98
M. Zinkevich et al. / Journal of Alloys and Compounds 438 (2007) 92–99
determination of the stability region of La2NiO4 by the anneal-
ing and quenching method [38] essentially confirmed the EMF
data [29]. It should be noted that fitting all experimental points
in Fig. 7 results in the Gibbs energy equations, which cannot be
reconciled with experimental values for the entropy and enthalpy
of formation.
4. Conclusions
In the present study, the heat capacity, the entropy and
the Gibbs energy of La3Ni2O7, La4Ni3O10, and LaNiO3 were
determined experimentally for the first time. On heating, these
phases remained almost stoichiometric until the decomposition
occurredaccordingtothesequenceofreactions (1) → (2) → (3).
The stability regions for each compound in terms of tempera-
ture and oxygen pressure were reliably determined. The data
obtained by equilibration with the gas phase at high temperatures
showed good consistency with the cryogenic heat capacity mea-
surements. The derived thermodynamic functions also account
well for a few measurements reported in the literature and give
a realistic representation of phase equilibria in the La–Ni–O
system. The latent heat associated with the first-order phase
transition in La3Ni2O7 at 490 K (orthorhombic to tetragonal)
was determined. Further studies are necessary to understand the
nature and mechanism of phase transformations in La4Ni3O10
and LaNiO3.
Fig. 7. Calculated La–Ni–O phase stability diagram in terms of temperature and
oxygen pressure in the gas phase (always present in the system) in comparison
with experimental data [29,34–37]. Circles, squares, and triangles are results
of this work (open symbols: dynamic experiments, filled symbols: isothermal
experiments).
taken from the literature can be observed. Also, the calcu-
lated line for the four-phase equilibrium La4Ni3O10, La3Ni2O7,
NiO, O2 is well-consistent with the boundary of stability for
La4Ni3O10, determined in a recent EMF study [9], despite the
authors claimed different mechanism of decomposition, i.e.,
La4Ni3O10 = 2La2NiO4 + NiO + 0.5O2 (Fig. 8).
Acknowledgments
At low temperatures, however, the experimental points shift
to lower oxygen pressures as compared to the lines. This may be
due to the slow kinetics of decomposition or increasing uncer-
tainty in mass spectrometric determinationof the oxygencontent
in the gas flow with decreasing oxygen partial pressure. Simi-
lar problems seemed to occur in the thermogravimetric studies
of the reaction: La2NiO4 ⇒ La2O3 + Ni + 0.5O2 [34,35] since
The authors thank to Mr. G. Kaiser for technical assistance.
One of the authors (N.S.) is grateful to the International Max
Planck Research School for Advanced Materials (IMPRS-AM)
for financial support.
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