Nitridochromate(IV): LiSr2[CrN3]

Abstract

The quaternary nitridochromate(IV) LiSr₂[CrN₃] crystallizes in a new structure type with the non-centrosymmetric space group P2₁ (no. 4) with a = 5.5685(7) Å, b = 5.3828(8) Å, c = 7.5381(1) Å, and β = 92.291(8)°. Predominant structural features of the compound are slightly nonplanar trigonal units [CrN₃]^5-, which are connected by three-fold coordinated lithium to form slabs in the (001) plane. Shorter Cr-N bond lengths in comparison with reported nitridochromates(III), as well as diamagnetic behavior and vibrational spectroscopy data indicate Cr(IV), which is in a good agreement with the charge balance. According to electronic structure calculations, the compound is a semiconductor with a band gap of 1.19 eV.

Figure 1

(A) Crystal structure of LiSr₂[CrN₃]. (B) [CrN₃]^5– complex anion with bond lengths depicted in red (Å) and N–Cr–N bond angles depicted in black (°).

Figure 2

Coordination of [CrN₃]^x− units in (A) LiSr₂[CrN₃], (B) Sr₃[Cr^IIIN₃], and (C) Ca₃[Cr^IIIN₃].

Figure 3

Crystal structure of LiSr₂[CrN₃], considering only metal positions (A), and its relation to the Li₃Bi-type (C) by shifting the atomic positions (B).

Figure 4

(A) Computed total and atom-resolved electronic density of states (DOS) for LiSr₂[CrN₃]. The upper panel presents the total DOS as well as Cr and N atom contributions to it. The lower panel presents Sr and Li contributions. (B) Projected DOS showing Cr 3d, 4s, and N 2p contributions in the energy range [−4.5, 4.0] eV.

Figure 5

Chemical bonding in LiSr₂[CrN₃]: (A) QTAIM shapes of atomic basins and the effective charges. (B) ELI-D bond basins with information on participating atoms (first line), their individual contributions (second line), and the total basin populations (third line for each atom). A spherical surface is typically a common boundary with a core basin; the flat surfaces are boundaries with other bond basins. (C) ELI-D distribution showing the bonding situation in the [CrN₃]^5– anion. The isosurface values are 1.425 (dark orange), 1.305 (orange), and 1.050 (cyan) depicting the Cr core, N–Cr bonds and Y1, and Cr–Sr1–Sr2 bond, respectively. Y1 denotes the Cr-dominated 11-atomic bond.

Figure 6

(A) IR (top) and Raman spectra (bottom) of LiSr₂[CrN₃]. (B) Computed phonon DOS for the fully-optimized LiSr₂[CrN₃]. The modes dominated by the vibrations of [CrN₃]^5– and [LiN₃] are identified. Maximum frequency is less than 900 cm^–1. (C) Fragment of Raman spectra of LiSr₂[CrN₃] (top) and Sr₃[CrN₃] (bottom).

Figure 7

Temperature dependence of magnetic susceptibilities χ(T) of LiSr₂[CrN₃] measured in 1, 3.5, and 7 T magnetic fields for temperatures between 1.8 and 300 K.