Vibrational properties of ferroelectric hexagonal tungsten bronzes AB_xW_3−xO₉, where A = K, Rb, Cs and B = Nb, Ta, Zr, Cr

Abstract

Powder x-ray diffraction, Raman and IR studies were performed for a number of ferroelectric hexagonal tungsten bronzes of the formula AB x W 3−x O 9 , where A = K, Rb, Cs and B = Nb, Ta, Zr and Cr. The observed vibrational modes were assigned to the respective motions of atoms in the unit cell. The x-ray data revealed that the partial substitution of W atoms by atoms with lower valence results in shortening of the W -W distances within the xy plane and elongation in the z direction. These changes in the crystal structure were correlated with the vibrational data. The measured spectra indicated also the presence of some kind of ordering in the potassium-and rubidium-zirconium bronzes, and the presence of a large number of defects for the chromium bronzes.

Oxygen vacancies and pseudo Jahn‐Teller destabilization in cesium‐doped hexagonal tungsten bronzes

“…The Raman profiles shown in Figure 9C,D closely resemble those reported in the past for HTBs, [43][44][45] showing three broad bands at (A) 150-360 cm −1 , (B) 500-850 cm −1 , and (C) 870-980 cm −1 . The A and B bands are attributed to the O− W−O bending and O−W−O stretching modes, respectively.…”

“…The A and B bands are attributed to the O−W−O bending and O−W−O stretching modes, respectively. The 900 and 960 cm −1 peaks in the band C are generally assigned as due to the terminal −W=O bond …”

“…As shown in the insets, the ratio of the 960 cm −1 peak relative to the 700 and 760 cm −1 peaks are observed to increase with increasing V O and Cs. The terminal −W=O bond has been interpreted variously as due to W vacancies, hydrogen coordination, sample surface, 42 and others. Mączka et al employed the bond valence model to show that the W=O bond could occur when W is vacant or substituted by atoms with weak interaction with oxygen, such as Ga.…”

Synthesis, Characterization and Photocatalytic Activity of KAl0.33W1.67O6 and Sn0.5Al0.33W1.67O6xH2O

“…Normal (A 2 B 2 O 7 ) and defect (AB ′ B″O 6 ) pyrochlores do not give bands above 700 cm −1 . The Raman bands observed above 700 cm −1 may arise due to (1) second order Raman scattering, (2) tetrahedral coordination of W ion , (3) octahedral coordination of W ion and (4) stretching modes of B ′ ‐O(B″‐O) due to vacancies in the B site . Maczaka et al .…”

Structural and optical properties of rare earth–doped (Ba0.77Ca0.23)1−x(Sm, Nd, Pr, Yb)xTiO3

“…Defect-induced bands are observed in other systems such as hexagonal bronze systems and titanosilicates. 39,40 In the hexagonal bronze systems, tungsten vacancies break down the force equilibrium among adjacent W-O bonds (associated with adjacent WO 4 octahedra), resulting in the shortening of W-O bonds and the appearance of high wavenumber bands at around 900 cm…”