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 Na 2 WO 4 using DSC and temperature‐programmed powder XRD. Na 2 WO 4 ‐I crystallises in space group Fddd (no. 70, a =650.28(1), b =1285.12(3), c =1105.26(2) pm, 63 rparam., R Bragg = 0.008), for Na 2 WO 4 ‐II a unit cell is suggested. Moreover, the crystal structure of the new triclinic polymorph Li 2 WO 4 ‐V prepared employing a LiF flux was elucidated in space group P (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., w R 2 =0.063). Finally, the crystal structure of Li 2 W 2 O 7 crystallising in space group P (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., w R 2 =0.025) was redetermined revealing yet unreported lithium disorder yielding new insights into its high ionic conductivity.