New Insights into Alkali Metal Tungstates: The High Temperature Polymorphism of Na 2 WO 4 , the New Polymorph Li 2 WO 4 ‐V and the Redetermined Crystal Structure of Li 2 W 2 O 7

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), Na2WO4-II a unit cell is suggested. Moreover, the crystal structure of new triclinic polymorph Li2WO4-V prepared employing LiF flux elucidated P 2, a=527.45(2), b=777.69(3), c=797.31(3) α=105.835(2), β=103.391(2), γ=90.581(2)°, 1080 irefl., 100 param., wR2=0.063). Finally, Li2W2O7 crystallising a=505.57(1), b=705.00(2), c=829.03(2) α=69.646(1), β=77.868(1), γ=77.868(1)°, 2386 refl., 105 wR2=0.025) redetermined revealing yet unreported lithium disorder yielding insights into its high ionic conductivity.