Structural and computational studies on a quenchable high-temperature polymorph of magnesium tungstate (MgWO4-II)

Abstract

Single-crystals of a quenchable high-temperature polymorph of magnesium tungstate (MgWO4-II) have been grown using the flux method. Polycrystalline material of the same compound could be obtained from solid-state reactions performed at 1200 °C. Basic crystallographic data of the previously unknown modification are as follows: space group P1‾, a = 6.5525(6) Å, b = 7.5883(7) Å, c = 7.6976(6) Å, α = 119.064(9)°, β = 95.545(7)°, γ = 107.645(8)°, V = 304.84(5)3 Å and Z = 4. The crystal structure was solved from single-crystal diffraction using direct methods and refined to a residual value of R1 = 2.16%. Both cation species exhibit an octahedral oxygen coordination environment. By sharing common edges and corners, the octahedra form a three-dimensional network, which can be built from infinite rod-like elements running along [010] having a 2 × 2 octahedra wide cross section. MgWO4-II is topologically equivalent to the VO2(HT) structure-type. A detailed analysis of the relationships with other ABO4-compounds is presented. Solid-state characterization has been supplemented by micro-Raman spectroscopy. Interpretation of the spectroscopic data, including the allocation of the bands to certain vibrational species, has been aided by DFT-calculations. The thermal expansion tensor between ambient temperature and about 700 °C has been determined. The calculations indicate quasi-two-dimensional expansion behavior.