2200
J. Phys. Chern. 1980, 84, 2200-2203
X-ray Spectroscopic Study of Chromium, Nickel, and Molybdenum Compoundst A. Manthiram, P. R. Sarode, W. H. Madhusudan, J. Gopalakrishnan, and C. N. R. Rao" Solid Stafe and Structural Chemistry Unit, Indian Institute of Science, Bangalore 5600 12, India (Received: November 13, 1979)
Chemical shifts in the K-absorption edges, AE,of a series of chromium, nickel, and molybdenum compounds have been investigated. The AE values in a given series vary in the same direction as the metal-core-levelbinding energies obtained from X-ray photoelectron spectroscopy. The AI3 values are related to the effective atomic charge of the metal by a parabolic relation. In the case of molybdenum compounds, the chemical shifts of the K, emission lines vary in the same manner as M.
1. Introduction A recent study' of the X-ray absorption spectra of manganese, iron, and cobalt compounds in this laboratory indicated that the variation of the chemical shifts, hE, of the K-absorption edge of transition metals could be satisfactorily correlated with the oxidation state or the effective charge, q , of the absorbing metal ion. The functional relation between AE and q was given by the expression
AE
= aq
+ bq2
(1)
which could also be justified on theoretical grounds. We considered it worthwhile to extend our studies of X-ray absorption spectra to other transition metal compounds in order to establish the nature of the variation of AE with q across the transition metal series and also to substantiate such correlations by relating AE with the metal-core-level shifts found in the X-ray photoelectron spectra (XPS). For this purpose, we have measured the K-absorption edges in several chromium and nickel compounds as well as the metal 2p binding energies from XPS. More importantly, we have investigated chemical shifts of both the K-absorption edge and the K, emission lines in a variety of molybdenum compounds (with the formal oxidation state of Mo varying between 0 and +6) particularly in view of the significance of such studies to biology.2 Having found that the chemical shifts vary linearly with the core-level binding energies of molybdenum obtained from XPS, we have correlated these chemical shifts with the effective atomic charge of molybdenum. 2. Experimental Section G O 3 , K2Cr04, K2Cr207,Ni(PPh3)2(CO)2,K 2 D W " I , and NiC12.6H20 were high purity samples obtained commerically. LaCr03, L~.gSro,,Cr03, Crz03, and LaNi03 were prepared by literature methods.,~~CrS and NiS were prepared by methods already reporteda5 Ba2Nip05was made by the method of Arjomand and Machin.6 GdzMo06, Ce2Mo06, GdMo04, SrMo04, and MgzMo,08 were prepared by solid-state reaction of the corresponding binary oxides a t elevated temperatures. MOO,, Gd2Mo05,SrMOO,, ZnMoO,, and Gd2Mo3Ogwere prepared by hydrogen reduction of the corresponding molybdenum(V1) compounds. MoS3 and MoSz were made from (NH4),MoS4 by literature r n e t h ~ d s .MoC13 ~ and Mo(CO), were obtained from Climax Molybdenum Co. X-ray absorption and emission spectra were recorded with a bent crystal X-ray spectrograph. The experimental details are given in our earlier paperes The dispersion in 'Communication No. 57 from the Solid State and Structural Chemistry Unit. 0022-3654/80/2084-2200$01 .OO/O
the setup was 4.90 X.U. mm-I on the films and 0.049 X.U. mm-' on the photometer records (1X.U. = 1.00202 X A). X-ray photoelectron spectra were recorded with an ESCA-3 Mark-I1 spectrometer of VG Scientific Ltd. U.K. by using A1 K, (1486.6 eV) radiation. 3. Results and Discussion Chromium and Nickel Compounds. Although the chemical shifts, AE, in the K-absorption discontinuities of a few chromium and nickel compounds have been reported in the literature,+" data on some of the key compounds were not available. We have measured the AE values for important key compounds including several chromium compounds with Cr in the 2+, 4+, and 6+ oxidation states and nickel compounds with Ni in the 0 and 3+ oxidation states. In Table I we have listed the AE values from our studies and also those taken from the literature. Although AE generally increases with the oxidation state of the metal, we cannot obtain an exact relation between the two since compounds with the metal in the same oxidation state show a wide range of AE values. This is because AE is determined by the effective charge on the metal rather than the formal oxidation state. T o establish that AE indeed reflects the effective charge on the metal atom, we have plotted the A E values against the metal 2p3,2 binding energies (obtained from XPS) for representative compounds (Figure 1). The plots are fairly linear with correlation coefficients of -0.95 and a standard deviation of 0.4. We, therefore, considered it appropriate to correlate the AE values with the effective atomic charge of the metal atom, q, calculated by Suchet's method.12 In principle, we could also relate the core-level binding energies from XPS with q , but the changes in these energies are considerably smaller than AI3 from K-absorption edge measurements. We have plotted the AE values of chromium and nickel compounds against q in Figure 2. Least-squares analysis of the data in this figure with both linear (AE= aq) and parabolic functions of q shows that the parabolic function as described by eq 1 gives higher correlation coefficients, y, and lower standard deviations, u. The least-squares-fit parameters are shown in Table 11, where we have also listed the parameters for the data on manganese, iron, and cobalt compounds studied earlier by us.' The parabolic relation seems to be superior to the linear relation in all of the systems studied. From theoretical considerations,' it was shown earlier that AE = 2afzeff"'fq+ afl2 (2) where Zeffmxfis the effective nuclear charge seen by an electron of the metal atom in the final level and af is a constant related to the quantum defect. Comparing eq 1 and 2, we find that the ratio of aj2b should correspond 0 1980 American Chemical Society
The Journal of Physical Chemistry, Vol. 84, No. 17, 1980 2201
X-ray Spectroscopy of Cr, Ni, and Mo Compounds
TABLE I: Chiemical Shifts of K-Absorption Discontinuities of Some Chromium and Nickel Compoundsu chromium compds
no. 1
formal oxidation state
compd CrS LaCrO, Cr103 La,,,Sr,.,CrO, CrO, K,CrO, CrO, K,Cr 72' CrSe CrTe CrF, CrC1, CrBr, CoCr,O, CoCr,S, CuCr,S, CuCr,Se, CuCr, 0, CrA NaCrO,
2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23
A E , ~eV
2t
4.96 9.10 9.20 10.50 14.00 20.90 20.97 21.20 2.52 0.69 9.50 8.02 2.71 9.01 6.72 6.80 1.70 8.99 7.60 9.08 9.23 20.09 19.81
%It ?it
3.1t 4 t Bt Bt 6+
f! + 2!t
%It
3t 3t ?;t
9t 3149t %I t
?It
K,CrF,
3t B t
Na,Cr,07 Cs, Cr ,0,
6t Bt
nickel compds
compd Ni(PPh,),(CO), NiO La,NiO, Ki,[Ni(CN),I N1C1,*6H1O NiS LaNiO, Ba,Ni,O, NiSe NiF, NiBr, NiI, NiMn,O, KNiF, NiAs NiAs, Nip, NiSO, Zn[ Ni( CN), ]
4 1.21 1.89 1.89 2.10 2.41 3.48 3.48 3.48 0.72 0.24 1.92 1.65 0.72 1.77 1.55 1.55 0.54 1.77 1.67 1.89 1.92 3.48 3.48
formal oxidation state A E , eV ~ 0 2t 2t 2t 2t 2t 3t 3+ 2t 2t 2t 2t 2t 2t 3t 4t 4t 2t 2t
4
0.7 6.69 6.31 9.00 5.80 5.32 11.00 10.62 2.10 7.00 3.00 0.49 6.94 7.20 2.40 1.30 11.39
1.22 1.22 0.54 0.24 1.83
11.30 9.51
1.44
1.22 1.22 1.44 1.07 1.03 1.80
1.80 0.50 1.22
0.50
Data for compounds 1-8 are obtained by us while the data for the remaining compounds are from the literature. Chemical shift, A E = EK-edge(compdI)- EKdge(metal). EKedge(Cr metal) = 5988.85 i: 0.50 eV; EKmedge(Nimetal) = 8331.20 f 0.50 eV.
r-'
I
r
I
/
t'$
15-
z
LL--
u
I
l z
L
853 .I2
L
u V E
-e> 1 0 6
12
16
20
-
'"7-
w
2L
a
12 0
AE(eV) k
1
5
q
1
575t 4
I
I
8
12
dI 16
20
2L
A E lev) Figure 1. Plots of metal (2p3,,) binding energies from XPS against chemical shifts of the K-absorption edge, A€" for chromium and nickel compounds: (a) rilckel compounds, (b) chromium compounds. Numbers refer to the compounds in Table I. Full lines are the least-squares fits of the data.
Figure 2. Plots of the chemical shifts, A€, of chromium and nickel compounds against the effective atomic charge, q : (0) chromium compounds; (0)nickel compounds. Numbers refer to the compounds in Table I. The solid curves were obtained by the least-squares fii of the data to eq 1.
to Zeffm,! The values of a / 2 b for the various transition metals in their compounds are listed in Table 11. There appears to be no obvious systematics in the a / 2 b values across the first-row transition metal series. Molybdenum Compounds. Results of our measurements On are presented in II1, where we have listed chemical shifts of both the Mo Kabsorption edge, AE, and the Mo K, emission line, 6E.
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The Journal of Physical Chemistry, Vol. 84, No. 17, 1980
Manthiram et al.
TABLE 11: Least-Squares Fit Parameters of the Chemical Shift in K-Absorption Edge A E with the Effective Charge q on the Metal Atoma
A E = aq element
n
a
U
Cr Mn Fe
a
23 5.45 1.35 12 5.10 0.94 12 5.80 0.74 co 9 7.36 1.06 Ni 16 5.88 0.58 Mo 16 5.25 1.04 10 0.07 0.018 Mob n is the number of compounds studied. 6E
AE = aq
no.
compd
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24
MOO, Gd,MoO, SrMoO, MoO,(acac), MoS, ("4)ZMoS4 H0.36MoO3 H1.0M003
232
formal oxidaCion state AE,a eV 6E,&eV 6t 6t 6t 6t 6t 5.64t 5t 5t 5t 5+ 4+ 4t 4+ 4t 4t 4t 4+ 4t 4t 3t 3t 2+ 0
t 15.22
+15.01 t 15.88
t13.00 t 1.51 + 7.52 t 13.20 +12.50 t 12.41 t12.61 t 10.70 +12.31 t 11.81 t 12.41 t 11.40 t 9.92 t 8.63 t 8.60 t 6.33 t 1.94 t 7.04 t4.11 t1.32 t 0.24
q
-0.23 -0.21 -0.20 -0.17
2.90 2.70 2.80
-0.11
1.49 2.72 2.42 2.44 2.10 2.04 1.95 1.97 1.95 1.94 1.95 1.71 1.69 1.71 0.47 1.31
0.45 0.44 0.44 0.92 0.46 1.04 0.013
,
I
( A E ) and Ka, Emission Line ( 6 E )of Some
Molybdenum Compounds
U
a/2b 2.59 1.21 1.75 2.51 2.58 8.20
Y a b 0.978 3.56 0.69 0.974 2.85 1.18 0.977 3.89 1.11 0.976 5.42 1.08 0.983 4.59 0.89 0.961 4.59 0.28 0.926 0.038 0.013 values (Mo Kcu emission line).
TABLE 111: Chemical Shifts of Mo K-Absorption Edge
+ bqz Y 0.998 0.995 0.993 0.984 0.991 0.964 0.968
,
I
-
-> 2
4
i
230-
i
-0.16 Ce,MoO, GdMoO, -0.14 MoClSC MOO, -0.13 Gd,MoO, Mg,M03O, SrMoO, -0.12 ZnMoO, -0.12 Gd,Mo30, MoC1,C -0.09 MoS, MoSeZC -0.08 MoC1, -0.04 MoI, MoI, -0.02 -0.02 Mo(CO), a EKmedge(molybdenum metal) = 20 003.59 0.50 eV. E K ~emission , (molybdenum metal) = 1 7 479.32 f 0.20 eV. The shifts in both KCY , and KCY,emission lines are almost the same. Data from the
1 1
I 5
I 10
I 15
AE leVl
Figure 3. Plot of Mo(Bd,,) binding energy from XPS against chemical shift, A€, of the Mo K edge. Plot of Mo(Sd,,) binding energy against the shift, 6 € , of the Mo K, emission is shown in the insert. Numbers refer to the compounds listed in Table 111. Full line gives the leastsquares fit of the data.
I
I
I
'01
-1
+_
Some of the molybdenum compounds show rather unusual AB (as well as 6E) values differing significantly from the values in other compounds containing the metal in the same oxidation state. For example, Mooz and MgzMo308 show slightly higher AE values than other 4+ molybdenum oxides. This is probably due to the presence of metalmetal bonding in these compounds. Copper compounds with metal-metal bonding like cupric acetate also show relatively high hE values.13 In molybdenyl acetylacetonate the oxidation state of molybdenum has been reported 14,15 as 4+ or 6+. The AE value of 13 eV shows that the oxidation state of molybdenum is 6+. Amorphous MoS3 gives a very low AE value indicating that the oxidation state of molybdenum is not 6+ as one would believe from the molecular formula. Accordingly, we find the following interesting features in the XPS of MoS3: (i) In the valence band region, the spectrum is closer to MoS2 than to (NH4)2M~S4, showing a distinct Mo(4d) band at 1.9 eV and a split S(3p) band (4.0 and 5.0 eV). (ii) The sulfur (3s) and (2p) bands both are split (12.7, 16.3 eV and 161.6, 162.8 eV, respectively) similar to ZrS26 and C U S . ~These features together with the small AI3 value of the K-absorption edge indicate an oxidation state con-
A E (ev)
Figure 4. Plot of chemical shifts of the Mo K, emission, 6 E , against the chemical shifts of the Mo K-absorption edge, A€. Numbers refer to compounds in Table 111. Full line gives the least-squares fit of the data.
siderably lower than 6+ (possibly 4+) and the presence of both S2-and S2-species in the compound. MoS, may, therefore, be formulated as M O ~ + S ~ - ( Ssimilar ~ ~ - ) to the zirconium16 and niobium1' analogues. The presence of molybdenum in the 4+ state together with the amorphous nature of this compound may be responsible for the low AE value. I t has indeed been shown8 that amorphous solids give considerably smaller shifts of the K-absorption edge than the corresponding crystalline compounds. In Figure 3, we have shown the plots of AE as well as 6E against the Mo(3d5I2)binding energies (from XPS) in representative compounds; the plots are quite linear with correlation coefficients of 0.98 and 0.95, respectively (and
The Journal of Physical Chemistry, Vol. 84, No. 17, 1980 2203
X-ray Spectroscolpy of Cr, Ni, and Mo Compounds
correlation coefficient are similar for linear and parabolic fits of the AE data in the case of Mo compounds. The chemical shifts of the Mo K, emission line, 6E, however, give a better fit with the parabolic function than the linear function, the correlation coefficient being considerably higher in the former case (see Table 11). The value of a/2b is largest in molybdenum compounds among the transition metal compounds studied by us, and it is possible that Zeffmpfincreases as we go down the triad. We feel that the correlations of AE and 6E shown in Figure 5 should be useful in the estimation of the effective atomic charge on molybdenum in new compounds. 5
-4
Acknowledgment. We thank the Council of Scientific and Industrial Research and the Department of Science and Technology, Government of India, for support of this research. References and Notes
i/
1 1
2
3
9
Flgure 5. Plot of chemical shifts of the Mo K-absorption edge, A€, against the effective atomic charge, q. Plot off the chemical shift, 6 € , against q Is shown in the inset. Numbers refer to compounds listed in Table 111. The solid curves were obtained by the least-squares fit of the data to eq 1.
standard deviations of 0.36 and 0.70). It is interesting that AE and 6E are themselves linearly related to each other (Figure 4), the relation being IdlEl = 0.0127AE 0.0008 eV (3)
+
with a standard deviation of 0.017 and a correlation coefficient of 0.96. The AE values are, however, much higher in magnitude than the 6E values or the shifts in the core-level binding energies in XPS. Variation of aE with q is shown in Figure 5. From Figure 5, we see that aE varies almost linearly with q; a least-squares fit of the data to the parabolic equation (eq l),therefore, gives a small value of b (Table 11),the value being much smaller than that in chromium, which is the first member of this triad. Accordingly, the values of the standard deviation and the
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