What is it?

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

Some confirmed electrides:

        Sodium-hP4 [high pressure]

       Na-TriPip222 [organic]

       Y₂C [inorganic]

Applications:

​        Electron emitters

​       Superconductors

​       Battery anodes

​       Catalysts

​

• Electride materials are:

Sodium - hp4

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

Ref: Yanming Ma, et.al. Nature, (2009), 182-185, 458(7235)

GoalS

• The definition describes a phenomena without telling the underlying mechanism.

• Due to various properties and types of materials that can be categorized as electride, to find a common descriptor to identify them can be very challenging.   

theory

Electrides are materials with (electron-deficient) multicentered bonding formed by orbitals of surrounding atoms.

Theory

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

Na-s

Descriptor-ELF

• Principle: The More localized the electron -> smaller probability of finding a like-spin electron around that point in space (due to Pauli exclusion).

• ELF tells us the degree of localization of that charge density quantum mechanically .

Ref: Andreas Savin, et al., ELF: The Electron Localization Function, Angew. Chem. Int. Ed. Engl., 1997,36,1808-1832

Ref: A. D. Becke and K.E. Edgecombe, A simple measure of electron localization in atomic and molecular systems, J. Chem. Phys., 1990, 92, 5397.

ELF-plot

Y₂C

• ELF gives exactly the location of the multicentered orbital.

• But how do translate this process to an automated process?

Ref: Huaqing Huang, et al., Topological Electride Y2C, Nano Lett., 2018, 18(3), 1972-1977

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.

ResultS

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

Elemental Metals

Electrides

Metallic bonding also have this feature and the transition to electride bonding is smooth!

ResultS

More "electride-like":

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

ResultS

• Cutting the graph at (arbitaraily):
  • Maxima value > 0.75
  • \( \nabla^2 \) ELF > -0.5
  • Occupation > 1.6

• We get: 300 electrides (mostly new)
  • 213 non-magnetic materials
  • 87 magnetic materials

ResultS

Ca₆Ge₂O [mp-1019564]

Descriptors:

ELF:       0.9877

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

Occ.:      1.7344

Properties:

Insulator;

Has charge maxima @ interstitial site;

Ref: Jiacheng Gao, et al., Unconventional materials: the mismatch between electronic charge centers and atomic positions, Sci. Bull., In press..

S-orbitals

S-ORBITALS

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

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

ResultS

HOMO

LUMO-1

LUMO-2

LUMO-3

• The HOMO and LUMOs act like atomic orbitals.
• HOMO is s-shaped.

More stories can be found on the origin of these orbitals. In our view they should be categorized as SAMOs (Super Atomic Molecular Orbitals). But other argue they are metal atoms' extended s-orbitals.

Na-Tripip222

Organic ones

Point Defects

• F-center defects (metal's s orbtial)

There are p-shaped orbitals exist in F-cetner defect as well.

⭐️Unifying theory

• Inorganic electrides (atomic orbital)
• High pressured electrides (atomic orbitals)
• Organic electrides (super atomic molecular orbital)
• F-center defects (metal's s orbtial)

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

ResultS

• Experiementally, charge maxims are more easily measured.

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

Interstitial charge maxima

Results

COBI [Ca₆Ge₂O]

COBI [Ca₆Ge₂O]