- Email: [email protected]
Study of YVO4 by appearance potential spectroscopy
Vol. 40, pp. 887-897 PergamonPressLtd., 1979. Printedin Great Britain
I. Phys. Chem. Solids
STUDY OF YVO, BY APPEARANCE I. CURE-U
POTENTIAL
and P. 0.
SPECTROSCOPY
NILSSON
Chalmers Universityof Technology, 412 96 Gothenburg, Sweden
(Receiued 21, February 1979; accepted 17 April 1979) Ah&a&-The luminescentorthovanadate compound YVO, has been studied by soft X-ray Appearance Potential Spectroscopy. Vanadium b,3 APS spectra, which give information about unoccupied electron states, have been interpreted in conjunction with existing experimental data on soft X-ray emission and with cluster model calculations. The main features of the APS spectrum of vanadium in YVO, as compared with the APS spectrum of pure vanadium metal are discussed, as well as the extent to which the vanadiumb,3 APS-signal in YVO, can be explained by the energy level diagram given by a non-self-consistent calculation in a VO:- cluster model. 1. INTRODUCTION
This paper reports on an experimental evidence for the existence of many-body effects in the excitation of Appearance Potential Spectra in a rather complicated chemical compound, namely yttrium orthovanadate (YVO,). YVO, is a luminescent material that crystallizes in the tetragonal system with the zirconium (ZrSiO,) structure (Fig. 1) and the 0:: space group symmetry. The crystal lattice of YVO, has thoroughly been investigated and its lattice parameters have been determined to a great degree of accuracy [l]. In the crystal lattice of YV04 the nearest neighbours of each vanadium atom are four oxygen atoms situated at a common V-O bond length of 1.706 8, on a slightly distorted tetraheder. The yttrium atom is coordinated to two sets of four oxygen atoms having the Y-O bond lengths equal to 2.299 and 2.443 a respectively. Two types of clusters are, therefore, characteristic for yttrium orthovanadate compound, namely VO:- and YOF-. If activated with a rare-earth ion, the lumines-
Fig. 1. Crystal structure of WO,.
cent properties change remarkably. So, for example, YVO,: Eu is a highly-efficient red-emitting phosphor which is at present widely used as a coating in luminescent lamps and colour television tubes [2]. Activation of YVO, with other rare-earth ions results in the production of very good laser materials for the infrared (YVO,:Nd [3]), and visible region of the optical spectrum (YVO,:Er, YVO, :Tm, [4, 51). Interesting is the case of YVO, :Tb in which the terbium ion acts as a quencher of luminescence, rather than an activator [6]. If Tb is added instead to a phosphate matrix, it luminesces intensely in green [7]. Cerium is similar in that it also shows an intense luminescence in the ultraviolet region when included in phosphate matrices, but fluoresces very weakly in vanadates. Common for all these compounds is the fact that the rare-earth activator substitutionally replaces one of Y atoms in the crystal lattice, leaving the crystal structure unchanged, with lattice parameters very close to those of YVO, [8]. The interest in the rare earth orthovanadates, as host matrices of such phosphors, has justified an extensive approach to study of their optical properties and electronic structure. Earlier experimental works performed some 15 years ago have focused on the study of the optical spectra of rare-earth orthovanadates and their comparison with the corresponding spectra of oxide and phosphide compounds [2, g-111. These works have a more descriptive character and leave unsolved a number of problems related to the mechanism of luminescence in these complex compounds, the presence or absence of optical activity of rareearth ions in various crystal lattice hosts, the detailed ordering of energy level diagrams of the compounds, etc. In general, there is little known about the bonding mechanism in vanadium compounds, and any attempt at a realistic description of these practically
887
888
I. C~RELARUand P. 0. NILSON
important but structurally complicated systems is welcome. YVO, belongs to a group of orthovanadates for which the study of the optical spectra have shown that no additional bands due to the rare-earth ion are present in the visible and near ultraviolet region of absorption and emission. This result has justified an interpretation based on a cluster approach, in which the luminescent properties of the compound are entirely attributed to optical transitions between various electronic states within the VO:cluster. Recent calculations of the electronic structure of VOZ- and R0i3- molecular clusters of some rare-earth orthovanadates (here R stands for the rare-earth ion R= Y, Ce, Nd, Eu, Tb, Dy, Gd and Yb) have been performed by Ellis et al. [12-141 using semi-empirical MWH and CNDO methods. These authors obtained an energy level diagram of both the occupied and unoccupied states, and a charge distribution that are apparently consistent with the existing experimental data on excitation, reflection and luminescence spectra of these compounds. On the basis of their energy level scheme, the authors tentatively explain the luminescence of the compound, when excited by X-rays, as being due to a three-step electron transition process within the tetrahedral vanadate ion VOZ-, in the following way. In the fist step, an electron is excited from the V 2p-level to a 2e vacant molecular orbital, after which a second electron transition occurs from the G 2p-band to the V 2p-level, with hole formation within the 0 2p-band. Finally, the electron excited to the 2e molecular orbital undergoes a luminescent transition to the 0 2p-band (Fig. 2). A condition for the realization of the optical process is, therefore, the formation of excited states on the 2e-molecular orbital. Experimental evidence for the existence of such “excited states” has recently been given by Kurmaev et al. [15] and Lazukova et al. [16] in the study of VL, X-ray emission spectra of YVO,. In addition to the main VI+ emission band, these authors found a rather intensive structure situated at higher energies (peak B, figure 11). They interpret it as being due to re-emission of radiation through transitions from the “excited” 2e molecular orbitals directly to the V 2p,,,-level. Since Appearance Potential Spectroscopy (APS) probes the unoccupied part of the electron states, it appears as a convenient method for testing the existence of the “excited states” which are believed
Fig. 2. Luminescence of YVO, explained as a three-step process of electron transition within the tetrahedral vanadate ion VO:-.
Fig. 3. Basic electron transitions involved in the APS process within the one-particle
model.
to be responsible for the luminescence of rare-earth orthovanadates. As known, in Appearance Potential Spectroscopy the specimen is bombarded by a quasimonoenergetic beam of electrons whose energy is continuously increased within a range of values covering the excitation of various core-levels. At each excitation edge, a sudden increase in the total X-ray yield is observed, due to the contribution of the characteristic X-ray emission superposed onto the continuously raising bremsstrahlung radiation (Fig. 3). The presence of the core hole together with both the incident and the excited electrons being captured into some available empty states situated just above the Fermi level makes the excitation and decay processes in APS in general complicated many-body processes. In the simplest case of itinerant conduction band states, a simple one-particle model has been developed [17], that could successfully explain the observed APS-spectra. In this model the assumptions is made that the matrix element involved in the transitions is constant. This approximation is supposed in most cases to only slightly influence the intensities, while the energy position of the peaks remain unaffected. The X-ray yield is then determined mainly by the self-convolution of the density of empty electron states above the Fermi level [17] E--E< Y tof= deN(e)N(E -EC -E). (1) d In order to enhance the edge features, the derivative instead of the direct total yield is usually recorded: d I APSo:Y,,. dE Let us consider a model distribution of the empty density of states that shows some structure at a distance x above the Fermi level (Fig. 4). The self-convolution of this density of states function N(E) results in a model APS-signal that reproduces the structure present in the DOS distribution at the same distance above the Fermi level and with the same width, but shows also an additional structure, much more intense, occurring at the distance 2x from the Fermi level, and containing a negative dip. It is therefore expected that the shape. of APS spectra is determined by the self-convolution of the density of empty conduction-band states, while the position of the excitation threshold provides a measure of the core-level binding energy.
Study of YVO,
by appearance
(al
Fig. 4.(a) Model-density of states of a simple-metal. (b) Model-Al’s signal within the one-particle model.
In the
case
of transition
metals
the
Fermi
level
lies within a highly peaked density of states, part of which is occupied and another part is empty (Fig. 5) The distance x to the lirst peak above the Fermi level is then equal to zero and the two “structures” in the derivative of the selfconvolution of the density of empty states overlap resulting in a typical APS-spectrum as shown in Fig. 5. If localized empty states are present, the oneparticle model sketched above fails to explain the experimental spectra even qualitatively. As a consequence of many-body interactions in the final states of the APS process, large bremsstrahlung resonances appear, and the energy position of the electronic states becomes highly occupationdependent [18-201. The main processes involved are illustrated in Fig. 6. In this case, neither the shape nor the position of the features in the APS spectra are consistent with the description given by the one-particle model through a simple correlation with the self-convolution of the density of empty states. An interpretation based on the specific configurations involved in the process appears to be a more realistic approach [19, 201. For illustration, we show in Fig. 7 the energy level diagram of localized 4f states in lanthanum, with different configurations, as obtained by three different spectroscopic methods, namely continuum isochromat spectroscopy (CIS) [ 181, electronexcited APS (EXAPS) [21] and photon-excited
889
potential spectroscopy
-
-75 C
ib)
Fig. 6. Basic electron transitions involved in the APS process in a transition metal. (a) bremsstrahlung emission; (b) bremsstrahhmg resonance emission; (c) characteristic emission. APS (XEAPS) [21]. It is seen that the energy position of the 4f level is highly dependent on the final state configuration of the system in these three processes. None of these “experimental” 4f-levels coincides with the position of the empty ground state 4f -level which would be tested in a oneelectron model. In isochromat spectroscopy, in which the final state contains one electron in a 4f -state and no core-hole is created, the 4f-level appears at 5.5 eV above the Fermi-level. In both electron-excited APS and photon-excited APS one hole is created on the core-3d-level, and the 4flevel is pulled down, appearing at 0.2 eV below the Fermi-level when populated with two electrons (EXAPS) and at 1.8 eV below the Fermi level when populated with one electron (XEAPS). We were aware of all these difficulties inherent in the interpretation of APS spectra of YVO,. We believe, however, that our data may throw some light on the complexity of the problem of understanding the luminescent properties of rare-earth orthovanadates in terms of their electronic structure. In the present paper, we report on the measurement of the APS spectrum of YVO, and interpret it in conjunction with the existing X-ray emission spectra [15, 161 and cluster-model calculations
ICI)
3d’0+~-3d’0+Lf’
---
+S.SeV
(CI)
1
qo
N(El
L I
1.8 eV IXEAPSI
Al=
APSIE
EF
3d”
0.2 ev (EXAPS)
E
Fig. 5.(a) Model-density of states of a transition metal. (b) Model-AI’s signal for a transition metal.
u---
835.8
(XPS)
Fig. 7. Energy level diagram of 4f-states in lanthanum as obtained by continuum isochromat spectroscopy (CI) and appearance potential spectroscopy in both electronexcitation-(EXAF’S) and photon-excitation (XEAF’S) variations.
I. CURELARUand P. 0. NILSON
8!m
([12, 161). An additional XPS measurement was performed in order to determine the binding energy of V,,-level in YVO,. 2. EwERlMENT
compound YV04, prepared as a polycrystalline powder by synthesisof oxides Y,O, and V205: The
Y,O, +v,o,
+ 2 YVO&
(3)
was loaded without any glue on a pure copper sample holder on the surface of which a mesh of scratches had been performed by hand. the specimen-material investigated in our APS experiment was part of the same product that was used by Kurmaev et al. in their X-ray emission experiment [15]. No chemical analysis has been performed on the specimen, but both the long-scan APS and XPS spectra, as well as the X-ray emission spectrum do not show any significant impurity contamination. The APS spectra were recorded as the derivative of the total soft X-ray yield by using the phase lock-in amplification technique with a modulation amplitude of 1 V peak-to-peak. The pressure inside the vacuum chamber during the measurement was in the low lop9 torr region. The APS spectra were excited by an electron beam of -1 mA emitted by a V-shaped tungsten filament, chemically etched at its top in order to localize the emission and minimize the voltage-drop across the emitting region of the filament. The energy scale of the spectra is given by the voltage applied on the specimen with respect to the mid-point of the emitting tungsten-filament, corrected for the work function and the thermal spread of the emitted electrons. In our experiment, the total correction is estimated to be equal to +5.0 eV. The accuracy in peak position determination, due mainly to the inherent width of the excited core level, the experimental broadening, the scale correction factor (+5.0 eV) and the absolute voltage calibration is in our case estimated
to be about ~tO.3 eV. Vanadium in YVO, being in a state of total oxidation, no noble-gas ionbombardment prior to spectra recording seems to be necessary, and the APS spectrum recorded with state can be the specimen in the “as-received” considered as being representative for the YVO, compound. However, we performed an argon ion bombardment as we routinely use to do in our APS experiments with the aim to remove the possibly adsorbed oxygen and oxygen-containing species. We observed a significant change in the shape and position of the APS spectra which indicates a change in the chemical composition of the specimen and deserves a special attention. The parameters of ion bombardment were the following: ion energy 1 500 eV, pressure -lo5 torr, duration -10 h. No thorough investigation of these changes in the spectra was pursuited at the present stage of the work, but the spectra will be shown in the last part of the paper to point out the interest for such an investigation. In the following, only the APS spectrum of vanadium in YVO, in the “asreceived” state was chosen for interpretation. The binding energy of V,, core-level in YVO, was measured using a Hewlett-Packard HP-5950A electron spectrometer equipped with a monochromatized Al& X-ray source. 3. RESULTS AND DISCUSSION
3.1. APS long spectrum Figure 8 shows the APS long-scan spectrum. One sees the structure corresponding to carbon at -280eV, the b and b lines of vanadium at -5OOeV and a hardly identifiable contribution from oxygen just above the vanadium L,,* lines. No other contaminants are observed. The carbon structure is due to the carboncontaining residual fragments arising from cracking of hydrocarbons by the electron beam. That this is the strongest peak in the whole long scan spectrum
-
VVO, APS
iI-%‘J
J,1 200
300
400
500
800
700
800
Fig. 8. APS long-scan spectrum of YVO,.
=JGw
891
Study of YVO, by appearance potential spectroscopy
does not mean that the amount of these contaminants is large, having in view the high sensitivity of APS for carbon. As expected, the features at the appearance potentials are superposed on a raising bremsstrahlung background. Unusual is the drastic increase in the slope of this background above -5OOeV. We do not have a good explanation for this. One can tentatively assume that the excitation probability of either V-b., or O-core holes in YVO, increases with the energy of the incident electrons. Unexpected also is the very low intensity of the oxygen 1s core-level line. Oxygen adsorbed on surfaces and oxygen bound in other compounds, e.g. oxides, usually gives a clear line-shaped structure in the APS spectrum. In the APS spectrum of YVO,,
V-APS
vvq
A
/
one observes an increase in the intensity of the APS signal in the region where the 0-1s core-hole excitation should appear, but the signal continues to
increase further on, without revealing a line-like structure. We can therefore not make any interpretation of the behaviour of oxygen in the APS spectrum of YVO,. The yttrium “appearance” features expected at -150-160 eV (3d-core levels), 300-312 eV (3pcore levels) and 394 eV (3s core level) could not be detected in our experiment. Its 2p and 2s levels that are situated above 2000eV have not been measured. Interesting is the fact that, although YVO, is an insulator, no shift of the lines due to charging effects is observed, as shown by the position of the carbon 1s line that is found at 282.2eV in our spectra for YV04, as compared to 282.5 eV in the APS spectra of a number of metallic samples. One may possibly understand this if one assumes that the yield for secondary electron emission is so high that part of the low energy secondary electrons can act as a flood beam to neutralize the extra-charge formed on the specimen during excitation of APS spectra. Of course, the problem of charging effects+r rather of their absence-in the APS spectrum of YVO, would deserve a dedicated study. Based on the observation that no charge shift is manifested in the position of the C Is-line, we considered for interpretation the vanadium lines so as they appear in our APS spectra, without making any charge effect correction. Our interpretation will be restricted to only the vanadium LAPS line. 3.2. Comparison of APS spectra for V in YVO,
1
530
E (el
Fig. 9. AF’S spectrum of pare vanadium [17] as compared with the spectrum of vanadium in YVO, (present work). above the Fermi level, in YVO, such features are absent. This change of the whole spectrum when the specimen turns from the pure metallic state into the insulating chemical compound witnesses for a significant change in the local electronic structure
and
pure V-metal
Figure 9 shows in the lower part the APS spectrum of pure vanadium, taken from the work of Park and Houston [17], and in the upper part the APS b.z line-spectrum of vanadium in YV04, measured in our experiment. Both position and shape of the spectra are sign&antly diEerent. While pure vanadium shows the characteristic features of the APS spectrum of a transition metal, as determined by its density of states distribution
Fig. 10. Integrated APS spectra of pure vanadium and of vanadium in YOV,.
I. CUREIARU and P. 0. NILSSON
892
t
-
APS
APS spectrum of YVO, (this work), V-L, X-ray emission spectrum of YVO,[lS] energy level diagram of molecular orbit& of the cluster VOi- in YVO, ([12]).
Fig. 11. V-L,.,
common energy scale. The calculated energy level diagram of Lazukova et al. [16], aligned as to give the best agreement with the X-ray emission spectrum is also shown. The theoretical cluster calculations were performed by the semi-empirical MWH method. The type of molecular orbitals, the energy eigenvalues and the percent atomic orbital composition for the cluster VO:- are shown in Table 1. The molecular orbital levels are labelled according to the irreducible representations of the cubic 0, space symmetry group. The cluster calculations indicate that the highest occupied valence states in the cluster VO:-, that have a predominant 3d-character, are le, 2t, and 3f,, while the lowest unoccupied states with largest 3d% composition belong to the group of 2e MO. As seen in Fig. 11, the L3 X-ray emission of
of the compound, and a complete failure of the one-particle excitation model, due to the formation of localized levels. The most striking feature is the absence of the negative dip in the derivative APS spectrum, which indicates a completely different trend in the variation of the total X-ray emission yield with energy. This variation is illustrated in Fig. 10 that shows the integrated AT’S spectra of pure vanadium and of vanadium in WO,. No quantitative comparison is possible, since the intensity units are arbitrary and different in the two experiments. 3.3. Comparison calculations
of APS, WE-spectra
and
and cluster
Figure 11 collects our V-b,, AT’S spectrum of YVO, and the Vg X-ray emission spectrum measured by Kurmaev et al. [lS], aligned on a
Table 1. Molecular orbitals, energy eigenvalues and the percent atomic orbital composition for the cluster vo:MO
e(eV)
CW,,)
cw,,
1
cw,,
)
c&J
CW,,)
le
-17.551 -17.254
0.254* 0.280*
2r,
-17.018 -16.962 -16.962
0.205* 0.203* 0.203*
0.011 0.015 0.015
2a1
-16.801
3t*
-16.134 -16.088 -16.088
0.067* 0.056* 0.056*
0.001 0.001
-15.308 -15.170 -15.170 -12.142 -11.940
0.765 0.792
-
0.002
0.485 0.468
_
4r*
-11.092 -10.231 -10.231
0.739 0.738 0.738
0.0395 0.064 0.064
_ -
0.533 0.529 0.530
0.02 0.032 0.028
3a,
+ 9.980
-
-
1.668
-
0.759
r1 2e
* C2(3d) + C’(4.s)
-
Study of YVO, by appearance potential spectroscopy
vanadium in YVO, is dominated by a strong peak (labelled A) at 511.3 eV, followed by a second significant but less intensive peak (labelled B) at 516.0 eV. When comparing the X-ray emission spectrum with the cluster MO-diagram one assumes that the intensity maximum in the metal L-band X-ray emission is mainly due to transitions from orbitals containing maximum contribution of the metal 3d-states. In Fig. 11, the 2t, molecular orbital having the highest weight due to the Vjd atomic orbital has therefore been brought into coincidence with the main intensity maximum A in the X-ray emission spectrum (the height of the vertical columns in the theoretical energy level diagram in Fig. 11 is a measure of the atomic orbital percent composition for the cluster). It should be pointed out here that the theoretical cluster energy level diagram gives one-electron eigenvalues of an approximate ground state Hamiltonian. Moreover, the alignment procedure used as shown before implies neglection of any possible final state relaxation effects. Such effects due to the presence of the core hole are known to result in a change of the energy of the molecular orbitals, but not of their relative position [22]. We can therefore expect that such a “frozen-orbital” description of the excitation/desexcitation process will still be able to describe correctly the relative position of the possible contribution of various groups of orbitals to the total X-ray emission spectrum. In this respect, the fact that the second peak (B) in the X-ray emission spectrum falls within the bunch of 2estates is quite pleasing. If we adopt an atomic- like approach, the X-ray emission process can be viewed simply as an electronic transition between valence and core states. We therefore attribute peak A in the X-ray emission spectrum to transitions of the type: 2p5(le, 2t,, 3t2)n --* 2p6(le, 2t,, 3t,)“-1+hv.
(4)
In an XPS measurement we determined the binding energy of vanadium 3psn core level in YV04 at 517.8 eV. This value was obtained after a correction by 10 eV for the charging effect, as determined by the carbon 1s line arising from carbon containing impurities on the YVO, sample surface (the C-Is line appeared in the recorded XPS spectrum at 295 eV, as compared to the value of 285 eV generally accepted in the literature). We applied the same 10 eV correction to the vanadium 2p,,, binding energy, although this may rise some controversy as to whether an assumption that the charge accumulated on the specimen during measurement of XPS spectra is uniformly distributed on its surface, over the YVO, sites that give rise to the vanadium 2~,,~ signal and over those sites from where the adsorbed carbon-containing residual species provide the carbon 1s signal. Our experimental binding energy value of V-2p,,* determined as above is, however, consistent within the XPS experimental limits of accuracy with the value JF’CS Vol. 40, No. 12-B
893
518.4 eV obtained by Jdrgensen and Berthou [23]t. One currently admits, by deli&ion, that the value of the V-~P,,~ core level binding energy determines the position of the Fermi level of the sample material. In our case we observe that the Fermi level determined in this way at 517.8 eV falls above the calculated unoccupied 2e and 4t2 states. We propose that, in the process of excitation of X-ray emission spectra, these states are pulled down due to the effect of many body interactions involving the core hole created on the vanadium 2p,,,-level. When pulled down below the Fermi level, these states get occupied, whereafter these ‘Lresonance”-states decay through X-ray emission. We therefore interpret the additional strucuture (B) in the X-ray emission spectrum as originating from electron transitions of the type: 2p5(2e, 4t,)’ + 2p6(2e, 4Q-C hu.
(5)
Both 2e- and 4r,-states have a predominant V3,, character and are consequently expected to contribute to V-L, X-ray emission. The fact that peak B is centered at the 2e-levels and not at the center of gravity of the (2e, 4r2) group may suggest that the probability of the process of re-emission of radiation is greatest for the electrons excited to the lirst 2e-vacant orbitals, and is successively lower for electrons excited to higher orbitals. Before we conclude this paragraph, we would like to make some additional comments on the interpretation of the X-ray emission spectra. A general theory based on a many-body model for X-ray emission processes in real systems has not yet been developed. In the case of simple metals (Na), von Barth and Grossman have shown [24,25] that the experimental spectra are very well described by a one-body theory using a final state potential, i.e. the potential of the ground state after the core hole has disappeared. This is in fact the ground state potential from which the band structure is obtained. The agreement of the ground state calculation with experiment can be explained by the many-body theory of Nozieres and De Dominicis [26]. This means that for absorption spectra, one can expect a satisfactory agreement with theory only if one uses again the final state potential, i.e. the potential in the presence of the fully screened core hole. Moreover, wave functions in the presence of the core hole should be found and used instead of the wave functions obtained from a band structure calculation. This approach is however not applicable
in the
t In their original publication [23], these authors give a binding energy for 2p,,a electrons of vanadium in YVO, equal to 523.4 eV, but they relate it to the carbon Is-line at 29O.OeV instead of the commonly accepted value of 285.0eV. For comparisoh witb our XPS data, we flrat subatracted 5.0eV from the binding energy reported by Jergensen and Berthou.
I. CURELARUand P. 0. NIL~~~N
894
case of YVO, where the studied states are highly localized and thus occupation dependent. Instead we use, as discussed above, an atomic description, and the initial state in the emission process becomes important, The initially empty (2e, 4t,) states of the cluster VOi- in YVO,, involved in the process of X-ray emission, should also answer for th6 occurrence of the measured APS spectrum. We recall that, in general, when some localized empty states exist within a band distribution of other empty states (as for example in the case of lanthanum discussed above) the appearance potential spectrum is expected to contain tsvo separate structures, one due to excitation processes in which one electron gets captured on a localized level, the second electron entering an itinerant state, and a second one due to excitation processes in which both electrons enter into the localized state. The first APS structure in YVO, would then involve population of the empty states through the transition: 2p6(2e, 4t,)‘+
e-(incident --* 2p5(2e, 4t$
+ e-(itinerant) (6) followed by X-ray emission through decay of these excited states: 2p5(2e, 4fJ1 + 2p6(2e, 4tJ0 + hu.
(7)
It is seen that the configurations involved in this APS process of X-ray emission (reaction (7) above) are equivalent with the configurations involved in the X-ray emission process corresponding to peak B in the experimental spectrum of Kurmaev et al. One therefore expects that this first APS structure would appear at exactly the same energy as peak B. However, our experimental APS spectrum does not show it as a clear-cut peak, but only as a low-energy tail of the second structure. We interpret this Bs being due to the very low density of wntinuum-like states in this energy region. Inspection of Table 1 shows that the percent contribution of s-like states near the “Fermi’‘-level coming from the 2e-and 4t,-MO is very low. The orbital 3a1 which contains a high density of s-states arising from V, and Oza atomic orbitals is separated by almost 20 eV from the “Fermi’‘-level, and is therefore not available for electrons with energy near the edge of excitation of APS processes. Although the experimental APS curve does not show any peak-like structure at the energy corresponding to peak B in the X-ray emission spectrum, the low energy branch of the “Hewnd” APS structure (peak C) extending from about 513 eV to 517 eV is an indication that such APS transitions are present. The only peak-like structure that appears in our APS spectrum (peak C) is seen to fall at higher energy than the group of (2e, 4t&empty orbitals. According to APS theory, this “second” structure would correspond to excitation processes in which
both the incident and the core-electrons enter the localized empty states through the following transitions: 2p6(2e, 4t,)‘+e-(incident)
+ 2p5(2e, 4t2)2.
(8)
The high intensity of this “second” APS structure is due to the fact that it contains two diflerent wntributions, one from the “characteristic” APS emission 2ps(2e, 4fJ2 + 2p6(2e, 4t,)‘+ and one from an enhanced strahlung emission Cl83: 2p6(2e, 4t,)‘+
hv
(9)
(resonance)
brems-
e-(incident) + 2p”(2e, 4tz)l + hv.
As seen, both these channels have the same final state wniiguration. 3.4 Elgect of argon ion-bombardment Figure 12 shows a set of APS spectra of WOI, measured in the “as-received” state (l), immediately after bombardment with argon ions (2) and also 2 and 4 h afterwards ((3) and (4), respectively). One can see that the spectrum measured just after ion bombardment is shifted about 1.5 eV toward lower energy values, while the spectra measured successively afterwards, at an interval of about 2 h, are gradually shifted back toward the position of the spectrum corresponding to the sample in the “as-received” state. Two alternative explanations may be forwarded
----
after Ion Bomb.
Fig. 12. APS spectra of V in WO,: (1) specimen in the “as-received” state; (2) immediately after ion bombardment; (3) 2 h after ion bombardment; (4) 4 h after ion bombardment.
Study of YVO, by appearance potential spectroscopy for this shifting effect. Since YVO, is an insulator, it gets positively charged during bombardment with argon ions, and the shift of the spectrum recorded just after ion bombardment may be due to such a charging effect. The spectra recorded later on then correspond to the specimen in which charging has gradually been compensated by bombardment with electrons from the emitting filament, during excitation of the APS process itself. The second alternative explanation may be connected to a possible chemical modification of YVO, compound submitted to prolonged bombardment with argon ions. Since vanadium atom in YVO, is in a state of total oxidation, ion bombardment may cause a reduction of the oxidation state, followed by a gradual recovery of the initial state by heating due to electron bombardment during the process of measurement of APS spectra. However, the spectra never came back to the initial position (before ion bombardment) and, which is most important, their shape continued to change drastically. The most significant feature is the growth of the dip that follows after peak C, it becoming even negative in the last spectrum. An attempt to connect this increasing dip with an increasing character reminding the APS spectrum of pure vanadium-metal is quickly disappointing, since the shift of the whole spectrum is then in the wrong direction, not approaching but moving away from the position of the APS spectrum of pure vanadium. This discussion is highly speculative and cannot be pursuited without a systematic experimental investigation as a basis. We content ourselves here to show in Fig. 13
Fig. 13. Integrated Aps spectra from Fig. 12.
895
the integrated APS spectra from Fig. 12, with the aim to illustrate the significant change in the energy variation of the APS total X-ray emission yield. In a dedicated experiment aimed at an understanding of the above experimental observation, one should take into account the thermal and chemical stability of YVO, under electron bombardment. We think it is useful to mention that the melting temperature of YVO, is 1810-1850°C. We have not measured purposely the temperature of the YVOT specimen during recording of APS spectra, but our previous experience with other specimens has shown that this temperature increases at most up to 200300°C. Nothing is known at present about the stability of the compound under noble gas ion bombardment. According to existing experimental data [27], compounds on the base of YV04, that are used as X-ray luminescent screens, are known to be stable when exposed to radiation. 4.
CONCLUSIONS
We have shown that due to the presence of localized empty states in the compound YV04, the experimental vanadium APS spectrum is significantly different from the APS spectrum of pure vanadium-metal. Many-body effects in the final state of the excitation processes result in a shift of the energy position of these localized states, that is specifically occupation-dependent. The simple APS theory that explains satisfactorily the APS spectra of metallic samples, which are characterized by extended (band-like) empty states above the Fermi level, is no longer valid. Neither the position nor the intensity of experimental APS features can be reproduced by a model based on the selfconvolution of the density of states above the Fermi level. An atom-like approach is here more realistic, in which each feature observed in the APS spectra is interpreted in terms of the wspecific configuration-interactions involved in the excitation and decay processes. For the interpretation of our spectra, we use a theoretical energy level diagram of the cluster VO:-. The first unoccupied molecular orbitals (2e) of this cluster, that have a predominant VBd-character, have been earlier tentatively inferred by Kurmaev et al. in explaining an extrastructure observed on the high energy side of the main VL, X-ray emission line [15, 161. This interpretation is qualitively confirmed by our APS experiment. However, we propose that both 2eand 4r,-MO participate in the process, instead of only the 2e-states as discussed by Kurmaev [lS, 161 and Gubanov [12]. The fact that the extra-structure (peak B) in the X-ray emission spectrum is centered at the 2e-states may be attributed to a significantly higher decay cross section of these excited states, as compared to the 4&-states. Our experimental APS spectrum shows an increasing intensity of emitted X-rays in the energy region where both the 2e and 4&-states are located.
8%
I.CUREUFW andP.O.Nmso~ Table 2. Configuration-transitionsinvolvedin WE, APS, and XPS for YVO,
SXE
APS
Peak A
2ps(le, 2t,, 3t,)n + 2p6(le, 2b, 3Q”-’ + hv
Peak B
2p5(2e, 4t,)l--, 2p6(2e, 4tJ”+ hv
Fit structure (observed as a “tail”)
2p5(2e, 4tJ’ + 2p6(2e, 4tJ0+ h
Peak C (second structure)
2ps(2e, 4tJ2 + 2p6(2e, 4tJ’+ hv(characteristic) 2p6(2e, 4Q+e-(incident) -+ 2p6(2e, 4t# C hv(bremsstrablung)
XPS
Marked by arrow in Fig. 11
2p6+hv+2p5+e-
We measured in a separate experiment the XPS binding energy of V,, core-level in YVO, and found a value of 517.8eV (after correction by POeV for charging effects) that agrees well with the value 518.2 eV reported earlier by Jlrgensen and Berthou [23]. If one accepts this binding energy value as defining the “Fermi’‘-level of YV04, one observes that both peak B in the X-ray emission spectrum and the onset of the APS spectrum lie below this “Fermi’‘-level. This result proves that the orbitals 2e and 4t, which are situated above the “Fermi”level in the ground state of YVO.+, change their energy position being pulled down, below the “Fermi”-level, when core-holes are created in the processes of excitation of both APS and X-ray emission spectra. The first structure expected in APS to coincide with the position of peak B in X-ray emission is not manifested in our spectra as a peak because it implies the existence of delocalized (itinerant) states whose density is very low within the (2e, 4r,) orbitals. The second APS structure (peak C) contains contributions from the characteristic APS processes in which both electrons enter the localized (2e, 4t,) states, as well as from an enhanced bremsstrahlung emission. Table 2 shows the configuration-transitions proposed by us to explain the APS, SXE and XPS spectra. A few facts remained not completely understood in the present work, namely the significant continuing increase of the total X-ray emission yield observed in the long scan APS beyond -500 eV, the poorly structured O1. APS signal, and the lack of any charging effect under electron bombardment, in spite of the fact that YVO, is an insulating material. An improvement of cluster calculations would be desirable. A more realistic energy level diagram is expected to be provided by iteration of calculations to self-consistency, by inclusion of the effect of more distant neighbours, inclusion of spin-orbit and
relativistic effects and by adopting a many-body (configuration-interaction) approach. Complementary, useful information would be provided in an investigation of YVOa by isochromat spectroscopy in which some other, different configurations are involved.
Acknowledgements-We want to express our thanks to Professor G. Brogren for his interest in the project and to Dr. J. Olefjord for the permission to use the XPS spectrometer of the Department of Engineering Materials, Chalmers University of Technology, Gothenburg. We benefited from extended discussions and cormspondence with E. Z. Kurmaev and V. A. Gubanov. We are indebted to N. I. Lazukova et al. for putting at our disposal their cluster calculations prior to publication. Financial support from the Swedish National Research Council is gratefully acknowledged.
REFERENCES
1. Baglio J. A. and Gashurov G., Acta Crystallog. B24, 292 (1968). 2. Levine A. K. and Palilla F. C., Appl. Phys. Len. 5, 118 (1964). 3. O’Connor J. R., Appl. Phys. Len. 9, 407 (1966). 4. O’Connor J. R., Trans. Met. Sot. AIME 239, 362 (1967). 5. Rubin J. J. and van-Uitert L. G., J. Appl. Phys. 37, 2920 (1966). 6. Kodos M. Ya. et al., Zh. Prikl. Spektrosk. (J. Appl. Spectrosc., Minsk) 16, 1023 (1972). 7. Rapp R. C.,J. Electrochem. Sot. 115,841 (1968). 8.Gambino R. and Guare C. J., Nature 198,1084 (1963). 9. van-Uitert L. G. et al., J. Chem. Phys. 36, 702, 1793 (1962). 10. Chang N. C., J. Appl. Phys. 34, 3500 (1963). 11. Wichersheim K. A. and Lefever R. A., J. Electrochem. Sot. 111,47 (1964). 12. Gubanov V. A., Ellis D. E. and Fotiev A. A., J. Solid State Chem. 21,303 (1977). 13.EIlis D. E. and Painter G. S., Phys. Reu. B2, 2887 (1970).
Study of YVO, by appearance potential spectroscopy 14. Averill F. W. and Ellis IX E., J. Chem. Phys. 59,
6412 (1973). 15. K~aev E. Z., Gubanov V. A. and C~rk~benko V. M., J. ~~ect~n Spectr. Related P~~rn., to be published. 16. Lazukova N. I. et at., Zh. Struktunwi Khimii, USSR., in press.
17. Park R. L. and Houston J. E., Phys. Rev. B6, 1073 (1972). 18. Liefeld R. J., Burr A. F. and Chamberlain M. B., Phys. Rev. A9, 316 (1974). 19. Wendin G. and Nuroh K., Phys. Reu. Lea 39, 48 (1977) 20. Nuroh K. and Wendm G., Institute of Theoretical Physics, Chalmers U~ive~i~ of Technology, Internal Reports 78-35, X-36, 89-39 (to be published). 21. Kanski J., Nilsson P. 0 and Curelaru I., J. pkys. F6* 1073 (1976).
897
22. ~W.~OV I... N., Zh. Strukturnoi Khimii (USSR) 18, 607 (1977). 23. J0r~e~en C. K. and Berthou I-I., Det. K. Dan. Viden&. Selsk. Mut.-Fys. Me&d. 38, (IS), 1 (1972). 24. van Barth U. and Grossman G., in heeding of ike 5th Intemationof Conference on Vacuum Ultrauiolet Radiation Physics, Montpellier, France, 1977, M. C.
Castex, M. Poucy, N. Pouey, eds. 25. von Barth U. and Grossman G., in Proceedings of the 8th Annual International Symposium on Electronic Structure of Metals and Altoys, Gauss&, RDR, 1978, to be published. 26. No&res P. and De Dominicis C. T., Phys. Rev. 2’7% 1097 (1969). 27. See Fotiev A. A., ShuIgiu B. V., Moskvin A. S. and Gavrilov F. F., Vanodievye K~st~Zlop~~p~~, Nauka, Moscow, 1976 and references herein.
I. Phys. Chem. Solids
STUDY OF YVO, BY APPEARANCE I. CURE-U
POTENTIAL
and P. 0.
SPECTROSCOPY
NILSSON
Chalmers Universityof Technology, 412 96 Gothenburg, Sweden
(Receiued 21, February 1979; accepted 17 April 1979) Ah&a&-The luminescentorthovanadate compound YVO, has been studied by soft X-ray Appearance Potential Spectroscopy. Vanadium b,3 APS spectra, which give information about unoccupied electron states, have been interpreted in conjunction with existing experimental data on soft X-ray emission and with cluster model calculations. The main features of the APS spectrum of vanadium in YVO, as compared with the APS spectrum of pure vanadium metal are discussed, as well as the extent to which the vanadiumb,3 APS-signal in YVO, can be explained by the energy level diagram given by a non-self-consistent calculation in a VO:- cluster model. 1. INTRODUCTION
This paper reports on an experimental evidence for the existence of many-body effects in the excitation of Appearance Potential Spectra in a rather complicated chemical compound, namely yttrium orthovanadate (YVO,). YVO, is a luminescent material that crystallizes in the tetragonal system with the zirconium (ZrSiO,) structure (Fig. 1) and the 0:: space group symmetry. The crystal lattice of YVO, has thoroughly been investigated and its lattice parameters have been determined to a great degree of accuracy [l]. In the crystal lattice of YV04 the nearest neighbours of each vanadium atom are four oxygen atoms situated at a common V-O bond length of 1.706 8, on a slightly distorted tetraheder. The yttrium atom is coordinated to two sets of four oxygen atoms having the Y-O bond lengths equal to 2.299 and 2.443 a respectively. Two types of clusters are, therefore, characteristic for yttrium orthovanadate compound, namely VO:- and YOF-. If activated with a rare-earth ion, the lumines-
Fig. 1. Crystal structure of WO,.
cent properties change remarkably. So, for example, YVO,: Eu is a highly-efficient red-emitting phosphor which is at present widely used as a coating in luminescent lamps and colour television tubes [2]. Activation of YVO, with other rare-earth ions results in the production of very good laser materials for the infrared (YVO,:Nd [3]), and visible region of the optical spectrum (YVO,:Er, YVO, :Tm, [4, 51). Interesting is the case of YVO, :Tb in which the terbium ion acts as a quencher of luminescence, rather than an activator [6]. If Tb is added instead to a phosphate matrix, it luminesces intensely in green [7]. Cerium is similar in that it also shows an intense luminescence in the ultraviolet region when included in phosphate matrices, but fluoresces very weakly in vanadates. Common for all these compounds is the fact that the rare-earth activator substitutionally replaces one of Y atoms in the crystal lattice, leaving the crystal structure unchanged, with lattice parameters very close to those of YVO, [8]. The interest in the rare earth orthovanadates, as host matrices of such phosphors, has justified an extensive approach to study of their optical properties and electronic structure. Earlier experimental works performed some 15 years ago have focused on the study of the optical spectra of rare-earth orthovanadates and their comparison with the corresponding spectra of oxide and phosphide compounds [2, g-111. These works have a more descriptive character and leave unsolved a number of problems related to the mechanism of luminescence in these complex compounds, the presence or absence of optical activity of rareearth ions in various crystal lattice hosts, the detailed ordering of energy level diagrams of the compounds, etc. In general, there is little known about the bonding mechanism in vanadium compounds, and any attempt at a realistic description of these practically
887
888
I. C~RELARUand P. 0. NILSON
important but structurally complicated systems is welcome. YVO, belongs to a group of orthovanadates for which the study of the optical spectra have shown that no additional bands due to the rare-earth ion are present in the visible and near ultraviolet region of absorption and emission. This result has justified an interpretation based on a cluster approach, in which the luminescent properties of the compound are entirely attributed to optical transitions between various electronic states within the VO:cluster. Recent calculations of the electronic structure of VOZ- and R0i3- molecular clusters of some rare-earth orthovanadates (here R stands for the rare-earth ion R= Y, Ce, Nd, Eu, Tb, Dy, Gd and Yb) have been performed by Ellis et al. [12-141 using semi-empirical MWH and CNDO methods. These authors obtained an energy level diagram of both the occupied and unoccupied states, and a charge distribution that are apparently consistent with the existing experimental data on excitation, reflection and luminescence spectra of these compounds. On the basis of their energy level scheme, the authors tentatively explain the luminescence of the compound, when excited by X-rays, as being due to a three-step electron transition process within the tetrahedral vanadate ion VOZ-, in the following way. In the fist step, an electron is excited from the V 2p-level to a 2e vacant molecular orbital, after which a second electron transition occurs from the G 2p-band to the V 2p-level, with hole formation within the 0 2p-band. Finally, the electron excited to the 2e molecular orbital undergoes a luminescent transition to the 0 2p-band (Fig. 2). A condition for the realization of the optical process is, therefore, the formation of excited states on the 2e-molecular orbital. Experimental evidence for the existence of such “excited states” has recently been given by Kurmaev et al. [15] and Lazukova et al. [16] in the study of VL, X-ray emission spectra of YVO,. In addition to the main VI+ emission band, these authors found a rather intensive structure situated at higher energies (peak B, figure 11). They interpret it as being due to re-emission of radiation through transitions from the “excited” 2e molecular orbitals directly to the V 2p,,,-level. Since Appearance Potential Spectroscopy (APS) probes the unoccupied part of the electron states, it appears as a convenient method for testing the existence of the “excited states” which are believed
Fig. 2. Luminescence of YVO, explained as a three-step process of electron transition within the tetrahedral vanadate ion VO:-.
Fig. 3. Basic electron transitions involved in the APS process within the one-particle
model.
to be responsible for the luminescence of rare-earth orthovanadates. As known, in Appearance Potential Spectroscopy the specimen is bombarded by a quasimonoenergetic beam of electrons whose energy is continuously increased within a range of values covering the excitation of various core-levels. At each excitation edge, a sudden increase in the total X-ray yield is observed, due to the contribution of the characteristic X-ray emission superposed onto the continuously raising bremsstrahlung radiation (Fig. 3). The presence of the core hole together with both the incident and the excited electrons being captured into some available empty states situated just above the Fermi level makes the excitation and decay processes in APS in general complicated many-body processes. In the simplest case of itinerant conduction band states, a simple one-particle model has been developed [17], that could successfully explain the observed APS-spectra. In this model the assumptions is made that the matrix element involved in the transitions is constant. This approximation is supposed in most cases to only slightly influence the intensities, while the energy position of the peaks remain unaffected. The X-ray yield is then determined mainly by the self-convolution of the density of empty electron states above the Fermi level [17] E--E< Y tof= deN(e)N(E -EC -E). (1) d In order to enhance the edge features, the derivative instead of the direct total yield is usually recorded: d I APSo:Y,,. dE Let us consider a model distribution of the empty density of states that shows some structure at a distance x above the Fermi level (Fig. 4). The self-convolution of this density of states function N(E) results in a model APS-signal that reproduces the structure present in the DOS distribution at the same distance above the Fermi level and with the same width, but shows also an additional structure, much more intense, occurring at the distance 2x from the Fermi level, and containing a negative dip. It is therefore expected that the shape. of APS spectra is determined by the self-convolution of the density of empty conduction-band states, while the position of the excitation threshold provides a measure of the core-level binding energy.
Study of YVO,
by appearance
(al
Fig. 4.(a) Model-density of states of a simple-metal. (b) Model-Al’s signal within the one-particle model.
In the
case
of transition
metals
the
Fermi
level
lies within a highly peaked density of states, part of which is occupied and another part is empty (Fig. 5) The distance x to the lirst peak above the Fermi level is then equal to zero and the two “structures” in the derivative of the selfconvolution of the density of empty states overlap resulting in a typical APS-spectrum as shown in Fig. 5. If localized empty states are present, the oneparticle model sketched above fails to explain the experimental spectra even qualitatively. As a consequence of many-body interactions in the final states of the APS process, large bremsstrahlung resonances appear, and the energy position of the electronic states becomes highly occupationdependent [18-201. The main processes involved are illustrated in Fig. 6. In this case, neither the shape nor the position of the features in the APS spectra are consistent with the description given by the one-particle model through a simple correlation with the self-convolution of the density of empty states. An interpretation based on the specific configurations involved in the process appears to be a more realistic approach [19, 201. For illustration, we show in Fig. 7 the energy level diagram of localized 4f states in lanthanum, with different configurations, as obtained by three different spectroscopic methods, namely continuum isochromat spectroscopy (CIS) [ 181, electronexcited APS (EXAPS) [21] and photon-excited
889
potential spectroscopy
-
-75 C
ib)
Fig. 6. Basic electron transitions involved in the APS process in a transition metal. (a) bremsstrahlung emission; (b) bremsstrahhmg resonance emission; (c) characteristic emission. APS (XEAPS) [21]. It is seen that the energy position of the 4f level is highly dependent on the final state configuration of the system in these three processes. None of these “experimental” 4f-levels coincides with the position of the empty ground state 4f -level which would be tested in a oneelectron model. In isochromat spectroscopy, in which the final state contains one electron in a 4f -state and no core-hole is created, the 4f-level appears at 5.5 eV above the Fermi-level. In both electron-excited APS and photon-excited APS one hole is created on the core-3d-level, and the 4flevel is pulled down, appearing at 0.2 eV below the Fermi-level when populated with two electrons (EXAPS) and at 1.8 eV below the Fermi level when populated with one electron (XEAPS). We were aware of all these difficulties inherent in the interpretation of APS spectra of YVO,. We believe, however, that our data may throw some light on the complexity of the problem of understanding the luminescent properties of rare-earth orthovanadates in terms of their electronic structure. In the present paper, we report on the measurement of the APS spectrum of YVO, and interpret it in conjunction with the existing X-ray emission spectra [15, 161 and cluster-model calculations
ICI)
3d’0+~-3d’0+Lf’
---
+S.SeV
(CI)
1
qo
N(El
L I
1.8 eV IXEAPSI
Al=
APSIE
EF
3d”
0.2 ev (EXAPS)
E
Fig. 5.(a) Model-density of states of a transition metal. (b) Model-AI’s signal for a transition metal.
u---
835.8
(XPS)
Fig. 7. Energy level diagram of 4f-states in lanthanum as obtained by continuum isochromat spectroscopy (CI) and appearance potential spectroscopy in both electronexcitation-(EXAF’S) and photon-excitation (XEAF’S) variations.
I. CURELARUand P. 0. NILSON
8!m
([12, 161). An additional XPS measurement was performed in order to determine the binding energy of V,,-level in YVO,. 2. EwERlMENT
compound YV04, prepared as a polycrystalline powder by synthesisof oxides Y,O, and V205: The
Y,O, +v,o,
+ 2 YVO&
(3)
was loaded without any glue on a pure copper sample holder on the surface of which a mesh of scratches had been performed by hand. the specimen-material investigated in our APS experiment was part of the same product that was used by Kurmaev et al. in their X-ray emission experiment [15]. No chemical analysis has been performed on the specimen, but both the long-scan APS and XPS spectra, as well as the X-ray emission spectrum do not show any significant impurity contamination. The APS spectra were recorded as the derivative of the total soft X-ray yield by using the phase lock-in amplification technique with a modulation amplitude of 1 V peak-to-peak. The pressure inside the vacuum chamber during the measurement was in the low lop9 torr region. The APS spectra were excited by an electron beam of -1 mA emitted by a V-shaped tungsten filament, chemically etched at its top in order to localize the emission and minimize the voltage-drop across the emitting region of the filament. The energy scale of the spectra is given by the voltage applied on the specimen with respect to the mid-point of the emitting tungsten-filament, corrected for the work function and the thermal spread of the emitted electrons. In our experiment, the total correction is estimated to be equal to +5.0 eV. The accuracy in peak position determination, due mainly to the inherent width of the excited core level, the experimental broadening, the scale correction factor (+5.0 eV) and the absolute voltage calibration is in our case estimated
to be about ~tO.3 eV. Vanadium in YVO, being in a state of total oxidation, no noble-gas ionbombardment prior to spectra recording seems to be necessary, and the APS spectrum recorded with state can be the specimen in the “as-received” considered as being representative for the YVO, compound. However, we performed an argon ion bombardment as we routinely use to do in our APS experiments with the aim to remove the possibly adsorbed oxygen and oxygen-containing species. We observed a significant change in the shape and position of the APS spectra which indicates a change in the chemical composition of the specimen and deserves a special attention. The parameters of ion bombardment were the following: ion energy 1 500 eV, pressure -lo5 torr, duration -10 h. No thorough investigation of these changes in the spectra was pursuited at the present stage of the work, but the spectra will be shown in the last part of the paper to point out the interest for such an investigation. In the following, only the APS spectrum of vanadium in YVO, in the “asreceived” state was chosen for interpretation. The binding energy of V,, core-level in YVO, was measured using a Hewlett-Packard HP-5950A electron spectrometer equipped with a monochromatized Al& X-ray source. 3. RESULTS AND DISCUSSION
3.1. APS long spectrum Figure 8 shows the APS long-scan spectrum. One sees the structure corresponding to carbon at -280eV, the b and b lines of vanadium at -5OOeV and a hardly identifiable contribution from oxygen just above the vanadium L,,* lines. No other contaminants are observed. The carbon structure is due to the carboncontaining residual fragments arising from cracking of hydrocarbons by the electron beam. That this is the strongest peak in the whole long scan spectrum
-
VVO, APS
iI-%‘J
J,1 200
300
400
500
800
700
800
Fig. 8. APS long-scan spectrum of YVO,.
=JGw
891
Study of YVO, by appearance potential spectroscopy
does not mean that the amount of these contaminants is large, having in view the high sensitivity of APS for carbon. As expected, the features at the appearance potentials are superposed on a raising bremsstrahlung background. Unusual is the drastic increase in the slope of this background above -5OOeV. We do not have a good explanation for this. One can tentatively assume that the excitation probability of either V-b., or O-core holes in YVO, increases with the energy of the incident electrons. Unexpected also is the very low intensity of the oxygen 1s core-level line. Oxygen adsorbed on surfaces and oxygen bound in other compounds, e.g. oxides, usually gives a clear line-shaped structure in the APS spectrum. In the APS spectrum of YVO,,
V-APS
vvq
A
/
one observes an increase in the intensity of the APS signal in the region where the 0-1s core-hole excitation should appear, but the signal continues to
increase further on, without revealing a line-like structure. We can therefore not make any interpretation of the behaviour of oxygen in the APS spectrum of YVO,. The yttrium “appearance” features expected at -150-160 eV (3d-core levels), 300-312 eV (3pcore levels) and 394 eV (3s core level) could not be detected in our experiment. Its 2p and 2s levels that are situated above 2000eV have not been measured. Interesting is the fact that, although YVO, is an insulator, no shift of the lines due to charging effects is observed, as shown by the position of the carbon 1s line that is found at 282.2eV in our spectra for YV04, as compared to 282.5 eV in the APS spectra of a number of metallic samples. One may possibly understand this if one assumes that the yield for secondary electron emission is so high that part of the low energy secondary electrons can act as a flood beam to neutralize the extra-charge formed on the specimen during excitation of APS spectra. Of course, the problem of charging effects+r rather of their absence-in the APS spectrum of YVO, would deserve a dedicated study. Based on the observation that no charge shift is manifested in the position of the C Is-line, we considered for interpretation the vanadium lines so as they appear in our APS spectra, without making any charge effect correction. Our interpretation will be restricted to only the vanadium LAPS line. 3.2. Comparison of APS spectra for V in YVO,
1
530
E (el
Fig. 9. AF’S spectrum of pare vanadium [17] as compared with the spectrum of vanadium in YVO, (present work). above the Fermi level, in YVO, such features are absent. This change of the whole spectrum when the specimen turns from the pure metallic state into the insulating chemical compound witnesses for a significant change in the local electronic structure
and
pure V-metal
Figure 9 shows in the lower part the APS spectrum of pure vanadium, taken from the work of Park and Houston [17], and in the upper part the APS b.z line-spectrum of vanadium in YV04, measured in our experiment. Both position and shape of the spectra are sign&antly diEerent. While pure vanadium shows the characteristic features of the APS spectrum of a transition metal, as determined by its density of states distribution
Fig. 10. Integrated APS spectra of pure vanadium and of vanadium in YOV,.
I. CUREIARU and P. 0. NILSSON
892
t
-
APS
APS spectrum of YVO, (this work), V-L, X-ray emission spectrum of YVO,[lS] energy level diagram of molecular orbit& of the cluster VOi- in YVO, ([12]).
Fig. 11. V-L,.,
common energy scale. The calculated energy level diagram of Lazukova et al. [16], aligned as to give the best agreement with the X-ray emission spectrum is also shown. The theoretical cluster calculations were performed by the semi-empirical MWH method. The type of molecular orbitals, the energy eigenvalues and the percent atomic orbital composition for the cluster VO:- are shown in Table 1. The molecular orbital levels are labelled according to the irreducible representations of the cubic 0, space symmetry group. The cluster calculations indicate that the highest occupied valence states in the cluster VO:-, that have a predominant 3d-character, are le, 2t, and 3f,, while the lowest unoccupied states with largest 3d% composition belong to the group of 2e MO. As seen in Fig. 11, the L3 X-ray emission of
of the compound, and a complete failure of the one-particle excitation model, due to the formation of localized levels. The most striking feature is the absence of the negative dip in the derivative APS spectrum, which indicates a completely different trend in the variation of the total X-ray emission yield with energy. This variation is illustrated in Fig. 10 that shows the integrated AT’S spectra of pure vanadium and of vanadium in WO,. No quantitative comparison is possible, since the intensity units are arbitrary and different in the two experiments. 3.3. Comparison calculations
of APS, WE-spectra
and
and cluster
Figure 11 collects our V-b,, AT’S spectrum of YVO, and the Vg X-ray emission spectrum measured by Kurmaev et al. [lS], aligned on a
Table 1. Molecular orbitals, energy eigenvalues and the percent atomic orbital composition for the cluster vo:MO
e(eV)
CW,,)
cw,,
1
cw,,
)
c&J
CW,,)
le
-17.551 -17.254
0.254* 0.280*
2r,
-17.018 -16.962 -16.962
0.205* 0.203* 0.203*
0.011 0.015 0.015
2a1
-16.801
3t*
-16.134 -16.088 -16.088
0.067* 0.056* 0.056*
0.001 0.001
-15.308 -15.170 -15.170 -12.142 -11.940
0.765 0.792
-
0.002
0.485 0.468
_
4r*
-11.092 -10.231 -10.231
0.739 0.738 0.738
0.0395 0.064 0.064
_ -
0.533 0.529 0.530
0.02 0.032 0.028
3a,
+ 9.980
-
-
1.668
-
0.759
r1 2e
* C2(3d) + C’(4.s)
-
Study of YVO, by appearance potential spectroscopy
vanadium in YVO, is dominated by a strong peak (labelled A) at 511.3 eV, followed by a second significant but less intensive peak (labelled B) at 516.0 eV. When comparing the X-ray emission spectrum with the cluster MO-diagram one assumes that the intensity maximum in the metal L-band X-ray emission is mainly due to transitions from orbitals containing maximum contribution of the metal 3d-states. In Fig. 11, the 2t, molecular orbital having the highest weight due to the Vjd atomic orbital has therefore been brought into coincidence with the main intensity maximum A in the X-ray emission spectrum (the height of the vertical columns in the theoretical energy level diagram in Fig. 11 is a measure of the atomic orbital percent composition for the cluster). It should be pointed out here that the theoretical cluster energy level diagram gives one-electron eigenvalues of an approximate ground state Hamiltonian. Moreover, the alignment procedure used as shown before implies neglection of any possible final state relaxation effects. Such effects due to the presence of the core hole are known to result in a change of the energy of the molecular orbitals, but not of their relative position [22]. We can therefore expect that such a “frozen-orbital” description of the excitation/desexcitation process will still be able to describe correctly the relative position of the possible contribution of various groups of orbitals to the total X-ray emission spectrum. In this respect, the fact that the second peak (B) in the X-ray emission spectrum falls within the bunch of 2estates is quite pleasing. If we adopt an atomic- like approach, the X-ray emission process can be viewed simply as an electronic transition between valence and core states. We therefore attribute peak A in the X-ray emission spectrum to transitions of the type: 2p5(le, 2t,, 3t2)n --* 2p6(le, 2t,, 3t,)“-1+hv.
(4)
In an XPS measurement we determined the binding energy of vanadium 3psn core level in YV04 at 517.8 eV. This value was obtained after a correction by 10 eV for the charging effect, as determined by the carbon 1s line arising from carbon containing impurities on the YVO, sample surface (the C-Is line appeared in the recorded XPS spectrum at 295 eV, as compared to the value of 285 eV generally accepted in the literature). We applied the same 10 eV correction to the vanadium 2p,,, binding energy, although this may rise some controversy as to whether an assumption that the charge accumulated on the specimen during measurement of XPS spectra is uniformly distributed on its surface, over the YVO, sites that give rise to the vanadium 2~,,~ signal and over those sites from where the adsorbed carbon-containing residual species provide the carbon 1s signal. Our experimental binding energy value of V-2p,,* determined as above is, however, consistent within the XPS experimental limits of accuracy with the value JF’CS Vol. 40, No. 12-B
893
518.4 eV obtained by Jdrgensen and Berthou [23]t. One currently admits, by deli&ion, that the value of the V-~P,,~ core level binding energy determines the position of the Fermi level of the sample material. In our case we observe that the Fermi level determined in this way at 517.8 eV falls above the calculated unoccupied 2e and 4t2 states. We propose that, in the process of excitation of X-ray emission spectra, these states are pulled down due to the effect of many body interactions involving the core hole created on the vanadium 2p,,,-level. When pulled down below the Fermi level, these states get occupied, whereafter these ‘Lresonance”-states decay through X-ray emission. We therefore interpret the additional strucuture (B) in the X-ray emission spectrum as originating from electron transitions of the type: 2p5(2e, 4t,)’ + 2p6(2e, 4Q-C hu.
(5)
Both 2e- and 4r,-states have a predominant V3,, character and are consequently expected to contribute to V-L, X-ray emission. The fact that peak B is centered at the 2e-levels and not at the center of gravity of the (2e, 4r2) group may suggest that the probability of the process of re-emission of radiation is greatest for the electrons excited to the lirst 2e-vacant orbitals, and is successively lower for electrons excited to higher orbitals. Before we conclude this paragraph, we would like to make some additional comments on the interpretation of the X-ray emission spectra. A general theory based on a many-body model for X-ray emission processes in real systems has not yet been developed. In the case of simple metals (Na), von Barth and Grossman have shown [24,25] that the experimental spectra are very well described by a one-body theory using a final state potential, i.e. the potential of the ground state after the core hole has disappeared. This is in fact the ground state potential from which the band structure is obtained. The agreement of the ground state calculation with experiment can be explained by the many-body theory of Nozieres and De Dominicis [26]. This means that for absorption spectra, one can expect a satisfactory agreement with theory only if one uses again the final state potential, i.e. the potential in the presence of the fully screened core hole. Moreover, wave functions in the presence of the core hole should be found and used instead of the wave functions obtained from a band structure calculation. This approach is however not applicable
in the
t In their original publication [23], these authors give a binding energy for 2p,,a electrons of vanadium in YVO, equal to 523.4 eV, but they relate it to the carbon Is-line at 29O.OeV instead of the commonly accepted value of 285.0eV. For comparisoh witb our XPS data, we flrat subatracted 5.0eV from the binding energy reported by Jergensen and Berthou.
I. CURELARUand P. 0. NIL~~~N
894
case of YVO, where the studied states are highly localized and thus occupation dependent. Instead we use, as discussed above, an atomic description, and the initial state in the emission process becomes important, The initially empty (2e, 4t,) states of the cluster VOi- in YVO,, involved in the process of X-ray emission, should also answer for th6 occurrence of the measured APS spectrum. We recall that, in general, when some localized empty states exist within a band distribution of other empty states (as for example in the case of lanthanum discussed above) the appearance potential spectrum is expected to contain tsvo separate structures, one due to excitation processes in which one electron gets captured on a localized level, the second electron entering an itinerant state, and a second one due to excitation processes in which both electrons enter into the localized state. The first APS structure in YVO, would then involve population of the empty states through the transition: 2p6(2e, 4t,)‘+
e-(incident --* 2p5(2e, 4t$
+ e-(itinerant) (6) followed by X-ray emission through decay of these excited states: 2p5(2e, 4fJ1 + 2p6(2e, 4tJ0 + hu.
(7)
It is seen that the configurations involved in this APS process of X-ray emission (reaction (7) above) are equivalent with the configurations involved in the X-ray emission process corresponding to peak B in the experimental spectrum of Kurmaev et al. One therefore expects that this first APS structure would appear at exactly the same energy as peak B. However, our experimental APS spectrum does not show it as a clear-cut peak, but only as a low-energy tail of the second structure. We interpret this Bs being due to the very low density of wntinuum-like states in this energy region. Inspection of Table 1 shows that the percent contribution of s-like states near the “Fermi’‘-level coming from the 2e-and 4t,-MO is very low. The orbital 3a1 which contains a high density of s-states arising from V, and Oza atomic orbitals is separated by almost 20 eV from the “Fermi’‘-level, and is therefore not available for electrons with energy near the edge of excitation of APS processes. Although the experimental APS curve does not show any peak-like structure at the energy corresponding to peak B in the X-ray emission spectrum, the low energy branch of the “Hewnd” APS structure (peak C) extending from about 513 eV to 517 eV is an indication that such APS transitions are present. The only peak-like structure that appears in our APS spectrum (peak C) is seen to fall at higher energy than the group of (2e, 4t&empty orbitals. According to APS theory, this “second” structure would correspond to excitation processes in which
both the incident and the core-electrons enter the localized empty states through the following transitions: 2p6(2e, 4t,)‘+e-(incident)
+ 2p5(2e, 4t2)2.
(8)
The high intensity of this “second” APS structure is due to the fact that it contains two diflerent wntributions, one from the “characteristic” APS emission 2ps(2e, 4fJ2 + 2p6(2e, 4t,)‘+ and one from an enhanced strahlung emission Cl83: 2p6(2e, 4t,)‘+
hv
(9)
(resonance)
brems-
e-(incident) + 2p”(2e, 4tz)l + hv.
As seen, both these channels have the same final state wniiguration. 3.4 Elgect of argon ion-bombardment Figure 12 shows a set of APS spectra of WOI, measured in the “as-received” state (l), immediately after bombardment with argon ions (2) and also 2 and 4 h afterwards ((3) and (4), respectively). One can see that the spectrum measured just after ion bombardment is shifted about 1.5 eV toward lower energy values, while the spectra measured successively afterwards, at an interval of about 2 h, are gradually shifted back toward the position of the spectrum corresponding to the sample in the “as-received” state. Two alternative explanations may be forwarded
----
after Ion Bomb.
Fig. 12. APS spectra of V in WO,: (1) specimen in the “as-received” state; (2) immediately after ion bombardment; (3) 2 h after ion bombardment; (4) 4 h after ion bombardment.
Study of YVO, by appearance potential spectroscopy for this shifting effect. Since YVO, is an insulator, it gets positively charged during bombardment with argon ions, and the shift of the spectrum recorded just after ion bombardment may be due to such a charging effect. The spectra recorded later on then correspond to the specimen in which charging has gradually been compensated by bombardment with electrons from the emitting filament, during excitation of the APS process itself. The second alternative explanation may be connected to a possible chemical modification of YVO, compound submitted to prolonged bombardment with argon ions. Since vanadium atom in YVO, is in a state of total oxidation, ion bombardment may cause a reduction of the oxidation state, followed by a gradual recovery of the initial state by heating due to electron bombardment during the process of measurement of APS spectra. However, the spectra never came back to the initial position (before ion bombardment) and, which is most important, their shape continued to change drastically. The most significant feature is the growth of the dip that follows after peak C, it becoming even negative in the last spectrum. An attempt to connect this increasing dip with an increasing character reminding the APS spectrum of pure vanadium-metal is quickly disappointing, since the shift of the whole spectrum is then in the wrong direction, not approaching but moving away from the position of the APS spectrum of pure vanadium. This discussion is highly speculative and cannot be pursuited without a systematic experimental investigation as a basis. We content ourselves here to show in Fig. 13
Fig. 13. Integrated Aps spectra from Fig. 12.
895
the integrated APS spectra from Fig. 12, with the aim to illustrate the significant change in the energy variation of the APS total X-ray emission yield. In a dedicated experiment aimed at an understanding of the above experimental observation, one should take into account the thermal and chemical stability of YVO, under electron bombardment. We think it is useful to mention that the melting temperature of YVO, is 1810-1850°C. We have not measured purposely the temperature of the YVOT specimen during recording of APS spectra, but our previous experience with other specimens has shown that this temperature increases at most up to 200300°C. Nothing is known at present about the stability of the compound under noble gas ion bombardment. According to existing experimental data [27], compounds on the base of YV04, that are used as X-ray luminescent screens, are known to be stable when exposed to radiation. 4.
CONCLUSIONS
We have shown that due to the presence of localized empty states in the compound YV04, the experimental vanadium APS spectrum is significantly different from the APS spectrum of pure vanadium-metal. Many-body effects in the final state of the excitation processes result in a shift of the energy position of these localized states, that is specifically occupation-dependent. The simple APS theory that explains satisfactorily the APS spectra of metallic samples, which are characterized by extended (band-like) empty states above the Fermi level, is no longer valid. Neither the position nor the intensity of experimental APS features can be reproduced by a model based on the selfconvolution of the density of states above the Fermi level. An atom-like approach is here more realistic, in which each feature observed in the APS spectra is interpreted in terms of the wspecific configuration-interactions involved in the excitation and decay processes. For the interpretation of our spectra, we use a theoretical energy level diagram of the cluster VO:-. The first unoccupied molecular orbitals (2e) of this cluster, that have a predominant VBd-character, have been earlier tentatively inferred by Kurmaev et al. in explaining an extrastructure observed on the high energy side of the main VL, X-ray emission line [15, 161. This interpretation is qualitively confirmed by our APS experiment. However, we propose that both 2eand 4r,-MO participate in the process, instead of only the 2e-states as discussed by Kurmaev [lS, 161 and Gubanov [12]. The fact that the extra-structure (peak B) in the X-ray emission spectrum is centered at the 2e-states may be attributed to a significantly higher decay cross section of these excited states, as compared to the 4&-states. Our experimental APS spectrum shows an increasing intensity of emitted X-rays in the energy region where both the 2e and 4&-states are located.
8%
I.CUREUFW andP.O.Nmso~ Table 2. Configuration-transitionsinvolvedin WE, APS, and XPS for YVO,
SXE
APS
Peak A
2ps(le, 2t,, 3t,)n + 2p6(le, 2b, 3Q”-’ + hv
Peak B
2p5(2e, 4t,)l--, 2p6(2e, 4tJ”+ hv
Fit structure (observed as a “tail”)
2p5(2e, 4tJ’ + 2p6(2e, 4tJ0+ h
Peak C (second structure)
2ps(2e, 4tJ2 + 2p6(2e, 4tJ’+ hv(characteristic) 2p6(2e, 4Q+e-(incident) -+ 2p6(2e, 4t# C hv(bremsstrablung)
XPS
Marked by arrow in Fig. 11
2p6+hv+2p5+e-
We measured in a separate experiment the XPS binding energy of V,, core-level in YVO, and found a value of 517.8eV (after correction by POeV for charging effects) that agrees well with the value 518.2 eV reported earlier by Jlrgensen and Berthou [23]. If one accepts this binding energy value as defining the “Fermi’‘-level of YV04, one observes that both peak B in the X-ray emission spectrum and the onset of the APS spectrum lie below this “Fermi’‘-level. This result proves that the orbitals 2e and 4t, which are situated above the “Fermi”level in the ground state of YVO.+, change their energy position being pulled down, below the “Fermi”-level, when core-holes are created in the processes of excitation of both APS and X-ray emission spectra. The first structure expected in APS to coincide with the position of peak B in X-ray emission is not manifested in our spectra as a peak because it implies the existence of delocalized (itinerant) states whose density is very low within the (2e, 4r,) orbitals. The second APS structure (peak C) contains contributions from the characteristic APS processes in which both electrons enter the localized (2e, 4t,) states, as well as from an enhanced bremsstrahlung emission. Table 2 shows the configuration-transitions proposed by us to explain the APS, SXE and XPS spectra. A few facts remained not completely understood in the present work, namely the significant continuing increase of the total X-ray emission yield observed in the long scan APS beyond -500 eV, the poorly structured O1. APS signal, and the lack of any charging effect under electron bombardment, in spite of the fact that YVO, is an insulating material. An improvement of cluster calculations would be desirable. A more realistic energy level diagram is expected to be provided by iteration of calculations to self-consistency, by inclusion of the effect of more distant neighbours, inclusion of spin-orbit and
relativistic effects and by adopting a many-body (configuration-interaction) approach. Complementary, useful information would be provided in an investigation of YVOa by isochromat spectroscopy in which some other, different configurations are involved.
Acknowledgements-We want to express our thanks to Professor G. Brogren for his interest in the project and to Dr. J. Olefjord for the permission to use the XPS spectrometer of the Department of Engineering Materials, Chalmers University of Technology, Gothenburg. We benefited from extended discussions and cormspondence with E. Z. Kurmaev and V. A. Gubanov. We are indebted to N. I. Lazukova et al. for putting at our disposal their cluster calculations prior to publication. Financial support from the Swedish National Research Council is gratefully acknowledged.
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