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Synthesis Crystal Structure and Magnetic Properties of the Semihard Itinerant Ferromagnet RhFe3N.

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Communications
Magnetic Storage Materials
DOI: 10.1002/anie.200502579
Synthesis, Crystal Structure, and Magnetic
Properties of the Semihard Itinerant Ferromagnet
RhFe3N**
Andreas Houben, Paul Mller, Jrg von Appen,
Heiko Lueken, Rainer Niewa, and
Richard Dronskowski*
Since the invention of magnetic data recording by Poulsen
more than a century ago and the development of the first
magnetic hard disc by the IBM company,[1] there has always
been a persistent demand for magnetic storage devices. At
present, the capacity of hard discs doubles every year and,
fortunately enough, this expansion outpaces the performance
enhancement of computer chips predicted by Moore"s law.[2]
If data storage should continue to keep ahead of data
production, intense research for new materials needed in
high-performance magnetic recording devices will remain a
challenge for solid-state chemistry.[3] But this is merely the
logical continuation of the impressive history of solid-state
magnetochemistry, as demonstrated by well-known example
of the maghemite phase.[4]
Within the realm of nitrogen-containing materials, the
binary nitride g’-Fe4N has been studied intensively both by
experimental[5–7] and also theoretical[8, 9] methods. In addition
to its remarkable chemical inertness, the phase exhibits
fascinating magnetic properties that make g’-Fe4N a possible
material for high-performance magnetic recording heads. In
particular, the very large specific saturation magnetization of
ss = 208 A m2 kg1 (which is close to that of a-Fe, ss =
218 A m2 kg1) and also the low coercive field of HC =
460 A m1 have attracted major attention.[6] g’-Fe4N adopts a
perovskite-like crystal structure (Figure 1) with the space
group Pm3̄m and the lattice parameter a = 3.7900(6) 8.[7]
It was realized some time ago that the magnetic properties
of g’-Fe4N may be influenced by replacing the iron atoms on
Wyckoff positions 1a and/or 3c. The newly introduced atoms
mainly control the crystal growth, yielding magnetic particles
with a pronounced anisotropic shape and a high coercive field
which makes them suitable for high-density storage materi[*] Dipl.-Chem. A. Houben, Dr. P. M#ller, Dipl.-Chem. J. von Appen,
Prof. Dr. H. Lueken, Prof. Dr. R. Dronskowski
Institut f#r Anorganische Chemie
Rheinisch-Westf0lische Technische Hochschule Aachen
Landoltweg 1, 52056 Aachen (Germany)
Fax: (+ 49) 241-80-92642
E-mail: drons@HAL9000.ac.rwth-aachen.de
Dr. R. Niewa
Max-Planck-Institut f#r Chemische Physik fester Stoffe
N@thnitzer Strasse 40, 01187 Dresden (Germany)
[**] We thank Klaus Kruse for measurement of the magnetization data,
Dipl.-Chem. Manfred Speldrich for competent help with their
graphical representation, and Dr. Roland Winde (Umicore, Wolfgang-Hanau) for the kind supply of metallic rhodium.
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2005 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Angew. Chem. Int. Ed. 2005, 44, 7212 –7215
Angewandte
Chemie
Figure 1. The crystal structure of g’-Fe4N in space group Pm3̄m
(a = 3.7900(6) F).[7] The large nitrogen atom (gray) occupies the very
center (Wyckoff position 1b), and the iron atoms (dark and light gray)
are found at the corners (1a) and face centers (3c). In RhFe3N, the 1a
position is completely substituted by Rh.
als.[10–12] We know—without claiming completeness—of a
couple of reports dealing with berthollide ternary nitrides of
the general form MxFe4xN with M = Ru,[10] Os,[10] Co,[11] Ir,[10]
Cu,[13] Ag,[14] and Zn[15] (x ! 1) as well as Sn (0 x 1.2),[16–18]
Mn,[12] and Ni[5, 19–21] (0 x 4). In contrast, replacing Fe with
Pd,[22] Pt,[5] Au,[14] or In[15] leads to daltonide ternary nitrides
MFe3N of the simple perovskite type.
There have been early attempts to relate preference for
substitution at the Wyckoff positions 1a and 3c[23] to the
relative affinities of Fe and M atoms to the nitrogen atom and,
even more importantly, the relative sizes of the metal-atom
radii. Only if the radius of the substituting M atom is smaller
than that of the Fe atom (rFe = 1.24 8), are both Wyckoff
positions statistically occupied. This is found for Mn with
rMn = 1.17 8. Otherwise, the larger atoms clearly prefer
position 1a because the coordination sphere of the 1acentered cuboctahedron in g’-Fe4N is much larger than that
of the 3c position. This is easily rationalized by the fact that
the 1aFe–3cFe distance (2.68 8) is longer than the 3cFe–1bN
distance (1.90 8) or, alternatively, by the fact that the 1a
position does not have nearest nitrogen neighbors but is
subject to an fcc-like environment of Fe atoms.
These classical ideas have been corroborated recently and
quantified by quantum-mechanical total-energy calculations
from first principles, and the Aachen group has also predicted,
by means of parameter-free enthalpy calculations, the existence of an enthalpically stable and also ferromagnetic
ternary nitride phase, namely RhFe3N. Based on densityfunctional theory and the GGA exchange-correlation functional,[24] the plausible chemical reaction [Eq. (1)] will be
FeRh þ Fe þ FeN ! RhFe3 N
ð1Þ
exothermic with an energy (enthalpy at 0 K) gain of about
DE = 7 kJ mol1 and a larger 23 kJ mol1 when the starting
materials are Rh, Fe, and FeN. Also, the GGA lattice
parameter of RhFe3N in space group Pm3̄m is predicted to
Angew. Chem. Int. Ed. 2005, 44, 7212 –7215
be a = 3.87 8, but one has to keep in mind that the GGA is
known to overestimate this value by ca. 1 %. In addition, the
calculations clearly predict that only the 1a position of
RhFe3N will be occupied by the rhodium atom, fully and
exclusively. In other words, RhFe3N, yet to be made, must be a
daltonide phase. Its magnetic saturation moment msA is
predicted to be 9.2 mB per formula unit, with local moments
of 0.96 and 2.76 mB for Rh and Fe, respectively.[24]
We have now succeeded in preparing RhFe3N for the first
time, and there are at least two synthetic routes. In the first
approach elemental Rh and Fe or e-Fe3N are mixed in a
planetary ball mill under Ar atmosphere for 40 h until the
products are nearly X-ray amorphous. Subsequent nitridation
in flowing ammonia at temperatures above 600 8C leads to
two-phase samples containing RhFe3N and FeRh. Although
the percentage of FeRh decreases upon increasing Fe/Rh
molar ratio, the relative intensities of the ternary nitride and
its unit cell parameter remain practically unaltered, thereby
indicating the absence of a significant homogeneity range in
the sense of (Rh1xFex)Fe3N.
Alternatively, one may follow the coupled-reduction
route[25, 26] developed by Klemm and co-workers, in which
the reduction of a metal oxide is accompanied by the
simultaneous formation of an intermetallic alloy. Ternary
nitrides such as RhFe3N are accessible by this method starting
with an NH3/H2 ammonolysis (1:1 ratio) of iron(iii) oxide,
Fe2O3, and elemental rhodium; in fact, the best results are
achieved upon using these starting materials. Nonetheless,
other iron oxides may also be utilized in combination with
rhodium, and even a solid mixture of iron/rhodium oxides
eventually leads to RhFe3N but with slightly lower yields. We
also note that the coupled-reduction principle also explains, in
retrospect, the early existence of NiFe3N, PdFe3N, and PtFe3N
by the original thermodynamic considerations made four
decades ago.[25, 26]
Probably because of the minute enthalpy of formation
(see above) of RhFe3N, its synthesis is always accompanied by
the formation of competing phases. At high temperatures the
second phase FeRh (Pm3̄m; a = 2.9825(3) 8) is observed,
whereas at lower reaction temperatures different iron nitrides
come into play, mainly e-Fe3N (P6322; a = 4.699(2) 8, c =
4.382(4) 8). The above-mentioned ammonolysis of oxide/
element precursors results in the highest RhFe3N yield
(85 wt. % or 75 mol %) at 1100 8C, whereas at around
1000 8C, the yield decreases to approximately 70 wt. %.
Neverthess, we believe that it should be possible to obtain
pure RhFe3N samples by non-equilibrium synthetic techniques.
The phase and structure determination is based on highresolution X-ray powder diffraction. An aluminum foil was
inserted into the X-ray beam line to reduce the effect of Fe
fluorescence radiation that results from the CuKa1 radiation. A
typical diffraction pattern including the Rietveld refinement[27] is shown in Figure 2.[28]
Although g’-Fe4N and RhFe3N have very similar lattice
parameters, the intensities of the 100 and 110 Bragg
reflections help to easily discriminate between the two
phases. Both reflections are quite weak for the binary nitride
but of medium intensity for the ternary nitride. Also, the 100
2005 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.angewandte.org
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Communications
Figure 2. Diffraction pattern and Rietveld refinement of RhFe3N (first
phase: Pm3̄m; a = 3.8292(2) F) and FeRh (second phase: Pm3̄m;
a = 2.9825(3) F). The vertical bars designate the positions of the Bragg
reflections.
intensity is almost twice as large as the 110 intensity for the
case of g’-Fe4N. But, on the other hand, the 100 intensity is
only about one-third as strong as the 110 intensity for RhFe3N.
As predicted from the theoretical calculations, the refinement
clearly indicates that among the two possible Wyckoff
positions (1a and 3c) only the 1a site is taken by the Rh
atom, and no substitution of Fe by Rh is found for position 3c.
In addition, position 1a is fully occupied by rhodium. Thus,
RhFe3N is indeed a daltonide compound.
In accord with the larger radius of rhodium (rRh = 1.34 8)
compared to that of iron (rFe = 1.24 8), the lattice parameter
of RhFe3N (a = 3.8292(2) 8) is also slightly larger (by about
1 %) than that of g’-Fe4N. The theoretical prediction by the
GGA method (a = 3.87 8) overestimates the lattice parameter by 1 %, which is characteristic of the GGA method.[24, 29]
The magnetic properties of RhFe3N samples, which are
unavoidably admixed with ca. 15 wt. % FeRh, were determined by SQUID magnetometry (MPMS-5S, Quantum
Design) in the temperature range 300–650 K at an applied
field of B0 = 0.01 T. At a temperature of 5 K, a hysteresis loop
was recorded in the field range 5 T. To allow for the
magnetic contribution of FeRh, a corresponding pure sample
of FeRh was also prepared and magnetically characterized
under the same conditions, and its signal was subtracted from
the original susceptibility data. Diamagnetic corrections and
contributions from conduction electrons cancel to a large
extent and have therefore been neglected.
The results of the magnetization measurements before
and after subtraction of the magnetic FeRh contribution are
presented in Figure 3 in a plot of the atomic magnetic moment
ma per formula unit as a function of the temperature. On
heating RhFe3N above 400 K one finds behavior typical of a
ferromagnetic compound, and TC is estimated to be
505(25) K. (TC is given by the intersection of the linear fit
of the steepest part of the magnetization curve with the T
axis.[30]) The accuracy is somewhat limited not only because of
the uncertainty of the amount of FeRh in the sample but also
because of the annealing effects of the latter phase.[31] The
hysteresis loop at 5 K is given in Figure 4. Its evaluation
results in a coercive field HC approximately one order of
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Figure 3. Variation of the magnetic moment ma per formula unit
RhFe3N at a field of B0 = 0.01 T as a function of the temperature, with
actual measured data (~), the FeRh contribution (~), and the RhFe3N
contribution (&).
magnitude larger than that of g’-Fe4N. This finding in
combination with a remanence of m0 MR = 0.52(1) T allows
us to categorize RhFe3N as a semihard ferromagnetic
material.[32] At B0 = 5 T the specific saturation magnetization
reaches ss = 163(2) A m2 kg1, corresponding to msa = 8.3(1) mB
per formula unit. Why HC of RhFe3N is enhanced relative to
that of g’-Fe4N is unknown as yet, and the 11 % discrepancy
between the experimental value for msa (8.3 mB) and the
theoretical (9.2 mB) is probably a result of a theoretically
overestimated (GGA) local moment of the Rh atom. Neutron
diffraction measurements are needed to fully answer that
question, and corresponding experimental studies are underway.
The density-functional (GGA) prediction of the bulk
modulus K0 (which serves as a simple measure of the phase"s
mechanical hardness) of the binary g’-Fe4N results in the
theoretical value of 172 GPa, which compares satisfactorily
with the recently determined experimental value[33] of
155(3) GPa; the small theoretical overestimation must be
considered quite acceptable for the GGA functional. Interestingly, the bulk modulus of RhFe3N is predicted even to be
larger, namely 187 GPa. Consequently, the ternary phase is
not only a magnetically semihard ferromagnet, but it is also
mechanically harder than g’-Fe4N itself; this is a truly
promising combination of physical properties for recording
heads.
In summary, the new ternary iron rhodium nitride RhFe3N
was synthesized and structurally characterized. RhFe3N is a
daltonide phase in which exclusively Rh occupies the 1a
position, just as predicted by density-functional theory. The
2005 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Angew. Chem. Int. Ed. 2005, 44, 7212 –7215
Angewandte
Chemie
Figure 4. Hysteresis loop of RhFe3N at 5 K (top) and part of the loop
showing remanent magnetization and coercive field (bottom). In the
bottom part, the solid lines only serve to guide the eye.
magnetic properties indicate that the new phase is a semihard
itinerant ferromagnet with a saturation moment of 8.3 mB per
formula unit and a Curie ordering temperature of TC 505 K.
Received: July 22, 2005
Published online: October 18, 2005
.
Keywords: ferromagnetism · rhodium · solid-state synthesis ·
structure elucidation · ternary nitrides
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Angew. Chem. Int. Ed. 2005, 44, 7212 –7215
2005 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.angewandte.org
7215
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