Summary
Through high-throughput first-principles simulations, this study systematically compares the thermodynamic stability of delafossite (D1), ordered rock salt variant (D2), and infinite-layer (IL) oxides at ABO2 stoichiometry, constructing phase diagrams encompassing 2,346 elemental combinations. The results demonstrate that for nickelates, palladates, and platinate, the delafossite structure exhibits stability comparable to or even superior to the infinite-layer phase, with competition between these two phases and the perovskite phase. Electronic structure analysis reveals that delafossite compounds feature an inverted cation order, with the Fermi surface dominated by dz2 orbital contributions, distinctly different from the dx2-y2 characteristics of the infinite-layer phase. Among all candidate systems, the La-Ni combination is the thermodynamically optimal choice for stabilizing the infinite-layer structure. Furthermore, hole doping via Ca, Sr, and Ba systematically enhances the relative stability of the infinite-layer phase across the three transition metal families. These findings elucidate the fundamental challenges in synthesizing substrate-free bulk infinite-layer oxides and provide guidance for the experimental exploration of novel superconducting compounds.
Materials
Methods
- High-throughput first-principles density functional theory (DFT) simulations with on-site Coulomb repulsion (DFT+U)
- Convex hull analysis
Keywords
- delafossite
- competing phase
- thermodynamic stability
- reversed cation order
- dz2 dominated fermi surface
- hole doping stabilization
Highlights
- The delafossite structure is found to be a strong competitor to the infinite-layer phase, with reversed cation order.
- BaAgO2 is identified as a stable isoelectronic analog to infinite-layer cuprates with similar electronic structure.
- The results reveal fundamental challenges in realizing bulk substrate-free infinite-layer oxides.
Conclusions
- The delafossite structure rivals the infinite-layer phase in thermodynamic stability for nickelates, and even more for palladates and platinates.
- Among all candidates, the La-Ni combination stands out as a thermodynamic optimum for stabilizing the infinite-layer motif.
- Hole doping via Ca, Sr, and Ba systematically enhances the stability of the infinite-layer phase in all three transition-metal families.
- The delafossite compounds exhibit a strongly dz2-dominated Fermi surface, in contrast to the dx2-y2 character in infinite-layer phases.
Main claims
- Delafossite structure (D1) is a strong competitor to the infinite-layer phase for nickelates, palladates, and platinates, often with lower formation energy.
- Evidence: From abstract: 'delafossite structure rivals the infinite-layer phase in thermodynamic stability for the nickelates, and even more for the recently suggested palladate and platinate analogs.',From Section III.A: 'Table I even reveals a minor energetic preference of 0.02-0.09 eV for the D1 structure' for nickelates; for palladates/platinates D1 is ≈0.3-0.8 eV more stable.
- The La-Ni combination is the thermodynamic optimum for stabilizing the infinite-layer phase among rare-earth/transition-metal pairings.
- Evidence: From abstract: 'Among all candidates, the La-Ni combination stands out as a thermodynamic optimum for stabilizing the infinite-layer motif.',From Section III.B: 'Our statistical analysis highlights LaNi as the most favorable rare-earth/transition-metal pairing for stabilizing the IL phase.'
- Hole doping via Ca, Sr, and Ba systematically enhances the relative stability of the infinite-layer phase and lowers the reduction energy.
- Evidence: From abstract: 'hole doping via Ca, Sr, and Ba systematically enhances the stability of the infinite-layer phase in all three transition-metal families.',From Section III.A: 'Ca-, Sr-, and Ba-based compounds consistently favor the IL phase as their ground state' and 'such substitution tends to lower the reduction energy relative to the parent P phase.'
- In delafossite compounds, the Fermi surface is dominated by d_z2 states due to reversed cation order, whereas infinite-layer phases have dx2-y2 character.
- Evidence: From abstract: 'delafossite compounds… exhibit a strongly d_z2-dominated Fermi surface, in stark contrast to the dx2-y2 character observed in the infinite-layer phases.',From Section III.C: 'In the D1 structure, exclusively the d_z2 states contribute to the Fermi surface' vs. 'the dx2-y2-derived bands exhibit the highest dispersion' in IL.
- BaAgO2 is identified as a stable isoelectronic infinite-layer compound with electronic structure similar to CaCuO2, making it a candidate for superconductivity.
- Evidence: From Section III.A: 'BaAgO2 is isoelectronic to the IL cuprates and presents a highly similar band structure' and from Appendix B: 'Both Fermi surfaces are predominantly of dx2-y2 character and exhibit a highly similar shape.'
Workflow
- High-throughput DFT simulations — Constructed a database of 7,038 unique delafossite and ordered rock-salt compounds.
- Materials: 2,346 elemental combinations at A and B sites; D1, D2, IL, P geometries
- Methods: DFT+U with projector-augmented wave (PAW) formalism; VASP code; GGA-PBE exchange-correlation functional
- Observations: Ground-state energies for 16,422 individual structure optimizations; Formation energies (Ef) for ABO2 and ABO3 compounds
- Phase diagram construction — Delafossite structure rivals infinite-layer phase in thermodynamic stability for nickelates, palladates, and platinates.
- Materials: Formation energies from DFT simulations
- Methods: Comparison of Ef for D1, D2, IL, and P phases; Vector-based color mapping to visualize stability sectors
- Observations: Relative stability sectors: D1 (65%), D2 (20%), IL (15%); Only 9% of IL compounds have negative reduction energy
- Electronic structure analysis — Delafossite compounds exhibit reversed cation order and strongly d_z2-dominated Fermi surface, contrasting with infinite-layer dx2-y2 character.
- Materials: Neodymium-based nickelates, palladates, platinates (NdNiO2, NdPdO2, NdPtO2); Also CaCuO2 and BaAgO2
- Methods: Orbital-resolved band structure calculations; Fermi surface visualization
- Observations: In D1: Fermi surface dominated by d_z2 states from transition metal at A-site; In IL: Fermi surface dominated by dx2-y2 states from transition metal at B-site