Phase transitions in Ca2−xBaxNdSbO6 complex perovskites: a combined crystallographic and vibrational investigation

Abstract

An investigation into crystal structures and vibrational degrees of freedom depending on a compositionally altered A-site chemistry is performed for complex perovskites Ca_2−xBa_xNdSbO₆ (0 ≤ x ≤ 2) in order to study the sequence of phase transitions and changes in the local coordination environment. X-ray crystallographic investigations show that the samples ‘toggle’ between the pure states of monoclinic Ca₂NdSbO₆ (CNS) and rhombohedral Ba₂NdSbO₆ (BNS) with changes in composition. A novel observation is made from the crystallographic results which display crystallographic site swapping of the Nd³⁺ ion when the structure transforms from CNS to BNS. The degree of this conformational site swapping of the Nd³⁺ ion has been correlated to the Ca/Ba size ratio at the A site of the perovskite lattice. The Raman spectra of the series Ca_2−xBa_xNdSbO₆ show remarkable signatures of a discontinuous monoclinic–rhombohedral phase transformation. A high temperature Raman analysis is carried out between 303 K and 673 K to explain the presence of a nearby rhombohedral (R)-cubic (Fmm) phase boundary. The thermal variations of the Raman spectra show a quasi-harmonic nature owing to the presence of weak rhombohedral distortion which is not effective in activating a change in the Raman profile.