Framework Polymorphism and Modular Crystal Structures of Uranyl Vanadates of Divalent Cations: Synthesis and Characterization of M(UO2)(V2O7) (M = Ca, Sr) and Sr3(UO2)(V2O7)2

Uranyl vanadate compounds with divalent cations, M(UO2)(V2O7) (M = Ca, Sr) and Sr3(UO2)(V2O7)2, were synthesized by flux crystal growth, and their crystal structures were solved using single‐crystal X‐ray diffraction data. Ca(UO2)V2O7 and Sr(UO2)V2O7 were synthesized from reactants with molar ratios M:U:V of 1:1:2 and identical heating conditions, and increasing the M:U:V ratio to 3:1:4 resulted in Sr3(UO2)(V2O7)2. Crystallographic data for M(UO2)V2O7 compounds are: a = 7.1774(18) Å, b = 6.7753(17) Å, c = 8.308(2) Å; V = 404.01(18) Å3; space group Pmn21, Z = 2 for Ca; a = 13.4816(11) Å, b = 7.3218(6) Å, c = 8.4886(7) Å; V = 837.91(12) Å3; space group Pnma, Z = 4 for Sr. Compound Sr3(UO2)(V2O7)2 has a = 6.891(3) Å, b = 7.171(3) Å, c = 14.696(6) Å, α = 85.201(4)˚, β = 78.003(4)˚, γ = 89.188(4)˚; V = 707.9(5) Å3; space group P1, Z = 2. The framework structure of Sr(UO2)(V2O7) is related to that of Pb(UO2)(V2O7) reported previously, while that of Ca(UO2)(V2O7) has a different topology. The topological polymorphism of the [(UO2)(V2O7)]‐type framework may be due to the differing ionic radii of the guest M2+ cations. Compound Sr3(UO2)(V2O7)2 has a modular structure based on two different types of electroneutral layers: [Sr(UO2)(V2O7)] and [Sr2(V2O7)]. Structural complexities were calculated, and Raman spectra were collected and their peaks were assigned.