The Crystal Structure of Polylithionite-1M from Darai-Pioz, Tajikistan: the Role of Short-range Order in Driving Symmetry Reduction in 1M Li-rich Mica

The crystal structure of polylithionite-1M from Darai-Pioz, (K0.97Na0.03Rb0.01)Σ1.01(Li2.04Al0.84 Ti4+0.09Fe3+0.03)Σ3.00(Si3.98Al0.02)O10[F1.68(OH)0.33]Σ2, a 5.1974(4), b 8.9753(6), c 10.0556(7) Å, β 100.454(1)°, V 461.30(6) Å3, space group C2, Z = 2, was refined to R1 = 1.99% using MoKα X-radiation. In the space group C2, there are three octahedrally coordinated M sites in the 1M mica structure: the M(1) site is occupied by Li+ and minor vacancy that is likely locally associated with Ti4+ at the M(2) site; the M(2) site is occupied dominantly by Al3+, with other minor divalent to tetravalent cations; the M(3) site is completely occupied by Li+. In the space group C2, the structure is completely ordered. Each non-bridging O2– ion is surrounded by an ordered arrangement of 2Li+ + Al3+ + Si4+ with an incident bond-valence sum of 1.95 vu (valence units). The F– ion is coordinated by Li+ + Li+ + Al3+ with an incident bond-valence sum of 0.84 vu (values around F– generally tend to be lower than ideal). Thus, the valence-sum rule is satisfied, both long range and short range. In the space group C2/m, there is long-range order but not short-range order. There are three different short-range arrangements, one of which has bond-valence deficiencies of 0.38 and 0.49 vu around the non-bridging O2– ion and the F– ion, destabilizing the structure relative to the more ordered arrangement of the C2 structure, which conforms more closely to the valence-sum rule. The drive to lower the symmetry in polylithionite-1M from C2/m to C2 comes from the short-range bond-valence requirements of the structure.