Pressure-Induced B1 to B2 Phase Transition in CeN Studied by ab initio Correlation Matrix Renormalization Theory Calculations

TL;DR

CeN exhibits a mixed-valence Ce $4f$ system that undergoes a pressure-driven B1 to B2 structural transition. The authors apply CMRT, a parameter-free ab initio many-body method based on a multiband Gutzwiller framework, to compute energetics, DOS, and Ce valence under pressure. CMRT reproduces the ambient B1 ground state, predicts a first-order transition at about $64\,\text{GPa}$ with an $11\%$ volume collapse, and reveals spectral-weight redistribution and increased $4f$ itinerancy consistent with valence fluctuations. The work demonstrates CMRT’s predictive power for correlation-driven transitions in f-electron systems and offers a computationally efficient alternative to more parameter-dependent approaches like DFT+DMFT. Overall, the study advances understanding of how lattice compression couples to $4f$ electron delocalization in CeN and related materials.

Abstract

We apply correlation matrix renormalization theory (CMRT) to cerium nitride (CeN) under pressure. For B1 (NaCl-type) phase, CMRT gives an equation of state consistent with ambient pressure experiments. It produces electronic density-of-state (DOS) characterized by a sharp 4f quasi-particle resonance peak pinned at the Fermi level and two subbands formed by strong hybridization between the localized Ce-4f electrons and the itinerant Ce-5d and N-2p electrons below the Fermi level, consistent with XPS experiments. Upon compression, CMRT predicts a first-order B1 to B2 (CsCl-type) transition with ~11% volume collapse in agreement with experiments. Across the transition, the 4f spectral weight broadens, the 4f orbital occupancy increases, and the hybridization with conduction states enhances, signaling a crossover from partially localized to more itinerant 4f behavior. These features are in excellent agreement with experimental observations, demonstrating that CMRT provides a parameter-free description and prediction of correlation-driven structural and electronic transitions in rare-earth compounds.

Figures (5)

• Figure 1: CMRT-calculated total energy of CeN as a function of unit-cell volume for the B1 (squares, black solid line) and B2 (circles, red solid line) phases.
• Figure 2: (a) Relative volume ($V/V_{0}$) vs Pressure (P) of CeN calculated using CMRT (B1 : Solid Line and B2: Dash line). $V_{0}$ denotes the equibrium volume of the B1 phase (at $P = 0$ GPa). The vertical blue solid line indicates the transition pressure (TP) corresponding to the B1 → B2 phase transformation. The points marked are the experimental data from Ref.12(black circle) and Ref. 23 (red circle); (b)Enthalpy differences and Estimation of phase transition pressure from B1 to B2 of CeN.
• Figure 3: Calculated spectra of B1-type(a) and B2-type(b) CeN. The experimental XPS spectrum reported in Ref. Baer1978 for the B1 phase is included in the upper panel for comparison, while the lower panel presents only the calculated results for the B2 phase. The calculated results include projections onto the Ce $4f$ and $5d$ states, as well as the N $2p$ state.
• Figure 4: Electronic density of $4f$ state at different volumes of CeN with the B1-type (a) and B2-type (b) structures.
• Figure 5: (a) Occupation probabilities of $4f^{n}$ (n=0,1,2,3,4) configurations and (b) the number of occupied $4f$ electrons $n_f$ of B1-type and B2-type CeN as a function of the specific cell volume. The green solid and blue dash vertical lines indicate the cell volumes immediately before and after the B1→B2 transition, respectively.