Crystal chemistry of anhydrous Li uranyl phosphates and arsenates. I. Polymorphism and structure topology: Synthesis and crystal structures of α-Li[(UO2)(PO4)], α-Li[(UO2)(AsO4)], β-Li[(UO2)(AsO4)] and Li2[(UO2)3(P2O7)2]

Four new Li uranyl phosphates and arsenates have been prepared by high-temperature solid-state reactions: α-Li[(UO2)(PO4)] (1), α-Li[(UO2)(AsO4)] (2), β-Li[(UO2)(AsO4)] (3) and Li2[(UO2)3(P2O7)2] (4). The structures of the compounds have been solved by direct methods: 1—triclinic, image, a=5.0271(1) Å, b=9.8799(2) Å, c=10.8920(2) Å, α=108.282(9)°, β=102.993(8)°, γ=104.13(1)°, V=470.69(2) Å3, Z=4, R1=0.0415 for 2786 unique reflections with |F0|⩾4σF; 2—triclinic, image, a=5.129(2) Å, b=10.105(3) Å, c=11.080(3) Å, α=107.70(2)°, β=102.53(3)°, γ=104.74(3)°, V=501.4(3) Å3, Z=4, R1=0.055 for 1431 unique reflections with |F0|⩾4σF; 3—triclinic, image, a=5.051(1) Å, b=5.303(1) Å, c=10.101(1) Å, α=90.31(1)°, β=97.49(1)°, γ=105.08(1)°, V=258.80(8) Å3, Z=2, R1=0.0339 for 2055 unique reflections with |F0|⩾4σF; 4—triclinic, image, a=5.312(1) Å, b=6.696(1) Å, c=12.542(1) Å, α=94.532(9)°, β=99.059(8)°, γ=110.189(7)°, V=409.17(10) Å3, Z=2, R1=0.0565 for 1355 unique reflections with |F0|⩾4σF. The structures of all four compounds are based upon 3-D frameworks of U and T polyhedra (T=P, As). Phases 1 and 2 are isostructural and consist of U2O12 dimers and UO6 square bipyramids linked by single TO4 tetrahedra. The structure of 3 consists of 3-D framework of corner-sharing UO6 bipyramids and AsO4 tetrahedra. In the structure of 4, the framework is composed of U2O12 dimers, UO6 bipyramids and P2O7 dimers. In all the compounds, Li+ cations reside in framework cavities. The topologies of the 3-D frameworks can be described as derivatives of the PtS (cooperite) network.