Masazumi Arao

Features of structural phase transition in La1−xSrFeO3

Abstract The structural phase transition in the insulating phase of La 1− x Sr x FeO 3 has been examined by X-ray powder diffraction. Based on the Rietveld analysis of the obtained profiles, the R 3 c – Pbnm structural transition was confirmed to take place in this oxide system. The interesting feature of the transition is that it is characterized by the appearance of the antiferroelectric displacement of the La/Sr atom, in addition to the change in the rotational–displacement pattern, R 25 x + R 25 y + R 25 z → R 25 x + R 25 y + M 3 z . It is thus understood that the low-temperature Pbnm phase can be identified as an antiferroelectric phase in oxides having a simple perovskite structure.

Features of the Jahn-Teller displacements in LaTiO3

Abstract Atomic displacements involved in a crystal structure of LaTiO3 have been investigated by means of transmission electron microscopy mainly at room temperature. Electron diffraction patterns taken from LaTiO3 were found to be characterized by the presence of the forbidden reflections for the Pnma structure. The extinction rule of these reflections including fundamental ones indicated that a space group of the crystal structure was determined to be P21/a with a monoclinic symmetry. A characteristic feature of the P21/a structure is that there exist the Jahn–Teller atomic displacement with q=〈1/2 1/2 0〉 c as well as the Jahn–Teller distortion. An additional displacement with q=〈1/2 0 0〉 c should also be needed.

Features of crystal structures in the correlated-electron system Sr1−1LaxTiO3

The details of crystal structures in Sr1−xLaxTiO3 with the simple perovskite structure have been investigated by means of electron diffraction at room temperature. When the Sr content decreases, the crystal structure was found to change as follow; Pm3m (x=0), R3c (0.05≤×<0.6), and Pm3m (0.6<×≤1.0). That is, the Pbnm structure results from the condensation of both M3 and R25 modes in the Pm3m structure, which are rotation modes of the TiO6 octahedron, while only the R25 displacement is involved in the R3c structure.

Stability of the Orbital-Modulated State in the Layered Perovskite Manganite Sr2-xRxMnO4 (R = Pr, Nd)

To understand a stability of the orbital-modulated (OM) state in Sr2-xRxMnO4 (R = Pr, Nd), the crystallographic features of oxide samples with 0.20 < x ≤ 0.50, prepared by the coprecipitation technique, have been investigated by transmission electron microscopy. When the Pr content x corresponding to the eg-electron concentration increased from x = 0, the OM state appeared in 0.20 < x ≤ 0.40 as a neighboring state of an orbital ordered (OO) state around x = 0.20. The interesting features are that around x =　0.44 there is a nanometer-sized coexisting state consisting of the OM and disordered tetragonal (DT) states, and that an average size of OM-state regions in the coexisting state decreases with increasing the eg-electron concentration. In addition, the so-called CE-type antiferromagnetic state found in Sr1.5La0.5MnO4 is absent in the Nd and Pr cases. This suggests that the stability of the OM state is not directly associated with charge ordering of eg electrons, but basically due to only their orbital ordering.

Crystallographic features of RTiO3 (R: Rare-earth ion)

Abstract The crystallographic features of the simple perovskite oxides RTiO3 with R=La, Nd, Sm and Y, which are known to be correlated-electron insulators, have been examined mainly at room temperature by transmission electron microscopy. Electron diffraction patterns indicated that the oxides with R=La and Nd have the space group Pbnm. In electron diffraction patterns of SmTiO3 and YTiO3, on the other hand, the twin splitting of reflections due to the Pbnm structure is seen along one of the 〈1 0 0〉 c directions, where the subscript c denotes the cubic Brillouin zone. That is, it is understood that their crystal system is lowered from an orthorhombic one to a monoclinic one. The analysis of the patterns also suggested that the Jahn–Teller atomic displacement with q =〈 1 2 1 2 0〉 c should be involved in the crystal structure for R=Sm and Y, in addition to the full symmetric Jahn–Teller lattice distortion in the structure for R=La and Nd.

Role of the Tilting of Oxygen Octahedra in the Stabilization of the Orbital-Modulated State in the Layered Perovskite Manganite Ca2-xNdxMnO4

There exists the orbital-modulated (OM) state in the layered manganite Ca2-xNdxMnO4 (CNMO) with 0.20 ≤ x ≤ 0.50. To understand the effect of the tilting of MnO6 octahedra to the stability of the OM state, the crystallographic features of CNMO samples prepared by a solid-state reaction have been investigated mainly by transmission electron microscopy. For 0.30 ≤ x ≤ 0.50, it was found that the (LTO → Pccn/LTT) structural transition occurred on cooling from room temperature, where the LTO and Pccn/LTT structures are, respectively, characterized by tilting displacements of oxygen octahedra about one and two of the <110> directions. The notable feature of the (LTO → Pccn/LTT) transition is that its progress strongly suppresses the growth of the OM state for 0.30 ≤ x ≤ 0.45. As a result of the suppression, the OM state exhibits a re-entrant behavior for its appearance. This is an indication that the Pccn/LTT tilting is not favorable for the stabilization of the OM state.

Crystallographic Features of the Charge-Orbital-Ordered State in the Highly-Correlated Electronic System Ca1-xPrxMnO3

The charge-exchange-type orbital-ordered (CEOO) state accompanying antiferromagnetic ordering has been reported in the highly-correlated electronic system Ca1-xPrxMnO3 (CPMO) with the simple perovskite structure. The feature of the CEOO state in CPMO is that the state is present in the wide Pr-content range of 0.30 ≤ x ≤ 0.70. Although the Zener-polaron model was proposed for the CEOO state at x = 0.60, the detailed features of the CEOO state, particularly for lower Pr contents, have not been understood sufficiently. We have thus investigated the crystallographic features of the CEOO state in CPMO with 0.40 ≤ x ≤ 0.50, mainly by transmission electron microscopy. It was found that, when the temperature was lowered from the disordered state, incommensurate satellite reflections characterizing the CEOO state appeared in electron diffraction patterns below about 250 K in CPMO. The careful analysis of the reflections indicated that the CEOO state for 0.40 ≤ x ≤ 0.45 was different from that for 0.45 ≤ x ≤ 0.50. Concretely, the former CEOO state accompanied only a transverse lattice modulation, while both transverse and longitudinal modulations with different wave vectors appeared in the latter state. This implies that the state for 0.40 ≤ x ≤ 0.45 can be regarded as an orbital-modulated (OM) state without a charge modulation. It is thus understood that an increase in the Pr content leads to the OM-to-CEOO state change below about 250 K around x = 0.45.

Crystallographic Features of the C-Type Orbital-Ordered, and Charge- and Orbital-Ordered States in the Simple Perovskite Manganite Ca1-xLaxMnO3

The C-type orbital-ordered (CTOO), and charge-and orbital-ordered (COO) states are present in the simple perovskite manganite Ca1-xLaxMnO3, which has a three-dimensional highly-correlated electronic system. In this study, the crystallographic features of the CTOO and COO states have been investigated mainly by transmission electron microscopy to understand responses of a lattice system to these orderings. Of these two states, the cooling from the disordered orthorhombic Pnma (DO) state around x = 0.20 resulted in the CTOO state with the monoclinic P21/m symmetry. As a result of the monoclinic distortion as a response of the lattice system, the CTOO state consisted of a banded structure that was characterized by an alternating array of two monoclinic domains with different β values. In 0.30 < x < 0.50, on the other hand, the appearance of the COO state from the DO state on cooling accompanied a transverse lattice modulation with q = []DO as a response to orbital ordering in the COO state. The subsequent cooling in the COO state led to the antiferromagnetic ordering with a large lattice dilatation. In other words, no change in the crystal symmetry occurs in the appearance of the antiferromagnetic ordering.

Crystallographic Features of the A-Type and CE-Type Antiferromagnetic States in the Simple-Perovskite Manganite System Sr1-xNdxMnO3

The electronic states of Sr1-xNdxMnO3 with the simple perovskite structure are characterized by a three-dimensional highly-correlated electronic system. To understand the detailed features of the A-and CE-type antiferromagnetic states in this system, their crystallographic features for x = 0.47, 0.48, and 0.50 have been investigated mainly by both x-ray powder diffraction and transmission electron microscopy. It was found at room temperature that the crystal structure for x = 0.47 was determined to have the monoclinic C2/m symmetry, while the orthorhombic Imma structure was confirmed for x = 0.48 and 0.50. The in-situ observation for x = 0.47 indicated that, in the heating process from room temperature, the C2/m-to-Imma transition occurred in the paramagnetic state, and that the A-type antiferromagnetic state appeared below about 200 K on cooling. In addition, the cooling from room temperature for x = 0.48 and 0.50 resulted in the direct transitions from the orthorhombic Imma state to the A-and CE-type antiferromagnetic states, respectively. Based on these features, we simply discussed the physical origin of the appearance of the paramagnetic state with the monoclinic symmetry for x = 0.47.

Crystallographic Features of Ferroelectric States around the Tetragonal /Rhombohedral Phase Boundary in Pb1-xLax(Zr1-yTiy)O3

To understand dielectric properties around the ferroelectric tetragonal (FT)/rhombohedral (FR) phase boundary in Pb1-xLax(Zr1-yTiy)O3, the crystallographic features of ferroelectric states around the boundary have been investigated by transmission electron microscopy. It was found that, when the Ti content approached to the phase boundary in the FT-phase area, a dielectric property became the relaxor behavior. The corresponding change in the crystallographic features is that the usual FT state is converted into the nanometer-sized coexisting state of the ferroelectric monoclinic (FM) and FR phases. Because the crystal structure of FR-phase regions in the coexisting state involves the R25-type rotational displacement of oxygen octahedra, the FR phase can be identified as the low-temperature ferroelectric rhombohedral phase that is present in Pb(Zr1-yTiy)O3. It is thus understood that the relaxor behavior found in Pb1-xLax(Zr1-yTiy)O3 should be associated with the presence of the nanometer-sized coexisting state that consists of two ferroelectric phases.