Neodymium-doping Concentration Induced Face-shared to Corner-shared Transition in Strontium Cobaltite

The topotactic connection style of oxygen octahedron/tetrahedron in transition metal oxides (TMOs) is an important feature that modulates their corresponding physical properties. Using a simple chemical doping technique, we obtained Sr 1- x Nd x CoO 3- δ with a crystal structure transition from face-shared octahedron to corner-shared octahedron/tetrahedron. The Rietveld analyses of the x-ray diffraction (XRD) patterns show that the crystal structure changes from rhombohedral to cubic and the connection style transforms from face-shared to corner-shared with the increase neodymium (Nd) content. During this process, the ferromagnetic behavior is greatly improved due to the larger amount of the corner-shared cubic SrCoO 3- δ phase. The synchrotron radiation x-ray absorption spectroscopies of the Co L -edge and O K -edge show that Nd doping mainly affects the electronic structure of oxygen rather than the valence state of Co. Thereby, the Nd changes the connection style of oxygen octahedron/tetrahedron, which then alters the magnetic interactions.


Introduction
Transition metal oxides (TMOs) are a material class with abundant and novel physical properties, such as superconductivity, [1] multiferroicity, [2,3] magnetoresistance, [4]  concentration. [6] They determined the lowest-energy structure for a series of perovskites (AMnO3 where A=Ca, Sr, Ba) at low temperatures using density-functional calculations, and pointed out that the charge of Mn is much lower than that of the traditional ionic model charge due to the covalence of Mn-O, which reduces the repulsion of Mn-Mn and facilitates the sharing of octahedral faces. Unfortunately, the crystal structure of topotactic connection style is not easily adjusted due to the phenomenon of growth competition. [7] Previous works have reported high pressure oxygen annealing, [8] electrochemical oxidation, [9] and high pressure (~GPa) treatment [5,10] methods to adjust the topotactic connection style. Therefore, it is important to develop a simple and effective method to realize the transformation and understand its effect on the material physical properties.
Strontium cobaltite, as a type of multivalent oxide, exhibits a transformation from face-shared Sr2Co2O5 [9] to corner-shared SrCoO3 [9] or SrCoO2.5 [11] based on the concentration and distribution of oxygen vacancies, as shown in Fig. 1. The cubic SrCoO3 (C-SCO) consists of corner-shared CoO6 octahedrons and is ferromagnetic (FM), while the rhombohedral Sr2Co2O5 (R-SCO) consists of face-shared CoO6 octahedrons and the tetragonal SrCoO2.5 (T-SCO) consists of corner-shared CoO6 and CoO4, which are both antiferromagnetic (AFM). Due to the metastability of Co 4+ , the C-SCO is not completely close-packed and that C-SCO on its own is prone to have oxygen vacancies. [12][13][14] As a result, the reversible topotactic phase transition between C-SCO and T-SCO has sparked interest due to the enhanced catalytic activities by oxygen sponges for sensors and solid oxide fuel cells. [15,16] However, controlling the concentration and distribution of oxygen vacancies in an SCO system requires unique annealing conditions [16,17] or chemical oxidation. [18] What's more, the faceshared R-SCO is rare and difficult to convert between the above phases. [9]  In this paper, we report the effect of neodymium (Nd) doping on the magnetic properties of strontium cobaltite due to changes in the crystal structure and oxygen coordination environment. Rietveld analyses provided evidence that varying the Nddoping in the crystal lattice can induce changes to the topotactic connection style. As a result, the magnetic properties of strontium cobaltite were changed by varying the Nd concentration. The synchrotron radiation x-ray absorption spectroscopies (XAS) of the Co L-edge and O K-edge provide evidence that the electronic structure of the oxygen vacancies in the SCO system are related to change the connections of CoO6/CoO4 and their magnetic interactions. Therefore, this work proposes a simple method to realize the transformation from face-shared structure to corner-shared octahedron and to improve the ferromagnetic behavior of strontium cobaltite.

Experiment
The polycrystalline samples of Sr1-xNdxCoO3 (SNCO-x, x=0, 0.05, 0.1, 0.2, 0.3) were prepared using the standard solid-state reaction method. The SrCO3, Nd2O3 and Co2O3 were used as raw materials and weighed according to their molecular formulas.
Then, the powders were ball-milled for 24 h, followed by calcination at 900℃ for 12 h. Finally, the obtained powders were pressed into pellets, followed by sintering at 930℃ for 12 h. The crystal structures of all the samples were characterized using the

Rietveld analyses of the x-ray powder diffraction (XRD) patterns, measured in a Rigaku
SmartLab diffractometer with Cu Kα radiation (λ=1.5406 Å). The temperature and field dependent magnetic moments were determined using a vibrating sample magnetometer (PPMS-VSM, Quantum Design). The electrical structures of Co L edge and O K edge were characterized by XAS collected at the BL12B-a of the National Synchrotron Radiation Laboratory (NSRL). The total electron yield (TEY) mode using the current measurements from a sample was used. First, the spectrum recorded for the sample was normalized to the photon flux by division through a spectrum obtained for a freshly sputtered gold wafer. Then substrate a line to set the pre-edge to be zero. Finally, the spectra were normalized to yield an edge-jump to one.   Considering the ionic radii of Nd 3+ , Sr 2+ , and Co 4+ /Co 3+ /Co 2+ , Nd should replace Sr which is close to its ionic radius, but not Co which is quite different from its ionic radius.

Results and discussion
And the ionic radius of Nd 3+ is smaller than that of Sr 2+ , which may be an important reason for the phase transition of crystal structure from R-SCO to C-SCO. In order to understand the changes of Nd-doping on the electron structure of SCO, Co L edge and O K edge XAS were characterized. As shown in Figure 3  As shown in Fig. 3(b), the O K-edge spectrums have multiple differences, which originate from the excited electrons of the O 1s core state to the hole states of the 2p characters. In TMOs, the O 2p state is hybridized with the s, p, and d states of its neighboring atoms. Figure 3(b) shows all the typical final hybrid states. [19] Among them, the O2comes from the adsorbed oxygen at the oxygen vacancies. [20,21] Another obvious feature is the relative changes of the CoO6 t2g and CoO6 eg peaks, which reflect the electrical structure of Co in the different samples. The CoO6 t2g peak intensity increases, and the CoO6 eg peak intensity reduces with more Nd in the lattice. to corner-shared transition. Therefore, the changes of the electronic structure with doping concentration will lead to ferromagnetic enhancement in Sr1-xNdxCoO3.
The zero field cooling (ZFC) and field cooling (FC) curves for all the samples are shown in Fig. 4(a), confirming the enhanced ferromagnetic property. The ZFC and FC curves for the un-doped sample are almost completely coincident at a value that is nearly zero when compared to the other samples. The increased Nd-doping into the lattice shows that the heavily-doped sample has a larger irreversibility between the ZFC and FC curves. All the doped samples have a broad peak below the bifurcation temperature in the ZFC curves, which reflects the glassy-like magnetic behavior in these magnetic systems. [23,24] In general, a broad peak in the ZFC curve is explained by two opposing magnetic effects. On one hand, the magnetization is expected to increase at lower temperatures due to the reduced spin fluctuations. On the other hand, anti-parallel spins tend to reduce the magnetization at lower temperatures. As a result, the two competitive interactions produces a peak. [25] The FC curve shows an increase below the bifurcation temperature, which is similar to ferro-or ferrimagnets. To determine the Curie-Weiss temperature (Tc) for all the samples, we used the Curie-Weiss law (Eq. 1) to fit the curves using its inverse form (Eq. 2), as shown in Fig.   4(b): where χ, C, T and Tc represent the magnetic susceptibility, Curie-Weiss constant (C > 0), temperature and Curie point, respectively. All the doped samples have the same fitted Curie point of Tc≈200 K, as shown in Fig. 4(b), which reflects their identical ferromagnetic source. By fitting the linear portion of the 1/χ-T curve to obtain the C value, the effective magnetic moment can be converted according to the Langevine paramagnetic theory, where the effective magnetic moment is = √ ( + 1). The calculated results are listed in Fig. 4(b), showing that µeff gradually increases with the Nd-doping. This means that the structural changes bring about direct changes in the electronic structure.
To quantify the influence of the electronic structure on the magnetic properties of the the T-SCO AFM matrix, which cannot be detected using XRD. [16] We compared the normalized values of µeff, Ms, and Mr, and found that they all increased at the same rate, as shown in the inset of Fig. 4(d). Therefore, the ferromagnetism is caused by the increased µeff of Co, which is related to the face-shared to corner-shared transition of CoO6. In general, the µeff is related to the arrangement of the 3d electrons. A larger µeff can be attributed to more unpaired 3d electrons when ignoring the orbital magnetic moment.

Conclusions
In summary, Nd modified SCO was prepared using a solid state reaction method.
The Rietveld analyses of the XRD patterns show the different phase compositions. The MT and MH curves were applied to analyze the magnetic properties in detail. It was observed that more Nd doping caused more C-SCO phase appear, which cannot be completely converted to C-SCO because of the appearance of T-SCO. Due to the transformation from the face-shared to corner-shared octahedron, the ferromagnetic properties were enhanced, which may be caused by the 3d electronic structure change. Figure 1 Different crystal structures and space groups of SrCoOx: face-shared CoO6 in rhombohedral Sr2Co2O5