Two‐dimensional zeolite‐like network in the new caesium copper aluminate Cs 2 CuAl 4 O 8

Monoclinic dicaesium copper tetraaluminate, Cs 2 CuAl 4 O 8 , space group P 2 1 / c , a = 8.4551 (7), b = 10.012 (1), c = 17.073 (2) Å, β = 101.643 (9)°, Z = 6, was obtained by high‐temperature crystallization from a phosphate flux. Its microporous crystal structure presents the first example of double layers built from [AlO 4 ] tetrahedra combined in 4‐, 6‐ and 8‐rings, topologically similar to those found in the ATT ‐type zeolites and isostructural minerals armstrongite, davanite and dalyite. These layers show a rare arrangement of three [AlO 4 ] tetrahedra sharing one oxygen vertex. The aluminate slabs are further linked by chains of edge‐sharing [CuO 4 ] square planes to form a mixed anionic three‐dimensional framework with Cs + cations in channels and cavities. An unusually short Cu...Cs distance of 3.166 Å is ascribed to the strong Jahn–Teller effect of Cu 2+ . The magnetic subsystem demonstrates properties of an alternating antiferromagnetic chain with a gap in the spectrum of magnetic excitations.