Electrical resistivity in metallic rare earth sesquisulfides

Journal of the Less-Common

Metals,

ELECTRICAL RESISTIVITY SESQUISULFIDES* S. M. A. TAHER,

A. BRAUN

Physics Department,

111 (1985)

361

IN METALLIC

361

368

RARE EARTH

and J. L. SCHWARTZ**

Wichita State University,

Wichita, KS 67208

(U.S.A.)

J. B. GRUBER

Department (Received

ofPhysics, January

San Jose State University,

29, 1985;

San Jose, CA 95192

in revised form March

(U.S.A.)

4, 1985)

Summary The electrical resistivities for several rare earth sulfides (Rs _ %VXS4, R E rare earth, 0 < x < l/3, V = rare earth vacancy) are reported between 1.5 and 300 K. Most of these materials are found to be magnetic semiconductors exhibiting magnetic scattering of conduction electrons. The results of the resistivity measurements are discussed with special reference to the electron localization and magnetic effects, Conduction mechanisms are found to be dependent on rare earth concentrations.

Introduction Rare earth sesquisulfides, lanthanum through dysprosium, crystallize in the Th,P, defect-type structure in the high-temperature y-phase, where -i of the sites in the metal sublattice are unoccupied, i.e., are stoichiometric vacancies [l]. The presence of such vacancies allows considerable amounts of rare earth in excess of the stoichiometric formula R2S3 to be dissolved. The outer shell electrons of the excess rare earth do not form valence bonds but their ionization energy decreases under the influence of the dielectric properties of the medium. Consequently these electrons can easily be transferred to the conduction band [2]. Introduction of rare earths allows the carrier concentration to be varied over a wide range between lOi cmp3 and 102’ cmm3. The vacancies are distributed at random, and the rapidly fluctuating local E field due to the negative charges on the vacancies causes electron localization [3]. Additionally, the indirect exchange *Paper presented at the International Rare land, March 4 - 8, 1985. **Present address: Boeing Military Aircraft U.S.A. 0022-5088/85/$3.30

Earth

Conference,

Company,

0 Elsevier

Wichita,

Sequoia/Printed

ETH Zurich, Kansas

Switzer-

67277-7730,

in The Netherlands

362

interaction between magnetic ions via the conduction electrons tends to establish magnetic ordering in the crystal [ 4,5]. The object of this paper is to report an investigation of the electrical resistivity measurements, p, of several samples belonging to the isostructural series of Th,P, defect-type structure, and to comment on the influence of vacancy and carrier concentration on the electrical resistivity of these materials.

Experimental

detail

Samples were prepared either by direct reaction of the elements in closed quartz tubes (between 600 and 900 “C) or by passing hydrogen sulfide over rare earth oxides (R203) and growing ingots from the melt between 1400 and 2100 “C using Bridgman techniques [6, 71. X-ray diffraction patterns revealed that all the samples possessed the high-temperature y-phase or Th,P, b.c.c. structure. The lattice parameters between RS1.33 and RSI.s differ by less than 0.01 A. Since varying quantities of sulphur were lost in all the samples during the melting process, wet chemical analyses were carried out to determine the sulphur-to-rare earth ratio *. The precision of the chemical analyses was within +0.002% in y of RS,. Attempts to determine the carrier density by measuring the Hall coefficient in the fields up to 7.7 kOe were successful only in the case of samples where IZ= 1018 cme3 at room temperature. For other samples having an electron density greater than 1018 cme3 the Hall e.m.f. against the measured voltage could not be identified. The conduction electron density was estimated on the assumption that each excess rare-earth atom contributed one electron to the conduction band at high temperatures (>250 K) from IZ, = 4(12/y - 8) ae3 where a is the lattice parameter and IZ, is the electron density. The measured electrical conductivity at high temperature (>250 K) is found to be linearly proportional to the calculated values of electron concentration for all samples, confirming the results of the chemical analysis. Thermo-electric power measurements confirm that the carriers are n-type. At room temperature the density of conduction electrons in these solid solutions ranges from lo*’ to lO*l carriers cme3. The electrical resistivity was measured by a four point d.c.-technique, with pressure contact at the gold-coated bars of samples having dimensions 1 mm X 2 mm X 8 mm. The overall accuracy of the resistivity measurements is typically 2% from 2

*The chemical analyses were carried out by G. V. Austin and R. Z. Bachman, Analytical Services Group, Ames Laboratory, Iowa State University, according to the following procedures. The sesquisulfides were dissolved in 4.8 M hydrochloric acid and the evolved HzS gas was collected in a solution of NaOH and Hz02. The sulfate which formed was determined as BaSOa. The rare earth cations in solution were quantitatively analyzed by titrating with ethylenediaminetetraacetic acid (EDTA).

~

363

to 40 K and 1% from 40 to 300 K. The resistivity of several samples was also measured under different magnetic fields up to 7.7 kOe.

Discussion

of data

The electrical resistivities, p, measured under conditions with and without a magnetic field, H, as a function of temperature, are reported in Figs. 1 - 6 for RS, (R = La, Ce, Nd, Pr, Tb and Dy). Except in the cases of lanthanum and praseodymium sulfides the resistivity of each sample on cooling passes through a minimum before the Curie temperature is reached. The variation of p as a function of temperature depends on the different rare earth concentrations in the samples. The application of a magnetic field in the sulphur-rich samples (except La and Pr) causes a significant reduction in resistivity at lower temperatures. Such behaviour is due to the electron-magnon scattering, as seen in other magnetic materials [8]. The behaviour of the resistivity in all the samples appears to be similar in the temperature region where Pmin occurs, except for PrS,, where little or no effect is observed. Our resistivity data, however, could not identify any Schottky anomaly in PrS,, , as reported earlier [ 91.

UII,

0

20

I

40

I

80

I

a0

I

100

I

120

T(K)

Fig. 1. Electrical

resistivity

p us. T for lanthanum

sesquisulfides.

364

06.

OZJ. 0

I,,

50

s

3

4.

1,

100

*

*

3

-

150

*

a,

*

4

200

18

j

.J

250

T(K)

Fig. 2. Electrical

0

I

40

resistivity

80

p us. 2’ for praseodymium

120

160

sesquisulfides.

200

T (K)

Fig. 3. Electrical

resistivity

p us. T for cerium sesquisulfides.

For high-vacancy concentrations (n, < 1020 cm--s) there are localized electronic states just below the conduction band where Ef lies below E, [lo].At low temperatures the electron conduction in these samples is due to the thermally-activated hopping, as shown in most of the sulphur-rich samples [8,10 - 121. In nonmagnetic LaSie4, we find that activation energy is about 0.2 X 10W3 eV. The samples having a high concentration of electrons become superconducting at different temperatures, very similar to the ones already reported [ 11, 13,141. However in NdS,.4, the activation energy it is much is 10e3 eV, and in TbS, and DyS,, of similar R/S composition, larger, which can only be due to the magnetic effect contributing to the

365

_-

200

Id”

I””

25(

T(K)

Fig.

4.

Electrical

resistivity

p us. 7’ for neodymium

sesquisulfides.

10.1

E 2 z

5.c

1.1 04

0.1

50

00

150

I 2(

Vi

Fig. 5. Electrical

resistivity

p vs. T for terbium

sesquisulfides.

magnetic binding energy (which is related to IC.f (CGcluster - (S>,,,,iJ where IC.f denotes the exchange interaction term and L!!Oclusteris the magnetization due to the cluster of spins in the neighbourhood of an electron throughout

366

0

0

20

40

60

60

100

120

140

160

160

200

220

240

T(K)

Fig. 6. Electrical

resistivity

p us. T for dysprosium

sesquisulfides.

the lattice [15, 161.) The increase in binding energy with increase in 4f electrons may be due to the decrease in lattice parameter which affects the Zcvfterm. The high-temperature electrical resistivity of rare earth-rich samples behaves linearly with temperature, whereas in the sulphur-rich samples non-degenerate electron scattering by lattice is more obvious, as is shown by fitting the data to p = A + BT3’* [2] for LaS1.49, TbS1.49 and DYS~+~.+ Activation energies for the samples are calculated by using p = p,(T) exp(AE/kT) where p,(T) is the high-temperature approximation. These energies are shown in Table 1 at temperatures where Pmin occurs. In the metallic samples the thermal activation due to hopping is negligible and the primary mechanism is magnetic in origin where the activation energy is found to decrease with the increase in free-carrier concentration. The addition of excess rare earth over the R3S3 stoichiometry is expected to enhance and contribute to the I,.r and the Mcluster through the addition the Wattice of free electrons; consequently AE decreases with increase in n,, which is observed in the materials that we studied. However, that spin-clustering is predominant at low electron concentration is evident by the significant quenching of the magnetic binding energy under the magnetic field. We have measured the electrical resistivity of the samples in the more metallic region of some of the defect R3_xVxS4 solid solution systems. The data for the very lightly doped materials are affected by Coulomb forces and can be explained by the hypothesis that the localized states exist below a critical energy, and electron conduction is thermally activated. The data for other materials with excess metals over R2S3 stoichiometry (except lanthanum sulfides) are affected both by Coulomb and magnetic forces. The anomalous resistivity behaviour observed in these materials is due to the magnetic interactions because application of the magnetic field

1.406

1.367

1.457

1.393

1.4985

Dys1.475

ms1.471

4Ts1.467

ms1.5

Tbs1.455

ms1.41

Tbfh.37

NdSl.49

NdSl.47

NdSl.45

NW.40

NdSl.37

PrSl.471

fis1.44

Las CeS CeS PrS RS

Las 1.465

LaS1.43

LaSl.37

Sample

--

Electrical

TABLE

1

0 0 1.15 21.65 0.91 4.85 0.375 0.42 0.73 1.05 0.28 0.314 0.79 0.81 5.3 0.22 0.338 1.341 5.34 1.64 3.78 14.3

cm)

4.58 2.36 1.15 0.32 3.82 1.47 4.91 3.38 2.10 1.00 4.96 3.69 1.78 1.02 0.35 5.24 3.52 1.71 0.37 1.37 1.11 0.95

Pmin

(m!d

of RS,,

Carrier concentration (x102’ cm-3)

properties Temperature

6.0 2.5 1.2 75.0 1.8 24.0 31.5 3.0 1.8 1.1 1.3 1.3 2.0 3.8 48.5 59.0 69.0 66.0 84.0 28.0 61.0 103.0

Of Pmin (K)

180 250 - 250 250 - 250 250 250 250 250 250

59 - 250 70 - 250 150 - 250 -

28 40 31.5 15 1.8 130 57 52 38 42 -

-

region

80 - 280 60 - 200

W)

Linear

1.38

energy

10-S lop4 10~_5 10-S 10-S x 10-3 x lo-’ x lo-’ 10-S

x x x x x

x 1O-4

x 1O-4

x 10~~’

5.257

16.4

9.1 12.6 17.4 37.3 47.0 4.15 1.86 46.6 513 x

-

-

-

-

-

(ev)

Activation maximum

6.1

11.0 5.25 28.0 33.0 10.0 5.0 4.5 3.8 12.0

49.8 -

-

-

23.0 -

-

Temperature of maximum activation

368

quenches the activated conduction. The magnitude of the activated conduction depends on the electron concentration, and the magnetic binding energy of the electrons calculated from the data is found to decrease with increase in carrier concentration. Thus we conclude that the electron transport in most of the metallic Th,P,-type rare earth sulfides is to a large extent dominated by the magnetic effect, the magnitude of which is determined by the electron concentration.

Acknowledgments We thank K. A. Gschneidner, Jr., Ames Lab., ISU, for the opportunity to prepare sulfide crystals. We also thank B. Beaudry, Ames Lab., ISU, for preparing the samples. This work was supported by the American Chemical Society - Petroleum Research Fund Grant #13740-B3 and the Wichita State University Research Committee.

References 1 F. Holtzberg and S. Methfessel, J. Appl. Phys., 37 (1966) 1433, and references therein. 2 M. Cutler and J. F. Leavy, Phys. Rev., 133 (1964) A1153. 3 P. W. Anderson, Phys. Rev., 109 (1958) 1492. 4 T. Kasuya and A. Yanase, Rev. Mod. Phys., 40 (1968) 684. 5 T. Penney, M. W. Shafer and J. B. Torrence, Phys. Rev. B, 5 (1972) 3669. 6 K. A. Gschneidner, Jr., B. J. Beaudry, T. Takeshita, S. S. Eucker, S. M. A. Taher, J. C. Ho and J. B. Gruber, Phys. Rev. B, 24 (1981) 7187. 7 J. C. Ho, S. M. A. Taher, J. B. Gruber and K. A. Gschneidner, Jr., Phys. Rev. B, 26 (1982) 1369. 8 S. von Molnar and F. Holtzberg, AIP Conf. Proc., 10 (1973) 1259, and references therein. 9 R. G. Mitarov, V. V. Tikhanov, L. N. Vasilev, A V. Golubkov and I. A Smirnov, Phys. Status Solidi A, 30 (1975) 457. 10 M. Cutler and N. F. Mott, Phys. Rev., 181 (1969) 1336. 11 G. Becker, J. Feldhaus, K. Westerholt and S. Methfessel, J. Magn. Magn. Mater., 6 (1977) 14. 12 S. M. A. Taher and J. B. Gruber,Phys. Rev. B, 16 (1977) 1624. 13 J. C. Ho, S. M. A. Taher, K. A. Gschneidner, Jr., B. J. Beaudry and J. B. Gruber, Bull. Am. Phys. Sot., 23 (1978) 94. 14 K. Ikeda, K. A Gschneidner, Jr., B. J. Beaudry and U. Atzmony, Phys. Rev. B, 24 (1982) 4604. 15 T. Kasuya, in S. T. Keller, J. C. Hensel and F. Stern (eds. ), Proc. 10th Znt. Conf. on Physics of Semi-conductors, Cam bridge, Mass., U.S. Atomic Energy Commission Division of Technical Information, Springfield, VA, 1970, p. 51. 16 J. B. Torrence, M. W. Shafer and T. R. McGuire, Phys. Rev. Lett., 29 (1972) 1168.