Pressure- and photoinduced transformation into a metastable phase in RbMn† Fe„ CN…
6
‡
Y. Moritomo and M. Hanawa
Department of Applied Physics, Nagoya University, Nagoya 464-8603, Japan and PRESTO, JST
Y. Ohishi, K. Kato, and M. Takata
SPring-8/JASRI, Hyogo 679-5198, Japan
A. Kuriki
Department of Crystalline Materials Science, Nagoya University, Nagoya 464-8603, Japan
E. Nishibori and M. Sakata
Department of Applied Physics, Nagoya University, Nagoya 464-8603, Japan
S. Ohkoshi, H. Tokoro, and K. Hashimoto
Research Center for Advanced Science and Technology, University of Tokyo, Tokyo153-8904, Japan
Received 25 February 2003; revised manuscript received 14 May 2003; published 15 October 2003
Pressure and photoirradiation effects on the structural properties were investigated for a transition metal
cyanide, RbMn Fe(CN)
6
, with perovskitelike structure by means of high-angle resolved synchrotron-
radiation x-ray powder diffraction. We have found that 1 photoirradiation by a 532-nm pulse laser at 91 K and
2 application of hydrostatic pressure to about 2 GPa at 300 K change the system from the tetragonal
low-temperature phase ( I 4
¯
m2; Z =2) to a metastable state with different crystal symmetry ( P 4
¯
n 2; Z =2). We
discuss the difference between the photoinduced and pressure-induced processes and suggest possible origin
for the photoinduced demagnetization.
DOI: 10.1103/PhysRevB.68.144106 PACS numbers: 64.60.My, 61.50.Ks
I. INTRODUCTION
Transition-metal cyanides the so-called Prussian-blue
compounds, A (I) M (II) N (III)( CN )
6
( A =Na, K, Rb, Cs;
N =Mn, Co, Cr; M =Fe, Cr,
1–9
have been attracting re-
newed interest of materials scientists, because they show a
photoinduced magnetization/demagnetization. For example,
Sato et al.
3
reported the enhancement of magnetization in
K
0.2
Co
1.4
Fe(CN)
6
6.9H
2
O by irradiation of a red light 660
nm at 5 K and suppression of magnetization by irradiation
of a blue light 450 nm at 5 K. This photoinduced magne-
tism has been ascribed to the photoinduced structural change
into some metastable state and the resultant variation of the
electronic configuration of the transition metals. Then, deter-
mination of the crystal structure, including atomic coordi-
nates, is indispensable for a deeper comprehension of
the photoinduced phenomena as well as their practical
application.
The Prussian-blue compounds are well known
from the earliest times. Nevertheless, except for
Fe
4
Fe(CN)
6
3
15H
2
O,
10
difficulty in crystal growth has pre-
vented detailed structural investigations of these compounds.
In addition, the compounds contain considerable nonsto-
ichiometric H
2
O molecules, which makes the precise struc-
tural analysis difficult. Recently, Ohkoshi et al.
11
synthesized
a Mn-Fe compound, RbMn Fe(CN)
6
, which does not con-
tain extra H
2
O molecules. Neutron powder-diffraction
experiment
12
revealed that RbMn Fe(CN)
6
is ferromag-
netic below T
C
=12 K, where the local spin moments are
taken only by the high-spin HS Mn
2 +
( S =2) ions. Fur-
thermore, Tokoro et al.
13
observed suppression of magneti-
zation by irradiation of a visible pulse laser 532 nm at 3 K.
Therefore, RbMn Fe(CN)
6
is one of the most suitable com-
pounds for detailed structural investigation to reveal the ori-
gin for the photoinduced demagnetization.
At room temperature, RbMn Fe(CN)
6
is face-centered
cubic ( F 4
¯
3 m : Z =4).
11,14
Reflecting the stability of the HS
Mn
2 +
state, the valence state is HS Mn
2 +
( d
5
;
S =5/2) –low-spin LS Fe
3 +
( d
5
; S =1/2).
11
With decrease
of temperature below 220 K, however, a structural transition
from the cubic phase to the body-centered tetragonal ( I 4
¯
m 2;
Z =2) phase takes place, accompanying the Jahn-Teller type
distortion of the MnN
6
octahedra. Therefore, the cubic-to-
tetragonal structural transition is considered to be driven by
charge transfer
14
from the Mn
2 +
site to the Fe
3 +
site. Con-
sistently, Osawa et al.
15
proposed that the low-temperature
valence state is HS Mn
3 +
( d
4
; S =2)–LS Fe
2 +
( d
6
; S
=0), based on their x-ray emission and absorption spectra.
Hereafter, we denote the high-temperature cubic phase
( F 4
¯
3 m ; Z =4) and the low-temperature tetragonal phase
( I 4
¯
m 2; Z =2) as HT and LT phases, respectively.
In this paper, we report the crystal structure of a thermally
quenched metastable state of RbMn Fe(CN)
6
determined
by synchrotron-radiation x-ray powder diffraction XRD.
The metastable state belongs to the tetragonal space group
( P 4
¯
n 2; Z =2), which is different from those of the HT
phase and the LT phase. In the metastable phase, the ligand
CN
-
ions approach the Mn site accompanied by the tilting of
the in-plane ions along the c direction. We further found that
PHYSICAL REVIEW B 68, 144106 2003
0163-1829/2003/6814/1441067/$20.00 ©2003 The American Physical Society 68 144106-1