Correlation between the metal–insulator transition and the electronic density of states near the Fermi level in oxygen‐deficient perovskite‐type NdBaCo₂O_5.5

Abstract

PACS 61.10.Ht, 71.20.Ps, 71.30.+h X-ray absorption spectra near the absorption edges of Nd, Ba, and Co ions have been taken to study the metal -insulator (M -I) transition occurring at about 360 K (T MI ) in NdBaCo 2 O 5.5 . In the metallic and insulating phases, the electronic density of states (DOS) just above the Fermi level (E F ) consists of the highdensity unoccupied bands of Nd -5dσ and Ba -5dσ. The narrow charge-transfer Co -3d7 (e g )L band and the broad charge-transfer Co -3d(t 2g ) n L m bands occupy positions just below and near E F , respectively, where L represent a hole in the O -2p valence band. The negative charge-transfer Nd -4d n L′ m and Ba4d n L′ m hybridized bands are also just below E F , where L′ represent a hole in the fully occupied 4d band of the Nd and Ba ions. It is suggested that the metallic conductivity above T MI arises from the itinerant (p -d) hybridized holes having an 3d(e g )-symmetry in the top of the O -2p valence band. The structural deformation localized around the Ba ions but not the Nd and Co ions affects the crystal-field splitting of the Co3d orbitals in the CoO polyhedra and the hybridization between the outer orbitals of the cations (Nd, Ba, Co ions) and the O -2p orbitals.

Metal–insulator transition induced by electronic and structural modulations in oxygen‐deficient perovskite‐type TbBaCo2O5.5

“…Thus, the fine structures in the XANES and EXAFS are intrinsic. Figure 1 and its inset show the XAS near the Tb-L Ш edge in the energy region of 5 eV to 50 eV and of -50 eV to 150 eV, respectively, as those in NdBaCo 2 O 5.5 [22], taken from room temperature (RT) to 390 K. In the figures, the ordinate (I/I 0 ) is a count ratio between the incident X-ray photons (I 0 ) and the fluorescence X-ray photons (I) emitted from the sample, while the abscissa is the energy difference (E) from E 0 (Tb). The XAS curves in Fig.…”

“…Furthermore, such a deformation of the CoO polyhedra affects a hybridization between the outer orbitals of the cations (Tb,Ba,Co ions) and the 2p orbitals of oxygen ions (O-2p orbitals). More recently, Hidaka et al [22] argued from X-ray absorption spectra that the M-I transition occurring at about 360 K in NdBaCo 2 O 5.5 is mainly characterized by the modulation of the density of states (DOS) for Nd, Ba and Co ions near the Fermi level and by local distortions of oxygen polyhedra around Ba ions, but not around Tb and Co ions, respectively. Thus, we deduce that the M-I transition in TbBaCo 2 O 5.5 is strongly correlated to the electronic density of states (DOS) near the Fermi level, especially the hybridized bands below E F and the high-density unoccupied bands above E F , in addition to the structural deformation localized in two-dimensional networks of BaO, CoO 2 , and TbO 0.5 stacked along the c-axis.…”

“…The oxygen-deficient perovskite-type LnBaCo 2 O 5.5 (Ln: lanthanides) have been studied for the unusual electric and magnetic properties of the metal-insulator (M-I) transition occurring in the region from about 290 K to 360 K [1][2][3][4][5][6][7][8][9][10][11] and of the magnetic transitions below room temperature [12][13][14][15][16][17][18][19][20][21][22]. The lanthanide cobaltites exhibit the ferromagnetic transition, the antiferromagnetic transition, the spin-state transition, and the large magnetoresistance appearing near the antiferromagnetic-ferromagnetic transition temperature.…”

Switching from unusual to usual ferromagnetism in “112” LnBaCo2O5.50±δ: By calcium doping

“…In this hybridized band, which is situated just below the Fermi level, the exchange of p-electron from the top part of the O-2p valence band to the empty d-orbital of e g band leads to the hybridized (p-d) holes and this hole is itinerant in nature. These authors anticipated that the unusual magnetic properties of NdBaCo 2 O 5.5 are strongly related to the itinerancy of (p-d) hybridized holes [34]. The itinerant O-2p majority holes couple antiferromagnetically to the Co spins and effectively align them ferromagnetically via the p-d exchange [35].…”

“…Takubo et al [33] have shown by using X-ray photoemission spectroscopy that even in the insulating phase e g orbitals are occupied. Based on the pre-edge peak in the X-ray absorption near edges structure (XANES), Hidaka et al [34] have suggested that there is a strong hybridization between the Co outer orbital and O-2p orbital. In this hybridized band, which is situated just below the Fermi level, the exchange of p-electron from the top part of the O-2p valence band to the empty d-orbital of e g band leads to the hybridized (p-d) holes and this hole is itinerant in nature.…”