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Platinum Metal Chalcogenides AARON WOLD Brown University, Providence, R. I. 02912
A number of binary platinum metal chalcogenides show interesting electrical and magnetic properties. These are classified according to structure types and their properties related to the formal valence of the platinum metal. A simple one-electron model proposed by Goodenough to explain the properties of the first row transition metal chal cogenides can be applied to the corresponding platinum metal compounds. This theory is successful in correlating new experimental data on these compounds—e.g., crysta structure, resistivity, Seebeck coefficient, and magnetic susceptibility. *"phe electrical and magnetic properties of simple binary platinum metal oxides (15, 19), as well as a number of ternary compounds (4), have shown that these materials can be of considerable interest to both solid state chemists and physicists. A number of the platinum metal oxides are remarkably good electrical conductors and, in addition, magnetic ordering has been observed (4) for several perovskites containing ruthenium. Many of these compounds can be prepared in the form of single crystals (IS), making them suitable for both transport and magnetic measurements. The platinum metal chalcogenides in general are easier to prepare than the corresponding oxides. Whereas special conditions of temperature and pressure are required to prepare many of the oxides, the platinum metals react most readily with S, Se, and Te. A number of additional differences concerning the chemistry of the chalcogenides and the oxides are summarized as follows: The metal-sulfur (selenium, tellurium) bond has considerably more covalent character than the metal-oxygen bond and, therefore, there are important differences in the structure types of the compounds formed. Whereas there may be considerable similarity between oxides andfluorides,the structural chemistry of the sulfides tends to be more closely related to that of the chlorides. The latter compounds 17 Rao; Platinum Group Metals and Compounds Advances in Chemistry; American Chemical Society: Washington, DC, 1971.
18
P L A T I N U M
GROUP
M E T A L S
A N D
C O M P O U N D S
f o r m as p r i m a r i l y l a y e r - t y p e structures, a n d a n u m b e r of i m p o r t a n t s t r u c t u r e types are f o u n d i n sulfides that h a v e n o c o u n t e r p a r t a m o n g the o x i d e structures—e.g., p y r i t e , m a r c a s i t e , n i c k e l arsenide. F i n a l l y , m a n y of t h e p l a t i n u m m e t a l c h a l c o g e n i d e s b e h a v e l i k e a l l o y s ; t h e elements d o a p p e a r to h a v e t h e i r n o r m a l valencies a n d , i n a d d i t i o n , the
not
compounds
h a v e a w i d e r a n g e of c o m p o s i t i o n a n d s h o w m e t a l l i c luster, r e f l e c t i v i t y , and conductivity. Goodenough
(10)
has a c c o u n t e d for the m e t a l l i c p r o p e r t i e s of
a
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n u m b e r of first r o w t r a n s i t i o n m e t a l c h a l c o g e n i d e s , a n d these ideas are also a p p l i c a b l e to the p l a t i n u m m e t a l chalcogenides.
H e has s h o w n that
t h e f o r m a t i o n of c o n d u c t i o n b a n d s is p o s s i b l e as a result of strong covalent m i x i n g between the cation e
9
a n d a n i o n s,p
a
orbitals. T h i s m i x i n g
a l l o w s sufficient i n t e r a c t i o n b e t w e e n o c t a h e d r a l l y c o o r d i n a t e d cations o n opposite sides of a n a n i o n so that the c o n d i t i o n s for l o c a l i z e d d-electrons break down.
T h e r e s u l t i n g d - l i k e c o l l e c t i v e - e l e c t r o n b a n d s , w h i c h are
a n t i b o n d i n g w i t h respect to the a n i o n array, are d e s i g n a t e d σ * b a n d s . M e t a l l i c b e h a v i o r c a n o c c u r w h e n these b a n d s exist a n d are p a r t i a l l y o c c u p i e d b y electrons. M e t a l l i c c o n d u c t i o n m a y o c c u r also i f c o n d u c t i o n b a n d s are f o r m e d b y the d i r e c t o v e r l a p of t r a n s i t i o n m e t a l t
2g
orbitals.
H e r e a g a i n , these b a n d s m u s t b e p a r t i a l l y o c c u p i e d .
et al. (19)
—
t
2g
Rogers
h a v e a p p l i e d the G o o d e n o u g h m o d e l to a c c o u n t for the elec
t r i c a l b e h a v i o r of a n u m b e r of p l a t i n u m m e t a l oxides.
T h e s e concepts
m a y also b e a p p l i e d to t h e p l a t i n u m m e t a l chalcogenides. It w i l l not b e possible, i n this p a p e r , to d e a l w i t h a l l of the p l a t i n u m m e t a l chalcogenides.
I n s t e a d , a n u m b e r of examples w i l l be c h o s e n a n d
t h e i r e l e c t r i c a l as w e l l as m a g n e t i c p r o p e r t i e s c o r r e l a t e d w i t h the a t o m i c positions i n the v a r i o u s structures f o r m e d . T h e first g r o u p of c o m p o u n d s to b e d i s c u s s e d c r y s t a l l i z e w i t h the p y r i t e structure, w h i c h is s h o w n i n F i g u r e 1.
T h i s s t r u c t u r e is s i m i l a r to the N a C l structure i f w e r e p l a c e
N a b y F e a n d each C I b y an S group. H o w e v e r , the S - S distance w i t h i n 2
R.
Figure 1.
W.
Pyrite structure
G. Wyckoff,
"Crystal Structures," Wiley
(20)
Rao; Platinum Group Metals and Compounds Advances in Chemistry; American Chemical Society: Washington, DC, 1971.
2.
Phtinum
W O L D
Table I.
Metal
19
Chalcogenides
Platinum Metal Compounds with the Pyrite Structure Compounds
Properties
Low Spin RuS , OsS , RhPS, RhPSe,
RuSe , OsSe , RhAsS, RhAsSe, RhAsTe, IrPS, IrAsS, IrPSe, IrAsSe, IrAsTe, N i o . P d o .eAsa, PtAs , PtP„ PtBi RhSe~ , RhS~ , IrS^ , IrSe^ IrTe~ 2
2
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2
2
2
2
rf
S =
6
0 Semiconductor Semiconductor Semiconductor Semiconductor Semiconductor Semiconductor Semiconductor Semiconductor Metallic Semiconductor Metallic Semiconductor Semiconductor Metallic
RuTe OsTe RhSbS, RhBiS RhSbSe, RhBiSe RhSbTe, RhBiTe IrSbS, IrBiS IrSbSe, IrBiSe IrSbTe, IrBiTe PdAs , PdSb PtSb 2
2
2
2
2
2
3
3
3
RhTe~
3
3
3
