The Layered Ordered Perovskites LnBaCo2O5.50 & LaBaCo2O6 : Structure Magnetism and
Transport Properties




B. Raveau1, V. Caignert1, A.Kundu1, M. Seikh1, Ph. Boullay1, E. Rautama1,2, Ch. Simon1, V.
Pralong1, M. Karpinnen2.



1 Laboratoire CRISMAT – UMR 6508 – ENSICAEN / CNRS, Bd du Maréchal Juin 14050 CAEN Cedex -
FRANCE
2 Laboratory of Inorganic and Analytical Chemistry, Helsinki University of Technology – PO Box 6100 – FI 020157
KK FINLAND



First investigated for their magnetoresistance and their original metal-insulator properties, the
“112” cobaltites LnBaCo2O5.50+δ (Ln = lanthanide) show in fact very complex magnetic and
transport transitions. The first characteristic of these oxides deals with the fact that their
magnetic properties are very sensitive to a small deviation from the “O5.50” stoichiometry.
Exploring the system with Ln = Eu, Sm, we show that three ferromagnetic (FM) states can be
obtained, depending on δ. For a cobalt valency close to +3 (δ ∼ 0) an unusual ferromagnetic
ordering FM1 takes place around 250 K. For δ < - 0.1 (Co2+/Co3+), a new ferromagnetic state
FM2 is observed, involving phase separation, whereas for δ > + 0.1 a different ferromagnetism
FM3 appears due to Co3+/Co4+ superexchange interactions.
The second property of these materials concerns the doping of the Ln site with calcium. In the
corresponding oxides, Ln1-xCaxBaCo2O5.50-δ (x ≤ 0.2), the cobalt valence remains +3 due to small
oxygen deficiency with respect to “O5.50”. A large expansion of the ferromagnetic state is
observed in the whole temperature range from 10 K to 320 K, TC increasing with the calcium
content. Moreover the metal-insolator transition temperature is not affected by this Ca doping,
whereas the resistivity and magnetoresistance decrease drastically for a small doping. A new
scenario is proposed to explain these results, based on the presence of canted antiferromagnetic
domains and on a disproportion presence mechanism of Co3+ into Co2+ and Co4+.
The possibility to synthesize an ordered layered stoichiometric perovskite, starting from the
“112” ordered oxygen deficient perovskite has been revisited. Three forms of the lanthanum
barium cobaltite perovskite have been synthesized: the disordered La0.5Ba0.5CoO3, the perfectly
ordered layered LaBaCo2O6 and a new one called nanoscale ordered LaBaCo2O6. The H REM
and XRD studies of the latter show that it consists of 112-type 90° oriented domains fitted into
each other at a nanometre scale, which induce large strains so that the crystallographic
parameters are different from the other two forms and local atomic distortions are obtained. The
three forms are ferromagnetic and exhibit the same TC (174-179 K), but differently from the two
other forms, the nanoscale ordered LaBaCo2O6 is a hard ferromagnet (Hc ∼ 4.2 kOe), due to
spin-locking effect induced by the strains. The magnetoresistance properties of these phases are
also investigated.

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