Structure analysis of NdBaMn₂O₆ by a combination of convergent-beam electron diffraction and first-principle calculations

概要

A-site ordered perovskite-type manganites, RBaMn2O6 (R = Y and rare-earth elements), have drawn considerable attention because of their large variety of structural and physical properties. According to the distortion type of MnO6 octahedra, the crystal structures of RBaMn2O6 can be divided into two groups; tilting of MnO6 octahedra and no tilting of MnO6 octahedra. The position of the R = Nd is at the boundary of the two structural groups. However, there are still controversies about the existence of the tilting of MnO6 octahedra and the ordering of 𝑑𝑥 2−𝑦 2 orbital states for NdBaMn2O6. One expected reason for these controversies is the imperfection of the single crystals examined. Thus, local structure analysis without the influence of imperfection is desired for a precise understanding of the space groups and crystal structures. In this study, the crystal structures of NdBaMn2O6 at high temperature (HT ≈ 450 K), room temperature (RT ≈ 293 K), and low temperature (LT ≈ 95 K) phases are investigated by developing a new technique with a combination of convergent-beam electron diffraction (CBED) and firstprinciple calculations based on density functional theory (DFT).

To determine the crystal structure of NdBaMn2O6 at each temperature phase, the experimental CBED patterns are compared with the simulated CBED patterns obtained from the DFT evaluation. It is found that the crystal structure of the RT phase exhibits no tilting of the MnO6 octahedra. In the LT phase, all of the MnO6 octahedra are apically compressed. The arrangement of shorter and longer basal Mn-O distances at each Mn site is inconsistent with the Jahn-Teller coupling of 𝑑𝑥 2−𝑦 2 orbital ordering. The observed SAED patterns at LT phase confirm that the unit cell has four-fold periodicity along the c-axis. Moreover, in the RT phase, the newly developed method is applied for the first time to examine structural coexistence. The symmetry analysis of CBED patterns from 20 segments reveals the coexistence of non-centrosymmetric C2mm (No. 38) and centrosymmetric Cmmm (No. 65) phases. The DFT calculations suggest that both structural phases are energetically stable, and their comparable total energies could be the origin of their coexistence. It is expected that the new technique can be applied to multiphase materials as well as locally deformed structures such domain boundaries and interfaces.