Phase transitions in Ca2-xBaxNdSbO6 complex perovskites: A combined crystallographic and vibrational investigation
An investigation into the crystal structures and vibrational degrees of freedom depending on a compositionally altered A-site chemistry are performed for the complex perovskites Ca2-xBaxNdSbO6 (0 ≤ x ≤ 2) in order to study the sequence of phase transitions and changes in the local coordination environment. The 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 a 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+ has been correlated to the Ca/Ba size ratio at the A site of the perovskite lattice. The Raman spectra of the series Ca2-xBaxNdSbO6 shows 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 (R3 @#x0305;)-cubic (Fm3 @#x0305;m) 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.