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High pressure synthesis of arsenopyrite-type ternary compounds
Mat. Res. Bull. Vol. I, pp. 3-12, 1966. P e r g a m o n P r e s s , Inc. the United States.
Prin ted in
HIGH PRESSURE SYNTHESIS OF ARSENOPYRITE-TYPE
TERNARY COMPOUNDS
Mario D. Banus and Mary C. Lavine Lincoln Laboratory, * M a s s a c h u s e t t s Institute of Technology Lexington, Massach u setts
(Received July 5th, 1966;
Communicated by J. B. Goodenough)
ABSTRACT
A s e r i e s of t e r n a r y compounds analogous to FeAsS (arsenopyrite) have been p r e p a r e d at a t m o s p h e r i c p r e s s u r e by Hulliger (1, 2) and others (3, 4). They w e r e able to synthesize only 20 of the possible 36 i s o e l e c t r o n i c compounds with the general fo rmu la MXY, w h e r e M i s Fe, R u o r O s ; X i s P, As, S b o r Bi; Y i s S , Se or Te. Four of the m i s s i n g compounds (OsSbSe, OsSbTe, OsBiSe and RuBiSe) have now been synthesized in a t e t r a h e d r a l anvil h i g h - p r e s s u r e unit at 35-50 kbar. The role of high p r e s s u r e is d i s c u s s e d in r e l a t i o n to vapor p r e s s u r e of the e l e m e n t s , size ratio and c o m p r e s s i b i l i t y of the atoms, and stability of the phases The X - r a y powder patterns of OsSbSe, OsSbTe and OsBiSe have been indexed as monoclinic s t r u c t u r e s of the same type adopted byHulliger (2) and the lattice p a r a m e t e r s a r e also consistent. Resistivity m e a s u r e m e n t s on OsSbSe and OsSbTe suggest that they are extrinsic semiconductors.
Introduction A s e r i e s of compounds with the g en eral f o r m u l a s MX2, MXY and MY2 can be synthesized w he r e M is the Group VIIIB metal, X is a Group VB and Y is a Group VIB metal.
The s t r u c t u r e s of these compounds a r e pyrite (C2),
m a r c a s i t e (C18) or a r s e n o p y r i t e (EO 7) depending p r i m a r i l y on the total number of e l e c t r o n s in the compound.
Thus, FeAsS (arsenopyrite) and CoAs 2 (safflorite)
*Operated with support f r o m the U. S. Air F o r c e .
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e a c h with 19 v a l e n c e e l e c t r o n s , a r e n a t u r a l l y o c c u r r i n g e x a m p l e s of c o m p o u n d s h a v i n g the m o n o c l i n i c EO 7 s t r u c t u r e .
The c o m p o u n d s with total n u m b e r of
v a l e n c e e l e c t r o n s of 20 or 21, a s f o r e x a m p l e FeS 2 (pyrite), CoAsS (colbaltite), P t P 2 , RhSe 2 and NiPS all have the p y r i t e C2 or C2' s t r u c t u r e .
Finally, the
c o m p o u n d s with total v a l e n c e e l e c t r o n s equal to 18, F e P 2 , etc. have the m a r casite structure. The t e r n a r y a r s e n o p y r i t e - t y p e c o m p o u n d s have b e e n s t u d i e d by H u l l i g e r (1, 2), H u l l i g e r and M o o s e r (3), and Hahn and K l i n g e r (4).
Of the 34 p o s s i b l e
c o m p o u n d s (not n a t u r a l l y o c c u r r i n g ) l i s t e d in Table 1, t h e y w e r e able to s y n t h e s i z e only 20 by p r o l o n g e d s i n t e r i n g of c o l d - p r e s s e d p o w d e r s at a t m o s p h e r i c pressure. TABLE 1 FePS
4
RuPS
2
OsPS
2
FeAsS
*
RuAsS
2
OsAsS
2
FeSbS
*
RuSbS
2
OsSbS
2
FeBiS
-
RuBiS
-
OsBiS
-
FePSe
4
RuPS
2
OsPSe
2
FeAsSe
4
RuAsSe
2
OsAsSe
2
FeSbSe
4
RuSbSe
2
OsSbSe
§
FeBiSe
-
RuBiSe
-
OsBiSe
§
FePTe
-
RuPTe
-
OsPTe
-
FeAsTe
4
RuAsTe
2
OsAsTe
2
FeSbTe
4
RuSbTe
2
OsSbTe
§
FeBiTe
-
RuBiTe
-
OsBiTe
§
* N a t u r a l l y o c c u r r i n g m i n e r a l s , a r s e n o p y r i t e (FeAsS) and g u d m u n d i t e (FeSbS). §This w o r k . The r e m a i n i n g c o m p o u n d s , p a r t i c u l a r l y t h o s e c o n t a i n i n g Os or Ru, might, h o w e v e r , be a m e n a b l e to s y n t h e s i s u n d e r high p r e s s u r e . S y n t h e s i s u n d e r high p r e s s u r e w a s i n d i c a t e d s i n c e one or m o r e of the f o l l o w i n g c o n s i d e r a t i o n s could apply: 1. The s i z e r a t i o of the a t o m s m i g h t be u n f a v o r a b l e f o r the
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d e s i r e d c o m p o u n d at 1 a t m and the c o m p r e s s i b i l i t i e s of the a t o m s could be s u c h that at h i g h e r p r e s s u r e the s i z e r a t i o s would i m p r o v e .
(The s y n t h e s i s of Nb3In (4) and Nb3Bi (5)
a r e e x a m p l e s of t h i s c a s e . ) 2. The v a p o r p r e s s u r e of one c o m p o n e n t in the s y s t e m m i g h t be v e r y high at the s y n t h e s i s t e m p e r a t u r e ,
and v e r y high
p r e s s u r e s would t h e r e f o r e be r e q u i r e d to c o n t a i n it.
(The
p r e p a r a t i o n of C r O 2 by K a f a l a s (6) is an e x c e l l e n t e x a m p l e of t h i s c a s e . ) 3. P r e s s u r e
m i g h t a l t e r the p h a s e d i a g r a m by r a i s i n g the
t e m p e r a t u r e of a p e r i t e c t o i d d e c o m p o s i t i o n or by i n c r e a s ing the s o l u b i l i t y of a c o m p o n e n t to p e r m i t the s y n t h e s i s . E x a m p l e s of t h e s e c a s e s a r e d i f f i c u l t to f u r n i s h b e c a u s e p r o o f r e q u i r e s the d e t e r m i n a t i o n of a p h a s e d i a g r a m u n d e r pressure.
The InSb-Sn (7) s y s t e m and F e - C (8) s y s t e m a r e
c a s e s w h e r e two c o m p o n e n t s y s t e m s have b e e n s t u d i e d u n d e r pressure. T h e r e is, h o w e v e r , the d i s a d v a n t a g e in h i g h - p r e s s u r e s y n t h e s i s that in c o m p o u n d s w h e r e s o l i d - s o l i d d i f f u s i o n is the p r i m a r y m e c h a n i s m , p r e s s u r e s t r o n g l y d e c r e a s e s the d i f f u s i o n r a t e .
In g e n e r a l p r a c t i c e it is not f e a s i b l e to
r u n h i g h - p r e s s u r e e q u i p m e n t f o r a s i n g l e s a m p l e at t e m p e r a t u r e s of 8 0 0 - 1 0 0 0 ° C f o r m o r e than a few d a y s at a t i m e .
D e t e r i o r a t i o n of the h e a t e r a s w e l l as w o r k
load on the e q u i p m e n t a r e s o m e of the p r o b l e m s e n c o u n t e r e d in l o n g - t e r m r u n s . Experimental The s p e c i m e n s t r e a t e d at high p r e s s u r e w e r e all p r e p a r e d in the f o l l o w i n g manner.
C a r e f u l l y w e i g h e d p o w d e r s ( h i g h e s t p u r i t y c o m m e r c i a l l y available) of
the e l e m e n t s w e r e m i x e d and cold p r e s s e d at ~ 7 k b a r . (0.3 in. d i a m e t e r by - 0 . 4
The r e s u l t i n g c y l i n d e r s
in. long) w e r e p l a c e d in b o r o n n i t r i d e c a n s i n s i d e
g r a p h i t e h e a t e r s l e e v e s and t h e n p l a c e d i n s i d e p y r o p h y l i t e t e t r a h e d r a 3 . 0 0 in. on an edge. incorporated.
Suitable p o w e r l e a d s and a s h e a t h e d Cr-A1 t h e r m o c o u p l e w e r e The s a m p l e s w e r e s q u e e z e d in a 2000-ton, t e t r a h e d r a l - a n v i l
p r e s s to 35-45 k b a r ( r o o m t e m p e r a t u r e c a l i b r a t i o n ) p r i o r to r a i s i n g the t e m perature.
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In s y n t h e s i s u n d e r high p r e s s u r e two h e a t i n g c y c l e s can be u t i l i z e d .
If
the e l e m e n t s , p o s s i b l e i n t e r m e d i a t e s , and final p r o d u c t a r e all s o l i d s at the m a x i m u m t e m p e r a t u r e or a r e all m o l t e n a n d c o m p l e t e l y soluble, t h e n r a p i d h e a t i n g to the s y n t h e s i s t e m p e r a t u r e is p o s s i b l e and p r e f e r a b l e .
If, h o w e v e r ,
s o m e of the e l e m e n t s or i n t e r m e d i a t e s a r e liquid, but the p r o d u c t is solid, slow h e a t i n g with a n n e a l s at i n t e r m e d i a t e t e m p e r a t u r e s m a y be p r e f e r r e d in o r d e r to p r e v e n t the f o r m a t i o n of s u b s t a n t i a l liquid p h a s e s that c a n i s o l a t e the c o m p o n e n t s and d e c r e a s e the i n t e r d i f f u s i o n s u r f a c e .
T h i s is a g g r a v a t e d by the
p r e s e n c e of t e m p e r a t u r e g r a d i e n t s , w h i c h in s p e c i m e n s of t h i s s i z e can a m o u n t to 1 5 - 3 0 o c . In T a b l e 2, the m e l t i n g p o i n t s of the e l e m e n t s at 45 k b a r a r e i n d i c a t e d , w h e r e known, a l o n g w i t h the s l o p e of the d T / d p c u r v e .
It would s e e m (based
TABLE 2
M (8 e l e c t r o n s )
X (5 e l e c t r o n s )
Y (6 e l e c t r o n s )
P (590oc)
S 330°C - P o s
As 940°C - P o s
Se 680oc - Pos
Ru (2450°C)
Sb 590°C ° N e g to 57 k b a r
Te ~ 530oC - P o s
Os (2700oc)
Bi 375°C - P o s above 25 k b a r
Fe 1640°C - P o s
on the e l e m e n t s ) t h a t the M - A s - S e s y s t e m s would have the h i g h e s t m e l t i n g p o i n t s , while M - B i - S m i x t u r e s would have the l o w e s t .
H o w e v e r , the s i t u a t i o n
is c o m p l i c a t e d by the i n t e r m e d i a t e s f o r m e d p r i o r to c o m p l e t e r e a c t i o n .
At
a t m o s p h e r i c p r e s s u r e s the As c h a l c o g e n i d e s have m . p . 's in the 300°C r a n g e , the Sb c o m p o u n d s in the 600°C r a n g e s and the Bi c o m p o u n d s in the 600-700°C range.
The Os and Ru c h a l c o g e n i d e s d e c o m p o s e a r o u n d 600°C.
p r e s s u r e on the above t e m p e r a t u r e s is not known.
The e f f e c t of
U n d e r t h e s e c o n d i t i o n s the
p r e f e r r e d c y c l e would s e e m to be slow h e a t i n g with a p p r o p r i a t e i n t e r m e d i a t e
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anneal temperatures selected by estimation of melting points of the intermediate compounds. The preferred synthesis conditions for the four compounds that were successfully synthesized are listed in Table 3. In most cases more complete reaction could be obtained by recycling as follows: The ingot from the first cycle (generally containing 2 or 3 phases), was ground, repelleted, and repressed with heating times of 6-24 hours.
The second cycle gave information
about the decomposition temperature of the compounds under pressure since samples from cycle I, which contained large amounts of arsenopyrite compound, were converted to the elements when heated above ~ 1100°C for OsSbSe and OsSbTe and ~ 1000°C for OsBiSe. These data set the upper limits for the holding temperatures for the preferred conditions. In all cases, the temperature cycle was the critical parameter as long as the applied pressure was greater than 37 kbar; therefore the pressure was not varied over wide ranges.
TABLE 3
1st Cycle
Compound
Pressure (kbar)
Heat Rate (hrs)
Holding Temp. (°C)
2nd Cycle
Time (hrs)
Heat Rate
Holding Temp. (°C)
Time (hrs)
OsSbSe
45-47
Slow 1-2
9501050
6-8
Slow hold at 550 °
9501000
12 -24
OsSbTe
42 -47
Fast 1/2
9751050
6-8
Fast 1/2
950I000
26-24
OsBiSe
44-47
Slow 2-3
900 ± 20
6-8
Slow
950 ± 25
6-24
RuBiSe
45-46
Slow 1-2
850900
8
The O s - S b - T e m i x t u r e w a s the only t e r n a r y that could be h e a t e d p r o m p t l y to the m a x i m u m t e m p e r a t u r e with s u c c e s s f u l r e s u l t s .
I n c o m p l e t e r e a c t i o n s in
the s u c c e s s f u l p r e p a r a t i o n s gave m i x t u r e s of MY2, X2Y 3 and s o m e of the desired monoclinic compound.
U n d e r all c o n d i t i o n s a t t e m p t e d , the t e r n a r y
ARSENOPYRITE-TYPE TERNARY COMPOUNDS
8
a
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b FIG. 1
Two a r e a s f r o m the same ingot of OsSbSe (HPTGR). (a) Bar cut f r o m center of ingot, single phase and uniform. (b) A fragment f r o m the edge showing definite second phase inclusions which has been shown by m i c r o p r o b e analysis to be Os.
mixtures, which w e r e not synthesized successfully, contained only MY2, X2Y 3 and the e l e m e n t s .
At p r e s s u r e s of 35-37 kbar, s amp les containing Bi generally
"blew out" even with slow heating. The extent of r e a c t i o n after various synthesis conditions was d e t e r m i n e d p r i m a r i l y f r o m X - r a y D e b y e - S c h e r r e r powder patterns;
additional information
was obtained f r o m visual and metallographic examination and f r o m e l e c t r o n beam m i c r o p r o b e analysis of polished sections of the ingots.
Samples, whose
powder p a t t e r n s indicated that only the monoclinic phase was p r e s e n t when examined by the optical methods, frequently showed small islands of impurities. These i m p u r i t i e s w e r e usually found at the p e r i p h e r y of the cylindrical ingots, so that bars cut f r o m the center for e l e c t r i c a l m e a s u r e m e n t s appeared to be f r e e of u n r e a c t e d m a t e r i a l s or i n t e r m e d i a t e s .
(See Fig. 1.)
The Knoop m i c r o h a r d n e s s of the t e r n a r y phase ( m e a s u r e d with a Leitz Miniload h a r d n e s s t e s t e r ) was 672 k g / s q m m for OsSbSe and 196 k g / s q mm for OsSbTe. phases.
OsBiSe and RuBiSe w e r e never obtained as apparently pure monoclinic
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P r o p e r t i e s of the New P h a s e s The s t r u c t u r e , l a t t i c e p a r a m e t e r s , and e l e c t r i c a l p r o p e r t i e s of the new t e r n a r y p h a s e s w e r e studied.
The D e b y e - S c h e r r e r p o w d e r p a t t e r n s w e r e m a d e
in a 114.6 m m N o r e l c o c a m e r a with C u I ~ r a d i a t i o n and e x p o s u r e s of 8-12 h o u r s . The d - v a l u e s and sin2e v a l u e s w e r e c a l c u l a t e d by a p r o g r a m f o r the 7090 c o m p u t e r f r o m the line p o s i t i o n s d e t e r m i n e d with a f i l m - m e a s u r i n g d e v i c e .
The
m a j o r s o u r c e of e r r o r is in the d i f f u s e n e s s of the lines, which set the e r r o r l i m i t s of • 0.03 f o r the l a t t i c e p a r a m e t e r v a l u e s shown in Table 4.
The r e s u l t s
of the X - r a y s t u d y on the t h r e e c o m p o u n d s a r e c o n s i s t e n t with the a s s i g n m e n t of the m o n o c l i n i c a r s e n o p y r i t e s t r u c t u r e a s s t a t e d by Hulliger (2).
The lattice
p a r a m e t e r s a r e a l s o c o n s i s t e n t and in logical s e q u e n c e with his v a l u e s f o r o t h e r m e m b e r s of this s e r i e s .
The d e n s i t y v a l u e s w e r e obtained u s i n g a B e r m a n
b a l a n c e and toluene a s the d i s p l a c e m e n t liquid.
The high d e n s i t y of OsBiSe
(as shown by the high n u m b e r of a t o m s / u n i t cell) is the r e s u l t of s o m e f r e e Os with its d e n s i t y of 2 2 . 5 g / c c .
The s a m p l e s of RuBiSe w e r e n e v e r u n c o n t a m i n a t e d
enough to allow d e t e r m i n a t i o n of the l a t t i c e p a r a m e t e r s . TABLE 4 a
Compound
o
b
o
c
o
8
(deg)
Density (g/ca)
(~)
(/~)
(/~)
OsSbSe
6.38
6.34
6.42
113.4
10.404
11. 855
OsSbTe
6.57
6.62
6.66
113.1
10.963
12. 008
OsBiSe
6.52
6.46
6.57
113.8
11.034
13.2
n
The e l e c t r i c a l p r o p e r t i e s of the t e r n a r y compounds w e r e d e t e r m i n e d on s m a l l r e c t a n g u l a r b a r s cut f r o m the c e n t e r of the ingot p a r a l l e l to the a x i s .
The
r e s u l t s of t h e s e m e a s u r e m e n t s on f o u r s p e c i m e n s that w e r e e s s e n t i a l l y f r e e of u n r e a c t e d m a t e r i a l s a r e shown in Table 5.
The b e s t s p e c i m e n is obviously
OsSbSe (HPTGR) s i n c e its high r e s i s t i v i t y r a t i o i n d i c a t e s the l e a s t a m o u n t of impurities.
A t t e m p t s w e r e m a d e to m e a s u r e the Hall valtage.
In f i e l d s of
6.3 k g a u s s and with c u r r e n t s of up to 10 ma, no Hall v o l t a g e s could be m e a s u r e d down to 5 × 10 -5 uv.
The t h e r m o e l e c t r i c - p o w e r r e s u l t s a r e c o n s i s t e n t with
H u l l i g e r ' s r e s u l t s on o t h e r c o m p o u n d s .
H o w e v e r , all t h e s e e l e c t r i c a l p r o -
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p e r t i e s a r e s e n s i t i v e to i m p u r i t y l e v e l s and to the n u m b e r of g r a i n b o u n d a r i e s , so t h e y s h o u l d be r e g a r d e d m e r e l y a s i n d i c a t i o n s .
A t t e m p t s w e r e m a d e to
d e t e r m i n e the o p t i c a l a d s o r p t i o n edge to o b t a i n a value of the e n e r g y gap. H o w e v e r , s a m p l e s a s t h i n a s 60 u w e r e c o m p l e t e l y opaque to light with w a v e l e n g t h s f r o m 0 . 5 to 25 u.
H u l l i g e r (2) w a s unable to m e a s u r e e n e r g y gaps
e i t h e r by e l e c t r i c a l or by d i f f u s e r e f l e c t a n c e t e c h n i q u e s f o r O s A s S e and O s A s T e . TABLE 5 P300OK (ohm-cm)
077OK (ohm-cm)
o77OK/P300OK
Type
(uv/°C)
OsSbSe HP7GR
I. 59
3 . 7 6 × 102
237
P
137
OsSbSe HP9GR
1 . 2 4 x I0 - I
i . 18
9.5
P
157
OsSbTe HP7
8 . 2 2 × 10 -2
4 . 5 9 × i0 - I
5.6
P
138
OsSbTe HP10GR
3 . 8 × 10 -2
2 . 2 8 × I0 - I
6.0
P
188
Sample
Discussion It is difficult to u n a m b i g u o u s l y a s s i g n the f u n c t i o n of high p r e s s u r e in the s y n t h e s e s of t h e s e t e r n a r y c o m p o u n d s .
The m o s t p e r t i n e n t e v i d e n c e is f r o m
a s t u d y of the s t a b i l i t y of OsSbSe and O s S b T e w h e n h e a t e d in v a c u u m - s e a l e d q u a r t z c a p s u l e s at c o n d i t i o n s s i m i l a r to t h o s e u s e d by H u l l i g e r to s y n t h e s i z e his compounds.
A f t e r h e a t i n g at 600°C f o r 6 h o u r s , t h e r e w a s no change in
the s a m p l e s that could be d e t e c t e d v i s u a l l y or with X - r a y p o w d e r p a t t e r n s . H o w e v e r , the s a m e s a m p l e s a f t e r 6 h o u r s at 800°C gave a m e t a l l i c m i r r o r and an X - r a y p o w d e r p a t t e r n that w a s p r i m a r i l y Os.
Thus, t h e s e c o m p o u n d s a r e
d e f i n i t e l y not s t a b l e u n d e r the a t m o s p h e r i c p r e s s u r e s y n t h e s i s c o n d i t i o n s . H o w e v e r , we cannot s a y w h e t h e r t h i s i n s t a b i l i t y is c a u s e d by v a p o r p r e s s u r e of the X and Y c o m p o n e n t s or by s i z e r a t i o of the a t o m s . F o r e x a m p l e , c o n s i d e r the d i f f e r e n c e in the m e t a l l i c r a d i i (10) b e t w e e n the Group V(X) a t o m s and the Group VI(Y) a t o m s :
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Sb-Se = 0 . 2 2 ~, S t - T e = 0 . 0 2 /~, B i - T e = 0 . 1 4 ~, and Bi-Se = 0 . 3 4 ~ . The Os t e r n a r y c o m p o u n d s of all but B i - T e w e r e s y n t h e s i z e d u n d e r p r e s s u r e , e v e n t h o u g h its s i z e d i f f e r e n c e is i n t e r m e d i a t e in the s e r i e s .
If v a p o r p r e s s u r e
of the X or Y e l e m e n t is the c o n t r o l l i n g f a c t o r , t h e n why can RuSbSe and RuSbTe be m a d e at a t m o s p h e r i c p r e s s u r e while the Os a n a l o g s r e q u i r e ~ 45 k b a r for t h e i r s y n t h e s i s ? An e x p l a n a t i o n m a y be that the u p p e r t e m p e r a t u r e l i m i t of s t a b i l i t y ( p e r i t e c t i c d e c o m p o s i t i o n t e m p e r a t u r e ) of t h e s e c o m p o u n d s is below the 700-900 ° r a n g e n e e d e d to c a r r y out the d i f f u s i o n c o n t r o l l e d s y n t h e s i s at atmospheric pressure.
At h i g h e r p r e s s u r e s ,
this decomposition temperature
is r a i s e d to the 1 0 0 0 - 1 1 0 0 ° C r a n g e and thus the s y n t h e s i s is s u c c e s s f u l . The e l e c t r i c a l p r o p e r t i e s of t h e s e c o m p o u n d s as s y n t h e s i z e d a r e u n u s u a l . T h e y show s e m i c o n d u c t i n g b e h a v i o r s i n c e t h e y have r e l a t i v e l y high r e s i s t i v i t i e s , n e g a t i v e t e m p e r a t u r e c o e f f i c i e n t of r e s i s t i v i t y and high t h e r m o e l e c t r i c p o w e r . H o w e v e r , t h e y do not have m e a s u r a b l e Hall c o e f f i c i e n t s , s u g g e s t i n g that t h e y have v e r y high c a r r i e r c o n c e n t r a t i o n s , e x c e e d i n g 1 0 2 0 / c c b a s e d on a o n e c a r r i e r e q u a t i o n f o r the Hall c o e f f i c i e n t .
The high c a r r i e r c o n c e n t r a t i o n is not
c o n s i s t e n t with the o t h e r e l e c t r i c a l p r o p e r t i e s , w h i c h s u g g e s t s that the o n e c a r r i e r m o d e l d o e s not apply.
Since t h e y have not b e e n p r e p a r e d a s s i n g l e
c r y s t a l s and a r e not i m p u r i t y f r e e , it is not c l e a r w h e t h e r t h e s e e l e c t r i c a l p r o p e r t i e s a r e t h o s e of the c o m p o u n d s t h e m s e l v e s or a r e a r e s u l t of the high i m p r u i t y level. We w i s h to thank Dr. V. S a d a g o p a n of the M. I. T. M a t e r i a l s S c i e n c e C e n t e r for b r i n g i n g t h e s e c o m p o u n d s to our a t t e n t i o n and we a r e g r a t e f u l to M r . T. Stack f o r his a s s i s t a n c e in p r e p a r i n g s a m p l e s , to M r s . M. J. Button f o r r e a d i n g the X - r a y p o w d e r p a t t e r n s and to M i s s M. C. F i n n for the e l e c t r o n beam microbe analyses.
We a l s o thank D r . A. J. S t r a u s s f o r his a d v i c e and
his help with the e l e c t r i c a l m e a s u r e m e n t s . References 1. F. H u l l i g e r , P h y s . L e t t e r s 4, 282 (1963). 2. F. H u l l i g e r , N a t u r e 201, 381 (1964). 3. F. H u l l i g e r and E. M o o s e r , J. P h y s . C h e m . Solids 2___66,429 (1965). 4. H. Hahn and W. K l i n g e r , N a t u r w i s s 52, No. 17, 494 (1965).
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5. M. D. Banus, T. B. Reed, H. C. C-atos, M. C. Lavine and J. A. Kafalas, J. Phys. Chem. Solids 23, 971 (1962). 6. D. H. Killpatrick, J. Phys. Chem. Solids 2_55, 1213 (1964). 7. D. S. Chapin, J. A. Kafalas and J. M. Honig, J. Phys. Chem. 69, 1402 (1965) 8. M. D. Banus, Susan Nye Vernon and H. C. Gatos, J. Appl. Phys. 36, 864 (1965). 9. S. V. Radcliff, M. Schatz and S. A. Kulin, J. Metals 12, 731 (1960). 10. L. Pauling, The Nature of the Chemical Bond, Third Edition, p. 403. Cornell University P r e s s , Ithaca, New York (1960).
Prin ted in
HIGH PRESSURE SYNTHESIS OF ARSENOPYRITE-TYPE
TERNARY COMPOUNDS
Mario D. Banus and Mary C. Lavine Lincoln Laboratory, * M a s s a c h u s e t t s Institute of Technology Lexington, Massach u setts
(Received July 5th, 1966;
Communicated by J. B. Goodenough)
ABSTRACT
A s e r i e s of t e r n a r y compounds analogous to FeAsS (arsenopyrite) have been p r e p a r e d at a t m o s p h e r i c p r e s s u r e by Hulliger (1, 2) and others (3, 4). They w e r e able to synthesize only 20 of the possible 36 i s o e l e c t r o n i c compounds with the general fo rmu la MXY, w h e r e M i s Fe, R u o r O s ; X i s P, As, S b o r Bi; Y i s S , Se or Te. Four of the m i s s i n g compounds (OsSbSe, OsSbTe, OsBiSe and RuBiSe) have now been synthesized in a t e t r a h e d r a l anvil h i g h - p r e s s u r e unit at 35-50 kbar. The role of high p r e s s u r e is d i s c u s s e d in r e l a t i o n to vapor p r e s s u r e of the e l e m e n t s , size ratio and c o m p r e s s i b i l i t y of the atoms, and stability of the phases The X - r a y powder patterns of OsSbSe, OsSbTe and OsBiSe have been indexed as monoclinic s t r u c t u r e s of the same type adopted byHulliger (2) and the lattice p a r a m e t e r s a r e also consistent. Resistivity m e a s u r e m e n t s on OsSbSe and OsSbTe suggest that they are extrinsic semiconductors.
Introduction A s e r i e s of compounds with the g en eral f o r m u l a s MX2, MXY and MY2 can be synthesized w he r e M is the Group VIIIB metal, X is a Group VB and Y is a Group VIB metal.
The s t r u c t u r e s of these compounds a r e pyrite (C2),
m a r c a s i t e (C18) or a r s e n o p y r i t e (EO 7) depending p r i m a r i l y on the total number of e l e c t r o n s in the compound.
Thus, FeAsS (arsenopyrite) and CoAs 2 (safflorite)
*Operated with support f r o m the U. S. Air F o r c e .
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e a c h with 19 v a l e n c e e l e c t r o n s , a r e n a t u r a l l y o c c u r r i n g e x a m p l e s of c o m p o u n d s h a v i n g the m o n o c l i n i c EO 7 s t r u c t u r e .
The c o m p o u n d s with total n u m b e r of
v a l e n c e e l e c t r o n s of 20 or 21, a s f o r e x a m p l e FeS 2 (pyrite), CoAsS (colbaltite), P t P 2 , RhSe 2 and NiPS all have the p y r i t e C2 or C2' s t r u c t u r e .
Finally, the
c o m p o u n d s with total v a l e n c e e l e c t r o n s equal to 18, F e P 2 , etc. have the m a r casite structure. The t e r n a r y a r s e n o p y r i t e - t y p e c o m p o u n d s have b e e n s t u d i e d by H u l l i g e r (1, 2), H u l l i g e r and M o o s e r (3), and Hahn and K l i n g e r (4).
Of the 34 p o s s i b l e
c o m p o u n d s (not n a t u r a l l y o c c u r r i n g ) l i s t e d in Table 1, t h e y w e r e able to s y n t h e s i z e only 20 by p r o l o n g e d s i n t e r i n g of c o l d - p r e s s e d p o w d e r s at a t m o s p h e r i c pressure. TABLE 1 FePS
4
RuPS
2
OsPS
2
FeAsS
*
RuAsS
2
OsAsS
2
FeSbS
*
RuSbS
2
OsSbS
2
FeBiS
-
RuBiS
-
OsBiS
-
FePSe
4
RuPS
2
OsPSe
2
FeAsSe
4
RuAsSe
2
OsAsSe
2
FeSbSe
4
RuSbSe
2
OsSbSe
§
FeBiSe
-
RuBiSe
-
OsBiSe
§
FePTe
-
RuPTe
-
OsPTe
-
FeAsTe
4
RuAsTe
2
OsAsTe
2
FeSbTe
4
RuSbTe
2
OsSbTe
§
FeBiTe
-
RuBiTe
-
OsBiTe
§
* N a t u r a l l y o c c u r r i n g m i n e r a l s , a r s e n o p y r i t e (FeAsS) and g u d m u n d i t e (FeSbS). §This w o r k . The r e m a i n i n g c o m p o u n d s , p a r t i c u l a r l y t h o s e c o n t a i n i n g Os or Ru, might, h o w e v e r , be a m e n a b l e to s y n t h e s i s u n d e r high p r e s s u r e . S y n t h e s i s u n d e r high p r e s s u r e w a s i n d i c a t e d s i n c e one or m o r e of the f o l l o w i n g c o n s i d e r a t i o n s could apply: 1. The s i z e r a t i o of the a t o m s m i g h t be u n f a v o r a b l e f o r the
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d e s i r e d c o m p o u n d at 1 a t m and the c o m p r e s s i b i l i t i e s of the a t o m s could be s u c h that at h i g h e r p r e s s u r e the s i z e r a t i o s would i m p r o v e .
(The s y n t h e s i s of Nb3In (4) and Nb3Bi (5)
a r e e x a m p l e s of t h i s c a s e . ) 2. The v a p o r p r e s s u r e of one c o m p o n e n t in the s y s t e m m i g h t be v e r y high at the s y n t h e s i s t e m p e r a t u r e ,
and v e r y high
p r e s s u r e s would t h e r e f o r e be r e q u i r e d to c o n t a i n it.
(The
p r e p a r a t i o n of C r O 2 by K a f a l a s (6) is an e x c e l l e n t e x a m p l e of t h i s c a s e . ) 3. P r e s s u r e
m i g h t a l t e r the p h a s e d i a g r a m by r a i s i n g the
t e m p e r a t u r e of a p e r i t e c t o i d d e c o m p o s i t i o n or by i n c r e a s ing the s o l u b i l i t y of a c o m p o n e n t to p e r m i t the s y n t h e s i s . E x a m p l e s of t h e s e c a s e s a r e d i f f i c u l t to f u r n i s h b e c a u s e p r o o f r e q u i r e s the d e t e r m i n a t i o n of a p h a s e d i a g r a m u n d e r pressure.
The InSb-Sn (7) s y s t e m and F e - C (8) s y s t e m a r e
c a s e s w h e r e two c o m p o n e n t s y s t e m s have b e e n s t u d i e d u n d e r pressure. T h e r e is, h o w e v e r , the d i s a d v a n t a g e in h i g h - p r e s s u r e s y n t h e s i s that in c o m p o u n d s w h e r e s o l i d - s o l i d d i f f u s i o n is the p r i m a r y m e c h a n i s m , p r e s s u r e s t r o n g l y d e c r e a s e s the d i f f u s i o n r a t e .
In g e n e r a l p r a c t i c e it is not f e a s i b l e to
r u n h i g h - p r e s s u r e e q u i p m e n t f o r a s i n g l e s a m p l e at t e m p e r a t u r e s of 8 0 0 - 1 0 0 0 ° C f o r m o r e than a few d a y s at a t i m e .
D e t e r i o r a t i o n of the h e a t e r a s w e l l as w o r k
load on the e q u i p m e n t a r e s o m e of the p r o b l e m s e n c o u n t e r e d in l o n g - t e r m r u n s . Experimental The s p e c i m e n s t r e a t e d at high p r e s s u r e w e r e all p r e p a r e d in the f o l l o w i n g manner.
C a r e f u l l y w e i g h e d p o w d e r s ( h i g h e s t p u r i t y c o m m e r c i a l l y available) of
the e l e m e n t s w e r e m i x e d and cold p r e s s e d at ~ 7 k b a r . (0.3 in. d i a m e t e r by - 0 . 4
The r e s u l t i n g c y l i n d e r s
in. long) w e r e p l a c e d in b o r o n n i t r i d e c a n s i n s i d e
g r a p h i t e h e a t e r s l e e v e s and t h e n p l a c e d i n s i d e p y r o p h y l i t e t e t r a h e d r a 3 . 0 0 in. on an edge. incorporated.
Suitable p o w e r l e a d s and a s h e a t h e d Cr-A1 t h e r m o c o u p l e w e r e The s a m p l e s w e r e s q u e e z e d in a 2000-ton, t e t r a h e d r a l - a n v i l
p r e s s to 35-45 k b a r ( r o o m t e m p e r a t u r e c a l i b r a t i o n ) p r i o r to r a i s i n g the t e m perature.
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In s y n t h e s i s u n d e r high p r e s s u r e two h e a t i n g c y c l e s can be u t i l i z e d .
If
the e l e m e n t s , p o s s i b l e i n t e r m e d i a t e s , and final p r o d u c t a r e all s o l i d s at the m a x i m u m t e m p e r a t u r e or a r e all m o l t e n a n d c o m p l e t e l y soluble, t h e n r a p i d h e a t i n g to the s y n t h e s i s t e m p e r a t u r e is p o s s i b l e and p r e f e r a b l e .
If, h o w e v e r ,
s o m e of the e l e m e n t s or i n t e r m e d i a t e s a r e liquid, but the p r o d u c t is solid, slow h e a t i n g with a n n e a l s at i n t e r m e d i a t e t e m p e r a t u r e s m a y be p r e f e r r e d in o r d e r to p r e v e n t the f o r m a t i o n of s u b s t a n t i a l liquid p h a s e s that c a n i s o l a t e the c o m p o n e n t s and d e c r e a s e the i n t e r d i f f u s i o n s u r f a c e .
T h i s is a g g r a v a t e d by the
p r e s e n c e of t e m p e r a t u r e g r a d i e n t s , w h i c h in s p e c i m e n s of t h i s s i z e can a m o u n t to 1 5 - 3 0 o c . In T a b l e 2, the m e l t i n g p o i n t s of the e l e m e n t s at 45 k b a r a r e i n d i c a t e d , w h e r e known, a l o n g w i t h the s l o p e of the d T / d p c u r v e .
It would s e e m (based
TABLE 2
M (8 e l e c t r o n s )
X (5 e l e c t r o n s )
Y (6 e l e c t r o n s )
P (590oc)
S 330°C - P o s
As 940°C - P o s
Se 680oc - Pos
Ru (2450°C)
Sb 590°C ° N e g to 57 k b a r
Te ~ 530oC - P o s
Os (2700oc)
Bi 375°C - P o s above 25 k b a r
Fe 1640°C - P o s
on the e l e m e n t s ) t h a t the M - A s - S e s y s t e m s would have the h i g h e s t m e l t i n g p o i n t s , while M - B i - S m i x t u r e s would have the l o w e s t .
H o w e v e r , the s i t u a t i o n
is c o m p l i c a t e d by the i n t e r m e d i a t e s f o r m e d p r i o r to c o m p l e t e r e a c t i o n .
At
a t m o s p h e r i c p r e s s u r e s the As c h a l c o g e n i d e s have m . p . 's in the 300°C r a n g e , the Sb c o m p o u n d s in the 600°C r a n g e s and the Bi c o m p o u n d s in the 600-700°C range.
The Os and Ru c h a l c o g e n i d e s d e c o m p o s e a r o u n d 600°C.
p r e s s u r e on the above t e m p e r a t u r e s is not known.
The e f f e c t of
U n d e r t h e s e c o n d i t i o n s the
p r e f e r r e d c y c l e would s e e m to be slow h e a t i n g with a p p r o p r i a t e i n t e r m e d i a t e
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anneal temperatures selected by estimation of melting points of the intermediate compounds. The preferred synthesis conditions for the four compounds that were successfully synthesized are listed in Table 3. In most cases more complete reaction could be obtained by recycling as follows: The ingot from the first cycle (generally containing 2 or 3 phases), was ground, repelleted, and repressed with heating times of 6-24 hours.
The second cycle gave information
about the decomposition temperature of the compounds under pressure since samples from cycle I, which contained large amounts of arsenopyrite compound, were converted to the elements when heated above ~ 1100°C for OsSbSe and OsSbTe and ~ 1000°C for OsBiSe. These data set the upper limits for the holding temperatures for the preferred conditions. In all cases, the temperature cycle was the critical parameter as long as the applied pressure was greater than 37 kbar; therefore the pressure was not varied over wide ranges.
TABLE 3
1st Cycle
Compound
Pressure (kbar)
Heat Rate (hrs)
Holding Temp. (°C)
2nd Cycle
Time (hrs)
Heat Rate
Holding Temp. (°C)
Time (hrs)
OsSbSe
45-47
Slow 1-2
9501050
6-8
Slow hold at 550 °
9501000
12 -24
OsSbTe
42 -47
Fast 1/2
9751050
6-8
Fast 1/2
950I000
26-24
OsBiSe
44-47
Slow 2-3
900 ± 20
6-8
Slow
950 ± 25
6-24
RuBiSe
45-46
Slow 1-2
850900
8
The O s - S b - T e m i x t u r e w a s the only t e r n a r y that could be h e a t e d p r o m p t l y to the m a x i m u m t e m p e r a t u r e with s u c c e s s f u l r e s u l t s .
I n c o m p l e t e r e a c t i o n s in
the s u c c e s s f u l p r e p a r a t i o n s gave m i x t u r e s of MY2, X2Y 3 and s o m e of the desired monoclinic compound.
U n d e r all c o n d i t i o n s a t t e m p t e d , the t e r n a r y
ARSENOPYRITE-TYPE TERNARY COMPOUNDS
8
a
Vol. 1. No. 1
b FIG. 1
Two a r e a s f r o m the same ingot of OsSbSe (HPTGR). (a) Bar cut f r o m center of ingot, single phase and uniform. (b) A fragment f r o m the edge showing definite second phase inclusions which has been shown by m i c r o p r o b e analysis to be Os.
mixtures, which w e r e not synthesized successfully, contained only MY2, X2Y 3 and the e l e m e n t s .
At p r e s s u r e s of 35-37 kbar, s amp les containing Bi generally
"blew out" even with slow heating. The extent of r e a c t i o n after various synthesis conditions was d e t e r m i n e d p r i m a r i l y f r o m X - r a y D e b y e - S c h e r r e r powder patterns;
additional information
was obtained f r o m visual and metallographic examination and f r o m e l e c t r o n beam m i c r o p r o b e analysis of polished sections of the ingots.
Samples, whose
powder p a t t e r n s indicated that only the monoclinic phase was p r e s e n t when examined by the optical methods, frequently showed small islands of impurities. These i m p u r i t i e s w e r e usually found at the p e r i p h e r y of the cylindrical ingots, so that bars cut f r o m the center for e l e c t r i c a l m e a s u r e m e n t s appeared to be f r e e of u n r e a c t e d m a t e r i a l s or i n t e r m e d i a t e s .
(See Fig. 1.)
The Knoop m i c r o h a r d n e s s of the t e r n a r y phase ( m e a s u r e d with a Leitz Miniload h a r d n e s s t e s t e r ) was 672 k g / s q m m for OsSbSe and 196 k g / s q mm for OsSbTe. phases.
OsBiSe and RuBiSe w e r e never obtained as apparently pure monoclinic
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P r o p e r t i e s of the New P h a s e s The s t r u c t u r e , l a t t i c e p a r a m e t e r s , and e l e c t r i c a l p r o p e r t i e s of the new t e r n a r y p h a s e s w e r e studied.
The D e b y e - S c h e r r e r p o w d e r p a t t e r n s w e r e m a d e
in a 114.6 m m N o r e l c o c a m e r a with C u I ~ r a d i a t i o n and e x p o s u r e s of 8-12 h o u r s . The d - v a l u e s and sin2e v a l u e s w e r e c a l c u l a t e d by a p r o g r a m f o r the 7090 c o m p u t e r f r o m the line p o s i t i o n s d e t e r m i n e d with a f i l m - m e a s u r i n g d e v i c e .
The
m a j o r s o u r c e of e r r o r is in the d i f f u s e n e s s of the lines, which set the e r r o r l i m i t s of • 0.03 f o r the l a t t i c e p a r a m e t e r v a l u e s shown in Table 4.
The r e s u l t s
of the X - r a y s t u d y on the t h r e e c o m p o u n d s a r e c o n s i s t e n t with the a s s i g n m e n t of the m o n o c l i n i c a r s e n o p y r i t e s t r u c t u r e a s s t a t e d by Hulliger (2).
The lattice
p a r a m e t e r s a r e a l s o c o n s i s t e n t and in logical s e q u e n c e with his v a l u e s f o r o t h e r m e m b e r s of this s e r i e s .
The d e n s i t y v a l u e s w e r e obtained u s i n g a B e r m a n
b a l a n c e and toluene a s the d i s p l a c e m e n t liquid.
The high d e n s i t y of OsBiSe
(as shown by the high n u m b e r of a t o m s / u n i t cell) is the r e s u l t of s o m e f r e e Os with its d e n s i t y of 2 2 . 5 g / c c .
The s a m p l e s of RuBiSe w e r e n e v e r u n c o n t a m i n a t e d
enough to allow d e t e r m i n a t i o n of the l a t t i c e p a r a m e t e r s . TABLE 4 a
Compound
o
b
o
c
o
8
(deg)
Density (g/ca)
(~)
(/~)
(/~)
OsSbSe
6.38
6.34
6.42
113.4
10.404
11. 855
OsSbTe
6.57
6.62
6.66
113.1
10.963
12. 008
OsBiSe
6.52
6.46
6.57
113.8
11.034
13.2
n
The e l e c t r i c a l p r o p e r t i e s of the t e r n a r y compounds w e r e d e t e r m i n e d on s m a l l r e c t a n g u l a r b a r s cut f r o m the c e n t e r of the ingot p a r a l l e l to the a x i s .
The
r e s u l t s of t h e s e m e a s u r e m e n t s on f o u r s p e c i m e n s that w e r e e s s e n t i a l l y f r e e of u n r e a c t e d m a t e r i a l s a r e shown in Table 5.
The b e s t s p e c i m e n is obviously
OsSbSe (HPTGR) s i n c e its high r e s i s t i v i t y r a t i o i n d i c a t e s the l e a s t a m o u n t of impurities.
A t t e m p t s w e r e m a d e to m e a s u r e the Hall valtage.
In f i e l d s of
6.3 k g a u s s and with c u r r e n t s of up to 10 ma, no Hall v o l t a g e s could be m e a s u r e d down to 5 × 10 -5 uv.
The t h e r m o e l e c t r i c - p o w e r r e s u l t s a r e c o n s i s t e n t with
H u l l i g e r ' s r e s u l t s on o t h e r c o m p o u n d s .
H o w e v e r , all t h e s e e l e c t r i c a l p r o -
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p e r t i e s a r e s e n s i t i v e to i m p u r i t y l e v e l s and to the n u m b e r of g r a i n b o u n d a r i e s , so t h e y s h o u l d be r e g a r d e d m e r e l y a s i n d i c a t i o n s .
A t t e m p t s w e r e m a d e to
d e t e r m i n e the o p t i c a l a d s o r p t i o n edge to o b t a i n a value of the e n e r g y gap. H o w e v e r , s a m p l e s a s t h i n a s 60 u w e r e c o m p l e t e l y opaque to light with w a v e l e n g t h s f r o m 0 . 5 to 25 u.
H u l l i g e r (2) w a s unable to m e a s u r e e n e r g y gaps
e i t h e r by e l e c t r i c a l or by d i f f u s e r e f l e c t a n c e t e c h n i q u e s f o r O s A s S e and O s A s T e . TABLE 5 P300OK (ohm-cm)
077OK (ohm-cm)
o77OK/P300OK
Type
(uv/°C)
OsSbSe HP7GR
I. 59
3 . 7 6 × 102
237
P
137
OsSbSe HP9GR
1 . 2 4 x I0 - I
i . 18
9.5
P
157
OsSbTe HP7
8 . 2 2 × 10 -2
4 . 5 9 × i0 - I
5.6
P
138
OsSbTe HP10GR
3 . 8 × 10 -2
2 . 2 8 × I0 - I
6.0
P
188
Sample
Discussion It is difficult to u n a m b i g u o u s l y a s s i g n the f u n c t i o n of high p r e s s u r e in the s y n t h e s e s of t h e s e t e r n a r y c o m p o u n d s .
The m o s t p e r t i n e n t e v i d e n c e is f r o m
a s t u d y of the s t a b i l i t y of OsSbSe and O s S b T e w h e n h e a t e d in v a c u u m - s e a l e d q u a r t z c a p s u l e s at c o n d i t i o n s s i m i l a r to t h o s e u s e d by H u l l i g e r to s y n t h e s i z e his compounds.
A f t e r h e a t i n g at 600°C f o r 6 h o u r s , t h e r e w a s no change in
the s a m p l e s that could be d e t e c t e d v i s u a l l y or with X - r a y p o w d e r p a t t e r n s . H o w e v e r , the s a m e s a m p l e s a f t e r 6 h o u r s at 800°C gave a m e t a l l i c m i r r o r and an X - r a y p o w d e r p a t t e r n that w a s p r i m a r i l y Os.
Thus, t h e s e c o m p o u n d s a r e
d e f i n i t e l y not s t a b l e u n d e r the a t m o s p h e r i c p r e s s u r e s y n t h e s i s c o n d i t i o n s . H o w e v e r , we cannot s a y w h e t h e r t h i s i n s t a b i l i t y is c a u s e d by v a p o r p r e s s u r e of the X and Y c o m p o n e n t s or by s i z e r a t i o of the a t o m s . F o r e x a m p l e , c o n s i d e r the d i f f e r e n c e in the m e t a l l i c r a d i i (10) b e t w e e n the Group V(X) a t o m s and the Group VI(Y) a t o m s :
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ARSENOPYRITE-TYPE TERNARY COMPOUNDS
11
Sb-Se = 0 . 2 2 ~, S t - T e = 0 . 0 2 /~, B i - T e = 0 . 1 4 ~, and Bi-Se = 0 . 3 4 ~ . The Os t e r n a r y c o m p o u n d s of all but B i - T e w e r e s y n t h e s i z e d u n d e r p r e s s u r e , e v e n t h o u g h its s i z e d i f f e r e n c e is i n t e r m e d i a t e in the s e r i e s .
If v a p o r p r e s s u r e
of the X or Y e l e m e n t is the c o n t r o l l i n g f a c t o r , t h e n why can RuSbSe and RuSbTe be m a d e at a t m o s p h e r i c p r e s s u r e while the Os a n a l o g s r e q u i r e ~ 45 k b a r for t h e i r s y n t h e s i s ? An e x p l a n a t i o n m a y be that the u p p e r t e m p e r a t u r e l i m i t of s t a b i l i t y ( p e r i t e c t i c d e c o m p o s i t i o n t e m p e r a t u r e ) of t h e s e c o m p o u n d s is below the 700-900 ° r a n g e n e e d e d to c a r r y out the d i f f u s i o n c o n t r o l l e d s y n t h e s i s at atmospheric pressure.
At h i g h e r p r e s s u r e s ,
this decomposition temperature
is r a i s e d to the 1 0 0 0 - 1 1 0 0 ° C r a n g e and thus the s y n t h e s i s is s u c c e s s f u l . The e l e c t r i c a l p r o p e r t i e s of t h e s e c o m p o u n d s as s y n t h e s i z e d a r e u n u s u a l . T h e y show s e m i c o n d u c t i n g b e h a v i o r s i n c e t h e y have r e l a t i v e l y high r e s i s t i v i t i e s , n e g a t i v e t e m p e r a t u r e c o e f f i c i e n t of r e s i s t i v i t y and high t h e r m o e l e c t r i c p o w e r . H o w e v e r , t h e y do not have m e a s u r a b l e Hall c o e f f i c i e n t s , s u g g e s t i n g that t h e y have v e r y high c a r r i e r c o n c e n t r a t i o n s , e x c e e d i n g 1 0 2 0 / c c b a s e d on a o n e c a r r i e r e q u a t i o n f o r the Hall c o e f f i c i e n t .
The high c a r r i e r c o n c e n t r a t i o n is not
c o n s i s t e n t with the o t h e r e l e c t r i c a l p r o p e r t i e s , w h i c h s u g g e s t s that the o n e c a r r i e r m o d e l d o e s not apply.
Since t h e y have not b e e n p r e p a r e d a s s i n g l e
c r y s t a l s and a r e not i m p u r i t y f r e e , it is not c l e a r w h e t h e r t h e s e e l e c t r i c a l p r o p e r t i e s a r e t h o s e of the c o m p o u n d s t h e m s e l v e s or a r e a r e s u l t of the high i m p r u i t y level. We w i s h to thank Dr. V. S a d a g o p a n of the M. I. T. M a t e r i a l s S c i e n c e C e n t e r for b r i n g i n g t h e s e c o m p o u n d s to our a t t e n t i o n and we a r e g r a t e f u l to M r . T. Stack f o r his a s s i s t a n c e in p r e p a r i n g s a m p l e s , to M r s . M. J. Button f o r r e a d i n g the X - r a y p o w d e r p a t t e r n s and to M i s s M. C. F i n n for the e l e c t r o n beam microbe analyses.
We a l s o thank D r . A. J. S t r a u s s f o r his a d v i c e and
his help with the e l e c t r i c a l m e a s u r e m e n t s . References 1. F. H u l l i g e r , P h y s . L e t t e r s 4, 282 (1963). 2. F. H u l l i g e r , N a t u r e 201, 381 (1964). 3. F. H u l l i g e r and E. M o o s e r , J. P h y s . C h e m . Solids 2___66,429 (1965). 4. H. Hahn and W. K l i n g e r , N a t u r w i s s 52, No. 17, 494 (1965).
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5. M. D. Banus, T. B. Reed, H. C. C-atos, M. C. Lavine and J. A. Kafalas, J. Phys. Chem. Solids 23, 971 (1962). 6. D. H. Killpatrick, J. Phys. Chem. Solids 2_55, 1213 (1964). 7. D. S. Chapin, J. A. Kafalas and J. M. Honig, J. Phys. Chem. 69, 1402 (1965) 8. M. D. Banus, Susan Nye Vernon and H. C. Gatos, J. Appl. Phys. 36, 864 (1965). 9. S. V. Radcliff, M. Schatz and S. A. Kulin, J. Metals 12, 731 (1960). 10. L. Pauling, The Nature of the Chemical Bond, Third Edition, p. 403. Cornell University P r e s s , Ithaca, New York (1960).