CONVENCIONES DE LLAMADO

8.1 Influence o f the Anion in the Bonding o f Metal Ceramics

8.1.1 Contribution of O 2p States to the Bottom of the Valence Band

It has been clear that for each o f the materials we have studied in this thesis, the covalent mixing between the metal and anion atomic orbitals in the electronic bands is more significant than previously considered. These are not ionic materials in the sense that charge is simply transferred from the cation to the anion. In each o f the Pb, Sn, Bi, Hg and T1 ceramics examined, tlie interactions o f O 2p with the valence metal states have been considerable. However, the anion p states interact to different extents in each case. To help quantify the idea o f overlap between the metal s and O 2p states in the valence band o f the metal oxides studied, it is possible to make an estimate o f the O 2p

contribution (f^^) to the lowest valence band state from both the D F T calculations and spectroscopic experiments, as foUows.

The calculated electronic density o f states and experimental X-ray photoem ission (XPS) and emission (XES) spectroscopy measurements are shown for Htharge P bO in Figure 8.1. The metal s states are located at the bottom o f the valence band (highest binding energ}^), band III. If all the states in the remaining valence band were o f O 2p character, it follows that the intensity o f band III {Ii„) relative to the total valence band

would simply be n /p , where n is the number o f metal s electrons per formulae unit and p is the total num ber o f valence electron pairs per unit ceU. For example, for PbO n= 2 and p=8, while for BijO, n = 4 and p=22. It foUows that the fractional contribution o f O

(the intensity o f peak III derived from the X E S data, w hich targets the O 2p states)

and (the intensity o f the total valence b and derived from XPS data), as show n in E quation 8.1. y j X E S I T X P S 1 _ ' ■ ' / / / Tota l ' (n/p) 4 ' ' / / / T o ta l / / o -1N J o i n ----

C om parative data derived from the D F T calculations simply requires integration o f the partial electronic density o f states (PE D O S).

EDOS XPS -p —'—I ' r ■2 12 10 8 6 4 2 0 •2 XES PEDOS 0 2p — ■ Pb 6s Pb 6p r ' — '— I— ' I 2 12 10 8 6 4 2 0 •2

Energy (eV) Energy (eV)

F igu re 8.1 V alence b and XPS and O K shell X E S spectra from a - P b O com pared w ith

the E D O S and the P E D O S derived from ou r D F T calculations.

T he results o f this analysis for P b O , SnO and B i2 0 „ H g O and T ljO j are show n in Figure 8.2. D iscrepancies betw een the calculated and experim ental contributions arise from the approxim ation in the experim ental estim ate th at bands I and II are o f pure O

present, hence the systematic overestimation in the experimental contribution to Figure 8.2. c o HgO 3 X> • f i B O U a, <N

o

Figure 8.2 Closed circles: estimated O 2p contribution to the lowest valence band state

derived from intensities in the XES spectra o f the metal oxides. Triangles; the ratio derived by integrating the PE D O S from our D FT calculations across the lowest valence

band state.

Both the experimental and calculated data clearly indicate that there is a progressive decrease in the am ount o f O 2p character present in the lowest valence band state

(highest binding energ)’ valence state) in moving across the Periodic Table. F or a-B i2 0 „ there is in fact only about 20% O 2p character present and we can therefore deduce that

the states at the bottom o f the valence band have dom inant cation 6s atomic character.

For PbO , there is still less than 50% O 2p character present. As the binding energy o f

the metal j' states decreases on going to the left o f the Periodic Table, the O 2p

contribution continues to increase. For TljO ,, the O 2p contribution is almost 60%. At

this threshold the metal s states are drawn into the main O 2p block due to the decreased

binding energy’ o f the T1 valence states. As such, the O 2p states dom inate the bottom

valence band for HgO. The effect o f changing the metal period can also be seen on transition from SnO to PbO . The binding energ)' o f the 6j' valence states o f Pb is greater

than the 5j states of Sn. This decreases the overlap between the metal

and O

states