Valence values of the cations in selenospinel Cu(Cr,Ti)₂Se₄

Abstract

A long-standing issue about the Cu valency in selenospinel CuCr2Se4 was investigated by soft X-ray absorption spectroscopy (XAS) and magnetic circular dichroism (XMCD). Using the sensitivity of XAS and XMCD to the valence value of transition metal ion and its local symmetry, we checked the valence value of each cation in selenospinel CuCrxTi2−xSe4 (x = 1.0, 1.1, 1.5, and 2.0) and obtained spectroscopic evidence that a small amount of the Cu cation changes the valency from Cu(I) to Cu(II) as the Cr concentration increases from 1.0 to 2.0. Dependence of the Cu(II) concentration and the mean field magnetic exchange energy on the Cr concentration suggests the Cu d-hole could play a crucial role in the intriguing magnetic/electrical properties of CuCr2Se4.

Room-temperature intrinsic ferromagnetism in epitaxial CrTe2 ultrathin films

“…In this case, the Cr 3+ ( d 3 ) configuration with half-filled t 2g states causes the reduction of free energy 49 , which is in good agreement with the reported theoretical value of magnetic moment, 3 μ B /Cr atom 50 . The observed XAS spectral line shape is in line with that of spinel Cu(Cr,Ti) 2 Se 4 polycrystals with trivalent Cr cations on O h sites 51 , further providing a spectroscopic fingerprint of 1T-type CrTe 2 with predominately Cr 3+ cations. In this case, approximately three electrons are removed from the Cr atoms, and distributed over the Te.…”

Electronic Modulation in Site-Selective Occupation of Quasi-2D Triangular-Lattice Cs2CuCl4–xBrx Perovskite Probed by Surface-Sensitive Characterization

“…The L 3 position of the detected energy situated at a lower photon energy (∼930.3 eV) than the binding energy determined by XPS (see Figure 2) due to the core hole effect. 33 The variation in peak intensities of Cu L 3 edges is somewhat invariant throughout this compound series. On the contrary, L 2 edges peak position encounters a relatively non-negligible modification upon increasing number of the halogen substitution.…”

Comparing the Pressure-Induced Structural Behavior of CuCr2O4 and CuCr2Se4 Spinels

“…We note that even though CuCr 2 Se 4 also contains a magnetic cation at the A site, it does not adopt the Cr 3 S 4 -type structure at ambient conditions as the rest of the ACr 2 Se 4 members with magnetic A cations. We tend to attribute this “inconsistency” to the valence state of Cu, as a significant portion of the Cu cations are in a monovalent valence state with their outer 3d orbitals fully occupied . Nevertheless, compression of both CuCr 2 Se 4 and ZnCr 2 Se 4 induces transitions toward a CrMo 2 S 4 -type structure, a superstructure of Cr 3 S 4 .…”

“…We tend to attribute this "inconsistency" to the valence state of Cu, as a significant portion of the Cu cations are in a monovalent valence state with their outer 3d orbitals fully occupied. 21 Nevertheless, compression of both CuCr 2 Se 4 and ZnCr 2 Se 4 induces transitions toward a CrMo 2 S 4 -type structure, 13 a superstructure of Cr 3 S 4 . The remaining Cr spinel selenides, i.e., CdCr 2 Se 4 and HgCr 2 Se 4 , undergo transitions toward the tetragonal I4 1 /amd structure.…”

“…The exact valence state of copper and the origin behind the magnetic properties of these two compounds have been debated for several decades. The currently accepted model involves a hole-mediated magnetic exchange between the localized electrons of Cr 3+ and delocalized holes from the Se 4p states, with the Cu cations exhibiting both monovalent and divalent valence states. − In addition, CuCr 2 Se 4 has the highest Curie temperature among Cr-bearing spinels ( T C = 415 K), thus making it appealing for use in magnetic devices …”