Extended Interactions in Transition Metal Oxides and Chalcogenides

1 Extended Interactions in Transition Metal Oxides and Chalcogenides AARON WOLD

Downloaded by 5.62.159.180 on February 17, 2017 | http://pubs.acs.org Publication Date: June 1, 1974 | doi: 10.1021/bk-1974-0005.ch001

D i v i s i o n of E n g i n e e r i n g , B r o w n U n i v e r s i t y , Providence, R.I. 02912

As e a r l y as 1937 de Boer and Verwey (1) had i n d i c a t e d that i n t r a n s i t i o n metal o x i d e s , in which the d-bands o f the t r a n s i t i o n metal ions were partially filled, the p o t e n t i a l energy b a r r i e r between atoms was high enough t o reduce the c o n d u c t i vity by an enormous amount. This was probably the first i n d i c a t i o n that the Block-Wilson band theory of s o l i d s (1,2) could not d e s c r i b e i n a realistic way the t r a n s p o r t p r o p e r t i e s o f t r a n s i t i o n metal compounds. Indeed, the classical band theory had p r e d i c t e d that these compounds with partially filled d-bands would show high electrical c o n d u c t i v i t y (σ = n e μ); the num ber o f c a r r i e r s n would show temperature independence and the m o b i l i t y μ would decrease as the temperature Τ increased. The electrical c o n d u c t i v i t y observed f o r these compounds would be de r i v e d from these latter temperature dependencies. In 1957 (3) Morin r e p o r t e d on the electrical p r o p e r t i e s of s e v e r a l vanadium and t i t a n i u m o x i d e s , namely VO, V O , VO and Ti 0 . These oxides c o n t a i n 3,2,1 and 1-d e l e c t r o n s per t r a n s i t i o n metal c a t i o n r e s p e c t i v e l y . These compounds show m e t a l l i c behavior a t high temperatures and then become semiconducting when cooled through a critical temperature Τ , (114°K, 153°K, 340°K and 450°K). For most o f these compounSs, the electrical resistivity was observed t o drop by s e v e r a l orders o f magnitude over a s m a l l temperature range. The oxides of vanadium show a r e d u c t i o n i n symmetry a s s o c i a t e d with the resistivity changes. The lattice parameters o f T i O vary q u i t e r a p i d l y i n the vici nity of the t r a n s i t i o n temperature ( T ) but there is no change i n symmetry (4). Figure 1 summarizes these f i n d i n g s . I t should a l s o be i n d i c a t e d that f o r pure s t o i c h i o m e t r i c s i n g l e c r y s t a l s , the temperature range over which the t r a n s i t i o n occurs is g r e a t l y reduced and the magnitude o f the d i s c o n t i n u i t y i n electrical con d u c t i v i t y i s increased by a f a c t o r o f 1 0 . The 3d t r a n s i t i o n metal oxides may t h e r e f o r e be e i t h e r me tallic at a l l temperatures, semiconducting at all temperatures or undergo a semiconductor

°3>

2°3

MnS, MnS , FeS « 2

2

-CLASS I I I T r a n s i t i o n a l Compounds

3d-compounds:

Vo, V Og, V 0

T i

2°3>

NiS, Ud-compound s :

T i

3°5*

V ^ , ν^0 , ν 0

2 $

?

F

e

β

2 V

CrS, FeS.

NbO . 9

Interrante; Extended Interactions between Metal Ions ACS Symposium Series; American Chemical Society: Washington, DC, 1974.

E X T E N D E D INTERACTIONS

4

V Q '.(3d) 2

V0 ;(3d)'

2

TLCUSd)

2

3

B E T W E E N M E T A L IONS

1

log ο* IOV ^. I Ο* Electrical Conductivity! I O ΙΟ σ (σ ohm"'cm") - i o 4 _ _ J L i o IO -ΙΟ" T ^450°K Τ T ^34Q°K τ Τ,*Μ50*Κ Τ (Z>i?J-?J),(2S)*C29-30)J(7),(35-37) PÔ 6?i 17-21Y22 - 23V References (22),(3j-33)*,(34) 0

1

J

J

t

t


X X IO*

XX ΙΟ*

XX ΙΟ*

Magnetic 16501 TOO Susceptibility], 1050 X 200 J (x in eg*, ΙΟΟ ΙΟΟ mole"' ) "673Ύ°Κ io iSolooT^ 33 ΙΟΟ 3 4 0 450Τ°Κ J(I2)(42-43)(26)Î)/221)(I3-I4)/3J)*(45) Γ(12)(3β)*.(3?-4Ι) References Κ4ΪΧ(44)* ' Ι20

Specific Heat) 81 (C in cals, mole** deg"')| p

^Lo700colt/»o»«

Τ

Figure

Ι.

*M.o750ce»»/eol*

20

15

28 20 Τ Κ

Τ Κ

κ

Electrical conductivity, cific heat vs. temperature

magnetic susceptibility, and spe for V>0, , VO>, and Ti 0. {

Figure

IL

:

Structure

s

of ReO 2

30 "

Re * 6

[2>[6] σ ·

j[2][2][2]

ζ-JT 1

(E

E

m,

FERML LEVEL

*[2][21Κ

Ε - E, )

}

1

*cf

/

[2>[3]

Figure

III.

Electron

energy diagram for ReO.,


1.

WOLD

Oxides

and

5

Chalcogenides

cubic bronze, Ma WO^, and the ordered p e r o v s k i t e Sr^M ReO have been i n v e s t i g a t e d by Goodenough and co-workers (9)· The one e l e c t r o n energy diagram f o r ReO« i s shown i n Figure III. Because o f the o c t a h e d r a l c r y s t a l f i e l d the d - e l e c t r o n energy s t a t e s o f the rhenium are s p l i t i n t o a l e s s s t a b l e , doubly degenerate s t a t e (e ) and a more s t a b l e t r i p l y degenerate ( t ^ )· The outer e l e c t r o n Inergy l e v e l s of the anion are s i m i l a r l y s flown on the r i g h t s i d e o f the f i g u r e . The overlap o f c a t i o n t and anion p^ o r b i t a l s r e s u l t s i n the formation o f bonding and Intibonding bands extending throughout the c r y s t a l (10^. The Fermi l e v e l may be l o c a t e d by l o o k i n g a t the number o f outer elec^roçs per molecule. Consider the ReO^ u n i t f o r rhenium 5d 6s . There are [7] χ [1] = 7 e l e c t r o n s . The oxygen 2s 2p c o n t r i b u t e [6] χ [3] = 18 e l e c t r o n s r e s u l t i n g i n 25 e l e c t r o n s per molecule. Upon f i l l i n g the a v a i l a b l e energy l e v e l s , the l a s t e l e c t r o n occupies the ir*band. This band i s p a r t i a l l y f i l l e d and causes the observed m e t a l l i c behavior. The o r i g i n o f the m e t a l l i c c o n d u c t i v i t y o f cubic Na^WO^ has been p o s t u l a t e d by s e v e r a l models: 1. The d i r e c t overlap of sodium 3p o r b i t a l s (11) 2. The d i r e c t overlap o f tungsten t o r b i t a l s l e a d i n g t o tungsten-tungsten bonds (12) 3. The covalent mixing o f tungsten t o r b i t a l s and oxygen ρ orbitals t o form p a r t i a l l y f i l l e d b a n d s ( 1 3 ) . Goodenough and h i s co-workers (9_) performed a unique ex periment which i n d i c a t e d t h a t the t h i r d p o s s i b i l i t y was indeed the most probable mechanism f o r the observed m e t a l l i c con d u c t i o n i n these m a t e r i a l s . The compound Sr M c crystallizes i n the ordered p e r o v s k i t e s t r u c t u r e (see F i g u r l IV;. The B - s i t e c a t i o n s , Mg and Re, order such that each rhenium has only magne sium nearest c a t i o n neighbors and v i c e versa ( 9 ) . Such an arrangement would s t i l l allow f o r the overlap of rhenium t ~ or b i t a l s across a cube f a c e . Consequently, m e t a l l i c behavior would be expected i f Model 2 a p p l i e d . However, i f the e l e c t r i c a l pro p e r t i e s are a r e s u l t of c a t i o n t and oxygen interactions (Model 3 ) , Sr MgReOg should be a iemi-conductor, s i n c e Mg does not possess t e l e c t r o n s . The observed semi-conductive be havior (E = 0.§leV) and temperature dependent paramagnetism con f i r m t h a t the e l e c t r o n s are l o c a l i z e d r a t h e r than occupying c o l l e c t i v e energy bands ( 9 ) . Thus, model 3 i s c o n s i s t e n t with the i n t e r a c t i o n s present i n p e r o v s k i t e - l i k e m a t e r i a l s and best e x p l a i n s the observed p r o p e r t i e s . g

g


2

δ

g

π

R e 0

2

2

2

2

II. Vanadium (IV) Oxide and S u b s t i t u t e d Compounds. The high temperature, m e t a l l i c VO phase has a t e t r a g o n a l r u t i l e l i k e (1*0 s t r u c t u r e (space group p4 /mnm). As can be seen from Figure V the s t r u c t u r e may be d e s c r i b e d as s t r i n g s o f edge-shared octahedra j o i n e d by corners extending i n the c d i r e c t i o n . The V-V d i s t a n c e s are equivalent w i t h i n the s t r i n g s (2.87 8 ). 2



6



©Sr ©Mg • Re O o Figure

IV.

Structure

of

Sr MgReO« 2

V 0 (tetragonal) 2

• Vanadium Ο Oxygen Figure V.

Rutile

structure


1.

WOLD

Oxides

and

Chalcogenides

7

Below 340°K, VO^ has been reported to have a d i s t o r t e d r u t i l e s t r u c t u r e with monoclinic symmetry (P2^/c. This d i s t o r t i o n (see F i g u r e VI) causes the vanadium atoms l o c a t e d i n the s t r i n g s of VOg octahedra t o occur as doublets ( s i m i l a r t o MoO^). The V-V d i s t a n c e s are no longer equal but are 2.658 and 3.12^ (1^0· As pointed out by Heckingbottom and L i n e t t (15), i n the low temper ature semiconducting phase the c a x i s i s t i l t e d causing the vana dium atoms to be d i s p l a c e d from the center o f i t s octahedra. The band s t r u c t u r e f o r t e t r a g o n a l V 0 has a l s o been examined by Goodenough (14) and i s shown i n Figure V I I . Because of edge sharing of VO^ octahedra i n the t e t r a g o n a l phase, an o r t h o r hombic component o f the c r y s t a l f i e l d removes the d-state de generacy. The two e o r b i t a l s , normally o c c u r i n g i n an o c t a h e d r a l f i e l d , are s p f i t i n t o two da o r b i t a l s and the three t ~ o r b i t a l s s p l i t i n t o two d o r b i t a l s which mix with the anion ρ o r b i t a l and a d n o r b i ! a l d i r e c t e d along the c r u t i l e a x i s . When the 17 outer e l e c t r o n s per molecule i n V0- are placed i n t o t h i s energy scheme the f i n a l e l e c t r o n enters the over lapping π* and d i i bands. These o v e r l a p p i n g , non-degenerate bands, being o n l y ' p a r t i a l l y f i l l e d , r e s u l t i n m e t a l l i c conductiv i t y i n the t e t r a g o n a l phase (14). Below the t r a n s i t i o n temperature VO^ has a m o n o c l i n i c a l l y d i s t o r t e d r u t i l e s t r u c t u r e c h a r a c t e r i z e d by the formation o f V-V p a i r s along the a monoclinic a x i s ( «i ) * a consequent d o u b l i n g g f the c r y s t a l o g r a p h i c u n i t c e l l ' ( 1 6 ) . This displacement o f the V ion from i t s center of symmetry i s b e l i e v e d to be caused by a f e r r o e l e c t r i c - type d i s t o r t i o n (14). The e f f e c t o f t h i s d i s t o r t i o n on the band s t r u c t u r e o f VO^ i s shown i n Figure V I I I . The d j i band i s s p l i t i n two and the Fermi energy i s lowered below'the bottom of the π» band. As a consequence o f doubling the c e l l , the lower X band i s f i l l e d completely and semi-conducting behavior r e s u l t l (14).


2

δ

c

a x : L S

r u t

a n c

e

+

I I I . O x y f l u o r i d e s . At low l e v e l s of f l u o r i n e , the oxyf l u o r i d e s u s u a l l y possess s t r u c t u r e s q u i t e s i m i l a r to the parent oxide. The c e l l parameters (orthorhombic indexing) a = 7.356 A, o °*> °- r e p o r t e d f o r WO F are q u i t e s i m i l a r to those observed by S l e i g h t (17) f o r monoclinic W0 a = 7.301 X b = 7.538 8, c = 3.844 ^7 3 = 90.89°. S l e i g h t d i d not r u l e out the p o s s i b i l i t y that WO^ gg^Q ^ could be monoclinic with the d e v i a t i o n o f the monoclinic angïe from 90° being too s m a l l to d e t e c t . Higher s u b s t i t u t i o n s of f l u o r i n e i n the WO^ F^ systems s t a b i l i z e s the c u b i c Re0 s t r u c t u r e (17, 18).~¥his s t r u c t u r e i s analogous t o the p e r o v s k i t e type a l k a l i metal bronzes with a l l A s i t e s vacant. In the tungsten o x y f l u o r i d e system the cubic phase extended from χ = 0.17 to 0.66. A one-electron energy diagram would be much simpler f o r 3 x x ^ corresponding cubic tungsten bronzes Na^WO^. I t Is not necessary t o consider i n t e r a c t i o n s i n v o l v i n g A s i t e c a t i o n s s i n c e these p o s i t i o n s are empty·

b

=

7 , 1 + 6 9

c

=

3 , 8 1 4 6

0

g 6

Q

Q l +

3

3

W 0

F

t

n

a

n

o r

t

n

e


8


£

S

/


y

i


= 2c

yf

—

/A

rutile axes Figure

V

Figure

7=

monoclinic axes

VI. Monoclinic structure temperature) for VO.

I

V0_

(low

(

VII. Electron energy for tetragonal VO

diagram

:


1.

WOLD

Oxides

and

Chalcogenides

9

The s u b s t i t u t i o n o f f l u o r i n e ajlso r e s u l t s i n a d d i t i o n a l elec trons because o f the formation o f W i o n s . Thus, the compounds WOg F have χ a d d i t i o n a l e l e c t r o n s per molecule. Although i t mighi £e expected that the more e l e c t r o n e g a t i v e f l u o r i d e ion would have a l o c a l i z i n g e f f e c t on the a d d i t i o n a l e l e c t r o n s , me t a l l i c c o n d u c t i v i t y has been observed f o r the c u b i c o x y f l u o r i d e bronzes. By comparison with the WO. F system, i n which l a r g e f l u o r i n e s u b s t i t u t i o n s s t a b i l i z e a high symmetry c u b i c phase, there may a l s o be an analogous decrease i n the semiconductor metal t r a n s i t i o n with i n c r e a s i n g f l u o r i n e s u b s t i t u t i o n i n 2 - x . For V 0 , the monoclinic t o t e t r a g o n a l t r a n s i t i o n temper ature decreases with i n c r e a s i n g anion s u b s t i t u t i o n i . e . the high temperature t e t r a g o n a l phase becomes more s t a b l e (19). For the system V 0 ^F a c t i v a t i o n energies were c a l c u l a t e d from the r e s i s t i v i t y data f s e e F i g u r e IX). the values are given i n Table II f o r d i f f e r e n t values of χ i n V0F . I t can be seen 2—χ χ from these curves t h a t a m e t a l l i c t o semiconductor t r a n s i t i o n occurs a t a temperature which decreases with i n c r e a s i n g values of x. T h i s change i n the e l e c t r i c a l p r o p e r t i e s o f V 0 ^F^ can be explained by the corresponding t r a n s i t i o n from the monoclinic phase t o the t e t r a g o n a l phase which has been observed by means of low-temperature X-ray a n a l y s i s . A l i n e a r r e l a t i o n s h i p e x i s t s be tween the value of χ and the t r a n s i t i o n temperature (T ) shown i n F i g u r e X; t h i s e x t r a p o l a t e s t o the c o r r e c t t r a n s i t i o n temperature f o r pure V0 . For the higher f l u o r i n e compounds, the t r a n s i t i o n r e g i o n i s c o n s i d e r a b l y broadened and the t r a n s i t i o n p o i n t was chosen as the f i r s t d e v i a t i o n from l o g - l i n e a r behavior. A s i m i l a r l i n e a r r e l a t i o n s h i p e x i s t s between the volume of the t e t r a gonal c e l l and the value o f x, again e x t r a p o l a t i n g t o the value of the pure VC> phase a t χ = 0 (see F i g u r e X I ) . The same behavior has been observed i n compounds corresponding t o the formula V W 0 (0$x^0.067) by Nygren and I s r a e l s s o n (19). x-x x . i s seen, t h e r e f o r e , that the a d d i t i o n o f f l u o r i n e tends to s t a b i l i z e the high temperature, higher symmetry, r u t i l e phase. The compositions c o n t a i n i n g l a r g e r amounts of s u b s t i t u t e d f l u o r i n e shows p r i m a r i l y m e t a l l i c behavior. However, f o r a l l compositions s t u d i e d there s t i l l appears t o be a d i s c o n t i n u i t y i n the r e s i s t i v i t y a t the expected t r a n s i t i o n temperature ( T ) . The m e t a l l i c behavior observed i n these m a t e r i a l s may be ex p l a i n e d on the model presented by Goodenough (13) and Rogers (20). In t h e i r model the band formed between the overlap o f the t σ o r b i t a l s p a r a l l e l t o the c r y s t a l l o g r a p h i c c d i r e c t i o n s p l i t s 'into a more s t a b l e , p a i r - l o c a l i z e d , bonding V-V s t a t e and a higher l e s s s t a b l e σ* s t a t e . The lower l y i n g V-V l e v e l i f f i l l e d with one e l e c t r o n per vanadium and hence the semiconducting p r o p e r t i e s of the monoclinic V 0 may be e x p l a i n e d . The s u b s t i t u t i o n of f l u o r i n e f o r oxygen i n V 0 r e s u l t s i n the c r e a t i o n o f a d d i t i o n a l unpaired d - e l e c t r o n s . Despite the tendency f o r the more e l e c t r o negative anion t o l o c a l i z e d - e l e c t r o n s , i t i s apparent t h a t f o r V 0

P


2

2

2

2

2

2

t

t

2

2

2


10


BETWEEN

M

monoclinic


tetragonal

V

Figure

2°4

VIII. Electron energy tetragonal and monoclinic

diagram VO,

for

/

VO F 2-X X ι /'

L0Gp|

X-0-208

0

2 b

L 10

20

x. SO

7\

X0

I

-2

2

J

I

Figure

I

6

4

IX.

8

Log

P

I

10

vs. W/Ί

L

12

Ιθ/

Γ
. ,F,


M E T A L IONS

WOLD

1.

Oxides

and

11

Chalcogenides

T a b l e 2.

C e l l Parameters of VO_» F C o m p o u n d s V

C e l l Parameters (A)

X

A c t i v a t i o n Energy (eV) Τ (°K)

Compound


V0„

4.530 - 4

2 %9 - 3 e

340

0.5 j u s t below Τ

V0

1.97 0.03

F

4.552 - 4

2.853 ί 3

0.07

298

V0

1.96 0.04

F

4.554 ί 4

2.854 ί 3

0.06

282

VO

F 1.86 0.14

+ 4.562 - 4

+ 2.876 - 5

< 0.01

155

VO

F 1.79 0.21

+ 4.569 - 4

+ 2.886 - 3

< 0.01

65

Figure

X.

Transition

temperature

vs. composition

for

VO>.F.

r


12

EXTENDED

INTERACTIONS B E T W E E N M E T A L

IONS


the amount o f f l u o r i n e which has been s u b s t i t u t e d , the conduction paths have not been s u b s t a n t i a l l y a l t e r e d . IV. T r a n s i t i o n Metal Chalcogenides. Binary and ternary t r a n s i t i o n metal chalcogenides e x h i b i t a v a r i e t y o f d i f f e r e n t s t r u c t u r e s which a r e o f t e n q u i t e complex. This has been a t t r i b u ted i n p a r t t o the considerable covalent nature o f the metals u l f u r bonds. Many o f these compounds a l s o e x h i b i t semimetallic or m e t a l l i c p r o p e r t i e s and t h i s i n d i c a t e s that not a l l o f the bonding e l e c t r o n s behave as i n simple covalent c r y s t a l s . A c h a r a c t e r i s t i c o f s u l f i d e minerals i s t h a t p a r t i a l or complete replacement o f e i t h e r the c a t i o n s or anions i s p o s s i b l e without a change i n the c r y s t a l s t r u c t u r e o c c u r i n g . Among the many chalco genides studied a t Brown U n i v e r s i t y the t r a n s i t i o n metal d i c h a l cogenides, with the p y r i t e s t r u c t u r e , have been p a r t i c u l a r l y a t t r a c t i v e s i n c e t h e i r e l e c t r o n i c p r o p e r t i e s can be r a d i c a l l y changed by s u b s t i t u t i n g f o r e i t h e r the c a t i o n or anion i n the host compound. This group o f compounds combine a simple s t r u c t u r e (Figure XII) with a wide v a r i e t y o f magnetic and e l e c t r i c a l properties(21). Of p a r t i c u l a r i n t e r e s t i s the e f f e c t o f c a t i o n and anion sub s t i t u t i o n on the ferromagnetic compound CoS^. A one-electron energy scheme f o r CoS has been described by B i t h e r e t a l (22) and i s shown i n F i g u r e ^ X I J I . ^ In t h i s model σ-d sp o r b i t a l s on the metal atom and sp o r b i t a l s on the anions a r e assumed. S u l f u r has s i x valence e l e c trons that a r e shared among four t e t r a h e d r a l bonds. Each s u l f u r c o n t r i b u t e s one e l e c t r o n t o the S-S bond and the remaining f i v e t o the three M-S bonds. Since a l l the M-S bonds are e q u i v a l e n t , each s u l f u r c o n t r i b u t e s 1 2/3 e l e c t r o n s t o each o f them. Cobalt i s coordinated t o s i x s u l f u r atoms a t the apices o f an octahedron and these s u l f u r atoms thus c o n t r i b u t e 6 x 1 2/3 = 1 0 e l e c t r o n s t o the bonding i n each octahedron. These ten e l e c t r o n s plus the two 4 s e l e c t r o n s o f the t r a n s i t i o n metal j u s t f i l l the ground s t a t e a(s-p) and σ e manifold o f s t a t e s . The remaining d - e l e c trons o f the metal occupy the next-lowest a v a i l a b l e l e v e l s . Since c o b a l t possesses seven d - e l e c t r o n s , the one unpaired e l e c t r o n i n d i c a t e d by the magnetic moment o f CoS^ suggests that the t l e v e l s a r e f i l l e d with s i x e l e c t r o n s ( s p i n - p a i r e d ) . The u n p a i r i d e l e c t r o n t h e r e f o r e occupies the σ* e s t a t e . In the presence o f a s u f f i c i e n t l y strong covalent i n f e r a c t i o n the σ* e l e v e l w i l l broaden i n t o a band o f c r y s t a l l i n e s t a t e s ; the e l e c trons a r e no longer bound to s p e c i f i c s i t e s but a r e f r e e t o move through the c r y s t a l under the i n f l u e n c e o f an e l e c t r i c f i e l d . This concept already has been discussed by Goodenough (23,24). t o e x p l a i n the occurrence o f m e t a l l i c c o n d u c t i v i t y i n oxides and sulfides. For both compounds CoS and CoSe , as w e l l as members o f the 2

2

g

0

0



1.

WOLD

Oxides

and

Chalcogenides

13

Figure Large

CoS

Co

s-s

6

]^s,p)

8 y P3

XII. Pyrite structure. halls are sulfur; small halls, cobalt.

•,\σΚβς)

^

f

3a*(S-S)

2

\\ÉrfCoS2> / e g

"χ

»

\ \b — \V"

Figure

XIII.

À

^ u

Inn* il * 9 \\ 2

Electron

energy diagram for CoS,


14

E X T E N D E D INTERACTIONS B E T W E E N

METAL

IONS

system CoS-_ Se , the only p a r t i a l l y f i l l e d s t a t e s are the o*e s t a t e s . This implies that i n these m a t e r i a l s the o*e states c o n s t i t u t e a band; the degree of covalence i s l a r g e . % h e occur rence o f an energy d i f f e r e n c e between the t ^ and a*e s t a t e s that i s l a r g e r than the intraatomic exchange enerfy ( l o w - l p i n con f i g u r a t i o n s i n l i g a n d f i e l d parlance) f o r the s o l i d s o l u t i o n s and CoS^ i s c o n s i s t e n t with the presence o f strong covalent i n t e r actions. The stronger the bonding between the c a t i o n i c e or b i t a l s and the a n i o n i c o r b i t a l s , the l a r g e r w i l l be the s p l i t t i n g between the t ^ and the o*e s t a t e s . Since the l e v e l s are e s s e n t i a l l y noibonding theyâre l i t t l e a f f e c t e d inênergy; hence, the energy d i f f e r e n c e between the t ^ and the o*e s t a t e s (which corresponds to the l i g a n d f i e l d s p l i f t i n g , or 10D§) becomes l a r g e r with i n c r e a s i n g covalence. CoAsS has been reported (25) to be a digmagnetic semi conductor where the low s p i n s t a t e c o b a l t (d ) i s a l s o present i n an octahedral f i e l d . S u b s t i t u t i o n of a r s e n i c f o r s u l f u r i n CoS> r e s u l t s i n a continued depopulation o f e l e c t r o n s from the σ* a n t i bonding band ( 2 6 ) u n t i l f o r the end member CoAsS the σ* band i s empty. The p o s s i b i l i t y c^f r e p o p u l a t i n g t h i s band by p r o g r e s s i v e s u b s t i t u t i o n of n i c k e l d f o r c o b a l t d has r e c e n t l y been studied (27). The s u b s t i t u t i o n of a small amount o f n i c k e l (x = 0.05) f o r c o b a l t i n the system Co^ ^NiÂsS changed the e l e c t r i c a l pro p e r t i e s from semiconducting to m e t a l l i c . H a l l - e f f e c t measure ments a l s o showed that the number o f c a r r i e r s ( e l e c t r o n s ) was p r o p o r t i o n a l to the n i c k e l concentration. These r e s u l t s are con s i s t e n t with the i n t r o d u c t i o n of e l e c t r o n s i n t o the σ* band as n i c k e l i s s u b s t i t u t e d i n t o CoAsS.


g

This work was supported by the U.S. Army Research O f f i c e , Durham, the National Science Foundation and the M a t e r i a l s Research Laboratory Program at Brown U n i v e r s i t y .



1. WOLD

Oxides

and

Chalcogenides

15

Literature Cited 1 de Boer, J. Η., and Verwey, E. J. W., Proc. Phys. Soc. London (1937) A49, 59 2 Block, F., Z e i t s , f., Phys. (1928) 52, 555 3 Morin, F. J., Phys. Rev. L e t t e r s (1959) 3, 34 4 Pearson, A. D., J. Phys. Chem S o l i d s (1958) 5, 316 5 A d l e r , D., Rev. Mod. Phys. (1968) 40, 714 6 Goodenough, J . Β., Mat. Res. B u l l (1967) 2, 37 7 Goodenough, J . G., Mat. Res. B u l l (1967) 2, 165 8 Braeken, Z. Krist., (1931) 78, 484 9 F e r r e t t i , Α., Rogers, D., Goodenough, J . B., J. Phys. Chem. S o l i d s (1965) 26, 2007 10 Banks, Ε., and Wold, Α., "Preparative Inorganic Reactions", 4, 237 I n t e r s c i e n c e P u b l i s h e r s , New York, 1968 11 MacKintosh, A. R., J. Phys. Chem., (1963) 38, 1991 12 Sienko, M. J., "Paper #21 in Non S t o i c h i o m e t r i c Compounds", 39, 224 Adv. Chem., Amer. Chem. Soc., Wash., D. C. 1963 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27

Goodenough, J. B., B u l l Soc. Chim. F r . , (1965) 1200 Goodenough, J. B., S o l i d State Chem., (1971) 3, 490 Heckingbottom, L i n e t t , J. V., Nature (London) (1962) 194, 678 Hagg, G., Z., Physik Chem. (1935) B29 , 192 S l e i g h t , A. W., Inorg. Chem., (1969),8, 1764 P i e r c e , J . W., McKinzie, H. L., V l a s s e , Μ., Wold, Α., J. S o l i d State Chem., (1969) 1, 332 Nygren, Μ., I s r a e l s s o n , Mat. Res. Bull., (1969) 4, 881 Rogers, D. B., Shannon, R. D., S l e i g h t , A. W., G i l l d o n , J . L., Inorg. Chem., (1969) 8, 841 H u l l i n g e r , F., J. Phys. Chem. S o l i d s (1965) 26, 639 B i t h e r , Τ. Α., Bouchard, R. J., Cloud, W. Η., Donohue, P. A. and Siemond, W. J., Inorg. Chem. (1968) 7, 2208 Goodenough, J . B., "Magnetism and the Chemical Bond", I n t e r science P u b l i s h e r s , Inc., New York, (1963) Goodenough, J . B., Proc. Colloque I n t . Orsay 1965 (1967) C.N.R.S. No. 157, 263 Nahigian, H., Steger, J., McKinzie H., A r n o t t , R. J., Wold, Α., to be published Goodenough, J. B., J . S o l i d State Chem. (1971) 3, 26 Steger, J. J., Nahigian,Η., A r n o t t , R. J., Wold, Α., t o be published


Extended Interactions in Transition Metal Oxides and Chalcogenides

Recommend Documents