Inorganic Chemistry, VoZ. 10, No. 4 , 1971 719
NOBLEMETALOXIDES
CONTRIBUTION No. 1617 FROM THE CENTRAL RESEARCH DEPARTMENT, EXPERIMENTAL STATION, E. I. DU PONTDE NEMOURS AND COMPANY, WILMINGTON, DELAWARE19898
Chemistry of Noble Metal Oxides. 11. Crystal Structures of PtCoO,, PdCoOz, CuFeO,, and AgFeO, BY C. T. PREWITT, R . D. SHANNON,* AND D. B. ROGERS
Received November 21, 1969 Refined crystal structure parameters for PtCoOz,PdCo02,CuFeOz (delafossite), and AgFeOz help to explain the high electrical conductivity of PtCoOzand PdCoO2 and support the assignment of valence states for each compound. The coordination polyhedron for P t and P d is a hexagonal bipyramid with oxygen atoms a t the apices and either Pt or Pd in the equatorial positions. The conductivity and short Pt-Pt (Pd-Pd) distances indicate strong metal-metal interactions. The relatively longer Cu-Cu and Ag-Ag distances are consistent with the semiconducting behavior of CuFeOa and AgFeOp. The refinement results show that atom parameters for other similar phases such as PdRhOz can be reliably determined using only the Shannon and Prewitt effective ionic radii and the unit cells of these phases.
Introduction The structure of delafossite, CuFe02, was confirmed by Pabst' to be the same as the structure of synthetic CuFeOz determined by Soller and Thompson.* Although some differences in structural parameters will be noted in the present work, the structure of CuFeOz is essentially correct. However, a certain amount of confusion about delafossite has been introduced into the literature. For example, Wyckoff3 grouped the delafossite structure with the CsClzI and the NaFeOz type structures. Although the rhombohedral space groups of these three structures are the same, the structures themselves are different and should not be indiscriminately grouped together. Doubt about the existence of CuFeO2 was raised by Buist, Gadalla, and White,*who were unable to synthesize CuFeOz and reported that this composition did not exist. This idea was refuted by Wiedersich, Savage, Muir, and S w a r t h o ~ t who , ~ used X-ray, Mossbauer, and chemical analysis techniques to verify the existence of CuFeOz. The work of Wiedersich, et al., has been further substantiated in part I of the present series of papers.6 In addition to the work of Soller and Thompson,2 Pabst,l and Muir, et aZ.,' on the structure of CuFeOz, a neutron diffraction investigation8 confirmed the structure but did not significantly improve the crystallographic parameters. However, the Mossbauer and neutron diffraction work has established that the structure contains Cu+ and Fea+rather than Cu2+and Fez+. Although a number of other AB02 compositions with the same structure are known where A is Cu, Ag, or H and B is Al, Co, Cr, Fe, Ga, or Rh, almost all the structural results reported are the result of an assumed oxy(1) A. Pabst, Amev. Mineual., 81, 538 (1946). (2) W. Soller and A. J . Thompson, Phys. Rev., 47, 644 (1935). (3) R . W. G. Wyckoff, "Crystal Structures," Val. 2, 2nd ed, Wiley, New York, N . Y . ,1884. (4) D. S. Buist, A. M . M. Gadalla, and J. White, Minevat. Mag., 36, 731 (1966). ( 5 ) H. Wiedersich, J . W. Savage, A. H. Muir, Jr., and D. G . Swarthout. ibid., 36, 643 (1968). ( 6 ) R . D. Shannon, D. B. Rogers, and C. T. Prewitt, Inovg. Chem., 10, 713 (1971). (7) A. H. Muir and H. Weidersich, J . Phys. Chem. Solids, 28, 65 (1967). (8) A. Apostolov, God. Sofii Uniu., Fiz. F a k . , 69, 47 (1966).
gen z parameter close to l / g . The only modern leastsquares determinations of oxides with the delafossite structure are those of Hamilton and Ibersg for HCrOz and those of Delaplane, Ibers, Ferraro, and Rush for HCo02."3 I n addition, many of the cell parameters reported in the literature are inaccurate so that calculated interatomic distances are unreliable. In part I6 we reported the synthesis of several new phases having the delafossite structure which contain Pd or Pt in the A position. I n this paper we report the results of crystal structure refinements used to investigate the composition and valence state of PtCoOz and PdCo02 and to provide reliable interatomic distances for CuFeOz and -4gFeOz. This information together with data from the Shannon and Prewitt" table of effective ionic radii and the cell parameters in part I enable us to predict the oxygen parameters very closely for any of the compositions given in part I. Experimental Section Crystals of PtCoOz and CuFeOz used for data collection were prepared hydrothermally as described in part I. Crystals of PdCoOz were produced by the metathetical reaction PdCla 2CoO .-r PdCoOa CoClz, also described in part I. The AgFeOa crystals were prepared hydrothermally by Croft, et aZ.Ia Unit cell dimensions for PtCoOz, PdCoOz, AgFeOz, and CuFeOz taken from part I6 are reproduced in Table I. The space group of these compounds has been thought to be R%n-D8d5 and we have seen no evidence to the contrary, either through examination of X-ray precession photographs taken of each type of crystal or in the least-squares structure refinements. Systematic absences in the diffraction data occur only when --h k 1 = 3%. There are three formula weights per hexagonal cell.
+
+
+ +
Structure Factor Data Since single crystals of these compositions generally are found as thin hexagonal or triangular platelets, crystals were selected and mounted to take advantage of this morphology in order to minimize the transmission factors for the diffraction data used in the least-squares refinements. The crystals referred to in Table I were (9) W. C. Hamilton and J. A . Ibers, Acta Cuystallogr., 16, 1209 (1963). (10) R.G. Delaplane, J. A. Ibers, J. R . Ferraro, and J. J. Rush, J. Chem. Phys., 60, 1920 (1869). (11) R . D. Shannon and C. T. Prewitt, Acta Cuystatlogr., Sect. B , 26, 925 (lQ69). (12) W. J. Croft, N . C. Tombs, and R.E. England, ibid., 17, 313 (1964).
PREWITT, SHANNON, AND ROGERS
720 Inorganic Chemistry, Vol. 10, No. 4 , 1971 CELL
TABLE I DATAFOR ABOz DELAPOSSITE STRUCTURES" P
v,A3
A
(Ag Ka), cm -1
PtCoOn
fi 2.8300 f 4
17.837 f:2
123.80
536
PdCoOs CuFeOn AgFeO2
2.8300 f:2 3.0351 & 1 3.0391 & 2
17.743 2~ 2 17.166 f 2 18.590 f:2
123.04 136.94 148.69
106 102 86
Crystal
a,
c,
Crystal shape
Equilateral triangular plate Hexagonal plate Hexagonal plate Rectangular plate
Dimensions (max X min), m m
Transmission factors
0.10 X 0.009
0.08-0.61
0.15 X 0.018 0.26 X 0.0056 0.10 X 0.034 x 0.022
0.31-0.90 0.58-0.94 0.44-0.82
Space group R8m. The c axes are always perpendicular to the crystal plates.
mounted so that the 4 axis on the Picker diffractometer was normal to the c axis and parallel to a or a* of each crystal. Then the transmission factors were large - except for hkl with very small 1. Reflections were recorded using Pd-filtered radiation from an Ag target X-ray tube, a scintillation detector with PH,4, and a 28 scan range of 2" plus the K a l - K a z dispersion. All crystals were single except for the PtCoOz platelet crystals which were always twinned by a 60" rotation around c. An attempt was made to grind spheres of untwinned, hydrothermally grown PtCoOz crystals, but only irregular egg-shaped crystals were obtained. The small transmission factors and small volumes of these crystals resulted in very unreliable diffraction data so we decided to use the twinned flakes for intensity measurements. Absorption corrections were made using the method of Wuensch and Prewitt.I3 Even with very thin crystals, the absorption corrections are only approximate since even a small error in measurement of the crystal thickness can make a significant difference in the transmission factor. The dimensions of each crystal are given in Table I. For the absorption correction, each hexagonal plate was approximated as a disk. Corrections for the triangular and rectangular plates of PdCoOz and AgFeOz were calculated directly from the dimensions given in Table I.
Refinement
TABLE I1 STRUCTURE FACTORS FOR PtCoOz, PdCoOs, CuFeOs, A N D AgFeOsC
2 1 4 0
0 120 0 137 0 103 60 1 1 45 I 86 1 71 1 51 1 106 2 129 2 88 2 165 2 115 2 74 2 ai 3 63 3 71 3 a8 3 123 4 134 4 72 4 102 4 122 4 101 5 42 5 57 5 69 5 47 5 62 6 133 6 149 6 135 6 109 7 54 7 78 7 66 7 47
1
7
3 0 1
1
2 2
1 3 2 2 1
3 0 2 4 1 2 3 0 1 2 0 0 2
0 0 0 1 1 2 3 4 3 2 1 1
0
0
1 0 4 0 2
1
0 2 I 3 0 4 1 0
2 1
3 1 2 0 0 0 1
1
2 2 0 3 1 3 0
2
0
2 0 3 1 0
1
1 2 0 4 0 3 2
2
1 1 0 0 1 0 4 0 2 1
88
a 129 a aa a 1j4 8 109 8 82 9 71 9 68 9 102 9 133 10 '44 10 76 10 107 10 124 10 a5 11 48 11 66 11 81 11 53 11 69 12 163 12 135 12 96 12 99
171' 102. 59 48
201 305 173 54 43
86
sg
74 53 106 135 91 156 116 76 a3 59 53' 75.
71 55 151 239 149 329 201 121 134
111.
881
146 77 108*
123 811 45 60 f3' 49 66 183. 142. 91' 100
54 77 68. 49
93' 135. 91 166. 117. 83 65 59' 88.
36 33 44
68 237 124 171 205 128 21
29 25 21 28 2ea 231 141 161 44 67 56 41 93 227 154 308
199 139 68 59 96
1140.
110
159' 80 112. 128. 89* 46 62 a1 51 70 149' 123.
266 135 191 230 151 37 51 73 39 57 235 195 129 145
194 295 170 57 47 87 71
52 137 245 149 324 195 124 135 38
36 43 54 271 118 166 200 129 25 28 22 27 26 328 225 ILO
157 46 62 54 42 83 242 153 313 197
139 66 60 91 137 303 137 190
3 4 1 2 1 1 3 0 2 4 1 2 3
0 1 -1
2 4 3 2 2 1 0 0 0 1
0 1
0
0 0 0
0 1 1 1
1 1 1
2 2 2
1 2 0 1
2 3 4 4
2 2
0
3
3
2
3
2 2
3
1
3 3
1
4 1 0
0
1 0
4 2 0 3 1 0 2 1 0
0 1 2 2 3 4 3 2 1
3 1 2
0
0 0 4 1 1 1
2 0 1 1 3 0 2
2 3 4 3 2 2 1
4
0
3 4
4 4
4 4 4 5 5 5 5 5 5 6 6 6 6 6 6 7 7 7
175 97 311 150 97 0
0 47 26
n 102 235 121 354 185 91 106 23 19 22 16 23 27 240 7a 142 191 71 96 34 31 36 61 33 37 262 67 213 103 129 67 9 14 10
7
0
1 0
7 7
2 0
8
15 25 224 123 300 179 94 109 23 15 57
3 1 0 1
a
8
179 99 307 148
99 17' 9" 52 34 121 93 237 121 330 160 91 104 16 10
12 8 16 25 242 79 139 160 69 93 26 26 27 53 25 32 294 67
202 103
12L 67 3 7 10
46 20 222 121
304
122 68
119 66
210 103 65
201 99 66 31 22
30
21 61 45 21 105 160
87 230 124
63 71 4 17 14 34 17 '17 175 62 106 128 53 71 7
61
46 26 100 160 a5 227 123 64 74 9 21 ia
32 9 45 179 62 102 130 55 72 9
8
7
17 21
16 21 12 19 216 53 148 79 94 53 23 16 42 33 19 61 155
10
22 222 53 157 81 95 51 23 17 42 35 15
66 157 90 212 124 68 72 37 31 65 103 192 74 121 143 65 65 12 12 32 51 18 41
86 214
0 2 226 1 2 8 122 174 Structures were refined using a full-matrix, least1 3 66 150 2 3 8 94 0 4 40 0 4 8 106 75 squares program and weighting scheme previously de1 2 26 52 0 3 9 38 0 1 69 20 2 2 9 32 scribed by Prewitt and Sleight.I4 Reflections for which 44 3 1 1 1 9 62 54 2 0 91 57 3 0 9 101 98 the transmission factor was very small were eliminated 0 0 1 0 10 252 271 2b9 198 1 1 4 0 10 107 l o a 193 73 from the refinement. The refinements were routine 84 * 2 2 2 1 lo 130 116 176 172 0 3 0 2 10 224 216 91. 145 145 except for PtCoOz where the data came from a twinned 3 2 10 64 96 96 1 3 10 125 123 a3 crystal. Here i t was possible to select two unique sets 0 4 11 2 17 2 3 11 0 14 5 of reflections from each individual (or similarly oriented 1 2 11 14 13 30 4h 0 1 11 36 50 individuals) plus a set containing contributions from 3 1 11 8 21 5 22 2 0 11 25 37 0 0 12 230 238 both individuals. These were then combined into one 164 163 1 1 12 128 122 I87 175 2 2 12 102 set by using scale factors obtained from each individual 98 75 71 122 115 0 3 12 85 83 set. (See Table I1 for structure factors.) However, s = 0.76. s = 2.07. s = 2.68. s = 1.41. e In each the error calculated for the oxygen position in PtCoOz set the structure factors should be divided by the indicated scale is very large and this is reflected in the interatomic disfactor, s, to give an absolute scale. Asterisks indicate reflections tances discussed below. Table I11 gives the refined omitted from the final cycles of refinement because of absorption parameters and R's [R = Z1lFol - ~ F o ~ ~ / ZW]RF= o ; Or twinningproblems. ((ZwllFol - [F,~~)2/(Z~~Fc~)2)'"] for each of the structures. Discussion Figure 1is a diagram representing the delafossite-type (13) B. J. Wuensch a n d C. T. Prewitt, Z. Krislallogr., Krislallcgeomelrie, structurewith the actual dimensions taken from the Krislallphys., Krislallchem., 128, 24 (1965). PtCoOz refinement. As can be seen in Figure 1, the (14) C. T. Prewitt and A. W.Sleight, Inoug. Ckem., 7, 1090 (1968).
-
Inorganic Chemistry, Vol. 10,No. 4,1971
NOBLEMETALOXIDES
721
TABLE I11 RESULTSO F LEAST-SQUARES REFINEMENT^ PtCOOZ
PdCoOz
CuFeOz
AgFeOz
4 . 1 (1.5) X 2 . 6 (1.3) X lo-' 2 . 4 (0.9) X lo-' 4 . 1 (1.3) X 0.19 (7) 0.16 (4) 0.93 (4) 0 . 7 8 (3) 0.34 (4) 0.46 (3) 0.15 (11) 0.15 (4) 0.1140 (62) 0.1112 (8) 0.1066 (5) 0.1112 (5) 0 . 2 5 (0.09) 0.66 (9) 0.64 (48) 0.34 (15) 0.035 R 0.046 0.051 0.041 0.037 WR 0.074 0.057 0.058 a Standard errors are given in parentheses. C is the secondary extinction parameter.14 Coordinates for the A ion are 0, 0, 0, and for the B ion 0, 0, l/z, and for oxygen 0, 0, z . The R quoted for PtCoOl is for thg reflections included in the least-squares refinement; R for all reflections is 0.072. Units for the isotropic temperature factors ( B ' s )are A2. C B(A ion) B(B ion) z(oxygen) B (oxygen)
TABLE IV INTERATOMIC DISTANCES (A) A N D BONDANGLES(DEG) FOR AB02 DELAFOSSITE STRUCTURES
A-Aa B-BQ A-Ba A-0 B-0 O+-O+" O+-O-
i-.--I Figure l.-PtCoOz
(delafossite) structure,
platinum and cobalt atoms occupy alternate layers in the structure which are normal to c. The platinum atoms are linearly coordinated by two oxygen atoms whereas the cobalt atoms are octahedrally coordinated by oxygen atoms. If the surrounding platinum atoms are also considered as part of the coordination polyhedron, Pt is in a hexagonal bipyramid with oxygen atoms a t the apices and platinum atoms a t the six equatorial positions. This latter coordination has not been explicitly discussed in previous work on delafossite-type structures, but good evidence is given below and in part I11 that there is Pt-Pt interaction in this structure. Hexagonal bipyramidal coordination is rarely found so this in itself is an important aspect of the PtCoO2 structure. Table IV lists the interatomic distances and bond angles for each of the structures examined. Probably the most important information to be gained from these data is support for the formal valence to be assigned to each ion in the structure. Although Pabst pointed out that two-coordinated Cu in CuFe02 strongly suggests the presence of Cu+ rather than Cu2+,the interatomic distances in Table IV further substantiate the presence of monovalent Cu, Ag, Pt, and Pd in these delafossitetype structures. Part I discussed the evidence which established that Cu+ and Fe3+ are present in CuFe02 and this can be substantiated by comparing the Fe-0 distance of 2.03 A calculated from effective ionic radii for six-coordinated Fea+ and four-coordinated oxygen
PtCoOz
PdCoOz
CuFeOz
AgFeOz
2.830 2.830 3.392 2.03(11) 1.89(5) 2.830 2.49 (8)
Distances 2.830 2.830 3.379 1.973(13) 1.908(7) 2.830 2.558 (10)
3.035 3.035 3.355 1.835(8) 2.033(4) 3,035 2.706 (6)
3.039 3.039 3.588 2.067(8) 2.035(4) 3.039 2.707 (6)
Angles 121.1 A-0-B 119.9 95.7 B-0-B 97.3 95.7 O+-B-O+ 97.3 84.2 O+-B-O82.7 Standard errors for these distances parameters.
120.2 122.0 96.6 96.6 96.6 96.6 83.4 83.4 are fixed by the cell
(O(IV)2-) to the value of 2.033 A in Table IV for CuFe02 and 2.035 k for AgFeOz. Using the above radius for O(IV)2-l radii for Cu(II)+ and Ag(II)+ of 0.46 and 0.69 A are also determined. Using the same approach] the Co-0 distances of 1.89 f 5 and 1.908 7 in PtCoOz and PdCoO2 are consistent with 1.905 A calculated for CO(VI)~+LS-O(IV)~-.Then, if the crystals are stoichiometric, Pt and Pd must be formally monovalent. This deduction must be examined carefully because such valence states have not been reported previously. Although this premise is discussed in detail in part 111, it seems appropriate to point out several structural considerations here. As noted in parts I and 111, the electrical conductivities of both PdCoOz and PtCoO2 are very anisotropic and highest when measured in the plane containing the a axis. This suggests metal-metal interaction which is supported by the Pd-Pd and Pt-Pt distances of 2.83 and 2.83 A which are close to those found in the fcc metals (Pd-Pd, 2.75 A; Pt-Pt, 2.77 8). I n contrast] the Cu-Cu and Ag-Ag distances of 3.035 and 3.039 A in CuFeO2 and AgFeOz are considerably longer than in the metals (Cu-Cu, 2.56 k ; Ag-Ag, 2.89 A>. This is consistent because CuFe02 and AgFe02 exhibit semiconducting rather than metallic behavior. Because of uncertainty about whether the composition Pt+Co8+0z or Pt2+o.8C03+0.802 is correct, both models were tried in the least-squares refinements, but the results were inconclusive. Both refinements re-
PREWITT, SHANNON, AND ROGERS
722 Inorganic Chemistry, Vol. 10, No. 4, 1971
Key NaCI-CUBIC '-', - LO^+
6:6:6 0 ~2 Li F e 0 2 - T E T R A G O N A L 6:6:6 0 a NaFeOz-RHOMBOHEDRAL 6:6:6 Cu FeOz- D E L A F O S S I T E 2:6:4 R KFe02-ORTHORHOMBIC 8:4:6 m B - N a F e O p - O R T H O R H O M B I C 4:4:4
' \ , = , a a m MISCELLANEOUS \ \
-B I ~ +
B
\
I 00--pr3+
e e
- Nd3+ - sm3+
\
\
e e
-Eu3+
\ '>
0 9O--y3+
0 0
-TI 3+ 4I
m -Cd2+ -Mn*+
0 0
a LiFeOz Ixl
I I
@
0 80-
0
0
0
0
0
0
El
a
a
a N o FeO2
IXI
0
8 8 tN13+
1
DELAFOSSlTEe
I
i
a?
a
o
I
m
P 0 40
@
m
E
m
E
NaFeO2
'KFe02
m
m
E
E
R
LI'
Nu'
IKC
Rb'
c s+
=Pt'
'Cu'
'Pd
=Ag'
Figure P.--Plot of
YA
zrs.
sulted in R's of 0.04 and the only observed differences were in the refined temperature factors. The trouble here seems to be that the problems of data measurements plus the domination of the intensities by the heavy metals prevent a reliable determination of the meta1:oxygen ratio. There is evidence that when cation sites are shared between two sizes of cations the interatomic distances observed in diffraction experiments are intermediate between those expected for each of the two cations. Similarly, one would expect the Co-0 distance to be smaller than usual if the site contained 20% oxygen vacancies. Since this is not the case, the only conclusion that can be drawn from the
YB
for various AB02 phases.
X-ray measurements is that PtCoOz is nearly stoichiometric. Although the isotropic temperature factors given in Table I11 show a large variation, it is probably not wise to provide much physical interpretation of these numbers. The difficulty of making accurate absorption corrections certainly influences the temperature factors as does the secondary extinction correction. The effective ionic radii of Shannon and Prewittl' can be used to estimate structure parameters for other ABOz delafossites without having to resort to diffraction intensity measurements. If we take, for example, the unit cells of PdRh02, CuRhOz, and AgRhOz from
Inorganic Chemistry, Vol. 10,No. 4, 1971 723
NOBLEMETALOXIDES Table I1 of part I and the Rh(VI)*+radius of 0.665 A, oxygen z parameters of 0.1076, 0.1072, and 0.1117 are found. This results in 1.95 A for Pd-0, 1.83 for Cu-0, and 2.08 A for Ag-0 which compare well with the equivalent distances in Table IV. Radii are also effective in comparing the delafossite structure types with those of other AB02 phases. Figure 2 shows a plot of r A V S . r B for the various known AB02’s. With few exceptions, this plot gives a good
idea of the structure to be expected for a particular composition. It also suggests new compositions which might be synthesized. Acknowledgment.-The authors are grateful to Professor A. Pabst of the University of California for reviewing the manuscript. He also provided us with synthetic crystals of AgFeOz prepared by Dr. W. J. Croft and natural crystals of delafossite, CuFeOz.
CONTRIBUTION NO. 1678 FROM THE CENTRAL RESEARCH DEPARTMENT, EXPERIMENTAL STATION, E.I. DU PONTDE NEMOURS AND COMPANY, WILMINGTON, DELAWARE19898
Chemistry of Noble Metal Oxides. 111. Electrical Transport Properties and Crystal Chemistry of ABO, Compounds with the Delafossite Structure BY D. B. ROGERS, R. D.
SHANKON,*
C. T . PREWITT, AND J. L. GILLSON
Received April 15, 1970 A schematic model of chemical bonding for the ternary oxides ABOp (where A is Cu, Ag, Pd, or P t and B is a trivalent ion such as Cr3+,Fe3+, Cos+, or Rh3+)is described. This model is shown to rationalize several features of the crystal chernistry of these compositions, including their common adoption of the delafossite-type structure. Electrical conductivity measurements as functions of temperature and crystallographic orientation are described and the results are shown to be qualitatively explainable on the basis of the proposed bonding scheme. All of the subject oxides exhibit anisotropic electrical conductivity that is metallic when A is P t or Pd but semiconducting when A is Cu or Ag.
Introduction Recent applications of low-temperature or highpressure synthesis techniques have led to the discovery of several new polynary oxides containing elements of the platinum and coinage groups. I n part I of this series some aspects of the general chemistry of such oxides were considered, and synthesis of ternary compositions having the general formula AB02 (where A is Pd, Pt, Cu, or Ag and B is one of the other transition elements) were described. Structural refinements of several of these AB02 compounds, which are isotypic with the mineral delafossite (CuFeOz), were reported in part 11. A s discussed in that paper, the A ions in delafossite have only two anion near neighbors, while the B ions occur in regular octahedral interstices. Each oxygen is coordinated by four cations, one A and three B, a t the corners of a tetrahedron. The platinum and palladium analogs (PtCoOz, PdCoOz, PdCrOz, and PdRhOz) were the first of the delafossite type to be investigated by us. In addition to being among the first well-characterized oxides in which P t or Pd occurs in combination with other transition metals, the structure and properties of these compositions presented several other unusual features. (1) Twofold, linear coordination by anions had not previously been observed for platinum or palladium. (2) Magnetic susceptibility measurements and Co-0 distances indicated that cobalt in PtCoO; and in
PdCoOz was in a trivalent, low-spin state. This conclusion was supported by the fact that chromium and rhodium could be wholly substituted for cobalt in the palladium analog. Since a trivalent oxidation state is the most stable for both Cr and Rh, there was little doubt that palladium in these compounds was formally present in the highly unusual monovalent state. (Similar reasoning would indicate Pt+ in PtCoOz; however, as pointed out in part I, the possibility of nonstoichiometry in the platinum composition, giving Pt2+0.8C03+0.802, cannot be excluded.) (3) Preliminary electrical resistivity measurements showed that singlecrystal, hexagonal plates of the platinum composition exhibited exceptionally high electrical conductivity (loa ohm-’ em-*) in the plane of the plate (ie., perpendicular to the crystallographic c axis). Such a high metal-like conductivity is, in itself, quite uncommon for oxides, but even more unusual was an apparently large anisotropy of this property. Consideration of these features of the platinum and palladium delafossite phases has led to the development of a qualitative model for the chemical bonding in these oxides that appears to rationalize their crystal chemistry and electrical behavior. General application of this model suggests that PdCoOz, PdCrOz, and PdRhOz should have electrical properties analogous to those of PtCoOB, but that copper and silver analogs should be semiconducting. In order to check this point, we made a careful study
