Electrical transport and magnetic properties of Nd2(WO4)3

1. Phys. Chcm.

Solids, 1977.Vol. 38, pp. 16146. PergamonPress. Printed in Great Britain

ELECTRICAL TRANSPORT AND MAGNETIC PROPERTIES OF Nd,(WOJ, H. B. LAL and NASEEB DAR Department of Physics, University of Gorakhpur, Gorakhpur 273001, India (Received

8 October

1975; accepted

in revisedforw

21 June 1976)

Abstract-Measurements of the molar magnetic susceptibility (x,_) of a powdered sample of Nd,(WO,), in the temperature range 300-900 K, and the electrical conductivity (a) and dielectric constant (6’) of pressed pellets of the compound in the temperature range 4.2-l 180 K are reported. xm obeys the Curie-Weiss law with a Curie constant C = 3.13 K/mole, a paramagnetic Curie temperature 0 = -60 K and a moment of Bohr magnetons, p = 3.49 for the Nd” ion. The electrical conductivity data can be explained in terms of the usual band model and impurity levels. Both the (r and c’ data indicate some sort of phase transition round 1025 K. The conductivity follows Mott’s law u = A exp (-B/T”‘) in the temperature range 200 < T < 3000 K with B = 45.00 (K)“’ and A = 1.38 x IO-’ R-’ cm-‘. The dielectric constant increases slowly up to 600 K, as is usual for ionic solids. The increase becomes much faster above 600 K, which is attributed to space-charge polarization of thermally generated charge carriers.

1. INTRODUCTION

In recent years rare-earth compounds have been of considerable interest both theoretically and technically[ l41. For the last few years we have been studying the transport and magnetic properties of rare-earth tungstates[5-71. This paper reports our study of the magnetic susceptibility, electrical conductivity and dielectric constant of Nd,(WO&, for which such data have not been previously reported. Our prepared sample was in powder form, and we have used pressed pellets for the electrical conductivity and dielectric constant measurements. We were unable to prepare single crystals due to experimental limitations. A fine-grained powder of neodymium tungstate has a light pinkish colour and a density of 7.04 g cm-‘, and melts in the temperature range 1400-1525K. Below 1318K it exists in a monoclinic form[8] with unit cell dimensions a = 7.765 A, b = 11.614A, c = 11.5448, and p = 109.8”,to an accuracy of 0.001 A. The volume of the unit cell is equal to 799.55A’. The structure is described by the space group C2(C:,,) with four molecules per unit cell. The lattice can be viewed more conventially in a pseudoorthorhombic form [9] with unit cell dimensions a2 = 7.75 A, b, = 11.59A and c2 = 21.67 A. In this form the unit cell contains 8 molecules, has double the volume of the monoclinic unit cell, and belongs to space group F 2/d. 2. SAMPLE PREPARATION AND EXPERMENTAL TECHNIQUES

The magnetic susceptibility (x,,,) was measured Faraday’s method [111employing a sensitive magnetic balance by (- 10m5 g) and an electromagnet with pole pieces of typical design. For the measurement of electrical conductivity (u) and dielectric constant (E’), powder specimens were pelletized at pressures ranging from 2 x IO6 to 10 x lo6 g cme2 using a hand-operated hydraulic press and a suitable die. The d.c. conductivities of the pellets were determined by recording the current with a Keithley Digital Multimeter. Both capacitance and conductance were measured with a Weine-Kerr Bridge whose internal operating frequency was 1.542kHz. The procedural details are described elsewhere [12]. 3.RESULTS AND DISCUSSION

The variation of inverse molar magnetic susceptibility (xm)-’ with temperature (K) is shown in Fig. 1. The plot is straight line consistent with Curie-Weiss law (x = C/T 0) behaviour at higher temperature [ 131.From this straight line the value of Curie Constant C and paramagnetic Curie temperature 0 are obtained as 3.125K/mole and -60 K, respectively. The only magnetic ion in this solid is Nd’+, which has the ground state ‘I9,2. This yields L = 6, S = 3/2 and J = 9/2, from which a value for the effective moment p = g{J(J t I)}“* of 3.62 pLg is obtained. From the experimental value of C, one can evaluate the effective moment using the formula [ 141.

Nd,(WO,), was prepared by the standard method reported earlier by Brixner and Sleight [lo]. Stoichiometric amounts of high purity Nd,O, (99.9%) from Fluka AG, Switzerland, and analytical grade tungsten trioxide (WO,) from E. Merck, Germany, both dried at 9OOK, were thoroughly mixed and heated in a closed platinum crucible at about 1200K for 20-24 hr. The reaction products were then homogenized by ball milling in an agate mortar, pelletized and refired between 1350 and 1400K for 6-10 hr. This yields a good final product as evident from the melting point and X-ray powder diffraction photographs. JPCS VOL. 38 NO. 2-E

p2 = [3kC/2NB2]

(1)

where k is the Boltzman constant, N is Avagadro’s number and /3 is the Bohr magneton = 0.927ergsG-‘, which yields p = 3.49 pB. There is thus good agreement between the theoretical and experimental values of p. The nature of the magnetic ordering and the interactions which are responsible for Curie-Weiss behavior at high temperatures can only be understood by low temperature studies. The magnetic exchange interactions between rare-earth ions are usually very weak [ 151and it 161

162

H. B. ‘LAL and N. DAR

,

I

I

I

I

1

1

I

I

too

200

300

400

500

600

700

800

9oC

molar

magnetic

TEMPERATURE Fig.

I. Variation

of the inverse

susceptibility Nd,(WO,),.

is therefore quite probable that the Curie-Weiss behaviour observed in this compound arises from crystal field effects as has been observed in many other rare-earth solids [ 161. The density of the pellets approaches a constant value for P > 6 x 106g cm-*. The quantities (T and E’ are very much dependent on the thermal history of the samples. However, consistent and reproducible results are obtained for pellets annealed around 1200 K for about two days. Both v and E’ for annealed pellets approach constant values for P > 6 x IO6g cm-*. Neither of these values depend upon the electrode materials within the accuracy of our apparatus. A little inconsistency is observed using silver electrodes at high temperature (T >900 K). This may be due to oxidation of the electrodes because the measurements were made in air. No such discrepancy was noticed, within experimental limits, using platinum-foil electrodes, which were therefore used for measurements of (T and E’ above room temperature. However, at low temperatures (4.2-300 K) silver-paint electrodes were used because they adhere very well to the surface and are to some extent better than the platinum foil electrodes in this temperature range, where the resistivity of the sample is very high. For low temperature measurements, pellets were first heated at 150°C for a few hours and then introduced into the helium vessel. Temperatures were measured by a Au-Au Fe

T (K I(u”,)

’ YStemperature

for a powdered

sample

of

(0.7%) thermocouple. Since LJ was not found to be frequency dependent, we conclude that grain-boundary effects are insignificant. The independence of the density, dielectric constant and electrical conductivity on pressures greater than 6 x lo6 g cm-* lead us to believe that the measured parameters probably approach the crystalline value. We have measured the d.c. electrical conductivity of a pellet formed at a pressure of 10 x IO6g cm-* g cmm2in the temperature range 4.2-l 180 K, and the a.c. conductivity at 1.542 kHz. The results are shown in Figs. 2-4. Figure 2 shows the variation of log u vs lO’/T in the temperature range 300-1180 K. From the nature of the curve the conductivity results can be divided into the following regions (i) 500-1OOOK (ii) A small temperature range round 1025 K (iii) Above 1050 K (iv) 300 such that, u, + u2 = C, exp (- W,/ZkT) + C, exp (- WJ2kT)

(2)

c: = WI = 1.3eV. C, = 2.46 x IO-’ 0-l cm-‘, with 26 0-l cm- ’ and W, = 2.5 eV respectively. Usually conductivity data in solids are explained in terms of band theory. The relevant bands for conduction in this solid are empty Nd3+: 5d and W”’ :Sd bands, a filled O*- : 2p band, and a narrow Nd” : 4f’ band that is

Electrical

3

transport

and magnetic

L

f

I

I

4

t-0

I.2

I.4

I.6

I ,8

properties

1

2.0

163

of Nd2(W0,),

o

AC (f+542kHlj)

l

DC VALUES

,

I

I

2.2

2.4

2.6

VALUES

1

2.8

I

3.0

8

3.2

ro 3f T tKli’--

Fig. 2. Variation of logo‘ vs lO’/T (K)-’ for a Nd,(WO& pelletfrom 300to 1180 K.

12.5 > 0

TEMPfRATURE Fig. 3. Variation

150

I00

50

of log o‘ vs temperature

correlation split from the Nd”’ : 4f4 band by nearly 20 eV. The only known oxide of neodymium is Nd&, and the Nd ions are presumed to exist in trivalent state. The fact that neither Nd2+nor Nd”’ ions are found in oxides places the tilted Nd” : 4f’ band below the top of the 0“ : 2p band and the empty Nd2+:4f4 band above the bottom of the conduction band. The existance of p~amagnetic susceptibility compatible with free ion Nd’+:4f’ cores is consistent with this model, which eliminates the Nd:4f

for a

T tI0-w

Nd*(WO,)~ pelletfrom4.2to 200K.

electrons from consideration in any discussion of the transport properties. Thus the 02-:2p band is probably the valence bound, and one of the cation Sd empty bands is the conduction band of the solid. The charge transfer excitation of an electron from the valence 02- : 2p band to one of the cation 5d bands is expected to be of the order of 3eV. This is the reason why pure stoichiome~c, Nd2(W04), is a good insulator. Of the experimentally observed activation energies W, is too low and is

164

H. B. LAL and N.

DAR

T (KI

I/T

Fig.4.

Variation

f4 (K- $/ 4-

of log0 vs (I/T”‘) (K)-"' for a Nd,(WO.),pelletfrom200to 300K.

certainly associated with some impurity levels. Our Nd,O, starting material contains about 0.1% impurity. The stated impurity contents by the manufacturers are Pr,O,, and Sm,O,. Thus impurity levels lying deep in the band gap, may be associated with the P?’ : 4f* levels. The energy W, = 2.5 eV is too large to be attributed to impurities, and probably represents the energy band gap of the solid. The log (T vs lO’/T curve shows a discrepancy in the temperature range lOOO-1050K. A Similar type of discrepancy is observed in l’ vs T curve in the same temperature range. This type of discrepancy indicates some sort of phase transition of the solid, which is perhaps a structural change. The log or vs lO’/T plot above I050 K is a straight line which can be expressed as follows (T= C, exp (- WJ2kT) = 69.9 exp (-2.6/2kT).

(3) (4)

This energy of 2.6 eV may interpreted as the energy band gap of the solid in this new phase. The conductivity curve has a positive slope below 400 K (Fig. 2) and a broad maximum around 300 K. This unusual feature of the log (T vs l/T curve may be due to moisture or hydrate formation. On heating above room temperature the compound starts showing a loss in weight. The maximum loss after prolonged (-10 hr) heating round 5OOK is 9%. It regains about 6% of this weight when cooled slowly in a 60% humid atmosphere to room temperature. (The time of cooling was about 10hr, which is approximately the total time needed for recording the conductivity data from SOOK to room temperature). Very little gain is observed if the sample is quenched into a dessicator and then weighed after 10hr. This 6% loss of weight, would correspond to about 3.5 Water molecules per formula unit in the rare-earth tungstate. Such a large content of water may correspond to hydrate formation hygroscopically with a formula of the form (Nd,(WO,), 3.5 H,O. Formation of a trihydrate has been observed in all heavy rare-earth tungstates[8] but no such hydrate formation occurs in light rare-earth tungstates[8]. Since the diffraction result suggests a

structure for the compound as reported in the literature[8, lo] for which there is no ambiguity in composition, we rule out hydrate formation. Therefore the discrepancy in the log u vs l/T curve around and above room temperature is due to the moisture content of the pellet. The values of u below 300 K are shown in Figs. 3 and4. It is seen from Fig. 4 that in the temperature range 200-300 K, the log (T vs l/T l/4 curve is a straight line and can be expressed by the equation, u = A exp (-B/T”‘) = 1.38x 10-j exp (-45/T)“4.

(5)

This apparently reflects shallow donor levels associated with 5d bands. The Mott T’” law is typically associated with a variable range hopping mechanism among such states. The conductivity remains constant in the temperature range 100-200K and below 100K, starts decreasing slowly. This may be due to the existance of some very shallow traps in this solid, which are fully ionized above a temperature of 100K. The dielectric constant (c’) was measured at 1.542kHz on a pellet made at a pressure of 10 x IO6g cm-* from 4.2 to 1180K. The results are shown in Figs. 5 and 6. Figure 5 shows the variation of e’ from 4.2 to 600 K, from which it is seen that l’ increases slowly with temperature upto 300 K, as is usual for ionic solids[l7]. The dielectric constant in the temperature range 3OCM90K shows a slightly unusual trend, namely, a broad peak. As has already been pointed out in the discussion of the conductivity results, measurements around room temperature in air are effected by moisture. Below 300 K, we made all the measurements in a helium atmosphere, first heating the pellets to about .lSoOCfor several hours before

introducing them into the helium vessel. This avoids the presence of moisture in that range. The dielectric constant increases at a relatively faster rate above 500°K (from an average increase of 0.008/K in the temperature range 4.2-330 K, to an average increase of 0.2/K in the temperature range 500 to 800 K). This increase in e’ can be attributed to thermally excited charge carriers. These carriers produce a space charge which can significantly increase the value of e’[18].

Electrical

transport

and magnetic

properties

165

of NdJW0.L

L

Y w 0

c

II

1

0

I

I

loo

200

300

TEMPERATURE Fig. 5. Variation

of dielectric

Constant

1

500

61

T (K)-

(E’) vs temperature

f ~I.542

1

400

for a Nd,(WO,h

pellet from 4.2 to 600 K.

kH1

100

9oc

8OC

600

800 TEMPERATURE

Fig. 6. Variation

of dielectric

Constant

(6’) vs temperature

900

1100

T (K)--c for a Nd,(WO,),

pellet from 600 to

1180K.

H. B. LAI. and N. DAR

166

Acknowledgements-The authors are grateful to Prof. Olof Beckman and Dr. Leif Lundgren, Institute of Technology. University of Uppsala, Sweden, for providing low temperature measuring facilities, help and useful discussions. We also thank the referee for his valuable suggestions which have helped considerably in improving the manuscript, some of which are incorporated in the discussion section. One of us (N.D.) thanks CSIR, India, for financial assistance.

REFERENCES I. Katz H. W., Solid Sfafe Magnetic and Dielectric Deoices. John Wiley, New York (1959). 2. Progress in Science and Technology of Rare-Earths (Edited by L. R. Eyring) Vols. I and 2. Oxford, Pergamon Press (1%6). 3. Methefessal, S. and Mattis D. C., Hand Buch Der Physik Vol. XVIII/l, 389. Springer-Verlag. Heidelberg (1968). 4. Taylor K. N. R.. Adc. Phys. 20,551 (1971); Confemp. Phys. II. 423 (1970). 5. Lal, H. B., Dar N. and Kumar A.. J. Phys. C: Solid Sfafe Phys. 7, 4335 (1974).

6. Lal H. B., Dar N. and Kumar A.. J. Phys. C: Solid Slafr Phys. 8, 2745 (1975). 7. La1 H. B. and Dar N.. 2. Naturforschunfi

3Oa, 1703 (1975).

8. Nassau K., Levinstein H. J. and Loianeono G. M., J. Phys. Chem. Solids 26, 1805 (1965). 9. Templeton D. H. and Zalkin A., Arta Cryst. 16, 762 (1963). IO. Brixner L. H. and Sleight A. W., Mat Res. Bull. 8. 1269(1973). II. Bates L. F., Modern Magnefism. University Press, Cambridge (19.(l). I?. Dar N., Ph.D. Thesis, Gorakhpur University, Gorakhpur (1976). 13. Morrish A. H.. The Physical Principle of Magnetism. p. 529. John Wiley, New York (1966). 14. Van Vleck J. H.. Theory of Hecfric and Magnefic Suscepfibilifies, p. 226. University Press, Oxford (1932). 15. Martin D. H., Magnefism in Solids. London Ileffe Books Limited (1967). 16. Thomas J. F. and Seinko M. J., J. Chem. Phys. 61.3920 (1974). 17. Smyth C. P., Dielecfric Behaoiour and Sfrucfure, p. 132. John Wiley, New York (1955). 18. Lal H. B. and Srivastava K. G., Cand. J. Phys. 47,3 (1969).