THERMODYNAMIC, SPECTROSCOPIC, X-RAY, ELECTRONIC AND MAGNETIC STUDY OF (Ti1-xNbx)O2 SOLID SOLUTIONS VERSUS TEMPERATURE AND COMPOSITION

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THERMODYNAMIC, SPECTROSCOPIC, X-RAY, ELECTRONIC AND MAGNETIC STUDY OF

(Ti1-xNbx)O2 SOLID SOLUTIONS VERSUS TEMPERATURE AND COMPOSITION

B. Poumellec, J. Marucco, B. Calès, B. Touzelin

To cite this version:

B. Poumellec, J. Marucco, B. Calès, B. Touzelin. THERMODYNAMIC, SPECTROSCOPIC, X- RAY, ELECTRONIC AND MAGNETIC STUDY OF (Ti1-xNbx)O2 SOLID SOLUTIONS VERSUS TEMPERATURE AND COMPOSITION. Journal de Physique Colloques, 1986, 47 (C1), pp.C1-825- C1-829. �10.1051/jphyscol:19861126�. �jpa-00225523�

JOURNAL DE PHYSIQUE

C o l l o q u e C I , s u p p l e m e n t a u n ° 2 , Tome 4 7 , f e v r i e r 1986 p a g e ci-825

THERMODYNAMIC, SPECTROSCOPIC, X-RAY, ELECTRONIC AND MAGNETIC STUDY OF ( T i1_xN bx) 02 SOLID SOLUTIONS VERSUS TEMPERATURE AND COMPOSITION

B. POUMELLEC, J . F . MARUCCO, B . CALES* a n d B . TOUZELIN

UR 446, Thermodynamique e t P h y s i c o - C h i m i e d e s Matériaux, Lab.

C.N.S., Bât. 415, F-91405 Orsay Cedex, France

*C.R. Physique Hautes Températures, C.N.R.S., F-45071 Orléans Cedex 2, France

Résumé - Diverses p r o p r i é t é s physiques ou physico-chimiques des solutions solides (Ti,Nb)0_ ont é t é étudiées en fonction de la température e t de la composition de manière a déterminer l ' é v o l u t i o n de l a s t r u c t u r e é l e c t r o n i q u e . La présence de n i o - bium à l ' é t a t 4d' modifie largement le comportement é l e c t r i q u e e t l a s t r u c t u r e de défauts de TiO„. Les niveaux de niobium s'accumulent au voisinage de la bande de conduction du T i . L ' é c a r t à l a stoechiométrie du matériau change de signe quand la concentration de niobium augmente, c e t t e modification e s t à r e l i e r à l'occupation p a r t i e l l e des niveaux 4d du niobium. Aux concentrations de niobium élevées des i n t e r a c t i o n s antiferromagnétiques entre les niobium ou l e s t i t a n e porteurs d ' é l e c - trons se développent. Les électrons sont a l o r s plus l o c a l i s é s que dans TiO e t la mobilité e s t thermiquement a c t i v é e .

Abstract - We have studied d i f f e r e n t physical or physico-chemical p r o p e r t i e s of (Ii,Nb)0 solid solutions versus temperature and composition in order to s i f t out the evolution of the e l e c t r o n i c s t r u c t u r e . The s u b s t i t u t i o n of titanium by niobium in the 4d' s t a t e changes l a r g e l y the e l e c t r i c a l behaviour and the defect s t r u c t u r e of TiO . The 4d e l e c t r o n i c levels of niobium p i l e up close to the bottom of the conduction band. The departure from stoichiometry changes of sign for the niobium contents> 4%, t h i s would be r e l a t e d to the p a r t i a l occupation of 4d l e v e l s . For higher niobium content, antiferromagnetic i n t e r a c t i o n occurs on the niobium or on the titanium atoms. Electrons are then more l o c a l i s e d than in TiO and t h e i r mobility in thermally a c t i v a t e d .

I-THERMODYNAMIC PROPERTIES / ! /

By means of t h e r m o g r a v i m e t r i c mea- s u r e m e n t s , i t h a s been p o s s i b l e t o o b t a i n i n f o r m a t i o n s on t h e n a t u r e of i n t r i n s i c and e x t r i n s i c d e f e c t s of NbxTi^_x02+y s o l i d s o l u t i o n s . Pure Ti02

i s an o x y g e n - d e f i c i e n t o x i d e . The main d e f e c t s a r e oxygen v a c a n c i e s , d o u b l y i o n i z e d VQ* o r s i n g l y i o n i z e d V0 and i n t e r s t i c i a l t i t a n i u m T i | ' . Nb02 i s a m e t a l - d e f i c i e n t o x i d e . The main d e f e c t s a r e niobium v a c a n c i e s . The s o l i d s o l u - t i o n s may be d i v i d e d i n t o two g r o u p s . I f x>.04, t h e behaviour i s analogous t o t h a t of Nb02 w i t h t h e same d e f e c t s , b u t t h e width of t h e homogeneity range d e - c r e a s e s w i t h t h e t i t a n i u m c o n t e n t and Nbp 0 4T :' - 0 . 9 6o2 i-s a s t o i c h i o m e t r i c o x i d e . I f * x < . 0 4 , t h e o x i d e s a r e b o t h m e t a l - d e f i c i e n t and o x y g e n - d e f i c i e n t a c c o r d i n g t o t h e oxygen p a r t i a l p r e s -

Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:19861126

~ 1 - 8 2 6 JOURNAL DE PHYSIQUE

sure. In the oxygen-deficient domain, the main defects are oxygen va- cancies. I n fhe metal-deficient domain, the main defects are metal vacancies VTi. 4

11-X-RAY ABSORPTION SPEmROScOPY /2/ I T i . N b l 0 2 K EDGE OF T I T R N I U M

The K-edcre spectra of titanium in ' (Ti,Nb)02 was ;eco;ded on the synchrotron radiation at Lure-DCI-Orsay. Only the near edge structures have been analized. They show the energy position of the band maxima of unoccupied levels if we assume the one electron approximation valid. The analysis of the second derivatives leads to three

important conclusions. z

First, a partial electron transfer Q occurs from Nb4d to Ti3d wave function with t

increasing niobium content. This implies close energies for these wave functions. 4 ^m

Second, the crystal field splitting -X

which is the splitting between the levels arising from t2 and e decreases from 3 . 3 e ~ to 2.5eV wRen nioBium content in- creases. This is related to the screening * by the d-electrons and the increase of the oxygen-metal bond length.

Third, the change of the shape of the T i spectra with niobium content indicates that modifications occur around the titanium conti- nuously from Ti02 to Nb02. Hence the solid solutions are probably disordered.

111-ELECTRON TRANSPORT IN (Til-xNbx)OZ FOR x>4% /6/

The analysis of electrical conduc- tivity and thermopower yields different conclusions a s for pure or doped Ti02 where a large polaron was pointed out /4,5/. In the present case, the experi- 2

mental results are consistent with a >

small polaron model. Mobilities shown 5' on the opposite figure are thermally , activated whatever the niobium content. ;

Thermopower (not shown here but in /6/) 2

indicates that polarons are bound to 5 larger species with an energy greater ' than O.leV increasing with the niobium content. However, it seems that mobile species would be single small polarons only for low temperatures, because of

the high calculated energy at high temperature for the associated phonon according to the small polaron theory. Hence we have proposed the existence of bipolaron or tripolaron for higher niobium content or high temperature. This assumption accounts for the high temperature thermopower and the variation of the activation energy of the mobility with the temperature for x<.5. For Nb02, owing to the magnitude of the thermopower (-80pV/K), a small polaron transport may still be assumed, but with a higher orbital degeneracy and the possibility of two elec-

trons on the same site.

IV-X-RAY DIFFRAC!l'ION STUDY

W e h a v e measured bv means of X-ravdiffraction onPowder,the"a8' an9 "c" parameters of the rutile phases

(~48)

(C4h) versus the temperature and the niobium content.

Plots of variations of c and a parameters relatively to Ti02 are

shown on the opposite figure versus the composition for 5,

r o o m and 1300K. W e have noticed a s other a u t h o r s that a tetragonal-rutile transition occurs for higher n i o b i u m c o n t e n t and near lOOOK but without disconti- nuity of the parameters.

The influence of x is much greater than that of T.

,

"a" parameter increases fas- ter than the "c" parameter 7

and follows a vegard law. It is not the case on the "c" parameter which becomes approximately constant for x>50% for all temperatures.

From that, we have concluded that a relative compression stands along c. This is related to antiferromagnetic interactions along the c axis between adjacent sites.

With increasing T, the thermal expansion of Ti02 is isotropic, on the contrary, Nb02 thermal expansion is strongly anisotropic, as "a"

is almost T independent and "c' has the same dependence as Ti02 (this is not shown here but in a subsequent paper).

These observations leads to the conclusion that interaction between adjacent sites occurs even at high temperatures and essential- ly along the "cw axis. Because of the disappearance of long range order but without discontinuity on the parameters, w e conclude to a thermal disorder of electron pairings.

V-MAGNETIC PROPERTIES / 7 - 8 / A-EXPERIMENTAL

Maqnetic susceptibility was measured with three differents appar- atus: aeliquid helium magietometer for 4K<T<300K, a gazeous helium m a g n e t o m e t e r for 3 0 0 K < T < 6 0 0 K and an argon m a g n e t o m e t e r for 400K<T<1250K. The different results are very well consistent with each other.

B-RESULTS - ^{Squared Bohr} lil-xNb,02 W e have shown on the Magneton

opposite figure the squared . number Bohr magneton number versus

t h e t e m p e r a t u r e and the -

niobium content. The analy- sis will be made by compa- - rison with the results ob- tained previously on (Ti,- -

Ta)02 and (Ti,V)02 solid solutions.

In the case of (Ti,V)02 the increase of magneton number is related to the monoclinic-tetragonal tran-

sition occuring at about 1000

350K for vanadium contents T(K)

larger than 0.8. For lower vanadium contents there is no transition and the increase of magneton number is related to the effect of tem- perature on the antiferromagnetic interactions in vanadium clusters.

The same phenomenon is noticeable in (Ti,Nb)02 for niobium con- tents larger than .5 and consequently we conclude to the existence of such clusters in these solid solutions. Nevertheless we notice that the transition occurs at much higher temperature (T>1100K).

The (Ti,Ta)02 solid solutions are limited to tantalum contents smaller than .5, the results are known for T<300K. For this range of temperatures and tantalum contents, no transition is visible. Never- theless (Ti,Nb)02 solid solutions look alike to (Ti,Ta)02 for low

C1-828 JOURNAL DE PHYSIQUE

t e m p e r a t u r e a n d f o r low n i o b i u m c o n t e n t . W e n o t i c e i n t h e b o t h c a s e s t h a t t h e m a g n e t o n number i s n o n - z e r o a n d i n d e p e n d e n t o f x f o r T=O. I t i s a u i t e d i f f e r e n t i n (Ti.V)O?. - ^{. . .}

C-DISCUSSION F o r x<.45

I n t h e c a s e o f ( T i , V ) O Z t h e m a g n e t i c p r o p e r t i e s a r e r e l a t e d t o t h e a n t i f e r r o m a g n e t i c c o u p l i n g b e t w e e n h a l f s p i n s w i t h q u e n c h e d o r - b i t a l momentum i n l i n e a r c l u s t e r a l o n g t h e c a x i s . B e c a u s e o f t h e d i f f e r e n t m a g n e t o n n u m b e r s a t l o w t e m p e r a t u r e b e t w e e n ( T i , V ) 0 2 a n d o t h e r s o l i d s o l u t i o n s we c o n c l u d e t h a t t h e r e i s n o S = 1 / 2 c e n t e r s i n ( T i , N b ) 0 2 . A b e t t e r e x p l a n a t i o n o f t h e n o n z e r o m a g n e t o n n u m b e r f o r T=O is a p a r t i a l b a l a n c e o f s p i n momentum a n d o r b i t a l momentum t h r o u g h t h e s p i n - o r b i t c o u p l i n g and t h e d i s t o r t i o n a r o u n d m e t a l l i c s i t e s . T h i s l e a d s t o s t a b i l i z e a n o r b i t a l s i n g l e t b u t t h e s p i n o r b i t c o u p l i n g mixed d i f f e r e n t wave f u n c t i o n s a n d t h u s e f f e c t i v e momentum h a s n o t t h e s p i n o n l y v a l u e n e i t h e r t h e z e r o v a l u e a t O K . T h e r e f o r e i t c a n b e d e d u c e d t h a t t h e s q u e e z i n g o f t h e m e t a l l i c o c t a h e d r o n a l o n g t h e c a x i s i s g r e a t e r t h a n t h e t e t r a g o n a l d i s t o r t i o n . T h i s e f f e c t a p p e a r s e i t h e r f o r ( T i , N b ) 0 2 o r f o r ( T i , T a ) 0 2 . S o e f f e c t i v e momentum a t v e r y low t e m p e r a t u r e comes f r o m e l e c t r o n s o n t i t a n i u m .

M o r e o v e r f o r h i g h e r t e m p e r a t u r e s a c o m p a r i s o n b e t w e e n t h e magni- t u d e o f e f f e c t i v e momentum o f TiS8Nb 202 and T i 8Ta.202 s h o w s t h a t t h e f i r s t i s s m a l l e r t h a n s e c o n d o n e

.

F o r T>750K f o r T i *Nb 20 t h e t h e r m o p o w e r becomes c o n s t a n t . T h i s i m p l i e s t h a t t h e numbek o f e l e c t r o n i c m o b i l e s p e c i e s d o e s n o t c h a n g e a n y m o r e , t h a t i s t o s a y a l l e l e c t r o n s a r e m o b i l e s . No b r e a k w a s o b - s e r v e d o n t h e t e m p e r a t u r e d e p e n d e n c e o f t h e e f f e c t i v e m a g n e t i c momen- tum. W e t h u s c a n c o n c l u d e t h a t n o c h a n g e o f m a g n e t i c p r o p e r t i e s a p - p e a r s s u c h a s t h e f o r m a t i o n o f a t r i p l e t s t a t e c o m i n g f r o m t h e l o c a l i - s a t i o n o f t w o e l e c t r o n s o n t h e s a m e s i t e . On t h e o t h e r h a n d , i f a l l m o b i l e s p e c i e s were m o n o e l e c t r o n i c w e w o u l d f i n d a n e f f e c t i v e momentum a p p r o x i m a t l y c o n s t a n t s i n c e e l e c t r o n s a r e f r e q u e n t l y pn t i t a n i u m s i t e s and t h e s p i n - o r b i t c o u p l i n g is r a t h e r s m a l l ( 150cm- ). C o n s e q u e n t l y , t h e a s s u m p t i o n o f p o l y e l e c t r o n i c m o b i l e s p e c i e s a t h i g h t e m p e r a t u r e is r a t h e r l i k e l y .

F o r x > . 4 5

F o r T=O, t h e m a g n e t o n n u m b e r i s z e r o , c o n s e q u e n t l y t h e o r b i t a l momentum may b a l a n c e t h e s p i n momentum e v e r y w h e r e t h e r e i s n o c l u s t e r a n d t h e d i s t o r t i o n o f t h e m e t a l l i c s i t e may s t a b i l i z e a n o r b i t a l d o u b l e t or t r i p l e t . T h i s d o e s n o t e x c l u d e a n t i f e r r o m a g n e t i c c o u p l i n g s o c c u r i n g e l s e w h e r e s i n c e t h e y l e a d a g a i n t o a z e r o e f f e c t i v e m a g n e t i c momentum.

A s m e n t i o n e d p r e v i o u s l y , t h e low m a g n e t o n number f o r i n t e r m e d i a t e t e m p e r a t u r e s c o m p a r i n g t o s p i n o n l y f o r m u l a , i m p l i e s a l a r g e number o f s p i n p a i r i n g s . T h o s e o n e s a r e d e s t r o y e d w i t h i n c r e a s i n g t e m p e r a t u r e l e a d i n g t o a n i n c r e a s e o f t h e Bohr m a g n e t o n number.

N e v e r t h e l e s s , i n Nb02 a t h i g h t e m p e r a t u r e t h e e f f e c t i v e momentum is c o n s i d e r a b l y l o w e r t h a n t h e s p i n o n l y v a l u e w i t h S=1/2 or t h a n t h e v a l u e o b t a i n e d by t a k i n g i n t o a c c o u n t t h e s p i n o r b i t c o u p l i n g . T h u s , e x c h a n g e c o u p l i n g s a r e s t i l l p r e s e r v e d b u t p r o b a b l y o n t h e s a m e site.

S i n c e t h e t h e r m o p o w e r i s i n d e p e n d e n t o f T , t h e p o l y e l e c t r o n i c s p e c i e s a r e s t i l l m o b i l e .

VI-CONCLUSION

A s a c o n c l u s i o n , w e w i l l p r o p o s e d t e n t a t i v e l y t h e f o l l o w i n g e v o l u t i o n o f t h e e l e c t r o n i c s t r u c t u r e n e a r t h e c o n d u c t i o n band v e r s u s t h e c o m p o s i t i o n a n d t h e t e m p e r a t u r e .

T h e d i f f e r e n t p o i n t d e f e c t s o f T i 0 2 a p p e a r i n g i n t h e g a p a r e r e p l a c e d by t i t a - n i u m v a c a n c i e s w h e n n i o b i u m i s a d d e d . T h e Nb l e v e l s s t a n d s 0.2eV b e l o w t h e c o n d u c t i o n b a n d . A s t h e n i o b i u m c o n t e n t i n c r e a s e s

a n t i f e r r o m a g n e t i c c h a i n s a r e c r e a t e d , t h e i r 3eV l e n g t h d e p e n d i n g o n t h e a m o u n t o f n i o b i u m .

H o w e v e r , i t r e m a i n s s i n g l e e l e c t r o n s o n Nb o r T i f o r x < 5 0 % f o r a l l t e m p e r a t u r e s . F o r x > 5 0 % , t h e a m o u n t o f s i n g l e e l e c t r o n s i s v e r y s m a l l a t l o w t e m p e r a t u r e s . I t i n - c r e a s e s a g a i n a t h i g h t e m p e r a t u r e s when t h e d e n s i t y o f s t a t e s o f e l e c t r o n s i n c l u s t e r s

d e c r e a s e s . a n t i b o h d i n

3

ACKNOWLEDGMENTS: W e a r e g r a t e f u l t o D r temperature B r o h a n o f N a n t e s U n i v e r s i t y , f o r h i s a s s i s -

t a n c e f o r l o w t e m p e r a t u r e m a g n e t i c m e a s u r e - m e n t s a n d D r C a s a l o t a n d M o n n e r e a u f o r t r a n s m i t t i n g t h e i r n u m e r i c a l v a l u e s .

Lpolaron states REFERENCES

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