Synthesis and Characterization of CaPd<sub>3</sub>O<sub>4</sub> Crystals

doi:10.4236/jcpt.2012.21003

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Journal of Crystallization Process and Technology, 2012, 2, 16-20

http://dx.doi.org/10.4236/jcpt.2012.21003 Published Online January 2012 (http://www.SciRP.org/journal/jcpt)

Synthesis and Characterization of CaPd3O4 Crystals

Hiroaki Samata1*, Satoshi Tanaka1, Soichiro Mizusaki2, Yujiro Nagata2, Tadashi C. Ozawa3,

Akira Sato4, Kosuke Kosuda4

1Graduate School of Maritime Sciences, Kobe University, Kobe, Japan; 2College of Science and Engineering, Aoyama Gakuin Uni-

versity, Sagamihara, Japan; 3International Center for Materials Nanoarchitectonics, National Institute for Materials Science, Tsukuba,

Japan; 4Materials Analysis Station, National Institute for Materials Science, Tsukuba, Japan.

Email: *[email protected]

Received November 15th, 2011; revised December 21st, 2011; accepted December 29th, 2011

ABSTRACT

A new method for the crystal growth of alkaline-earth palladate CaPd3O4 was developed. The crystals were synthesized

on a voltage-applied electrode in a molten chloride solvent. The maximum length of the crystal was about 1.5 mm. The

X-ray diffraction data were refined well by assuming a cubic structure of the space group Pm 3n, and the lattice con-

stant a was 5.7471 (10) Å. The temperature dependence of the resistivity showed semiconductor-like characteristics

with a very small activation energy Ea of 0.45 meV at low temperatures, and the resistivity at 300 K was 0.1 ·cm. The

temperature dependence of the molar magnetic susceptibility showed the Curie-Weiss paramagnetic behavior with a

small molar Curie constant Cmol of 5.0(1) × 10–3 emu K/(mol·Oe), indicating the existence of localized spin moments.

Keywords: Single Crystal; Electrochemical Technique; CaPd3O4

1. Introduction

Alkaline-earth palladate CaPd3O4 has a cubic NaPt3O4-type

structure and exhibits a semiconductor-like temperature-

dependent resistivity [1,2]. Hase and Nishihara calculated

the band structure of CaPd3O4 by the FLAPW method

within LDA and suggested that CaPd3O4 is a potential

excitonic insulator in which electrons and holes bind as

excitons [3]. Ichikawa and Terasaki reported that CaPd3O4

was a degenerate semiconductor with a low carrier con-

centration [4]. Chemical substitution on insulating com-

pounds occasionally produces interesting physical prop-

erties, and insulator-metal transitions have been observed

in the Ca1–xNaxPd3O4 and Ca1–xLixPd3O4 systems [2,4-6].

The aliovalent ion-substituted CaPd3O4 is considered to

be a good representative of a thermoelectric material [4].

Thermoelectric energy conversion in solids has great po-

tential as an energy-saving technology, but this type of

conversion is not widely used due to the poor perform-

ance of the materials.

We have previously investigated the effects of aliova-

lent ion substitution on the properties of SrPd3O4, which

is isostructural with CaPd3O4, and we observed the exis-

tence of insulator-metal transitions and bipolar conduc-

tivity [7,8]. In this system, the substitutions of Na+ and

Bi3+ for Sr2+ introduced hole and electron carriers, re-

spectively. The power factor of Sr1–xNaxPd3O4 (x ≤ 0.2)

was not sufficient for practical use as a thermoelectric ma-

terial, but increasing the amount of substitution and de-

vising an appropriate synthesis method may enhance its

thermoelectric performance [8].

A strategy for searching for novel thermoelectric ma-

terials was suggested by the analysis of single-crystal data

for Ce-based compounds [9]. Moreover, the use of a high-

quality single crystal was effective for practical applica-

tions of thermoelectric materials [10]; therefore, the de-

velopment of new crystal growth techniques is important

for practical use as well as for the theoretical analysis of

thermoelectric materials.

The growth of CaPd3O4 crystals was previously attempted

using the conventional flux method; potassium hydroxide

was used as the flux, and the crystallographic properties

were characterized [11]. However, no other study has in-

vestigated the crystal growth, and the intrinsic properties

of this material have not been elucidated in detail. Elec-

trosynthesis in a molten flux effectively prepares single

crystals of transition metal oxides [12], and we recently re-

ported the crystal growth of certain oxides on electrodes

in molten chloride solvents [13]. In the current study, we

describe the results of the synthesis and the characteriza-

tion of CaPd3O4 crystals.

2. Experimental

Single crystals of CaPd3O4 were synthesized on the sur-

face of voltage-applied platinum rods submerged in mol-

*Corresponding author.

Synthesis and Characterization of CaPd3O4 Crystals 17

ten chloride solvent. A schematic of the apparatus used

for the crystal growth is shown in Figure 1 of [13]. An

alumina crucible with a capacity of 20 or 30 cc was used

to hold the solvent, which was a mixture of CaCl2 and

NaCl. An appropriate amount of PdO powder (99.9%) was

placed in the crucible, and the chloride mixture was lay-

ered over the PdO powder. The total amount of chlorides

was 20 or 30 g, and the molar ratio of NaCl/CaCl2 varied

in the range of 0 to 5.3; a PdO to CaCl2 molar ratio of

0.04 to 0.12 was used. The crucible was placed in an alu-

mina container, and two parallel platinum rods (0.5 mm

in diameter) were inserted into the solvent at a distance

of 7 mm from each other. The container was covered by

an alumina plate and placed in an electric furnace with an

ambient atmosphere. The depositions were performed at a

constant temperature in the range of 780˚C to 1050˚C,

which accounted for the melting points of CaCl2 (772˚C)

and NaCl (801˚C). After the solvent was melted at each

synthesis temperature, an electrical voltage in the range of

0.2 to 0.4 V was applied to the electrodes using a DC power

supply. The synthesis was carried out over 48 to 96 hours;

the applied voltage was then turned off, and the electrodes

were quickly extracted from the molten solvent. The crys-

tals were washed thoroughly in distilled water to remove

the solvent from the surface of the crystals, which were

then dried on a hot plate. Selected crystals were annealed

at 500˚C for 7 days under flowing oxygen gas at 1 atm.

The chemical composition of the crystals was charac-

terized by electron-probe microanalysis (EPMA; JEOL,

JXA-8500F). The crystal structure was identified by sin-

gle-crystal X-ray diffraction and powder X-ray diffract-

tion. Single-crystal X-ray diffraction data were acquired

on a Bruker SMART APEX S diffractometer equipped

with a pyrolytic graphite incident monochromator and a

CCD camera at room temperature. The crystal was mo-

unted on a glass fiber, and X-rays were generated using a

(a) (b)

Figure 1. Photographs of as-grown CaPd3O4 crystals syn-

thesized by applying a voltage of 0.2 V at 950˚C for 48 h: (a)

cry stals on the platin um anode rod and ( b) a crystal remove d

from the electrode.

Mo target at 50 kV and 30 mA. A total of 3597 reflections

were collected for the full sphere using a 0.3˚ ω-scan with

a 40 s exposure. The crystal structure was solved by di-

rect methods using SHELXS-97, and the structure was

refined utilizing the SHELXL-97 software package, which

used 130 unique reflections and 7 parameters [14]. Pow-

der X-ray diffraction data were acquired using Cu K



generated at 40 kV and 20 mA in a 2



range of 20˚ - 80˚

at room temperature (Rigaku, RINT2000). The data were

refined by the Rietveld method using the values obtained

from the refinement of single-crystal X-ray diffraction

data [15].

The electrical resistivity of the crystals was measured

using a conventional DC two-probe method in the tem-

perature range of 10 to 300 K; a refrigerator was used to

maintain temperatures lower than room temperature. The

electrical contacts were established by attaching gold leads

onto the surface of the crystal using silver paste to form

an ohmic contact. Magnetic measurements were carried

out using a superconducting quantum interference device

magnetometer (Quantum Design, MPMS2) in the tempe-

rature range of 5 to 300 K in an applied magnetic field of

10 kOe.

3. Results and Discussion

Figure 1(a) shows a photograph of the CaPd3O4 crystals

grown by applying a voltage of 0.2 V at 950˚C for 48

hours; a molar ratio of PdO:CaCl2:NaCl of 0.06:1:2 was

used. The electrical current was approximately 1 mA at the

end of crystal growth. These conditions were the most sui-

table for the growth of large crystals within the range of

the present experimental conditions. Green-tinted black

crystals grew on the surface of the anode. Because the crys-

tals were grown in a molten flux, the natural surface of

the crystal was observed, as shown in the photograph. Fi-

gure 1(b) shows a photograph of a crystal removed from

the electrode. The maximum length of the crystal was about

1.5 mm. The EPMA measurements indicated that the Ca/Pd

molar ratio of the as-grown crystal was 0.35. This value

agrees with the calculated value of 0.33 that was assu-

med based on the chemical formula of CaPd3O4. The mea-

sured value of 0.35 did not vary in the direction of crystal

growth. Because an alumina crucible, platinum electrodes,

and chloride solvents were used in the synthesis, Al, Pt,

Na, and Cl could have potentially been incorporated into

the crystal; however, the EPMA measurements showed

no inclusion of these elements. Tiny CaPd3O4 crystals also

grew on the bottom of the crucible, but the size of each

crystal was 0.01 mm or less, which is significantly smaller

than the crystals grown on the anode. Based on this dif-

ference in size, we conclude that the application of the

electrical voltage to the electrodes had a significant effect

on the growth of large crystals. Because positive Pd ions

Synthesis and Characterization of CaPd3O4 Crystals

in the solvent were attracted by the cathode during the

synthesis, the cathode was covered with metallic palladium.

An increase in the amount of NaCl used in the synthesis

resulted in a decrease in the amount of metallic palladium

on the cathode; thus, NaCl seemed to inhibit the reduct-

ion of PdO.

The crystal structure of the CaPd3O4 crystal was char-

acterized by the refinement of single-crystal XRD data,

and the results are summarized in Tables 1 and 2. The

data were refined well by assuming a cubic structure of

the space group Pm 3n. The refined lattice constant a was

5.7471(10) Å, and this value agrees well with those re-

ported in previous studies [1-2,4-6]. The crystal structure

was also characterized by powder XRD. Figure 2 con-

tains the powder XRD profiles and the results of the re-

finement of the data. The data were refined well with re-

liability factors Rwp = 8.10, Re = 8.07, and S = 1.004, and

all peaks were indexed as a NaPt3O4-type structure. The

refined a of 5.7471 Å is in fair agreement with that ob-

tained by the refinement of the single-crystal XRD data.

This result demonstrates that all crystals have the same

structure. Figure 3 shows the crystal structure of CaPd3O4.

In this figure, palladium atoms are bonded with neighbor-

ing oxygen atoms; the distance between palladium and

oxygen atoms is 2.0319(2) Å, and each palladium atom is

surrounded by four coplanar oxygen atoms that form a

PdO4 square planar unit. These units are perpendicularly

connected to each other via shared corner oxygen and form

a three-dimensional framework. The large natural surface

shown in Figure 1(b) was confirmed as the (100) plane of

the cubic crystal.

Figure 4 shows the temperature dependence of electri-

cal resistivity, which was measured for an as-grown crys-

Table 1. Crystallographic data and structure refinement data

for CaP d3O4.

Formula weight 423.28 g/mol

Space group Pm3n (No. 223)

a 5.7471(10) Å

V 189.822(6) Å3

Z 2

Abs. coeff. 15.277 mm–1

F(000) 380

Crystal size 80 m  80 m  40 m

Final R indices [I > 2σ(I)] a R1 = 0.0199, wR2 = 0.0439

R indices (all data) R1 = 0.0203, wR2 = 0.0445

0c 0

1RFF=-

åå

() ()

; wR2 =

{}

22 2

00c

wF FwF

ùé ù

úê ú

ûë û

åå

(

)

(

)

122

0 0.00771.34wF P



-éù

=+ +

êú

ëû

Table 2. Atomic coordinates and isotropic displacement pa-

rameters (Å2) for CaPd3O4.

x y z Uiso

Ca 0 0 0 0.0078(3)

Pd 1/4 0 1/2 0.0053(2)

O 1/4 1/4 1/4 0.0075(6)

Figure 2. Powder X-ray diffraction profile of CaPd3O4 crys-

tals and the results of refinement of the diffraction data by

the Rietveld method.

Figure 3. Crystal structure of CaPd3O4. Palladium atoms

are bonded with four neighboring oxygen atoms, which are

represented by gray bars. The solid line shows the unit cell.

P, where

()

max, 0 23.PF F

ëù

tal with a size of 0.103 × 0.130 × 0.647 mm3. The current

was flowed parallel to the [100] direction of a rectangular

parallelepiped crystal. The resistivity showed semicon-

ductor-like characteristics; the resistivity decreased as the

temperature increased. The resistivity at 300 K was 0.10

·cm. Because this measurement was performed with a

single crystal, the result was free from the effects of the

Synthesis and Characterization of CaPd3O4 Crystals 19

grain boundary scattering. The inset of Figure 4 shows an

Arrhenius plot. The data at lower temperatures can be fit-

ted by the thermal-activation process expressed by

exp ,









(1)

where 0



, Ea, and kB are the pre-exponential factor, the

activation energy, and the Boltzmann constant, respecti-

vely. An Ea of 0.45 meV was obtained; this value is

lower than that obtained for sintered material (1.1 meV)

[2] and is inconsistent with the result of the band calcula-

tion, which predicted a semimetallic band structure with

a zero band gap [3].

Figure 5 shows the temperature dependence of the mo-

lar magnetic susceptibility



for as-grown crystals and the

crystals annealed at 500˚C for 7 days under an oxygen

atmosphere of 1 atm. The measurements were conducted

in an applied magnetic field of 10 kOe during the heating

process after zero-field-cooling (ZFC) and field-cooling

(FC) process. The FC data were nearly identical to the ZFC

data and are not shown. Both plots illustrate paramagnetic

behavior with an increase of the susceptibility at low tem-

peratures; χ can be fitted by the modified Curie-Weiss

law expressed by

mol

TΘ





 (2)

where 0



, Cmol, and



are the temperature-independent

magnetic susceptibility, the molar Curie constant, and the

asymptotic Curie temperature, respectively. The solid line

shows the results which were obtained by fitting the mo-

dified Curie-Weiss formula to the data, using the Mar-

quardt-Levenberg algorithm [16]. The values 04.1







emu/(mol·Oe), Cmol = 5.0(1) × 03 emu K/

(mol·Oe), and



= –1.4(2) K were obtained for the as-

grown crystals. The effective number of the Bohr mag-

neton per chemical formula unit p was calculated using

the formula



215



Figure 4. Temperature dependence of the electrical resistiv-

ity of an as-grown crystal. The inset of the figure shows the

Arrhenius plot.

Figure 5. Temperature dependence of the molar magnetic

susceptibility





measured for as-grow n crystals and oxygen-

annealed crystals (10 kOe, ZFC).

mol



 (3)

where N, μB, and kB are the Avogadro constant, the Bohr

magneton, and the Boltzmann constant, respectively. The

value of p was determined to be 0.20/f.u. In CaPd3O4, Pd

is the only magnetic ion. Based on the stoichiometry of the

crystal, the valence of the Pd ion is 2+, and the electron

configuration is . In this structure with PdO4 tetragons,

the

d-energy level is the highest, and the 2

d, dxy, dyz,

and dzx levels are filled by electrons in a low-spin con-

figuration [8]. The band calculation also suggests that the

empty 22

y- forms the highest band [3]. Based on this

configuration, diamagnetism was expected for CaPd3O4,

and negative magnetic susceptibility was actually observed

at a high temperature range; however, the Curie-Weiss pa-

ramagnetic contribution was observed, suggesting the

existence of localized spin moments. Because the meas-

urements were performed for single crystals, the Curie-

Weiss paramagnetic behavior was not the effect of impu-

rity magnetic phases. Because the paramagnetic contribu-

tion didn’t decrease after the oxygen annealing of the

crystals, the Curie-Weiss paramagnetic behavior was not

the effect of the oxygen vacancies. The Curie-Weiss pa-

ramagnetic behavior is attributable to the existence of Pd3+,

which was induced by the existence of cation vacancies.

As previously reported, the molar ratio of Ca/Pd was de-

termined to be 0.35 by EPMA; this value is slightly lar-

ger than the value of 0.33 that is based on the stoichiome-

tric composition. This disparity suggests that Pd vacan-

cies are present, which would introduce Pd3+ in order to

maintain the charge neutrality of the system [4,6,17]. In

this case, the Curie-Weiss paramagnetic behavior would

be induced by the unpaired spin at the dxy level of Pd3+;

however, the exact cause of the Curie-Weiss paramagnetic

behavior remains unclear at present.

Synthesis and Characterization of CaPd3O4 Crystals

4. Conclusion [7] T. Taniguchi, Y. Nagata, T. C. Ozawa, M. Sato, Y. Noro,

T. Uchida and H. Samata, “Insulator-Metal Transition In-

duced in Sr1–xNaxPd3O4 for Small Na-Substitutions,”

Jour- nal of Alloys and Compounds, Vol. 373, No. 1-2,

2004, pp. 67-72. doi:10.1016/j.jallcom.2003.11.004

Single crystals of alkaline-earth palladate CaPd3O4 were

grown by an electrochemical technique that used voltage-

applied platinum electrodes inserted into a molten chlo-

ride solvent. Crystals with natural surfaces were obtained

on the anode, reflecting that the growth proceeded in a

liquid phase. The maximum length of the crystal was about

1.5 mm. The crystal structure parameters were refined well

by assuming a cubic structure of the space group Pm 3n,

and the lattice constant a was 5.7471(10) Å. The resistiv-

ity showed semiconductor-like characteristics with a very

small activation energy Ea of 0.45 meV at lower tempe-

ratures. The temperature dependence of the molar mag-

netic susceptibility showed the Curie-Weiss paramagne-

tic behavior, indicating the existence of localized spin mo-

ments.

[8] T. C. Ozawa, A. Matsushita, Y. Hidaka, T. Taniguchi, S.

Mizusaki, Y. Nagata, Y. Noro and H. Samata, “Synthesis

and Characterization of Electron and Hole Doped Ternary

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83. doi:10.1016/j.jallcom.2007.03.137

[9] J. Kitagawa, T. Sasakawa, T. Suemitsu, Y. Echizen and T.

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5. Acknowledgements

The work performed at Kobe University and Aoyama Ga-

kuin University was supported by Grants-in-Aid for Sci-

entific Research from the Ministry of Education, Culture,

Sports, Science and Technology of Japan.

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