Crystal Growth and Physical Properties of Sr4Co3O7.5+xCl2 Single Crystals (x ∼ 0.14)

We report on the single crystal growth and physical properties of the triple-layer cobalt oxychloride Sr 4 Co 3 O 7 . 5 + x Cl 2 (x∼ 0.14) with 4-3-10 Ruddlesden–Popper type structure that was synthesized by a KCl-SrCl 2 flux method. The crystal structure was determined by means of single crystal X-ray diffraction. In this quasi two dimensional (2D) material two pyramidal CoO 5 layers and a central Co oxide layer with random oxygen deficiencies are forming the layered Co oxide blocks. These blocks are separated by Cl − -ions which are interacting via Van der Waals forces, thus, enhancing the quasi 2D nature of this compound. The soft X-ray absorption spectra at the Co-L 2 , 3 edge and O-K edge indicate that Co ions are in high spin +3 state which is in agreement with the single crystal X-ray diffraction measurements that indicate basically pyramidal oxygen environments for the Co 3 + ions in this compound.


Introduction
Quasi-two-dimensional (2D) layered cobaltates attracted enormous attention in the past, and display a plethora of interesting physical properties, such as superconductivity in Na x CoO 2 ·yH 2 O [1], high thermoelectric power in NaCo 2 O 4 and Ca 3 Co 4 O 9 [2], hour-glass shaped magnetic excitation spectra in La 2−x Sr x CoO 4 [3][4][5][6][7][8], extraordinary charge storage properties in Li x CoO 2 [9] etc. Besides spin, lattice and orbital degrees of freedom also the spin state degree of freedom [10][11][12][13][14] is relevant for the rich physical properties of these cobaltates. For the Co 3+ ion a S = 0 low spin (LS), a S = 1 intermediate spin (IS) and a S = 2 high spin (HS) state can occur. The IS state, with one electron in the e g orbitals, is Jahn-Teller active and, hence can gain energy and become the ground state in the presence of sufficiently large distortions of the local structure. In oxygen-deficient GdBaCo 2 O 5.5 the Co 3+ -ions have a square pyramidal oxygen coordination leading to the proposal that the Co 3+ IS state occurs in this system [15]. In contrast to that, a soft X-ray absorption spectroscopic study at the Co-L 2,3 and the O-K edge as well as a density-functional theory study indicate that the HS state has to be considered for Co 3+ -ions with pyramidal coordination [16,17]. Layered cobaltates within the Ruddlesden-Popper series (AE n+1 Co n (O,Cl) 2n+1 ; AE = Alkali earth ion) exhibit also ordering of oxygen-deficiencies [18,19]. The single-layer (Sr 2 CoO 3 Cl) and double-layer (Sr 3 Co 2 O 5 Cl 2 ) compounds are hosting Co ions with pyramidal oxygen coordination. In the triple-layer compound Sr 4 Co 3 O 7.5+x Cl 2 there are two different Co sites [20]: between two pyramidal CoO 5 layers there is another oxygen deficient Co oxide layer, see Figure 1. These three Co oxide layers are separated by Cl-ions from the other triple-layers and interact via Van der Waals forces only. Therefore, Sr 4 Co 3 O 7.5+x Cl 2 should exhibit enhanced quasi-two-dimensional (2D) properties. However, the expected intrinsic anisotropic physical properties of the layered AE n+1 Co n (O,Cl) 2n+1 materials have not been reported so far, which might be due to the lack of sizeable single crystals. The availability of single crystals would be also desirable for a clarification of the structural details and especially the Co coordination in the intermediate Co oxide layer.  Figure 2 shows one of our flux-grown Sr 4 Co 3 O 7.5+x Cl 2 single crystals. Powder X-ray diffraction measurements indicate the absence of impurity phases for our crushed crystals, see Figure 3. Le Bail fits yield lattice parameters (a = 3.9008(3) Å, c = 31.727(3) Å) that are roughly in agreement with the ones reported in Reference [18]. Subsequently, more accurate single crystal X-ray diffraction measurements were performed. The X-ray scattering intensities within the 0KL, H0L and HK0 planes of reciprocal space are shown in Figure 4. These measurements indicate the untwined, single crystalline nature of our Sr 4 Co 3 O 7.5+x Cl 2 crystals. In total, 28051 reflections were collected up to 2Θ max = 92.94 • with an internal R-value of 3.51% and a redundancy of 38.9. The structural results of our crystal structure refinement are listed in Table 1. The Co1-ions are pyramidally coordinated by 5 oxygen ions whereas the central Co2 ions has a formal octahedral coordination. However the occupancy close to 3/4 shows that almost one of the four surrounding oxygen ions in the basal plane is missing. Therefore, most of the Co2 ions are also square pyramidally surrounded by five oxygen ions (three within the central Co-O layer and one in each of the Co-O layers above and below. Thus, compared to the CoO 5 pyramids within the outer layers where the apical oxygen ions are pointing in c-direction, the CoO 5 pyramids within the central Co oxide layers are tilted by 90 • to the basal plane. The atomic displacement parameters of the corresponding apical oxygens of these central CoO 5 pyramids are enlarged within the ab-plane, thus, indicating rotations of these CoO 5 pyramids around the tetragonal c-axis, see Figure 1. These pyramidal rotations are obviously induced by the random distribution of oxygen deficiencies. The pyramidal environment will induce shifts of the Co ions towards the apical oxygens. Therefore, also these Co-ions should not be located at the central points of the octahedra but shift towards the apical oxygen. Indeed, this is also indicated by the anisotropic atomic displacement parameters of the central Co-ions, see Figure 1. (I obs.

Results and Discussion
-I cal.
) Bragg pos. According to the refined value of the oxygen occupancy the total oxygen content of our measured single crystal amounts to O 7.636 (18) . That the bond valence sums (BVS) of the Co ions (see Table 1) are somewhat smaller than the expected Co 3+ oxidation state might be either a consequence of systematic errors of the BVS formalism for our pyramidally coordinated Co ions or an indication for a more covalent character of the Co ions in this compound.
We also would like to note that the clattice parameter of these Sr 4 Co 3 O 7.5+x Cl 2 single crystals could be increased by 0.03 Å after a treatment in de-ionized water. Figure 5a shows the temperature dependence of resistivity of Sr 4 Co 3 O 7.5+x Cl 2 for the in-plane (black circles) and out-of plane (red circles). The temperature dependence of ρ(T) is semiconductive/ insulating and obeys well the Arrhenius equation 1/ρ = σ = A·exp(∆/kT) around room-temperature, where A is a constant pre-exponential factor, k is the Boltzmann constant and ∆ is the activation energy with ∆= 0.66 eV and ∆= 0.90 eV for the in-plane and out-of plane directions, respectively. The large out-of-plane resistivity is in agreement with the quasi-2D nature of this material.
The magnetic susceptibility as a function of temperature for H||ab (black circles) and H||c (red circles) taken with H = 0.1 T is shown in Figure 5b. The small absolute value of the magnetic signal can be due attributed to AFM interactions of the HS Co 3+ ions. In the inset of Figure 5b the magnetization curves for H||ab and H||c are shown. These measurements, indeed, confirm that Sr 4 Co 3 O 7.5+x Cl 2 is not a simple paramagnet, but, exhibits antiferromagnetic together with ferrimagnetic correlations (probably due to canted AFM moments). Hence, the AFM ordering temperature seems to be well above room-temperature.   Figure 6a shows the isotropic Co-L 2,3 XAS spectrum of Sr 4 Co 3 O 7.5+x Cl 2 taken at room temperature together with those of Sr 2 CoO 3 Cl [17], EuCoO 3 [17], and CoO serving as HS-Co 3+ , LS-Co 3+ , and HS-Co 2+ references. It is well known that the soft XAS at the Co-L 2,3 edges is very sensitive to the spin, orbital and valence states of the ion [14,17,21,22]. As shown in Figure 6a, the center of gravity of the L 2,3 white lines of Sr 4 Co 3 O 7.5+x Cl 2 locates at the same photon energies as observed for the Co 3+ reference samples Sr 2 CoO 3 Cl and EuCoO 3 , thus, indicating the presence of a Co 3+ valence state in Sr 4 Co 3 O 7.5+x Cl 2 . The absence of any pronounced spectral feature at 777.8 eV indicates a negligible amount of Co 2+ impurities in Sr 4 Co 3 O 7.5+x Cl 2 . Furthermore, the multiplet spectral feature of Sr 4 Co 3 O 7.5+x Cl 2 is very different from that of EuCoO 3 , implying the different electronic structures of these two compounds. On the other hand, the spectral features at both the Co-L 3 and the Co-L 2 edges of the Sr 4 Co 3 O 7.5+x Cl 2 are almost identical to those of the HS-Co 3+ reference sample Sr 2 CoO 3 Cl, which indicates the presence of a high-spin Co 3+ state in Sr 4 Co 3 O 7.5+x Cl 2 . More spectroscopic evidence for the HS nature of the Co 3+ ions in the Sr 4 Co 3 O 7.5+x Cl 2 can be found from the O-K XAS spectrum.  [17] and EuCoO 3 [17] for comparison. In (a) a CoO reference that was measured simultaneously is addionally shown. Figure 6b presents the O-K XAS spectra of Sr 4 Co 3 O 7.5+x Cl 2 and of a HS Co 3+ reference sample Sr 2 CoO 3 Cl and of a LS Co 3+ reference sample EuCoO 3 . The structures below 532 eV originated from transitions from the O 1s to the O 2p orbitals that are hybridized with the Co 3d orbitals. One can see that the pre-edge peak in the O-K XAS spectra of Sr 4 Co 3 O 7.5+x Cl 2 and Sr 2 CoO 3 Cl locates at the same energy and is shifted by about 1.2 eV to lower energy with respect to the one of the LS reference sample EuCoO 3 . This, further, confirms a HS Co 3+ in Sr 4 Co 3 O 7.5+x Cl 2 . The low photon energy of the pre-edge peak in the HS Co 3+ oxides as compared with that of LS Co 3+ in EuCoO 3 is attributed to the fully occupied t 2g levels in the latter, where only transitions to the e g -related states are possible, whereas the transitions to the unoccupied t 2g orbitals for the HS Co 3+ oxides are also allowed [17].

Materials and Methods
A Sr 4 Co 3 O 7.5+x Cl 2 single crystal was grown by a KCl-SrCl 2 flux method. Starting materials for this synthesis were SrO 2 (98% Aldrich, Darmstadt, Germany), SrCl 2 (99.5% Alfa Aesar, Kandel, Deutschland), KCl (99.99% Alfa Aesar, Kandel, Deutschland) and Co 3 O 4 (99.7% Alfa Aesar, Kandel, Deutschland) with a molar ratio of 1.5:1.5:0.99:0.33. A mixture of these materials was placed in a Al 2 O 3 crucible and heated to and kept at 850 • C for 12 h. Then, the sample was heated with a rate of 1 • C/h to 950 • C and kept at this temperature for 12 h. Afterwards the sample was cooled to 850 • C with a cooling rate of 0.5 • C/h, followed by furnace cooling to room-temperature. Plate-like single crystals with size up to 2 mm × 2 mm in diameter could be obtained by mechanical separation from the flux, see Figure 2. The phase purity and single crystalline nature of the as-grown Sr 4 Co 3 O 7.5+x Cl 2 crystals was ascertained by means of powder and single crystal X-ray diffraction (XRD) using a D8 powder diffractometer(Bruker-AXS, Karlsruhe, Germany) with Cu-K radiation (λ = 1.5418 Å) as well as a Bruker D8 Venture single crystal diffractometer (Bruker-AXS, Karlsruhe, Germany) with Mo-K radiation (λ = 0.71069 Å). Structural refinements were performed with the Jana2006 and TOPAS (Ver.4.2, Bruker-AXS, Karlsruhe, Germany) software packages [23][24][25]. The electrical resistivity was measured using a commercial four-probe setup within a Physical Property Measurement System (PPMS, Quantum Design Inc., San Diego, CA, USA ). The four electrodes were connected with silver paste to a shiny and almost rectangular shaped single crystal (with inter-electrode distances below 1 mm) and a current of 500 mA was applied. The magnetization of our samples was studied using a SQUID magnetometer (MPMS-5XL, Quantum Design Inc., San Diego, CA, USA). The chemical composition of a crystal was also determined by energy dispersive X-ray spectroscopy (EDX) measurements and the following composition was detected at the surface of one sample: 43.94% Sr, 35.72% Co and 20.34% Cl. These EDX results are less accurate than the ones of our single crystal X-ray diffraction measurements. The X-ray absorption spectroscopy (XAS) experiments were performed at the Dragon beam line of the National Synchrotron Radiation Research Center (NSRRC) in Taiwan. The Co-L 2,3 and the O-K spectra were measured in the total electron yield (TEY) mode with a photon energy resolution of 0.3 and 0.2 eV, respectively. Clean crystal surfaces were obtained by cleaving the samples in situ in a vacuum of 1×10 −9 mbar. CoO and NiO single crystals were measured simultaneously in a separate chamber to serve as energy reference for Co-L 2,3 edge and for O-K edge, respectively.

Conclusions
We successfully grew single crystals of the triple-layer cobalt oxychloride Sr 4 Co 3 O 7.5+x Cl 2 by using a KCl-SrCl 2 flux method. The anisotropic transport and magnetic properties are well in accordance with the layered crystal structure. Sr 4 Co 3 O 7.5+x Cl 2 is an AFM semiconductor. Due to the presence of oxygen vacancies the Co ions at both Co sites are basically pyramidally coordinated with oxygen ions. Thus, the Co ions appear to be in the Co 3+ HS state as observed from the Co-L 2,3 and the O-K XAS spectra.