Photon Factory Activity Report 2019 #37 (2020)

BL-8B/2019G039

Particle size dependence of crystal structure for nanoparticles ofperovskite manganite TbMnO3
1

Takayuki 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
Perovskite manganites RMnO3 (R: rare earth element)
are well known as one of strongly correlated electronicmaterials. The materials exhibit quite interesting properties,
such as colossal magnetoresistance effect and multiferroic
behavior. The crystallographic structure of RMnO3 isorthorhombic distorted perovskite structure. The
nanoparticles of strongly correlated materials such as theRMnO3 are expected to exhibit characteristic size effects
on crystal structure and magnetic property owing to thestrongly electron correlation and strong coupling amongspin, orbital, and lattice. The nanoparticles, because of
their large fraction of surface atoms and finite size effects,
possess interesting magnetic properties and crystalstructures, which differ from those of the bulk crystals. Inparticular, distorted perovskite manganite RMnO3 isexpected to show anomalous size effects, resulting in acomplex phase diagram.
In our previous works, we have studied the correlation
between crystal structure and magnetic properties for theRMnO3 nanoparticles based on systematic experimentalstudies. The nanoparticles of (La,Sr)MnO3 [1,2], DyMnO3[3], BiMnO3 [4], and GdMnO3 [5] 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. We focused on the size effect of
crystal structure and magnetic property on TbMnO3nanoparticles. The bulk crystal of TbMnO3 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. We synthesized
the TbMnO3 nanoparticles with particle size of
approximately 10–20 nm and investigated their magnetic
properties and crystallographic structure. The magnetic
measurement results indicated that the TbMnO3nanoparticles exhibited the superparamagnetic behaviors
and their parameters, such as blocking temperature andcoercive field, depended on the particle size. These values
exhibited a pronounced changes at approximately particles
size 16 nm. We report on the size dependence of thecrystallographic structure for the nanoparticles obtained
from X-ray diffraction experiments in the PFACR2019.
2 Experiments
The TbMnO3 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 withdiameter approximately 5–30 nm are separated by silica
walls. In this study, the TbMnO3 nanoparticles weresynthesized by soaking the SBA-15 with pore size ofapproximately 8 nm in a stoichiometric aqueous solutionof Tb(CH3COO)3ꞏ4H2O and Mn(CH3COO)2ꞏ4H2O. Then,the soaked samples were dried and calcinated in oxygen
atmosphere.
PowderX-ray
diffraction(XRD)
measurements for the nanoparticles were carried out atroom temperature using the a Debye-Scherrer camera at
the beamline BL-8B. The incident X-ray wavelength wascalibrated using the XRD pattern of the CeO2 powder.
3 Results and Discussion
We observed the powder XRD patterns of the TbMnO3nanoparticles synthesized in SBA-15 using incident X-ray
energy of 18 keV at room temperature. The XRD patternsof the nanoparticles exhibited some broad Bragg peaks,
which were corresponding to the space group Pbam of theTbMnO3 bulk crystal. The average particle size of theTbMnO3 nanoparticles was estimated based on the peakpositions and the full width at half maximum of the some
Bragg peaks using Scherrer’s equation. The lattice
constants for the nanoparticles were estimated from thepeak position of the observed Bragg peaks.
Figure 1 shows the particle size dependence of the lattice
constants for the TbMnO3 nanoparticles. The lattice
constants, a, b, and c, for the nanoparticles were slightly
different from those for the bulk crystal and depended on
the particle size. Above approximately 16 nm, the lattice
constants a, b and c were almost constant. Below
approximately 16 nm, the lattice constants a and b
increased, whereas the lattice constant c decreased withdecreasing particle size. The results indicated that thecrystallographic structure of the TbMnO3 nanoparticles
were distorted anisotropically from that of bulk crystal, andthe magnitude of the strain increased with decreasing
particle size below approximately 16 nm.
The TbMnO3 nanoparticles have the Jahn–Teller
distorted orthorhombic structure (O’-type structure: b > a> c/√2) similar to the TbMnO3 bulk crystal. The
orthorhombic distortion b/a of the nanoparticles wasslightly different from that of the bulk crystal, b/a = 1.101,
as shown in Fig. 2. The value depended on the particle size:
Above approximately 16 nm, the orthorhombic distortion

Photon Factory Activity Report 2019 #37 (2020)

c [Å]

b [Å]

a [Å]

was almost constant value ~1.099. Below approximately
16 nm, however, the value increased with decreasingparticle size, such that it became larger than that of bulk
crystal below approximately 14 nm. The change in theorthorhombic distortion suggested modulation of thesymmetry of the MnO6 octahedra and the Jahn–Teller
distortion from those for the bulk crystal. Thus, it isconsidered that the magnitude of the Jahn–Teller effect for
the nanoparticles changes with the particle size. The resultsof crystal structural analysis indicate that there is a strong
correlation between the magnetic properties and thedistortion of crystallographic structure of the nanoparticles,
which is similar to the size effects on the DyMnO3 andGdMnO3 nanoparticles. The modulation of thecrystallographic structure of the nanoparticles induces thechanges in the magnetic interactions between the Mn spins,
which result in the pronounced changes in the magnetic
properties for the TbMnO3 nanoparticles below
approximately 16 nm.

b/aFig. 1: Particle size dependence of lattice constants for
TbMnO3 nanoparticles. The horizontal dashed lines are thelattice constants of the bulk crystal.

Fig. 2: Particle size dependence of orthorhombic distortionb/a for TbMnO3 nanoparticles. The horizontal dashed lines
is the value of b/a of the bulk crystal.
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).
* tajiri@fukuoka-u.ac.jp