Valence state of Fe, Cu, Cr ions in films of the CuxFe1−xCr2S4 solid solutions

Solid State Communications, Vol. 64, No. 6, pp. 989-992, 1987. Printed in Great Britain.

0038-1098/87 $3.00 + .00 © 1987 Pergamon Journals Ltd.

V A L E N C E STATE OF Fe, Cu, Cr IONS IN FILMS OF T H E CuxFe~_xCrzS4 SOLID S O L U T I O N S E.M. Zub and S.V. Sukhvalo Institute of Physics of Solids and Semiconductors the BSSR Academy of Sciences, Minsk, USSR

(Received 10 May 1987 by E.F. Bertaut) Nature of the CuxFel_xCr2S 4 solid solution formation as well as the dependences of their electrical and galvanomagnetic properties on composition of the film samples have been elucidated. The interrelation between the behaviour of the above mentioned characteristics of the Cu~ Fel_xCr2 $4 system films and the valence state of chromium, iron and copper ions has been revealed. It is established that the valence distribution of ions for the Cux Fel _~Cr2 $4 system films is as follows: Cu+ ( Fe2+ Fe3+ )1-x( Cr3 + Cr2+ )2 $42-.~ (Cu + Cu 2+ )~Fe~ +_~(Cr 3+ Cr 2+ )2 S]_-

INTRODUCTION

0 ~< x ~< 0.5,

0.5
2+ C u Cuz~_ 1

D E V E L O P M E N T OF SOLID STATE electronics calls for creation of new semiconducting and magnetic materials with desired and reproducible properties. Chromium chalcogenide spinels rank high among these materials. By virtue of the crystallochemical properties the above mentioned spinels retain their structure at considerably large deviation of composition from the stoichiometry, i.e. high concentration of structure defects of the vacancy - - and interstitial-type can be observed in such materials. Wide modification of properties of nonstoichiometric spinels is possible on this basis. In particular the magnetic and electrical properties of chalcospinels depend mainly on the type and concentration of the defects available. The most marked variation of properties has been observed at such deviation of spinels from the stoichiometry that leads to valency transformation of ions. Similar features can often be revealed when the ions of transition elements with alternating valency are present in the cationic system. In view o f the above said the CuxFe~_xCr2S4 solid solution system obviously presents interest. In this system the Cu, Fe, Cr ions can possess variable valency that gives rise to peculiarities of their kinetic and magnetic properties. Employing the results of the investigations of electrical and magnetic properties o f the system solid solutions different models of the electronic structure have been suggested by a number of authors: Fe~-+z~Fe3+Cu~+C~+S]-

0
(1) 989

+

I- x

Fe3+ 6,,.3+,~2 I x x"~2 ~4

C n 2 + 1~2~2+ 1~3+ ~ 2 -

"x -~1-x'-~-2 o4

0.5 ~< x ~< 1,

0 ~< x ~< 0.7,

Cu~+Fe~+_xCr~+SL_,S~_2x

0.5 ~ x ~< 1

(Fe2+Fe3+)l_xeu+(Cr3+Cl~+):S~-

0 < x < 0.5

(2) (3) (4) (5)

It can be seen that up to now the opinions of the valence state of ions in the above mentioned system differ. Based on the experimental results of a X-ray spectral analysis and the investigation of magnetic, electrical and galvanomagnetic properties we have attempted to find out the evolution of the valence state of ions while forming the solid solutions of the Cu~Fel_xCr2S 4 system via ionic substitutions in the tetrahedral subsystem. EXPERIMENTAL The synthesis of the Cux Fe,_ xCr2 S4 solid solution films involved 2 stages: (1) The polycrystaUine chalcospinels were obtained by solid phase synthesis of highly homogenized mixture of elementary compounds. (2) The CuxFel_xCr2S4 films (x = 0; 0.2; 0.3; 0.4; 0.5; 0.6; 0.8; 1.0) were obtained by the discrete evaporation technique where the mixture of solid solutions with composition close to the stoichiometric one was used as evaporating material. The phase composition and stoichiometry of the obtained films were checked by the electronographical and micro-Xray spectral methods. The electrical and magnetic resistances were measured by the potentiometer

990

V A L E N C E STATE OF Fe, Cu, Cr IONS

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method, the concentration and carrier mobility were determined from the Hall e.m.f, measurements. RESULTS A N D DISCUSSION The knowledge of the stoichiometry and phase composition of the solid solutions of chalcospinels provides important data on crystallochemical peculiarities of their structure and the process of their formation under varying conditions of synthesis and cationic substitutions. The obtained electronographical data speak for the fact that the single phase character os solid solutions is preserved over the whole composition range and in a relatively wide range of technological modes of the formation of specimens. The stoichiometry of the spinels is, on the contrary, very sensitive to cationic substitutions and the synthesis conditions of the samples. Thus the following picture of deviation degree from the stoichiometry (Fig. 1) depending on the substitution of iron ions by copper ions has been revealed. For sulphur ions a deficit with a sharp maximum at x = 0.5 has been observed over the whole range of x. The deviations for chromium ions depending on the range x can correspond both to the excess and the deficit of ions, with the deficit being observed in the range 0.2 < x < 0.6. For copper ions an excess has been found for all x. In the range x -- 0.5 drastic anomalies in the correlation of quantities of the ions in cationic and anionic sublattices and

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-8

0,2

X

0

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Fig. 1. Deviation from the stoichiometry against composition for Cu, Fe, Cr, S elements in the Cux Fe~ _xCr2 $4 system.

in octahedral and tetrahedral positions have also been noted. The observed extremums of the above mentioned values in the range x = 0.5 give evidence for the essential redistribution of ions both in cationic and anionic subsystems to characterize the concentration ordinate x = 0.5 as a special one.

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V A L E N C E STATE OF Fe, Cu, Cr IONS

The considered peculiarities of the change of the degree of deviation from the stoichiometry, while substituting iron ions by copper ions, are consistent with those of physical properties in the process of formation of the CuxFe]_xfr2S4 solid solution system [6]. Thus the thermoelectric coefficient depending on composition of the components has p-type conductivity in ranges 0 < x < 0.5,
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991

can obtain the qualitative data on the character of valence changes caused by substitution of ions domineering defects and the behaviour of magnetic and electrical properties. In order to perform this task we solved the electron neutrality equations. The estimated valence state values for the CuxFel_xCrES 4 solid solution system are as follows: Cu+(Fe2+Fe3+)]_~(Cr3+Cr2+)2S42_-~

0 < X < 0.5

(Cu+Cu2+)xFe~_+x(Cr3+Cr2+)2S2_- ~

0.5 < x < 1.

The analysis of this model of valence states in the CuxFel_xCr2S 4 system allows one to note that.

(1) Large vacancies for sulphur result in the presence of a large quantity of chromium ions of lower valence. In this connection the Cr 3÷ - C r 2+ heterovalent ions are available at all compositions x of copper ions. (2) Substitution of divalent iron ions by single valence copper ions leads to the appearance of heterovalent Fe 2+-Fe 3+ ions in the range of high concentration of iron (i.e. near FeCr 2$4) and heterovalent Cu +-Cu 2+ ions in the range of high concentration of copper. In the range 0.2 ~< x ~< 0.5 the presence of the Fe 2+-Fe 3+ ions in the cationic sublattice at Fe 3÷ > Fe 2÷ stipulates the n-type conductivity and at Fe 3+ < Fe 2÷ - - p - t y p e conductivity. The first case is realized at x > 0.2, the second-- at x < 0.2. In the vicinity of x = 0.2 the change of the sign of charge carriers occurs (Fig. 2(a)). The appearance of the Cu r - C u 2+ ions and disappearance of Fe 2÷ ions lead to the transition from n- to p-type conductivity at x > 0.5. (3) The observed evolution of valence states of ions in the solid solution system agrees with the behaviour of the electrical conductivity value. The factors such as sulphur deficit and substitution of Fe 2÷ ions by copper ions of lower valence (Cu ÷) in solid solutions with p-type conductivity should lead to opposite effects: in the first case to the decrease in electrical conductivity, in the second - - to its increase. It is experimentally established that when there is p-type conductivity in the range 0 < x < 0.2 the electrical conductivity increases, which is indicative of the dominating role of the Fe 2+ ions in the heterovalent pair Fe2+-Fe 3+. In the range 0.5 < x < 1, where p-type dominates, the decrease of the sulphur deficit and the increase of Cu 2÷ ions quantitity should favour the rise of electrical conductivity which agrees with the experimentally observed trend of tr. (4) The composition x - 0.5 is critical in many respects. Near this concentration ordinate the valence transformations proceed according to phase transitions: Fe3+-Fe 2+ ~ Fe3+; Cu + "--t. C u + - f u 2+.

992

VALENCE STATE OF Fe, Cu, Cr IONS

Besides, the presence of the largest deviation from stoichiometry for sulphur in the vicinity of this composition, as it has already been noted, conditions the maximum quantity of Cr 2+ ions. At x = 0.5 there occurs a sharp (by several orders of magnitude) rise of electrical conductivity. The analysis has revealed that the origin of similar anomaly lies in high degree of crystallochemical and structure disorder of the samples, due to realization of valence transformations in the range x = 0.5 for iron and copper ions, and in extremely high vacancy concentration in closely packed sulphur ions. While interpreting the anomalies at x = 0.5 we assumed the realization of phase transition of the Anderson type [7] as a working hypothesis. Thus the observed regularilies of the change of properties for the CuxFel_xCr2S4 solid solution against composition of the compounds are determined by simultaneous effect of two factors: cationic substitution in the .4 sublattice on the one hand, and

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the type and relation of heterovalent iron and copper ions on the other. REFERENCES 1. 2. 3.

4. 5. 6. 7.

F.K. Lotgering, R.P. Stapele, G.H.A.M. van Steen & J.S. van der Wieringen, J. Phys. Chem. Sol. 30, N6, 799 (1969). G.B. Goodenough, J. Phys. Chem. Sol. 30, 261 (1969). V.N. Zaritskii, R.A. Sadykov, Ya.I. Kostyuk, T.G. Aminov, R.A. Sizov, & V.T. Kalinnikov, Izv. A N SSSR. Neorg. materialy, 19, N2, 200 (1983). K. Ando & Y. Nishihara, J. Phys. Chem. Sol. 41, N l l , 1273 (1980). K. Ando, Solid State Commun. 36, N2, 165 (1980). E.M. Zub, E.V. Kuchis & S.V. Sukhvalo, Phys. Status Solidi (a), 82, 569 0984). P.W. Anderson, Phys. Rev. 109, N5, 1492 (1958).