Inorganic Compounds with Unusual Properties

5 Hg2.86

AsF —A Novel Structure with 6

Unusual Electrical Properties

Downloaded by UNIV OF BATH on July 3, 2016 | http://pubs.acs.org Publication Date: June 1, 1976 | doi: 10.1021/ba-1976-0150.ch005

B. D. CUTFORTH, W. R. DATARS, R. J. GILLESPIE, and A. VAN SCHYNDEL Departments of Chemistry and Physics, McMaster University, Hamilton, Ontario, Canada

The structure of Hg AsF can be described as octahedral AsF6- ions arranged on a lattice that contains linear non -intersecting channels in two mutually perpendicular directions. Within these channels are infinite chains of mercury atoms, each with a fractional formal charge of +0.35 and a mercury-mercury distance of 2.64 (1) A. Conductivity experiments confirm the metallic nature of the compound. The resistivity, measured by a four-probe ac technique, decreases by a factor of 10 between room temperature and 4.2°K. No metal-insulator transition was detected. 2.86

6

3

ecent studies o n c o m p o u n d s c o n t a i n i n g m e r c u r y i n o x i d a t i o n states lower than + 1 demonstrated that H g ( A s F ) 3

6

2

(1)

and H g ( A s F ) 4

e

2

c a n b e p r e p a r e d b y t r e a t i n g m e r c u r y w i t h a s t o i c h i o m e t r i c a m o u n t of

(2) AsF

in liquid S 0 .

5

2

polycations.

B o t h c o m p o u n d s c o n t a i n l i n e a r , discrete, m e r c u r y

H g ( A l C l ) has also b e e n p r e p a r e d ( 3 ) b y the r e a c t i o n of 3

4

2

mercury w i t h a molten H g C l - A l C l 2

d e t e r m i n a t i o n (4) linear H g 3

3

mixture, a n d subsequent structural

r e v e a l e d that the c o m p o u n d c o n t a i n e d a v e r y n e a r l y

cation.

2 +

T h e i n i t i a l p r o d u c t o b t a i n e d i n t h e r e a c t i o n of m e r c u r y w i t h A s F

5

w a s a c r y s t a l l i n e s o l i d w i t h a d i s t i n c t g o l d e n m e t a l l i c lustre. X - r a y c r y s t a l lography (5) Hg

2

8 6

AsF , 6

d e m o n s t r a t e d that the c o m p o u n d c o u l d b e f o r m u l a t e d as

a n d i t r e v e a l e d t h e presence of

infinite l i n e a r c h a i n s

of

m e r c u r y atoms i n t w o m u t u a l l y p e r p e n d i c u l a r d i r e c t i o n s . It w a s r e a l i z e d t h a t the e l e c t r i c a l p r o p e r t i e s of s u c h a c o m p o u n d w o u l d b e of c o n s i d erable interest b e c a u s e the c o m p o u n d

might be

a highly anisotropic

metallic conductor. 56 King; Inorganic Compounds with Unusual Properties Advances in Chemistry; American Chemical Society: Washington, DC, 1976.

5.

CUTFORTH

E T

57

Hg 6^ P(

AL.

s

2t8

I n recent years, the s t u d y of o n e - d i m e n s i o n a l m e t a l l i c systems has a t t r a c t e d a great d e a l of interest. A n u m b e r of excellent r e v i e w s h a v e b e e n p u b l i s h e d (6, 7, 8, 9, 10)

w h i c h were mostly concerned w i t h two

classes of o n e - d i m e n s i o n a l c o m p o u n d s : t h e p a r t i a l l y o x i d i z e d t e t r a c y a n o p l a t i n a t e complexes s u c h as K [ P t ( C N ) ] B r 3 o * 3 H 0 , a n d t h e c h a r g e 2

4

0

2

transfer o r g a n i c complexes i n v o l v i n g t e t r a c y a n o q u i n o d i m e t h a n e

(TCNQ)

of w h i c h the best e x a m p l e to d a t e is t e t r a t h i o f u l v a l i n i u m t e t r a c y a n o q u i n o d i m e t h a n e [ ( T T F ) ( T C N Q ) ] . S t r u c t u r a l studies of the p a r t i a l l y o x i d i z e d t e r a c y a n o p l a t i n a t e complexes

r e v e a l e d t h a t the p l a t i n u m - p l a t i n u m

(11)

d i s t a n c e i n the b r o m i d e is a n u n u s u a l l y short 2.88 A . I n s u c h a p a r t i a l l y o x i d i z e d c o m p o u n d , electrons h a v e b e e n r e m o v e d f r o m the h i g h e s t o c c u Downloaded by UNIV OF BATH on July 3, 2016 | http://pubs.acs.org Publication Date: June 1, 1976 | doi: 10.1021/ba-1976-0150.ch005

p i e d b a n d a r i s i n g f r o m o v e r l a p of the p l a t i n u m (d^)

o r b i t a l s , thus g i v i n g

a p a r t i a l l y filled o n e - d i m e n s i o n a l c o n d u c t i o n b a n d . T h e a n i s o t r o p i c m e t a l l i c n a t u r e of this c o m p o u n d studies (12).

w a s c o n f i r m e d b y extensive

M e a s u r e m e n t s of ( T T F ) ( T C N Q )

(13,

14)

conductivity also

demon-

strated that the c o n d u c t i v i t y is m e t a l l i c at r o o m t e m p e r a t u r e w i t h a l a r g e a n i s o t r o p y r a t i o . W e demonstrate i n this p a p e r that H g . 8 6 A s F is h i g h l y 2

6

c o n d u c t i n g a l t h o u g h the o n e - d i m e n s i o n a l n a t u r e of the c o n d u c t i v i t y has not yet b e e n c o n f i r m e d .

Experimental T h e c o m p o u n d was p r e p a r e d b y a m e t h o d w h i c h d i f f e r e d f r o m t h a t d e s c r i b e d p r e v i o u s l y ( 5 ) . B e c a u s e of the m e t a l l i c n a t u r e of t h e c o m p o u n d a n d its resultant i n s o l u b i l i t y i n a v a i l a b l e solvents, crystals s u i t a b l e for c o n d u c t i v i t y studies w e r e p r e p a r e d b y l o w t e m p e r a t u r e d i s p r o p o r t i o n a t i o n of a n S 0 s o l u t i o n of the H g ( A s F ) c a t i o n . A s the t e m p e r a t u r e w a s l o w e r e d f r o m r o o m t e m p e r a t u r e to — 3 0 ° C over a f e w days, the r e d s o l u t i o n of H g ( A s F ) d i s p r o p o r t i o n a t e d to y i e l d s o l u b l e H g ( A s F ) a n d Hgo.scAsFc w h i c h d e p o s i t e d as l a r g e ( 1 m m ) crystals o n the s i d e of the b o r o s i l i c a t e glass tube. T h e crystals w e r e w a s h e d w i t h S 0 to r e m o v e traces of H g ( A s F ) a n d H g ( A s F ) , a n d t h e y w e r e t h e n stored u n d e r v a c u u m i n a sealed b o r o s i l i c a t e glass t u b e . 2

4

4

6

e

2

2

3

6

2

3

2

3

0

2

4

G

2

Resistance was m e a s u r e d w i t h f o u r p l a t i n u m w i r e p r o b e s i n s p r i n g contact w i t h the s a m p l e . A l t e r n a t i n g c u r r e n t w i t h a f r e q u e n c y of a p p r o x i m a t e l y 25 H z was passed t h r o u g h the s a m p l e b e t w e e n t w o c u r r e n t probes. A phase-sensitive detector was u s e d to a m p l i f y the v o l t a g e b e t w e e n the t w o p o t e n t i a l p r o b e s a n d to o b t a i n the i n - p h a s e c o m p o n e n t of the s i g n a l . T h e s a m p l e was m o u n t e d i n a h o l d e r w h i c h w a s sealed b y a r u b b e r O - r i n g onto a b o r o s i l i c a t e glass tube. ( T h e s e m a n i p u l a t i o n s w e r e p e r f o r m e d i n a v e r y t i g h t d r y box b e c a u s e of the extremely h y g r o s c o p i c n a t u r e of the c o m p o u n d . ) A H contacts w e r e c h e c k e d before r e m o v i n g t h e sealed s a m p l e f r o m the d r y box. T h e s a m p l e h o l d e r was t r a n s f e r r e d to a s t a i n less-steel h e l i u m d e w a r for resistance measurements. T h e t e m p e r a t u r e w a s m o n i t o r d b y a c o p p e r - c o n s t a n t a n t h e r m o c o u p l e m o u n t e d next to the s a m p l e . B i s m u t h a n d a n t i m o n y samples w e r e also m e a s u r e d to d e t e r m i n e the r e s i s t i v i t y for o u r contact c o n f i g u r a t i o n f r o m t h e v o l t a g e - c u r r e n t

King; Inorganic Compounds with Unusual Properties Advances in Chemistry; American Chemical Society: Washington, DC, 1976.

58

INORGANIC

COMPOUNDS

WITH

UNUSUAL PROPERTIES

r e l a t i o n s h i p . T h e error i n the absolute v a l u e of the r e s i s t i v i t y w a s ~ 5 0 % because of u n c e r t a i n t y i n t h e p r o b e p o s i t i o n a n d i n the effective s a m p l e thickness. Results and

Discussion

T h e s t r u c t u r e of H g

2 8 C

AsF

( F i g u r e 1)

e

is a n a r r a y of

octahedral

A s F " anions c o n f o r m i n g to the space g r o u p s y m m e t r y ( t e t r a g o n a l , J 4 i / e

amd).

T h e r e is a n a r r a y of n o n - i n t e r s e c t i n g channels w i t h i n the l a t t i c e

r u n n i n g a l o n g directions a a n d b.

Infinite chains of m e r c u r y atoms l i e

i n these channels w i t h a m e r c u r y - m e r c u r y distance of 2.64(1) A , a d i s tance

not c o m m e n s u r a t e

w i t h the l a t t i c e d i m e n s i o n s .

The

mercury-


m e r c u r y b o n d l e n g t h is c o n s i d e r a b l y shorter t h a n t h a t f o u n d i n m e t a l l i c m e r c u r y ( 3.005 A ). T h e chains are electron deficient ( e a c h m e r c u r y m a y b e assigned a f o r m a l p o s i t i v e c h a r g e of + 0 . 3 5 ) .

T h e s e facts suggest a

m e t a l l i c state. It seems reasonable to s u p p o s e that the crystals m a y d i s p l a y a n i s o t r o p i c c o n d u c t i v i t y , i.e. t h e y m i g h t b e h i g h l y c o n d u c t i n g a l o n g the a a n d b axes a n d less so a l o n g the c axis.

Figure 1. An isometric view of Hg AsF showing the chains of mercury atoms (circles) running through the lattice composed of AsFf ions (octahedra) 286

6

ο

R e s i s t i v i t y as a f u n c t i o n of t e m p e r a t u r e is p l o t t e d i n F i g u r e 2 for a typical sample.

M e a s u r e m e n t s w e r e m a d e w i t h o u t r e g a r d for c r y s t a l

o r i e n t a t i o n because i t was not possible to o b t a i n crystals w i t h sufficiently w e l l defined m o r p h o l o g y to a l l o w o r i e n t a t i o n of the crystals a l o n g the p r i n c i p a l axes.

The compound

is h i g h l y c o n d u c t i n g , the r e s i s t i v i t y at

r o o m t e m p e r a t u r e b e i n g of the order of 130 μ,Ωαη. T h i s corresponds to a c o n d u c t i v i t y of 8 Χ ΙΟ

3

( Ω α η ) " , a v a l u e t h a t is c o n s i d e r a b l y larger t h a n 1

that f o u n d for the p a r t i a l l y o x i d i z e d K [ P t ( C N ) ] B r o 3 o * 3 H 0 w h i c h is 2

t y p i c a l l y about a f e w h u n d r e d ( Ω α η ) " (12).

1

4

2

a l o n g t h e h i g h l y c o n d u c t i n g axis

I t is e v e n m o r e e x t r a o r d i n a r y t h a t the r e s i s t i v i t y decreases

r a p i d l y w i t h d e c r e a s i n g t e m p e r a t u r e . T h e r e s i s t i v i t y at 4.2° Κ w a s μΩυτη w h i c h w a s the l o w e r l i m i t of d e t e c t i o n .


so

~0.1

5.

CUTFORTH

ET

Hg

AL.

2

59

AsF

g6

e

I

I

I

I

I

I

I

I

I

I

I

I

I20h

ιool· ε o è


b ^

8

0

•

60-

40h

20 [

. *

I

100

I

I

I

300

200 T(°K)

Figure 2.

The resistivity of Hg AsF tion of temperature 2

86

6

as a func-

L o w t e m p e r a t u r e ( < 7 7 ° K ) d a t a represent the average for t w o d i f ferent samples; t h e y are p l o t t e d i n F i g u r e 3 together w i t h t h e m e a s u r e d t e m p e r a t u r e - d e p e n d e n t r e s i s t i v i t y of a b i s m u t h s a m p l e u s e d f o r c a l i b r a t i o n . T h e decrease i n r e s i s t i v i t y b y a f a c t o r of almost 1 0 over t h e t e m 3

p e r a t u r e r a n g e s t u d i e d is c o n s i d e r a b l y l a r g e r t h a n t h a t o b s e r v e d i n metals that are not h i g h l y p u r i f i e d , a n d i t is consistent w i t h a m e t a l l i c state a r i s i n g f r o m t h e i n t e r a c t i o n of m e r c u r y atoms i n a c h a i n f o r m i n g a p a r t i a l l y filled b a n d . T h e increase i n c o n d u c t i v i t y w i t h d e c r e a s i n g t e m p e r a t u r e n o r m a l i z e d to the c o n d u c t i v i t y at r o o m t e m p e r a t u r e is p l o t t e d i n F i g u r e 4. P e i e r l s ( 1 5 ) s h o w e d that a p a r t i a l l y filled o n e - d i m e n s i o n a l b a n d c a n a l w a y s l o w e r its energy b y s p l i t t i n g i n t o filled a n d e m p t y b a n d s .

I f the

c o m p o u n d u n d e r s t u d y is i n fact o n e - d i m e n s i o n a l , the f a i l u r e to observe the t r a n s i t i o n is s o m e w h a t p u z z l i n g . H g . 8 6 A s F seems to f u l f i l l t h e neces2

6

sary c o n d i t i o n s f o r a o n e - d i m e n s i o n a l m e t a l l i c system. T h e s e c o n d i t i o n s


60

INORGANIC

COMPOUNDS

m a y b e s u m m a r i z e d b r i e f l y as f o l l o w s : l i n e a r p a r a l l e l r o w s of atoms; ( b )

(a)

WITH

UNUSUAL PROPERTIES

the s t r u c t u r e m u s t c o n t a i n

the i n t e r a c t i o n b e t w e e n these

atoms

m u s t b e sufficiently s t r o n g to m a k e b a n d f o r m a t i o n p o s s i b l e ; a n d ( c )

the

atoms c o m p o s i n g the c h a i n m u s t h a v e a n o d d or f r a c t i o n a l f o r m a l v a l e n c y , w h i c h w o u l d l e a d to a p a r t i a l l y filled c o n d u c t i o n b a n d . It is not y e t clear to w h a t extent t h e r a t h e r short i n t e r c h a i n s e p a r a t i o n of 3.085 A affects c o n d u c t i o n

a l o n g the c axis.

It m a y w e l l be

that

a l t h o u g h this d i s t a n c e is c o n s i d e r a b l y longer t h a n the i n t r a c h a i n m e r c u r y m e r c u r y distance of 2.64 A , is is s t i l l short e n o u g h to p r o v i d e a c o n d u c t i o n p a t h b e t w e e n the chains a l o n g the c axis. I f this is t r u e , i t m a y m e a n that, a l t h o u g h t h e c o n d u c t i v i t y is a n i s o t r o p i c , the a n i s o t r o p y m a y b e too


s m a l l for detection. T h e fact t h a t the c o n d u c t i v i t y w a s consistently h i g h f o r a l a r g e n u m b e r of samples m o u n t e d r a n d o m l y w i t h respect to o r i e n t a t i o n does not necessarily m e a n that t h e c o n d u c t i v i t y is n o t a n i s o t r o p i c . It c a n b e s h o w n t h a t i f the o r i e n t a t i o n is n o t w i t h i n a f e w degrees of the c r y s t a l axes, t h e n the c o m p o n e n t a l o n g t h e h i g h l y c o n d u c t i n g d i r e c t i o n w i l l d o m i n a t e the c o n d u c t i v i t y .

40 60 T(°K) Figure 3. The low temperature region of the resistivity of Hg 6^ ^6 d bismuth as a function of temperature 2

8

s

an


5.

CUTFORTH

ET

Hg

AL.

1

'

ι

61

AsF

2

86

ι

6

ι

ι

ι

ι

I

I

I

I

I

I

I

I

I

2.5

2.0[


f

en o

1.0

•s

0.5

I

I

I

I

100

I

200 T(°K)

300

Figure 4. The conductivity of Hg AsF as a function of temperature normalized to the room temperature conductivity 2

86

6

I t is h o p e d that i n the near f u t u r e sufficiently w e l l defined crystals w i l l be o b t a i n e d w h i c h w i l l p e r m i t us to investigate t h e p o s s i b l e e l e c t r i c a l a n i s o t r o p y of this m o s t interesting, h i g h l y c o n d u c t i n g m a t e r i a l .

Literature Cited 1. Cutforth, B. D., Davies, C. G., Dean, P. A. W., Gillespie, R. J., Ireland, P. R., Ummat, P. K., Inorg. Chem. (1973) 12, 1343. 2. Cutforth, B. D., Gillespie, R. J., Ireland, P. R., J. Chem. Soc. Chem. Commun. (1973) 723. 3. Torsi, G., Fung, K. W., Begun, G. H., Mamantov, G., Inorg. Chem. (1971) 10, 2285. 4. Ellison, R. D., Levy, Η. Α., Fung, K. W., Inorg. Chem. (1972) 11, 283. 5. Brown, I. D., Cutforth, B. D., Davies, C. G., Gillespie, R. J., Ireland, P. R., Vekris, J. E„ Can. J. Chem. (1974) 52, 791. 6. Krogmann, K., Angew. Chem. Int. Ed. Engl. (1969) 8, 35. 7. Thomas, T. W., Underhill, A. E., Chem. Soc. Rev. (1972) 1, 99.


62

INORGANIC COMPOUNDS WITH UNUSUAL PROPERTIES

Shchegolev, I. F., Phys. Status Solidi A (1972) 12, 9. Zeller, H. R., Festkoerperprobleme (Adv. Solid State Phys.) (1973) 13, 31. Miller, J. S., Epstein, A. J., Prog. Inorg. Chem. (1975) 20. Krogmann, K., Hansen, H. D., Z. Anorg. Allg. Chem. (1968) 67, 358. Zeller, H. R., Beck, A., J. Phys. Chem. Solids (1974) 35, 77. Ferraris, J., Cowan, D. O., Walatka, V., Jr., Perlstein, J. H., J. Amer. Chem. Soc. (1973) 95, 948. 14. Coleman, L. B., Cohen, M. J., Sandman, D. J., Yamagishi, F. G., Garito, A. F., Heeger, A. J., Solid State Commun. (1973) 12, 1125. 15. Peierls, R. E., "Quantum Theory of Solids," p. 108, Oxford University, London, 1955. RECEIVED January 24, 1975.


8. 9. 10. 11. 12. 13.


Inorganic Compounds with Unusual Properties

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