Li–B–C Thermodynamic Stability Map

First-principles stability audit of LiBC delithiation and proposed lithium borocarbide phases

Overview

Lithium borocarbides sit at an interesting intersection of energy materials and fundamental solid-state chemistry: LiBC was proposed as a route toward high-Tc superconductivity via hole-doping, yet experimentally delithiation produces metastable, hard-to-control LixBC derivatives rather than a clean equilibrium pathway.

In this project, we performed an ab initio thermodynamics + stability audit of reported and proposed Li–B–C compounds, connecting:

  • (T, P) delithiation conditions for LiBC in equilibrium with Li2 gas
  • stability of BC3 polymorphs (including kinetic/dynamical feasibility)
  • reassessments of LiBC3 and the proposed Li2B2C phase

My Role

I was the main author and technical coordinator for the study. My responsibilities included:

  • Coordinating efforts among multiple collaborators (structure candidates, analysis ownership, and manuscript integration)
  • Designing the computational plan and baselines, and defining rubrics used to draw conclusions
  • Launching and monitoring large batches of DFT calculations across multiple HPC systems (consistent settings, job recovery, and reruns)
  • Synthesizing results into a coherent, reproducible stability narrative

Approach & Stability Rubric

Rather than relying on a single energy ranking, the conclusions were drawn using a layered rubric:

1) Thermodynamic stability

  • Computed formation energies and evaluated convex-hull stability against competing phases across the Li–B–C system.
  • Used distance-to-hull as the primary criterion for equilibrium stability.

2) Finite-temperature corrections

  • Incorporated configurational entropy for disordered Li in LixBC and harmonic vibrational contributions where relevant.
  • Evaluated how free-energy corrections shift relative stability and delithiation boundaries.

3) Delithiation thermodynamics

  • Modeled equilibrium with Li2 gas and mapped x-dependent (T,P) phase boundaries by comparing Gibbs free energies over a broad temperature range.
  • Quantified sensitivity of predicted boundaries to systematic DFT uncertainties and neglected terms.

4) Dynamical stability checks

  • Used phonon-based screening to rule out candidates with imaginary modes, separating “thermodynamically plausible” from “dynamically unstable.”

Structure Discovery & Verification

For phases where literature models proved unstable or inconsistent, we expanded the search beyond reported prototypes:

  • Performed unconstrained evolutionary searches initialized from random structures
  • Applied motif-driven construction when evolutionary operators struggled with particular structural families
  • Re-optimized and cross-checked the best candidates under consistent computational settings

Key Results

LiBC delithiation: a narrow thermodynamic window shaped by Li2 gas entropy

Delithiation is largely driven by entropy gain in the diatomic Li2 gas phase, and equilibrium boundaries can be mapped by matching free energies of LiBC vs. LixBC + Li2 mixtures. The resulting stability regions for intermediate x are narrow, helping explain why experimentally accessible compositions are strongly influenced by kinetics and process history.

BC3: layered h-BC3 is dynamically unstable

The commonly discussed layered h-BC3 model exhibits imaginary phonon modes, indicating dynamical instability and undermining its feasibility as a stable or even metastable target.

LiBC3: Li intercalation stabilizes one polymorph more effectively

Li intercalation brings one LiBC3 polymorph substantially closer to stability, while other proposed variants inherit the energetic penalty of their parent frameworks and remain unfavorable; some reported structures are also dynamically unstable.

Li2B2C: improved candidates still remain far above the convex hull

Evolutionary searches and motif-driven construction produced significantly improved polymorphs compared to earlier proposals. However, the best candidates still remain well above the convex hull, indicating that Li2B2C is unlikely to form under equilibrium conditions.

Publication

Kharabadze, S., Thorn, A., Sandoval, E. D., Hajinazar, S., Kolmogorov, A. N. Thermodynamic stability of Li–B–C compounds from first principles. (Physical Chemistry Chemical Physics).