Inorganic Chemistry
ARTICLE
Reduction in autoclaves was initially tried as a convenient
alternative to reaction in the presence of solvents under reflux.
Additionally, a large number of solvents can be used at tempera-
tures above their boiling points, i.e., solvothermal conditions.
Reduction of La4Ni3O10 by NaH in diglyme (bp = 162 °C)
at 200 °C led to the most reduced nickelate, La4Ni3O8, as the
major phase. Therefore, at 200 °C the reducing activity of NaH
in diglyme under solvothermal conditions is comparable with
that of solid NaH and higher than NaH in triglyme at ambient
pressure.
The solvothermal reduction performed with NaH in pentane
(bp = 36.1 °C) at 200 °C resulted in pure phase La4Ni3O8. While
the reaction is kinetically hindered and it takes 10 days to prepare
the single phase, simplicity of procedure and the purity of the
product are advantageous over solid state reduction with NaH
in this case (Figure 2).
free energy of the system and can hinder the homogeneous
nucleation. It is possible that the pressure during reduction at
solvothermal condition decreases the nucleation energy of the
reduced nickelates—an effect that should be particularly impor-
tant at lower temperatures. While it is not possible to uniquely
attribute the pressure effect to increased reduction activity, our
results give direction for further development of this low-
temperature topotactic oxygen deintercalation method. As we
established, higher pressure is favorable for this reaction to occur.
At the same time, the autogenous pressure becomes lower at
lower temperatures; therefore, reduction under an externally
generated pressure of inert gas might allow extending reaction to
lower temperatures.
’ CONCLUSION
A novel solvothermal topotactic oxygen deintercalation meth-
od employing metal hydrides and organic solvents was described
and validated via reduction of La4Ni3O10 to La4Ni3O8 at 150 °C.
The relative reduction activities of different conventional solid
state reduction techniques were compared to determine the
reductive power of the proposed solvothermal method. Intro-
duction of pressure is favorable and leads to formation of pure
La4Ni3O8 at lower temperatures than previously reported.
Experiments are underway to investigate whether new meta-
stable complex oxides can be synthesized by the proposed
method.
Representative PXD patterns for the reduction products
prepared by four different experimental methods are shown in
Figure 3. Pure La4Ni3O8 without any peaks of admixture phases
can be prepared by reduction with H2 at 325 °C or by reduction
with NaH at solvothermal condition at 200 °C. Highly crystalline
products are formed in both cases. The full width at half
maximum (FWHM) for the main La4Ni3O8 reflections, (1 1 0)
and (1 0 7), were 0.113(8)° and 0.131(5)° 2Θ for the H2
reduction product and 0.111(4)° and 0.146(3)° 2Θ for La4-
Ni3O8 prepared by the solvothermal reduction method. Cell
parameters were refined as a = 3.9708(1) Å and c = 26.106(1) Å
after reaction with H2 at 325 °C and a = 3.9705(2) Å and c =
26.103(2) Å after solvothermal preparation at 200 °C.
La4Ni3O8 was also obtained as a main phase by the solvother-
mal reduction with NaH in pentane at 150 °C. The solubility of
both NaH and NaOH is expected to be much lower in nonpolar
solvents like pentane than in the polar glyme ethers; therefore,
the reducing power of NaH in pentane cannot be attributed to
reagent/product solubility.
’ AUTHOR INFORMATION
Corresponding Author
*Phone: 517-355-9715, ext. 388. Fax: 517-353-1793. E-mail:
’ ACKNOWLEDGMENT
There is a clear correlation between formation of La4Ni3O8
and autogenous pressure in autoclaves (Table 1). The pressure
inside of the autoclaves was calculated by employing the Clausiusꢀ
Clapeyron equations.27 Above the critical temperature of
pentane (196.7 °C), the van der Waals equation was used with
V.V.P. thanks Michigan State University for a start-up package
in support of this work.
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a = 18.99 L2 atm molꢀ2 and b = 0.146 L molꢀ1 27 The effect of
.
pressure could be related to many factors: changes in relative
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dx.doi.org/10.1021/ic200677p |Inorg. Chem. 2011, 50, 6696–6700