Unusual mixed spin-state of Co3+ in the ground state of LaSrCoO4: Combined high-pressure and high-temperature study
Introduction
Cobalt oxides have attracted intense attention in past decades due to their fascinating transport and magnetic properties [1], such as the metal-insulator transition and spin-state transition in LaCoO3 (LCO) [2], [3], [4], [5], superconductivity in NaxCoO2·yH2O [6], the giant magnetoresistance in RENi0.3Co0.7O3 (RE = La, Nd, Sm) [7], and the spin-blockade behavior in HoBaCo2O5.5 [8] and La1.5Sr0.5CoO4 [9]. Special to the cobalt oxides relative to other transition-metal oxides is the spin-state degree of freedom. For example, in an octahedral coordination, Co3+ ion with configuration d6 could exist in three possible spin states: a HS state (S = 2, t2g4eg2), a LS state (S = 0, t2g6eg0), and even an IS state (S = 1, t2g5eg1) [1], [10], [11]. This richness of electronic and magnetic properties is closely related to the variation of spin states with different eg occupation. The high performance of widely used electrochemical cobalt-oxide catalysts was found recently to be attributed to an eg occupancy near unity, namely IS-Co3+ [12], [13], [14], [15].
The existence of the various spin states originates from the subtle balance between Hund’s exchange energy and the crystal-field-splitting energy. The spin states of Co3+ ion can hence be altered on tuning the crystal-field interaction on varying the temperature [16], [17], external pressure [17], [18] or the internal pressure through chemical substitution [7], [19], [20], [21]. For example, the pressure-induced spin-state transition was observed in BiCoO3, LCO, SrCo0.5Ru0.5O3-δ, Pr0.5Ca0.5CoO3, (Pr0.7Sm0.3)0.7Ca0.3CoO3, and Sr2Co0.5Ir0.5O4 [18], [22], [23], [24], [25]; a temperature-induced spin-state transition was observed in LCO accompanying an insulator-to-metal transition and a non-magnetic-to-magnetic transition [16], [17], [26].
The nature of the magnetic spin state (IS or HS) was strongly disputed in the literature for over four decades. In the case of CoO6 octahedra without distortion, calculations of Co3+ energy levels show that the LS (HS) state can be stabilized with a large (small) value of the crystal field 10 Dq, whereas the IS has invariably greater energy and can never be stabilized in LCO [26]. As the IS state with occupation eg1 is Jahn-Teller active, it can gain energy and become the ground state in the presence of a sufficiently large distortion of the local structure. For this reason, the spin state of Co3+ ions, in layered cobaltates in which an elongated distortion of CoO6 octahedra is favored and might stabilize the IS state, has been the subject of intensive debate. The band formation has been proposed to provide another possible route for the stabilization of IS-Co3+ [11], [27].
In the case of layered La2−xSrxCoO4, contradictory scenarios for the Co3+ ions were considered to interpret the complicated structural, magnetic, and transport properties of the system as a function of Sr doping: LS-Co3+, IS-Co3+ [28], [29], [30], [31], [32], HS-to-IS transition [28], mixing of HS/IS [33], [34], LS for x = 0.5 [9], [35] and a mixture of LS/HS for x > 1 [35], [36], [37]. A recent systematic study indicated that Co3+ ions are in the non-magnetic LS state for strontium concentration x < 0.5 but in a mixed spin state with a growing population of HS states for concentration x increasing in the range 0.5 ≤ x ≤ 1.1 [19], [37], [38].
The crystal structure, electrical transport, and magnetic susceptibility of the half-doped compound La2−xSrxCoO4 (x = 1), LaSrCoO4 (LSCO), have been reported, whereas there is a lack of detailed discussion regarding the electronic structure and spin state of Co3+ in LSCO [30], [36]. In this study, we focus on LSCO compound with oxidation state pure Co3+ to explore its detailed electronic structure and spin state as a function of pressure and temperature. The role played by particular structural factors influencing the spin-state transitions in similar layered compounds can be studied on appropriately choosing a LSCO compound with pure Co3+ ion and paramagnetic insulating behavior [30], [38], [39], [40]. In light of the mixed LS/HS Co3+ ions in LSCO [37], we tried to tune systematically the LS and HS content on increasing and decreasing the crystal-field interaction via high pressure and heating, respectively. For this purpose, the Co-Kβ X-ray emission spectroscopy (XES) and high resolution partial fluorescence yield (HRPFY) X-ray absorption spectroscopy (XAS) at Co-K edge were recorded as a function of high pressure up to 31.3 GPa and high temperature up to 724 K.
Section snippets
Experimental
Polycrystalline samples of layered perovskite LSCO were synthesized by solid state reaction method. Appropriate amount of the starting materials of La2O3, SrCO3 and Co3O4 were mixed and thoroughly ground in an agate motar. The powders were then pressed into pellets and sintered at 950 °C for 6 days with several intermediate grindings. The pellets were subsequently annealed in O2 atmosphere of 5000 bar at 450 °C for 4 days. Powder X-ray diffraction measurement of LSCO has been performed using Cu
Powder X-ray diffraction measurement
Fig. 1 shows powder X-ray diffraction (PXRD) pattern of LSCO. The FullProf program package was used for Rietveld fits [42]. As expected, the material has tetragonal crystal structure with space group I4/mmm and lattice constants of a = 3.8077(6) Å and c = 12.4978(24) Å. Only a tiny impurity of 1.7( ± 0.2) % was detected from PXRD refinement.
Experimental and theoretical Co-L2,3 edge XAS spectra
The valence state and spin state of Co ion in LSCO were first determined on recording the Co-L2,3 edge XAS spectra. The spectral weight center of the Co-L3
Conclusion
The spin state of Co3+ ion in LSCO below 300 K was verified from the experimental Co-L2,3 and the O-K XAS spectra, and the Co-L2,3 XMCD spectrum. Unlike LCO with a pure LS-Co3+ ground state, the ground state of LSCO consists of a mixed HS/LS Co3+ even at 10 K. A weighted ratio 2:3 of HS:LS for LSCO relative to 1:3 for LCO was obtained from the Co-L2,3 edge XAS spectra at 300 K and AP. The spin-state transition was studied systematically with Co-Kβ XES spectra as a function of pressure and
CRediT authorship contribution statement
Conception and design of study: Zhiwei Hu, Jin-Ming Chen and Liu Hao Tjeng. Acquisition of data: Shu Chih Haw, Hong Ji Lin, Jenn Min Lee, Hirofumi Ishii, Nozomu Hiraoka, Anna Melendez-Sans, Alexander C. Komarek, Kai Chen, Chen Luo, Florin Radu. Analysis and/or interpretation of data: Shu Chih Haw, Zhiwei Hu, Jin-Ming Chen, and Liu Hao Tjeng. Drafting the manuscript: Shu Chih Haw, Zhiwei Hu, and Jin-Ming Chen. Revising the manuscript critically for important intellectual content: Shu Chih Haw,
Declaration of Competing Interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgements
This work is supported by the Ministry of Science and Technology under grants MOST 108–2113-M-213–004 and 109–2113-M-213–001. We acknowledge support from the Max Planck-POSTECH-Hsinchu Center for Complex Phase Materials. We acknowledge also financial support for the VEKMAG project and for the PM2-VEKMAG beamline by German Federal Ministry for Education and Research (BMBF 05K10PC2, 05K10WR1, 05K10KE1) and by HZB.
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