Structural and magnetic study of PrBaCo
2
O
5+
„ ¶ 0.75… cobaltite
Carlos Frontera, José Luis García-Muñoz, and Anna E. Carrillo
Institut de Ciència de Materials de Barcelona, CSIC, Campus Universitari de Bellaterra, E-08193 Bellaterra, Spain
Clemens Ritter
Institut Laue Langevin, 6, rue Jules Horowitz, F-38042 Grenoble Cedex 9, France
David Martín y Marero
Institut Laue Langevin, 6, rue Jules Horowitz, F-38042 Grenoble Cedex 9, France,
and Instituto de Ciencia de Materiales de Aragón, CSIC, Zaragoza, Spain
Alberto Caneiro
Centro Atómico de Bariloche, 8400-San Carlos de Bariloche, Argentina
(Received 10 June 2004; revised manuscript received 22 September 2004; published 23 November 2004)
We have investigated the structural and magnetic properties of PrBaCo
2
O
5+
0.75 layered cobaltite. By
means of neutron and synchrotron x-ray powder diffraction we have determined the ordered distribution of
oxygen vacancies in PrO
planes. This order doubles both a and b lattice parameters from the perovskite cell
parameter, forming a tetragonal (rather than orthorhombic) structure. On cooling, a paramagnetic to ferromag-
netic and an incomplete ferromagnetic to antiferromagnetic transition take place. This second transition leads
to a coexistence of ferromagnetic and antiferromagnetic order, forming a canted G-type antiferromagnetic
structure. Magnetic data above the ferromagnetic transition evidence a large value of the effective paramag-
netic moment of Co, implying that, well below room temperature, Co
3+
ions in an octahedral environment
present a high-spin state, in contrast to RBaCo
2
O
5.5
cobaltites.
DOI: 10.1103/PhysRevB.70.184428 PACS number(s): 75.25.+z, 71.27.+a, 75.80.+q
I. INTRODUCTION
The study of transition-metal oxides is, nowadays, very
extended in the field of materials science. Among them, co-
balt oxides are the subject of vivid studies. In cobaltites the
spin state degree of freedom of Co ions introduces new in-
teresting effects in the case of narrow-band oxides. Among
other families like misfit cobaltites for thermopower
applications
1,2
or mixed-valence cobalt perovskites as candi-
dates to present charge ordering like Pr
0.5
Ca
0.5
CoO
3
,
3
layered
cobaltites with general formula RBaCo
2
O
5+
(R rare earth,
0 1) have been the subject of several studies during the
last years.
4–15
These compounds have been demonstrated to
be complex systems presenting very intriguing phenomena
like charge ordering,
4,5
metal-insulator transitions,
6,7
orbital
order,
8
and also a large thermoelectric power.
12
Following
the paper by Troyanchuk et al.,
6
the cornerstone paper by
Maignan et al.
7
evidenced that the oxygen content of these
compounds can be tailored by heat treatments in different
atmospheres. Maignan et al.
7
also established two special
structural features: the ordering in alternating planes of Ba
and R ions and the tendency of oxygen vacancies to place at
the R planes forming ordered patterns.
The most appealing features of narrow-band cobaltites are
the spin-charge-orbital coupling and the fact that Co pre-
sents, with great facility, transitions between different spin
states. As an example one can mention the case of LaCoO
3
,
object of great controversy during the 1980s and 1990s (Ref.
16). The competition between the Hund exchange and the
crystal field, together with the distortions in octahedral coor-
dination or the presence of pyramidal coordination, can sta-
bilize three different spin states: low spin (LS), intermediate
spin (IS), and high spin (HS) for both Co
3+
(LS with t
2g
6
and S =0, IS with t
2g
5
e
g
1
and S =1, and HS with t
2g
4
e
g
2
and
S =2) and Co
4+
(LS with t
2g
5
and S =
1
2
, IS with t
2g
4
e
g
1
and
S =
3
2
, and HS with t
2g
3
e
g
2
and S =
5
2
). (Ref. 17)
The oxygen content of these layered cobaltites, , controls
not only the mean valence of Co ions (which can vary from
3.5+ for =1 to 2.5+ for =0), but also the coordination of
Co (pyramidal or octahedral) and has therefore a strong in-
fluence on the spin state of Co. This leads to the magnetic
and transport properties of these compounds being mainly
dominated by the oxygen content. For instance, it has been
found that for low there is a great tendency to form charge-
ordered structures and this phenomenon has been reported
for YBaCo
2
O
5
(Ref. 4) and TbBaCo
2
O
5
(Ref. 5). Metal-
insulator and spin-state transitions as well as successive mag-
netic transitions due to competing ferromagnetic-
antiferromagnetic interactions are present in compounds with
= 0.5 for a wide variety of rare earths.
7–9,14,15
In Ref. 10 we investigated the origin of this metal-
insulator transition above room temperature (RT)(for = 0.5).
We have attributed the changes of the electron mobility on
heating to a sudden change from the Co
3+
LS state t
2g
6
to
Co
3+
HS state t
2g
4
e
g
2
in Co
3+
O
6
octahedra.
10
For =0.5 it is
well established that oxygen vacancies order forming filled
and empty rows of oxygen sites along a in the RO
planes.
Associated with this ordering, IS/LS Co
3+
states where found
to be orderly distributed in the structure below T
MI
(IS/HS
states above the spin-state transition at T
MI
).
10
This makes it
such that the spin-state transition takes place in an ordered
PHYSICAL REVIEW B 70, 184428 (2004)
1098-0121/2004/70(18)/184428(9)/$22.50 ©2004 The American Physical Society 70 184428-1