Zirconium pivalate: Synthesis and volatility

Article abstract of DOI:10.1134/S0036023606110118

Zirconium pivalate is synthesized using an original procedure starting from hydrous zirconium oxochloride. The volatility of zirconium pivalate is characterized for the first time in the range from 130 to 210°C using mass spectrometry: the enthalpies of s

Full text of DOI:10.1134/S0036023606110118

ISSN 0036-0236, Russian Journal of Inorganic Chemistry, 2006, Vol. 51, No. 11, pp. 1750–1754. © Pleiades Publishing, Inc., 2006.
Original Russian Text © N.P. Kuz’mina, A.E. Altsybeev, I.P. Malkerova, A.S. Alikhanyan, I.E. Korsakov, 2006, published in Zhurnal Neorganicheskoi Khimii, 2006, Vol. 51,
No. 11, pp. 1859–1864.
COORDINATION
COMPOUNDS
Zirconium Pivalate: Synthesis and Volatility
a
a
b
b
a
N. P. Kuz’mina , A. E. Altsybeev , I. P. Malkerova , A. S. Alikhanyan , and I. E. Korsakov
a
Moscow State University, Vorob’evy gory, Mosocw, 119992 Russia
b
Kurnakov Institute of General and Inorganic Chemistry, Russian Academy of Sciences,
Leninskii pr. 31, Moscow, 119991 Russia
Received December 7, 2005
Abstract—Zirconium pivalate is synthesized using an original procedure starting from hydrous zirconium
oxochloride. The volatility of zirconium pivalate is characterized for the first time in the range from 130 to
2
10°C using mass spectrometry: the enthalpies of sublimation and the vapor pressure are determined as func-
tions of temperature. Experiments are carried out to compare the thermal stabilities of zirconium pivalate and
zirconium acetylacetonate. The property of zirconium pivalate to serve as the precursor for MOCVD of zirconia
films is shown.
DOI: 10.1134/S0036023606110118
Volatile zirconium compounds are interesting as zirconium pivalate is also expected to be volatile. To
they may serve as precursors for the chemical vapor our knowledge, there is no data on zirconium pivalate.
deposition (CVD) of ZrO and yttrium-stabilized cubic
2
Our goals in this work were to synthesize Zr(Piv)4,
to characterize its volatility, and to test it as the precur-
ZrO (YSZ), which are used as heat-resistant coatings,
2
oxygen sensors, and buffer layers [1]. Volatile zirco- sor for the CVD technology. In addition, we carried out
nium β-diketonates, in particular, tetrakisacetylaceto- a comparative investigation of the thermal stability and
nate Zr(acac) , are the most frequently used precursors thermohydrolysis resistance of zirconium acetylaceto-
4
for film manufacturing by CVD [2]. Amorphous ZrO2 nate and zirconium pivalate.
films have recently attracted the attention of researchers
because of their use as films with high dielectric con-
EXPERIMENTAL
stants in semiconductor devices [3]. Volatile precur-
sors, which readily decompose to ZrO at low tempera-
The starting chemicals used were zirconium
2
tures or in the presence water vapor, are required for the oxochloride octahydrate (chemically pure grade, Rus-
CVD or atomic layer deposition (ALD) of such films. sia), acetic acid (chemically pure grade, Russia), acetic
In the ALD technology, the deposition is most fre- anhydride (chemically pure grade, Russia), and pivalic
acid (from Merck). The solvents used (m-xylene, tolu-
ene, benzene) were absolutized by distillation from
sodium hydroxide.
quently carried out from anhydrous zirconium halides,
nitrates, or alkoxides [4]. The high moisture sensitivity
of these compounds provides for the formation of
amorphous ZrO films at 250–350°ë but creates some
Zr(MeCOO) synthesis. ZrOCl · 8H O (20 mmol) was
2
4
2
difficulties with manipulation and storage. Zirconium dissolved under reflux in 100 mL of an MeCOOH +
β-diketonates have higher thermal stability and mois- (åÂëé) O mixture (1 : 9). After the mixture was
2
ture resistance; therefore, when they are used for the
deposition of amorphous films, strong oxidizers (O3)
cooled to room temperature, a white crystalline deposit
appeared; then, it was filtered and vacuum dried at
are required in order to keep films from carbon contam- 50°ë: Yield: ~80%.
ination [5]. Search for new precursors for preparing
For ZrC H O anal. calcd. (%): C, 29.36; H, 3.67;
8
12
8
amorphous ZrO films is underway; in our opinion, zir-
2
Zr, 27.8.
conium carboxylates may be used for this purpose.
Found (%): C, 29.52; H, 3.72; Zr, 27.6.
–
1
Zirconium carboxylates (Zr(RCOO) ) have been
4
IR spectrum (ν, cm ): 1540, 1455 (ν
–
) .
COO
described long ago, but relevant data are few [6]. It is
known that zirconium carboxylates decompose to
oxide at temperatures below 500°ë and readily hydro-
lyze in a moist atmosphere. The volatility of zirconium
carboxylates has not been mentioned in the literature.
Based on the volatility data for pivalates (trimethylace-
1H NMR spectrum (δ, ppm): 2.09, 1.96, 1.89, 1.84,
(
ëç3).
Zr(Piv) synthesis was carried out in two solvents
4
(
m-xylene or toluene) using the same procedure. A sus-
pension of zirconium acetate (15 mmol) and pivalic
acid (60 mmol) was refluxed in 100 mL of the solvent
tates) of some other elements (Ag(Piv) [7], Ln(Piv)3,
Ba(Piv) , and Cu(Piv) [8], where HPiv is pivalic acid), until a clear solution was formed. The solvent was dis-
2
2
1
750

ZIRCONIUM PIVALATE: SYNTHESIS AND VOLATILITY
1751
tilled off in two stages: first the solvent–acetic acid Table 1. Sputtering parameters
azeotrope was distilled off and then the excessive sol-
vent was. A yellow precipitate was vacuum dried at Sublimation temperature, °C
250–300
5
0°ë. Yield: ~90%.
The products obtained from toluene and m-xylene Deposition temperature, °C
500–800
had different compositions.
In toluene, Zr(åÂëéé) (Piv) was produced.
Pressure, mbar
For ZrC H O anal. calcd. (%): C, 35.77; H, 4.95; O2 partial pressure, mbar
12
5
3
1
1 18 8
Zr, 24.4
Sputtering time, h
0.5
Found (%): C, 35.60; H, 4.95; Zr, 24.6.
–1
IR spectrum (ν, cm ): 1540, 1560, 1455, 1440
νCOO ).
(
–
and exposed at a fixed temperature (200–400°ë) for
.5 h in a water-saturated oxygen flow. The composi-
0
1H NMR spectrum (δ, ppm): 1.98, 1.88, 1.85 (CH3); tion of the thermohydrolysis products was determined
.24, 1.03 (t-Bu).
1
by elemental analysis.
In m-xylene, Zr(Piv) was produced.
MOCVD. Yttrium-stabilized zirconia (YSZ) films
were deposited using CVD in a hot-walled reactor
equipped with a powder dropping dispenser [11]. The pre-
cursor used was a Zr(Piv) + Y(Piv) (80 : 20 mol/mol)
mixture. The substrates used were polished single-crys-
talline MgO (100) wafers. The main process parame-
ters are listed in Table 1.
4
For ZrC H O anal. calcd. (%): C, 48.48; H, 7.27;
2
0 36 8
Zr, 18.38.
4
3
Found (%): C, 48.01; H, 7.07; Zr, 18.4.
–
1
IR spectrum (ν, cm ): 1560, 1440 (ν
–
) .
COO
1H NMR spectrum (δ, ppm): 1.23, 1.09 (t-Bu).
Zr(acac) was prepared by the vacuum sublimation
X-ray powder diffraction patterns from the films
were recorded on a DRON-3M diffractometer using
4
(
180°ë, 0.01 mmHg) of a hydrated Zr(acac) · nH O Ni-filtered CuK radiation.
4
2
α
sample, which was prepared as in [9].
Y(Piv)3 was synthesized using a procedure
described in [10] and refined by sublimation (330°ë,
RESULTS AND DISCUSSION
0
.01 mmHg).
The known methods for the synthesis of zirconium
tetrakiscarboxylates are based on ligand exchange reac-
tions [6]:
Methods. Carbon and hydrogen in the compounds
were determined by elemental microanalysis on a CHN
analyzer. Zirconium was determined gravimetrically.
ZrX + 4RCOOH
Zr(RCOO) + 4HX.
(1)
4
4
IR spectra were recorded as Nujol or hexachlorob-
utadiene mulls on a PE-FTIR-1600 spectrometer in the
range 400–4000 cm .
Thermal analysis in air was performed on a Q 1500
derivatograph from 20 to 600°ë; sample sizes were
Anhydrous zirconium chloride ZrCl was proposed
4
–
1
for use as the ZrX reagent [12]. This synthesis version
4
is inconvenient: ZrCl is readily hydrolysable, and the
4
synthesis should be carried out under strictly anhydrous
conditions. According to [13], reaction (1) yields basic
carboxylate when oxochloride ZrOCl · 8H O is used.
5
0−100 mg; the heating rate was 10 K/min.
1H NMR spectra were recorded on a Bruker AC
2
2
A synthesis version is known where Zr(acac) , which
4
2
00P spectrometer (CDCl –TMS).
3
is hydrolysis resistant, is used instead of zirconium chlo-
Zr(Piv) vaporization was studied using the Knudsen
4
ride [6]. However, mixed-ligand complexes of composi-
effusion technique with the mass-spectrometric analysis tion Zr(acac) (RCOO)
can form in this version: che-
x
4 – x
of the gas phase on an MS 1301 mass spectrometer with late acetylacetonates are more stable than carboxylates,
a mass number range extended to 1300 emu. Standard and acetylacetone itself is a very weak acid. In general, a
molybdenum Knudsen cells, in which the ratio of the danger exists that mixed-ligand carboxylates will form in
vaporization surface area to the effusion surface area all processes based on reaction (1). A way to avoid this
was ~600, were used in the experiments. Temperature danger is to completely remove the HX product from
was measured with a Pt/Pt(Rh) thermocouple and the reaction mixture.
adjusted with an accuracy of ±1 K.
This problem may be solved if zirconium acetate is
Vacuum sublimation was carried out in a glass
ampule inside a gradient-free tubular electrical furnace
at 0.5 mmHg for 60 min.
used as the precursor and if acetic acid produced by
exchange reaction (1) is removed from the reaction
mixture in the form of an azeotrope with the solvent.
Thermohydrolysis. A quarts boat with a sample The efficiency of this approach was demonstrated in the
~100–150 mg) was placed in a horizontal glass reactor synthesis of bismuth carboxylates [14]: benzene and
(
RUSSIAN JOURNAL OF INORGANIC CHEMISTRY Vol. 51 No. 11 2006

1
752
KUZ’MINA et al.
Table 2. Ion intensities in the mass spectrum of Zr(Piv) at difference in stability between bismuth pivalate and
4
1
89°C (U = 70 eV)
Ion
bismuth acetate than between the respective zirconium
compounds. This also the reason why bismuth trispiv-
alate is produced by the exchange reaction in toluene.
If this interpretation is valid, better removal of acetic
acid from the reaction sphere may provide higher
degrees of substitution of pivalate for acetate ligands.
To displace the equilibrium of reaction (3) to the
right, we used m-xylene instead of toluene: the azeo-
trope of m-xylene with acetic acid has a higher boiling
temperature (115°ë) and contains 72.5% MeCOOH.
Synthesis in this solvent yielded zirconium tetrakispiv-
alate.
Our two-stage version of the synthesis differs from
the known processes in the following: at the first stage,
available hydrous zirconium oxochloride is the reagent;
at the second stage, m-xylene, which forms the high-
boiling and MeCOOH-rich azeotrope with acetic acid,
is used to provide the full substitution of pivalic ligands
for acetate ligands. This version may be recommended
I, %
[
[
[
[
[
[
[
[
[
[
Zr(Piv)O]+
5.9
7.8
4.5
5.4
9.6
6.5
2.2
3.8
5.9
Zr(Piv)OH]+
Zr(Piv)O2]+
+
Zr(Piv)O C]
2
+
Zr(Piv)O C(CH ) ]
2
3 2
Zr(Piv)2]+
+
Zr(Piv) O ]
2
2
+
Zr(Piv) O C]
2
2
+
Zr(Piv) O C(CH ) ]
2
2
+
3 2
Zr(Piv)3]
100
toluene, which form azeotropes with acetic acid, were
used to eliminate it.
In our opinion, it is pertinent to employ this for synthesizing zirconium compounds of other fatty or
approach for the synthesis of zirconium pivalate. We aromatic acids.
propose to synthesize zirconium acetate from hydrous
The moisture resistance of zirconium pivalate was
oxochloride in acetic anhydride, which will help to evaluated from the weight change during storage in air.
remove trace water from the reaction mixture and to The weight of a Zr(Piv) sample remained unchanged
4
avoid oxoacetate formation:
for 10–15 min; after 30 min of exposure in air, the
weight loss was ~1%. This means that zirconium piv-
alate is a hydrolysable compound, but one can handle it
in air for several minutes. This feature distinguishes zir-
conium pivalate from anhydrous zirconium halides,
nitrate, and alkoxides.
ZrOCl ⋅ 8H O + xMeCOOH + (MeCOO) O
2
2
2
(
2)
Zr(MeCOO) + 2HCl↑.
4
Zirconium acetate is readily formed by reaction (2),
which is confirmed by elemental analysis, IR spectros-
copy, and H NMR spectroscopy.
Zr(Piv)
ments, it was found that zirconium pivalate sublimed at
volatility. In vacuum sublimation experi-
4
1
2
50°ë and 0.01 mmHg. About 95% of the starting
The ligand-exchange reaction
amount sublimed. The sublimate was zirconium piv-
1
(
Zr(MeCOO) + 4HPiv
Zr(Piv)4)
alate, which was shown by elemental analysis and H
NMR spectra.
4
(
3)
+
4MeCOOH ⋅ xSolvent↑
The Zr(Piv) mass spectrum indicates that the vapor
4
was carried out in toluene, by analogy with the synthe-
sis of bismuth(III) carboxylates [14]; the azeotrope of
toluene with acetic acid boils at 105°ë and contains
consists practically of monomeric molecules (Table 2).
Vaporization may be represented as follows:
Zr(Piv)4 s
Zr(Piv)4 gas.
3
4% acetic acid.
Bismuth pivalate is formed by the exchange reaction
The coincidence (to the experimental error) of the
enthalpies of sublimation derived for various ions
in toluene; for zirconium, we only obtained a mixed-
ligand complex Zr(MeCOO) (Piv) under these condi-
tions. This difference may be due to the specifics gener-
ated by the positions of zirconium and bismuth in the
Periodic Table. Zirconium, a d element, is a stronger
(
Table 3) also allows us to infer that only one molecular
3
species, namely Zr(Piv) , exists in the saturated vapor
4
of zirconium pivalate.
This distinguishes Zr(Piv) from other pivalates
4
complex former than bismuth, but in compounds with whose saturated vapor consists of polynuclear species.
O-donor ligands it forms mainly ionic bonds. Reaction For example, the saturated vapor of Ag(Piv) and
(
3) in toluene did not go to the end: the difference Ln(Piv) consists of dimers and trimers [15, 16].
3
+
3
between the stabilities of acetates and pivalates is not
large in zirconium compounds with mainly ionic bond-
From the ion current intensities for Zr(Piv) and
+
Zr(Piv) measured as a function of temperature from
ing, and the pK values for pivalic and acetic acids are
2
a
1
30 to 210°ë, the enthalpy of sublimation of the com-
similar (5.05 and 4.75, respectively). In the compounds
of bismuth (a post-transition element), the covalence of
the metal–ligand bonds is higher than in the zirconium
compounds. This is likely responsible for the greater
pound was determined in the least-squares fits using the
Clausius–Clapeyron equation:
∆ H°(Zr(Piv)4s, 298.15 ä) = 167.9 ± 3.0 kJ/mol.
S
RUSSIAN JOURNAL OF INORGANIC CHEMISTRY Vol. 51 No. 11 2006

ZIRCONIUM PIVALATE: SYNTHESIS AND VOLATILITY
1753
Table 3. Worksheet for the calculations of the enthalpies of Table 4. Analysis of the products of Zr(Piv) thermohydrolysis
4
sublimation from Clausius–Clapeyron equation
Com-
pound
∆mfound,
0
T
0
T
Reaction
∆mcalcd, % T, °C
∆S H for
∆ H for
%
S
Temperature
range, K
+
3
+
2
Zr(Piv) ion,
Zr(Piv) ion,
2
00
31.9
55.7
75.1
kJ/mol
kJ/mol
Zr(Piv)4 Zr(Piv)4
ZrO2
75.2
74.8
300
413–482
444–482
403–481
433–479
412–483
175.0 ± 1.5
158.0 ± 2.1
173.5 ± 1.0
177.9 ± 1.9
165.8 ± 0.4
160.8 ± 1.7
154.7 ± 1.7
174.2 ± 0.6
173.5 ± 0.5
4
2
00
00
35.7
52.9
64.3
Zr(acac) Zr(acac)
ZrO2
300
400
4
4
The equation for the saturated vapor pressure of 400°ë. Based on the above data, we may state that zir-
Zr(Piv) as a function of temperature may be written as conium pivalate is less stable at high temperatures and
4
is more reactive to water vapor.
logP[atm] = –(8780.8 ± 428.6)/T
Film growth by MOCVD. At this stage, zirconium
pivalate was tested as the precursor for the thermal
CVD of crystalline YSZ films.
A set of experiments was carried out on YSZ film
deposition in the range of temperatures from 500 to
+
(13.57 ± 0.2)(403 ≤ T ≤ 482 K).
The full isothermal evaporation data allowed us to
calculate the Zr(Piv) partial vapor pressure at 189°ë
from the Hertz–Knudsen equation; its value was 5.3 ×
4
8
00°ë. The film deposited at 500°ë was X-ray amor-
–
6
1
0 atm.
phous. X-ray powder diffraction patterns (Fig. 2) show
The volatility of zirconium pivalate may be com- that the films deposited at 600°ë and above were poly-
pared to that of Zr(acac) . The latter is a well-known crystalline; they were mixtures of the cubic and tetrag-
4
precursor, which has the same bulk formula as zirco- onal phases.
nium pivalate. Mass-spectrometric data show that
acetylacetonate sublimes in a range of 62–47°ë, and its
standard enthalpy of sublimation is 150.2 ± 5.4 kJ/mol
These experiments demonstrated the feasibility of
using Zr(Piv) as the precursor for the CVD of zirconia
4
films. We hope that zirconium pivalate, with its moder-
ate thermal stability and tendency to hydrolysis, will be
recognized as a more promising precursor for growth of
[
17]. This means that our synthesized zirconium piv-
alate is exceeded in volatility by acetylacetonate, which
may be due to the different intermolecular bonds in
β-diketonates and carboxylates.
Thermal stability and thermohydrolysis of
Percentage weight loss, %
Zr(Piv) and Zr(Acac) . Zirconium pivalate in air
4
4
0
decomposes in two stages from 180 to 400°ë (Fig. 1).
The first stage involves oxopivalate formation; the sec-
ond is zirconium oxide formation. The observed weight
loss (75.2%) corresponds to oxide formation. For zirco-
nium acetylacetonate, the decomposition starts at a
lower temperature (~140°ë), occurs in three stages,
and ends with oxide formation at ~600°ë, i.e., at a
higher temeprature than for zirconium pivalate (Fig. 1).
The different thermal stabilities of the two com-
pounds also manifested themselves in model thermohy-
drolysis experiments, which were carried out at 200–
00°ë. The extents of hydrolysis for zirconium pivalate
and zirconium acetylacetonate were evaluated from the
composition of the products (Table 4).
1
2
1
2
0
0
30
4
5
6
7
0
0
0
0
80
0
100 200 300 400 500 600
4
T, °C
In a water-saturated oxygen flow, zirconium piv-
Fig. 1. Weight loss curves for (1) Zr(acac) and (2) Zr(Piv)
in air.
4
4
alate, unlike Zr(acac) , fully decomposed to oxide at
4
RUSSIAN JOURNAL OF INORGANIC CHEMISTRY Vol. 51 No. 11 2006

1
754
KUZ’MINA et al.
Iabs
MgO
1
1
07
06
ZrO2
ZrO2
ZrO2
tdepos = 700 °C
1
1
1
05
tdepos = 700 °C
tdepos = 600 °C
04
03
2
0
25 30 35 40 45 50 55 60 65 70 75 80
θ, deg
2
Fig. 2. X-ray powder diffraction patterns for ZrO (Y O ) films deposited at various temperatures.
2
2 3
amorphous films than well-known zirconium acetylac-
etonate.
8. E. Iljina, A. Kor’jeva, N. Kuzmina, et al., Mater. Sci.
Eng., No. 188, 234 (1993).
9
. N. Zharkova, N. Dzubenko, and L. Martynenko, Russ. J.
Coord. Chem, No. 23, 611 (1997).
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