New Insights into Alkali Metal Tungstates: The High Temperature Polymorphism of Na2WO4, the New Polymorph Li2WO4-V and the Redetermined Crystal Structure of Li2W2O7

Abstract

Trimorphous temperature-dependent polymorphism (I: above 591°C, II: 589-591°C, III: below 589°C) was confirmed for sodium tungstate Na2WO4 using DSC and temperature-programmed powder XRD. Na2WO4-I crystallises in space group Fddd (no. 70, a=650.28(1), b=1285.12(3), c=1105.26(2) pm, 63 rparam., RBragg = 0.008), for Na2WO4-II a unit cell is suggested. Moreover, the crystal structure of the new triclinic polymorph Li2WO4-V prepared employing a LiF flux was elucidated in space group P (Formula presented.) (no. 2, a=527.45(2), b=777.69(3), c=797.31(3) pm, α=105.835(2), β=103.391(2), γ=90.581(2)°, 1080 irefl., 100 param., wR2=0.063). Finally, the crystal structure of Li2W2O7 crystallising in space group P (Formula presented.) (no. 2, a=505.57(1), b=705.00(2), c=829.03(2) pm, α=69.646(1), β=77.868(1), γ=77.868(1)°, 2386 refl., 105 param., wR2=0.025) was redetermined revealing yet unreported lithium disorder yielding new insights into its high ionic conductivity.