Les pyrochlores ferroelectriques derives de Cd2Nb2O6S: Mise en évidence des transitions de phase par des techniques d'optique non linéaire

Covalency-driven Structural Evolution in the Polar Pyrochlore Series Cd2Nb2O7–xSx

“…The drop from cubic to tetragonal is supported by the refined lattice parameters of a = b = 7.4737 (7) and c = 10.5670(2) Å (where the unrefined transformed cell from cubic symmetry has cell edges of a = b = 7.4720 and c = 10.5670 Å), indicating that the unit cell has a slight tetragonal distortion which improves the overall fit. Additionally, this refined tetragonal distortion is supported by metrically comparing the square root of 2 multiplied by the refined a lattice parameter, which yields a value of 10.5693 (2). This is significantly different from the refined c lattice parameter (by 0.0024 Å, which is approximately 10 times the standard deviation of the refined values), supporting the drop from cubic to tetragonal symmetry.…”

“…Although the pyrochlore structure supports numerous chemical compositions, various forms of ferroelectricity (proper, improper, relaxor, etc.) have been reported only in Cd 2 Nb 2 O 7 and related compounds, Ho 2 Ti 2 O 7 , , La 2 Zr 2 O 7 , Nd 2 Ti 2 O 7 , and frustrated ferroelectricity in Y 2 (Nb 0.86 Y 0.14 ) 2 O 6.91 , CaYNb 2 O 7 , and Y 2 NbTiO 7 . Many of these studies illustrate the complex behavior of the pyrochlore lattice upon cooling, and unlike perovskite-structured compounds, the origin of coherent polar displacements in pyrochlore oxides is not well understood.…”

“…The structure is commonly described as two interpenetrating sublattices of corner-shared BO 6 octahedra and A 2 O′ chains, illustrated in Figure . , The general structure can be written as A 2 B 2 O 6 O′, as the oxygen atoms in the structure are found on two distinct crystallographic Wyckoff sites: the BO 6 oxygen atoms are located at the ( x , 1/8, 1/8) 48f site, whereas the A 2 O′ oxygen is located at the (3/8, 3/8, 3/8) 8b site. The pyrochlore structure can host a large array of cation and anion substitution, ,− in addition to tolerating cation and anion vacancies and the structural and site disorder. − However, structural distortions such as cation off-centering are frustrated in the pyrochlore structure due to the triangular arrangement of cations on the lattice, − inhibiting a transition from a paraelectric to ferroelectric (FE) phase in a majority of pyrochlore oxide materials.…”

“…Cd 2 Nb 2 O 6 S and a solid solution Cd 2 Nb 2 O 7– x S x ( x = 0–1) were reported to exhibit acentricity through second harmonic generation (SHG), and the origin of polarity across the solid solution was attributed to the dynamic displacement of the Cd 2+ cations. , A total of four phase transitions were determined in the solid solution (labeled as α, β, γ, and δ), but detailed characterization of the space groups and atomic positions of the low temperature phases was not reported. The lack of literature on the low-temperature structure of the solid solution Cd 2 Nb 2 O 7– x S x may be a result of the complex patterns that are seen upon phase transformations.…”

Structural Transformation and Second-Harmonic-Generation Activity in Rare-Earth and d0 Transition-Metal Oxysulfides RE3NbS3O4 (RE = Ce, Sm, Gd, Dy)

“…On the basis of MQ 4 (M = Si, Ga, Ge; Q = S, O) units, a series of melilite-type oxysulfides and derivates like Sr 2 ZnGe 2 S 6 O, BaGeS 2 O, Sr 5 Ga 8 O 3 S 14 , Ba 2 In 2 Si 3 O 10 S, and [(Ba 19 Cl 4 )­(Ga 6 Si 12 O 42 S 8 )] have been found. In addition, it can be found that most oxysulfides contain transition metals such as Ba 5 (VO 2 S 2 ) 2 (S 2 ) 2 , Cd 2 Nb 2 O 7– x S x , and BiMOQ (M = Cu, Ag; Q = S, Se, Te) . RE 2 O 2 S (RE = rare-earth metal) is representative of RE oxysulfides constructed by RE–O/S polyhedra.…”