Structure and Thermodynamic Stability of LuFe2O4

Article abstract of DOI:10.1134/S1070427219020022

Synthesis of a multiferroic of composition LuFe₂O₄ at a temperature of 1363 K and low oxygen pressures, its crystal structure, and thermodynamic properties are reported. The LuFe₂O₄ ferrite was obtained by using

Full text of DOI:10.1134/S1070427219020022

ISSN 1070-4272, Russian Journal of Applied Chemistry, 2019, Vol. 92, No. 2, pp. 181−185. © Pleiades Publishing, Ltd., 2019.
Russian Text © L.B. Vedmid’, O.M. Fedorova, V.M. Dimitrov, 2019, published in Zhurnal Prikladnoi Khimii, 2019, Vol. 92, No. 2, pp. 149−153.
INORGANIC SYNTHESIS
AND INDUSTRIAL INORGANIC CHEMISTRY
Structure and Thermodynamic Stability of LuFe2O4
L. B. Vedmid’a,b, O. M. Fedorovaa,*, and V. M. Dimitrova
a Institute of Metallurgy, Ural Branch, Russian Academy of Sciences, Yekaterinburg, Russia
b Ural Federal University, Yekaterinburg, Russia
*e-mail: fom55@mail.ru, elarisa100@mail.ru
Received September 28, 2018; revised November 22, 2018; accepted November 29, 2018
Abstract—Synthesis of a multiferroic of composition LuFe2O4 at a temperature of 1363 K and low oxygen
pressures, its crystal structure, and thermodynamic properties are reported. The LuFe2O4 ferrite was obtained by
using an improved method of synthesis in a controlled atmosphere. The stability range of this compound was
determined in relation to the partial pressure of oxygen in the gas atmosphere under a thermal treatment. At room
–
temperature, the sample has a rhombohedral structure (R3m space group). The X-ray diffraction method was
used to determine the structural characteristics of the compound, and its thermodynamic properties were found
by the static method in a vacuum circulation installation.
Keywords: multiferroics, partial oxygen pressure, mixed valence, structure, thermodynamic properties.
DOI: 10.1134/S1070427219020022
aliovalent iron ions Fe2+, Fe3+, are, as a rule, obtained in
Materials based on transition metal oxides, which
a reducing mixture of gases CO–CO2–H2–H2O at tem-
possess a mixed valence of cations and combine fer-
peratures above 1423 K [3–6]. The oxide LuFe2O4 is a
romagnetic and ferroelectric properties, are attributed to
representative of materials of this kind. At present, there
the special class of multiferroics, which makes promising
are no data on the possibility of synthesizing the com-
their technological application in information and energy-
pound LuFe2O4 at temperatures below 1473 K. Lutetium
saving technologies [1]. Their particular properties are
ferrite attracts researchers’ interest because the charge
due to the relationship between the spin, charge, and
ordering occurs in this material at room temperature,
orbital kinds of ordering. The mechanisms by which the
and its structure changes as the temperature is lowered
ferromagnetic and ferroelectric ordering appears in this
[5]. There is published evidence about the crystal struc-
class of compounds differ rather strongly, to the point
ture and physical properties of YbFe2O4 [7, 8], YFe2O4,
of mutual exclusion. [2]. To obtain compounds of this
ErFe2O4 [9], TmFe2O4 [10], but no data are available on
the thermodynamic stability of these compounds.
kind and prognosticate their properties, it is necessary
to have information about their existence conditions
The goal of our study was to find conditions for
synthesis of LuFe2O4 at temperatures below 1423 K,
obtain thermodynamic characteristics at lowered oxygen
pressures, and examine the structure in more detail in a
wide temperature range. The availability of evidence of
this kind is important for practical application of this
class of compounds.
and structural and thermodynamic characteristics. At
present, there is information about the classification by
the presence of compounds of the homological series
LnFeO3·nFeO in Ln–Fe–O systems (Ln is a rare-earth
element) in the temperature range 1423–1523 K and some
phase diagrams have been reported [3, 4]. Depending on
the presence of a rare-earth ion Ln3+ in compounds and on
thermodynamic conditions, the classification subdivides
the systems into four groups. The compounds LnFeO3 and
Ln3Fe5O12 are synthesized in air at temperatures higher
than 1273 K for all Ln. The homologs LnFe2O4 (Ln = Y,
Ho–Lu) and Ln2Fe3O7 (Ln = Y, Yb, Lu), which contain
EXPERIMENTAL
LuFe2O4 samples were synthesized from a mixture of
oxides Fe2O3 (analytically pure) and Lu2O3 (99.9%), both
181

182
VEDMID’ et al.
Fig. 1. Fragment of the phase diagram of the Lu–Fe–O system at a temperature of 1363 K. Phase designations: А = LuFe2O4±δ, Р =
LuFeO3±δ, G = Lu3Fe5O12, W = FeO, M = Fe3O4.
(a)
preliminarily dried at a temperature of 550°C. Equimolar
amounts of the starting oxides were mixed and ground in
an agate mortar for 1 h. The resulting mixture was used to
compact pellets 10 mm in diameter on a hydraulic press
under a pressure of 150 kPa cm–2, and the pellets were
calcined in a closed-volume installation [11] in a quartz
reactor at T = 1363 K in a gas atmosphere with variable
oxygen pressure during 18 h. Under the experimental
conditions, the maximum possible synthesis temperature
is 1363 K, which is determined by using quartz as the
reactor material. The partial oxygen pressure in the gas
atmosphere is monitored during the whole run with the
use of an oxygen cell including an oxygen sensor and
an oxygen pump. The operation of the oxygen cell is
controlled by a programmed oxygen-pressure regulator.
The gas medium was formed from a purified mixture of
an inert gas (Ar) and O2. In this case, only an oxygen
exchange can occur as a result of the interaction of a
sample with components of the gas mixture, in contrast
to the case of a mixture of gases, CO–CO2–H2–H2O,
in which carbides can be formed. An X-ray diffraction
(XRD) analysis of quenched LuFe2O4 samples was
made with a Shimadzu XRD 7000 diffractometer (CuKα
radiation) at angles 10° < 2θ < 70° with a step of 0.2°.
A full-profile XRD analysis was made by the Rietveld
method with EXPGUI software complex [12], with the
model described in [8] served as the starting model. The
(b)
2θ, deg
Fig. 2. Diffraction patterns of LuFe2O4 samples at (a) 300 and
(b) 150 K. Miller indices are shown for (a) rhombohedral and
(b) monoclinic structures.
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STRUCTURE AND THERMODYNAMIC STABILITY OF LuFe2O4
183
thermal stability of LuFe2O4 samples was examined in
UCP
a vacuum circulation installation by the static method
[13]. Changes in the thermal properties of the compounds
were recorded by the method of differential scanning
calorimetry (DSC) on a Netzsch DSC 204 F1 instrument.
Measurements were made in aluminum crucibles in
the atmosphere of nitrogen in the temperature range
110–373 K at a heating rate of 10 deg min–1.
RESULTS AND DISCUSSION
UCP
The process in which compounds of the type RFe2O4
(R = Dy–Lu) can be represented by the scheme
R3+Fe3+O32– + Fe2+O2– → R3+Fe3+Fe2+O42– → RFe2O4. (1)
Fe–O1 (ap)
Fe–O2 (ap)
Fixing one parameter, temperature, while varying
another parameter, oxygen pressure, when synthesizing
samples made it possible to find the stability range of the
LuFe2O4 compound. When the partial oxygen pressure
at a temperature of 1363 K changes to more than PO2
=
10–11.24 atm, the XRD analysis demonstrates the presence
of LuFe2O4, LuFeO3, and Fe3O4 phases. Lowering the
oxygen pressure to below PO2 = 10–12.04 atm results in
that the Ln2O3 and FeO phases appear in addition to the
main phase LnFe2O4 (Fig. 1). At the boundaries of this
range of oxygen pressures, samples retain a single-phase
rhombohedral structure.
Temperature, K
Fig. 3. Temperature dependences of the unit cell parameters
(UCPs) and bond lengths in LuFe2O4.
octahedron are oriented perpendicularly to the principal
axis [14].
The single-phase LuFe2O4 samples have a
rhombohedral structure and belong to the R3– m space
group with hexagonal packing; their unit cell parameters
are the following: a = 3.4403(1) Å and c = 25.272(2) Å
(Fig. 2a).
Figure 3 shows the temperature dependence of the
unit cell parameters of LuFe2O4 with rhombohedral
structure and that of the lengths of Lu–O2 bonds and
Fe–O1 and Fe–O2 apical bonds. It can be seen in Fig. 3
that the parameter a of lutetium ferrite slightly decreases
in the temperature range 150–210 K, with the lengths of
Fe–O1 and Fe–O2 bonds also insignificantly decreasing.
Then an increase in the parameter a is observed up to a
temperature of 400 K. This rise is accounted for by the
thermal expansion of a sample.
When LuFe2O4 is cooled to below 250 K, a transition
–
from the rhombohedral (R3m space group) to the
monoclinic (C2/m space group) structure is observed.
In the XRD patterns, this process is manifested in that
peaks appear at angles in the 2θ range 25–28° (Fig. 2b).
According to our data, the temperature of this transition
somewhat differs from that obtained in [5]. A probable
reason may be the difference between the methods used
to produce the samples. In the spatial structure of the
ferrite LuFe2O4, the anionic polyhedra containing in
the disordered state aliovalent iron cations form layers
perpendicular to the principal axis z of the unit cell.
Iron cations are situated within a trigonal bipyramids in
fivefold environment of oxygen anions. The environment
of Lu is an octahedron with six oxygen anions moved
apart in the x–y plane. The triangular faces of this
The parameter c remains nearly unchanged at
temperatures of 150 to 290 K, and then sharply decreases
in the temperature range 295–313 K. This behavior of the
parameter c is correlated with the behavior of the Lu–O2
bond length. Within the same range, the DSC curve shows
a minimum. The decrease in the parameter c of lutetium
ferrite at 310 K and the endothermic effect in the DSC
curve in the same temperature range may be indicative
of the charge ordering in LuFe2O4. This temperature
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VEDMID’ et al.
log PO2 [atm]
DSC, mW mg–1
104/T, K–1
Temperature, K
Fig. 5. Temperature dependence of the equilibrium partial
oxygen pressure in dissociation of the LuFe2O4 compound
Black squares (1) our data, white square (2) data of [6].
Fig. 4. DSC curve for the LuFe2O4 compound.
coincides with the data of [5], in which study the charge
ordering in this material was confirmed by neutron
diffraction analysis. The second endothermic effect at a
temperature of 242.5 K is associated with the structural
transition from R3– m space group to C2/m (Fig. 4).
the changes in the standard enthalpy and entropy of
formation of the compounds from elements:
ΔHТ° = –1610.556 kJ mol–1, ΔSТ° = 237.294 J mol–1 K–1.
CONCLUSIONS
To determine the phase equilibria and obtain tem-
perature dependences of the equilibrium partial oxygen
pressure for these equilibria, we examined the process of
dissociation of the LuFe2O4 compound with the use of the
vacuum circulation installation in the temperature range
990–1190 K. In the experimental conditions under the ac-
tion of a reduced oxygen pressure PO2 = 10–11.24–10–12.04
atm, the compound LuFe2O4 dissociates with evolution
of oxygen into simple oxides by the reaction
The conditions of synthesis of the compound LuFe2O4
at a temperature of 1363 K were studied. This made
it possible to supplement with new data the P–T–C
constitution diagram of the Lu–Fe–O system. The values
of the thermodynamic characteristics of formation f the
LuFe2O4 compound from simple oxides were determined.
It was found that LuFe2O4 has in the temperature range
400–150 K a rhombohedral crystal structure and belongs
to the R–3m space group, At 295–313 K, the parameters
a and c change significantly, which is not accompanied
by a transition to another space group. This change can
be attributed to the change in the Lu–O and Fe–O1 and
Fe–O2 bond lengths. In the temperature range 295–313 K,
the length of the Lu–O2 bond increases and the apical
lengths of the Fe–O1(ap) and Fe–O2 bonds decreases,
which causes compression of FeO5 bipyramids and an
increase in the height of the LuO6 octahedron. The fact
that the temperature ranges of charge ordering and sharp
decrease in the parameter c coincide may indicate that
these two processes are related.
LuFe2O4 = 1/2Lu2O3 + 2FeO + 1/4O2.
(2)
In this case, the temperature dependence of the oxygen
pressure is linear (Fig. 5, line 1) and is expressed by the
equation
log (PO2, Pa–1) = 12.3–27991/T ± 0.041.
(3)
Point 2 represents the results of [6], obtained at a
higher temperature of 1473 K. The change in the free
Gibbs energy for reaction (2) was calculated by Eq. (3)
to be
ΔGТ° = 133.93 – 0.0349T ± 0.12 kJ mol–1.
(4)
FUNDING
Using the values of ΔHТ° and ΔSТ° for reaction
(2), calculated by equation (4), and the values of the
thermodynamic functions for the reaction in which the
simple oxides Lu2O3 and FeO are formed, we calculated
The study was carried out under the State assignment
to the Institute of Metallurgy, Ural Branch, Russian
Academy of Sciences, on the equipment of the Ural-M
Collective Use Center.
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pp. 218–222.
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