Thermal Expansion of Ln₆WO₁₂ (Ln = Y,Ho,Er,Yb) and Ln₂WO₆ (Ln = Gd,Dy,Ho) – an In Situ Synchrotron X‐ray Diffraction Study

Abstract

The thermal expansion (TE) behavior of cubic and rhombohedral Ln6WO12 (Ln = Y, Ho, Er, Yb) and monoclinic Ln2WO6 (Ln = Gd, Dy, Ho) was investigated by synchrotron X‐ray diffraction from room temperature to approximately 1500°C. The volumetric and lattice parameter expansions were measured for all compositions in both systems, and the respective expansion coefficients were derived and fitted over the observed temperature range with a second‐order polynomial. The relative TE evolution along the unit cell edges in each phase was described from a crystallographic perspective, with the reasons for TE dynamics explained in terms of the fundamental constituent units of the structure. Analysis of the TE ellipsoid of the monoclinic Ln2WO6 revealed a continuous change in both its shape and orientation, with the latter effect strongly manifested in the (010) plane. A noticeable reversal of the relative expansion rates between a, and b and c lattice parameters was attributed to the rotation of the ellipsoid cross section in this plane, bringing the larger of the two eigenvectors closer to a, while the smaller one to c. Investigating the structural dynamics of the constituent layers in Ln2WO6 suggested that the bridging role of the Ln polyhedron centered on the only general position in the unit cell may be the reason for the rotating TE ellipsoid.

Ferroelectric State in an α-Nd2WO6 Polymorph Stabilized in a Thin Film

“…and β ≈ 107.7°). 28 On the other hand, for the smallest cations (Ln = Ho to Lu), it results in a P2/c space group Scheelite-type superstructure with the following relationships…”

“…On the one hand, Ln 2 WO 6 compounds with large cations (Ln = Ce to Dy) usually crystallize in a monoclinic fluorine-type superstructure ( , a M ≈ 3 a F , b M ≈ 2 b F , c M ≈ c F , and β ≈ 107.7°) . On the other hand, for the smallest cations (Ln = Ho to Lu), it results in a P 2/ c space group Scheelite-type superstructure with the following relationships ; ; ; . , …”

Crystal Growth and Structure Analysis of Ce18W10O57: A Complex Oxide Containing Tungsten in an Unusual Trigonal Prismatic Coordination Environment

“…Rare-earth-containing tungstates are a multifunctional class of oxide materials that continue to attract significant interest for the numerous physical properties they exhibit, including high-temperature proton and electron conductivity, − catalysis, − lasing, fluorescence, piezo- and ferroelectricity, − and negative thermal expansion behavior. − Oxides in the La 2 O 3 –WO 3 family have been extensively studied by a number of groups, including those of Yoshimara, Yanovskii, and Ivanova. − By contrast, the Ce 2 O 3 –WO 3 family is relatively unexplored due to the difficulty of achieving the +3 oxidation of cerium in the oxide products. Generally, the synthesis of Ce 2 O 3 -containing oxides is complicated by the instability of Ce 2 O 3 in air, as it will readily reoxidize to CeO x or CeO 2 .…”

“…Most of the reported tungsten-containing oxides contain tungsten in the typical tetrahedral and octahedral coordination environments. Ternary oxides such as CaWO 4 , which adopt the common Scheelite structure, exhibit the tetrahedral coordination environment, whereas Ln 6 WO 12 (Ln = Y, Ho, Er, Yb), La 2 W 2 O 9 (Ln = La, Pr, Nd, Sm, Gd), M 2 (UO 2 )­(W 2 O 8 ) (M = Na, K), Ba 3 WO 6 , Ba 2 WO 5 , Ba 3 W 2 O 9 , Ba 3 MWO 6 (M= Mg, Zn), and Ba 3 Fe 2 WO 9 all contain tungsten in an octahedral coordination environment. The latter coordination environment is rather common in tungstates and is found as isolated, ,, corner-shared, ,− and edge-shared , motifs; very few examples, such as Ba 3 W 2 O 9 , A 6 Th 6 (WO 4 ) 14 O (A = K and Rb), and La 18 W 10 O 57 , contain tungsten in a face-shared coordination environment due to the electrostatic repulsion between the highly charged metal centers. , An exception to this generalization is found for cases where tungsten is present as W­(III), W­(IV), or W­(V), , which sufficiently reduces the tungsten–tungsten repulsion and where potential metal–metal bonding interactions can stabilize the arrangement.…”

“…Ternary oxides such as CaWO 4 , which adopt the common Scheelite structure, exhibit the tetrahedral coordination environment, whereas Ln 6 WO 12 (Ln = Y, Ho, Er, Yb), La 2 W 2 O 9 (Ln = La, Pr, Nd, Sm, Gd), M 2 (UO 2 )­(W 2 O 8 ) (M = Na, K), Ba 3 WO 6 , Ba 2 WO 5 , Ba 3 W 2 O 9 , Ba 3 MWO 6 (M= Mg, Zn), and Ba 3 Fe 2 WO 9 all contain tungsten in an octahedral coordination environment. The latter coordination environment is rather common in tungstates and is found as isolated, ,, corner-shared, ,− and edge-shared , motifs; very few examples, such as Ba 3 W 2 O 9 , A 6 Th 6 (WO 4 ) 14 O (A = K and Rb), and La 18 W 10 O 57 , contain tungsten in a face-shared coordination environment due to the electrostatic repulsion between the highly charged metal centers. , An exception to this generalization is found for cases where tungsten is present as W­(III), W­(IV), or W­(V), , which sufficiently reduces the tungsten–tungsten repulsion and where potential metal–metal bonding interactions can stabilize the arrangement. In a small number of reported structures, tungsten is found in other less common coordination environments that include distorted cubic coordination in Y 2 WO 6 , trigonal bipyramidal coordination in Ca 3 WO 5 Cl 2 , seven-coordinated WO 7 polyhedra in Re 10 W 22 O 81 (Ce, Nd), , and a trigonal prismatic arrangement in the oxychloride Ln 3 WO 6 Cl 3 (Ln = La, Pr). , …”