Extended Interactions between Metal Ions

13 Spectroscopic and Magnetic Properties o f CsMI

3

T y p e Transition Metal Iodides G. L. McPHERSON and L. J. SINDEL

Downloaded by UNIV LAVAL on July 3, 2014 | http://pubs.acs.org Publication Date: June 1, 1974 | doi: 10.1021/bk-1974-0005.ch013

Tulane University, New Orleans,La.70118

As t h e two previous papers demonstrate, there has been a great deal of interest in transition m e t a l salts of t h e general formula M(I)M' (II)X (where M(I) is a large univalent cation, Μ' (II) a divalent transi tion m e t a l ion, and X a halide ion). These materials often crystallize in hexagonal lattices in which t h e most prominent structural feature is a parallel array of infinite, linear c h a i n s of octahedra sharing faces. The c h a i n s r u n parallel t o t h e crystallographic c-axis with the transition metal i o n s at t h e centers and t h e halide i o n s at t h e c o r n e r s of t h e octahedra (see Figure 1). The magnetic properties o f these types of salts a p p r o a c h t h o s e o f a o n e - d i m e n s i o n a l system o f interacting spins. The hexagonal linear chain struc ture is observed with t h e widest variety o f transition metals and halogens when M(I) is a cesium ion. T h i s paper discusses t h e magnetic and spectroscopic proper ties o f several cesium metal triiodides which adopt this structure. A l t h o u g h t h e properties o f t h e s e salts are inherently interesting, it is especially informative to compare t h e iodides t o t h e analogous chlorides and b r o m i d e s . 3

The cesium metal triiodides, CsMgI , CsVI , CsCrI , CsMnI and CsNiI have been shown by X-ray studies to adopt the linear chain structure, Table I contains a summary of the crystallographic data for these salts. Although the space groups are not unambiguously determined it is very likely that a l l of the materials except the chromium salt are isostructural with CsNiCl (space group P6 /mmc). Crystallographic studies of CsCrCl3 and CsCrBr3 suggest that the structures of the chromium salts differ somewhat from that of CsNiCl ; however, the basic linear chain feature is still retained. Un doubtedly there are minor structural variations among 3

3

3

3

3

1,2

3

3

3

3

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

4

13.

MCPHERSON

AND

siNDEL

Metal

183

Iodides

the CsMX s a l t s w h i c h r e s u l t f r o m t h e d i f f e r e n c e s i n the s i z e o f t h e h a l i d e i o n . C e r t a i n l y , a v e r y impor tant s t r u c t u r a l parameter i s the i n t r a c h a i n metalmetal separation. This separation i s equal t o half of the l a t t i c e dimension i n t h e c d i r e c t i o n and i s e x p e c t e d t o be l a r g e s t i n t h e i o d i d e s a l t s . 3

f f

Table

I.


Crystal

Structural

System:

n

Properties

Salts

2n

hhi,

Space Group:

P6 /mmc, P 6 / m c ,

Lattice

3

Hexagonal

Extinctions:

Mol./unit

of CsMI

3

3

o r P^2c

Ζ = 2

cell:

Constants :

CsVI

a

c

8.21

6.81

a

8.12

6.85

b

8.18

6.95

a

8.00

6.76

a

8.20

7.01

a 3

CsCrI

3

CsMnI

3

CsNiI

3

CsMgI

3

^Reference "Reference

ίέϊ

In a d d i t i o n t o the f a i r l y subtle s t r u c t u r a l v a r i a t i o n s , s i g n i f i c a n t changes i n t h e n a t u r e o f t h e m e t a l - h a l o g e n bond would be e x p e c t e d i n g o i n g f r o m a c h l o r i d e t o a bromide and f i n a l l y t o a n i o d i d e lattice. These bonding d i f f e r e n c e s a r e d r a m a t i c a l l y demonstrat ed b y e l e c t r o n s p i n r e s o n a n c e m e a s u r e m e n t s . The e p r spectra of V , Μη , and N i doped i n t o t h e i s o s t r u c t u r a l magnesium s a l t s , C s M g C l , C s M g B r , and C s M g I , have b e e n s t u d i e d . The g- and m e t a l h y p e r fine tensors indicate a considerable v a r i a t i o n i nthe metal-halogen bonding i n the three l a t t i c e s . Table I I g i v e s a summary o f h y p e r f i n e c o n s t a n t s and g - v a l u e s for the three l a t t i c e s . 2 +

2

+

3

9

3

6

3

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

EXTENDED INTERACTIONS

Jg4

Table

II.

H y p e r f i n e C o n s t a n t s * and

CsMgCl

V (77°K) g

obs Scale

Mn (77°K) 2 +

fobs

A

Ni (77°K)

4

g-Values "

CsMgBr

3

2 +

CsMgI

3

1.975 1.957 75.

1.994 1.950 70.

2.056 1.942 65.

2.002

2.004 77.

2 .008 75.

2.25

2.25 2.40

2.16

80.


2 +

Sobs Scale

BETWEEN M E T A L IONS

2.58

3

2.40

*The h y p e r f i n e c o n s t a n t s r e p r e s e n t t h e a v e r a g e o f t h e p a r a l l e l and p e r p e n d i c u l a r components and a r e i n u n i t s of 10" cm" . (Data t a k e n from (6)) 4

1

+

T h e gobs v a l u e s r e p r e s e n t t h e a v e r a g e o f t h e p a r a l l e l and p e r p e n d i c u l a r components o f o b s e r v e d g - t e n s o r . The g a x values f o r the d and d systems are c a l c u l a t e d from simple c r y s t a l f i e l d theory. (Data t a k e n from (6)) 3

C

8

c

One n o t i c e s t h a t t h e r e a r e c o n s i d e r a b l e d i s c r e p a n c i e s between the observed g-values o f V and N i * and t h o s e c a l c u l a t e d f r o m t h e f o l l o w i n g s i m p l e crystal field expression. 2

2

(g = 2 . 0 0 2 3 - § λ ) F u r t h e r m o r e , t h e d i s a g r e e m e n t b e c o m e s more p r o n o u n c e d i n going from c h l o r i d e t o bromide t o i o d i d e . Presum a b l y t h e d i s a g r e e m e n t between t h e o b s e r v e d and c a l c u l a t e d values a r i s e s from a l i g a n d c o n t r i b u t i o n t o the g-value. The l i g a n d c o n t r i b u t i o n i n c r e a s e s a s t h e s p i n o r b i t c o n s t a n t o f t h e l i g a n d i n c r e a s e s and a l s o as the d e r e a l i z a t i o n o f the u n p a i r e d e l e c t r o n s from the metal to the ligands i n c r e a s e s . " I n view o f the observed g-values, i t appears that the metalh a l o g e n b o n d i n g b e c o m e s more c o v a l e n t p r o c e e d i n g through the s e r i e s from c h l o r i d e t o i o d i d e . The Mn and V hyperfine constants support t h i s conclusion, s i n c e t h e c o n s t a n t s show a s t e a d y d e c r e a s e i n g o i n g from c h l o r i d e t o i o d i d e . A decrease i n the metal 7

1 0

5 5

5 1


13.

MCPHERSON

AND

siNDEL

Metal

185

Iodides

h y p e r f i n e constant suggests an i n c r e a s e i n the metal to ligand delocalization. (Reference ( 6 ) g i v e s a more thorough d i s c u s s i o n of the epr parameters.) Although the trend i n the nature of the metal-halogen bonding i n the CsMX s e r i e s i s p e r h a p s i n t u i t i v e l y obvious, the epr s t u d i e s p r o v i d e a very s a t i s f y i n g experimental verification. P l o t s of the r e c i p r o c a l of the molar s u s c e p t i b i l i t y versus the a b s o l u t e temperature f o r C s N i I , CsMnI , and C s C r I a r e shown i n F i g u r e s 2, 3 , a n d 4, respec tively. Both CsNiI and C s M n I o b e y t h e C u r i e - W e i s s l a w a b o v e 190°K, b u t show s i g n i f i c a n t d e v i a t i o n a t 77°K. The l o w t e m p e r a t u r e d e v i a t i o n s a n d t h e l a r g e negative Weiss constants i n d i c a t e that these s a l t s are antiferromagnetic. The c h r o m i u m s a l t , C s C r I , o b e y s t h e C u r i e - W e i s s l a w t h r o u g h o u t t h e 77° t o 300°K r e g i o n . T h i s m a t e r i a l , h o w e v e r , has a l a r g e n e g a t i v e W e i s s constant which indicates that i t i s a l s o antiferromag netic. The m a g n e t i c p r o p e r t i e s o f C s V I d i f f e r f r o m those of the three p r e v i o u s l y mentioned s a l t s . The v a n a d i u m s a l t has a s m a l l p a r a m a g n e t i c s u s c e p t i b i l i t y ( 2 . 3 χ 10" esu/mole) which i s e s s e n t i a l l y independent of temperature. This o b s e r v a t i o n suggests that the antiferromagnetic i n t e r a c t i o n s i n this material are s i g n i f i c a n t l y s t r o n g e r than those of the other s a l t s . These i n t e r a c t i o n s a r e e f f e c t i v e e v e n a t room tempera ture. A l t h o u g h t h e s e s u s c e p t i b i l i t y s t u d i e s do n o t completely c h a r a c t e r i z e the magnetic behavior of the i o d i d e s , t h e r e i s l i t t l e doubt about the antiferromag n e t i c nature of these m a t e r i a l s . The m a g n e t i c s u s c e p t i b i l i t i e s o f a number o f t h e a n a l o g o u s b r o m i d e s and c h l o r i d e s have a l s o b e e n studied. D a t a has b e e n r e p o r t e d f o r C s V C l s , CsCrCl , CsMnBr , CsNiBr , and CsNiCl . ~ A f u n d a m e n t a l q u e s t i o n t o be c o n s i d e r e d when d i s c u s s i n g the m a g n e t i c p r o p e r t i e s o f t h e CsMX s a l t s i s whether the magnetic exchange i n t e r a c t i o n s are d i r e c t (through space) or i n d i r e c t (through l i g a n d ) . In p r i n c i p l e b o t h mechanisms a r e p o s s i b l e , s i n c e t h e metal-metal separations w i t h i n a chain are f a i r l y s h o r t (~3Â) and e a c h m e t a l i o n s h a r e s t h r e e h a l i d e ligands w i t h the neighboring metal ions i n the chain. The c o m p a r i s o n o f t h e s u s c e p t i b i l i t y d a t a f o r t h e C s M X s a l t s s h o w n i n T a b l e I I I g i v e s some q u a l i t a t i v e insight into this question. 3

3

3

3


3

3

3

3

3

1 1

1 2

3

1 3

3

1 4

1 4

3

3

3

3


1 6

INTERACTIONS

BETWEEN

METAL


EXTENDED


IONS

MCPHERSON

AND

siNDEL

Metal

187

Iodides

100


50

-100°

0°

100° 200° 300° T(°K)

Figure 3. Reciprocal molar susceptibility of CsMnlg vs. absolute temperature. Curie-Weiss constants for the linear re gion: C = 4.97; θ = -165°.

Figure 4. Reciprocal molar suscepti bility of CsCrI vs. absolute temperature. Curie-Weiss constants: C = 3.08; θ = -163°. s

T(°K)


188

EXTENDED INTERACTIONS BETWEEN METAL IONS

Table I I I .

M a g n e t i c P r o p e r t i e s o f CsMX Cs7X χ,(297°Κ) 3

M-M D i s t a n c e

(Α)

Cl

3.01

1370

I

3.40

2220

Salts

3

Χ(77 Κ) ό

a

a

1440 2360

CsCrX (A.) X(297°K) 4500° 3

Cl Br Downloaded by UNIV LAVAL on July 3, 2014 | http://pubs.acs.org Publication Date: June 1, 1974 | doi: 10.1021/bk-1974-0005.ch013

I

M-M D i s t a n c e 3.11 3.25 3.42

12800

CsMnI (Α) X(297°K) o

3.26 3.47

3

11000 10750

2.98 3.12 3.38

X(77°K) 18500 17200

e

CsNils (A) X(297°K)

M-M D i s t a n c e Cl Br I

D

67ΟΟ

M-M D i s t a n c e Br ι

X(77°K) 5300

3800 3650 2940

e

X(77°K) 9200^ 8l50 4350

α

d

d

X v a l u e s a r e i n u n i t s o f 10"" esu/mole ^Reference i l l ) R e f e r e n c e ΓΤ2) Reference Π 3 ) R e f e rence ( T i ) 6

D

c

d

D i r e c t exchange i s a f u n c t i o n o f t h e d i s t a n c e between i n t e r a c t i n g i o n s and would be expected t o d i m i n i s h as t h e m e t a l - m e t a l s e p a r a t i o n i n c r e a s e s . On the o t h e r hand, i n d i r e c t exchange depends more on t h e covalency of the metal-ligand-metal l i n k a g e . Since t h e m e t a l - m e t a l s e p a r a t i o n s i n t h e CsMX s e r i e s a r e d i r e c t l y dependent on t h e s i z e o f t h e h a l i d e i o n , t h e s t r e n g t h o f d i r e c t e f f e c t s would be e x p e c t e d t o f o l l o w the o r d e r : Cl>Br>I. I n c o n t r a s t , t h e i n d i r e c t e f f e c t s would be e x p e c t e d t o e x h i b i t t h e o p p o s i t e o r d e r . The data f o r the cesium n i c k e l t r i h a l i d e s i n d i c a t e t h a t the s t r e n g t h o f the a n t i f e r r o m a g n e t i c i n t e r a c t i o n s i s g r e a t e s t f o r C s N i I and s m a l l e s t f o r C s N i C l . This o b s e r v a t i o n s u g g e s t s t h a t i n d i r e c t exchange i s p r e dominant i n t h e s e s a l t s . T h i s c o n c l u s i o n i s q u i t e reasonable i n l i g h t o f simple c r y s t a l f i e l d theory. 3

3

3


13.

MCPHERSON

AND

Metal

SINDEL

189

Iodides

8

The u n p a i r e d e l e c t r o n s o f a d system i n an o c t a h e d r a l complex o c c u p y t h e e~ s e t o f o r b i t a l s w h i c h a r e d i r e c t ed t o w a r d t h e l i g a n d s . For a d s y s t e m s u c h as Mn d i r e c t as w e l l as i n d i r e c t i n t e r a c t i o n s m i g h t be exp e c t e d s i n c e t h e u n p a i r e d e l e c t r o n s o c c u p y t h e t 2 g and e orbitals. The d a t a i n d i c a t e t h a t t h e c o u p l i n g i n CsMnBr i s perhaps a l i t t l e stronger than i n CsMnI , b u t t h e s u s c e p t i b i l i t i e s o f t h e two s a l t s a r e v e r y similar. I t appears that there are considerably stronger interactions i n CsCrCl than i n C s C r I . It i s p o s s i b l e t h a t d i r e c t exchange i s dominant i n a d system s i n c e the m a j o r i t y of the unpaired e l e c t r o n s o c c u p y o r b i t a l s ( t g ) w h i c h a r e d i r e c t e d away f r o m t h e ligands. We h e s i t a t e t o s p e c u l a t e o n t h e C s V X salts s i n c e t h e s u s c e p t i b i l i t i e s a r e r a t h e r s m a l l and ess e n t i a l l y independent of temperature. These s m a l l s u s c e p t i b i l i t i e s may r e s u l t f r o m a t e m p e r a t u r e i n d e p e n d e n t p a r a m a g n e t i s m w h i c h has n o t h i n g t o do w i t h t h e normal paramagnetism a s s o c i a t e d w i t h the unpaired e l e c t r o n s o f t h e V"*" i o n . Clearly, rather strong antiferromagnetic i n t e r a c t i o n s are p r e s e n t i n these vanadium s a l t s . One v e r y i m p o r t a n t p o i n t h a s b e e n n e g l e c t e d i n the q u a l i t a t i v e d i s c u s s i o n of the magnetic p r o p e r t i e s o f t h e CsMX s a l t s . I t has b e e n f i r m l y e s t a b l i s h e d t h a t t h e i n t e r a c t i o n b e t w e e n two p a r a m a g n e t i c i o n s i s c r i t i c a l l y d e p e n d e n t on t h e m e t a l - l i g a n d - m e t a l angle. W h i l e t h e s t r u c t u r e s o f t h e s a l t s t h a t have b e e n d i s cussed are a l l s i m i l a r , t h i s c r i t i c a l angle undoubtedl y v a r i e s t o some e x t e n t f r o m l a t t i c e t o l a t t i c e . Unfortunately, s u f f i c i e n t precise c r y s t a l l o g r a p h i c data are not p r e s e n t l y a v a i l a b l e t o d i s c u s s t h i s important point. The e l e c t r o n i c s p e c t r a o f C s V I , C s C r I , C s M n I , and C s N i I a r e shown i n F i g u r e s 5 and 6. In general, t h e s p e c t r a show t h e l i g a n d f i e l d t r a n s i t i o n s t h a t w o u l d be e x p e c t e d f r o m o c t a h e d r a l c o m p l e x e s o f t h e s e t r a n s i t i o n metal ions. The s p e c t r u m o f C s C r I shows an i n t e n s e a b s o r p t i o n edge a t a p p r o x i m a t e l y 10,000 cm" . The m a t e r i a l a b s o r b s s t r o n g l y t h r o u g h o u t t h e visible region. T h i s i n t e n s e a b s o r p t i o n may be due t o charge-transfer transitions. Charge-transfer absorpt i o n would be e x p e c t e d t o a p p e a r a t l o w e r e n e r g i e s i n these i o d i d e s than i n s i m i l a r c h l o r i d e or bromide complexes. The s h o u l d e r o n t h e a b s o r p t i o n e d g e o f t h e CsCrI s p e c t r u m has b e e n t e n t a t i v e l y a s s i g n e d t o t h e s p i n a l l o w e d , E-+ T , l i g a n d f i e l d transition. S i m i l a r l y , the C s N i I s p e c t r u m has a n i n t e n s e a b s o r p t i o n edge w h i c h a p p e a r s a t a p p r o x i m a t e l y 13000 cm" and p r e s u m a b l y r e s u l t s f r o m c h a r g e - t r a n s f e r t r a n s 5

2

g

3

3

3

3

4

2


3

2

3

3

3

3

3

3

1

3

5

5

2

3

1


EXTENDED

INTERACTIONS

BETWEEN

METAL


190


IONS

13.

MCPHERSON AND

Metal

siNDEL

191

Iodides

itions. The s p e c t r a o f C s V T a n d C s M n I do n o t s e e m t o be p a r t i c u l a r l y u n u s u a l . T a b l e I V g i v e s t h e band a s s i g n m e n t s f o r t h e i o d i d e s a l t s b a s e d on a n o c t a h e dral ligand f i e l d . 3

Table

IV.

Spectroscopic CsVI

Assignment 4A

4 4

T

Assignments

3

CsCrI Energy 3

1

Energy

(cm"" )

7700 12000

2

2

-

3

Τχ(Ρ)

Assignment

1

(cm"" )

9000 (sh)

5E -* [ 5 T ] 2


13000(sh) 4

-

15300 18700

Ti(P)

CsMnI Assignment E n e r g y (cm" ) Αχ Ti(G) I78OO 3

1

6

4

-* « T (G) 2

4e,4

—

4

T

2

A 3 L

(g)

(d)

~» 4E (D)

Assignment 3

A

3 2

CsNils Energy

6500 8000

T [IE] 2

20900 22100 255ΟΟ

3

1

(cm" )

10900

T (F) X

26400

28600

4 T i (P)

s h = shoulder Brackets designate

assignments which are

uncertain.

3

4

Spectroscopic studies of C s C r C l , C s C r B r , CsMnBr CsNiCl and C s N i B r ' have b e e n r e p o r t e d . A com p a r i s o n o f t h e Dq v a l u e s o f t h e CsMX s a l t s i s p r e s e n t e d i n T a b l e V. 3

1 7

3

3

1 3

3

1 8

3

3

Table

V.

Dq

CsVX

V a l u e s f o r the CsCrX

3

1000

CI Br

—

I

770

114 5

1150 900

b

3

CsMX CsMnX

3

3

Salts CsNiX

3

695*

a

680 605

c

655 650

d

Reference Reference ^Reference ^Reference

(13 (Tf

The t r e n d s i n t h e Dq v a l u e s a p p e a r t o f o l l o w t h a t w h i c h would be p r e d i c t e d b y t h e s p e c t r o c h e m i c a l series. I t s h o u l d be m e n t i o n e d t h a t t h e Dq v a l u e f o r C s M n I was d e r i v e d f o l l o w i n g t h e p r o c e d u r e p r e s e n t e d 3


,

192

EXTENDED

INTERACTIONS

BETWEEN METAL

IONS

in

reference (13). The e l e c t r i c a l r e s i s t i v i t i e s o f s i n g l e c r y s t a l s o f C s N i I have b e e n s t u d i e d a s a f u n c t i o n o f tempera ture. The m a t e r i a l a p p e a r s t o be a s e m i c o n d u c t o r w i t h a room t e m p e r a t u r e r e s i s t i v i t y o f 1 0 t o 10 ohm cm a n d a n e n e r g y o f a c t i v a t i o n o f a p p r o x i m a t e l y 0 . 7 ev. For a m a t e r i a l t h a t i s an i n t r i n s i c semiconductor the band gap ( t h e e n e r g y s e p a r a t i n g t h e v a l e n c e and c o n d u c t i o n bands) should be e q u a l t o twice t h e energy of a c t i v a t i o n o f c o n d u c t i o n . The i n t e n s e a b s o r p t i o n edge a p p e a r s i n t h e s p e c t r u m o f C s N i I a t a p p r o x i m a t e l y 1.5 e v w h i c h i s a b o u t t w i c e t h e o b s e r v e d e n e r g y o f activation. T h i s s u g g e s t s t h a t a t room t e m p e r a t u r e the m a t e r i a l i s a n i n t r i n s i c s e m i c o n d u c t o r . In spite of the l i n e a r c h a i n s t r u c t u r e , the r e s i s t i v i t y o f the material i s essentially isotropic. I n c o n c l u s i o n , we h o p e t h a t we h a v e shown t h a t the c e s i u m m e t a l t r i i o d i d e s have r a t h e r i n t e r e s t i n g s o l i d s t a t e p r o p e r t i e s a n d t h a t t h e s e compounds w i l l be u s e f u l f o r f u r t h e r s t u d i e s i n t o t h e n a t u r e o f t h e linear chain M(I)M'(II)X compounds. 3

1 9

7

8


3

3

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