Charge-transfer insulators

Charge-transfer insulators are a class of materials predicted to be conductors following conventional <a href="/facts/Band_theory/W5o7oE2R">band theory</a>, but which are in fact insulators due to a charge-transfer process. Unlike in <a href="/facts/Mott_insulators/nPswoaFc">Mott insulators</a>, where the insulating properties arise from electrons hopping between unit cells, the electrons in charge-transfer insulators move between atoms within the <a href="/facts/Unit_cell/znygJqdx">unit cell</a>. In the Mott–Hubbard case, it's easier for electrons to transfer between two adjacent metal sites (on-site Coulomb interaction U); here we have an excitation corresponding to the <a href="/facts/Coulomb_energy/1skBKuk2">Coulomb energy</a> U with

 
 
 
 
 d
 
 n
 
 
 
 d
 
 n
 
 
 →
 
 d
 
 n
 −
 1
 
 
 
 d
 
 n
 +
 1
 
 
 ,
 
 Δ
 E
 =
 U
 =
 
 U
 
 d
 d
 
 
 
 
 {\displaystyle d^{n}d^{n}\rightarrow d^{n-1}d^{n+1},\quad \Delta E=U=U_{dd}}
 
.
In the charge-transfer case, the excitation happens from the <a href="/facts/Ion/3bePpDna">anion</a> (e.g., oxygen) p level to the metal d level with the charge-transfer energy Δ:

 
 
 
 
 d
 
 n
 
 
 
 p
 
 6
 
 
 →
 
 d
 
 n
 +
 1
 
 
 
 p
 
 5
 
 
 ,
 
 Δ
 E
 =
 
 Δ
 
 C
 T
 
 
 
 
 {\displaystyle d^{n}p^{6}\rightarrow d^{n+1}p^{5},\quad \Delta E=\Delta _{CT}}
 
.
U is determined by repulsive/exchange effects between the cation valence electrons. Δ is tuned by the chemistry between the cation and anion. One important difference is the creation of an oxygen p <a href="/facts/Electron_hole/YymdTv9d">hole</a>, corresponding to the change from a 'normal' 
 
 
 
 
 
 O
 
 2
 −
 
 
 
 
 
 {\displaystyle {\ce {O^2-}}}
 
 to the ionic 
 
 
 
 
 
 O
 
 −
 
 
 
 
 
 {\displaystyle {\ce {O-}}}
 
 state. In this case the ligand hole is often denoted as 
 
 
 
 
 
 L
 _
 
 
 
 
 {\textstyle {\underline {L}}}
 
.
Distinguishing between Mott-Hubbard and charge-transfer insulators can be done using the Zaanen-Sawatzky-Allen (ZSA) scheme.

Charge-transfer insulators open-in-new

Charge-transfer insulators