Study of structural properties of bismuth pyrostannate by Raman and IR spectroscopy

Chromium-substituted bismuth pyrostannates with a pyrochlore structure were synthesized by the solid-phase reaction method. The X-ray structural analysis performed at room temperature showed that the samples Bi₂(Sn_1-xCr_x)₂O₇, x = 0; 0.05, 0.1 are single-phase and belong to the Pc monoclinic structure. Polymorphic transformations of the synthesized samples were studied by Raman and IR spectroscopy. IR spectra were obtained at the temperature range 110–525 K and frequencies 350–1100 cm^–1. Raman spectra were measured at room temperature at frequencies of 100–3000 cm^–1. Heterovalent substitution of Sn⁴⁺ for Cr³⁺ modifies the spectra of pure Bi₂Sn₂O₇. The crystal structure of Bi₂Sn₂O₇ consists of two oxygen sublattices: SnO₆ and Bi₂O'. Chromium ions substituted tin ions in the SnO₆ oxygen octahedra, distorting the local structure in the vicinity of bismuth ions. Phonon modes are softening in the vicinity of phase transitions. А shift of the phase boundaries of polymorphic transitions is observed for Bi₂(Sn_1-хCr_х)₂O₇, x = 0.05, 0.1. The frequencies of stretching vibration modes were determined from IR and Raman spectra. The substitution of chromium for tin ions resulted in the appearance of two new modes at frequencies of 581 and 822 cm^–1 in the Raman spectra. The absence of an inversion center in the crystal structure of Bi₂(Sn_1-xCr_x)₂O₇ is confirmed by Raman spectroscopy. IR spectra of chromium-substituted samples consist of complex lines, which decompose into 2 and 3 Lorentzian lines. The softening and broadening of optical absorption modes are associated with the electronic contribution. Impurity states of electrons form polarons.