%) and OOOS (10%) structural fragments. Most CaBi crystals appeared to consist of intergrown type I and type II sub-crystals. As in Na-rich birnessite, the A 5 3a superstructure arises from the ordered distribution of Mn 31 -rich rows parallel to (010) and separated from each other along (100) by two Mn 41 rows. In Mn 31 -rich rows heterovalent Mn cations are regularly distributed according