22 Magnus Green Salt Solid Solutions Containing M i x e d Valence Platinum Chains: An Approach to 1-D Metals B. A. S C O T T , R. MEHRAN, a n d B. D . SILVERMAN IBM T. J. W a t s o n Research Center, Y o r k t o w n Heights, N.Y. 10598
M. A. RATNER N e w Y o r k U n i v e r s i t y , New York, N.Y. 10003
Introduction The c l a s s o f i n o r g a n i c s o l i d s composed o f the partially o x i d i z e d platinum chain s a l t s are o f s p e c i a l i n t e r e s t to participants in t h i s Symposium, as they represent perhaps the most extended type o f i n t e r a c t i o n r e a l i z a b l e in the 1-D systems: that l e a d i n g t o f r e e c a r r i e r s and m e t a l l i c - l i k e c o n d u c t i v i t y . Such systems are t y p i f i e d by the compound K Pt(CN) Br .3H O(KCP). 2
4
0.3
2
In terms o f the known formal o x i d a t i o n s t a t e s o f platinum (+2,+4) the stoichiometry o f KCP can be w r i t t e n K [Pt(II) Pt(IV) (CN) ]Br .3H O. However, it is c l e a r 2
0.85
0.15
4
0.3
2
1
from Professor Krogmann's c r y s t a l structure f o r t h i s compound that all P t - s i t e s are equivalent and separated by the extremely short i n t e r m e t a l l i c d i s t a n c e o f 2.88 Å along the [001] s t a c k i n g a x i s . Thus, strong metal-metal i n t e r a c t i o n i n v o l v i n g the 5d 2 z
2
o r b i t a l s , which are the highest l y i n g occupied states , along the c-axis permits the system t o a t t a i n a f r a c t i o n a l , o r partially o x i d i z e d valence o f +2.3. The x-ray s t r u c t u r e shows that the charge compensating Br ions are l o c a t e d in the i n t e r s t i c e s between the [Pt(CN) ] s t a c k s . The i n t e r s t i c e s a l s o contain s i t e s 4
f o r the alkali ions and water molecules. Table I , taken i n p a r t from recent reviews , shows some of the considerable number o f known partially o x i d i z e d platinum chain s a l t s . All o f these s a l t s are composed o f tetracyano- o r b i s o x a l a t o - p l a t i n u m complexes. In the s t r u c t u r e s of these com3,4
3
pounds
the common f e a t u r e is the presence o f [Pt(CN) ] o r
Pt(Ox)
stacks comprised of c l o s e l y spaced Pt ions with
2
4
t i o n numbers between 2.26 and 2.40.
oxida-
Charge compensation occurs
through the presence o f halide ions in the s t r u c t u r e , or vacanci e s on the a l k a l i - c a t i o n s i t e s . 2.3
For KCP the o x i d a t i o n number o f
corresponds to 1.7 e l e c t r o n s per platinum; i.e., = 5/6 filling
331
332
EXTENDED
INTERACTIONS
BETWEEN
METAL
IONS
of the 5d 2 band. This c o n d i t i o n leads to m e t a l l i c - l i k e ζ e l e c t r i c a l c o n d u c t i v i t y near room temperature. The e l e c t r i c a l p r o p e r t i e s of KCP, as w e l l as the r e s u l t s of a l a r g e number of s o l i d s t a t e measurements, are reviewed by Dr. Z e l l e r i n t h i s Symposium volume and w i l l not be discussed here. Rather, our primary concern w i l l be the phase diagram of KCP and r e l a t e d sys tems shown i n Table I. Why do these compounds occur with s t o i chiometrics corresponding to 1.6-1.74 5d 2 electrons/platinum? z
Is there a s p e c i a l s t a b i l i t y i n the degree of band f i l l i n g a s s o c i ated with these e l e c t r o n concentrations? Answers to these ques t i o n s are important to s o l i d s t a t e chemists, p r o v i d i n g the r u l e s necessary to design high c o n d u c t i v i t y i n t o new 1-D metal chain systems. In d i s c u s s i n g our approach to understanding the stoichiome t r y of the 1-D m e t a l l i c - t y p e p a r t i a l l y o x i d i z e d metal chain com plexes, i t i s u s e f u l to consider the o v e r s i m p l i f i e d "binary" phase diagram of the aqueous K P t ( I I ) ( C N ) ^ - K P t ( I V ) ( C N ) ^ B r s y s tem at room temperature, as shown i n F i g . 1. Here the d i v a l e n t platinum complex i s denoted by II and the t e t r a v a l e n t complex designated IV. KCP occurs at a mole f r a c t i o n , x»0.15 i n t h i s system, corresponding to n ( d 2 ) = l . 7 and the o x i d a t i o n s t a t e 2.3. 2
2
2
z
For χ < 0.15 compound II and KCP c r y s t a l l i z e out of s o l u t i o n , whereas f o r χ > 0.15 KCÇ and IV c r y s t a l l i z e on evaporation of the s o l v e n t . A l l of the known p a r t i a l l y o x i d i z e d platinum s a l t s can be represented on a phase diagram such as F i g . 1. For example, the b i s o x a l a t o s a l t s of Table I can be designated on a phase diagram i n which II - ( c a t i o n ) P t ( I I ) ( O ^ O ^ and 2
IV = " P t ( I V ) ( C 0 ) 2
platinum. The dized compound 0.20. We term system " gi x
m
a
c
M
4
f f 2
,
i . e . , a pseudo-compound of t e t r a v a l e n t
s p e c i f i c composition at which the p a r t i a l l y o x i appears on the diagram ranges between χ = 0.13 to the exact composition of the phase f o r a given > because there appears to be a s p e c i f i c minimum,
or "magic" e l e c t r o n concentration at which the 1-D m e t a l l i c phase i s s t a b l e . The reasons f o r t h i s are not yet c l e a r , but t h e o r i e s 3 have been proposed and others w i l l be discussed i n t h i s as w e l l as subsequent papers i n the Symposium. In the present paper we wish to i n d i c a t e an a l t e r n a t i v e ap proach to the 1-D m e t a l l i c s t a t e which attempts to reach i n a way which we b e l i e v e provides some clues to the s t o i c h i o metry of the p a r t i a l l y o x i d i z e d platinum chain compounds. This approach i s i l l u s t r a t e d i n F i g . 2, d e p i c t i n g a h y p o t h e t i c a l phase diagram between a platinum(II) and a platinum(IV) compound. We r e q u i r e that II be an i n s u l a t i n g phase with a s t a c k i n g of P t ( I I ) ο
ions at a separation < 3.3 A, and f u r t h e r that the s t r u c t u r e of II contain vacant i n t e r s t i t i a l s i t e s f o r the i n t r o d u c t i o n of a
seoir E TAL.
22.
Magnus
Green
Salt
333
TABLE I P a r t i a l l y Oxidized Chain Compounds Pt-Pt Distance
-3H O
+2.32
2.880 A
0.16
1.68
-3H O
+2.30
2.887
0.15
1.70
-7H O
+2.28
2.985
0.14
1.72
[Pt(CN) ]-1.8H 0
+2.26
2.96
0.13
1.74
[Pt(C O ) ].2H O
+2.40
2.80
0.20
1.60
+2.36
2.81
0.18
1.64
+2.38
2.81
0.19
1.62
+2.36
2.82
0.18
1.64
+2.32
2.85
0.16
1.68
K [Pt(CN) ]Cl 2
4
K [Pt(CN) ]Br 2
4
4
K
1 7 4
H
1 6 0
K
2
2
2
2
4
[ P t ( C
2
2
0
)
]
2 4 2 -
6 H
2
0
[Pt(C 0 ) ].xH 0 2
(NH ) 4
Ba
0 e 2 8
2
4
1.64 1 6 2
0 3 2
0 e 3 ( )
Mg[Pt(CN) ]Cl
L i
n(d 2)
O x i d a t i o n Number of Platinum
Complex
0 8 4
1 6 4
4
2
2
[Pt(C 0 ) ].H 0 2
4
2
2
[Pt(C O ) ].4H O 2
4
2
2
z
n ( d 2 ) = number of 5d 2 e l e c t r o n s per platinum z
z
χ • mole f r a c t i o n composition parameter i n pseudo-binary P t ( I I ) - P t ( I V ) system. (See text) KCP«K Pt(CN) Br ·3Η 0 2
4
0 3
2
KCP
π +
KCP +TSL
KCP
0
π
0.15
1.0
0.5
is:
KgPtiCN^
K Pt(CN) Br 2
4
2
Figure 1. Oversimplified "binary" phase diagram between II = K Pt(CN), and IV = K Pt(CN)., Br in water at room temperature 2
t
£
t
2
EXTENDED
334
INTERACTIONS
BETWEEN
METAL
IONS
charge compensating ion i n t o the l a t t i c e . Thus, we wish to create a s o l i d s o l u t i o n system through the i n t r o d u c t i o n of P t ( I V ) com plexes i n t o the stacks"* with concomitant charge compensation i n the i n t e r s t i t i a l s i t e s . By analogy with the inorganic semicon ductor s i l i c o n doped with aluminum, we might expect II to t r a n s form as follows with i n c r e a s i n g χ : I I ( i n s u l a t o r ) -> p-type semi conductor -* degenerate semiconductor - 1-D metal. We contend that the f i n a l transformation w i l l occur ( p o s s i b l y abruptly) at x = gi moment we neglect the i n f l u e n c e of d i s o r d e r , tf
x
e
m
a
F
o
r
ft
t n e
c
the very l i k e l y p o s s i b i l i t y of one or more m i s c i b i l i t y gaps (6 5 0 i n F i g . 2) and other perturbations which may turn o f f the m e t a l l i c conduction (e.g. P e i r e l s d i s t o r t i o n ) , and confine our selves instead to the f i r s t order problem: What are the s o l i d state factors s t a b i l i z i n g χ . ? Note also that we have assumed, ° magic * f o r g e n e r a l i t y , a f i n i t e homogeneity range, γ * 0, f o r the 1-D metal phase i n F i g . 2. F i g . 2 would transform i n t o the s p e c i a l case of F i g . 1 i f γ = 0 and χ . = δ = 0.15. magic In the present study we have chosen II as Magnus green s a l t (MGS), P t ( N H ) P t C l , and IV = K P t ( C N ) Y (Y - CI,Br) and K«PtCl . The t e t r a g o n a l Magnus s a l t s t r u c t u r e i s shown i n F i g . 3. e
1
3
4
4
2
4
2
6
II
I t i s c h a r a c t e r i z e d by the s t a c k i n g of square co-planar cations and P t C l " anions at a 3.24 4
Pt(NH^)
4
A separation along the c r y s -
t a l l o g r a p h i c c - a x i s , and the r e l a t i v e l y long i n t e r c h a i n Pt-Pt ° 6 distance of 6.35 A along the d i r e c t i o n . Previous studies 7-9 suggested that the e l e c t r i c a l and o p t i c a l p r o p e r t i e s of MGS could be described by M i l l e r ' s model i n which f i l l e d 5d 2 metal ζ
bands are separated by £ 0.6 eV from empty 6p bands. Recent band c a l c u l a t i o n s ^ , c o n d u c t i v i t y ' " ^ and i n f r a r e d measurements ^ suggest that the 5d 2-6p gap i s ^ 4.5 eV and that the measured 8 9 ïl 12 —6 —2 —1 —1 conductivities, ' ' ' which range from 10 to 10 Ω -cm , 14 may be impurity dominated. In our EPR studies of MGS we have suggested that the s i g n a l i s due to a s e l f - t r a p p e d 5d 2 hole whose energy l e v e l may l i e ^ 0.6 eV above the top of the 5d 2 z
1
1 1
1
Z
z
z
valence bands. Such states are formally equivalent to those i n troduced by the s u b s t i t u t i o n of P t ( I I I ) f o r P t ( I I ) i n the P t ( I I ) l a t t i c e and are introduced i n t o MGS due to the presence of Pt(IV) complexes during the preparation of the compound. This s i t u a t i o n i s depicted i n the band model of F i g . 4, which may be con s i d e r e d our s t a r t i n g point and equivalent to a s o l i d s o l u t i o n 14 -4 with a very small value of χ (-10 ) i n the phase diagram of F i g . 2. In the present work we have found i t p o s s i b l e to i n crease χ to f a i r l y l a r g e values and report the e f f e c t of such
22.
SCOTT
ET
Magnus
AL.
Green
Salt
335
β /-ID METAL
Br
—
ID
METAL + 12
1 SOLID ' Ι § SOLUTIONS ρ - 8 —
• wÊÊÊÊÊÊË Χ
magic
Figure 2. Hypothetical phase diagram between a Pt(H) and Pt(IV) compound illustrating the approach taken here for achiev ing the 1-D metallic state. Parameter δ is width of two-phase region, and γ is assumed homogeneity range for the 1-D metallic phase. In principle, many intermediate phases all separated by small immiscibility gaps could exist in such a system.
Figure PtCh, ' 2
3. Stacking ofPt(NH,) and ions in Magnus' green salt (6) 2+
/t
336
EXTENDED
INTERACTIONS
BETWEEN
METAL
IONS
systematic v a r i a t i o n s i n the t o t a l e l e c t r o n concentration on the p h y s i c a l p r o p e r t i e s of the system. I t i s our contention, then, that there w i l l be a s p e c i a l χ » χ . f o r which MGS w i l l magic behave as a 1-D "metal". The approach to x sys tem, and i t s i m p l i c a t i o n s f o r the e n t i r e c l a s s of p a r t i a l l y o x i d i z e d metal chain compounds, i s thus the main subject of t h i s work. r
i n
t n e
M
G
S
m a g i c
M a t e r i a l s Preparation A l l platinum s a l t s used i n t h i s study were prepared from 99.99% pure platinum metal by standard conversion techniques as 1 5
described by B r a u e r .
Samples of K P t C l 2
I ^ P t C l ^ and
6 >
e
K P t ( C N ) 3 H 0 were a l s o obtained from Strem Chemicals (Danvers, 2
4
2
Mass.) f o r purposes of comparison. The r e s u l t s reported h e r e i n were found to be independent of m a t e r i a l s source. The Magnus s a l t s o l i d s o l u t i o n s (MGS(SS)) were prepared ac cording to the f o l l o w i n g r e a c t i o n scheme (1)
(a)Pt(II)(NH ) 3
+ + 4
+ (b)Pt(II)Cl
= 4
+ (c)Pt(IV)L Y 4
= 2
+ MGS(SS),
where the tetrammine platinum(II) complex was present as the CI or CIO. s a l t , K P t C l , was the anion source, and the molar r a t i o 4 2 4 (a):(b) = 1. S u c c e s s f u l i n c o r p o r a t i o n of higher valence Pt i n t o MGS was found to occur_un Cl . 4
q
q
The maximum values of q obtained f o r the v a r i o u s systems i s shown i n Table I I . I t should be noted that the parameter q as used i n the formulae f o r MGS(SS) systems i s a measure o f the de gree of s u b s t i t u t i o n of MGS a n i o n i c s i t e s by P t ( I I I ) . I t i s of course r e l a t e d , but not e q u i v a l e n t t o , the composition parameter χ of F i g s . 1 and 2. For comparison, the p a r t i a l l y o x i d i z e d Pt(+2.3) s t a t e occurs at χ = 0.15 i n F i g s . 1 and 2 and would be equivalent to q = 0.6 i n Formulae (3) and (4) i f an "averaged" o x i d a t i o n s t a t e were a p p r o p r i a t e f o r the MGS s o l i d s o l u t i o n s . This p o i n t w i l l be considered again i n the subsequent d i s c u s s i o n . I f the top of the 5d 2 valence bands are mostly of a n i o n i c z
( P t ( I I ) C l ~ ) c h a r a c t e r , as shown i n F i g . 4, then the 5d 2 hole i s 4
z
l o c a t e d on the P t C l (5)
4
groups i n both (3) and ( 4 ) ; i . e . , (3) becomes
Pt (II) ( N H ) ( P t (II) C l 3
4
4
) ^ (Pt (II) (CN) ) (Pt ( I I I ) C l ) Y . 4
4
q
q
E.P.R. powder data f o r s o l i d s o l u t i o n s of the type (4) gave the f o l l o w i n g g-tensor parameters: g., = 1.94 and g = 2.50. 14 These values are i d e n t i c a l to those measured f o r MGS(SS) i n which K P t ( C N ) C l or K P t ( C N ) B r i s the oxidant; Thus, EPR suggests that the P t ( I I I ) s i t e i s independent of o x i d i z i n g agent. Whether the 5d 2 hole r e s i d e s p r i m a r i l y on ( P t C l ) or P t ( N H ) 2
4
2
2
4
2
g
4
3
4
groups depends on the r e l a t i v e energies of the c a t i o n and anion bands, of course, and we choose the former s i t e to be c o n s i s t e n t with the assignments i n M i l l e r ' s o r i g i n a l model.^ Our formulation of Magnus s a l t s o l i d s o l u t i o n systems places charge compensating h a l i d e ions i n i n t e r s t i t i a l s i t e s i n the s t r u c t u r e . Our main evidence f o r t h i s c o n s i s t s of i n f r a r e d data and x-ray d i f f r a c t i o n measurements of the l a t t i c e constants of MGS(SS). The f a r i n f r a r e d spectrum of the K P t ( C N ) B r - o x i d i z e d 2
4
2
MGS product with q = 0.2 shows no evidence f o r a Pt-Br s t r e t c h band. Such a band would be expected f o r s u b s t i t u t i o n along the chain, or f o r charge compensation through replacement of one of the l i g a n d s w i t h i n the Pt square c o o r d i n a t i o n plane (e.g., r e - _^ placement of NH ). The changes i n i n f r a r e d p a t t e r n above 700 cm 3
with i n c r e a s i n g q values are shown i n F i g . 6. These s p e c t r a were taken f o r constant s o l i d c o n c e n t r a t i o n i n the KBr matrix, and show only the i n t r o d u c t i o n of one new band due to the C Ξ Ν 1
s t r e t c h i n g vibrâtion_near 2200 cm" . This i s c o n s i s t e n t with the replacement of P t C l ~ anions i n the MGS s t r u c t u r e with Pt(CN) ~~ 4
4
as determined by chemical a n a l y s i s . Unit c e l l data as a f u n c t i o n o f q f o r the K P t ( C N ) B r 2
4
2
oxi-
dized s o l i d s o l u t i o n s , shown i n F i g . 7, i n d i c a t e s expansion of
340
EXTENDED
INTERACTIONS
BETWEEN
METAL
TABLE I I Solid Solution Limits Dopant ( O x i d i z i n g Agent)
K Ptci 2
0.03
6
K Pt(CN) Cl 2
4
K Pt(CN) Br 2
4
0.08
2
0.20
2
See Eqs. ( 4 ) , ( 5 ) . WAVELENGTH (MICRONS) 3
3000
4
5
6
2000
7
1500
8
9
1200 cm
Figure
6. Infrared KgPtfCN^Brj.
10
II
1000 900
12
13
800
14
15
700
-I
spectra for MGS(SS) formed by partial oxidation Composition parameter q is defined in Equation 5.
with
IONS
22.
seoir E TAL.
the MGS the C
q
Magnus
Green
Salt
341
c e l l along the t e t r a g o n a l a - a x i s and c o n t r a c t i o n along Q
stacking d i r e c t i o n .
Expansion
transverse to the l i n e a r
chains i s p l a u s i b l e f o r Br" i n t e r s t i t i a l s combined with the i n t r o d u c t i o n of P t ( C N ) groups i n a n i o n i c l a t t i c e s i t e s , whereas 4
c o n t r a c t i o n along C i s c o n s i s t e n t with the attendant i n t r o d u c t i o n of the s m a l l e r P t ( I I I ) ions i n the c h a i n s . In t h i s connec t i o n i t i s r e l e v a n t that a l l the p a r t i a l l y o x i d i z e d Pt s a l t s shown i n Table I have very short Pt-Pt spacings. Thus, our ob s e r v a t i o n of a decrease i n C f o r MGS(SS) i s e n t i r e l y expected Q
q
with p a r t i a l o x i d a t i o n ( i n c r e a s i n g q) o f t h i s phase. However, u n l i k e the m e t a l l i c - l i k e s a l t s o f Table I , we expect two d i f f e r ent types of platinum s i t e s along the c h a i n s : a m a j o r i t y con s i s t i n g o f the P t ( I I ) h o s t s , and the remainder P t ( I I I ) s i t e s . In f a c t , the expected d i s o r d e r along the chain as a consequence of t h i s arrangement i s c l e a r l y observed i n the x-ray d i f f r a c t i o n as a l o s s and broadening of (00il) r e f l e c t i o n s with i n c r e a s i n g q. E l e c t r i c a l and O p t i c a l P r o p e r t i e s . The formation o f mixed valence s o l i d s o l u t i o n s through the i n t r o d u c t i o n of P t ( I I I ) has profound e f f e c t s on the p r o p e r t i e s of MGS. Measurements of the d.c. e l e c t r i c a l c o n d u c t i v i t y of powder compactions shown i n Table I I I show a four order of magnitude change i n σ as the com p o s i t i o n was changed from o s t e n s i b l y pure MGS (undoped, q = 0) to a s o l i d s o l u t i o n c o n t a i n i n g the maximum degree of s u b s t i t u t i o n , q = 0.20. Moreover, the h o l e i o n i z a t i o n e n e r g i e s , E determined &9
from the a c t i v a t i o n energies f o r e l e c t r i c a l conduction become s m a l l e r with i n c r e a s i n g q. Although the measurements were per formed on compacted powders, these general trends are c l e a r l y evident and q u i t e s i m i l a r to the behavior of doped three-dimen s i o n a l semi-conductors as the impurity concentrations are i n creased · The o p t i c a l spectrum i n the near i n f r a r e d r e g i o n a l s o r e v e a l s some i n t e r e s t i n g f e a t u r e s . These measurements were c a r r i e d out on samples d i s p e r s e d i n KBr p e l l e t s . F i g . 8 d e p i c t s the spec trum f o r two d i f f e r e n t "pure" (q = 0) MGS samples. Sample A was measured i n two separate pressings of the same p e l l e t to d e t e r mine the v a r i a t i o n i n o p t i c a l d e n s i t y to be expected due to v a r i a t i o n s i n sample preparatory technique. The s p e c t r a were mea sured with no r e f e r e n c e i n the standard compartment of the spec trometer. We a s c r i b e the absorption r i s e i n the 1-2.4μ r e g i o n of the spectrum mostly to s c a t t e r i n g , as there do not appear to be any d i s t i n c t bands i n t h i s range. On the other hand, F i g . 9 shows the a b s o r p t i o n i n the MGS(SS) system, prepared with K P t ( C N ) B r oxidant, f o r the range of compositions q = 0.05 to 2
4
2
0.2. The new band appearing i n t h i s frequency r e g i o n i s i n t e n s e , and d i r e c t l y p r o p o r t i o n a l to the c o n c e n t r a t i o n of P t ( I I I ) centers i n the sample as determined by chemical a n a l y s i s based on carbon
342
EXTENDED
INTERACTIONS
BETWEEN
METAL
IONS
9.04
5 10 MOLE % Pt INTRODUCED
Figure 7. Lattice constants vs. mole % platinum introduced for MGS(SS) prepared by K Pt(CN)j,Br . Constants obtained by least-squares refinement of Guinier camera powder data. 2
3.00|
Q00 4000 1
1
1
1
1
8000
1
1
1
1
1
1
12000
1
1
ι
1
16000
r
1
1
20000
1
24000
WAVELENGTH, Â Figure 8. Spectra of nominally pure MGS in the near IR. Curves A correspond to spectra obtained in two separate pressings of the same pellet; curve Β obtained from a separate MGS preparation made up to the same concentration in pellet as A.
2
22.
SCOTT
ET
Magnus
AL.
Green
Salt
343
(CN) content. Excluding the q = 0.05 samples, whose chemical a n a l y s i s i s l e a s t r e l i a b l e , we f i n d an absorption c o e f f i c i e n t α = ( 6 . 0 + 0 . 3 ) χ 10 1/mol-cm, assuming the absorption i s en t i r e l y p o l a r i z e d along the c-axis of the c r y s t a l l i t e s ( i . e . , α » α ). (|
χ
In a d d i t i o n to the intense absorption peaking near 1.3μ, another important feature of F i g . 9 i s the gradual s h i f t of ab s o r p t i o n edge to lower energies with i n c r e a s i n g q. This i s sug gested by the general increase i n o p t i c a l d e n s i t y at 2.4μ as q i n c r e a s e s . Although we can not o b t a i n an accurate absorption edge from the p e l l e t measurements, t h i s q u a l i t a t i v e trend i s con s i s t e n t with the lowering of a c t i v a t i o n energy f o r e l e c t r i c a l conduction with q as shown i n Table I I I . I t i s r e l e v a n t to point out that e a r l y measurements of the IR spectrum of o s t e n s i b l y "pure" MGS a l s o reported absorption i n g the near IR region shown i n F i g . 9. Moreover, Collman e t . a l . reported photocurrent i n MGS with e x c i t a t i o n between 0.74-2.5μ. 13 Other measurements, i n c l u d i n g those on much purer MGS samples , 18 cast doubt on the presence of t h i s absorption, but i t i s c l e a r that samples prepared i n the presence of Pt(IV) complexes d e f i n i t e l y c o n t a i n the broad IR peak. We have i n f a c t found the iden t i c a l IR band i n MGS(SS) samples produced by K P t ( C N ) C l and K P t C l ^ o x i d a t i o n . However, we i n t e r p r e t the absorption as due to t r a n s i t i o n s from the 5d 2 valence band to the 5d 2 bound ζ ζ s t a t e s , r a t h e r than to an interband t r a n s i t i o n . ' 2
4
2
2
?
8
Discussion I t i s worthwhile to review some of the p r o p e r t i e s of the mixed valence MGS s o l i d s o l u t i o n s before examining t h e i r r e l a t i o n s h i p to the c l a s s of p a r t i a l l y o x i d i z e d platjinum s a l t s of Table I such as KCP. P r e v i o u s l y , we found from EPR measure14 ments that " e x t r i n s i c " MGS c r y s t a l s u s u a l l y c o n t a i n small con c e n t r a t i o n s of bound 5d 2 holes ( P t ( I I I ) ) l y i n g ^ 0.6 eV above the 5d 2 o r b i t a l s . The P t ( I I I ) holes whose occurence i s due to the presence of Pt(IV) complexes during sample p r e p a r a t i o n , have a f a i r l y wide extent-over at l e a s t s e v e r a l metal chain l a t t i c e 14 sites . In these s t u d i e s we have been able to s i g n i f i c a n t l y i n crease the concentration of P t ( I I I ) centers by p a r t i a l o x i d a t i o n with Pt(IV) complexes, r e s u l t i n g i n as many as one out of ten P t ( I I ) chain s i t e s replaced by P t ( I I I ) f o r the case of o x i d a t i o n with K P t ( C N ) B r (Table I I , q - 0.2). Concomitant with the i n c r e a s i n g P t ( I I I ) concentration i s an increase i n d.c. c o n d u c t i v i t y and the appearance of a strong IR t r a n s i t i o n due to 5d 2 band to z
2
4
2
z
EXTENDED INTERACTIONS
344
BETWEEN
METAL
TABLE I I I E l e c t r i c a l Data on Powder Compactions ((IV)
a
= K Pt(CN) Br ) 2
R T
4
, (ohm-cm)"
2
Ε
1
q
, eV a'
0.0
3.7 χ 1 0 ~
7
0.66
0.12
2.5 χ 1 0 "
4
0.27
0.20
6.3 χ 1 0 ~
3
0.15
>gl 4000
ι
ι 8000
ι
ι
I
ι
12000
16000
1
1 20000
1 — -J 24000
W/VELENGTH, Â
Figure 9. Near IR spectra for MGS(SS) prepared from Br . Composition parameter q defined in Equation results were obtained for partial oxidation of MGS with CL and K PtCl . 2
z
r>
K Pt(CN) Similar K>Pt(CN) É
5.
r
r
IONS
22.
SCOTT
ET
Magnus
AL.
5d 2 h o l e e x c i t a t i o n . z
Green
Salt
345
A c t i v a t i o n energies obtained from the d.c.
c o n d u c t i v i t y measurements i n d i c a t e a decrease i n Ε , the h o l e i o n i z a t i o n energy. In what f o l l o w s we w i l l consider the p o t e n t i a l the MGS system has f o r e x h i b i t i n g a t r a n s i t i o n to a "1-D metal" at dopant l e v e l s higher than we have been p r e s e n t l y able to achieve and f u r t h e r i m p l i c a t i o n s concerning the s t a b i l i z a t i o n of known 1-D metals such as KCP at s p e c i a l s t o i c h i o m e t r i e s . By c o n s i d e r i n g the low h o l e c o n c e n t r a t i o n l i m i t , one can more c l e a r l y understand j u s t what might be r e s p o n s i b l e f o r a t r a n s i t i o n to m e t a l l i c behavior at higher hole c o n c e n t r a t i o n . I f we e n v i s i o n a f u l l band (the MGS l i m i t ) and introduce a few l o c a l c a r r i e r s by s u b s t i t u t i o n , then the c a r r i e r , i n t h i s case a d 2 h o l e , w i l l not ζ * be f r e e l y mobile, but w i l l i n s t e a d be r a t h e r t i g h t l y bound to i t s site. This b i n d i n g may a r i s e simply from the coulomb a t t r a c t i o n between the P t ( I I I ) hole atid the charge compensating anion and r e s u l t i n the formation of an e x c i t o n i c or quasibound p a r t i c l e - h o l e 19 s t a t e r a t h e r than i n a f r e e c a r r i e r . This coulomb p a r t i c l e - h o l e a t t r a c t i o n can however be reduced i n s e v e r a l ways. As the concen t r a t i o n of dopant and d e n s i t y o f c a r r i e r s i n c r e a s e s , the s i n g l e c a r r i e r p o t e n t i a l r e s u l t i n g from the charge compensators w i l l tend to be screened. This mechanism, analogous to the Mott mechanism i n higher dimensional i s o t r o p i c conductors, should l e a d to a semi conductor-metal t r a n s i t i o n . A l s o , with an i n c r e a s e i n doping, the d e n s i t y of charge compensators which an i n d i v i d u a l d 2 hole sees w i l l become e f f e c t i v e l y constant, i . e . independent of the s i t e on which the hole f i n d s i t s e l f . This w i l l occur s i n c e the o v e r l a p ping coulomb t a i l s from the impurity or compensator d i s t r i b u t i o n w i l l average out the one e l e c t r o n (hole) p o t e n t i a l f l u c t u a t i o n s more e f f e c t i v e l y at high c o n c e n t r a t i o n s . For the case of the KCP l i m i t , there are ^ 0.3 h a l i d e ions f o r each P t . We suggest that f o r KCP i t i s t h i s s t o i c h i o m e t r i c r a t i o , corresponding to the composition g ^ 0.15 i n our phase diagram ( F i g . 1) at which the l o c a l e x c i t o n s t a t e goes over to the m e t a l l i c s t a t e . The f a c t that t h i s r a t i o occurs c o n s i d e r a b l y below a P t ( I I I ) / P t ( I I ) r a t i o of u n i t y comes about because the h o l e s , w h i l e l o c a l i z e d , are i n extended e i g e n s t a t e s which reach s e v e r a l Pt s i t e s along the c h a i n . Such i n t e r p r e t a t i o n can be i n f e r r e d from the EPR spec t r u m * ^ as w e l l as the l a t t i c e r e g u l a r i t y . This i n d i c a t e s that the hole-anion i n t e r a c t i o n reduced by h o l e - h o l e i n t e r a c t i o n s and smoothed due to i n c r e a s e d h a l i d e c o n c e n t r a t i o n could r e s u l t , at s u f f i c i e n t l y high dopant l e v e l , i n a s i n g l e hole wave f u n c t i o n with a coherence length s u f f i c i e n t l y l a r g e to r e s u l t i n m e t a l l i c l i k e behavior. I t i s i n t e r e s t i n g to note that f o l l o w i n g M o t t s o r i g i n a l a r gument, the dopant d e n s i t y ^ 0.X5 i s i n near agreement with 9
z
x
m
2
a
i
c
1
1 , 3
1
x
m
a
g
i
c
1
the o r i g i n a l Mott c r i t e r i o n f o r the t r a n s i t i o n . Following Mott s o r i g i n a l argument, based on Thomas-Fermi screening of the
346
EXTENDED
INTERACTIONS
BETWEEN
METAL
IONS
c a r r i e r s , the semiconductor -> metal t r a n s i t i o n f o r impurity con d u c t i o n i n a three dimensional system should occur approximately when n ^ /
3
a^ *υ .25.
As Mott and Twose p o i n t o u t , ^
1
a sharp break
i n the c o n d u c t i v i t y of both η-type and p-type germanium occurs i n 1/3 f a c t f o r n^ a^ ^ .3. In these expressions, i s the c a r r i e r d e n s i t y and a^ i s the e f f e c t i v e s i t e r a d i u s . I f we take a as 1/3 one-half of the Pt-Pt spacing along the c h a i n , then n
