What is it?

Electride materials are:

ionic compounds in which electrons are localized at interstitial sites and act as anions.

Electron (density) in an octahedral interstitial site

Some confirmed electrides:

        Sodium-hP4 [high pressure]

       Na-TriPip222 [organic]

       Y₂C [inorganic]

       

Applications:

​        Electron emitters

​       Superconductors

​       Battery anodes

​       Catalysts

​

Ref: Liu C, Electride: a review, J. Matter. Chem. C, 2020, 8, 10551.

Questions

• The definition describes a (not easily describable) phenomena without telling the underlying mechanism.

 

• To make the matter worse, due to various properties and types of materials can be categorized as electride, a common descriptor to find them from a database can be very challenging.   

Problems

• An empty anionic site that's surrounded by positive charged ionic cores? How come this thing is stable?

 

• It's really hard to use this definition to construct a usable descriptor that covers all know electrides.

 

• Can metals be idneitifed as electrides as they some times also have local accumulation of charge?

This definition is not (easily) applicable and does not tell us the whole story🤔!

CLASSIFICATION

• Organic electride
• Inorganic electride
  • Elemental electride
  • 2D electride
• Intermetallic electride

• Activated electride
• High-pressure electride
• Magnetic electride
• Topological electride

Based on properties

Based on compositions

Ref: Liu C, Electride: a review, J. Matter. Chem. C, 2020, 8, 10551.

theory

Theory: They are (electron-deficient) multicentered bonding between orbitals on surrounding atoms.

Theory

Only when the lowest bonding orbital is occupied, the system can behave like electrides, hence electron-deficient.

Theory

• An empty anionic site that's surrounded by positive charged ionic cores? How come this thing is stable?
  • Lower kinetic energy by forming a bonding orbital.
• Metals also have electrons localized at interstitial sites.
  • ”Weak“ vs “strong” multicentered orbital (not electron deficient)
• It's really hard to use this definition to construct a usable descriptor that covers all know electrides.
  • Descriptor: Use ELF value + topological analysis.

Descriptor-ELF

In layman's term:

• ELF tells the degree of Pauli exclusion force of a test electron feel at a specific point in space.

 

• Hence, it describes the degree localization: More localized -> larger repulsion force (Fermi Hole)

DESCRIPTOR-ELF

• Have ELF maxima @ center of atomic cage (with a high value).

• Small \( \nabla^2 \) ELF @ interstitial site.

• High ELF basin integrated charge number.

Descriptor-ELF

• Have Electron localization function (ELF) maxima @ interstitial region (with a high value).
• Small \( \nabla^2 \) ELF @ interstitial site.
• High ELF basin integrated charge number 

Descriptor:

electron deficient?

Multicentered bonding can have different types. Usually The strongest bonding orbital has the lowest energy and as the degree of bonding decrease, the energy increases.

Systems with electrons only occupying the lowest bonding orbital is preferred for electrides.

theory

Theory: They are (electron-deficient) multicentered bonding between orbitals on surrounding atoms.

• Have Electron localization function (ELF) maxima @ interstitial region (with a high value).
• Small \( \nabla^2 \) ELF @ interstitial site.
• High ELF basin integrated charge number 

Descriptor:

DESCRIPTOR-ELF

• There are reports that they can be used to identify multicentered bonding .

 

• It is a local property that's based on pure quantum properties of electrons (Fermi hole) and can be calculated easily.

 

• It's explanation is intuitive: ELF = 0.5 -> free electron, ELF =1 -> perfect localization.

Topological analysis?

ELF alone only describes local information of electron localization.  

• ELF maxima at the center of the interstitial region.
• ELF maxima to be high (so that electron are actually localized)
• ELF pockets that surrounding the interstitial sites to have a high occupation so that enough electrons are localized inside the pocket.

To use it to identify interstitial multicenter bonding state, we need:

COBI

• Crystal Orbital Bond Index (COBI) is a way to measure the bonding strength (number of bonds) between atoms.
•  
• For multicentered bonding, the magnitude of COBI represents the degree of occupation bonding, non-bonding, anti-bonding orbitals:
  • ICOBI > 0 : electron deficient
  • ICOBI < 0 : electron rich

Method

1. Apply Bader's AIM analysis on ELF to find local maxima points.
2. For all local maxima points, find the ones that are equidistant more than two surrounding atoms of the same type. (similar to the so-called "synaptic order")
3. Analysis the topology of the ELF around the maxima points by calculating the second derivative of ELF and integrated charge number under the ELF Bader basin.

DOES IT WORK?

ELF

Identified sites

Y₂C

ResultS

First, some numbers:

• A total of ~55000 entries on the Materials Project data base is sampled.
• Around 10000 entries were found to possess interstitial ELF maxima. Among them:
  • Elemental systems: 295
  • Binary systems: 5822
  • Ternary system: 4178
• The majority of them are metals.

ResultS

Ref: Liu C. et al., Electrides: a review, J. Mater. Chem. C, 2020,  8, 10551.

Elemental Metals

Electrides

ResultS

• Experiementally, charge maxims are more easily measured.

 

• Systems with charge maxima are also located at the upper right.

Interstitial charge maxima

ResultS

All predicted structure

Structures w/ CHG max

More "electride-like":

high ELF value + lower \(\nabla^2\) ELF + high occupation

Results

RESULTS

*Ref:Zhu et al., Matter, (2019) 1, 1293–1303 

S-orbtials

The s-orbitals of alkaline and alkaline earth metals are very dispersive + have no direction dependency.

That's why most discovered electrides are composed by them.

Na

K

Most Electrid-like

Ca6Ge2O

Ca₆Ge₂O-mp-1019564

ELF:       0.9877

\(\nabla^2 \)ELF: -0.0655

Occ.:      1.7344

COBI [Ca₆Ge₂O]

COBI [Ca₆Ge₂O]

MAKING ELECTRIDES

BCC-Sodium

Equipped with the knowledge of where these bonding originates, we can force a system with metallic bonding into an electide.

ELF:       0.5505

\(\nabla^2 \)ELF: -0.0993

Occ.:      0.1604

MAKING ELECTRIDES

ELF

Tetragonal-Sodium

ELF:       0.7266

\(\nabla^2 \)ELF: -0.0992

Occ.:      0.7470

Making electrides

Previously, we used stain to control the transition from metallic bonding to electride bonding. This time, we use doping to bring out electride nature inside an typical ionic material

NaCl-Doped [4e]

ELF:       0.7525

\(\nabla^2 \)ELF: -0.5243

Occ.:      0.4094

thank you 🤟

Powered by:

With ♥️ 

One more thing...

http://chengcheng-xiao.github.io/electride-db/