Metal-Urea Complex - A Precursor to Metal Nitrides

Article abstract of DOI:10.1111/j.1551-2916.2004.00352.x

A novel and general route to synthesize various metal nitrides (AlN, CrN, and ζ-Fe₂N) from metal-urea complexes is presented. These complexes, especially metal-urea chloride, have proved to be useful precursors to metal nitrides, because urea molecules construct a coordination sphere around the metal atom and form a stable structure, compared with the air-sensitive halide. Different anions in the second coordination sphere determine the reaction mechanism. The transformation from metal-urea chloride to nitride is thought to follow a nucleation-growth mechanism, while that from metal-urea nitrate is thought to follow a nitridation mechanism. We anticipate that this metal-urea complex will find applications in the fabrication of other, more complex, nitrides.

Full text of DOI:10.1111/j.1551-2916.2004.00352.x

J. Am. Ceram. Soc., 87 [3] 352–57 (2004)
Metal–Urea Complex—A Precursor to Metal Nitrides
Yu Qiu and Lian Gao*,†
journal
State Key Lab of High Performance Ceramics and Superfine Microstructure, Shanghai Institute of Ceramics,
Chinese Academy of Sciences, Shanghai 200050, People’s Republic of China
A novel and general route to synthesize various metal nitrides
(AlN, CrN, and ␨-Fe2N) from metal–urea complexes is pre-
sented. These complexes, especially metal–urea chloride, have
proved to be useful precursors to metal nitrides, because urea
molecules construct a coordination sphere around the metal
atom and form a stable structure, compared with the air-
sensitive halide. Different anions in the second coordination
sphere determine the reaction mechanism. The transformation
from metal–urea chloride to nitride is thought to follow a
nucleation–growth mechanism, while that from metal–urea
nitrate is thought to follow a nitridation mechanism. We
anticipate that this metal–urea complex will find applications
in the fabrication of other, more complex, nitrides.
to synthesize and handle. It is very desirable to develop a method
that uses inexpensive precursor materials and is applicable to
preparing various metal nitrides.
In this work, a novel route has been demonstrated that permits
the simple and general synthesis of AlN, CrN, and Fe2N from
metal–urea complex precursors. An important synthesis step is the
preparation of the metal–urea complex, which is a chemically
stable and inexpensive compound, compared with other precur-
sors, such as anhydrous chloride12 or organometallic materi-
als.13,14 Urea replaces the coordinate water around the metal atom
and decomposes at elevated temperatures, leaving metal chloride
adducts with NH3 (in the case of metal–urea chloride) or fine
oxide particles (in the case of metal–urea nitrate) before the
complex finally transforms to metal nitride under a flowing NH3
atmosphere.
I. Introduction
ETAL NITRIDES are promising materials, because they have a
wide variety of excellent properties, such as high hardness,
II. Experimental Procedure
M
(1) Materials and Processes
good wear resistance, high melting point, high-temperature stabil-
ity, high thermal conductivity, and large magnetization values, as
well as good electrical and optical properties. For example,
aluminum nitride (AlN) is used as an important ceramic substrate
material because of its high thermal conductivity (170–260
W⅐mϪ1⅐KϪ1 for hexagonal- (h-) AlN and 250–600 W⅐mϪ1⅐KϪ1
for cubic- (c-) AlN).1 AlN also is known as a piezoelectric material
with a high acoustic velocity and as a refractory compound that
resists oxidation.2 Chromium nitride (CrN) shows a paramagnetic
to antiferromagnetic transition at 280 K and has been identified as
an excellent coating material because of its stability, high hard-
ness, high melting point, and high wear resistance.3,4 Iron nitride
(FeNx) has attracted extensive research interest for a long time
because of its superior magnetic properties as well as improvement
in the corrosion and abrasion resistance of materials surfaces.
Many processing methods used to synthesize metal nitride have
been reported in recent literature, such as direct nitridation of the
element,5,6 oxide7 and halide routes,8 plasma-based processes,9,10
and heat-treatment of organometallic precursors under ammonia
gas.11 Synthesis technologies should result in the production of
materials that have optimum properties for performing specified
functions. One of the most critical factors in the synthesis of metal
nitrides is the starting material. There is increasing interest in
developing alternative chemical approaches to metal nitrides using
tailored molecular precursors, to lower the growth temperature,
and to achieve molecular control over the crystal growth and the
processing of the materials. Numerous efforts have been focused
on organometallic compounds. However, the applications of orga-
nometallic compounds have been limited, because they are expen-
sive, toxic, extremely unstable (even explosive), and very difficult
The starting chemicals used in this work were urea (CO(NH2)2,
Ͼ99.0%), aluminum chloride (AlCl3⅐6H2O, Ͼ97%), aluminum
nitrate (Al(NO3)3⅐9H2O, Ͼ99%), chromium nitrate (Cr(NO3)3⅐
9H2O, Ͼ99%), iron(III) nitrate (Fe(NO3)3⅐9H2O, Ͼ98.5%), anhy-
drous ethanol (C2H5OH, Ͼ99.5%), ammonia (NH3, Ͼ99%), and
nitrogen (N2, Ͼ99.999%).
Metal chloride or nitrate was dissolved in anhydrous ethanol to
obtain a concentrated solution. The solution then was added
dropwise into a saturated urea/ethanol solution at 75°–80°C with
stirring, until the final salt/urea molar ratio reached 1:6 (for
chloride) or 1:9 (for nitrate). The reaction gave a water-soluble
precipitate of a metal–urea complex, which was separated by
filtration and dried at 80°C. The precipitate was characterized and
used directly to synthesize metal nitrides without further purifica-
tion. Some of the complex for each batch (5.0 g) was loaded into
a quartz boat and heated under flowing NH3 or N2 (0.5 L/min) at
300°–1000°C for 1–15 h in a tube furnace to obtain metal nitrides.
NH3 gas was allowed to pass through the furnace for 1 h to remove
the air in the furnace before the furnace was turned on, and the
heating rate was kept at 10°C/min before it reached the set
temperature. The resultant powder was collected after the furnace
was cooled down under the flow of NH3 gas. Similar procedures
were conducted when high-purity N2 was used. Table I summa-
rizes the experimental conditions used.
(2) Characterization
The compositions of the metal–urea complexes were deter-
mined using chemical and elemental analysis (Model Vario EL,
Elementar, Germany). The complex structures were characterized
using Fourier-transform infrared spectroscopy (FTIR; Model
NEXUS, Nicolet Instrument Corp., Madison, WI) and high-
resolution solid-state 27Al magic angle spinning and 13C cross-
polarization/magic angle spinning nuclear magnetic resonance
(CP/MAS-NMR; Model Avance 300, Bruker Instruments, Inc.,
Billerica, MA) at room temperature. Simultaneous thermogravim-
etry–differential scanning calorimetry–mass spectroscopy (TG–
DSC–MS; Model STA 449C, Netzsch, Bayern, Germany, and
Model ThermoStar, Balzers, Liechtenstein) was used to record the
T. Parthasarathy—contributing editor
Manuscript No. 186486. Received December 16, 2002; approved October 20,
2003.
*Member, American Ceramic Society.
†Author to whom correspondence should be addressed.
352

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Metal–Urea Complex—A Precursor to Metal Nitrides
353
Table I. Summary of Experiments on the Synthesis of
Metal Nitride: Experimental Conditions and Results of
XRD Analysis
Table II. Absorption Maximums Observed for Urea and
Metal–Urea Complexes and Their Assignments
Observed frequency (cmϪ1 †
AUC CUN
)
Temperature
Time
(h)
Type of vibration
Urea
FUN
Complex
Gas
(°C)
XRD phase†
␯as(NH2)15
3435(s) 3473(s) 3460(s) 3460(s)
3346(s) 3365(s) 3357(s) 3357(s)
3259(sh) 3192(m) 3195(m) 3197(m)
␯s(NH2)15
AUC
NH3
800
800
900
900
1000
800
500
700
900
900
1000
700
300
500
600
600
1000
5
8
h-AlN Ͼ c-AlN
h-AlN, c-AlN
h-AlN Ͼ c-AlN
h-AlN, c-AlN
h-AlN, c-AlN
(ClAlNH)cn
Third N–H band15
␯(CO)15,16
3
1684(s)
/
/
/
␯(CN) ϩ ␦(NH2)16
␯(CO) ϩ ␦(NH2)15,16
␯as(CN)15,17
␯(NOϪ3 )
1628(sh) 1628(s) 1626(m) 1630(s)
1600(s) 1589(sh) 1572(s) 1572(s)
1468(s) 1498(s) 1498(m) 1498(m)
5
5
N2
NH3
5
CUN
5
Cr2O3
/
/
1385(s) 1385(s)
␳(NH2)15,17
␯s(CN)15,17
1155(m) 1172(m) 1163(w) 1163(w)
1057(vw) 1036(m) 1032(w) 1030(w)
5
CrN Ͼ Cr2O3
CrN Ͼ Cr2O3
CrN
CrN ϾϾ Cr2O3
Cr2O3 ϾϾ CrN
Fe2O3
␨-Fe2N Ͼ Fe2O3
␨-Fe2N
Fe2O3
␣-Al2O3
1
␯(NOϪ3 )
/
/
827(vw) 825(vw)
787(vw) 769(m) 764(vw) 766(vw)
632(m) 634(w) 613(w)
557(s) 557(m) 552(w) 544(w)
15
10
5
‡
␶(ONCN)17
␦(NCO)15
N2
NH3
/
␦(NCN)15
FUN
AUN
3
5
†s is strong, m is medium, w is weak, sh is shoulder, and / is not found.
1
N2
NH3
5
8
␦(NH2) vibration,16 the formation of oxygen-to-metal coordinate
bonds (CϭO 3 M) in AUC, CUN, and FUN are determined.
To further confirm the structure of the metal–urea complexes,
the 13C CP/MAS-NMR and 27Al MAS-NMR spectra of AUC were
measured and found to have only one sharp resonance peak at ␦ of
161.7 ppm for 13C NMR (Fig. 2(a)) and at ␦ of Ϫ7.6 ppm for 27Al
NMR (Fig. 2(b)). The 13C peak is assigned for the carbon atoms in
urea. The 27Al peak corresponds to the octahedral AlO6 coordinate
bond, which proves the existence of oxygen-to-metal bonds.
Meanwhile, the extremely narrow half-height width (42.5 Hz) of
the 27Al resonance peak shows a highly symmetrical structure.
Thus, the structure of AUC (Fig. 3) can be expected according to
the analysis above. Urea molecules locate in the first coordination
†Ͼ is more than; ϾϾ is much more than. ‡Characterized using FTIR.
thermal decomposition of aluminum–urea chloride under argon
gas flow of 20 mL/min at a heating rate of 10°C/min.
The X-ray diffraction patterns of the synthesized products were
obtained using powder X-ray diffractometry (XRD; Model D/Max
2550V, Rigaku, Tokyo, Japan) with CuK␣ radiation (␭ ϭ 1.5406
Å). Scanning electronic microscopy (SEM; Model JSM-6700F,
JEOL, Tokyo, Japan) and transmission electron microscopy
(TEM; Model 200CX, JEOL) were performed to observe the
microstructure of the products in selected cases.
III. Results and Discussion
(1) Characteristics of the Metal–Urea Complexes
Metal–urea complexes are water-soluble powders with various
colors that depend on the metal salt used. Aluminum–urea
chloride (Al[OC(NH2)2]6Cl3, AUC) and aluminum–urea nitrate
(Al[OC(NH2)2]6(NO3)3, AUN) are white powders, while chrom-
ium–urea nitrate (Cr[OC(NH2)2]6(NO3)3, CUN) and iron-urea
nitrate (Fe[OC(NH2)2]6(NO3)3, FUN) are dark green and reddish
brown powders, respectively. Measured and (calculated) chemical
and elemental analyses are as follows: AUC, 5.59 (5.46) alumi-
num, 21.84 (21.54) chlorine, 14.56 (14.60) carbon, 33.52 (34.05)
nitrogen, and 4.92 (4.90) hydrogen; CUN, 8.51 (8.69) chrom-
ium, 12.28 (12.04) carbon, 34.76 (35.11) nitrogen, and 4.10 (4.04)
hydrogen; and FUN, 9.60 (9.27) iron, 12.11 (11.97) carbon, 34.08
(34.89) nitrogen, and 4.18 (4.02) hydrogen. FTIR spectra of AUC,
CUN, and FUN are shown in Fig. 1, and the main bands in the
infrared spectra are summarized in Table II. Through this assign-
ment, particularly by the absence of a carbonyl band at 1684
cm–1,15 and by the decrease in the wavenumber of the ␯(CO) ϩ
Fig. 2. (a) Solid-state 13C CP/MAS-NMR and (b) 27Al MAS-NMR
spectra of AUC powder, operated at room temperature at 78.172 and 75.43
Fig. 1. Infrared spectra of (a) urea, (b) AUC, (c) CUN, and (d) FUN.
MHz, respectively. Spinning sidebands are denoted by an asterisk ( ).
*

354
Journal of the American Ceramic Society—Qiu and Gao
Vol. 87, No. 3
Fig. 3. Expected structure of AUC complex.
Fig. 5. Ion current intensity curves of AUC recorded using MS.
sphere near the aluminum atom, while the chloride anions locate in
the outer coordination sphere. This structure stabilizes the vivid
reactive AlCl3 in an ambient atmosphere. Similar structures can be
expected for CUN, FUN, and AUN.
temperature increases, the elimination of HCl takes place (Eq. (7))
with simultaneous formation of more stable oligomeric
(Cl2AlNH2)2.19,20
(2) Thermal Behavior of the Metal–Urea Complex
AUC was analyzed using TG–DSC–MS to reveal the effect of
the urea group during the decomposition of the complex. The
TG–DSC curves of the thermal decomposition of AUC under
argon atmosphere are shown in Fig. 4, and the temperature
variation of the major species formed as determined using MS is
shown in Fig. 5. Comparison of the TG and DSC curves leads to
the conclusion that the diagrams contain very complicated thermal
effects. The decomposition reaction takes place in three stages.
The first stage starts at 180°C, with three fast and strong endo-
thermic processes at 256°, 276°, and 305°C. The weight loss at the
first stage is characterized as 63.3%, and the main decomposition
products are identified by MS as NH2, NH3, HNCO, and H2NCO
(Fig. 5). The second stage is a weight loss (13.9%) between 350°
and 650°C. The third stage is a moderate weight loss (15.5%)
between 650° and 1000°C, which is accompanied by two endo-
thermic peaks at ϳ818° and 930°C. In consistence with these two
processes, the ion current intensity of HCl is detected to increase
successively above 300°C. NH3, NH2, and H2NCO also are
detected in the last two stages. The thermal behavior is explained
as follows.
(3) In the temperature range of 650°–1000°C, (ClAlNH)n
forms.2,21
(Cl2AlNH2)2 and (ClAlNH)6 are the most important species in
the chemical vapor deposition (CVD) process.20 Our research
results also prove that (ClAlNH)6 and (ClAlNH)4, with hexagonal
and cubic geometry, respectively, are important in the formation of
h- and c-AlN, which is discussed below. Jegier et al.22 also have
proposed a solid ((HGaNH)n) remains after the heat-treatment of
(H2GaNH2)3. The further decomposition of (ClAlNH)n leads to
AlN, which involves the elimination of HCl.23 The reaction in this
process is presented as Eqs. (8)–(10) in Table III. The decompo-
sition process of AUC is more complicated than proposed above:
the AlCl3 and NH3 decompose, and other species, such as
Cl2AlNH2, ClAlNH, and oligomeric (ClAlNH)n (n ϭ 2, 3),
form.23 We have examined the resulting amorphous AlN powder
after the pyrolysis at 800°C for 5 h under an inert N2 atmosphere
(see Table I). The infrared spectrum gives vibration bands of
␯(NH) at 3429 cm–1,24,25 of Al–N2 absorption at 2160 cm–1,26 of
␦as(NH) at 1628 cmϪ1 16,24,25
and of others in the 1600–1300
,
cmϪ1 region,26 i.e., ␯(AlN) at 719 cm–1,25,27,28 and ␯(AlCl) at 570
cm–1.25,29 This band assignment is evidence of intermediate solid
compounds (Cl2AlNH2)2 and (ClAlNH)n (n ϭ 4, 6) in the reaction.
When NH3 is used as the carrier gas, the decomposition behavior
is even more complicated. Weight losses and possible reactions
during the decomposition of AUC are listed in Table III.
In the case of metal–urea nitrate, the thermal decomposition
behavior is quite different from metal–urea chloride. Because of
the presence of the vigorous oxidizing group NOϪ3 in the outer
coordination sphere of the complex besides the reducing urea
group in the inner coordination sphere, fine oxide particles are
formed after decomposition (Table I). The thermal decomposition
of FUN under air has been studied by Carp et al.,16 and their
conclusion is further confirmed by our present study of CUN and
FUN. Both studies give oxide as the major phase under an inert N2
atmosphere. The transformations of CUN and FUN are discussed
below.
(1) During the first stage, the complex decomposes to AlCl3,
according to Eq. (1) in Table III.18 During the decomposition, a
simultaneous degradation of urea takes place (Eqs. (2) and (3)).16
Subsequently, AlCl3 is converted to the AlCl3⅐xNH3 adduct
according to Eq. (4),19 where x is determined as 3 using TG
analysis.
(2) AlCl3⅐NH3 forms when the temperature is Ͼ300°–400°C
(Eqs. (5) and (6)),19 which is unstable above 600°C. As the
(3) Synthesis Mechanisms of Metal Nitrides
The metal nitrides, including h- and c-AlN, CrN, and ␨-Fe2N,
are derived from the thermal treatment of the metal–urea com-
plexes under a flowing NH3 atmosphere. Figure 6 shows the XRD
patterns of metal nitrides obtained under various reaction condi-
tions. Nanocrystalline AlN can be synthesized at 800°C, which is
Fig. 4. TG and DSC curves of AUC at a heating rate of 10°C/min.

March 2004
Metal–Urea Complex—A Precursor to Metal Nitrides
355
Table III. Weight Loss and Possible Reactions during Thermal Decomposition of
AUC under Argon
Temperature range (°C)
Weight loss (%)†
Possible reactions
150–350
63.3 (62.64)
(1) Al[OC(NH2)2]6Cl3 3 AlCl3 ϩ 6CO(NH2)2
(2) CO(NH2)2 3 NH3 ϩ HNCO
(3) CO(NH2)2 3 NH2 ϩ H2NCO
(4) AlCl3 ϩ xNH3 3 AlCl3⅐xNH3 (x ϭ 3)
(5) AlCl3⅐3NH3 3 AlCl3⅐2NH3 ϩ NH3
(6) AlCl3⅐2NH3 3 AlCl3⅐NH3 ϩ NH3
350–650
13.9 (14.28)
15.5 (14.77)
1
(7) AlCl3⅐NH3 3 (Cl2AlNH2)2 ϩ HCl
2
1
1
650–1000
(8) (Cl2AlNH2)2 3 (ClAlNH)n(n ϭ 4, 6) ϩ HCl
2
1
n
(9) (ClAlNH)4 3 c-AlN ϩ HCl
4
1
(10) (ClAlNH)6 3 h-AlN ϩ HCl
6
†Measured and (calculated).
lower than the traditional chemical routes (usually Ͼ1200°C). The
AlN synthesized under NH3 is a mixture of hexagonal and cubic
phases, with particle sizes mostly Ͻ25 nm, as observed using TEM
(Fig. 7(a) and (b)). The cubic phase increases with increases in
temperature or prolonged reaction time, and the amount is almost
equal to that of the hexagonal phase when the temperatures is
Ͼ800°C or the heating time is 8 h. The broadening of the XRD
peaks makes it difficult to separate cubic (311) and (440) from
hexagonal (101) and (103) peaks; therefore, selective area diffrac-
tometry (SAD) is used (Fig. 7 inset) and the results support the
increase of c-AlN with temperature. The intensities of the (311),
(400), and (440) planes of c-AlN become apparently much
stronger under higher temperature. This result is notable, because
c-AlN is an unstable phase at a high temperature. Timoshkin et al.20
have concluded that, in CVD of AlN from AlCl3⅐NH3, the preferable
route is (Cl2AlNH2)2 3 c-(ClAlNH)4 or h-(ClAlNH)6 3 AlN,
wherein the formation of (ClAlNH)6 is thermodynamically fa-
vored. That route is the possible reason why h-AlN is easier to
prepare. In the present study, the formation of c-(ClAlNH)4
cluster, which is the basic unit for the growth of c-AlN, may be
thermally favored under heating conditions, according to the
reaction20
for FUN, which fully converts to Fe2N at 600°C (Fig. 6(c)), while
the reaction time is the most important parameter for CUN, which
needs a rather prolonged reaction time (15 h) for its complete
conversion, even at a temperature as high as 900°C (Fig. 6(b)).
This result can be explained by the stronger affinity of chromium
atom to oxygen atom. The shifts of ␦(NCO) and ␦(NCN) to higher
frequency are the evidences of a stronger O 3 Cr coordinate bond.
The typical morphology of CrN and Fe2N observed using SEM is
presented in Figs. 8(a) and (b), respectively. SEM shows that the
synthesis process produces porous, spongelike materials that
contain fine nitride particles and large holes and voids. TEM
clearly shows the nitride particles are angular and Ͻ1 ␮m, with
smooth edges, which indicates good crystallization. Holes and
voids are produced by gas that is released during the decomposi-
tion. These nitride materials are porous and have a high surface
area, and they can be used as the matrix materials for composite
materials or carrier materials for catalysts. Their morphology is
distinct from that of AlN, which indicates a synthesis mechanism
different from that of AUC. Different anions in the outer coordi-
nation sphere result in different decomposition and transformation
mechanisms. To prove this assumption, AUN has been prepared
and studied. AUN transforms to ␣-Al2O3 after it is heated at
1000°C for 8 h (Table I), which results in the same morphology as
that of Fe2N and CrN.
͑ClAlNH͒6 3 1͑ClAlNH͒
⌬H͑298.15͒ ϭ 51.7 kJ⅐molϪ1
1
4
2
2
Consequently, the formation of metal nitride from metal–urea
chloride and nitrate can be assumed following two different
mechanisms: (I) nucleation–growth and (II) nitridation, which are
schematically represented in Fig. 9.
The assembly of (ClAlNH)n (n ϭ 4, 6) cluster to corresponding
c- or h-AlN nucleus produces nanocrystalline AlN, which is a
process similar to the nucleation–growth mechanism.
The decomposition of CUN and FUN gives mainly metal oxide
under NH3 atmosphere, before the nitridation of oxide by NH3 gas,
as revealed by the XRD analysis (Table I). In the nitridation
process, the reaction temperature is the most important parameter
In mechanism I, the formation of AlN nanoparticles includes
four steps: decomposition, formation of AlN monomers (single
molecule in gas phase), nucleation, and growth of AlN particles.
AUC decomposes to intermediate compounds (ClAlNH)n (n ϭ 4,
Fig. 6. Powder XRD patterns of (a) AlN, (b) CrN, and (c) Fe2N synthesized under various conditions.

356
Journal of the American Ceramic Society—Qiu and Gao
Vol. 87, No. 3
Fig. 7. TEM micrographs for the AlN powders synthesized at (a) 800°
and (b) 900°C for 5 h. Insets are the associated SAD patterns. Innermost
rings can be indexed to the h(100) plane, and the other rings from the inner
to the outer can be indexed to h(002), c(311), c(222), c(400), h(110),
c(440), and h(112).
Fig. 8. SEM micrograph of (a) CrN synthesized at 900°C for 15 h and (b)
Fe2N synthesized at 600°C for 1 h.
temperature Ͼ200°C, replaces the water molecule in the inner
coordination sphere. AUC loses urea to yield oligomeric
(ClAlNH)n (n ϭ 4, 6), which can transform to nanocrystalline c-
and h-AlN at elevated temperature under flowing NH3 atmosphere
through the nucleation–growth mechanism. Metal–urea nitrate
decomposes mainly to fine oxide particles because of the presence
of oxidative nitrate ion, and it further transforms to nitride by
reacting with NH3. It has been proved that metal–urea complex is
a useful precursor to synthesize various metal nitrides. Metal–urea
chloride, especially, is a good candidate for a precursor material to
synthesize nanocrystalline metal nitride at a relatively low tem-
perature. The metal–urea complex route appears to be very
promising for elaboration of more complex nitrides.
6), which are the most important substances and the basic clusters
(units) in this process. The formation of AlN molecules from
(ClAlNH)n occurs via the elimination of HCl, with subsequent
nucleation and growth processes. The decomposition of
AlCl3⅐NH3 adduct or the assembly of (ClAlNH)n can take place at
the surface of the initial particles. Thus, the particle growth occurs
through the following reactions.
(1) Surface reaction and elimination of HCl molecules:
AlCl3⅐NH3—S 3 1n͑ClAlNH͒n—S ϩ 2HCl͑g͒
(2) Decomposition of intermediate solid compounds, assem-
bly, or diffusion through the solid phase, and elimination of HCl
molecules:
͑ClAlNH͒n—S 3 nAlN͑s͒ ϩ nHCl͑g͒
where S is an adsorption or assembly site on the particle surface.
In mechanism II, metal–urea nitrates first decompose to porous
and unmethodical oxide powders. The oxide powders then react
with NH3 gas to form nitrides. It is concluded that the property of
the anions in the outer coordination sphere of the complex results
in the various reaction mechanisms.
IV. Conclusions
Two types of metal–urea complexes, metal–urea chloride and
nitrate, are prepared as the starting materials for the synthesis of
metal nitrides. The reductive urea group, which can degrade at a
Fig. 9. Transformation mechanism from metal–urea complex to metal
nitride.

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Metal–Urea Complex—A Precursor to Metal Nitrides
357
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Study of the Coordination Compound [Fe(urea)6](NO3)3,” Thermochim. Acta, 390,
169–77 (2002).
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