Homo- and heterometallic terbium alkoxides - Synthesis, characterization and conversion to luminescent oxide nanostructures

Article abstract of DOI:10.1002/ejic.201000963

Terbium alkoxides in homometallic - [Tb₃(μ₃-OtBu) ₂(μ₂-OtBu)₃(OtBu)₄ (HOtBu) ₂] (1), [Tb{OC(tBu)₃}₃(THF)] (2) - and heterometallic configurations - [TbAl(μ<

Full text of DOI:10.1002/ejic.201000963

FULL PAPER
DOI: 10.1002/ejic.201000963
Homo- and Heterometallic Terbium Alkoxides – Synthesis, Characterization
and Conversion to Luminescent Oxide Nanostructures
[a]
[b]
[c]
[c]
Eva Hemmer, Volker Huch, Matthias Adlung, Claudia Wickleder, and
Sanjay Mathur*[
d]
Keywords: Lanthanides / Terbium / Aluminium / Alkoxides / Nanostructures / Luminescence / Sol-gel process / Optical
materials
Terbium alkoxides in homometallic Ϫ [Tb
OtBu) (OtBu) (HOtBu) ] (1), [Tb{OC(tBu) (THF)] (2) Ϫ and
heterometallic configurations Ϫ [TbAl(μ -OiPr) (OiPr) (iP-
rOH)] (3), [TbAl (μ -OiPr) (OiPr) ] (4) Ϫ were synthesized
3
(μ
3
-OtBu)
2
(μ
2
-
works present in 3 and 4 were, however, unstable under sol-
vothermal conditions and resulted in hydroxide–oxide com-
posites [Tb(OH) /Al O /Al(OH) ]. Compound 4 exhibited
3 2 3 3
sufficient vapour pressure to be used in the chemical vapour
deposition (CVD) process to grow Tb–O–Al thin films. Crys-
3
4
2
3
}
3
2
3
3
2
3
2
6
6
and characterized by single-crystal X-ray diffraction. Decom-
position of 1 and 2 under solvothermal conditions produced
2 4 2
talline compositions obtained on MgAl O and SiO (quartz)
Tb(OH)
3
nanorods, whereby the material formation and
substrates were garnet and perovskite phases, and were as-
cribed to the crystallographic relationship between the sub-
strate and CVD deposits. The optical properties of the pow-
ders obtained were studied by photoluminescence spec-
troscopy.
crystallization were influenced by the steric profile of the or-
ganic ligand, which controlled the hydrolysis and condensa-
tion reactions of the precursor molecules. Monophasic ter-
3
bium aluminate in the perovskite phase (TbAlO ) was ob-
tained by the sol–gel processing of 3. Heterometallic frame-
Introduction
Trivalent lanthanide alkoxides favour a trinuclear frame-
work, particularly when tert-butoxy groups are chosen as
ligands, resulting in molecular compounds of general for-
mula [Ln (OtBu) (HOtBu) ], in which two neutral alcohol
3
9
2
III
[1]
molecules are coordinated to the Ln centres (Scheme 1).
III
Formation of a crystalline Pr tert-butoxide was described
by Hubert-Pfalzgraf et al.; however, no crystallographic
data on the molecular structure was reported.[ We have
recently characterized the praseodymium and gadolinium
2]
[
3]
3 9 2
Scheme 1. General molecular structure of [Ln (OtBu) (HOtBu) ]
tert-butoxides, which were found to be analogous to the
(
the dotted lines to ROH groups represent the delocalization of the
[
4]
[1]
[5]
[6]
yttrium, lanthanum, cerium, erbium and dyspro-
III
alcohol molecules rotating around the Ln centres).
[
7]
sium compounds.
Herein we report new terbium-containing single- and
mixed-metal alkoxides, and their application in the prepara-
tion of terbium-based ceramics by solvothermal, sol–gel
and chemical vapour deposition techniques. Metal alk-
oxides are attractive precursors to metal oxides due to the
presence of pre-existing metal–oxygen units, which facilitate
the formation of nanocrystalline oxide materials at low tem-
[
[
a] Department of Materials Science and Technology, Tokyo
University of Science,
[
8]
peratures (Ͻ1000 °C). Their high solubility and/or volatil-
ity, which can be tuned by the electronic and steric factors,
allows easy purification by sublimation or recrystalli-
2
641 Yamazaki, 278-8510 Chiba, Japan
b] Institute of Inorganic Chemistry, Saarland University, Building
C4 1,
[
9]
sation. A further advantage of these preparations of pure
metal–oxide ceramics is the absence of other elements such
as Cl, which may be introduced in the crystalline framework
when precursors such as metal halides are used. Although
the synthesis of binary oxides from monometallic alkoxides
66123 Saarbruecken, Germany
[
[
c] Institute of Inorganic Chemistry, University of Siegen,
Adolf-Reichwein-Str., 57068 Siegen, Germany
d] Institute of Inorganic Chemistry, University of Cologne,
Greinstr. 6, 50939 Cologne, Germany
Fax: +49-221-470-5899
E-mail: sanjay.mathur@uni-koeln.de
Supporting information for this article is available on the
WWW under http://dx.doi.org/10.1002/ejic.201000963.
[
M(OR) ǞMO ] appears straightforward, formation of
x x
single phase mixed-cation ceramics (e.g., LiNbO , Co-
3
2148
© 2011 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Eur. J. Inorg. Chem. 2011, 2148–2157

Homo- and Heterometallic Terbium Alkoxides
Fe O , BaZrO ) from a mixture of component alkoxides is tified as [Tb (OtBu) (HOtBu) ] (1) by X-ray structure
2
4
3
3
9
2
not obvious due to the differential Lewis acidity and basic- analysis. Compound 1 crystallizes in the monoclinic space
ity of individual metal alkoxides. Consequently, besides the group P2 /c and is isotypical with the analogous yttrium
1
major phase, minor phase separation or element segrega- and lanthanum derivatives.[1,4] The molecular structure of 1
tion is generally observed in mixed-cation ceramics. The (Figure 1) contains three terbium atoms building a triangu-
fact that the redistribution of elements is thermodynami- lar framework supported by two doubly-bridging alkoxy li-
cally favoured and cannot be suppressed in a simple mix- gands along the sides of the triangle (O3, O4 and O5). Two
ture of constituent metal–organic derivatives limits the pre- additional μ -bridging alkoxy ligands (O1 and O2) are pres-
3
dictability of the final phase (composition) of precursor- ent below and above the triangular plane, stretched by the
based routes. Nevertheless, heterometal alkoxides enable the three Tb atoms, resulting in sixfold oxygen coordination
formation of homogenates, in which the phase-forming ele- and a distorted octahedral geometry around the Tb
[
22]
ments are bound chemically to each other. Preserving the centres.
One of the terbium atoms (Tb1) bears two alk-
elemental network given in the alkoxide precursor enables oxy ligands, whereas Tb2 and Tb3 are each coordinated
its transformation into ceramics with a controlled mixed- by an alkoxy group and a tert-butyl alcohol ligand. This
metal matrix. We have demonstrated the potential of molec- structural inhomogeneity is reflected in the corresponding
ular precursors in the preparation of single phase mixed- bond angles; the presence of a hydrogen bond [RO···
–
metal oxides with different chemical compositions such as HO(R)Tb] between anionic (OR ) and protonated (ROH)
[
10]
[11]
[4,12]
AB O , ABO3 and A B O
(A = Co, Ni, Cu, Mg, ligands is indicated by comparing the O8–Tb2–O9 and
O10–Tb3–O11 angles with the much smaller O6–Tb2–O7
2
4
3
5
12
Ba, Gd, Y; B = Al, Ti, Zr, Fe).
Besides nanoparticles (zero-dimensional) and thin films angle (Table 1).
two-dimensional), one-dimensional structures as nanorods,
(
nanotubes and nanowires are gaining increasing attention.
The preparation of highly luminescent Tb(OH) @SiO2
3
nanotubes by soft-template synthesis was described by
[
13]
Huong et al., and the synthesis of Eu(OH) nanorods by
3
microwave irridation for biomedical applications was re-
ported by Patra et al.[ Due to the crystallisation of lan-
14]
thanide hydroxide in the hexagonal lattice, Ln(OH) nano-
3
rods can also be grown by template-free solvothermal syn-
thesis and be easily transformed into anisotropic Ln O3
2
nanostructures, which are promising for optical applica-
[
15]
tions.
Herein, the main advantage of the solvothermal
process is the synthesis of crystalline and isolated fine struc-
tures at low temperatures, whereas most wet-chemical ap-
proaches require subsequent heat treatment for crystalli-
3 9 2
Figure 1. Molecular structure of [Tb (OtBu) (HOtBu) ] (1) (hydro-
zation inducing the risk of agglomerations by the sintering gen atoms are omitted for clarity).
[
16]
3+
effect.
Due to the intense emission of Tb ions near
5
44 nm, terbium-containing materials find applications as
Table 1. Selected bond lengths and angles of [Tb
3 9
(OtBu) -
[17]
green emitting phosphors
in luminescent materials
(
HOtBu) ] (1).
2
3
+
3+
3+
3+
3+
[18]
(Ce ,Tb :LaPO , Tb :CePO , Eu /Tb :SrB O ),
4 4 4 7
3
+
[19]
Bond lengths [Å]
Angles [°]
display applications (Tb :YAG)
stable biomolecule markers (Tb :Gd O ). Phase separa-
and as highly photo-
3
+
[20]
Tb1–O1
Tb1–O2
Tb1–O3
Tb1–O5
2.403(3)
2.343(3)
2.285(4)
2.320(4)
2.609(4)
2.114(4)
O6–Tb1–O7
O8–Tb2–O9
O10–Tb3–O11 84.2(2)
Tb1–O3–Tb2
Tb1–O5–Tb3
Tb2–O4–Tb3
72.29(14)
89.1(2)
2
3
tion is crucial for doped systems, where the functional be-
haviour of the material depends on the chemical homo-
geneity of the phase and an optimal distribution of the dop- Tb1–O6
ant ions.
Tb1–O7
The aim of this study is to develop new Tb-containing
98.35(14)
97.84(14)
101.11(13)
molecular compounds and investigate their precursor chem-
The profound influence of the steric profile of the alkoxy
ligand on the nuclearity of the molecule and the coordina-
tion sphere of the central metal atom was observed when
the tert-butoxy groups were replaced by bulkier tris-tert-
butoxy (tritox) ligands (Scheme 2).
istry in solvothermal, sol–gel and CVD[ processes.
21]
Results and Discussion
Reaction of [Ln{N(SiMe ) } ] (Ln = Nd, Ce) with three
3
2 3
Precursor Synthesis
equivalents of HOC(tBu) resulted in [Ln(tritox) (THF)] as
3
3
Terbium tert-butoxide was prepared by the alcoholysis of shown by NMR spectroscopy.[23,24] Formation of solvent-
terbium hexamethyldisilylamide [Tb{N(SiMe ) } ] with an free holmium tritox was confirmed by IR spectroscopy, al-
3
2 3
excess of tert-butyl alcohol.[ Colourless crystals obtained though synthesis of the homoleptic dysprosium tritox
1]
[
25]
from a mixture of hexane and THF at –20 °C were iden- failed.
Furthermore, the reaction between [Ln{N-
Eur. J. Inorg. Chem. 2011, 2148–2157
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2149

E. Hemmer, V. Huch, M. Adlung, C. Wickleder, S. Mathur
FULL PAPER
alkoxy groups O1–O3 2.113 Å) compared to those of 1 (ter-
minal alkoxy groups O6–O11 2.254 Å, μ -bridging O3–O5:
2
2
.332 Å, μ -bridging O1–O2 2.443 Å).
3
Table 2. Selected bond lengths and angles of [Tb{OC(tBu)
THF)] (2).
3 3
} -
(
Selected Distances [Å]
Selected Angles [°]
Tb1–O1
Tb1–O2
Tb1–O3
Tb1–O4
2.111(4)
O1–Tb1–O2
O1–Tb1–O3
O2–Tb1–O3
O1–Tb1–O4
119.33(18)
2.121(4)
2.107(4)
2.401(5)
116.98(19)
116.50(18)
98.96(18)
Scheme 2. Steric profiles of the tert-butoxy and tris-tert-butoxy (tri-
tox) ligand.
Lanthanide-containing heterometallic alkoxides have
been prepared by (i) condensation reactions between con-
stituent alkoxides and (ii) salt elimination reactions between
(
SiHMe ) } ] and HOC(tBu) resulting in the homoleptic
2 2 3 3
complexes Ln(tritox) (Ln = Y, Nd), as well as structurally
similar compounds Ln(OR) with aryl ligands have been
reported.
[27]
[28]
3
LnCl and KAl(OiPr)4 as described by Mehrotra et al.
3
3
In addition, heterometallic alkoxides with Ln = Y, Pr, Nd,
Gd, Er and Yb and M = Fe or Al have been prepared by
[
26]
We show here that the reaction of
[
Tb{N(SiMe ) } ] with HOC(tBu) in THF produces mo-
3
2 3
3
ligand exchange reactions between Ln and M-tert-butoxides
nomeric [Tb{OC(tBu) } (THF)] (2) with a pseudotetrahe-
dral coordination of the terbium atom constituted by three
tritox ligands and one coordinated THF molecule
[29]
3
3
with excess 2-propanol.
Samarium aluminium alkoxides
have been obtained by the reaction of samarium amide and
[30]
aluminium alkoxide with excess isopropanol. The hetero-
(recrystallisation was performed in an n-hexane/THF mix-
metallic Tb–Al compound [TbAl(OiPr) (iPrOH)] , with a
6
2
ture) (Figure 2). Attempts to obtain a THF-free compound
were not successful.
Tb:Al ratio of 1:1, was obtained by the equimolar reaction
of 1 and aluminium isopropoxide in an iPrOH/C H mix-
6
6
ture. A concentrated solution produced [TbAl(OiPr)6-
(
iPrOH)] (3) as colourless crystals, which was shown by
2
single crystal diffraction to be a centrosymmetric dimer
Figure 3) with an inversion centre lying in the middle of
the central Tb O ring [Tb1–O1: 2.2768(14) Å, Tb1–O1#1:
(
2
2
+
2
.2868(14) Å], whereby the Tb3 ions form the centre of a
distorted tetragonal bipyramid.
3 3
Figure 2. Molecular structure of [Tb{OC(tBu) } (THF)] (2) (hy-
drogen atoms are omitted for clarity).
The strong steric repulsion between the tritox ligands
leads to increased bond angles [O1–Tb1–O2 119.33(18)°,
O2–Tb1–O3 116.50(18)°, O1–Tb1–O3 116.98(19)°] com-
pared to the tert-butoxide derivative (Table 2). Angles be-
tween the coordinated THF molecule and the tritox ligands
[
O4–Tb1–O1 98.96(18)°, O4–Tb1–O2 102.06(18)°, O4–
Tb1–O3 95.85(19)°] are smaller than expected for tetrahe-
dral angles (109.5°) resulting in a distorted trigonal pyrami-
dal molecular geometry. The Tb–O bond lengths in the mo-
nonuclear 2 are shorter (average bond length 2.185 Å) than
6 2
Figure 3. Molecular structure of [TbAl(OiPr) (iPrOH)] (3) (hydro-
gen atoms are omitted for clarity).
The three μ -bridging oxygen atoms O1, O4 and O5, and
2
those found in 1 (average bond length 2.343 Å), which O2 of the terminal alkoxy ligand form the equatorial plane
shows the interplay of electronic and structural factors. The around the central terbium cation [O1–Tb1–O2: 105.95(7)°,
electron donating power of tert-butyl groups (basicity) de- O2–Tb1–O5: 95.51(6)°, O4–Tb1–O5: 63.43(5)°, O1–Tb1–
creases with increasing coordination of alkoxy oxygen O4: 94.07(5)°]. In an ionic formalism, the overall structure
atoms to the metal centre (terminal Ͻ μ -bridging Ͻ μ - can be described as the fusion of two tetrahedral [Al-
2
3
–
bridging). Due to the fact that 2 contains only terminal (OiPr)4] units to a central [(ROH)(OR)Tb(μ -OiPr) Tb-
2
2
alkoxy groups, the Tb–O distances are shorter (terminal (OR)(ROH)]2 unit. The Al–O bond lengths are shorter
+
[31]
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Homo- and Heterometallic Terbium Alkoxides
than the Tb–O bonds [Al1–O4: 1.7707(15) Å, Al1–O5: angles in 4 are comparable to these found in analogous Ln–
1
.7856(14) Å, vs. Tb1–O4: 2.3861(13) Å, Tb1–O5: Al compounds. In contrast to Pr and Nd alkoxy alumi-
[
12b]
2.3955(15) Å] demonstrating a better overlap of the Al and nates
the Tb–Al compound crystallizes without any co-
O orbitals due to similar van der Waals radii (Table 3).
ordinated alcohol molecules. Similar alcohol-free structures
[
27a]
[27c]
have been observed for Er–Al
and Eu–Al
derivatives.
6
Table 3. Selected bond lengths and angles of [TbAl(OiPr) -
4 3
Table 4. Selected bond lengths and angles of [Tb{Al(OiPr) } ] (4).
2
(iPrOH)] (3).
Bond lengths [Å]
Angles [°]
Bond lengths [Å]
Angles [°]
Tb1–O1
Tb1–O2
Al1–O1
Al1–O2
Al1–O3
Al1–O4
2.292(5)
2.292(4)
1.809(5)
1.802(5)
1.684(6)
1.696(6)
O1–Tb1–O2
O1–Al1–O2
O3–Al1–O4
66.17(15)
87.7(2)
118.8(3)
Tb1–O1
Tb1–O1#
Tb1–O2
Tb1–O3
Tb1–O4
Tb1–O5
Al1–O4
Al1–O5
Al1–O6
Al1–O7
2.2768(14)
2.2801(16)
2.0681 (15)
2.4478(18)
2.3861(13)
2.3955(15)
1.7707(15)
1.7856(14)
1.7422(17)
1.7005(16)
O1#1–Tb1–O3 160.86(5)
O1–Tb1–O2
O1–Tb1–O3
O1–Tb1–O4
O4–Al1–O5
O6–Al1–O7
105.95(7)
92.83(6)
94.07(5)
89.95(7)
111.37(8)
Optical Properties of Compound 1
In order to analyse the optical properties of terbium alk-
oxides, 1 was chosen for investigation by photolumines-
cence spectroscopy. Figure 5 shows the excitation and emis-
sion spectra of 1 obtained at 10 K. Under UV excitation
The reaction of terbium tert-butoxide and aluminium
isopropoxide in a 1:3 molar ratio produced [Tb{Al-
(
OiPr) } ] (4). This compound can also be prepared by a
4 3
salt elimination reaction between TbCl and three equiva-
3+
3
the characteristic Tb emission peaks are detected with a
lents of KAl(OiPr)4.[
a monomer with a central Tb ion surrounded by three
Al(OiPr) ] units resulting in a sixfold coordination around
27a]
X-ray structural analysis revealed
maximum at 543 nm, which can be attributed to the
3
+
5
7
D Ǟ F transition. Further peaks observed at 495, 591,
4
5
[
4
6
23 and 656 nm are assigned, based on previous investi-
the Tb centres (Figure 4).
5
7
5
7
5
7
gations, to the D Ǟ F , D Ǟ F , D Ǟ F and
4
6
4
[33]
4
4
3
5
7
D Ǟ F transitions, respectively.
4
2
Figure 5. Photoluminescence spectra of 1 at T = 10 K.
Figure 4. Molecular structure of [Tb{Al(OiPr)
atoms are omitted for clarity).
4
}
3
] (4) (hydrogen Material Synthesis and Characterization
Terbium Hydroxide Nanostructures by Solvothermal
Synthesis
The structural motif was originally proposed by Mehro-
tra[ and was also reported for aluminium lanthanide alk-
oxides-containing ligands other than OtBu and OiPr.
28]
The decomposition of suitable precursor molecules un-
der basic conditions favours the formation of crystalline
[
32]
The bond lengths (Table 4) of the central Tb atom and the elongated lanthanide hydroxide, Ln(OH) , structures in
3
34]
[
μ2-bridging oxygen atoms in 4 [2.281(4) Å (Tb1–O9) to particular for lighter rare earth elements. Decomposition
2
.302(4) Å (Tb1–O6)] correlate well with praseodymium behaviour of 1 and 3 was further investigated by thermo-
[12b,27a,27b]
and erbium derivatives.
The Al–O bond lengths gravimetric analysis and thermogravimetric differential
[
1.684(6) Å (Al1–O3) to 1.815(5) Å (Al2–O5)] and bond thermal analysis (TG-DTA) curves are given in the Sup-
Eur. J. Inorg. Chem. 2011, 2148–2157
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2151

E. Hemmer, V. Huch, M. Adlung, C. Wickleder, S. Mathur
FULL PAPER
Figure 6. X-ray powder diffraction pattern of powders obtained by solvothermal treatment of the homometallic compounds 1 (a) and 2
b) and the heterometallic precursors 3 (c) and 4 (d).
(
porting Information. Solvothermal treatment (250 °C, 24 h)
of an 2-propanolic solution of 1 and 2 in an alkaline (aq.
KOH, c = 1.87 mol/L) medium produced crystalline ter-
bium hydroxide, Tb(OH) , as shown by X-ray powder dif-
3
fraction (Figure 6, a and b).
Powder samples of 2 showed higher crystallinity as evi-
dent in the sharpness and intensities of the diffraction
peaks. In addition, a crystalline minor phase was also ob-
served, which could be assigned to terbium silicate,
Tb Si O . The peak at 31° supports the presence of the sec-
ondary phase that could result from silicon-containing re-
sidual species probably originating from terbium hexa-
methyldisilylamide or occluded hexamethyldisilazane,
which is liberated as a by-product.
2
2
7
Scheme 3. Possible formation of aqua complexes and condensation
directions of intermediate hydroxides produced in the hydrolysis of
1
and 2.
The lower crystallinity of powders obtained from 1 can
Transmission electron micrographs (TEM, Figure 7)
be explained by the different hydrolysis paths of compounds showed facetted and elongated particles with a rather broad
1
and 2 (Scheme 3). Apparently, the nucleophilic attack size distribution and a particle size ranging between 70 and
–
(OH ) in 1 follows a cascade of reactions (hydrolysis + con- 600 nm. The elongated microstructure is apparently fav-
densation), which is a function of the basicity of the OR oured by the hexagonal lattice of terbium hydroxide. The
groups depending upon their mode of ligation (terminal, lattice fringes (inset in Figure 7, a) seen in high-resolution
doubly- or triply-bridged positions). It can be envisaged TEM and the inter planar spacing of 5.8 Å corresponded
that hydrolysis preferentially occurs at the two μ -alkoxo to the (100) plane of crystalline Tb(OH)3.
3
groups that cap the Tb O plane. As a result, 2 shows a
The pH of the reaction solution strongly influences the
3
3
higher number of reaction sites than 1 and is therefore more morphology of the Ln(OH) structures obtained. Basic con-
3
–
prone to impurities.
ditions, providing a sufficient concentration of OH , induce
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Homo- and Heterometallic Terbium Alkoxides
room for the nucleophilic attack by water. The coordinated
water molecule is deprotonated on increasing the pH,
whereby terminal electronically- and sterically-unsaturated
hydroxo groups are formed that bridge other lanthanide
centres leading to an assembly of structurally well defined
Ln–hydroxo clusters (Scheme 3). Furthermore, the aspect
ratio of Ln(OH) is not only driven by pH, but is also ele-
3
ment specific. Although lighter rare earth hydroxides tend
to form elongated structures of high aspect ratio, the
heavier rare earth elements, among them Tb, form
[
36]
Ln(OH) of lower aspect ratio,
which is in agreement
3
with the elongated nanostructures described here (Fig-
ure 7).
Solvothermal processing of 3 produced only Tb(OH) as
3
the crystalline phase (Figure 6, c), whereas additional peaks
were found for 4 (Figure 6, d). As aluminium is present in
3
and 4, it is evident that the heterometallic frameworks are
fragmented under solvothermal conditions causing prefer-
ential crystallization of Tb(OH) and formation of Al-con-
3
taining amorphous species [Al(OH) , AlOOH and Al O ],
3
2
3
which require higher temperatures for ordering. The ad-
ditional peaks in powders produced by 4 can be assigned
[
12b,37]
to transition alumina, δ-Al O3
and Al(OH) , which
2
3
[
38]
was confirmed by IR spectroscopy. Discussion of the IR
data is provided in the Supporting Information.
The TEM analysis of powders produced by 3 revealed a
bimodal morphology distribution in which Tb(OH) nano-
3
particles are surrounded by amorphous aluminium oxide or
hydroxide phases (Figure 7, c). The solvothermal treatment
of 4 resulted in a resinous solid, which could not be dis-
persed for TEM sample preparation.
Tb–O–Al Phases by Sol–Gel Processing
We previously reported that sol–gel processing of [NdAl-
(
OiPr) (iPrOH)] and [NdAl (OiPr) (iPrOH)] led to mono-
6 2 3 12
phase NdAlO3 and biphasic oxide–oxide NdAlO /Al O
3
2
3
[
29a,29c]
composite, respectively.
However, treatment of the
Tb analogues under solvothermal conditions produced only
Tb(OH) as the crystalline material, which indicates that
3
the hydrolysis kinetics are much faster (than the condensa-
tion reaction) under solvothermal conditions, which favours
the phase separation into individual terbium and alumin-
ium compounds. Stoichiometric reaction of 3 and 4 with
Figure 7. TEM images of powders obtained by solvothermal de-
composition of precursors 1 (a), 2 (b) and 3 (c).
water (3–4 mol-equiv. H O per metal centre) produced
2
homogeneous gels, which, upon heat treatment (1000 °C,
2
h), confirmed the formation of terbium aluminate
(
TbAlO ) as the only crystalline phase in the case of 3 (Fig-
3
the one-dimensional growth of Ln(OH) resulting in nano- ure 8, a), which corresponds to the Tb:Al stoichiometry
3
rods or nanowires, whereas a further increase in pH induces present in the precursor. In contrast, the heat treated xero-
the formation of shorter nanorods or plates as observed for gel obtained from 4 revealed AlOOH as the major crystal-
[15a]
Sm(OH) and Gd(OH) by Li et al.
This phenomenon line phase (Figure 8, b). Additional XRD 2θ values corre-
3
3
is the result of a complex interaction between the chemical sponded to the peaks for the garnet (Tb Al O ) and mono-
3
5
12
potential and the ionic motion in the solution.[
15a,35]
In this clinic terbium aluminate Tb Al O (2θ = 29.2 and 30.5°)
4 2 9
case, the hydrolysis is ligand-controlled as the steric profile phases. This observation shows the influence of the second-
of the alkoxo ligands limits the number of ligands that can ary phase on the crystallization and structure of the major
[
29a,29c]
be arranged around the Ln centre; however, there is enough phase.
Formation of Y Al O and YAlO is reported
4 2 9 3
Eur. J. Inorg. Chem. 2011, 2148–2157
© 2011 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
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2153

E. Hemmer, V. Huch, M. Adlung, C. Wickleder, S. Mathur
FULL PAPER
as a kinetically stable phase rather than the thermodynami-
cally stable Y Al O (YAG) when a mixture of precursors
3
5
12
with 3:5 Y:Al ratio was used in spray pyrolysis.[
38a,39]
Figure 9. X-ray diffraction patterns of postannealed thin films de-
posited on a) quartz (SiO
the precursor.
2 2 4
) and b) MgAl O substrates using 4 as
Table 5. Lattice parameters of the substrate and film materials
Figure 8. X-ray diffraction pattern of powders obtained by the sol–
gel process using heterometallic Tb–Al precursors 3 (a) and 4 (b).
(
PDF [88–0154], [46–1045], [76–0111], [21–1152]).
Cryst. system a [Å] b [Å] c [Å] α [°] β [°] γ [°]
TbAlO
SiO
Al
MgAl
3
orthorhombic 5.2296 5.3058 7.4154 90
90
90
90
90
90
120
90
Thin Film Deposition using Compound 4
2
hexagonal
12 cubic
cubic
4.9134 4.9134 5.4052 90
12.000 12.000 12.000 90
8.0831 8.0831 8.0831 90
The heterometallic alkoxide 4 exhibited adequate volati- Tb
3
5
O
O
2
4
90
lity required for chemical vapour deposition. Analogous
lanthanum, neodymium and praseodymium compounds
have been used in liquid injection and metal–organic CVD
process for the deposition of amorphous LnAlO thin
3
Optical Properties of the Obtained Powders: Influence of
Precursor Chemistry
[
12b,27d]
films.
We investigated the deposition of thin films
using 4 as the precursor on quartz (SiO ) and spinel
2
(
MgAl O ) substrates. The as-deposited films were found
Emission spectra of the terbium-containing powders ob-
2
4
to be amorphous even at 1000 °C. However, post-thermal tained from 3 and 4 by sol–gel processing are shown in
treatment at 1350 °C under reduced pressure (p = Figure 10. Both spectra show characteristic peaks corre-
–
4
3+
1
0 mbar, t = 6 h) induced crystallization to produce ter- sponding to the 4fǞ4f transition in Tb ions with a maxi-
5
7
bium aluminate in the perovskite phase (TbAlO ) on the mum peak at 543 nm, which is due to the D Ǟ F transi-
quartz substrate (Figure 9, a). Under similar conditions tion.
postannealing induced crystallization of Tb Al O when 488, 586 and 621 nm, which correspond to the D Ǟ F ,
MgAl O was chosen as the substrate (Figure 9, b).
3
4
5
[
33]
Three additional emission bands were observed at
5
7
3
5
12
4
6
5
7
5
7
3+
D Ǟ F and D Ǟ F transitions. Tb ions were excited
2
4
4 4 4 3
The preferred crystallization of perovskite and garnet with light in the UV range (λex = 277 or 354 nm) which
5
phases can be ascribed to the crystallographic relationship provides enough energy for excitation in the D energy
3
between the CVD deposit and substrate materials (Table 5), level and relaxation should result in blue and green emis-
which showed that the respective lattice mismatches are sion. However, no emission in the blue range (350–475 nm)
minimized in the obtained combinations TbAlO3 (a = was observed, which is ascribed to cross-relaxation pro-
3
+
5
1
.2296 Å)/SiO2 (a = 4.9134 Å) and Tb Al O
(a = cesses due to the high concentration of Tb in the pow-
3
5
12
[
40]
2.000 Å)/MgAl O (a = 8.0831 Å).
ders. The spectra obtained for 4 showed better resolution
2
4
2154
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© 2011 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Eur. J. Inorg. Chem. 2011, 2148–2157

Homo- and Heterometallic Terbium Alkoxides
compared to 3, which may be due to the higher intensity of sol–gel processing of 3 and 4. The results clearly illustrate
the emission from the mixture of terbium aluminates and the importance of elementary chemical reactions involved
AlOOH (4) allowing detection of the luminescence at a in the conversion of a chemical precursor in the solid state,
smaller slit width [3: λex = 354 nm, slit(emission) = 3, slit- which offers the possibility of imposing a chemical control
(
(
excitation) = 3, 4: λex = 277 nm, slit(emission) = 1, slit- on the compounds and structures of the final ceramic mate-
excitation) = 2]. Furthermore, taking into account that the rial. The application of 4 in CVD processing led to thin
green emission is not very sensitive to concentration garnet or perovskite films subject to the nature of the sub-
quenching, the different resolution can be ascribed to the strate. The optical properties of the obtained terbium alu-
3
+
different environment the Tb ions are embedded in. The minate powders were investigated by photoluminescence
host lattice strongly influences the crystal field resulting in spectroscopy, which showed the influence of the precursor
different splittings of the J level. In addition, a lattice with- composition on the optical properties of the resulting mate-
out an inversion centre (lower symmetry) means that the rials.
parity selection rule can be lifted and higher intensities and
better resolutions observed.[ As described above, X-ray
diffraction analysis of the powders obtained from 3 revealed
41]
Experimental Section
phase-pure TbAlO in contrast to a small amount of crys- (I) Precursor Synthesis and Characterization: All manipulations
3
talline Tb Al O and Tb Al O in crystalline AlOOH and were performed in vacuo and in a nitrogen atmosphere using a
2
4
9
3
5
12
amorphous aluminium rich phases when 4 was used. This modified Schlenk assembly. Solvents used for amide and alkoxide
syntheses were purified and dried by standard procedures and
stored over sodium or molecular sieves.
suggests a lower symmetry for 4 explaining the better reso-
lution of the emission spectra obtained.
[
Tb{N(SiMe
3
)
2
}
3
]: [Tb{N(SiMe
3
)
2
}
3
] was synthesized from the re-
[20.0 mmol,
action between LiN(SiMe
3
)
2
(60.0 mmol) and TbCl
3
Sigma Aldrich, powder, water free, 99.9% dried under dynamic
vacuum at 120 °C for at least 1 h and activated by THF (50 mL)
at 60 °C for 2 h].[ LiN(SiMe
42]
3 2
) was obtained by the reaction of
n-butyllithium (477.0 mmol, 300 mL, 1.6 m in hexane, Acros Or-
ganics) and hexamethyldisilazane (477.0 mmol, 100 mL, Fluka,
Ͼ 98.0%) at room temperature for 24 h. The white powder was
purified by sublimation at 80 °C before further use.
[
Tb
tion of [Tb{N(SiMe
2 mL) at –195.80 °C. The reaction mixture was stirred at room
temperature for 24 h. Condensation and resolution in toluene
15 mL) followed by cooling at –20 °C produced colourless crystals
of 1.
3
(OtBu)
9
(HOtBu)
2
] (1): HOtBu (20 mL) was added to a solu-
) } ] (5.7 mmol) in hexane (25 mL) and THF
3 2 3
(
(
Figure 10. Emission spectra of TbAlO
and a mixture of Tb Al (TAM), Tb
4, λex = 277 nm, r.t.) obtained by sol–gel processing.
3
(TAP, 3, λex = 354 nm, r.t.)
[
Tb(tritox)
3
(THF)] (2): Synthesis of H-tritox (confirmed by NMR)
2
4
O
9
3
Al 12 (TAG) and AlOOH
5
O
[43]
was performed according to a literature procedure.
A solution
(
of diethylcarbonate (6.18 mL, Sigma–Aldrich, puriss., Ͼ99.5%) in
ethyl ether (50 mL) was slowly (2 h) added to a solution of tert-
butyllithium (90 mL, 1.7 m in pentane, Acros Organics) and stirred
at room temperature. For the hydrolysation of residual tert-buthyl-
lithium a mixture of water (50 mL) and acetic acid (10 mL) was
added (0 °C). The compound was isolated by washing with ethyl
ether under addition of sodium hydrogen carbonate. After drying
with magnesium sulfate, the product was obtained from the organic
phase as a waxy solid that was purified by sublimation at 80 °C.
Conclusions
New terbium-containing homo- and heterometallic alk-
oxides were synthesized and their molecular structures de-
termined by X-ray diffraction. When used in solvothermal
processing terbium tert-butoxide (1) and terbium tri-tert-
butoxide (2) produced terbium hydroxide. A higher ten-
dency for the formation of a secondary phase was observed
for 2, illustrating the influence of precursor configuration
on reactivity and nucleation behaviour due to different hy-
A solution of H-tritox (4.9 mmol) in hexane (20 mL) was dropped
into a solution of [Tb{N(SiMe ) } ] (1.6 mmol) in hexane (50 mL)
3 2 3
and stirred at room temperature for 24 h. After condensation the
compound obtained was dissolved in a hexane/THF mixture
(
5 mL/2 mL) and cooled to –20 °C. Colourless crystals were ob-
drolysis and condensation processes. When mixed-metal tained that were identified as [Tb{OC(tBu) } (THF)].
3
3
compounds [TbAl(OiPr) (iPrOH)] (3) and [TbAl (iPrO) ]
(6.2 mmol)[44]
6
2
3
12
[
TbAl(OiPr) (iPrOH)] (3): A solution of Al(OiPr)
6
2
3
(4) were used in solvothermal processing, phase segregation
in benzene (25 mL) was mixed with a solution of 1 (7.8 mmol) in
benzene (25 mL). 2-Propanol (20 mL) was added to the mixture,
occurred due the instability of the molecular framework
producing a mixture of crystalline Tb(OH) and amorph- which was stirred at 80 °C for 20 h. After condensation the com-
3
ous or crystalline aluminium-containing phases [γ-Al- pound obtained was diluted in toluene and cooled to –20 °C to
(
OH) , δ-Al O ]. In contrast, phase-pure terbium alumin- produce colourless crystals of 3.
3 2 3
ium, TbAlO , and a mixture of terbium aluminium garnet [TbAl (OiPr) ] (4): A solution of Al(OiPr) (17.3 mmol) in benz-
3
3
12
3
(
Tb Al O ), Tb Al O and AlOOH could be obtained by ene (25 mL) was mixed with a solution of 1 (7.7 mmol) in benzene
3 5 2 4 9
12
Eur. J. Inorg. Chem. 2011, 2148–2157
© 2011 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.eurjic.org
2155

E. Hemmer, V. Huch, M. Adlung, C. Wickleder, S. Mathur
FULL PAPER
(
25 mL). 2-Propanol (25 mL) was added to the mixture, which was
dition of aqueous potassium hydroxide solution (1 mL, c =
1.87 mol/L). The activated solution was introduced under ambient
conditions in Teflon inliners, which were enclosed in steel auto-
claves (DAB-2, Berghof Products + Instruments GmbH) and
heated to 250 °C for 24 h. After cooling to room temperature the
precipitates were washed several times with methanol and ethanol,
collected by centrifugation, dried in air and ground before further
characterization.
stirred at 80 °C for 20 h. After condensation of the solvent, resolu-
tion in toluene and cooling to –20 °C colourless crystals of 4 were
obtained.
Molecular structures of compounds 1–4 were determined by X-ray
single crystal analysis using a diffractometer AED 2 by Siemens.
The molecular structures were solved using SHELXS-86 and
SHELXS-93.
Sols of the precursors were obtained by solving 3 (1.71 mmol) in a
mixture of 2-propanol (100 mL) and toluene (50 mL), and 4
CCDC-816882 (for 1), -817587 (for 2), -816881 (for 3), and
-816883 (for 4) contain the supplementary crystallographic data for
(
(
2
2.39 mmol) in toluene (100 mL), followed by activation with water
3: 0.37 mL H O in 20 mL iPrOH, T = 80 °C; 4: 0.56 mL H O in
0 mL iPrOH, T = 25 °C). The sols obtained were stirred under
this paper. These data can be obtained free of charge from The
Cambridge Crystallographic Data Centre via www.ccdc.cam.ac.uk/
data_request/cif.
2
2
ambient conditions for several days until the solvent had evapo-
rated. The remains of the solvent were removed from the white
Crystal Data for 1: C44
K, 0.71073 Å, monoclinic, space group P2(1)/c,
9.625(4) Å, b = 10.831(2) Å, c = 27.509(6) Å, α = 90°, β =
101 3
H O11Tb , M = 1283.01 g/mol, T = 293(2)
λ
=
a
=
–3
xerogels at 160 °C under reduced pressure (p = 1ϫ10 mbar) for
1
9
=
6
h, followed by calicination.
3
3
9.39(3)°, γ = 90°, V = 5769(2) Å , Z = 4, ρcalcd. = 1.477 Mg/m , μ
–1
Thin Tb–O–Al films were deposited on SiO (quartz) and MgAl O
3.684 mm , F(000) = 2592, 35761 reflections in h(–22/22), k-
2
2
4
substrates (cleaned by ultra sound in iPrOH) with 4. Precursor flow
(
–12/12), l(–28/29), measured in the range 2.02°ՅθՅ24.04°, com-
pleteness to θmax = 94.5%, 8618 independent reflections, Rint
.0805, absorption correction: numerical, refinement method: full-
was set by a precursor temperature of 120–125 °C and a reduced
=
–
4
–6
pressure of 10 –10 mbar. Substrate temperature was 1000 °C and
deposition time was 90 min. The as-deposited films were annealed
0
2
matrix least-squares on F , data/restraints/parameters: 8618/0/556,
GooF = 1.061, R1 final = 0.0441, wR2 final = 0.1133, R1 all = 0.0496,
–
4
under reduced pressure (p = 1ϫ10 mbar) at 1350 °C for 6 h.
wR2 all
= 0.1198, largest difference peak and hole: 2.253/
Crystallinity and phase of the powders obtained were determined
by powder X-ray diffraction at room temperature with a D-5000
by Siemens (40 kV, 25 mA, step width: 0.02°) or a PW 1710 by
Philips (45 kV, 30 mA; step width: 0,02°) using Cu-Kα1 radiation.
X-ray diffraction patterns of the thin films were recorded with a
D-5000 by Siemens (40 kV, 25 mA, step width: 0.02°) at room tem-
perature using Cu-Kα1 radiation. Phase assignment was affected
using the program X’Pert HighScore by Philips Analytical B.V. (Al-
melo, Netherlands). TEM images were obtained with a trans-
mission electron microscope JEOL 200 CX by Philips. IR spectra
were recorded on KBr pellets using a 5 PC FTIR spectrometer by
Nicolet. Photoluminescence spectra were obtained with Fluorolog
3–22 spectrometer by Jobin Yvon using a 450-W xenon high-pres-
sure lamp and a photomultiplier R928P for the UV/Vis region.
–3
–2.924 eÅ .
Crystal Data for 2: C43
λ = 0.71073 Å, monoclinic, space group P2(1)/n, a = 12.964(3) Å,
b = 17.564(4) Å, c = 19.469(4) Å, α = 90°, β = 93.24(3)°, γ = 90°,
V = 4425.9(15) Å , Z = 4, ρcalcd. = 1.244 Mg/m , μ = 1.634 mm ,
F(000) = 1776, 40841 reflections in h(–16/17), k(–22/21), l(–25/25),
measured in the range 2.40°ՅθՅ28.06°, completeness to θmax
89 4
H O Tb, M = 829.06 g/mol, T = 203(2) K,
3
3
–1
=
95.8%, 10301 independent reflections, Rint = 0.0785, absorption
correction: numerical, refinement method: full-matrix least-squares
2
on F , data/restraints/parameters: 10301/0/460, GooF = 1.413,
R1 final = 0.0617, wR2 final = 0.1557, R1 all = 0.0682, wR2 all = 0.1590,
–3
largest difference peak and hole: 2.080/–1.557 eÅ .
Crystal Data for 3: C21 50AlO Tb, M = 600.51 g/mol, T = 293(2)
K, λ = 0.71073 Å, triclinic, space group P1, a = 10.823(2) Å, b =
H
7
¯
Supporting Information (see footnote on the first page of this arti-
cle): TG-DTA data for 1 and 3, FTIR spectra of powders obtained
from 3 and 4 by solvothermal and sol–gel processing.
1
7
2
1.951(2) Å, c = 12.971(3) Å, α = 82.34(3)°, β = 66.74(3)°, γ =
1.23(3)°, V = 1459.4(5) Å , Z = 2, ρcalcd. = 1.367 Mg/m , μ =
.484 mm , F(000) = 620, 47294 reflections in h(–17/16), k(–20/20),
3
3
–1
l(–21/22), measured in the range 1.71°ՅθՅ38.28°, completeness to
max = 90.3%, 14623 independent reflections, Rint = 0.0424, absorp-
Acknowledgments
θ
tion correction: numerical, refinement method: full-matrix least-
squares on F , data/restraints/parameters: 14623/0/289, GooF =
2
The authors thank the Deutsche Forschungsgemeinschaft (DFG)
for financial support (Schwerpunktprogramm SPP1166) and Prof.
Kohei Soga for helpful discussions.
1
0
.039, R1 final = 0.0304, wR2 final = 0.0757, R1 all = 0.0350, wR2 all =
–3
.0800, largest difference peak and hole: 2.385/–3.652 eÅ .
12Tb, M = 948.89 g/mol, T = 293(2)
K, λ = 0.71073 Å, orthorhombic, space group P2(1)2(1)2(1), a =
3
Crystal Data for 4: C36H84Al O
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Published Online: March 24, 2011
Eur. J. Inorg. Chem. 2011, 2148–2157
© 2011 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
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