Photon Factory Activity Report 2018 #36 (2019)

BL-8B/2017G027

Particle size dependence of crystal structure for nanoparticles ofperovskite manganese oxide
1Takayuki TAJIRI1,* and Makaki MITO2
Faculty of Science, Fukuoka University, 8-19-1 Nanakuma, Jonan-ku, Fukuoka 814-0180, Japan
2
Graduate School of Engineering, Kyushu Institute of Technology, 1-1 Sensui-cho, Tobata-ku,
Kitakyushu 804-8550, Japan

1 Introduction
The nanoparticles of strongly correlated materials suchas manganese oxide are expected to exhibit characteristicsize effects on crystal structure and magnetic property
owing to the strongly electron correlation and strong
coupling among spin, orbital, and lattice. Crystal structureof nanoparticles differs from that of the bulk crystal owing
to the changes in energy state and translation symmetry at
the surface of a nanoparticle because the fraction of atoms
at or near the surface increases with decreasing particle
size. In particular, distorted perovskite manganite RMnO3(R = rare earth) is expected to show anomalous size effects,
resulting in a complex phase diagram. In our previousworks, we have studied the correlation between crystal
structure and magnetic properties based on systematic
experimental studies. Nanoparticles of (La,Sr)MnO3 [1,2],DyMnO3 [3], and BiMnO3 [4], with particle sizes ofapproximately 10 nm, exhibited characteristic size effects
on the magnetic properties and crystallographic structure,
which are different from the usual size effects in othermagnetic materials. Now, we focused our attention on thesize effects on GdMnO3 nanoparticles. GdMnO3 has been
known as multiferroic material with magneto-electriceffect and shows a complex magneto-electric phase
diagram such as commensurate–incommensuratetransition with a spiral magnetic structure and A-type
antiferromagnetic order. We synthesized the GdMnO3nanoparticles and have investigated their magnetic
properties [5]. The magnetic measurement results for theGdMnO3 nanoparticles indicated that the magnetic
properties, such as transition temperature and coercive
field, exhibited the characteristic size dependences: thesevalues exhibited a pronounced changes approximately
particles size 14 nm. We report on the size dependence ofthe crystallographic structure for the nanoparticles
obtained from X-ray diffraction experiments.
2 Experiments
The GdMnO3 nanoparticles were synthesized in onedimensional pores of mesoporous silica SBA-15. The
SBA-15 was used as a template to equalize the particle sizein the fabrication of the nanoparticles. SBA-15 has a wellordered two-dimensional mesoporous structure withhexagonal symmetry, and the one-dimensional pores areseparated by silica walls. The pore size and silica walls areable to control by changing synthesis conditions [6]. The
GdMnO3 nanoparticles were synthesized by soaking theSBA-15 with pore size of approximately 8 nm in astoichiometric aqueous solution of Gd(CH3COO)3ꞏ4H2Oand Mn(CH3COO)2ꞏ4H2O. Then, the soaked samples weredried and calcinated in oxygen atmosphere [5].
Powder X-ray diffraction (XRD) measurements for thenanoparticles were carried out at room temperature using
the a Debye-Scherrer camera at the beamline BL-8B. The
incident X-ray wavelength was calibrated using the XRD
pattern of the CeO2 powder.
3 Results and Discussion
The powder XRD patterns for the GdMnO3nanoparticles synthesized in SBA-15 were collected using
incident X-ray energy of 18 keV at room temperature. The
powder diffraction patterns of the nanoparticles exhibitedsome broad Bragg peaks, which were attributed to theorthorhombic symmetry, with the space group Pbam of theGdMnO3 bulk crystal. The average particle size for theGdMnO3 nanoparticles was estimated based on the peak
positions and the full width at half maximum of the some
Bragg peaks using Scherrer’s equation. The results
indicated that successful synthesis of the GdMnO3nanoparticles with average particle sizes ranging fromapproximately 8 to 23 nm. The lattice constants for thenanoparticles were estimated from the peak position of theobserved Bragg peaks. Figure 1 shows the particle size
dependence of the lattice constants for GdMnO3nanoparticles. The lattice constants for the nanoparticles
are slightly different from those for the bulk crystal. The
lattice constant a was almost constant value, whereas thelattice constants b and c exhibited a relatively large
dependence below approximately 13 nm. As particle sizedecreases, the b increased whereas the c decreased
monotonically below approximately 13 nm. The size
dependence indicated that crystallographic structure of thenanoparticles distorted anisotropically from that for bulkcrystal.
The GdMnO3 nanoparticles have the Jahn–Teller
distorted orthorhombic structure (O’-type structure: b > a> c/√2) same as the GdMnO3 bulk crystal. The
orthorhombic distortion b/a for the nanoparticles is slightly
smaller than that for the bulk crystal, 1.103. The change inthe orthorhombic distortion suggested modulation of thesymmetry of the MnO6 octahedra and the Jahn–Teller
distortion from those for the bulk crystal.
The value of b/a for the nanoparticles increased rapidly
with decreasing particle size below approximately 14 nm
whereas it kept almost constant for above approximately
14 nm. Thus, it can be concluded that the magnitude of thePhoton Factory Activity Report 2018 #36 (2019)

Jahn–Teller effect for nanoparticles changes with theparticle size.
The results of crystal structure analyses indicated acorrelation between the magnetic properties and thedistortion of the crystallographic structures. It isconsidered that the modulation of the lattice constants andthe Jahn-Teller distortion varies the magnetic interactions
between the Mn spins, which result in the pronounced
changes in the magnetic properties for the GdMnO3nanoparticles below approximately 14 nm.

5.40

bulk

5.30
5.20
5.95
5.90

bulk

5.85
5.80
5.75
7.45

bulk

7.40
7.35
7.30
1012

14

16
1820
particle size [nm]

2224

b/a

Fig. 1: Particle size dependence of lattice constants for
GdMnO3 nanoparticles.

Fig. 2: Particle size dependence of orthorhombic distortionb/a for GdMnO3 nanoparticles.
References[1] T. Tajiri et al., J. Phys. Soc. Jpn. 75, 113704 (2006).
[2] T. Tajiri et al., J. Phys. Soc. Jpn. 77, 074715 (2008).
[3] T. Tajiri et al., J. Magn. Magn. Mater. 345, 288 (2013).
[4] T. Tajiri et al., J. Appl. Phys. 49, 06GH04 (2010).
[5] T. Tajiri et al., Physica B 536, 111–114 (2018).
[6] D. Zhao et al., SCIENCE 279, 548 (1998).
* tajiri@fukuoka-u.ac.jp