Raman study of phonon modes in bismuth pyrochlores

Abstract

have been measured at room temperature. The frequencies of the Raman modes, obtained from first-principles calculations, for Bi 2 Ti 2 O 7 are presented for comparison. The spectra of the four samples are similar and agree well with the first-principles calculations. Each bismuth pyrochlore shows more than the six modes expected for the ideal pyrochlore structure. The analysis shows that many of the additional modes could be explained as the relaxation of the selection rules due to the displacive disorder. The Raman modes are assigned by reference to spectra of other pyrochlore materials, comparison to infrared data, and the ab initio calculations.

Raman spectroscopy evidence of inhomogeneous disorder in the bismuth-oxygen framework of Bi25InO39and other sillenites

“…33,51 However, all vibrations are Raman-active in the sillenite structure and we do not expect small deviations to produce new Raman-active modes. Also, unlike the displacement disorder in other compounds such as the pyrochlores, the displacements of O(1) and Bi atoms would not result in a lower symmetry site since these sites have very low symmetry to begin with.…”

“…However, the IR and Raman comparison of the damping coefficients may be difficult if other modes are overlapping. More details on this type of analysis is outlined in reference [ 33 ]. Figure 4 shows the results for the imaginary susceptibilities in both samples, the fit to Equation 1, and the individual oscillators SiO 20 FIG .…”

Thermal annealing of natural, radiation-damaged pyrochlore

“…Figure 8 shows that the crystalline reference sample has infrared maxima centered at 553, 620, 664 and 829 cm −1 , corresponding to Raman shifts centered at 540, 605, 680 and 810 cm −1 . Six of the seven fundamental infrared-active modes were observed between 70 and 523 cm −1 in chemically comparable niobate pyrochlores [64,66]. The seventh mode was assigned to an infrared band between 550 and 850 cm −1 , which could correspond to the observed band at 553 cm −1 in the Schelingen sample.…”

“…The local structure of synthetic pyrochlore-type materials have been extensively studied by Raman spectroscopy and the observed Raman signals could be assigned to certain phonon modes on the basis of model calculations [6,7,10,45,48,49,52,[58][59][60][61][62][63][64][65][66][67][68][69][70][71].…”

Infrared phonon anomaly and magnetic excitations in single-crystal Cu3Bi(SeO3)2OMiller1, Stephens2, Martin3 et al. 2012Phys. Rev. BSelf Cite

“…When the inversion center in a structure is removed, the local centers of symmetry about which all the Raman modes have zero net dipole moment will be removed, and all Raman modes will therefore become infrared active. 16 To further investigate the 16 new modes observed at 115 K, preliminary 300 K Raman measurements were recorded 17 in the ab plane of crystal 1. Raman modes observed at 172.9, 323.5, and 484.4 cm −1 closely correspond to three of the new infrared modes observed below 115 K. If we take into account the fact that Raman modes can slightly increase their resonance frequencies upon cooling to low temperatures, then 300 K Raman modes at 538.3 and 583.1 cm −1 draw close redolence to two additional new modes observed in the infrared below 115 K. We therefore turn to a closer examination of a possible centrosymmetric to non-centrosymmetric transition in Cu 3 Bi(SeO 3 ) 2 O 2 Cl near 115 K. …”