Disproportionation of Co2+ in the Topochemically Reduced Oxide LaSrCoRuO5

Abstract Complex transition‐metal oxides exhibit a wide variety of chemical and physical properties which are a strong function the local electronic states of the transition‐metal centres, as determined by a combination of metal oxidation state and local coordination environment. Topochemical reduction of the double perovskite oxide, LaSrCoRuO6, using Zr, yields LaSrCoRuO5. This reduced phase contains an ordered array of apex‐linked square‐based pyramidal Ru3+O5, square‐planar Co1+O4 and octahedral Co3+O6 units, consistent with the coordination‐geometry driven disproportionation of Co2+. Coordination‐geometry driven disproportionation of d7 transition‐metal cations (e.g. Rh2+, Pd3+, Pt3+) is common in complex oxides containing 4d and 5d metals. However, the weak ligand field experienced by a 3d transition‐metal such as cobalt leads to the expectation that d7+ Co2+ should be stable to disproportionation in oxide environments, so the presence of Co1+O4 and Co3+O6 units in LaSrCoRuO5 is surprising. Low‐temperature measurements indicate LaSrCoRuO5 adopts a ferromagnetically ordered state below 120 K due to couplings between S=1/2 Ru3+ and S=1 Co1+.

Complex metal oxides have been the subject of extensive study due to the wide variety properties they exhibit.These range from electronic and magnetic behaviors such as ferroelectricity, superconductivity and magnetoresistance to an extensive array of catalytic and electrochemical phenomena.As the chemical and physical behaviors exhibited by metal oxides tend to depend strongly on the electric configurations of the metal cations they contain (defined by a combination of oxidation states and coordination environments), there has been an enduring interest in establishing composition-structure-property relations in extended oxide systems to explore these features.These studies have revealed that a number of elements exhibit 'disfavored' oxidation states in oxide environments, i.e. oxidation states that appear to be thermodynamically accessible (sufficient lattice energy to overcome the required ionization energy) but are unstable with respect to disproportionation, when the metal is located in an extended oxide framework.
The instability of some of these disfavored states, such as the disproportionation of Pb 3 + and Bi 4 + in Pb 2 O 3 and BiO 2 respectively, [1][2] can be accounted for by universal chemical concepts-in this instance the global instability of ns 1 electron configurations in main group metals leads to Pb 2 O 3 and BiO 2 being better described as Pb II Pb IV O 3 and Bi III Bi V O 4 respectively.
However, similar disproportionations are observed in transition-metal systems, where the instability of the metal oxidation state cannot be easily attributed to a global instability of a particular electron count but appears to arise from the favorability of particular combinations of delectron count and local coordination environment.For example, AgO is better described as Ag I Ag III O 2 , [3] with the disproportionation of Ag II attributed to the favorability of locating d 10 Ag I in a linear coordination and d 8 Ag III in square-planar coordination sites within the oxide framework. Here we describe the first observation of the disproportionation of d 7 Co II in an extended oxide, which occurs during the topochemical reduction of the double perovskite oxide LaSr-CoRuO 6 to LaSrCoRuO 5 .
The corresponding cobalt phase, LaSrCoRuO 6 , [11][12] exhibits an analogous phase transition at T � 450 °C.Rapidly quenching LaSrCoRuO 6 through its R-3 to P2 1 /n phase transition also enhances its reactivity enabling the preparation of the infinite layer phase LaSrCoRuO 4 via reaction with binary metal hydrides, as will be described in detail elsewhere.However, in contrast to the LaSrNiRuO 6-x system, quenched samples of LaSrCoRuO 6 can be reduced to a phase of intermediate oxygen content (shown to be LaSrCoRuO 5 by oxidative thermogravimetric analysis) by reaction with a Zr getter at 450 °C.
Synchrotron X-ray powder diffraction (SXRD) data collected from LaSrCoRuO 5 could be indexed using a bodycentered monoclinic unit cell (a = 5.40 Å, b = 5.41 Å, c = 8.16 Å, γ = 90.5 °) consistent with the retention of the perovskite framework from the LaSrCoRuO 6 parent phase.However, close inspection revealed a series of weak additional reflections in the SXRD data that could not be indexed by this cell. This expanded cell accounts for all the additional weak peaks observed in the SXRD data and can also index neutron powder diffraction (NPD) data collected at room temperature from LaSrCoRuO 5 .
Considering the A 2 BB'O 5 composition and the 2 p 2×2 p 2×2 cell expansion of the phase, a number of anionvacancy ordered and B-site cation ordered perovskite structural models were considered for LaSrCoRuO 5 .It was observed that a good fit to the SXRD and NPD data could be achieved using a model based on the anion-vacancy ordered structure of LaNi 0.9 Co 0.1 O 2.5 which consists of a network of apex-linked 6-coordinate octahedral, 5-coordinate square-based pyramidal and 4-coordinate square planar BO x units. [15]The model was modified to take account of the rock salt ordering of the Co and Ru cations, so that the Ru centers were exclusively located within 5-coordinate sites, while the Co centers occupied both 6-and 4-coordinate sites within a monoclinic unit cell (a = 10.8128(2)Å, b = 10.8231(2) Å, c = 8.1626(1) Å, γ = 90.55(1)°) with P112 1 space group symmetry, as shown in Figure 2. The model was refined against the NPD data to achieve a good fit (wRp = 6.33 %) as shown in Figure 1 and described in detail in the Supporting Information. [16]he crystal structure of LaSrCoRuO 5 shown in Figure 2 reveals that the cobalt cations occupy two distinct sites within the oxide framework.A 4-coordinate planar site and a 6-coordniate octahedral site.The location of the cobalt   ) [17] or Sr 2 CoO 2 Cu 2 S 2 (CoÀ O = 1.995(1)Å×4) [18] consistent with assignment of Co 1 + O 4 for the units present in LaSrCoRuO 5 .
In an attempt to further confirm the disproportionation of Co 2 + , cobalt EELS data collected from LaSrCoRuO 5 .These data show a single set of Co L 2 and L 3 peaks (Figure S14 in the Supporting Information) and thus represent the superposition of signals from both the squareplanar and octahedral cobalt sites.In the absence of a Co 1 + oxide standard we are unable to know if a Co 1 + /Co 3 + oxidation state combination would be expected to lead to a resolvable splitting of the L 2 and L 3 peaks.It should be noted that splitting of Co 2 + /Co 3 + signals is not resolvable for Co 3 O 4 . [22]The L 3 /L 2 intensity ratio (4.83) and L 3 -L 2 energy difference (15.06 eV) from the data are broadly consistent with Co 2 + .
Magnetization data collected from LaSrCoRuO 5 indicate that, in common with many other topochemically reduced phases containing cobalt, samples of LaSrCoRuO 5 contain small quantities of ferromagnetic, elemental cobalt not detectable by diffraction.The magnetization of LaSrCoR-uO 5 was therefore measured using the 'ferromagnetic subtraction' method described in the Supporting Information.A plot of the magnetic susceptibility of LaSrCoRuO 5 against temperature (Figure 3a) can be fit by the Curie-Weiss law in the range 140 < T/K < 300.However, the extracted Curie constant (C = 3.76 cm 3 K mol À 1 ; θ = + 82.7 K) is much larger than can be accounted for by a combination of Co 1 + , Co 3 + and Ru 3 + cations (even with the cobalt centers in high-spin states), suggesting strong magnetic interactions are present between the metal centers in this temperature range.

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On cooling below 120 K there is a large increase in the saturated ferromagnetic moment of the samples (Figure 3b), increasing from 0.03 μB per fu at 150 K (a value that is attributed to the presence of an elemental Co impurity) to � 1.25 μB per fu at 2 K, indicative of a ferromagnetic state, however NPD data collected at 5 K show no additional features indicative of magnetic order, as described in the Supporting Information.
The bond lengths of the square-planar and octahedral cobalt sites in LaSrCoRuO 5 are consistent with a high spin, S = 1 Co 1 + center, and a low spin, S = 0 Co 3 + respectively.Thus, the most significant magnetic couplings in the system will be between the square-planar Co As shown in Figure 3c, the Co 1 + and Ru 3 + centers are magnetically coupled by either (Ru4d x2À y2 )À O2pÀ (Co3d x2À y2 ) or (Ru4d z2 )À O2p-(Co3d x2À y2 ) σ-type super exchange or (Ru4d z2 )À (Co3d z2 ) direct exchange.Given that the Ru 4d x2À y2 and 4d z2 orbitals are empty and the corresponding Co3d orbitals are half filled, all of these interactions will be ferromagnetic, [23] consistent with the low-temperature magnetization data.
The disproportionation of Co 2 + evident in LaSrCoRuO 5 is surprising.As noted above, other transition metal cations with d 7 electron counts (e.g., Pd 3 + , Pt 3 + , Rh 2 + ) are observed to disproportionate in oxide environments, driven by the presence of 'preferred' coordination sites.However, to date, this behavior has been restricted to 4d and 5d transition metals where the stronger ligand fields (compared to 3d metals) provide a larger energetic stabilization for the d 6 octahedral and d 8 square-planar electron-count/coordination combinations.It is therefore unexpected to see Co 2 + , a common oxidation state with a modest ligand field in oxides, undergo a coordination-site driven disproportionation.
There are limited examples of 3d transition metal cations, such as Fe 4 + and Ni 3 + disproportionating in extended oxides.However, in these cases the disproportionation of the metal center (e.g.Fe 4 + in CaFeO 3 or BaFeO 3 ; Ni 3 + in TlNiO 3 ) [24][25][26] is driven by a metal-insulator phase transition driven by the presence of a single electron in the σ-band of these oxides phases, rather than coordination site preference.
The unique observation of coordination-site driven disproportionation of Co 2 + in LaSrCoRuO 5 suggests that the topochemical reaction which forms LaSrCoRuO 5 may act to 'select' this phase, as the disproportionated structure is a local energy minimum in composition-structure space in the reaction path between LaSrCoRuO 6 and LaSrCoRuO 4 . In combination these observations suggest further coordination-site driven disproportionated oxide phases could be accessible by this type of low-temperature reaction, presenting an opportunity to prepare a range of transition metal oxides with a rich variety of novel metal oxidation-state/coordination geometry-combinations.

Figure 2 .
Figure 2. Crystal structure of LaSrCoRuO 5 (top) and a projection of the transition metal coordination polyhedra at z � 0.75 (bottom).
cations in two distinct sites is reminiscent of the localcoordination-driven disproportionation of transition metals with d 7 electron counts observed for Pd 3 + and Pt 3 + and more recently Rh 2 + , and suggests the disproportionation of Co 2 + into Co 1 + (square-planar) and Co 3 + (octahedral).Analysis of the local coordination environments of the cobalt centers is hampered by the lack of reported Co 1 + O 4 units for comparison.However, the observed bond lengths of the CoO 4 units in LaSrCoRuO 5 (CoÀ O = 2.032(11) Å×2; 2.119(9) Å×2) are significantly longer than those in the Co 2 + O 4 units reported in Sr 3 Co 2 O 4 Cl 2 (CoÀ O = 2.007(1) Å×4

Figure 3 .
Figure 3. a) Paramagnetic susceptibility and b) saturated ferromagnetic moment of LaSrCoRuO 5 measured via the 'ferrosubtraction' method and plotted as a function of temperature, c) The direct exchange and super exchange pathways in LaSrCoRuO 5 .