<b>Absolute Structures of a Mirror Pair of Infinite Na(H₂O)₄</b>⁺<b>‑Connected ε‑Keggin–Al₁₃ Species</b>

The absolute structures of a pair of infinite Na­(H₂O)₄⁺-connected ε-Keggin–Al₁₃ species (Na−ε-K–Al₁₃) that were inversion structures and mirror images of each other were determined. Single crystals obtained by adding A₂SO₄ (A = Li, Na, K, Rb, or Cs) solution to NaOH-hydrolyzed AlCl₃ solution were subjected to X-ray structure analyses. The statistical results for 36 single crystals showed that all the crystals had almost the same unit cell parameter, belonged to the same <i>F</i>4̅3<i>m</i> space group, and possessed the same structural formula [Na­(H₂O)₄AlO₄Al₁₂(OH)₂₄(H₂O)₁₂]­(SO₄)₄·10H₂O. However, the crystals had two inverse absolute structures (denoted A and B), which had a crystallization ratio of 1:1. From Li⁺ to Cs⁺, with increasing volume of the cation coexisting in the mother solution, the degree of disorder of the four H₂O molecules in the Na­(H₂O)₄⁺ hydrated ion continuously decreased; they became ordered when the cation was Cs⁺. Absolute structures A and B are the first two infinite aluminum polycations connected by statistically occupied [(Na_1/4)₄(H₂O)₄]⁺ hydrated ions. The three-dimensional structure of the infinite Na−ε-K–Al₁₃ species can be regarded as the assembly of finite ε-K–Al₁₃ species linked by [(Na_1/4)₄(H₂O)₄]⁺ in a 1:1 ratio. In this assembly, each [(Na_1/4)₄(H₂O)₄]⁺ is connected to four ε-K–Al₁₃ and each ε-K–Al₁₃ is also connected to four [(Na_1/4)₄(H₂O)₄]⁺ in tetrahedral orientations to form a continuous rigid framework structure, which has an inverse spatial orientation between absolute structure A and B. This discovery clarifies that the ε-K–Al₁₃ (or ε-K–GaAl₁₂) species in Na­[MO₄Al₁₂(OH)₂₄(H₂O)₁₂]­(XO₄)₄·<i>n</i>H₂O (M = Al, Ga; X = S, Se; <i>n</i> = 10–20) exists as discrete groups and deepens understanding of the formation and evolution process of polyaluminum species in forcibly hydrolyzed aluminum salt solution. The reason why Na⁺ statistically occupies the four sites was examined, and a formation and evolution mechanism of the infinite Na−ε-K–Al₁₃ species was proposed.