Structural Phase Transition in a Multiferroic Hexagonal Manganite
We synthesized hexagonal YMnO3 doped with Ti ions and investigated the crystal structure by means of transmission electron microscopy. Hexagonal RMnO3 has recently attracted a lot of interest because of the coexistence of ferroelectricity and antiferromagnetism. Such "emultiferroic" materials in which ferroelectricity and ferromagnetism coexist are of interest with respect to the coupling between the ferroelectricity and the ferromagnetism, and their possible control by the application of magnetic and/or electric fields. First, we investigated the electron-diffraction patterns. Superlattice reflection spots with a modulation wave vector q = 1/3c* appeared for the Ti-doped YMnO3 (Fig. 1). We carried out high-resolution transmission electron microscopy (HRTEM) observation of the x = 0.25 compound to confirm the fundamental structure and the superstructure of Ti-doped YMnO3 (Figs. 2 and 3 ). We found that Ti-doped YMnO3 intrinsically consists of the hexagonal phase and the rhombohedral phase derived from formation of a superstructure with the 3d001 period along the c axis. The hexagonal-to-rhombohedral phase transition results from a change in the stacking manner of Mn(Ti)-trimers in Mn(Ti)O layers (Fig. 4).
Figure 1: Sets of electron diffraction patterns of YMn0.75Ti0.25O3 showing the (a) [001], (b) [110], (c) [110], and (d) [540] zone-axis.
Figure 2: [110] -zone HRTEM image of YMn0.75Ti0.25O3.
Figure 3: (a) [110]-zone HRTEM image. The boundary which crosses diagonally in (a) is a superstructure-twin boundary. The inset is the corresponding electron diffraction pattern. Images (b) and (c) are the FFT results for regions A and B in (a), respectively.
Figure 4: (a) Schematic representation of the crystal structure of hexagonal YMnO3 and a representation of the MnO (ab) plane at (b) z = 0 and (c) z = 1/2. (d) Proposed model of the crystal structure of YMn0.75Ti0.25O3. (e) to (j) MnO planes in the proposed model. Black and gray spheres represent Mn and O, respectively. The rhombuses and the dotted triangles show the unit cell and the formation of Mn-trimers, respectively.