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The electronic structure of the doped one-dimensional transition metal oxide Y1-xCaxBaNiO5 studied using x-ray absorption

A strong anisotropic distribution of the holes in Ni 3d and O 2p orbitals is observed in the polarization dependent O1s and Ni2p3/2 x-ray absorption spectroscopy of the linear-chain nickelate Y1-xCaxBaNiO5 (x = 0, 0.05, 0.1, 0.2), which demonstrates the one-dimensional nature of the electronic state in these compounds. Furthermore, the additional holes introduced by Ca-doping occupy both O 2p and Ni 3d orbitals along the NiO5 chains. By comparing the experimental Ni 2p3/2 absorption spectra of Y1-xCaxBaNiO5 to those from charge transfer multiplet calculations we can derive the orbital character of the additional holes to be of ca. 60% O2p and ca. 40% Ni 3d.

our results with those from other Ni-O based compounds we label the relevant O and Ni orbitals with z along the crystallographic a direction (i.e. along the Ni-O chains) and with x/y perpendicular to this direction.

III. Results and discussions
In Fig. 2  The energetically lowest lying structure in the spectra of Y 2 BaNiO 5 which is labeled A in Fig. 2 is found at 531.7 eV for E||a and at 531.4 eV for E||b. These energies are close to that observed for the corresponding spectral feature of NiO and these structures are thus assigned to transitions from the O 1s level into unoccupied states with O 2p character mixed with the Ni 3d-derived so-called upper Hubbard band (UHB). The shift of this feature to higher energies for E||a compared to E||b is attributed to the distortion of the NiO 6 octahedra along the a axis which results in differing band widths in the two directions and different on-site energies for the corresponding oxygen sites. This interpretation is in agreement with predictions from band structure calculations [18]. The larger intensity for E||a compared to E||b reflects the stronger covalence between the O 2p and Ni 3d levels, i.e. a larger contribution of O 2p character to the UHB along the a axis (chain) with a shorter Ni-O1 distance.
At a low doping level of x = 0.05, a weak pre-edge peak B appears at 528.3 eV for E||a and at 528.4 eV for E||b, originating from transitions into the Ca-doping induced O 2p hole states. Upon further Ca-doping, the intensity of peak B increases more strongly for E||a, which clearly indicates that the doping induced holes mainly occupy orbitals along the chain direction, thus underlining the one-dimensionality of the low lying electronic structure of Y 2- x Ca x BaNiO 5 . At a doping level of x = 0.1 a weak feature C is observed at 529.6 eV for E||a, similar to what has been found for x = 0.15 in a previous study [3]. Furthermore, similar spectral features were also observed in the case of La 2-x Sr x NiO 4 , where a weak structure corresponding to C was more clearly observed for higher Sr doping levels (≥ 0.4) [23].
Formally, the introduction of holes leads to the formation of trivalent Ni(III) sites with a 3d 7 configuration. Magnetic susceptibility measurements indicate the formation of spin S=1/2 sites upon Ca-doping [24]. As the covalence between Ni and O switches on, the Hamiltonian matrix and the ground state of a 3d n system are given by: where n = 7 stands for Ni(III) and L denotes a hole in O 2p states (for simplicity we neglect is comparable with the observed energy separation between peak B and C for E||a in Fig. 2. We thus assign peaks B and C for E||a to excitations into high and low spin Ni 3d 8  It is clearly seen that the spectral weight of peak A is reduced upon doping which is a result of a spectral weight transfer form the UHB (peak A) to the doping induced features B and C.
Such a spectral weight transfer is a direct signature of a strongly correlated electron system (here a charge transfer insulator) and has been studied experimentally [25,26] and theoretically [27.28] for other correlated materials. Figure 3 shows the Ni-L 3 XAS spectra of Y In addition, for E||a the dominant peak A is shifted by 0.7 eV to higher energy with respect to that for E||b while the high energy shoulder B appears at the same energy (855 eV) in both cases. Since the spectra for E||a and E||b result from transitions from Ni 2p 3/2 to Ni 3d(3z 2 -r 2 ) and 3d(xy) states, respectively, the energy shift of peak A of about 0.7 eV is basically a measure for the crystal field splitting between 3d(3z 2 -r 2 ) and 3d(xy) states caused by the pseudo-tetragonal distortion of the NiO 6 octahedra in Y 2 BaNiO 5 . Thus, the tetragonal splitting of the e g -derived 3d states is about half of the octahedral e g -t 2g crystal field splitting (10 Dq) which has been estimated to be 1.3-1.5 eV for the related three-dimensional compound NiO [29,30].
We note that the tetragonal splitting observed here for Y 2 BaNiO 5 is much smaller than that previously reported for La 2 NiO 4-δ of 1.3 eV [23]. We argue that in the latter case the   Fig. 3 demonstrates that upon doping, i.e. with increase of the formal valence from Ni(II) to Ni(III), spectral weight is transferred from the lower energy peak A to peak B, which reflects an increase of the effective Ni 2p core hole potential due to poorer screening resulting from decrease in the valence electron count.
Furthermore, the Ni-2p 3/2 spectra become broader due to an increase of the O 2p admixture in the wave function, since ∆ is expected to decreased by as much as 3 eV going from Ni(II) to Ni(III) compounds [31,33], as result of the increase of β 0 according to Eq. (3).
The polarization dependent Ni 2p 3/2 XAS spectra of undoped Y 2 BaNiO 5 can be well reproduced using charge-transfer atomic multiplet calculations applying using (ionic) tetragonal crystal-field values 10 Dq = 0.8 eV and Ds = 0.2 eV [31,33]. The 3d 9 L state is found to lie 3 eV above the 3d 8 Fig. 4  O 2p character for the additional holes as compared to an isolated NiO 6 cluster via the interoctahedra hybridization in the ground state of the former. It was found previously that the covalence was decreased (the 3d 7 weight was increased) by nearly 10% going from compounds with interconnected to systems with isolated NiO 6 clusters [31]. In addition, this delocalization of the electronic states along the chains offers the system a further opportunity to screen the Ni 2p core hole via ligand electrons in such a core level spectroscopy. As a result, the corresponding spectrum becomes broader and the higher energy spectral weight is suppressed due to a final state effects. A more quantitative analysis of the distribution of the additional holes in the doped compounds based upon theoretical simulation would be intensively, but is beyond the scope of the present paper.

IV. Summary
We