Evidence for the Existence of Oxygen Clustering and Understanding
of Structural Disorder in Prussian Blue Analogues Molecular Magnet
M
1.5
[Cr(CN)
6
]·zH
2
O (M = Fe and Co): Reverse Monte Carlo Simulation
and Neutron Diffraction Study
Pramod Bhatt,* Nidhi Thakur,
†
Mayuresh D. Mukadam, Sher Singh Meena, and Seikh M. Yusuf
Solid State Physics Division, Bhabha Atomic Research Centre, Mumbai 400 085, India
ABSTRACT: A detailed structural disorder investigation of
Prussian blue analogues M
1.5
[Cr(CN)
6
]·zH
2
O (M = Fe and
Co) has been done by carrying out a reverse Monte Carlo
(RMC) simulation on the powder neutron diffraction data. X-
ray diffraction, infrared spectroscopy, Mö ssbauer spectroscopy,
and dc magnetization measurements have also been employed
to investigate the structural and magnetic properties of the
compounds. The Rietveld refinement of the X-ray and neutron diffraction patterns reveals that both compounds are in a single
phase with a face-centered cubic crystal structure (space group Fm3m). The observation of characteristic absorption bands in the
range 1900-2200 cm
-1
of infrared (IR) spectra, which corresponds to the CN stretching frequency of M
II
NCCr
III
sequence, confirms the formation of Prussian blue analogues, M
1.5
[Cr(CN)
6
]·zH
2
O. The IR study also infers the presence of
cyanide flipping in the Fe
1.5
[Cr(CN)
6
]·zH
2
O compound. The Mö ssbauer study on the Fe
1.5
[Cr(CN)
6
]·zH
2
O compound
confirms the presence of high as well as low spin Fe
II
ions due to isomerization of some Cr
III
CNFe
II
linkages to the
Cr
III
NCFe
II
form. The magnetization data show a soft ferromagnetic nature of both compounds with a Curie
temperature of ∼17 and ∼22 K for Fe
1.5
[Cr(CN)
6
]·zH
2
O and Co
1.5
[Cr(CN)
6
]·zH
2
O, respectively. A large amount of structural
disorder is present in both compounds, which is manifested in the form of a diffuse scattering in neutron diffraction patterns. The
RMC results, obtained after the modeling, simulation, and analysis of the neutron diffraction data, propose that the water
molecules and the [Cr(CN)
6
] vacancies are mainly responsible for the structural disorder. Moreover, a clustering of the non-
coordinated oxygen atoms around the coordinated oxygen atoms is also ascertained by the RMC analysis. The correlation of
structural disorder with the water content and [Cr(CN)
6
] vacancies is also discussed.
■
INTRODUCTION
Periodic arrangements of atoms in a material decide the
crystalline nature, and the local deviation of atoms from their
atomic sites leads to the structural disorder in the materials.
Structural disorder occurs due to an uncertainty in the spatial
arrangements of atoms in a material. The disordered materials
display interesting physical properties that are not always seen in
their crystalline counterparts. Therefore, the disordered
materials have also received great attention and are equally
important from the scientific as well as technological point of
view when compared to crystalline materials. For example, the
disordered semiconductor materials, produced by metal doping,
are highly useful in microelectronics technology.
1
Further, the
high temperature superconductor can be synthesized with an
optimal amount of doping of defects/disorder in the materials.
2,3
The disorder can be introduced in the materials either by using
external stimuli, such as temperature, pressure, heat, magnetic
field, or light, or intrinsically due to doping of atomic species,
vacancies, impurities, defects, and flexing of molecules, and
subsequently affects their physical properties.
4-7
The physical properties of disordered materials mainly depend
on the types of disorder (dynamic, static, and subsitutional)
present in the materials. Hence, a deeper understanding of local
structure/disorder at the atomic scale is highly desirable. Some
examples of disordered materials
8
are (i) materials showing small
deviation from perfect crystallinity: In-Ga-As semiconductor
alloys,
9,10
(ii) crystalline materials showing significant structural
disorder: nanophase LiMoS
2
,
11
(iii) completely disordered
materials: Ca-Al-Si-O glasses,
12
(iv) materials showing no
positional but chemical disorder: metallic Cu
3
Au,
8
(v) disorder
in organic materials: ferrocene,
13
(vi) colossal magnetoresistance
(CMR) materials with local dynamic distortions,
14
(vii) zeolites
with host-guest systems,
15,16
and (viii) molecular disorder in
C
60
and related compounds,
17
etc. In addition, one class of the
crystalline materials which possesses structural disorder (static in
nature) are Prussian blue analogues (PBA), which have attracted
great attention recently because of their novel magnetic
functionalities
18
as well as other properties, such as magnetic
pole inversion,
19,20
photomagnetic behavior,
21
zero/negative
thermal expansion,
22
etc. Moreover, these compounds are also
the subject of increasing current interest because of the
possibility of producing molecule-based magnets with ordering
Received: December 17, 2012
Revised: January 21, 2013
Published: January 22, 2013
Article
pubs.acs.org/JPCC
© 2013 American Chemical Society 2676 dx.doi.org/10.1021/jp312395y | J. Phys. Chem. C 2013, 117, 2676-2687