Photon Factory Activity Report 2017 #35 (2018)

BL-8B/2017G027

Particle Size Dependence of Crystal Structure of Mn3O4 Nanoparticles
Takayuki TAJIRI1,* and Masaki MITO2
Faculty of Science, Fukuoka University, Fukuoka 814-0180, Japan
2
Graduate School of Engineering, Kyushu Institute of Technology, Kitakyushu 804-8550, Japan
11 Introduction
The nanoparticle of strongly correlated materials suchas manganese oxide are expected to exhibit characteristic
size effects on crystal structure and magnetic property
owing to the strongly electron correlation and strong
coupling among spin, orbital, and lattice. Spinel oxides
with a general formula AB2O4 has tetrahedral A site withfour oxygen and octahedral B site with six oxygenarranged in a pyrochlore latteice. Mn3O4 has geometricalfrustration derived from the antiferromagnetic interactions
between the B sites of spinel oxide. The Mn3O4 exhibitsinteresting phenomena such as magnetodielectric,
magnetoelastic and magnetocaloric behaviors, andsuccessive phase transformations owing to the strong
coupling among spin, orbital, and lattice degrees offreedom [1-3]. For Mn3O4, the tetrahedral A sites andoctahedral B sites are selectively occupied by Mn2+ andMn3+ ions, respectively. Since Mn3+ ions at the MnO6octahedral site with one electron in the doubly degenerate
eg states, the Mn3O4 exhibits a structural phase transition
from cubic to tetragonal at 1443 K due to the strong
instability of the Jahn-Teller distortion, whose magnitude
is estimated to be c/√2 a ~ 1.16 based on the ratio oflattice constants (a ~ 5.76 Å and c ~ 9.47 Å) [1,2].We
synthesized the Mn3O4 nanoparticles in the pores ofmesoporous silica, and their crystallographic structure
and magnetic properties were investigated. In this report,
we show the results of crystal structural analysis for theMn3O4 nanoparticles.
2 Experiment
The Mn3O4 nanoparticles were synthesized in the pores,
with a diameter of approximately 7 nm, of mesoporous
silica SBA-15. SBA-15 was used as a template to equalize
the particle size in the fabrication of the Mn3O4
nanoparticles
andtodeflocculate
synthesized
nanoparticles since it has a two-dimensional hexagonalmesoporous structure and its pores are separated by silicawall [4]. The Mn3O4 nanoparticles were synthesized bysoaking the SBA-15 in a aqueous solution of MnCl2·4H2O.
The soaked SBA-15 was then dried and calcinated in an
air atmosphere. The powder X-ray diffraction (XRD)
measurements for the synthesized nanoparticles werecarried out at room temperature by using a DebyeScherrer camera at the beamline BL-8B. The X-ray
wavelength was calibrated using the XRD pattern of CeO2powder.
3 Results and Discussion
We observed powder XRD patterns for the synthesized
Mn3O4 nanoparticles in the pores of SBA-15 at room
temperature. The diffraction patterns of the Mn3O4
nanoparticles exhibited broad Bragg peaks, which wereattributed to the tetragonal symmetry with space group
I41/amd the same as that of bulk crystal. Note that theparticle sizes of the Mn3O4 nanoparticles were estimatedbased on the peak positions and the full widths at half
maximum of the Bragg peaks using Scherrer’s equation.
These results indicated successful synthesis the Mn3O4
nanoparticles with mean particle size raging fromapproximately 7 to 30 nm.
The lattice constants of the nanoparticles wereestimated from XRD patterns. The lattice constants werecalculated from the relation between lattice constants andplane indices determined from the Bragg peak angles forthe all observed diffraction peaks. Figure 1 shows the size
dependences of the lattice constants for the nanoparticles.
The lattice constants for the nanoparticles were slightly
different from those for bulk crystal [1] and depended on
the particle size. The lattice constant a exhibits
pronounced increase with decreasing particle size below~12 nm, while it is almost constant value above ~12 nm.
As particle size decreases, lattice constant c gradually
decreases above ~12 nm, then the value increases
drastically below ~12 nm. These results indicate that,compared with bulk crystal, the crystallographic structure
for the nanoparticles distorts from that for bulk crystal,and the distortion of unit cell depends on the particle size.
We estimated the tetragonal distortion, c/√2 a, which has
been used to describe the Jahn-Teller distortion in Mn3O4.
The Mn3O4 bulk crystal has huge tetragonal distortion,c/(√2 a) = 1.162 [1]. The estimated tetragonal distortion
for the nanoparticles depends on the particle size asshown in Fig. 2. The Jahn-Teller distortion for Mn3O4nanoparticles is suppressed as compared with the bulk
crystal, and its magnitude changes with decreasing
particle size. As particle size decreases, the Jahn-Teller
distortion gradually decreases above ~12 nm, and then thevalue increases below ~12 nm. On the other hand, weinvestigated the magnetic properties for the synthesized
Mn3O4 nanoparticles, which exhibit the drastic changes inthe magnetic properties, the transition temperatures andcoercive field, below ~12 nm. The crystal structural
analysis and the magnetic measurement results suggestthat the Mn3O4 nanoparticles have strong correlation
between crystal structure and magnetic property. The
modulation of the lattice constants and the Jahn-Teller
distortion varies the magnetic interactions between theMn spins, which result in the pronounced changes in thetransition temperatures and coercive field for the Mn3O4
nanoparticles.

Photon Factory Activity Report 2017 #35 (2018)

Fig. 1. Particle size dependences of lattice constants for
the Mn3O4 nanoparticles.

Fig. 2. Particle size dependences of tetragonal distortion
for the Mn3O4 nanoparticles.
References[1] M. C. Kemei et al., Phys. Rev. B 90, 064418 (2014).
[2] H. Lv et al., J. Phys. Chem. C 116, 2165-2171 (2011).
[3] Y. Nii et al., Phys. Rev. B 87, 195115 (2013).
[4] D. Zhao et al., science 279, 548 (1998).
* tajiri@fukuoka-u.ac.jp