Raman signatures of monoclinic distortion in (Ba$_{1-x}$Sr$_{x}$)$_{3}$CaNb$_{2}$O$_{9}$ complex perovskites

TL;DR

This study uses Raman spectroscopy and group theory to investigate structural distortions in (Ba$_{1-x}$Sr$_{x}$)$_{3}$CaNb$_{2}$O$_{9}$ perovskites, revealing phase transitions and octahedral tilting effects relevant to their physical properties.

Contribution

It combines Raman scattering with group-theory calculations to identify structural phase transitions and octahedral tilting in complex perovskites, providing a non-invasive method for structural analysis.

Findings

Detected 36 Raman modes at 25 K in Sr$_{3}$CaNb$_{2}$O$_{9}$, matching theoretical predictions.

Identified a phase transition from trigonal to monoclinic structure at x ≥ 0.7.

Confirmed octahedral tilting as the main distortion mechanism through Raman spectra.

Abstract

Octahedral tilting is most common distortion process observed in centrosymmetric perovskite compounds (ABO$_{3}$). Indeed, crucial physical properties of this oxide stem from the tilts of BO$_{6}$ rigid octahedra. In microwave ceramics with perovskite-type structure, there is a close relation between the temperature coefficient of resonant frequency and tilt system of the perovskite structure. However, in many cases, limited access facilities are needed to assign correctly the space group, including neutron scattering and transmission electron microscopy. Here, we combine the Raman scattering and group-theory calculations to probe the structural distortion in the perovskite (Ba$_{1-x}$Sr$_{x}$)$_{3}$CaNb$_{2}$O$_{9}$ solid solution, which exhibits a structural phase transition at $x$ $\geq$ 0.7, from D$_{3d}^{3}$ trigonal to C$_{2h}^{3}$ monoclinic cell. Both phases are related by an octahedral tilting distortion ($a^{0}b^{-}b^{-}$ in Glazer notation). Low temperature Raman spectra corroborate the group-theoretical predictions for Sr$_{3}$CaNb$_{2}$O$_{9}$ compound, since 36 modes detected at 25 K agree well with those 42 (25A$_{g}$ $\oplus$ 17B$_{g}$) predicted ones.