Structural distortion and frustrated magnetic interactions in the layered copper oxychloride(CuCl)LaNb2O7

Abstract

We present a computational study of the layered copper oxychloride ͑CuCl͒LaNb 2 O 7 that has been recently proposed as a spin-1/2-frustrated square lattice compound. Our results evidence an orbitally degenerate ground state for the reported tetragonal crystal structure and reveal a Jahn-Teller-type structural distortion. This distortion heavily changes the local environment of copper-CuO 2 Cl 2 plaquettes are formed instead of CuO 2 Cl 4 octahedra-and restores the single-orbital scenario typical for copper oxides and oxyhalides. The calculated distortion is consistent with the available diffraction data and the experimental results on the electric field gradients for the Cu and Cl sites. The band structure suggests a complex three-dimensional spin model with the interactions up to the fourth neighbors. Despite the layered structure of ͑CuCl͒LaNb 2 O 7 , the spin system has pronounced one-dimensional features. Yet, sizable interchain interactions lead to the strong frustration and likely cause the spin-gap behavior. Computational estimates of individual exchange couplings are in qualitative agreement with the experimental data.

Magnetism of coupled spin tetrahedra in ilinskite-type KCu5O2(SeO3)2Cl3

“…6). However, Cl atoms are known to mediate long-range superexchange interactions, which render the spin lattice a lot more complex 32,34,35 . Our microscopic analysis reported below identifies additional long-range interactions indeed.…”

Interplay of atomic displacements in the quantum magnet(CuCl)LaNb2O7

“…24 The on-site Coulomb repulsion and the on-site exchange parameters were fixed at U d = 5.5 eV and J d = 1 eV, according to the previous studies. 17 The variation in U d in the range of 5.5Ϯ 1 eV and the type of the magnetic ordering had little influence on the resulting geometry and on the EFG values. Residual forces in the relaxed structures did not exceed 0.01 eV/ Å.…”

“…The resulting crystal structure is very similar to our previous DFT-based model that, for the sake of simplicity, did not consider the tilts of the NbO 6 octahedra. 17 The tilting distortion doubles the unit cell along a and is coupled to the shifts of the Cu atoms. Yet the local environment of Cu and the topology of the layer are retained.…”

“…The results for the Cu and Cl sites are similar to our previous report: the z axis of the EFG tensor lies in the ab plane for Cu and aligns along c for Cl. 17 Note, however, that the model IIa leads to a pronounced asymmetry of the Cu EFG ͑ = 0.43͒ due to the very irregular local environment of the Cu atom. The model IIb shows a lower asymmetry ͑ = 0.04͒ in agreement with the experimental result ͑ = 0.10͒.…”

“…17 Employing densityfunctional theory ͑DFT͒ calculations, we relaxed the crystal structure of ͑CuCl͒LaNb 2 O 7 and found that the system tends to undergo a Jahn-Teller-type distortion. The displacements of the Cl atoms lead to two short and two long Cu-Cl bonds.…”