CRYSTALGROWTH& DESIGN

Multipod Crystals of Perovskite SrTiO3Takeshi Toshima,*,‡ Hiroyuki Ishikawa,† Satoshi Tanda,‡ and Tomohiro Akiyama†
Department of Applied Physics, Hokkaido UniVersity, Sapporo 060-8628, Japan, and Centre for
AdVanced Research of Energy ConVersion Materials, Hokkaido UniVersity, Sapporo, 060-8628 Japan2008
VOL. 8, NO. 7
2066–2069

ReceiVed January 27, 2008; ReVised Manuscript ReceiVed May 12, 2008

ABSTRACT: We use our designed self-propagating high-temperature synthesis (SHS) method for the synthesis of the multipod crystalsof cubic perovskite strontium titanate (SrTiO3), which has a scale of about 5 µm in length and 200 nm in the cross section of legs. Electronmicroscopy and diffraction measurements show that the formation mechanism of these multipod crystals is not the same as previous reportsof hexapod and tetrapod crystals but fairly similar to snow crystals, in which multi-needle-like crystals lie a on two-dimensional plate andtwo needle-like crystals grow perpendicular to them. These SrTiO3 multipod shaped crystals should provide promising materials for
fundamental investigations on nanomicro-scale shape dependent unique properties such as piezoelectricity, opt-electricity, catalyticity,superconductivity, and so on. As the first success of synthesis of multipod-shaped perovskite crystals, our method shows a possibility thatany materials would form the multipod shapes.
Introduction. Manipulating the shapes of nanomicro crystals
remains an important goal of modern materials science. Whenbuilding nontrivial shapes, self-organization offers the potential forapplications to three-dimensional networks,1 and optical2 andmechanical3 devices. These nontrivial shapes are often obtained
as simple structures constructed with one- or two-element systems.4
For example, the synthesis of ZnO tetrapod5 crystals has beensuccessfully controlled, and they are used as functional materials
owing to their nontrivial shapes. Systematic manipulation ofmultipod shapes, such as tetrapod6 and hexapod7 crystals, hasenough potential for the above capabilities. We sought to provethat more functional materials, composed three or more elements,
would form the multipod shapes.
The ternary transition metal oxide systems, including SrTiO3 andhigh Tc superconductors, with a perovskite structure, are noteworthy
for their exceptional dielectric, piezoelectric, electrostrictive, electrooptic, superconducting, and catalytic properties with electromechanical devices, capacitors, actuators, transducers, high-kdielectrics, field-effect transistors, and so on. Since the perovskite
systems have complex crystal structures, they need a longer
formation time during the crystallization than that of one or twoelement systems. So, the crystals would tend to become usual
growth forms with the crystal habits due to being synthesized undernearly equilibrium conditions.8 However, multipod-shaped crystals
are only synthesized under nonequilibrium conditions.5–7,9,10
Therefore, perovskite systems with multipod shapes have never beenrealized.
Strontium titanate (SrTiO3) is an inorganic conductor with aperovskite structure (space group: Pm3m, cubic structure with a )3.905 Å) and is employed for research related to photoluminescence,11 catalyst,12 and substrates.13 SrTiO3 has normally beensynthesized under nearly equilibrium conditions (in the solid orliquid phase), because of not only the complex crystal structure
but also the high boiling point of the starting material (Ti, 3560K); it is difficult to realize a vapor phase (nonequilibrium)
crystallization. If vapor phase crystallization were realized, thendifficulty of the vaporization would be highly advantageous as a
responsiveness parameter in the crystallization.
In order to resolve the preceding problems, here we report ourdesigned self-propagating high-temperature synthesis (SHS) method

for multipod crystals of SrTiO3. We control the growth forms ofSrTiO3 crystal by choosing the shape of the starting material.
Choosing the starting materials with a larger surface area enhances
the rate of the chemical reaction and indicates the rapid heating
and quenching crystal growing conditions needed to synthesize themultipod shapes. We also propose their formation mechanism bycomparing those of other multipod crystals. Our results are close
to previous reports of hexapod crystals, but the formation mechanism is close to that of the snowflake crystal.

* To whom correspondence should be addressed. Present address: Toyama
National College of Technology, Toyama-shi, Japan. E-mail: t.toshima@toyama-nct.ac.jp.
‡Department of Applied Physics, Hokkaido University.
†Centre for Advanced Research of Energy Conversion Materials, HokkaidoUniversity.

Results and Discussion. Figure 1, panels A and B show
scanning electron microscopy (SEM) images of SrTiO3 single
crystals with cubic and octahedral shapes, respectively. In ourmethod, these crystals are obtained by using spherical shaped
powder. There are no appreciable differences between our method

Experimental Section. We synthesized the SrTiO3 crystals
using our designed SHS method, which is a very simple thermodynamic nonequilibrium process for material synthesis. The
experimental procedure is as follows: the one end of starting
mixtures (titanium powder, sodium perchlorate (NaClO4), andstrontium carbonate (SrCO3)) is ignited by the external heating inan argon atmosphere at room temperature. Then its heat energy
causes a combustion wave to propagate rapidly through the mixture.
Its production rate is 5.0 g/s and the desired product is formed
according to the following chemical reaction:
SrCO3 + Ti + 0.5NaClO4 f SrTiO3 + CO2(gas) +0.5NaCl(gas)(1)

This is an exothermic reaction. After the chemical reaction, theproduct is rinsed in water to remove the sodium chloride.
The key to our success of the controllable crystal growing
condition in vapor phase synthesis is the choosing of irregularly
shaped titanium powder (the diameter of the particle is less than
45 µm and has a nonsmooth surface). Powder consisting of small
particles with irregular shapes has a larger surface area than thesame weight of powder with a spherical (smooth) surface. Choosing
the starting materials with a larger surface area leads to two typesof enhancements in the chemical reaction, which are a highertemperature (The adiabatic temperature of the equation is 3860 K
in maximum, which exceeds the boiling point of the other materials
in the chemical reaction) and faster propagation of front line of
the chemical reaction (similar to a blast wave). Additionally, sucha self-propagating chemical reaction rapidly heats and quenches
the system. This conduction is the advantage for realization ofmultipod-shaped crystals.

10.1021/cg800098r CCC: $40.75  2008 American Chemical Society
Published on Web 06/13/2008

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Crystal Growth & Design, Vol. 8, No. 7, 2008 2067

Figure 1. SEM images of SrTiO3 crystals with (A) cubic shape and(B) octahedral shape. Red spheres are Sr atoms, blue spheres are Ti,and green spheres are O. Scale bar is 1 µm.

and previous reports. These structural geometries are explained by
the crystal morphology.
Figure 2, panels A and B, respectively, are SrTiO3 hexapod anddecapod crystals synthesized by using irregular shaped powder. The
angle between neighboring legs is approximately 90° in Figure 2A.
As seen in the inset of Figure 2B, eight legs lie on a plate (b-cplate), and two legs are perpendicular to them (along the a axis).
These results are clearly different from previous reports,9 in which
multi-needle crystals are connected at one end, their lengths are
not the same and the configuration rule of legs is not in force. It isdifficult to believe that our results are coincidental.
Figure 3, panels A and B, respectively, are X-ray diffraction(XRD) and energy dispersive X-ray spectroscopy (EDS) analysisof SrTiO3 crystals. From the analysis of (A), the production isalmost SrTiO3 crystals. Other productions (Sr2TiO4, Sr3Ti2O7, TiO2,and SrCO3) are very few. The ratio of atoms is shown in the insetof Figure 3B. The quantitative analysis of the production is aboutSr/Ti ) 51.15:48.85.
The discovery of multipod crystals poses the problem ofdetermining their formation mechanism. Our multipod crystals aresimilar in appearance to previously reported materials of hexapod7crystals, tetrapod5,6 crystals and snowflake10 crystals, where themechanisms are clearly different in each case. Which formation
mechanism is closest to our results?
The geometry of hexapod crystals is most similar to that of ourmultipod crystals. Their sizes are comparable to our results.
However, the time scale of the chemical reaction is far from ourFigure 2. SEM images of SrTiO3 crystals with (A) hexapod (6-pods)
shape and (B) decapod (10-pods) shape. Insets are models of configuration of the SrTiO3 legs. All of the red-colored legs in panel b lie on
the cyan-colored disk (b-c plate). Scale bar is 2 µm.

results. Our multipod crystals are obtained in few seconds, but otherapproaches require several hours. Additionally, we find that thecore and leg cross-sections are a similar size, whereas the core crosssections of previously reported crystals were smaller than that of
the legs.7
Tetrapod crystals also have a similar geometry. Their growing
condition is also reached in the vapor phase of the starting materials.
Additionally, the nondendritic shape is similar to our results, andthe relationship between the sizes of the core and legs is closer
than that of previously reported hexapod crystals. Tetrapod crystals
can have such a nontrivial geometry because they are polycrystals.
Four needle-like single crystals are joined at the center of a tetrapod
crystal owing to stacking faults with the wurtzite and zinc blende
structures.5,6 However, SrTiO3 crystal has a simple cubic structure.
Therefore, there are no stacking faults such as those found withtetrapod crystals. Figure 4A,B, respectively, are a transmission
electron microscopy (TEM) image and transmission electron
diffraction (TED) pattern of a SrTiO3 single crystal with a cubic
body and a leg obtained from the same batch of multipod crystals.
The leg grows in the [110] direction and forms an angle of 135°with the two sides of the cubic body. The TED patterns are exactly
the same for the cubic body and the end of leg. An analysis of theTEM image and TED pattern shows that the cubic body and theleg are seamless (a single-phase crystal).

2068 Crystal Growth & Design, Vol. 8, No. 7, 2008

Figure 3. (A) X-ray diffraction (XRD) analysis of SrTiO3 crystals.
Upper: Using Ti powder with spherical shapes and large radius (∼150µm). Middle: Spherical shapes and small radius (∼45 µm). Lower:
Irregular shapes and small radius (∼45 µm). (B) Energy dispersive
X-ray spectroscopy (EDS) analysis of SrTiO3 crystals obtained by using
the irregular shaped powder. The peak of carbon is due to substrate(we use the graphite as the substrate in this analysis). The peaks ofcopper and zinc are noises due to the experimental system.

Snowflake crystal growth, which is a well-known example ofvapor phase growth as expressed in the Nakaya-diagram, corresponds to the growth of snowflake crystal shapes that depend on
temperature and vapor saturation. Our synthesis is the supersaturated
state; the vapor density is not uniform around the crystal nucleus.
The edges of the crystal nucleus experience a higher vapor density
and grow more rapidly than at other positions.14 This is known as
the Berg effect. If legs grow at the six edges of a crystal nucleus
with an octahedral shape, a hexapod crystal will be obtained (insetof Figure 2A). This condition is exactly the same as that ofsnowflake crystal growth except for the accompanying chemicalreaction. The decapod crystal in Figure 2B has eight legs (red) on
a two-dimensional plate and two legs (blue) perpendicular to theplate, as seen in the inset. The snowflake crystals exhibit twodimensional tabular and columnar growth, and the columnar legs
are also perpendicular to the tabular legs. And the two-dimensional
tabular crystal growth takes on an infinite variety of form. Thisconfiguration accounts for our crystal growth mechanism in thesame way as that of snowflake crystals.

Communications

Figure 4. SrTiO3 single crystal (cubic body with a leg). (A) TEM image.
Inset is a sketch of this crystal. (B) TED pattern of SrTiO3 single crystal.
The TED analysis shows that the leg grows [110] direction.

Conclusion. In summary, single-crystalline SrTiO3 multipod
crystals have been synthesized by the SHS method. Choosing theshapes of starting materials changes the efficiency of the chemical
reaction and indicates the controllable vapor phase crystal growthcondition. Specifically, this is the first report of multipod crystals
with perovskite structure (three elements system). We compared
their formation mechanism with those of other crystals with a starlike geometry. Our crystals have a similar shape to hexapod andtetrapod crystals, but the formation mechanism is the same as thatof snowflake crystals. Multipod perovskite crystals will findapplications in various fields, for example, with SrTiO3, which isa good catalyst owing to its large specific surface, in threedimensional devices such as cellular phone antennas owing to their
fixed high dielectric constant, and the success of the multipod
shaped perovskite crystal shows the possibility of three-dimensional
high-Tc superconductor networks.
Acknowledgment. We wish to acknowledgement Prof. N.
Hatakenaka, Dr. K. Inagaki, Dr. Lihua Zhang, and Dr. D. Meacockfor stimulating discussions and useful information on related
subjects. This work has been partially supported by Grant-in-Aidfor the 21st Century COE program on “Topological Science andTechnology” from the Ministry of Education, Culture, Sport,
Science and Technology of Japan.

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CG800098R