Metal-to-Semiconductor Transition and Electronic Dimensionality Reduction of Ca2N Electride under Pressure

Abstract

The discovery of electrides, in particular, inorganic electrides where electrons substitute anions, has inspired striking interests in the systems that exhibit unusual electronic and catalytic properties. So far, however, the experimental studies of such systems are largely restricted to ambient conditions, unable to understand their interactions between electron localizations and geometrical modifications under external stimuli, e.g., pressure. Here, pressure-induced structural and electronic evolutions of Ca₂N by in situ synchrotron X-ray diffraction and electrical resistance measurements, and density functional theory calculations with particle swarm optimization algorithms are reported. Experiments and computation are combined to reveal that under compression, Ca₂N undergoes structural transforms from R $\overline{3}$ m symmetry to I $\overline{4}$ 2d phase via an intermediate Fd $\overline{3}$ m phase, and then to Cc phase, accompanied by the reductions of electronic dimensionality from 2D, 1D to 0D. Electrical resistance measurements support a metal-to-semiconductor transition in Ca₂N because of the reorganizations of confined electrons under pressure, also validated by the calculation. The results demonstrate unexplored experimental evidence for a pressure-induced metal-to-semiconductor switching in Ca₂N and offer a possible strategy for producing new electrides under moderate pressure.

Keywords: electrides; electronic dimensionality; metal‐to‐insulator transition; phase evolution.

Figure 1

Synchrotron XRD patterns of Ca₂N obtained under compression up to 50.1 GPa and decompression. a) The raw 2D XRD images at selected pressures during compression and decompression. b) Integrated XRD profiles under different pressures.

Figure 2

a) The enthalpy per atom for Ca₂N in R $\overline{3}$ m, I $\overline{4}$2d, and Cc symmetry as a function of pressure with respect to Fd $\overline{3}$ m‐type Ca₂N. Inset: the enthalpy per atom for Ca₂N in Cc symmetry as a function of pressure with respect to I $\overline{4}$2d type Ca₂N. b,c) Rietveld refinements on XRD patterns of Ca₂N under different pressure and the pressure dependence of formula unit volume. The marked peaks around 9° and 10.6° are caused by CaO with Fm $\overline{3}$ m structure (see Figure S6). d–g) Crystal structures of R $\overline{3}$ m (d), Fd $\overline{3}$ m (e), I $\overline{4}$2d (f), and Cc (g) Ca₂N. The blue and green spheres represent Ca and N atoms, respectively. The red guide lines in Fd $\overline{3}$ m structure are showing tetrahedral configurations, not bonding.

Figure 3

Electrical resistance changes with pressure and temperature. a) Temperature dependence of electrical resistance from 2 to 300 K. The curves obtained under pressures show the temperature dependence of electrical resistance changed from positive to negative. b) Electrical resistance values as a function of applied pressure at different temperature of 30, 100, 200, and 300 K, respectively.

Figure 4

Calculated band structure, total and partial density of states (TDOS and PDOS) for a) R $\overline{3}$ m, b) Fd $\overline{3}$ m, c) I $\overline{4}$2d, and d) Cc structures. The red curves in the band structure are the interstitial bands where the anionic electrons occupied mainly. For Fd $\overline{3}$ m structure, the spin–orbit coupling (SOC) calculations give a band gap of 0.2 meV. I $\overline{4}$2d and Cc were calculated at 11.2 and 20.6 GPa, respectively, the calculated band gap with HSE06 is found to be 1.44 and 2.88 eV, respectively.

Figure 5

Calculated electron localization functions (ELFs) for a) R $\overline{3}$ m, b) Fd $\overline{3}$ m, c) I $\overline{4}$2d, and d) Cc structures with isosurface value 0.55, 0.9, 0.9, and 0.9, respectively. The ELF maps around anionic electrons accompanied with the coordination environment of anionic electrons (Ae) are showed in the bottom. The distribution of anionic electrons reveals a transition from 2D layer (R $\overline{3}$ m), 1D chain (Fd $\overline{3}$ m), and 0D spheres (I $\overline{4}$2d and Cc).