ALBERT

Description: 〈span〉〈div〉Abstract〈/div〉The crystal structure of polylithionite-1〈span〉M〈/span〉 from Darai-Pioz, (K〈sub〉0.97〈/sub〉Na〈sub〉0.03〈/sub〉Rb〈sub〉0.01〈/sub〉)〈sub〉Σ1.01〈/sub〉(Li〈sub〉2.04〈/sub〉Al〈sub〉0.84〈/sub〉 Ti〈sup〉4+〈/sup〉〈sub〉0.09〈/sub〉Fe〈sup〉3+〈/sup〉〈sub〉0.03〈/sub〉)〈sub〉Σ3.00〈/sub〉(Si〈sub〉3.98〈/sub〉Al〈sub〉0.02〈/sub〉)O〈sub〉10〈/sub〉[F〈sub〉1.68〈/sub〉(OH)〈sub〉0.33〈/sub〉]〈sub〉Σ2〈/sub〉, 〈span〉a〈/span〉 5.1974(4), 〈span〉b〈/span〉 8.9753(6), 〈span〉c〈/span〉 10.0556(7) Å, β 100.454(1)°, 〈span〉V〈/span〉 461.30(6) Å〈sup〉3〈/sup〉, space group 〈span〉C〈/span〉2, 〈span〉Z〈/span〉 = 2, was refined to 〈span〉R〈/span〉〈sub〉1〈/sub〉 = 1.99% using Mo〈span〉K〈/span〉α X-radiation. In the space group 〈span〉C〈/span〉2, there are three octahedrally coordinated 〈span〉M〈/span〉 sites in the 1〈span〉M〈/span〉 mica structure: the 〈span〉M〈/span〉(1) site is occupied by Li〈sup〉+〈/sup〉 and minor vacancy that is likely locally associated with Ti〈sup〉4+〈/sup〉 at the 〈span〉M〈/span〉(2) site; the 〈span〉M〈/span〉(2) site is occupied dominantly by Al〈sup〉3+〈/sup〉, with other minor divalent to tetravalent cations; the 〈span〉M〈/span〉(3) site is completely occupied by Li〈sup〉+〈/sup〉. In the space group 〈span〉C〈/span〉2, the structure is completely ordered. Each non-bridging O〈sup〉2–〈/sup〉 ion is surrounded by an ordered arrangement of 2Li〈sup〉+〈/sup〉 + Al〈sup〉3+〈/sup〉 + Si〈sup〉4+〈/sup〉 with an incident bond-valence sum of 1.95 〈span〉vu〈/span〉 (valence units). The F〈sup〉–〈/sup〉 ion is coordinated by Li〈sup〉+〈/sup〉 + Li〈sup〉+〈/sup〉 + Al〈sup〉3+〈/sup〉 with an incident bond-valence sum of 0.84 〈span〉vu〈/span〉 (values around F〈sup〉–〈/sup〉 generally tend to be lower than ideal). Thus, the valence-sum rule is satisfied, both long range and short range. In the space group 〈span〉C〈/span〉2/〈span〉m〈/span〉, 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 〈span〉vu〈/span〉 around the non-bridging O〈sup〉2–〈/sup〉 ion and the F〈sup〉–〈/sup〉 ion, destabilizing the structure relative to the more ordered arrangement of the 〈span〉C〈/span〉2 structure, which conforms more closely to the valence-sum rule. The drive to lower the symmetry in polylithionite-1〈span〉M〈/span〉 from 〈span〉C〈/span〉2/〈span〉m〈/span〉 to 〈span〉C〈/span〉2 comes from the short-range bond-valence requirements of the structure.〈/span〉