*Corresponding author. Tel.: #81-3-3478-6811d5382; fax:#81-3-3401-5169.

E-mail address: masa@ginnan.issp.u-tokyo.ac.jp (M. Hasegawa)

Journal of Crystal Growth 203 (1999) 400}404

Growth and characterization of delafossite-typePd(Co

1~xMn

x)O

2(x"0.04, 0.11) single crystals

Masayuki Tanaka!, Masashi Hasegawa!,*, Humihiko Takei"

!Institute for Solid State Physics, University of Tokyo, Roppongi, Minato-ku, Tokyo 106-8666, Japan"Graduate School of Science, Osaka University, Machikaneyama, Toyonaka-shi, Osaka 560-0043, Japan

Received 22 October 1998; accepted 8 February 1999Communicated by T. Nishinaga

Abstract

Solid solution crystals of Pd(Co1~x

Mnx)O

2(x"0.04, 0.11) of high quality have been successfully grown using

a metathetical reaction. The maximum crystal sizes of Pd(Co0.96

Mn0.04

)O2

and Pd(Co0.89

Mn0.11

)O2

are about0.6a]0.6a]0.9c mm3 and 0.2a]0.2a]0.6c mm3, respectively. The crystals show silver metallic luster and are well-habited. The crystals have a polyhedral-barrel shape which is clearly di!erent from the hexagonal plate shape of purePdCoO

2crystals. In the crystals of Pd(Co

0.89Mn

0.11)O

2, a broad cusp at about 18 K is observed in the temperature

dependence of magnetization in a low magnetic "eld. ( 1999 Published by Elsevier Science B.V. All rights reserved.

PACS: 61.50.Jr; 75.30.Hx; 81.10.!h; 81.40.Rs

Keywords: Pd(Co,Mn)O2; Metathetical reaction; Delafossite type structure; Crystal morphology; Magnetic susceptibility

1. Introduction

Delafossite-type oxides are physically attractivebecause the structure has a two-dimensional tri-angular lattice which causes magnetic spin frustra-tion. We have succeeded in growing many kinds ofhigh quality crystals with delafossite structure, suchas CuFeO

x[1}3], PdCoO

2[4,5], PtCoO

2[4], etc.

Iron ions in CuFeOx

are magnetic, whereas Co3`

ions in the octahedral site of stoichiometricPdCoO

2are in a nonmagnetic low spin state [4}6].

It is interesting to investigate the substitution e!ect

of magnetic ions from nonmagnetic ions in the twodimensional triangular lattice. Thus, we focused oncrystals of Pd(Co,Mn)O

2where some parts of the

nonmagnetic Co3` ions were substituted by mag-netic Mn3` ions.

In this paper, crystal growth and characteriza-tion of the solid solution of Pd(Co,Mn)O

2are de-

scribed. E!ects of the Mn substitution on thegrowth features, crystal structures and magneticstates are clari"ed and discussed brie#y.

2. Experimental procedure

Crystals were grown using a metathetical reac-tion, which was used successfully for growing singlecrystals of PdCoO

2and PtCoO

2[6}8]. On the

0022-0248/99/$ - see front matter ( 1999 Published by Elsevier Science B.V. All rights reserved.PII: S 0 0 2 2 - 0 2 4 8 ( 9 9 ) 0 0 1 1 2 - 8

Fig. 1. SEM photograph of typical grown single crystals ofPd(Co

0.89Mn

0.11)O

2.

basis of this method, noble metal halides andtransition metal oxides are usually used as startingmaterials in accordance with the following chem-ical reaction formula:

PdCl2#2M(1!y)CoO#yMnON

PPdCo1~y

MnyO

2#(1!y)CoCl

2#yMnCl

2.

Powders of reagent grade Co (2N, Kou-jundoKagaku), PdCl

2(2N, Kokusan Kagaku) and MnO

(2N, Kou-jundo Kagaku) were used. The startingratios y"0.01 and 0.05 were tried for the reaction.The weighed (about 20 g in total) and well mixedpowder was sealed in an evacuated silica ampoule(12 mm in diameter, 10 cm in length) and heated atabout 7003C and then kept for 40 h. Heating andcooling rates were about 903C/h. Single crystalswere obtained by leaching the by-products, i.e. gen-erated halides with acetone, ethanol and dilutedwater. Residual cobalt oxides were then removedwith diluted nitric acid.

The grown single crystals were inspected by anoptical microscope and a scanning electron micro-scope (SEM) with an electron probe microanalyzer(EPMA). Then, they were characterized by meansof the X-ray powder di!raction (XPD) and X-rayprecession methods. The cell parameters were esti-mated from the XPD data using the least-squaresmethod, where silicon powder was used as an inter-nal reference. The chemical composition was deter-mined by EPMA. Temperature dependence of themagnetization in 10 kOe and 100 Oe was measuredby a superconducting quantum interference device(SQUID) magnetometer from 5 to 300 K. The mag-netic "eld was applied in a direction parallel orperpendicular to the c-axis of the crystals.

3. Results and discussion

3.1. Crystal growth

Solid solution crystals of Pd(Co0.96

Mn0.04

)O2

andPd(Co

0.89Mn

0.11)O

2having a well-habited polyhed-

ral-barrel shape were successfully obtained. The com-positions of the grown crystals have been determinedby SEM-EPMA. Fig. 1 shows an SEM photographof the typical single crystals of Pd(Co

0.89Mn

0.11)O

2.

The crystal morphology of Pd(Co0.96

Mn0.04

)O2

isalmost the same as that of Pd(Co

0.89Mn

0.11)O

2. The

crystals of Pd(Co0.96

Mn0.04

)O2

and Pd(Co0.89

Mn0.11

)O2

show silver metallic luster, suggestinga metallic conductivity as well as in PdCoO

2[4,5].

The X-ray precession method revealed that thecrystals were of the delafossite structure, as shownin Fig. 2. No extra spots were observed, indicatingrandom substitution of Mn atoms at Co sites in thetwo-dimensional triangular lattice. The elongationaxis of the crystals was the c-axis. The maximumsize of the grown crystal was about 0.6a]0.6a]0.9c mm3 for Pd(Co

0.96Mn

0.04)O

2and 0.2a]0.2a]

0.6c mm3 for Pd(Co0.89

Mn0.11

)O2. The sharp dif-

fraction spots without those of any other phasesindicate high quality (single phase, uniform composi-tion and good crystallinity) of the grown crystals, asshown in the precession photographs in Fig. 2.

It should be noted that the shape ofPd(Co

0.96Mn

0.04)O

2and Pd(Co

0.89Mn

0.11)O

2crystals is signi"cantly di!erent from that ofPdCoO

2crystals. As shown in Fig. 1, the solid

solution crystals have a polyhedral-barrel shape,whereas the pure PdCoO

2crystal has an hexagonal

plate shape [4}7], suggesting a change in thegrowth process of crystals by addition of Mn ions.

In the solid solution crystal, surfaces perpendicu-lar to the c-axis are almost smooth and #at, andpolygonal growth islands were sometimes ob-served. This result suggests that the crystals were

M. Tanaka et al. / Journal of Crystal Growth 203 (1999) 400}404 401

Fig. 2. X-ray precession photographs of the single crystal ofPd(Co

0.89Mn

0.11)O

2for (a) (h k 0) plane and (b) (h h l) plane.

Table 1Cell parameters of the grown crystals estimated by XPD

Formula a0

(As ) c0

(As )

PdCoO2

2.8317

17.740

PdCo0.96

Mn0.04

O2

2.8320

17.762

PdCo0.89

Mn0.11

O2

2.8376

17.812

grown through lateral advancement of steps on thec-plane. On the other hand, the six side walls of thecrystals always consisted of round smooth surfaceswhich are di$cult to be indexed by a simple integer.This indicates that the growth rate along the c-axis,R,, was larger in comparison to that perpendicular

to the c-axis, RM, by doping of MnO in the growth

system. As was discussed in Ref. [5], pure PdCoO2,

grown with well-habited hexagonal plate shape,crystallizes in the lateral advancement of steps onthe basal c-plane. When Mn ions arrive at thec-plane, the lateral growth would be hampered andconsequently, the ratio of the growth rates, R

M/R

,would become larger. This situation is convenientfor explaining growth of the Mn-doped PdCoO

2crystals in the polyhedral form.

3.2. Characterization

The cell parameters of the single crystal are sum-marized in Table 1. It is found that both parametersincrease upon increasing the substitution amountof Mn. Since it is reasonable that Mn is substitutedfrom Co in PdCoO

2on the basis of the crystal

chemistry on PdCoO2, Mn ions are considered to

be Mn3`. Although Co3` in PdCoO2

is in a lowspin state, substituted Mn3` ions are considered tobe in high spin state because they are usually in anhigh spin state in an octahedral coordination, forexample, Mn ions in perovskite type LaMnO

2.

Accordingly, the tendency of the cell parameter isreasonable because the radius of high spin Mn3`

ion (0.65 As [9]) in the octahedral site is consider-ably larger than that of substituted low spin Co3`

(0.525 As [9]).Temperature dependence of the magnetic suscepti-

bility parallel or perpendicular to the c-axis in 10 kOeshows a Curie}Weiss type behavior in the measuredtemperature range for both Pd(Co

0.96Mn

0.04)O

2and

Pd(Co0.89

Mn0.11

)O2. They showed a slight anisot-

ropy below around 60 K. Parameters estimated bythe least-squares "tting using the Curie}Weiss laware listed in Table 2. It is found that the e!ectivemagnetic moment increases upon increasingthe Mn concentration. Temperature dependence ofthe magnetic susceptibility at 100 Oe was also

402 M. Tanaka et al. / Journal of Crystal Growth 203 (1999) 400}404

Table 2Parameters estimated by the least-squares "tting using the Curie}Weiss law s"s

0#C/(¹!h), for the temperature dependence of

magnetic susceptibility between 20 and 300 K

Formula Direction s0

h C p%&&

(]10~4 emu/mol) (K) (emu K/mol) (lB)

PdCo0.96

Mn0.04

O2

c,

1.05 !2.64 0.106 0.921cM

1.19 !0.504 0.106 0.921

PdCo0.89

Mn0.11

O2

c,

0.689 !2.35 0.242 1.39cM

1.47 !1.29 0.236 1.37

Fig. 3. Temperature dependence of magnetic susceptibility in100 Oe magnetic "eld parallel or perpendicular to the c-axis for(a) Pd(Co

0.96Mn

0.04)O

2and (b) Pd(Co

0.89Mn

0.11)O

2.

measured. Pd(Co0.96

Mn0.04

)O2

shows just theCurie}Weiss type temperature dependence in themeasured temperature range as shown in Fig. 3a,whereas a broad cusp is observed around 18 K in

the magnetic susceptibility curves in both the direc-tions of Pd(Co

0.89Mn

0.11)O

2, as shown in Fig. 3b,

suggesting a magnetic phase transition.

4. Conclusions

In summary, solid solution crystals ofPd(Co

0.96Mn

0.04)O

2and Pd(Co

0.89Mn

0.11)O

2in

high quality were obtained using the metatheticalreactions. The maximum size is about 0.6a]0.6a]0.9c mm3 for Pd(Co

0.96Mn

0.04)O

2and 0.2a]0.2a]

0.6c mm3 for Pd(Co0.89

Mn0.11

)O2, respectively.

The crystals have silver metallic luster and arewell-habited. They have the polyhedral-barrelshape which is clearly di!erent from the hexagonalplate of PdCoO

2crystals. It is also found that the

crystals of Pd(Co0.96

Mn0.04

)O2

and Pd(Co0.89

Mn0.11

)O2

tend to grow along the c-axis, whereasthe PdCoO

2crystals tend to grow in the c-plane. It

should be noted that the crystals of Pd(Co0.89

Mn0.11

)O2

show a broad cusp at about 18 K in thetemperature dependence of magnetization underthe low magnetic "eld.

Acknowledgements

The authors are grateful to Professor Y. Ueda,Institute for Solid State Physics, the University ofTokyo, for his support in employing the SQUIDmagnetometer system. They also thank Ms. F. Sakaifor her help in chemical analysis and Mr. M. Koikefor his kind help throughout the experiments.

M. Tanaka et al. / Journal of Crystal Growth 203 (1999) 400}404 403

References

[1] T.R. Zhao, M. Hasegawa, M. Koike, H. Takei, J. CrystalGrowth 148 (1995) 189.

[2] T.R. Zhao, M. Hasegawa, H. Takei, J. Crystal Growth 154(1995) 322.

[3] T.R. Zhao, M. Hasegawa, H. Takei, J. Crystal Growth 166(1996) 408.

[4] M. Tanaka, M. Hasegawa, H. Takei, J. Phys. Soc. Japan 65(1996) 3973.

[5] M. Tanaka, M. Hasegawa, H. Takei, J. Crystal Growth 173(1997) 440.

[6] D.B. Rogers, R.D. Shannon, C.T. Prewitt, J.L. Gillson,Inorg. Chem. 10 (1971) 723.

[7] R.D. Shannon, D.B. Rogers, C.T. Prewitt, Inorg. Chem. 10(1971) 713.

[8] C.T. Prewitt, R.D. Shannon, D.B. Rogers, Inorg. Chem. 10(1971) 719.

[9] R.D. Shannon, C.T. Prewitt, Acta Crystallogr. B 25 (1969)925.

404 M. Tanaka et al. / Journal of Crystal Growth 203 (1999) 400}404