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 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 P1 (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., wR 2 = 0.063). Finally, the crystal structure of Li 2 W 2 O 7 crystallising in space group P1 (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., wR 2 = 0.025) was redetermined revealing yet unreported lithium disorder yielding new insights into its high ionic conductivity.

Critical Evaluation and Calorimetric Study of the Thermodynamic Properties of Na2CrO4, K2CrO4, Na2MoO4, K2MoO4, Na2WO4, and K2WO4

“…There are some uncertainties in the literature regarding the number of phases for Na 2 WO 4 , as well as their crystal structures and space groups. 33,36,43,[46][47][48] XRD analyses conducted by Pistorius 43 and Temperature-Programmed x-ray powder Diffraction (TPXRD) by Hämmer and Höppe 45 suggested the presence of three phases. However, the XRD analyses performed by Pistorius 43 have only identified with certainty two phases (I and III).…”

“…At room temperature, only Na 2 MoO 4 and Na 2 WO 4 exhibit an ordinary spinel structure among alkali metal sulfates, chromates, molybdates and tungstates. 30,35 The work of Hämmer and Höppe 45 was the only one able to characterize the phase II using TPXRD; Austin and Pierce 47 had indicated that both solid-solid transitions were displaced and non-reconstructive. Therefore, obtaining single crystals of the high-temperature phases of Na 2 WO 4 by quenching was impossible, as well as the use of Rietveld refinement and DSC.…”

“…Based on TPXRD data, it is suggested that Na 2 WO 4 (II) has a tetragonal unit cell with lattice parameters a = 1707.2 pm, c = 1293.6 pm, and a space group consistent with symmetry relationships to Na 2 WO 4 (I), i.e., I41/amd. 45 The latter is assumed to be a maximal subgroup of Fd3m and a direct supergroup of Fddd (space group of the phase III). 45 Even with indexing and Pawley fitting (R Bragg = 0.013, Rwp = 0.058), the structure of II was not reliably determined.…”

“…45 The latter is assumed to be a maximal subgroup of Fd3m and a direct supergroup of Fddd (space group of the phase III). 45 Even with indexing and Pawley fitting (R Bragg = 0.013, Rwp = 0.058), the structure of II was not reliably determined. Thus, Na 2 WO 4 (II) was assumed to crystallize in the I41/amd space group, with no symmetry relationship derivable between Na 2 WO 4 (I) and Na 2 WO 4 (III), and no proposed unit cell fitting into such a structure.…”

“…Thus, Na 2 WO 4 (II) was assumed to crystallize in the I41/amd space group, with no symmetry relationship derivable between Na 2 WO 4 (I) and Na 2 WO 4 (III), and no proposed unit cell fitting into such a structure. 45 Very limited high-temperature XRD studies have been conducted to identify the structure and space group of the phase III; Pistorius 43 and Hämmer and Höppe 45 reported an orthorhombic structure with different space groups (Pnam and Fddd, respectively).…”