METAL-NON METAL TRANSITION OF Ni1-xS AND NiS SUBSTITUTED by Se, As and Fe : TRANSPORT PROPERTIES AND STRUCTURAL ASPECT

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METAL-NON METAL TRANSITION OF Ni1-xS AND

NiS SUBSTITUTED by Se, As and Fe : TRANSPORT

PROPERTIES AND STRUCTURAL ASPECT

E. Barthelemy, C. Chavant, G. Collin, O. Gorochov

To cite this version:

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JOURNAL DE PHYSIQUE Colloque C4, supplkment au no 10, Tome 37, Octobve 1976, page C4-17

METAL-NON METAL TRANSITION OF Nil -,S AND

NiS SUBSTITUTED

by

Se, As

and

Fe

:

TRANSPORT PROPERTIES AND STRUCTURAL ASPECT

E. BARTHELEMY, C. CHAVANT, G. COLLIN and 0 . GOROCHOV

Laboratoire de Chimie Mintrale Structurale (L. A. 200) Universitt Rent Descartes Paris V

4 Avenue de l'observatoire, 75270 Paris Cedex 06, France

Rbsumb. - NiS (structure NiAs) prksente une transition mBtal-non mCtal a 270 K, accompagnke d'un changement des propriBtCs magnktiques, Blectriques et structurales. Quelques nouvelles propri6 tBs physiques effectuees sur monocristaux sont ici dkcrites.

La structure de NiS a 300, 263 et 173 K a BtB Btablie. Un important domaine critique apparait juste en dessous de la transition.

La structure cristalline de la phase ordonnke Nio,wS a BtB dBterminBe.

Le comportement semiconducteur extrinskque (2 porteurs p par lacune) de la phase non mBtalli- que est mis en Bvidence.

Abstract. - NiS (NiAs type) undergoes a metal-non metal transition at 270 K, transition which

is accompanied by a change in magnetic, electrical and structural properties. The authors report here some new physical measurements on single crystals.

The structure of NiS at 300,263 and 173 K has been pointed out. A large critical region occurs just below the transition.

The crystal structure of the ordered phase Ni0.94S has been determined.

Evidence for an extrinsic semiconducting behaviour of the non metallic phase is reported (2p carriers per Ni vacancy.)

The hexagonal form (high temperature phase) of the nickel monosulfide shows a metal-non metal transition as the temperature is lowered.

The stoichiometric phase has been reported to show the transition a t T, -- 265 K. T, decreases rapidly if the composition deviates from stoichiometry (T, = 0 for Ni, .,,S).

I t has been previously reported by Sparks and Komoto [l-41 that NiS is above T, a metallic material with a Pauli paramagnetism, and an antiferromagnetic semiconductor at low temperature. The powder neutron diffraction data they obtained at 4.2 K has shown that the spins are ferromagnetically coupled within the hexagonal layer, with adjacent layer anti- ferromagnetically coupled. The reduced value of the moment per nickel atom as well as the susceptibility measurements suggest an itinerant model, as it was pointed out by J. M. D. Coey et al. [5]. First structural data was obtained by Trahan et al. [6]. On the basis of powder X-ray diffraction data, they have concluded that a non centrosymmetric distortion occurs a t low temperature, below the transition. Moreover the previously reported change in the crystallographic parameters is confirmed. However MC Whan et al. [7], on the basis of crystal X-ray diffraction studies have not pointed out any change in the space group a t low temperature.

Results of electrical conductivity, thermoelectric power and Hall effect were reported by Ohtani et al. 18- 101, Townsend et al. [ll-121 and more recently by E. Barthelemy et al. l13-141. Differences between obtained results seem to be correlated with the methods used to prepare the samples. Seebeck and Hall measurements were in good agreement. The conductivity has been found to be p-type in the low temperature phase and n-type in the high temperature phase.

Mossbauer spectroscopy measurements have been made by Coey 1151 and Gosselin [l61 on samples countaining 2

%

57Fe.

Optical reflectivity spectra have been performed by A. S. Baker [l71 and those results suggest also an itinerant model for the low temperature phase.

Several theoretical studies have been made concer- ning the transition in nickei sulfide. White and Mott [l81 and later Koehler [l91 concluded that NiS undergoes a metal-non metal transition. More recently, Kasowsky [20] and Mattheiss [21] have proposed a band structure using respectively LCMTO and APW methods.

Our purpose is to report some transport and structural properties on well characterized single crystals. 1. Experimental results. - NiS has been prepared in powder form from the elements at 550 OC. Single

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C4-18 E. BARTHEL~MY, C. CHAVANT, G. COLLIN AND 0. GOROCHOV crystals have been grown up in sealed silica tubes by

the method of chemical vapor deposition in a temperature gradient ( T , = 720 OC,

T,

= 650 OC). using chlorine as transporting agent (p,,2 = 40 to 60 torrs at 300 K). Crystals were obtained at about 650 OC. They were non stoichiometric. In order to obtain the stoichiometric composition, the silica tubes were slowly cooled down to 550 OC after the reaction.

All crystals have been obtained by this method. Nickel, sulphur and the substituted element have been determined by X-ray measurements and micro- probe analysis.

The crystals grew as hexagonal platelets with a metallic golden aspect (Fig. 1).

FIG. 1. - Crystals of NiS.

2. Crystallographic data. - The various solid solutions have been performed a t 550 OC. The homogeneity range of nickel sulfide has been detailed elsewhere [13]. At 550 OC, it is possible to have 6

%

Ni vacancies and a superstructure occurs for the limiting composition of the solid solution.

By substitution, continuous solid solutions appear. The substitution of sulphur by selenium or arsenic has been described elsewhere [14]. The solid solution Nil -,Fe$ has been studied for small amounts of iron. Two phases have been observed, both with NiAs structure :

- a first phase with 0 d x 0.05,

- a second phase with 0.10 x 4 0.20, in which c parameter is greater than in the first case.

In the composition range 0.05 < x < 0.10, a mixture of the two phases is observed.

3. Physical properties. - Magnetic measurements and DTA hare been made on powders as well as on single crystals from 4.2 to 300 K.

Electrical properties have been made over the same temperature range, on single crystals, perpendicularly to the c axis.

Resistivity has been determined by Van der Pauw D C method and Hall effect with AC (408 Hz). Six contacts are made on the crystal which is immersed in epoxy adhesive in order to avoid the formation of microcracks as the crystal passes through the transition. Extremely small and thin crystals have been used for those measurements (200 X 200 X 20 p).

4. Physical data.

-

4 . 1 MAGNETIC AND DTA DATA.

- Magnetic measurements and DTA in Nil-,S have been reported elsewhere [14]. Tt decreases linearly as a

function of the nominal composition (Fig. 2). There is a very sharp decrease of

T,

with the vacancies ratio and by substitution of sulphur by arsenic. Tt decreases

more slowly in selenium substituted phases.

FIG. 2.

-

The transition temperature Tt as a function of compo-

sition for several N ~ I - ~ S , Nil-xFezS, NiS1_,Sez, N ~ S I - ~ A S ~ compositions.

Figure 3 reports the magnetic susceptibility for a small amount of iron. For 1

%

iron, Tt decreases

quickly, but for a larger amount of iron (2-3

%)

T,

increases.

FIG. 3. -The molar susceptibility versus temperature for Nio.gsFeo.01S.

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METAL-NON METAL TRANSITION O F Nil-,S AND NiS SUBSTITUTED C4- 19

FIG. 4. - The resistivity as a function of temperature for NiS single crystal.

tion, the resistivity remains constant. So NiS is a semimetal or an extrinsic semiconductor.

We have represented in figure 5 the variation of the

Hall coefficient. The carriers concentrations have been

determined in both phases (n = 4 X 1 0 2 2 / ~ m 3 at

300 K, p = 2 X 1020/cm3 at low temperature). As

previously reported by Ohtani et al. [10], there is a large change in the carriers concentration at the transition, but not in the mobility (p = 1 and 5 cm2. V-'. S-'

at high and low temperature respectively).

FIG. 6. - nH = RH. e versus 2 x (a = number of vacancies for Nil-& in the low temperature phase) ; (experimental values on single crystals and Ohtani results on sintered samples).

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C4-20 E. BARTHELEMY, C. CHAVANT, G. COLLIN AND 0. GOROCHOV For the stoichiometric phase, an important disper-

sion of the values is noted at low temperature. This results probably from the difficulty to reach the per- fect stoichiometry.

The observed values of the Hall coefficient corres- pond to one electron per formula unit in the metallic phase. As expected for a semiconducting process in the low temperature phase, the values of the Hall coefficient are always very close to two charge carriers per vacancy as shown in figure 6.

Similar data may be reported on substituted single crystals : metallic behaviour above the transition, semiconductor or semimetal at low temperature. Nickel vacancies as well as selenium or arsenic substitution have effects which may be interpreted starting from a band scheme close to this proposed by Koehler et al. [19], but in a semiconductor model.

Ni vacancies lead to an increase of the density of states at Fermi level (Fig. 7b). Se substitution gives an overlap of the two e, bands (Fig. 7c). As substitution leads simultaneously to the two effects (Fig. 7d).

5. Structural data. - 5.1 SUPERSTRUCTURE OF Ni,.,,S. - There is a continuous homogeneity range for Nil-,S (0 d

x

0.06) with a gradual change of

the a and c parameters of the hexagonal cell (Fig. 8).

A single crystal with X = 0.06 has been prepared,

which shows an hexagonal superstructure :

a' = 3 a,,,

C' = 3 C N i s

Z = 54 Space group P 6 ,

X-ray datas have been collected on a four circles diffractometer from 2 9 = 0 to 650. The refinement has. converged to a partially ordered structure with all

1 1 0 p l a n e of N i 5 , S S 4

@ N i a t o m

h a l f o c c u p ~ e d s i t e v a c a n c y

FIG. 9. - Projection on the (110) plane of NislSs4.

vacancies on the (110) plane of the hexagonal cell (Fig. 9). There are

-

2 sites completly empty on the ternary axis [+,

3,

z ] - [$,

+,

z

+

31

with z N 0

-

2 half filled sites on the 6 , axis

[O, 0, z ] - [O, 0, z

+

$1

with z

-

3 .

FIG. 8. - Unit cell parameters a and c and density d of Nil-$ versus composition (- calculated values ;

- - - -

experimental

values).

This solid solution can be well explained by vacancies on the nickel sites as shown from density measurements.

This leads to a composition NislSs4 for the whole superstructure cell. Distortions occur in the surroun- ding of the nickel atoms close to the vacant sites. Moreover we can mention that no long range ordering has been observed in the range 0

<

X

<

0.03, that

is to say with the compositions for which the metal- non metal transition is observed.

5.2 STRUCTURE OF HIGH AND LOW TEMPERATURE MODIFICATION OF STOICHIOMETRIC NiS. - The S ~ ~ U C -

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METAL-NON METAL TRANSITION OF Nil-$S AND NiS SUBSTITUTED C421 also been investigated on a single crystal. X-ray

reflections have been collected on a four circles diffractometer between 2 8 = 0 and 1200. All reflections have been kept for the refinement.

The metallic phase ( T = 298 K) and the non metallic phase ( T = 173 K) have been first studied far from the transition.

Parameters of NiS hexagonal cell

Structural data :

Atomic positions in the three structures :

Two Ni atoms in [0, 0, 01 and [O, 0,

41

Two S atoms in

&,

+,

$1

and [$,

3, $1

Values of independent anisotropic thermal parameters (A2) and refinement results :

number of independent B11 8 3 3 R factor reflections T = 298 K Ni 0.011 (2) 0.010 (2) 0.028 156 S 0.008 (3) 0.012 (4) T = 263 K Ni 0.010 (2) 0.009 (3) 0.038 158 S 0.009 (4) 0.010 (5) T = 173 K Ni 0.006 (2) 0.006 (2) 0.027 159 S 0.005 (2) 00.06 (3)

Both structures are pure NiAs type (Table I), without any distortion. Differences occur :

1) on the lattice parameters,

2) on the anisotropic thermal coefficients, especially with respect to the sulphur atoms in the high temperature metallic phase.

In addition to, the behaviour of the 002 and 110 reflections have been followed over the temperature

I

T ZOOK ZJOK 300K p 9000 s s o 0 0 -.

-

5700

.

9 b T ZOOK 250K 300K

FIG. 10. - 2 0 angle and intensity of 110 and 002 reflections as a function of temperature for NiS single crystal.

range 173-300 K (Fig. 10). It has been observed just below the transition :

1) an anomalous decrease of the intensities of these two reflections,

2) a gradual change of the 2 6' angles in the same region.

These effects enable the existence of a critical region just below the transition temperature with a width of

about 20 K.

A crystal structure refinement in the critical region (T = 263 K) has not shown any deviation from the NiAs type structure (Table I).

6. Conclusion. - Our experimental results are consistent with a Hubbard-Mott transition with no lattice distortion. However, a difference between metallic and non metallic phases is observed on anisotropic thermal coefficients.

A large critical region below the transition temperature has been pointed out.

A semiconducting behaviour is correlated to nickel vacancies at low temperature and a metallic conduction appears at high temperature.

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[3] SPARKS, J. T. and KOMOTO, T., Phys. Lett. 25A (1967) 398. [4] SPARKS, J. T. and KOMOTO, T., Rev. Mod. Phys. 40 (1968)

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F51 COEY, J. M. D., BRUSSETTI, R., KALLEL, A., SCHWEIZER, J. and FUESS, H., Phys. Rev. Lett. 32 (1974) 1257.

[6] TRAHAN, J., GOODRICH, R. G. and WATKINS, S. F., Phys

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NIER, P. D., Phys. Rev. B 5 (1972) 2552.

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[91 OHTANI, T., KOSUGE, K. and KACHI, S., J. Phys. Soc. Japan

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C4-22 E. BARTHELEMY, C. CHAVANT,

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[l11 TOWNSEND, M. G., TREMBLAY, R., HORWOOD, J. L. and RIPLEY, L. B., J. Phys. C Solidstate Phys. 4 (1971) 598. [l21 HORWOOD, J. L., RIPLEY, L. G., TOWNSEND, M. G. and

TREMBLAY, R. J., J. Appl. Phys. 42 (1971) 1476. [l31 BARTH~L~MY, E., GOROCHOV, 0 . and MCKINZIE, M., mate^.

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[l51 COEY, J. M. D., ROUX-BUISSON, H. and CHAMBEROD, A.,

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[l61 GOSSELIN, J. R., TOWNSEND, M. G., TREMBLAY, R. J., RIPLEY, L. G, and CARSON, D. W., J. Phys. C 6 (1973) 1661.

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[l81 WHITE, R. M. and MOTT, N. F., Phil. Mag. 24 (1971) 845. [l91 KOEHLER, R. F. Jr and WHITE, R. L., J. Appl. Phys. 44 (1973)

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