Chapter 25 Some Additional Topics
321
In Fig. 25-4, electron conduction mechanisms
10 are displayed for p and n type
semiconductors, which involve silicon doped with gallium and arsenic, respectively. Figure 25-5 gives the following speculative mechanism
10 for electron
conduction in the high temperature superconductor YBa 2 Cu 3 O 7 . It makes use of
the instability of Pauling “3-electron bonds” under compression (cf. Section 3-10).
One formulation of an A type layer of YBa 2 Cu 3 O 7 involves a
(CuO)(CuO)
+
(CuO)(CuO)
+
… arrangements of copper and oxygen ions. Each
(CuO) component involves a Pauling “3-electron bond”, which arises from the
overlap of the singly-occupied
2
2
x y
3d atomic orbital of Cu
2+ with the doublyoccupied 2pAO of O
2- , to give the ( CuO )
2
(* CuO )
1 molecular orbital configuration. Each (CuO)
+ component involves the ( CuO )
2 molecular orbital configuration.
In Fig. 25-5, the electron spins are indicated in structure (1), and each
(CuO)(CuO)
+ component involves a 5-electron 4-centre bonding unit. Lattice
vibrations of the type indicated in structure (2) involve a compression of the
(CuO) Pauling “3-electron bonds” and expansion of the (CuO)
+ electron-pair
bonds. The resulting (CuO) instability would lead to transfer of electrons from the
singly-occupied antibonding molecular orbitals into the vacant antibonding
molecular orbitals of the (CuO)
+
, as indicated in structure (2), to generate structure
(3). If a unidirectional flow of electrons is able to be established, i.e. if the next
vibration is able to generate structure (4) rather than structure (2), the process
(2) (3) (4) provides a speculative mechanism for the superconductivity.
At any stage of a vibration, the electronic wavefunction can be expressed as
= 2 + 3 .
5.
Dimerization and Polymerization of O 2
A computational study
11 predicts that at high pressures, O 2 forms an insulating
spiral-chain O 4 polymer structure. Figure 25-6 displays
12 (a) the formation of increased-valence structure (2) for the O 2 dimer from the O 2 monomers of (1) and
the O 2 dimer Lewis structure (3); (b) types of increased-valence structures for O 8
((4) and (5)) and spiral chain O 4 polymers ((6), (7) and (8)).
321
In Fig. 25-4, electron conduction mechanisms
10 are displayed for p and n type
semiconductors, which involve silicon doped with gallium and arsenic, respectively. Figure 25-5 gives the following speculative mechanism
10 for electron
conduction in the high temperature superconductor YBa 2 Cu 3 O 7 . It makes use of
the instability of Pauling “3-electron bonds” under compression (cf. Section 3-10).
One formulation of an A type layer of YBa 2 Cu 3 O 7 involves a
(CuO)(CuO)
+
(CuO)(CuO)
+
… arrangements of copper and oxygen ions. Each
(CuO) component involves a Pauling “3-electron bond”, which arises from the
overlap of the singly-occupied
2
2
x y
3d atomic orbital of Cu
2+ with the doublyoccupied 2pAO of O
2- , to give the ( CuO )
2
(* CuO )
1 molecular orbital configuration. Each (CuO)
+ component involves the ( CuO )
2 molecular orbital configuration.
In Fig. 25-5, the electron spins are indicated in structure (1), and each
(CuO)(CuO)
+ component involves a 5-electron 4-centre bonding unit. Lattice
vibrations of the type indicated in structure (2) involve a compression of the
(CuO) Pauling “3-electron bonds” and expansion of the (CuO)
+ electron-pair
bonds. The resulting (CuO) instability would lead to transfer of electrons from the
singly-occupied antibonding molecular orbitals into the vacant antibonding
molecular orbitals of the (CuO)
+
, as indicated in structure (2), to generate structure
(3). If a unidirectional flow of electrons is able to be established, i.e. if the next
vibration is able to generate structure (4) rather than structure (2), the process
(2) (3) (4) provides a speculative mechanism for the superconductivity.
At any stage of a vibration, the electronic wavefunction can be expressed as
= 2 + 3 .
5.
Dimerization and Polymerization of O 2
A computational study
11 predicts that at high pressures, O 2 forms an insulating
spiral-chain O 4 polymer structure. Figure 25-6 displays
12 (a) the formation of increased-valence structure (2) for the O 2 dimer from the O 2 monomers of (1) and
the O 2 dimer Lewis structure (3); (b) types of increased-valence structures for O 8
((4) and (5)) and spiral chain O 4 polymers ((6), (7) and (8)).