Full text of DOI:10.1557/JMR.2000.0205

Phase and microstructural development of sol-gel-derived
strontium barium niobate thin films
A.Y. Oral and M.L. Mecartney
Department of Chemical and Biochemical Engineering and Materials Science, University of
California—Irvine, Irvine, California 92697-2575
(Received 18 October 1999; accepted 22 March 2000)
Microstructural changes in sol-gel-derived SrxBa1−xNb2O6 (SBN) thin films were
monitored as a function of Ba-to-Sr ratio (from x ס 0 to x ס 1), choice of substrate
(Si or MgO), and processing variations. Sols were created using Ba, Sr, and Nb
alkoxides dissolved in acetic acid. The relatively high decomposition temperature for
the organics led to a tendency to form defects, but careful control of thermal process
parameters could be used to produce a uniform film microstructure. An unexpected
phase, interpreted as a hexagonal (pseudo-orthorhombic) variant of hexagonal
BaNb2O6, was encountered in Ba-rich sol-gel-derived SBN powders and thin films
annealed at 750 °C. Increased (001) orientation was observed for SBN thin films
deposited on (100) MgO when fast thermal processing was used.
I. INTRODUCTION
The basic formula for a ferroelectric tetragonal tung-
sten bronze is [(A1)2 (A2)4 C4] [(M1)2 (M2)8] O30.2 The
skeletal framework of the tungsten bronze structure is
formed by MO6 octahedra, which share corners to form
cavities of A1, A2, and C (Fig. 1). The smaller cations
prefer A1 sites because of the relatively smaller size of
A1 sites compared to A2 sites. Jamieson and Bernstein’s5
atom distribution model for SBN suggests that Ba cations
are only found in large A2 sites and Sr atoms preferably
occupy the small A1 sites. Nevertheless, some of the Sr
atoms can also occupy A2 sites, especially in Sr-rich
solid solutions. The C site is the smallest of the three
cavities created by the framework of octahedra. Very
small cations that may occupy the C site are lithium,
vanadium, and beryllium.1
Strontium barium niobate (SrxBa1−xNb2O6, SBN) is a
ferroelectric phase with a tungsten bronze structure that
is formed between BaNb2O6 and SrNb2O6. The region of
solid solution for the tungsten bronze structure is gener-
ally accepted to be 0.25 < x < 0.75.1 Two different lattice
types and symmetries have been observed in the solid
solution. The first phase is tetragonal, with space group
P4bm, having approximate lattice parameters of a,b ס
1.24 nm, c ס 0.39 nm. Numerous members of the fer-
roelectric tungsten bronze type of oxides all have a
related tetragonal symmetry with a unit cell of approxi-
mately 1.2 × 12 × 0.39 nm containing 10 corner-sharing
MO6 (M ס penta valent cation) octahedra.2 The tungsten
bronze structure is a layered type, with the oxygen ions
forming slightly puckered sheets at approximately z ס 0
1
and ⁄
2
, the M ions at approximately z ס 0 and the
1
remaining cations (Ba, Sr, Na, etc.) at z ס ⁄ .
2
A closely related structure to the tetragonal tungsten
bronze family has orthorhombic symmetry, with a unit
cell of approximately 1.75 × 1.75 × 0.8 nm. The ortho-
rhombic variant is related to the tetragonal unit cell by
the following equations:
√
aorth borth
2atetragonal
,
(1)
(2)
corth 2ctetr
.
Only a few researchers have identified this orthorhombic
structure (space group Cmm2) in the BaNb2O6–SrNb2O6
system and assigned approximate lattice parameters,
a,b ס 1.76 nm, c ס 0.78.3–6
FIG. 1. The tetragonal tungsten bronze structure. Niobium sits in the
center of the oxygen octahedra.
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A.Y. Oral et al.: Phase and microstructural development of sol-gel-derived strontium barium niobate thin films
SrxBa1−xNb2O6 is a ferroelectric material. Below the
Curie temperature, all the cations are displaced offset
from the planes of oxygen atoms along the c axis, result-
ing in maximum polarization along the [001] direction.
At thermodynamic equilibrium, when x ס 0.67, Sr ions
are in the A1 sites and Ba ions are at A2 sites. As the Sr
content (x) of the solution increases, SBN changes from
a normal ferroelectric to a relaxor ferroelectric. During
this transition, as the Ba content decreases, the Sr/Ba
ratio in the A2 sites increases.1 The Sr atoms in A2 sites
may be responsible for the broadening of the temperature
range for the paraelectric to ferroelectric transition, in-
ducing relaxor behavior.
The metaniobates, BaNb2O6 and SrNb2O6, that are the
end compositions of the SBN phase diagram are not fer-
roelectric although their chemical formulae are similar to
SBN. The relative positions of the octahedra in the crys-
tal structure determine whether a compound is ferroelec-
tric or not. In these metaniobates, the octahedra meet
along their edges.3 In all known ferroelectrics, the octa-
hedra only join at their vertices and it is believed that this
type of array is responsible for the strong internal fields
in the crystals. The size of the A+2 cation appears to be
the main factor determining the relative positions of the
octahedra. In metaniobate solid solutions, only if the
mean radius of the A+2 cations is close to radius of Pb,
should the solid solution possess ferroelectric properties
(PbNb2O6 is the only known ferroelectric metaniobate).7
The experiments described in this paper examine
the low-temperature crystallization of a range of
SrxBa1−xNb2O6 compositions from x ס 0 (BaNb2O6) to
x ס 1 (SrNb2O6) using a modified sol-gel approach.
FIG. 2. Flow chart of (a) sol preparation and (b) film fabrication.
Two different heat-treatment methods were used to
crystallize the thin films. The first one was fast thermal
processing. In this process, a pyrolysis heat treatment
was applied to each layer of the films at 350 °C following
deposition. After the last coating was applied, the films
were directly inserted into the furnace at 750 °C and heat
treated for 60 min. Powders were also produced by dry-
ing the sols in open containers in the atmosphere fol-
lowed by fast thermal processing and a heat treatment at
750 °C for 60 min.
The second heat treatment method was controlled fur-
nace annealing, which used the information obtained
from thermogravimetric analysis (TGA) to determine
various heating rates applied during annealing. Heating
rates as slow as 1 °C/min were used in regions corre-
sponding to sharp weight losses identified in the TGA
curves. In addition, the temperature was held for 60 min
at the midpoints of every temperature range with a major
weight loss (Fig. 3). The crystal structures and the crys-
tallization behaviors of the films and powders were ana-
lyzed by x-ray diffraction (XRD) using a Siemens D5000
apparatus, transmission electron microscopy (TEM) us-
ing a Philips CM20 apparatus, differential scanning calo-
rimetry (DSC), and TGA. Surface morphology of the
films was examined by scanning electron microscopy
(SEM) with a Philips XL 30 FEG apparatus.
II. EXPERIMENTAL PROCEDURE
Figure 2 illustrates the material preparation process. A
new type of sol was prepared by dissolving Ba–
isopropoxide and Sr–isopropoxide in acetic acid and
mixing with Nb–ethoxide. In previous studies,8,9 ethanol
was commonly used as a solvent for the preparation of
the sol-gel-derived SBN thin films. These sols are diffi-
cult to prepare due to their high sensitivity to the mois-
ture in air.4 This modified sol preparation method, using
acetic acid as a main solvent, eliminates these problems,
because acetic acid is a suitable chelating agent reducing
the susceptibility of sols to fast hydrolysis. The new sols
have a very low sensitivity to moisture and a shelf life of
several months. Furthermore, preparation of the new sols
requires only 20 min whereas ethanol-based sols can re-
quire many hours.
The final solution, with a concentration of 0.36 M, was
light brown and clear without any suspension of par-
ticles. The solutions were spun at 2000 rpm for 60 s onto
(111) Si and (100) MgO substrates. After five layers of
coating, the films were approximately 500 nm thick.
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A.Y. Oral et al.: Phase and microstructural development of sol-gel-derived strontium barium niobate thin films
FIG. 3. Heating profile for the controlled furnace annealing.
III. RESULTS AND DISCUSSION
A. Effect of heat treatment on
microstructural development
DSC and TGA results indicate that crystallization oc-
curs around 535 °C, represented by a strong exothermic
peak at 535 °C in the DSC graphs with no corresponding
weight loss in the TGA analysis. In addition are three
temperature ranges associated with weight losses. The
first weight loss, extending from room temperature to
around 150 °C, also represented by an endothermic peak
in DSC, should be caused by evaporation of solvents. A
second sharp weight loss between 300 and 400 °C,
matched by an exothermic peak in DSC graphs, is pos-
sibly caused by the burnout of organics. A final weight
loss at temperatures between 710 and 730 °C is most
likely caused by the decomposition of bound organics
and the combustion of free carbon in the films and is
FIG. 4. Scanning electron micrographs of SBN50 film deposited on
accompanied by exothermic peaks in the DSC. The
weight loss between 710 and 730 °C is significantly less
when slower heating rates are used (5% weight loss with
a heating rate of 10 °C/min and 0.5% weight loss with a
heating rate of 5 °C/min).
(111) Si substrates prepared by fast thermal processing: (a) the pres-
ence of voids and cracks seen at lower magnifications, (b) nanoscale
porosity evident at higher magnifications.
All films, however, contained nanometer-scale poros-
ity. TEM micrographs show that the regions in Fig. 5(b),
defined by a uniform orientation of the elongated
nanoporosity, correspond to single grains (Fig. 6). The
orientation of the pores appears to be related to the crys-
tal anisotropy because the pores are elongated in the
same direction inside each grain.
SBN50 films on Si substrates exhibited substantial mi-
crostructural variations when different heating rates were
applied during annealing. The films produced by fast
thermal processing were full of cracks and bubbles
(Fig. 4). In contrast, films that were produced by con-
trolled furnace annealing, at slow heating rates, were
uniform and defect free on a macroscopic scale (Fig. 5).
The bubbles in the fast thermal processing films are
likely caused by entrapped gases between the substrate
and the film during the decomposition of organics and
the combustion of free carbon at higher temperatures.
The macroscopic voids seen in the SEM micrographs
may be developed by fast solvent evaporation at low
temperatures. Cracks form due to the rapid shrinkage of
the film during rapid heating. Abrupt temperature
changes during fast thermal processing enhance the crack
formation. Films produced by controlled furnace anneal-
ing did not contain macroscopic voids due to slower
decomposition and evaporation at lower temperatures.
B. Crystalline phases
Four different crystalline phases were formed through-
out the solid solution (0 ഛ x ഛ 1). The same phases were
found after annealing at 750 °C for both thin films and
powders (Figs. 7 and 8). Orthorhombic BaNb2O6 was
identified near the Ba-rich end, as predicted by the equi-
librium phase diagram [Fig. 7(a)]. An unexpected crystal
structure was identified in films and powders that had a
small amount of Sr [Figs. 7(b) and 8(a)]. This new phase,
which has been previously identified as an orthorhombic
tungsten bronze phase by some researchers,4,9 exists as a
single phase at intermediate Sr values (0.15 ഛ x ഛ 0.45).
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A.Y. Oral et al.: Phase and microstructural development of sol-gel-derived strontium barium niobate thin films
FIG. 5. Scanning electron micrographs of SBN50 film deposited on
FIG. 6. (a) Bright-field TEM image of SBN50 thin films on (111) Si
substrate. (b) Dark-field TEM image of SBN50 thin films on (111) Si
substrate. Regions with similarly oriented porosity are single grains.
(111) Si substrate prepared by controlled furnace annealing: (a) uni-
form microstructure observed at low magnifications, (b) nanoscale
porosity with grainlike regions evident at higher magnifications.
This unexpected phase has d spacings and intensities
similar to hexagonal (pseudo-orthorhombic) BaNb2O6.
Barium metaniobate, BaNb2O6, has two polymorphs: or-
thorhombic and hexagonal. The hexagonal structure is a
sheared version of the tetragonal tungsten bronze struc-
ture with some edge-sharing octahedra replacing corner
sharing. The hexagonal form is a metastable structure
between the orthorhombic structure and tungsten bronze
structure and has been found when alkoxides are used to
produce barium metaniobate at low temperatures.10 Sol-
gel systems involve nonequilibrium processing, and
when metal alkoxides are used, there can be a correlation
between the structure of the molecular units in the sol and
gel form and the final crystal structure.8 Major structural
changes can be inhibited due to insufficient thermal en-
ergy at the very low calcination temperatures used in
sol-gel processing. Strong chemical bonds form in the
metal oxides after the removal of organics and nonequi-
librium phases can result. The hexagonal form of barium
metaniobate, produced from alkoxides, transforms to the
orthorhombic modification when heated up to 1250–
1310 °C.10 During cooling, the orthorhombic-to-
hexagonal transformation does not occur, which
indicates that the transformation is irreversible and the
hexagonal phase is metastable.10 The unexpected SBN
phase identified here for 0.15 ഛ x ഛ 0.45 may be a
distorted version of the hexagonal BaNb2O6 structure
with Ba ions replaced by smaller Sr ions.
Around the middle of solid solution (x ס 0.5), the
tetragonal tungsten bronze phase starts to form
[Figs. 7(c) and 8(b)]. The reason for this transformation
should be the increasing instability of the hexagonal
(pseudo-orthorhombic) BaNb2O6 structure due to in-
creasing concentration of smaller Sr ions. The BaNb2O6
structure has only one single-size interstitial site (large).
When two different cations (Ba,Sr) with a considerable
size difference are present in the equilibrium concentra-
tion, the tungsten bronze structure with different sizes of
interstitial sites becomes the most stable structure due to
the reduction of strain in the structure. In these sol-gel-
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A.Y. Oral et al.: Phase and microstructural development of sol-gel-derived strontium barium niobate thin films
FIG. 7. XRD patterns of SBN powders: (a) BN, (b) SBN20, (c) SBN60, and (d) SN.
FIG. 9. An approximate phase diagram for SBN solid solutions after
annealing sol-gel-derived SBN at 750 °C.
large Ba ions decreases, the need for the large interstitial
sites also decreases and the orthorhombic SrNb2O6 struc-
ture with smaller interstitial sites becomes more stable
than the tetragonal tungsten bronze structure. The phase
diagram for SBN powders and thin films after low-
temperature processing at 750 °C is summarized
in Fig. 9.
FIG. 8. XRD patterns of SBN thin films (a) SBN20 and (b) SBN60.
The new low-Sr phase of SBN found in this work and
identified as a hexagonal (pseudo-orthorhombic) phase
based on the diffraction match with hexagonal BN has
derived materials, the tetragonal tungsten bronze phase
was found to exist as a single phase only in a narrow
composition range around x ס 0.65.
Near the SrNb2O6 end (0.85 ഛ x), the equilibrium
orthorhombic SrNb2O6 phase (isostructural with
CaTa2O6) was identified [Figure 7(d)]. As the number of
been previously attributed to the incommensurate high-
temperature orthorhombic phase of SBN that occurs in
single crystals grown from the melt.6 It is unlikely, how-
ever, that the incommensurate orthorhombic structure en-
countered in single crystals is the same as the phase
identified in this paper as the low-temperature hexagonal
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A.Y. Oral et al.: Phase and microstructural development of sol-gel-derived strontium barium niobate thin films
(pseudo-orthorhombic) phase of SBN formed after sol-
gel processing. Single crystals are produced at high tem-
peratures (T > 1400 °C) from liquid SBN and should
represent an equilibrium high-temperature phase. The
low-temperature phase identified here is metastable and
transforms to the tetragonal tungsten bronze structure
after annealing at 1200 °C.
ions. The thermal energy supplied by the annealing at
750 °C may be insufficient to rearrange the local orien-
tation of edge-sharing Nb–O octahedra to form corner-
sharing Nb–O octahedra as required by the tetragonal
tungsten bronze structure. Annealing at higher tempera-
tures (1200 °C) supplies more thermal energy and results
in the formation of the equilibrium tetragonal tungsten
bronze structure. It should also be noted that due to the
edge sharing of Nb–O octahedra in the hexagonal
(pseudo-orthorhombic) structure, it is unlikely that this
metastable SBN phase is ferroelectric.
Francombe11 has reported that there is a slope change
in the lattice parameter shift at x ס 0.45 for single-
crystal SBN as the Sr/Ba ratio of the solid solution is
varied. This slope change may be due to the formation of
a new phase. Carruthers and Grasso12 detected an ortho-
rhombic splitting of the diffraction peaks in SBN barium-
rich solid solutions of SBN ceramics (x < 0.45). In this
current work, a nontetragonal tungsten bronze phase also
exists as a single phase in solid solutions with x < 0.45,
but the d spacings of this phase do not match the ortho-
rhombic splitting observed by Carruthers and Grasso.12
TEM high-resolution lattice images obtained from the
low-temperature hexagonal (pseudo-orthorhombic)
phase (Fig. 10) include a lattice spacing of 0.84 nm that
does not match the orthorhombic phase.
An orthorhombic distortion in barium-rich solid solu-
tions (x < 0.5) of SBN single crystals has also been
observed by Bursill and Lin13,14 and Viehland et al.6
This orthorhombic structure has a uniform mixing of
blocks of two orthorhombic cells with lattice parameters
a ס 17.58Å, b ס 35.16Å, c ס 7.83Å and a ס 17.58Å,
b ס 17.58Å, c ס 7.83Å in the ratio of 3:1.6,13,14 The
incommensurate structure forms due to shearing of oxy-
gen octahedra in the a–b plane. In the results presented
here, there is no evidence of the large d spacings that
should be present in diffraction patterns of the high-
temperature orthorhombic phase.13,14
C. Orientation
Thin films grown on (111) Si had a random orienta-
tion, with thin-film diffraction patterns similar to the
powders (Figs. 7 and 8). Oriented ferroelectric thin films,
however, are highly desired, because larger values of
remnant polarizations can be obtained.14
The use of (100) MgO substrates enhanced for forma-
tion of the tetragonal tungsten bronze phase [Fig. 11(a)].
There is a close lattice matching of three unit cells of
(100) MgO (a ס 0.42 nm) with the (100) and the (010)
planes of the SBN tetragonal tungsten bronze structure.
The lattice mismatch ranges from 1.8% to 1.3% as the Sr
content of the solution decreases from x ס 0.75 to x ס
0.25. In these experiments single-phase tetragonal tung-
sten bronze could be formed at x ס 0.73 on (100) MgO
while the orthorhombic SN phase began to form for SBN
thin films on Si substrates.
Despite the close lattice match, no apparent grain ori-
entation was observed on MgO when the films were
produced by controlled furnace annealing. In contrast, a
strong preferred (001) orientation was apparent for the
films produced by fast thermal processing [Fig. 11(b)].
This different in grain orientation suggests that the role of
the substrate is more pronounced at higher heating rates,
possibly due to the initiation of crystallization at the
substrate–film interface and some degree of epitaxial
growth. However, the faster crystallization also intro-
duced some of the orthorhombic SrNb2O6 phase.
However, Sakamato et al.9 have suggested that the
unknown phase in sol-gel-derived SBN50 powders an-
nealed at 700 °C should be orthorhombic. Their un-
known phase transformed to a tetragonal tungsten bronze
structure when annealed at 1200 °C, similar to the trans-
formation found in our experiments. The main diffraction
peak in the XRD patterns, around 2⌰ ס 30°, shifted
from the main lattice spacing of BN to SN as the Sr
content increased. From this shift the researchers sug-
gested that a possible orthorhombic solid solution could
form between BN and SN at low temperatures.9 Al-
though similar peak shifts were also encountered in the
work reported here, a distinct tetragonal tungsten bronze
phase was found to form in Sr-rich solid solutions. In
addition, it should be noted that the crystal structures of
BaNb2O6 and SrNb2O6 are not isostructural, so complete
solid solubility is not possible.
IV. CONCLUSIONS
(1) Good quality SBN thin films can be produced by
sols made by dissolution of alkoxides in acetic acid.
(2) The heating rate is a key aspect to the film quality.
Controlled slow heating rates with holds at the decom-
position temperatures allow decomposition reactions to
occur at lower temperatures and sintering at higher tem-
peratures to produce defect-free films.
(3) Two orthorhombic structures, a hexagonal
(pseudo-orthorhombic) structure, and one tetragonal
tungsten bronze crystal structure were identified through-
The hexagonal (pseudo-orthorhombic) structure en-
countered in SBN thin films and powders, produced by
sol-gel processing, could be a distorted version of hex-
agonal BaNb2O6 due to the replacement of Ba ions by Sr
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A.Y. Oral et al.: Phase and microstructural development of sol-gel-derived strontium barium niobate thin films
FIG. 11. XRD plots: (a) SBN73 thin films on (100) MgO substrate
after controlled furnace annealing, (b) (001)-oriented SBN73 thin
films on (100) MgO with some orthorhombic (O) SrNb2O6 phase
present in addition to the tetragonal tungsten bronze (T) phase after
fast thermal annealing.
out the solid solution for both powders and thin films on
(111) Si substrates. The Ba/Sr ratio was the main factor
determining crystal structure whereas choice of substrate
has a less-pronounced effect.
(4) Increased (001) orientation of SBN was observed
for films deposited on (100) MgO when fast thermally
processed.
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FIG. 10. (a) Nanoscale porosity in sol-gel-derived SBN50 powders
[similar to the nanoscale porosity found for thin films in Fig. 7(b)]. (b)
TEM HREM image of SBN50 powders in a region with the tetragonal
tungsten bronze phase. (100), (010) d spacings of 1.24 nm can be
observed. (c) TEM HREM image of SBN50 powders with the orthor-
hombic phase. Large fringes are moire´ fringes from overlapping
grains. A d spacing of 0.8 nm can be observed.
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