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The P K-near edge absorption spectra of phosphates
ELSEVIER
Physica B 216 (1995) 85- 95
The P K-near edge absorption spectra of phosphates R. Franke*, J. Hormes Physikalisches Institut der Universit&t Bonn, Nuflallee 12, D-53115 Bonn, Germany Received 12 April 1995; revised 19 June 1995
Abstract
The X-ray absorption near edge structure (XANES) at the P K-edge in several orthophosphates with various cations, in condensed, and in substituted sodium phosphates have been measured using synchrotron radiation from the ELSA storage ring at the University of Bonn. The measured spectra demonstrate that chemical changes beyond the PO4tetrahedra are reflected by energy shifts of the pre-edge and continuum resonances, by the presence of characteristic shoulders and new peaks and by differences in the intensity of the white line. We discuss the energy differences between the white line positions and the corresponding P 1s binding energies as a measure of half of the energy gap. The corresponding values correlate with the valence of the cations and the intensity of the white lines. The energy positions of the continuum resonances are discussed on the basis of an empirical bond-length correlation supporting a 1 / r 2 - dependence.
1. Introduction
The part of the X-ray absorption spectrum called XANES (X-ray absorption near edge structure) is determined mainly by the atomic geometrical arrangement in a local cluster around the absorbing atom [1,2]. Consequently, this spectroscopy has been currently used in the chemical and structural characterization of disordered systems like amorphous materials (see e.g. Ref. [3]). In this paper we show that chemical changes beyond the first coordination sphere also have a systematic influence on the XANES spectrum. For this purpose we have measured the P K-shell XANES spectra in a series of ortho-, condensed, and substituted phosphates. In a simple manner, orthophosphates are composed of tetrahedral PO4-groups with metallic ca* Corresponding author.
tions (Me) in the void spaces of the crystal structure [4]. The ionic compounds with sum formulas Me3PO4 and Me3(PO4)2 and covalent phosphates MePO4 belong to these species. In condensed phosphates the basic PO4-units are linked by sharing O-atoms forming PxOy-chains of different length and dimension. Moreover, we have investigated substituted Na-phosphates, in which one O-atom of the PO4-units is substituted by a heteroatom (F, S). In order to explain the origin of characteristic absorption features, XANES spectra are commonly subdivided into three sections on the energy scale as shown in Fig. 1 [5]. Region I, the pre-edge region of K-shell spectra below the absorption edge at E0 (II), often exhibits sharp intense resonances (white lines) resulting from electronic transitions of the core electron (i) into unoccupied p-like valence electronic states in the conduction band (CB). It has been shown that
0921-4526/95/$09.50 ~ 1995 Elsevier Science B.V. All rights reserved SSDI 0 9 2 1 - 4 5 2 6 ( 9 5 ) 0 0 4 4 6 - 7
86
R. Franke, J. Hormes/Physica B 216 (1995) 85 95
~
!
Eb,a~q
IZ
IZI
x~s
Eo
/~
Elte
E[i|F Ell)¥
Ew
Eel
E
Fig. 1. Energy relations of core-level binding energies E(i) F and E(i) v referred to the vacuum level E v and the Fermi level E F, respectively, the sample work function ~b, and the different energy regions I, II and II1 of a XANES spectrum with an absorption m a x i m u m at E(whl). The absorption edge E o corresponds to the ionization potential E(i) v. The Fermi level is assumed to be centered in the energy gap E~ between the highest occupied electronic state Eva of the valence band VB and the lowest unoccupied state EcB of the conduction band CB.
the intensity, the energy position and the line shape of the pre-edge resonances are determined by electronegativity and number of the nearest neighbors as well as by the size and the symmetry of the coordination polyhedron around the absorbing atom [6-8]. The weak absorption resonances observed in region III above the core-level ionization potential Eo are discussed as arising from constructive interferences of the outgoing and at the neighbors multiple scattered photoelectron wave function [2]. Consequently, the energy position of these features is very sensitive to internuclear distances. Because of the different physical origin it is interesting to distinguish between pre-edge and continuum resonances. In the case of free molecules the ionization potential E0 can be obtained directly from X-ray photoelectron spectroscopy (XPS) where the binding energy E(i)v( = Eo) is referred to the vacuum level E v (Fig. 1). But it is difficult to distinguish between both types of resonances in spectra of solids because of core-level binding-energy data E(i) v are commonly referred to the Fermi level E F. We assume that E v is centered between the lowest unoccupied electronic state EcB of the conduction band (CB) and the highest occupied state EvB of the valence band (VB). Since the most intense pre-edge resonance called white line in solids is due to a transition of an inner-shell electron into an unoccupied electronic state in CB, the resulting
energy position of the white line E(whl) is higher compared to E(i) F. The difficulty in the determination of absolute electron binding energies or the energy of the absorption edge, respectively, arises from the estimation of the sample work function ~b. Here, we approximate a constant value of q~ = 5 eV for the investigated insulating samples. This value allows in combination with recently published P Is binding energies [9] to distinguish between resonances present in regions I and III in the spectra reported here.
2. Experimental The P K-XANES measurements were carried out at the synchrotron radiation beamline BN3 of the Bonn electron stretcher and accelerator (ELSA) operated in storage ring mode with an electron energy of 2.3 GeV and with an average current of about 50mA [10]. Synchrotron radiation was monochromatized by a Lemonnier-type double crystal monochromator [11] equipped with two InSb(1 1 1) crystals (2d = 748.06 pm). These crystals give an energy resolution of about 0.8 eV at the P K-edge. The monochromatic flux was about 109 photons per energy interval and second. Spectra were taken in the standard mode, detecting the monochromatized beam before (monitor) and behind the sample (detector) with ionization chambers filled with air (45 mbar). The absorption spectra were obtained from the logarithmic ratio of the currents of the monitor and detector chambers. The calibration of the photon-energy scale was determined before and after scanning a sample, using the absorption maximum of Na4P207 as standard value (2152.4 eV). This value and thus the relative energy scale is reproducible to a value better than 0.05 eV. All presented XANES spectra have been taken at photon energies between 2130 and 2190 eV with a step width of 0.03 eV and an integration time of 0.1 s per step. The obtained spectra were normalized in order to compare the intensity of the absorption features quantitatively. For this purpose the pre-edge region of the raw data was fitted linearily and the so determined background was subtracted from the whole
R. Franke, J. Hormes / Physica B 216 (1995) 85 95
spectrum. The absorption jump was set to 1 at a photon energy of about 2180 eV at which the absorption did not exhibit pronounced structures. The intensities of the white lines are given in relative units and correspond to their area. These data were obtained by means of a least-squares fitting routine assuming Gaussian line shapes. The phosphates were available commercially as powders. The condensed phosphates were prepared by tempering acidic Na-phosphates for several hours [12]. Before the powders were prepared on adhesive tape (20 ~m thick) they had been dried at 200°C for 2 h and had been grinded in order to reach a homogeneous thickness. Details of the XPS measurements were published recently [9].
8 7
¥
3. Results
5
o
The P K-XANES spectra of the investigated covalent and ionic orthophosphates are presented in Figs. 2-4. The overall line shape is similar in all spectra. We observe the white line in the energy
9
!
]
!
87
21 45
21 50
21 55
21 60
21 65
21 70
21 75
energy/(eV)
Fig. 3. P K-XANES spectra of the investigated orthophos-
phates with transition-metal cations. T marks a pre-edge resonance.
, !
12 ........... ~....................................................
8
il iii iiiiii iiiiiii
7
6
5
i .... ".......... o! ......... ~ [ ] ~ ~z~ .......
)21 q
4
..............)1/~tI
o
;..............
[]
c
o ~_ o .a o
~
d~
7 6 5 4
2 2
1 1
0
21 45
21 50
21 55
21 60
21 65
21 70
21 75
energy/(eV)
0 21 45
21 50
21 55
21 60
21 65
21 70
21 75
energy/(eV)
Fig. 2. P K-XANES spectra of covalent orthophosphates. A, B,
and C mark continuum resonances, the vertical bars give the XPS P Is-binding energy.
Fig. 4. P K-XANES spectra of the investigated alkaline, earth alkaline, and condensed phosphates.
R. Franke, ,1. Hormes / Physica B 216 (1995) 85-95
88
region between 2151 and 2154 eV and weak absorption resonances above a photon energy of 2155 eV. In general, the K-XANES spectra of tetrahedral coordinated P in orthophosphates resemble those of the majority of K-shell spectra obtained from other atoms with the same coordination to four more electronegative ligand atoms [1, 5-1. Since the PO4-group has Td-symmetry the white line is assigned to a transition of the P Is electron into an unoccupied valence electronic state formed by the overlap of P sp 3 hybrid- and O 2p-orbitals (t~). The experimentally determined E(ls) F values [9] are given in Table 1 and are marked in the absorption spectra by bars on the low-energy side of the white line. We observe characteristic differences in the presented spectra. In comparison with the spectra of the covalent orthophosphates (BPO4, A1PO4, GaPO4 and lnPO4 (Fig. 2)) and the alkaline and earth-alkaline phosphates (Na3PO4, K3PO4 and Ca3(PO4)2 (Fig. 4)) the spectra of the ionic transition-metal phosphates (NaNiPO4, Fe3(PO4)2 and Ni3(PO4)2 (Fig. 3)) exhibit an additional weak spectral feature
denoted as T in the spectra. Previously, Greaves et al. [13] observed the same feature for Fe-phosphates. The energy difference between this resonance and the absorption maximum at E(whl) (Table 1) was determined by means of a leastsquares fit to be 2.9eV (NaNiPO4), 2.7eV (Ni3(PO4)2), and 4.5 eV (Fe3(PO4)2), respectively. Moreover, we observe a relative large F W H M of the white line in the spectrum of Mn3(PO4)2 (3.95 eV, Table 1). However, in contrast to the other spectra, here peak T is absent, which suggests that it is hidden in the broadened white line. It is obviously from Fig. 3 that E(ls) F in Fea(PO4)2 given as a vertical bar overlaps the new pre-edge peak. We observe broadened white lines in the spectra of GaPO4 and InPO4 in comparison to BPO4 and AIPO4 as it is indicated by the according F W H M in Table 1. This result is explained by the polycrystailine composition of these samples. The white line intensities of the spectra are given in Table 1. We obtain the largest values for the covalent phosphates and less intensities for the phosphates of type Me3(PO4)2 and MeaPO4.
Table 1 XPS E(P Is)F binding energies [9], the energy positions E(whl), the absorption intensities I (peak areas), and the full widths at half maximum (FWHM) of the white lines in the reported XANES spectra of phosphates, and the energy differences AE between white line and binding energies (Eq. (3~). Values are given in (eV). I is given in (rel. units) Sample
E{ ls) F
E(whl)
BPO 4 AIPO 4 GaPO 4 lnPO 4 NaNiPO 4 Mn3(PO4) 2 Fea(PO4) 2 Ni3(PO4) 2 Ca3(PO4) z K3PO 4 Na3PO 4 Na4P20 7 NaPO 3 Na2PO3F Na3PO3S
2149.55 2149.1 2148.8 2148.4 2147.7 2148.2 2148.3 2147.7 2147.3 2146.8 2147.05 2147.9 2149.0 2148.55 2146.85
2153.05 2152.8 2152.25 b 2152.35 b 2152.45 c 2152.35 c 2152.6 ¢ 2152.25 ~ 2152.1 c 2151.9 ~ 2152.2 ¢ 2152.4 ¢ 2152.8 c 2152.5 2152.1 b
IS
FWHM b
AE
10.7 (1) 12.2 (1) 11.0 (2)b 10.5 (2)b'd 10.3 (2)~ 8.8 13)¢ 9.7 {2)~ 7.7 (2)" 7.9 (2)' 6.0 (2)c 6.3 (2)d 8.2 (3)d 11.5 (3)d 11.2 (I) 9.4 (2)b
2.1 2.1 2.75 3.5 2.95 3.95 2.4 2.0 2.1 d 3.3 d 3.35 d 3.05 3.5 3.45 3.25
3.5 3,6 3,45 3.95 4.75 4.15 4.3 4.55 4.8 5.1 5.15 4.5 2.8 3.9 5.25
a In brackets: number of fitted peaks incl. the resonances T (transition-metal phosphates) and A (Ca- and alkaline phosphates). b Values correspond to the sum curve of the fitted peaks. c Values correspond to the main component of two fitted peaks. d Value determined without resonance A.
R. Franke, J. Hormes/Physica B 216 (1995) 85 95
Assuming that ~b = 5 eV (see Section 1) we assign all resonances denoted with A, B, B', C and C' as continuum resonances. In all reported XANES spectra resonance C can be identified at about 2168 eV (Table 2). We observe about the same energy of resonances A, B and C for Na3PO4 and Caa(PO4)2, whereas they are shifted in K3POa (Fig. 4). In contrast to the other spectra the feature denoted with A in the spectra of the alkali- and earth-alkaline phosphates (Fig. 4) is visible now as a strong shoulder on the high-energy side of the white line. Its intensity is about half of the whiteline intensity. The most pronounced absorption fine structure in the continuum region is observed in the spectra of B P O 4 , N i a ( P O 4 ) 2 , and C a 3 ( P O 4 ) 2. In comparison to the orthophosphate the formation of P - O - P chains in condensed phosphates corresponds to a chemical variation in the 2nd coordination sphere of P. Moreover, the multiple
89
bond character in the bond to bridging O-species is reduced compared to the terminal bonded O-atoms which is reflected by the corresponding larger bond lengths [4]. The spectra of Na4P207 and NaPO3 are compared with Na3PO4 in Fig. 4. As can be seen, the formation of bridging P - O - P units causes a slight splitting of the white line due to transitions into different unoccupied electronic states (P-O type.) A similar result was obtained by Seikeyama et al. [14] who analyzed the FWHM of the white line in P K-shell spectra of Na-phosphates. The typical shoulder A observed in the absorption spectra of the alkali phosphates (Fig. 4) is shifted to higher energies in the condensed phosphates and therefore is clearly separated from the white line. The continuum resonances occur at similar energy positions (Table 2). As indicated by arrows in Fig. 4, additional resonances B' are observed in the region of 2160-2166 eV going from Na3PO4 to Na4P207 and NaPO3.
Table 2 Modified "term values" E,v (X) with X = A, B, B', C, and C' (Eq. (5)) of continuum resonances in P K-XANES spectra of orthophosphates, condensed, and substituted sodium phosphates, selected interatomic distances from Ref. [4, 29-31], tabulated ionic radii of the cations r~o" [19], estimated P-cation distances rp Me in the studied orthophosphates (Section 4.1) and estimated individual potential contributions V(Me) (Eq. (2)) of the cations to the energy shift AE (whl) (Table 1) of the white-line position. Etv and V(Me) are given in (eV), r is given in (pm).
E,v(B, B')
E,~(C, C')
r
ri."
rp Me
V(Me)
9.1, 12.0
15.1 17.1 16.8 17.2 16.2 16.7 16.5 ] 6.3 16.8 13.9 15.9 17.6 16.9 17. l 5 15.6, 19.4
280" 311" 309 a 332 a
12 39 47 66 102, 70 82 77 70 110 138 102 102 102 102 102
235 262 270 289 309 305 300 293 333 361 325
1.85 1.65 1.6 1.5 1.4 1.4 1.4 1.45 1.3 1.2 1.35
Sample
Err(A)
BPO 4 AIPO 4 GaPO 4 InPO 4 NaNiPO 4 Mn3(PO4) 2 Fe3(PO,,)2 Ni3(PO4) 2 Ca3(PO4) 2 K3PO 4 NajPO 4 Na4P20 7 NaPO 3 NazPO3F Na3POaS
6.6 7.0 4.3 3.5 4.4 3.0
10.5 9.2, 12.1 10.9 8.8
5.2 2.5 2.8 3.7 4.7 4.35 4.8 5.6
11.1 10.3 6.1 9.15 8.2, 9.8 7.8, t0.0 9.9 8.0
'rl, Me: Me = B, A1, Ga, In, Ref. [4]. brP Ni: average values [30]. re ca: average values [29]. ~r e p: Ref. [4]. ~re v: Ref. [4]. frp s: Ref. [31].
285 b 330 c
275 d 275 d 158 e 205 f
R. Franke, d. Hormes / Physica B 216 (1995) 85-95
90 r 6
r
]
. . . . . . . . . . . . . .
140
14
50 2 5q
160
) 6 ~ 2 7C
2175
218C
eqergy/(eV) Fig. 5. P K - X A N E S s p e c t r a of N a 3 P O 4 a n d t h e i n v e s t i g a t e d substituted phosphates.
The XANES spectra of Na3PO3S and Na2PO3F are compared with that of Na3PO4 in Fig. 5. In spite of the drastic chemical changes in the first coordination shell due to varying electronegativities, bonding distances and bonding angles, accompanied by symmetry distortions [4], the measured spectra are very similar. Both substituted phosphates exhibit sharp white lines which have FWHM in the same magnitude but with larger intensity compared to that of orthophosphate (Table 1). This result is altogether astonishing, as we expected new resonances in the white line region arising from transitions into electronic states formed by the P-F - and the P-S - bond, respectively, as recently measured by Redeker et al. [15] in P-oxidesulfides. Only, an asymmetric white line is observed in the spectrum of NazPO3F.
4. Discussion
4.1. Pre-edge resonances The measured spectra demonstrate that chemical changes beyond the PO4-tetrahedra are reflected in
P K-XANES spectra by energy shifts of pre-edge and continuum resonances, by the presence of characteristic shoulders and new peaks and finally by differences in the white-line intensity. We have found that the white line shifts between 2153.05 and 2151.9 eV in orthophosphates depending on the choice of the cation and between 2152.8 and 2152.2eV depending on the P - O - P chain length in condensed phosphates. This shift do not follow the chemical trend of AE(ls) F in every case (Table 1). It is well-established that the energy position of the absorption edge shifts owing to the valence-charge transfer by the creation of chemical bonds. Moreover, many authors have suggested (see e.g. Refs. [1, 5]) that the chemical shift of a particular excited bound state (here: E(whl)) can be used to extract the effective atomic charge of the absorber if the symmetry of the excited state is the same in the considered species [16] as it is assumed in the investigated phosphates. It should be possible to interpret shifts AE(whl) from the orthophosphates as it has been shown for AE(ls) F [17] within the point-charge limit given by e2
AE(whl) = k'Sq(P) + 4~eo ~.q(O)/rp o e2
+~
~q(Me)/rp Me,
(1)
where q(P), q(O), and q(Me) (Me = metallic cation) represent the effective charges of the particular atoms and rp_o and rp-Meare the interatomic distances to the considered P-atom. The first term on the right-hand side of Eq. (1) contains k ~s as an adjustable parameter and accounts the energy shift with respect to the "free-ion model" [1]. The sum terms give the lattice sums of the electrostatic potentials at P-sites arising from all O-atoms and metallic cations (Madelung potential). Chemical shifts AE(ls) r in orthophosphates have been explained by this approach as arising mainly from changes in the electronic charge distribution in the chemical environment of P (Madelung potential terms) [17]. These investigations and X-ray emission data [16, 18] supported the hypothesis that q(P) is rather constant in orthophosphates due to the presence of the same nearest neighbors. Assuming constant values for q(P) and q(O), E(whl)
R. Franke, J. Hormes/Phvsica B 216 (1995) 85- 95
should increase with approximated individual potential contributions V(Me) of all cations per Patom in the formula unit (Table 2) which have been determined according to
91
2153.0 2152.8
:Ap÷
!
2152.6
V(Me) = e e Fq(Me) x n(Me)] 4 ~ o L rp Me n[P) J '
(2)
where q(Me) corresponds to the cation valence and the ratio n(Me)/n(P) means the cation count per P atom. Unfortunately, interatomic distances are not well defined and are not unique in most of the orthophosphates. We have estimated rp ue on the basis of modelled packages, where rp Me (Table 2) corresponds to the sum of the average P - O distance in orthophosphates (155 pm), the half of the O--anion radius in ionic crystals (135 pm [19]) and the cation radius (Table 2, [19]) according to the coordination number in phosphates. The obtained values for V(Me) are plotted versus E(whl) in Fig. 6. In general, the obtained trend and linear correlation confirm the assumed dependence of E(whl) on the cation distance and charge within the point-charge approximation. Only the values of GaPO4 and Fe3(PO4)2 deviate significantly from this correlation. Another interpretation of AE(whl) assumes that the energy position of the excited valence electronic state is close to EcB (Fig. 1). In this approximation chemical shifts AE(whl) are influenced by chemical shifts AE(ls) v and the varying magnitude of the energy gap E~ likewise. We define the energy difference AE by AE = E(whl) - E(ls) v
(3)
with AE being a rough measure of half of E~ (Fig. 1). This interpretation of AE can easily be confirmed, e.g., by the use of corresponding data from Na K-edge data of NaF as a pure ionic crystal. E(whl)= 1076.3 eV taken from Kasrai et al. [20] in combination with E(Na Is) v = 1072.1 eV from our own measurements yield AE = 4.2 eV, which is approximately half of E~ = 8.5 eV [21]. Other available AE data of amorphous red phosphorus (0.3 eV) [9,22] as a semiconductor with E ~ = 0 . 9 - 1 . 1 e V [21] and of transition-metal phosphides ( ~ zero) [23] containing P as metalloid with Eo = 0 support this approach as well. The
2152.4 2152.2
21 52.0 2151.8 . 1.1
g+rk~ .
.
.
~6 '
1:2 1'.3 114 1'.5 1.
, i , ii
1.7 1.8
-
1.9 2.0
V(Me)/(eV) Fig. 6. The c o r r e l a t i o n of the white-line energy p o s i t i o n E(whl) in P K - X A N E S spectra of o r t h o p h o s p h a t e s with i n d i v i d u a l potential c o n t r i b u t i o n s V(Me) of the metallic cations (Eq. (2)).
obtained AE data of the phosphates studied in this work are given in Table 1. We observe gradual differences between the particular values. We distinguish the investigated samples in orthophosphates of type MePO4 (AE-3.45-3.95eV), M%(PO4)2 (AE = 4.15-4.8 eV) and Me3PO4 (AE = 5.1 eV). This result is illustrated in Fig. 7. The magnitude of Ec in insulators increases with the ionic character of the anion-cation bonding [24]. The correlation presented in Fig. 7 shows indirectly that the larger the valence of the cation is the lower the ionic character of the cation-anion interaction will be. This is indicated by corresponding AE values. Available AE data from the covalent molecular crystals O = P(OCsH6)3 (2.75 eV) and P4Olo (1.25 eV) [25,26], having polar covalent bonds in the next neighborhood of P are lower than the AE values of all investigated phosphates and support this approximation. Consequently, in contrast to Eqs. (1) and (2) the electronic structure of orthophosphates cannot be regarded rigorously as composed of threefold negative charged PO4-units and metallic cations Me which are charged according to their formal valence ( + 1 , + 2 , or +3). It is well-known that in the covalent orthophosphates MePO4 a predominant polar covalent bond exists between the metallic component and the Oatoms of the PO4-group. This property leads to a decreasing coordination number (4) of the cations in contrast to 6-9 in the ionic phosphates (e.g.: in
92
R. Franke. d. Hormes / Physica B 216 (1995) 85-95 5.5
5.5
5.0
5.0
4.5
>
4.5
4.0 4.0 3.5
3.5 3.0
2 valence of catiom
Fig. 7. The dependence of the experimentally obtained energy difference AE between the white-line position E(whl) and P Isbinding energy E(ls) r (Eq. (3)) on the cation valence in the investigated orthophosphates.
Ca3(PO4)2),
which is expressed in a silica-like struc-
ture [4]. The diagram in Fig. 7 suggests that there are gradual differences in the ionic character of the cation-anion interaction in phosphates with the same sum formula, which can be measured by AE. These gradual differences should be correlated to the electronegativity of the cations. Fig. 8 shows the dependence of experimentally AE values on Pauling's electronegativity XP [27] of the cations in orthophosphates. It can be seen, that AE decreases in general with increasing gp reflecting the increasing covalent character of the cation-anion interaction. However, it should be noted, that the correlation (Fig. 8) is not well. No systematic relationship was found in the orthophosphates with cations in the sequence: B(2.04) ~ Ga(1.81) ---,In(1.78) ~ Ai(1.61) as well as in: Nin(1.91) ~ Fen(l.83) ~ Mnn(1.55), (Table l) where gp (in brackets) decreases gradually. In general, changes in the absorption intensity of pre-edge resonances are discussed on the basis of symmetry distortion, the size of the "molecular cage" around the absorber atom and the density of the unoccupied electronic states [5-8]. It can be seen from intensities (Table 1) that considerable differences exist for the here investigated samples. Neglecting any changes in the local geometry of the "PO4-cage" we conclude that increasing intensity going from the rather ionic to the covalent phosphates is due to an
0.75
:
1.oo
1.~5
L~o
~
1.,
2.6o
cation eloctzonegativity ~3~
Fig. 8. The dependence of the experimentally obtained energy difference AE (Eq. (3)) on Pauling's electronegativity of the cations [27]. The value Zp for NaNiPO 4 (NaNi) corresponds to the average Zp value of Na and Ni.
increasing density of unoccupied p-like electronic states in CB. This result overlaps with the outlined AE information. Fig. 9 shows that increasing intensity I of the white line in the investigated orthophosphates is correlated linearily with decreasing AE values. Both quantities characterize the magnitude of the charge transfer from the cation to the phosphate group: 1 represents the density of unoccupied p-like states which increases with a decreasing negative partial charge of the PO4-group. AE represents the influence on the energy splitting E~ between occupied and unoccupied electronic states. The presence of the typical pre-edge feature T in Fig. 3 can be explained by the considerable covalent cation-PO4-anion interaction. This resonance might be due to a P ls electron transition into unoccupied states with partial 3d-character in the CB. However, unlike the case Is --, 3p, a transition into d-like states actually is dipole-forbidden within the X-ray transition selectivity rule Al = -t- 1 [1, 16]. The fact that resonance T is nevertheless observed with a rather low intensity is ascribed to hybridization of Me 3d, O 2p, and P 3p valence orbitals giving some d-character to the p-like unoccupied states. 4.2. Continuum resonances
We assume that the observed weak continuum resonances have the same physical origin as in
93
R. Franke. J. Hormes / Physica B 216 (1995) 85-95
5.5
20 • f r e ~ il~Qpm a ~ l i
u
5.0
o
15
4.5
/
4.0
s
<3
~
/
3.5 3.0 2.5 5
9 ' 1'0 ' 1'1 ' l J2 ' 13
i
intenmy I of white iin~r©l.U,
Fig. 9. Relationship between the absorption intensities 1 of the white lines in P K-XANES spectra of orthophosphates and condensed phosphates (Table 1) and the energy difference AE between the white-line maxima E(whl) and appropriate electron binding energies E(ls) v (Table 1). 1Na3PO4, 2Na4P2OT, 3NaPO3
molecules, i.e. the multiple scattering (MS) of the photoelectron wave function at the neighbor atoms. Using the rule (Ep - Eb) × r 2 = constant,
14)
[5] where Ep is the energy of the continuum resonance and Eb is the energy of a bound valence state, Eb can be predicted when the bond length r is known. Although, MS events at one or more neighbored centeres are significant for the presence of continuum resonances in XANES spectra in contrast to single scattering resonances in the EXAFS region [2], many authors have obtained linear correlations of experimental energy differences between E~ and pre-edge resonances [5] and the ionization potential Eo [25, 28], respectively, with internuclear distances r in small molecules and solids. We have investigated, whether modified term values Err(X) according to Etv(X) = E(X) - E(whl),
i
3
,
5
l/r2/(pro2 x 105)-x
(5)
whereas X means the continuum resonance A, B, or C as depicted in the spectra are also correlated to interatomic distances in the studied phosphates. To this we refer to the empirical linear correlation of term values ( E p - E o ) and l / r 2 - data with r: P - O - and P-Pbond lengths in P-containing molecules reported by Kiiper et al. [22, 25]. We have derived Etv(X ) values
Fig. 10. Empirical correlation of modified term values Etv(X) of continuum resonances (Eq. (5), Table 2) and 1/r2 where r is an internuclear distance in the investigated sample (Table 2). The solid line connects P = O - and P - P -data derived from P K-XANES spectra of the molecule P4Oto given in Ref. [25] and Et,(C ) data (P~O) (Table 2) for the reported phosphates. The dashed line represents the correlation of the Etv(A) data for covalent phosphates, Ni3(PO4)2, and Ca3(PO4)2, Etv(B') data (P-P) for Na4P20 7 and NaPO 3 and Et~(C') (P-S) for Na3PO3S reported in this work.
after Eq. (5) for P4Ot0 [25] and have obtained a slope of 33 eV/10 -4 pm 2 (Err(X)versus l/r2). Note, that this value corresponds to the result if one uses term values on the basis of estimated ionization potentials Eo as used in Ref. [25]. In Fig. 10 the according data for the P - O - and P - P - like continuum resonances from P4Ot0 Ref. [25] are plotted (filled squares) and connected by a solid line with the mentioned slope. The local maxima (C) in all here reported spectra yield the largest values Eta(C) (Eq. (5), Table 2). The corresponding Eta(C) are plotted in Fig. 10 versus derived 1/r 2 values with respect to tabulated P ~ ) distances (r = 153-156 pm) in orthophosphates [4] which on principle support the correlation (sofid line). We conclude that resonance C is due to MS at the neighbored O-atoms. Two different nearest-neighbor distances exist in the substituted phosphates. In the spectrum of Na3PO3S a splitting of C and C' by about 3.8 eV is observed which points out to a difference of about 35 pm between P - O and P-S internuclear distances in this species applying the rule given in Eq. (4) and the constant mentioned above. The magnitude of this difference agrees with the tabulated bond-length difference ( ~ 4 5 pm [31]).
94
R. Franke. J. Hormes / Physica B 216 (1995) 85-95
Slightly different nearest-neighbor distances in Na2PO3F are expressed by a more asymmetric shape of resonance C in comparison to NaaPO4 (Fig. 5). The different line shapes in the XANES continuum region of ortho- and condensed phosphates indicate that they are affected by the next-nearest environment. We have pointed out to additional continuum features B' in the spectra of condensed phosphates compared to Na3PO4 (Fig. 4). It shall be emphasized that the Etv(B3 can be associated with the formation of P4D--P units with P-P distances of about 275 pm [4] as is obviously from the correlation in Fig. 10. Moreover, we have investigated the influence of different cations beyond the first coordination sphere of P to the near edge structure in the absorption spectra. Interatomic distances r in orthophosphates were available for the covalent phosphates [4], Ca3(PO4)2 [29], and Ni3(PO4)2 [30] (Table 2). As can be seen in Fig. 10, the corresponding values Et,(A) together with Et,(C') from Na3PO3S and Etv(B') from condensed phosphates are correlated to l/r 2 (dashed line) with a larger slope compared to the P-P and P4S) bond data of molecules [25]. Furthermore, the data presented in Table 2 show that similar energy positions E(A),E(B), and E(B') occur in the spectra of all investigated Na-phosphates (Table 2). Assuming a relation between the radii of the cations and their internuclear distances to P we expect relative large distances in Na-, Ca-, and K-phosphate (Table 2). The observed intensive shoulder A of the white line (Fig. 4) with the lowest values E,,(A) of about 2-3 eV can thus be explained as arising from MS processes including the far distant cation sites in these compounds.
5. Summary The P K-XANES spectra of orthophosphates with different cations exhibit significant differences in the energy positions and intensities of resonances, demonstrating deafly that chemical changes beyond the phosphate tetrahedra have a systematic influence on the observed spectra. The energy positions E(whl) of the pre-edge absorption maxima can be explained within the pointcharge approximation because they are correlated to
individual cation potential contributions V(Me). Moreover, E(whl) in combination with XPS P isbinding energies provide the quantity AE which is a rough measure of half of E6 in the studied sample. We have shown that increasing AE reflect an increasing ionicity in the POa-anion-cation interaction. The obtained gradual differences of AE correspond to the inverse trend in the determined absorption intensities I. This result suggests that both spectral quantities contain important overlapping information about the electronic structure. The continuum resonances are explained as arising from MS processes at the nearest neighbors (resonances C and C'), on the one hand, and at atoms in higher coordination spheres (resonances A, B, and B3 on the other hand.
Acknowledgements This work was partly supported by the Deutsche Forschungsgemeinschaft within the Sonderforschungsbereich SFB 334.
References [1] A. Meisel, G. Leonhardt and R. Szargan, X-Ray Spectra and Chemical Binding, Springer Series Chem. Phys., Vol. 37 (Springer, Heidelberg, 1989). [2] D.C. Koningsberger and R. Prins (eds.), X-Ray Absorption: Techniques of EXAFS, SEXAFS and XANES (Wiley, New York, 1988). [3] A. di Cicco, A. Bianconi, C. Coluzza, P. Rudolf, P. Lagarde, A.M. Flank and A. Marelli, J. Non-Cryst. Solids 116 (1990) 27. [4] DE.C. Corbridge, The Structural Chemistry of Phosphorus (Elsevier, Amsterdam, 1974). [5] A. Bianconi, in Ref. [2] p. 596. [6] J. Wong, F.W. Lytle, R.P. Messmer and D.H. Maylotte, Phys. Rev. 30 (1984) 5596. [7] P. Behrens, Trends Anal. Chem. 11 (1992) 218. [8] P. Behrens, J. Felsche, S. Vetter, G. Schulz-Ekloff, N.I. Jaeger and W. Niemann, J. Chem. Soc. Chem. Commun. (1991) 678. [9] R. Franke, Th. Chassr, P. Streubel and A. Meisel, J. Electron Spectrosc. Relat. Phenom. 56 (1991) 381. [10] KH. Althoff, W.v. Drachenfels, A. Dreist, D. Husmann, M. Neckenig, H.D. Nuhn, W. Sehauerte, M. Schillo, F.J. Schittko and C. Wermelskirchen, Particle Accelerators 27 (1990) 101. [11] M. Lemonnier, O. CoUet, C. Depautex, J.M. Esteva and D. Raoux, Nucl. Instr. and Meth. 152 (1978) 109. [12] Gmelin's Handbook of Inorganic Chemistry (Na), Vol. 4 (VCH, Weinheim, 1965).
R. Franke, .1. Hormes / Physica B 216 (1995) 85-95
[13] G.N. Greaves, R.N. Jenkins, S.J. Gurman, L.F. Gladden, C.A. Spence, P. Cox, B.C. Sales and L.A. Boatner, Philos. Mag. 58 (1988) 271. [14] H. Seikeyama, Y. Kitayama, N. Kosugi, H. Kuroda and T. Ohta, Photon Factory Activity Report 1984/1985, p. 198. [15] A. Redeker, G. Kiiper, J. Hormes, F. Frick, M. Jansen and M. Miihlh/iuser, Phosphorus, Sulfur, and Silicon 76 (1993) 239. [16] R.L Barinski and V.I. Nefedov, X-ray Spectroscopic Determination of Charges in Molecules (in German) (Verlag Chemie, Weinheim, 1968). [17] Th. Chass6, R. Franke, C. Urban, P. Franzhdd, P. Strcubel and A. Mcisel, J. Electron Spectrosc. Rdat. Phenom. 62 (1993) 287. [18] V.I. Nefedov, V.G. Yarzhemskii, A.W. Chuvaev and E.M. Trishkina, J. Electron Spectrosc. Relat. Phenom. 46 (1988) 381. [19] R.D. Shannon and C.T. Prewitt, Acta Cryst. B 25 (1969) 925. [20] M. Kasrai, M.E Flet, G.M. Bancroft, K.H. Tan and J.M. Chert, Phys. Rev. B 43 (1991) 1763.
95
[21] Landolt-B6rnstein New Series, K.-H. Hellwege (eel.), Group IIl: Crystal and Solid State Physics, Vol. 17 (Springer, Berlin, 1983) p. 28. [22] G. Kiiper, PhD Thesis, University of Bonn, Germany. [23] R. Franke, J. Hormes, M. Jaitschko and J. Albefing, Z. Anorg. Allg. Chem., submitted. [24] U. M~iller,Inorganic Structural Chemistry (in German) (B.G. Teubner, Stuttgart, 1992). [25] G. Kiiper, R. Chauvistr6, J. Hormes, F. Frick, M. Jansen, B. Liicr and E. Hartmann, Chem. Phys. 165 (1992) 405. [26] R. Franke, J. Electron~ Spectrosc. Relat. Phenom. (1995), to be published. [27] A.L Allred, J. Inorg. Nuclear Chem. 17 (1961) 215, [28] J. St6hr, NEXAFS Spectroscopy (Springer, Berlin, 1992), [29] B. Dickens, LW. Sehroder and W.E. Brown, J. Solid State Chem. 10 (1974) 232. [30] C. Calvo and R. Faggiani, Can. J. Chem. 53 (1975) 1516. [31] A.F. Wells, Structural Inorganic Chemistry (Clarendon Press, Oxford, 1984).
Physica B 216 (1995) 85- 95
The P K-near edge absorption spectra of phosphates R. Franke*, J. Hormes Physikalisches Institut der Universit&t Bonn, Nuflallee 12, D-53115 Bonn, Germany Received 12 April 1995; revised 19 June 1995
Abstract
The X-ray absorption near edge structure (XANES) at the P K-edge in several orthophosphates with various cations, in condensed, and in substituted sodium phosphates have been measured using synchrotron radiation from the ELSA storage ring at the University of Bonn. The measured spectra demonstrate that chemical changes beyond the PO4tetrahedra are reflected by energy shifts of the pre-edge and continuum resonances, by the presence of characteristic shoulders and new peaks and by differences in the intensity of the white line. We discuss the energy differences between the white line positions and the corresponding P 1s binding energies as a measure of half of the energy gap. The corresponding values correlate with the valence of the cations and the intensity of the white lines. The energy positions of the continuum resonances are discussed on the basis of an empirical bond-length correlation supporting a 1 / r 2 - dependence.
1. Introduction
The part of the X-ray absorption spectrum called XANES (X-ray absorption near edge structure) is determined mainly by the atomic geometrical arrangement in a local cluster around the absorbing atom [1,2]. Consequently, this spectroscopy has been currently used in the chemical and structural characterization of disordered systems like amorphous materials (see e.g. Ref. [3]). In this paper we show that chemical changes beyond the first coordination sphere also have a systematic influence on the XANES spectrum. For this purpose we have measured the P K-shell XANES spectra in a series of ortho-, condensed, and substituted phosphates. In a simple manner, orthophosphates are composed of tetrahedral PO4-groups with metallic ca* Corresponding author.
tions (Me) in the void spaces of the crystal structure [4]. The ionic compounds with sum formulas Me3PO4 and Me3(PO4)2 and covalent phosphates MePO4 belong to these species. In condensed phosphates the basic PO4-units are linked by sharing O-atoms forming PxOy-chains of different length and dimension. Moreover, we have investigated substituted Na-phosphates, in which one O-atom of the PO4-units is substituted by a heteroatom (F, S). In order to explain the origin of characteristic absorption features, XANES spectra are commonly subdivided into three sections on the energy scale as shown in Fig. 1 [5]. Region I, the pre-edge region of K-shell spectra below the absorption edge at E0 (II), often exhibits sharp intense resonances (white lines) resulting from electronic transitions of the core electron (i) into unoccupied p-like valence electronic states in the conduction band (CB). It has been shown that
0921-4526/95/$09.50 ~ 1995 Elsevier Science B.V. All rights reserved SSDI 0 9 2 1 - 4 5 2 6 ( 9 5 ) 0 0 4 4 6 - 7
86
R. Franke, J. Hormes/Physica B 216 (1995) 85 95
~
!
Eb,a~q
IZ
IZI
x~s
Eo
/~
Elte
E[i|F Ell)¥
Ew
Eel
E
Fig. 1. Energy relations of core-level binding energies E(i) F and E(i) v referred to the vacuum level E v and the Fermi level E F, respectively, the sample work function ~b, and the different energy regions I, II and II1 of a XANES spectrum with an absorption m a x i m u m at E(whl). The absorption edge E o corresponds to the ionization potential E(i) v. The Fermi level is assumed to be centered in the energy gap E~ between the highest occupied electronic state Eva of the valence band VB and the lowest unoccupied state EcB of the conduction band CB.
the intensity, the energy position and the line shape of the pre-edge resonances are determined by electronegativity and number of the nearest neighbors as well as by the size and the symmetry of the coordination polyhedron around the absorbing atom [6-8]. The weak absorption resonances observed in region III above the core-level ionization potential Eo are discussed as arising from constructive interferences of the outgoing and at the neighbors multiple scattered photoelectron wave function [2]. Consequently, the energy position of these features is very sensitive to internuclear distances. Because of the different physical origin it is interesting to distinguish between pre-edge and continuum resonances. In the case of free molecules the ionization potential E0 can be obtained directly from X-ray photoelectron spectroscopy (XPS) where the binding energy E(i)v( = Eo) is referred to the vacuum level E v (Fig. 1). But it is difficult to distinguish between both types of resonances in spectra of solids because of core-level binding-energy data E(i) v are commonly referred to the Fermi level E F. We assume that E v is centered between the lowest unoccupied electronic state EcB of the conduction band (CB) and the highest occupied state EvB of the valence band (VB). Since the most intense pre-edge resonance called white line in solids is due to a transition of an inner-shell electron into an unoccupied electronic state in CB, the resulting
energy position of the white line E(whl) is higher compared to E(i) F. The difficulty in the determination of absolute electron binding energies or the energy of the absorption edge, respectively, arises from the estimation of the sample work function ~b. Here, we approximate a constant value of q~ = 5 eV for the investigated insulating samples. This value allows in combination with recently published P Is binding energies [9] to distinguish between resonances present in regions I and III in the spectra reported here.
2. Experimental The P K-XANES measurements were carried out at the synchrotron radiation beamline BN3 of the Bonn electron stretcher and accelerator (ELSA) operated in storage ring mode with an electron energy of 2.3 GeV and with an average current of about 50mA [10]. Synchrotron radiation was monochromatized by a Lemonnier-type double crystal monochromator [11] equipped with two InSb(1 1 1) crystals (2d = 748.06 pm). These crystals give an energy resolution of about 0.8 eV at the P K-edge. The monochromatic flux was about 109 photons per energy interval and second. Spectra were taken in the standard mode, detecting the monochromatized beam before (monitor) and behind the sample (detector) with ionization chambers filled with air (45 mbar). The absorption spectra were obtained from the logarithmic ratio of the currents of the monitor and detector chambers. The calibration of the photon-energy scale was determined before and after scanning a sample, using the absorption maximum of Na4P207 as standard value (2152.4 eV). This value and thus the relative energy scale is reproducible to a value better than 0.05 eV. All presented XANES spectra have been taken at photon energies between 2130 and 2190 eV with a step width of 0.03 eV and an integration time of 0.1 s per step. The obtained spectra were normalized in order to compare the intensity of the absorption features quantitatively. For this purpose the pre-edge region of the raw data was fitted linearily and the so determined background was subtracted from the whole
R. Franke, J. Hormes / Physica B 216 (1995) 85 95
spectrum. The absorption jump was set to 1 at a photon energy of about 2180 eV at which the absorption did not exhibit pronounced structures. The intensities of the white lines are given in relative units and correspond to their area. These data were obtained by means of a least-squares fitting routine assuming Gaussian line shapes. The phosphates were available commercially as powders. The condensed phosphates were prepared by tempering acidic Na-phosphates for several hours [12]. Before the powders were prepared on adhesive tape (20 ~m thick) they had been dried at 200°C for 2 h and had been grinded in order to reach a homogeneous thickness. Details of the XPS measurements were published recently [9].
8 7
¥
3. Results
5
o
The P K-XANES spectra of the investigated covalent and ionic orthophosphates are presented in Figs. 2-4. The overall line shape is similar in all spectra. We observe the white line in the energy
9
!
]
!
87
21 45
21 50
21 55
21 60
21 65
21 70
21 75
energy/(eV)
Fig. 3. P K-XANES spectra of the investigated orthophos-
phates with transition-metal cations. T marks a pre-edge resonance.
, !
12 ........... ~....................................................
8
il iii iiiiii iiiiiii
7
6
5
i .... ".......... o! ......... ~ [ ] ~ ~z~ .......
)21 q
4
..............)1/~tI
o
;..............
[]
c
o ~_ o .a o
~
d~
7 6 5 4
2 2
1 1
0
21 45
21 50
21 55
21 60
21 65
21 70
21 75
energy/(eV)
0 21 45
21 50
21 55
21 60
21 65
21 70
21 75
energy/(eV)
Fig. 2. P K-XANES spectra of covalent orthophosphates. A, B,
and C mark continuum resonances, the vertical bars give the XPS P Is-binding energy.
Fig. 4. P K-XANES spectra of the investigated alkaline, earth alkaline, and condensed phosphates.
R. Franke, ,1. Hormes / Physica B 216 (1995) 85-95
88
region between 2151 and 2154 eV and weak absorption resonances above a photon energy of 2155 eV. In general, the K-XANES spectra of tetrahedral coordinated P in orthophosphates resemble those of the majority of K-shell spectra obtained from other atoms with the same coordination to four more electronegative ligand atoms [1, 5-1. Since the PO4-group has Td-symmetry the white line is assigned to a transition of the P Is electron into an unoccupied valence electronic state formed by the overlap of P sp 3 hybrid- and O 2p-orbitals (t~). The experimentally determined E(ls) F values [9] are given in Table 1 and are marked in the absorption spectra by bars on the low-energy side of the white line. We observe characteristic differences in the presented spectra. In comparison with the spectra of the covalent orthophosphates (BPO4, A1PO4, GaPO4 and lnPO4 (Fig. 2)) and the alkaline and earth-alkaline phosphates (Na3PO4, K3PO4 and Ca3(PO4)2 (Fig. 4)) the spectra of the ionic transition-metal phosphates (NaNiPO4, Fe3(PO4)2 and Ni3(PO4)2 (Fig. 3)) exhibit an additional weak spectral feature
denoted as T in the spectra. Previously, Greaves et al. [13] observed the same feature for Fe-phosphates. The energy difference between this resonance and the absorption maximum at E(whl) (Table 1) was determined by means of a leastsquares fit to be 2.9eV (NaNiPO4), 2.7eV (Ni3(PO4)2), and 4.5 eV (Fe3(PO4)2), respectively. Moreover, we observe a relative large F W H M of the white line in the spectrum of Mn3(PO4)2 (3.95 eV, Table 1). However, in contrast to the other spectra, here peak T is absent, which suggests that it is hidden in the broadened white line. It is obviously from Fig. 3 that E(ls) F in Fea(PO4)2 given as a vertical bar overlaps the new pre-edge peak. We observe broadened white lines in the spectra of GaPO4 and InPO4 in comparison to BPO4 and AIPO4 as it is indicated by the according F W H M in Table 1. This result is explained by the polycrystailine composition of these samples. The white line intensities of the spectra are given in Table 1. We obtain the largest values for the covalent phosphates and less intensities for the phosphates of type Me3(PO4)2 and MeaPO4.
Table 1 XPS E(P Is)F binding energies [9], the energy positions E(whl), the absorption intensities I (peak areas), and the full widths at half maximum (FWHM) of the white lines in the reported XANES spectra of phosphates, and the energy differences AE between white line and binding energies (Eq. (3~). Values are given in (eV). I is given in (rel. units) Sample
E{ ls) F
E(whl)
BPO 4 AIPO 4 GaPO 4 lnPO 4 NaNiPO 4 Mn3(PO4) 2 Fea(PO4) 2 Ni3(PO4) 2 Ca3(PO4) z K3PO 4 Na3PO 4 Na4P20 7 NaPO 3 Na2PO3F Na3PO3S
2149.55 2149.1 2148.8 2148.4 2147.7 2148.2 2148.3 2147.7 2147.3 2146.8 2147.05 2147.9 2149.0 2148.55 2146.85
2153.05 2152.8 2152.25 b 2152.35 b 2152.45 c 2152.35 c 2152.6 ¢ 2152.25 ~ 2152.1 c 2151.9 ~ 2152.2 ¢ 2152.4 ¢ 2152.8 c 2152.5 2152.1 b
IS
FWHM b
AE
10.7 (1) 12.2 (1) 11.0 (2)b 10.5 (2)b'd 10.3 (2)~ 8.8 13)¢ 9.7 {2)~ 7.7 (2)" 7.9 (2)' 6.0 (2)c 6.3 (2)d 8.2 (3)d 11.5 (3)d 11.2 (I) 9.4 (2)b
2.1 2.1 2.75 3.5 2.95 3.95 2.4 2.0 2.1 d 3.3 d 3.35 d 3.05 3.5 3.45 3.25
3.5 3,6 3,45 3.95 4.75 4.15 4.3 4.55 4.8 5.1 5.15 4.5 2.8 3.9 5.25
a In brackets: number of fitted peaks incl. the resonances T (transition-metal phosphates) and A (Ca- and alkaline phosphates). b Values correspond to the sum curve of the fitted peaks. c Values correspond to the main component of two fitted peaks. d Value determined without resonance A.
R. Franke, J. Hormes/Physica B 216 (1995) 85 95
Assuming that ~b = 5 eV (see Section 1) we assign all resonances denoted with A, B, B', C and C' as continuum resonances. In all reported XANES spectra resonance C can be identified at about 2168 eV (Table 2). We observe about the same energy of resonances A, B and C for Na3PO4 and Caa(PO4)2, whereas they are shifted in K3POa (Fig. 4). In contrast to the other spectra the feature denoted with A in the spectra of the alkali- and earth-alkaline phosphates (Fig. 4) is visible now as a strong shoulder on the high-energy side of the white line. Its intensity is about half of the whiteline intensity. The most pronounced absorption fine structure in the continuum region is observed in the spectra of B P O 4 , N i a ( P O 4 ) 2 , and C a 3 ( P O 4 ) 2. In comparison to the orthophosphate the formation of P - O - P chains in condensed phosphates corresponds to a chemical variation in the 2nd coordination sphere of P. Moreover, the multiple
89
bond character in the bond to bridging O-species is reduced compared to the terminal bonded O-atoms which is reflected by the corresponding larger bond lengths [4]. The spectra of Na4P207 and NaPO3 are compared with Na3PO4 in Fig. 4. As can be seen, the formation of bridging P - O - P units causes a slight splitting of the white line due to transitions into different unoccupied electronic states (P-O type.) A similar result was obtained by Seikeyama et al. [14] who analyzed the FWHM of the white line in P K-shell spectra of Na-phosphates. The typical shoulder A observed in the absorption spectra of the alkali phosphates (Fig. 4) is shifted to higher energies in the condensed phosphates and therefore is clearly separated from the white line. The continuum resonances occur at similar energy positions (Table 2). As indicated by arrows in Fig. 4, additional resonances B' are observed in the region of 2160-2166 eV going from Na3PO4 to Na4P207 and NaPO3.
Table 2 Modified "term values" E,v (X) with X = A, B, B', C, and C' (Eq. (5)) of continuum resonances in P K-XANES spectra of orthophosphates, condensed, and substituted sodium phosphates, selected interatomic distances from Ref. [4, 29-31], tabulated ionic radii of the cations r~o" [19], estimated P-cation distances rp Me in the studied orthophosphates (Section 4.1) and estimated individual potential contributions V(Me) (Eq. (2)) of the cations to the energy shift AE (whl) (Table 1) of the white-line position. Etv and V(Me) are given in (eV), r is given in (pm).
E,v(B, B')
E,~(C, C')
r
ri."
rp Me
V(Me)
9.1, 12.0
15.1 17.1 16.8 17.2 16.2 16.7 16.5 ] 6.3 16.8 13.9 15.9 17.6 16.9 17. l 5 15.6, 19.4
280" 311" 309 a 332 a
12 39 47 66 102, 70 82 77 70 110 138 102 102 102 102 102
235 262 270 289 309 305 300 293 333 361 325
1.85 1.65 1.6 1.5 1.4 1.4 1.4 1.45 1.3 1.2 1.35
Sample
Err(A)
BPO 4 AIPO 4 GaPO 4 InPO 4 NaNiPO 4 Mn3(PO4) 2 Fe3(PO,,)2 Ni3(PO4) 2 Ca3(PO4) 2 K3PO 4 NajPO 4 Na4P20 7 NaPO 3 NazPO3F Na3POaS
6.6 7.0 4.3 3.5 4.4 3.0
10.5 9.2, 12.1 10.9 8.8
5.2 2.5 2.8 3.7 4.7 4.35 4.8 5.6
11.1 10.3 6.1 9.15 8.2, 9.8 7.8, t0.0 9.9 8.0
'rl, Me: Me = B, A1, Ga, In, Ref. [4]. brP Ni: average values [30]. re ca: average values [29]. ~r e p: Ref. [4]. ~re v: Ref. [4]. frp s: Ref. [31].
285 b 330 c
275 d 275 d 158 e 205 f
R. Franke, d. Hormes / Physica B 216 (1995) 85-95
90 r 6
r
]
. . . . . . . . . . . . . .
140
14
50 2 5q
160
) 6 ~ 2 7C
2175
218C
eqergy/(eV) Fig. 5. P K - X A N E S s p e c t r a of N a 3 P O 4 a n d t h e i n v e s t i g a t e d substituted phosphates.
The XANES spectra of Na3PO3S and Na2PO3F are compared with that of Na3PO4 in Fig. 5. In spite of the drastic chemical changes in the first coordination shell due to varying electronegativities, bonding distances and bonding angles, accompanied by symmetry distortions [4], the measured spectra are very similar. Both substituted phosphates exhibit sharp white lines which have FWHM in the same magnitude but with larger intensity compared to that of orthophosphate (Table 1). This result is altogether astonishing, as we expected new resonances in the white line region arising from transitions into electronic states formed by the P-F - and the P-S - bond, respectively, as recently measured by Redeker et al. [15] in P-oxidesulfides. Only, an asymmetric white line is observed in the spectrum of NazPO3F.
4. Discussion
4.1. Pre-edge resonances The measured spectra demonstrate that chemical changes beyond the PO4-tetrahedra are reflected in
P K-XANES spectra by energy shifts of pre-edge and continuum resonances, by the presence of characteristic shoulders and new peaks and finally by differences in the white-line intensity. We have found that the white line shifts between 2153.05 and 2151.9 eV in orthophosphates depending on the choice of the cation and between 2152.8 and 2152.2eV depending on the P - O - P chain length in condensed phosphates. This shift do not follow the chemical trend of AE(ls) F in every case (Table 1). It is well-established that the energy position of the absorption edge shifts owing to the valence-charge transfer by the creation of chemical bonds. Moreover, many authors have suggested (see e.g. Refs. [1, 5]) that the chemical shift of a particular excited bound state (here: E(whl)) can be used to extract the effective atomic charge of the absorber if the symmetry of the excited state is the same in the considered species [16] as it is assumed in the investigated phosphates. It should be possible to interpret shifts AE(whl) from the orthophosphates as it has been shown for AE(ls) F [17] within the point-charge limit given by e2
AE(whl) = k'Sq(P) + 4~eo ~.q(O)/rp o e2
+~
~q(Me)/rp Me,
(1)
where q(P), q(O), and q(Me) (Me = metallic cation) represent the effective charges of the particular atoms and rp_o and rp-Meare the interatomic distances to the considered P-atom. The first term on the right-hand side of Eq. (1) contains k ~s as an adjustable parameter and accounts the energy shift with respect to the "free-ion model" [1]. The sum terms give the lattice sums of the electrostatic potentials at P-sites arising from all O-atoms and metallic cations (Madelung potential). Chemical shifts AE(ls) r in orthophosphates have been explained by this approach as arising mainly from changes in the electronic charge distribution in the chemical environment of P (Madelung potential terms) [17]. These investigations and X-ray emission data [16, 18] supported the hypothesis that q(P) is rather constant in orthophosphates due to the presence of the same nearest neighbors. Assuming constant values for q(P) and q(O), E(whl)
R. Franke, J. Hormes/Phvsica B 216 (1995) 85- 95
should increase with approximated individual potential contributions V(Me) of all cations per Patom in the formula unit (Table 2) which have been determined according to
91
2153.0 2152.8
:Ap÷
!
2152.6
V(Me) = e e Fq(Me) x n(Me)] 4 ~ o L rp Me n[P) J '
(2)
where q(Me) corresponds to the cation valence and the ratio n(Me)/n(P) means the cation count per P atom. Unfortunately, interatomic distances are not well defined and are not unique in most of the orthophosphates. We have estimated rp ue on the basis of modelled packages, where rp Me (Table 2) corresponds to the sum of the average P - O distance in orthophosphates (155 pm), the half of the O--anion radius in ionic crystals (135 pm [19]) and the cation radius (Table 2, [19]) according to the coordination number in phosphates. The obtained values for V(Me) are plotted versus E(whl) in Fig. 6. In general, the obtained trend and linear correlation confirm the assumed dependence of E(whl) on the cation distance and charge within the point-charge approximation. Only the values of GaPO4 and Fe3(PO4)2 deviate significantly from this correlation. Another interpretation of AE(whl) assumes that the energy position of the excited valence electronic state is close to EcB (Fig. 1). In this approximation chemical shifts AE(whl) are influenced by chemical shifts AE(ls) v and the varying magnitude of the energy gap E~ likewise. We define the energy difference AE by AE = E(whl) - E(ls) v
(3)
with AE being a rough measure of half of E~ (Fig. 1). This interpretation of AE can easily be confirmed, e.g., by the use of corresponding data from Na K-edge data of NaF as a pure ionic crystal. E(whl)= 1076.3 eV taken from Kasrai et al. [20] in combination with E(Na Is) v = 1072.1 eV from our own measurements yield AE = 4.2 eV, which is approximately half of E~ = 8.5 eV [21]. Other available AE data of amorphous red phosphorus (0.3 eV) [9,22] as a semiconductor with E ~ = 0 . 9 - 1 . 1 e V [21] and of transition-metal phosphides ( ~ zero) [23] containing P as metalloid with Eo = 0 support this approach as well. The
2152.4 2152.2
21 52.0 2151.8 . 1.1
g+rk~ .
.
.
~6 '
1:2 1'.3 114 1'.5 1.
, i , ii
1.7 1.8
-
1.9 2.0
V(Me)/(eV) Fig. 6. The c o r r e l a t i o n of the white-line energy p o s i t i o n E(whl) in P K - X A N E S spectra of o r t h o p h o s p h a t e s with i n d i v i d u a l potential c o n t r i b u t i o n s V(Me) of the metallic cations (Eq. (2)).
obtained AE data of the phosphates studied in this work are given in Table 1. We observe gradual differences between the particular values. We distinguish the investigated samples in orthophosphates of type MePO4 (AE-3.45-3.95eV), M%(PO4)2 (AE = 4.15-4.8 eV) and Me3PO4 (AE = 5.1 eV). This result is illustrated in Fig. 7. The magnitude of Ec in insulators increases with the ionic character of the anion-cation bonding [24]. The correlation presented in Fig. 7 shows indirectly that the larger the valence of the cation is the lower the ionic character of the cation-anion interaction will be. This is indicated by corresponding AE values. Available AE data from the covalent molecular crystals O = P(OCsH6)3 (2.75 eV) and P4Olo (1.25 eV) [25,26], having polar covalent bonds in the next neighborhood of P are lower than the AE values of all investigated phosphates and support this approximation. Consequently, in contrast to Eqs. (1) and (2) the electronic structure of orthophosphates cannot be regarded rigorously as composed of threefold negative charged PO4-units and metallic cations Me which are charged according to their formal valence ( + 1 , + 2 , or +3). It is well-known that in the covalent orthophosphates MePO4 a predominant polar covalent bond exists between the metallic component and the Oatoms of the PO4-group. This property leads to a decreasing coordination number (4) of the cations in contrast to 6-9 in the ionic phosphates (e.g.: in
92
R. Franke. d. Hormes / Physica B 216 (1995) 85-95 5.5
5.5
5.0
5.0
4.5
>
4.5
4.0 4.0 3.5
3.5 3.0
2 valence of catiom
Fig. 7. The dependence of the experimentally obtained energy difference AE between the white-line position E(whl) and P Isbinding energy E(ls) r (Eq. (3)) on the cation valence in the investigated orthophosphates.
Ca3(PO4)2),
which is expressed in a silica-like struc-
ture [4]. The diagram in Fig. 7 suggests that there are gradual differences in the ionic character of the cation-anion interaction in phosphates with the same sum formula, which can be measured by AE. These gradual differences should be correlated to the electronegativity of the cations. Fig. 8 shows the dependence of experimentally AE values on Pauling's electronegativity XP [27] of the cations in orthophosphates. It can be seen, that AE decreases in general with increasing gp reflecting the increasing covalent character of the cation-anion interaction. However, it should be noted, that the correlation (Fig. 8) is not well. No systematic relationship was found in the orthophosphates with cations in the sequence: B(2.04) ~ Ga(1.81) ---,In(1.78) ~ Ai(1.61) as well as in: Nin(1.91) ~ Fen(l.83) ~ Mnn(1.55), (Table l) where gp (in brackets) decreases gradually. In general, changes in the absorption intensity of pre-edge resonances are discussed on the basis of symmetry distortion, the size of the "molecular cage" around the absorber atom and the density of the unoccupied electronic states [5-8]. It can be seen from intensities (Table 1) that considerable differences exist for the here investigated samples. Neglecting any changes in the local geometry of the "PO4-cage" we conclude that increasing intensity going from the rather ionic to the covalent phosphates is due to an
0.75
:
1.oo
1.~5
L~o
~
1.,
2.6o
cation eloctzonegativity ~3~
Fig. 8. The dependence of the experimentally obtained energy difference AE (Eq. (3)) on Pauling's electronegativity of the cations [27]. The value Zp for NaNiPO 4 (NaNi) corresponds to the average Zp value of Na and Ni.
increasing density of unoccupied p-like electronic states in CB. This result overlaps with the outlined AE information. Fig. 9 shows that increasing intensity I of the white line in the investigated orthophosphates is correlated linearily with decreasing AE values. Both quantities characterize the magnitude of the charge transfer from the cation to the phosphate group: 1 represents the density of unoccupied p-like states which increases with a decreasing negative partial charge of the PO4-group. AE represents the influence on the energy splitting E~ between occupied and unoccupied electronic states. The presence of the typical pre-edge feature T in Fig. 3 can be explained by the considerable covalent cation-PO4-anion interaction. This resonance might be due to a P ls electron transition into unoccupied states with partial 3d-character in the CB. However, unlike the case Is --, 3p, a transition into d-like states actually is dipole-forbidden within the X-ray transition selectivity rule Al = -t- 1 [1, 16]. The fact that resonance T is nevertheless observed with a rather low intensity is ascribed to hybridization of Me 3d, O 2p, and P 3p valence orbitals giving some d-character to the p-like unoccupied states. 4.2. Continuum resonances
We assume that the observed weak continuum resonances have the same physical origin as in
93
R. Franke. J. Hormes / Physica B 216 (1995) 85-95
5.5
20 • f r e ~ il~Qpm a ~ l i
u
5.0
o
15
4.5
/
4.0
s
<3
~
/
3.5 3.0 2.5 5
9 ' 1'0 ' 1'1 ' l J2 ' 13
i
intenmy I of white iin~r©l.U,
Fig. 9. Relationship between the absorption intensities 1 of the white lines in P K-XANES spectra of orthophosphates and condensed phosphates (Table 1) and the energy difference AE between the white-line maxima E(whl) and appropriate electron binding energies E(ls) v (Table 1). 1Na3PO4, 2Na4P2OT, 3NaPO3
molecules, i.e. the multiple scattering (MS) of the photoelectron wave function at the neighbor atoms. Using the rule (Ep - Eb) × r 2 = constant,
14)
[5] where Ep is the energy of the continuum resonance and Eb is the energy of a bound valence state, Eb can be predicted when the bond length r is known. Although, MS events at one or more neighbored centeres are significant for the presence of continuum resonances in XANES spectra in contrast to single scattering resonances in the EXAFS region [2], many authors have obtained linear correlations of experimental energy differences between E~ and pre-edge resonances [5] and the ionization potential Eo [25, 28], respectively, with internuclear distances r in small molecules and solids. We have investigated, whether modified term values Err(X) according to Etv(X) = E(X) - E(whl),
i
3
,
5
l/r2/(pro2 x 105)-x
(5)
whereas X means the continuum resonance A, B, or C as depicted in the spectra are also correlated to interatomic distances in the studied phosphates. To this we refer to the empirical linear correlation of term values ( E p - E o ) and l / r 2 - data with r: P - O - and P-Pbond lengths in P-containing molecules reported by Kiiper et al. [22, 25]. We have derived Etv(X ) values
Fig. 10. Empirical correlation of modified term values Etv(X) of continuum resonances (Eq. (5), Table 2) and 1/r2 where r is an internuclear distance in the investigated sample (Table 2). The solid line connects P = O - and P - P -data derived from P K-XANES spectra of the molecule P4Oto given in Ref. [25] and Et,(C ) data (P~O) (Table 2) for the reported phosphates. The dashed line represents the correlation of the Etv(A) data for covalent phosphates, Ni3(PO4)2, and Ca3(PO4)2, Etv(B') data (P-P) for Na4P20 7 and NaPO 3 and Et~(C') (P-S) for Na3PO3S reported in this work.
after Eq. (5) for P4Ot0 [25] and have obtained a slope of 33 eV/10 -4 pm 2 (Err(X)versus l/r2). Note, that this value corresponds to the result if one uses term values on the basis of estimated ionization potentials Eo as used in Ref. [25]. In Fig. 10 the according data for the P - O - and P - P - like continuum resonances from P4Ot0 Ref. [25] are plotted (filled squares) and connected by a solid line with the mentioned slope. The local maxima (C) in all here reported spectra yield the largest values Eta(C) (Eq. (5), Table 2). The corresponding Eta(C) are plotted in Fig. 10 versus derived 1/r 2 values with respect to tabulated P ~ ) distances (r = 153-156 pm) in orthophosphates [4] which on principle support the correlation (sofid line). We conclude that resonance C is due to MS at the neighbored O-atoms. Two different nearest-neighbor distances exist in the substituted phosphates. In the spectrum of Na3PO3S a splitting of C and C' by about 3.8 eV is observed which points out to a difference of about 35 pm between P - O and P-S internuclear distances in this species applying the rule given in Eq. (4) and the constant mentioned above. The magnitude of this difference agrees with the tabulated bond-length difference ( ~ 4 5 pm [31]).
94
R. Franke. J. Hormes / Physica B 216 (1995) 85-95
Slightly different nearest-neighbor distances in Na2PO3F are expressed by a more asymmetric shape of resonance C in comparison to NaaPO4 (Fig. 5). The different line shapes in the XANES continuum region of ortho- and condensed phosphates indicate that they are affected by the next-nearest environment. We have pointed out to additional continuum features B' in the spectra of condensed phosphates compared to Na3PO4 (Fig. 4). It shall be emphasized that the Etv(B3 can be associated with the formation of P4D--P units with P-P distances of about 275 pm [4] as is obviously from the correlation in Fig. 10. Moreover, we have investigated the influence of different cations beyond the first coordination sphere of P to the near edge structure in the absorption spectra. Interatomic distances r in orthophosphates were available for the covalent phosphates [4], Ca3(PO4)2 [29], and Ni3(PO4)2 [30] (Table 2). As can be seen in Fig. 10, the corresponding values Et,(A) together with Et,(C') from Na3PO3S and Etv(B') from condensed phosphates are correlated to l/r 2 (dashed line) with a larger slope compared to the P-P and P4S) bond data of molecules [25]. Furthermore, the data presented in Table 2 show that similar energy positions E(A),E(B), and E(B') occur in the spectra of all investigated Na-phosphates (Table 2). Assuming a relation between the radii of the cations and their internuclear distances to P we expect relative large distances in Na-, Ca-, and K-phosphate (Table 2). The observed intensive shoulder A of the white line (Fig. 4) with the lowest values E,,(A) of about 2-3 eV can thus be explained as arising from MS processes including the far distant cation sites in these compounds.
5. Summary The P K-XANES spectra of orthophosphates with different cations exhibit significant differences in the energy positions and intensities of resonances, demonstrating deafly that chemical changes beyond the phosphate tetrahedra have a systematic influence on the observed spectra. The energy positions E(whl) of the pre-edge absorption maxima can be explained within the pointcharge approximation because they are correlated to
individual cation potential contributions V(Me). Moreover, E(whl) in combination with XPS P isbinding energies provide the quantity AE which is a rough measure of half of E6 in the studied sample. We have shown that increasing AE reflect an increasing ionicity in the POa-anion-cation interaction. The obtained gradual differences of AE correspond to the inverse trend in the determined absorption intensities I. This result suggests that both spectral quantities contain important overlapping information about the electronic structure. The continuum resonances are explained as arising from MS processes at the nearest neighbors (resonances C and C'), on the one hand, and at atoms in higher coordination spheres (resonances A, B, and B3 on the other hand.
Acknowledgements This work was partly supported by the Deutsche Forschungsgemeinschaft within the Sonderforschungsbereich SFB 334.
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