TlInAs2O7, RbInAs2O7, and (NH4)InAs2O7: Synthesis and crystal structures of three isotypic microporous diarsenates – representatives of a novel structure type

The new isotypic diarsenates TlInAs2O7 (I), RbInAs2O7 (II) and (NH4)InAs2O7 (III) were grown hydrothermally at 493 K and low pH (≤1.5). Single-crystal structure determination demonstrated that they crystallize in a novel triclinic (P1¯) structure type with a = 7.827(2)/7.845(2)/7.858(2) Å; b = 8.625(2)/8.678(2)/8.649(2) Å; c = 10.494(2)/10.492(2)/10.515(2) Å; α = 88.83(3)/88.85(3)/88.96(3)°; β = 89.98(3)/89.93(3)/89.94(3)°; γ = 74.38(3)/74.31(3)/74.34(3)°; V = 682.1(3)/687.5(3)/688.0(3) Å3 (Z = 4) for I, II, and III, respectively. The crystal structures were refined to R1(F) = 0.033, 0.023, and 0.020 for 4310, 4622, and 4532 unique reflections for I, II, and III, respectively. The novel microporous framework structure, designated MCV-4 (for: Institute of Mineralogy and Crystallography Vienna), is built of As2O7 groups (As–O–As ≅ 119.2–120.5°) which are corner-linked to InO6 octahedra. The M1+ cations occupy two different tunnels extending along [0 1 0] and [1 0 1], with effective pore diameters of 3.60–3.68 and 2.03–3.81 Å, respectively. Adjacent tunnels are linked via As2O7 groups. The topology has a pseudo-monoclinic symmetry and is closely related to that of KAlP2O7-type RbScAs2O7 (P21/c) and RbAlAs2O7 (P1¯). The influences of structural features on the crystal symmetry are discussed.