Structure and cation distribution in perovskites with small cations at the A site: the case of ScCoO3

We synthesize ScCoO3 perovskite and its solid solutions, ScCo1−xFexO3 and ScCo1−xCrxO3, under high pressure (6 GPa) and high temperature (1570 K) conditions. We find noticeable shifts from the stoichiometric compositions, expressed as (Sc1−xMx)MO3 with x = 0.05–0.11 and M = Co, (Co, Fe) and (Co, Cr). The crystal structure of (Sc0.95Co0.05)CoO3 is refined using synchrotron x-ray powder diffraction data: space group Pnma (No. 62), Z = 4 and lattice parameters a = 5.26766(1) Å, b = 7.14027(2) Å and c = 4.92231(1) Å. (Sc0.95Co0.05)CoO3 crystallizes in the GdFeO3-type structure similar to other members of the perovskite cobaltite family, ACoO3 (A3+ = Y and Pr-Lu). There is evidence that (Sc0.95Co0.05)CoO3 has non-magnetic low-spin Co3+ ions at the B site and paramagnetic high-spin Co3+ ions at the A site. In the iron-doped samples (Sc1−xMx)MO3 with M = (Co, Fe), Fe3+ ions have a strong preference to occupy the A site of such perovskites at small doping levels.


Introduction
ABO 3 perovskite-type compounds, where A 3+ = Y and La-Lu and B 3+ = V, Cr, Mn, Fe, Co, Ni and Ni 0.5 Mn 0.5 , and their solid solutions have been attracting a lot of attention for decades from the viewpoints of fundamental physics and practical applications [1][2][3]. For example, some ACrO 3 compounds exhibit spin-reorientation transitions [4], and doped ACrO 3 are good oxygen-ion conductors and show sensitivity toward methanol, ethanol, some gases and humidity [5]. ACoO 3 compounds have been investigated a lot because of spin-state transitions in Co 3+ ions and metalinsulator transitions [6]; ACoO 3 also exhibit thermoelectric Content from this work may be used under the terms of the Creative Commons Attribution 3.0 licence. Any further distribution of this work must maintain attribution to the author(s) and the title of the work, journal citation and DOI. [7] and catalytic properties [8]. The large A site of ABO 3 perovskites is usually occupied by larger cations (such as, rare earths), and the small B site by smaller cations (such as, transition metals). The A/B inter-site mixing is very rare in simple perovskites. In complex perovskites, the A/B inter-site and intra-site mixing can occur, and the cation distribution could significantly modify properties of materials, such as, magnetic and dielectric properties. For example, the appearance of Mg 2+ at the A site in BaMg 1/3 Ta 2/3 O 3 [9] and Mn 2+ at the A site in SrTiO 3 [10] results in increased dielectric loss, and the degree of Ni 2+ and Mn 4+ ordering at the B site in ANi 0.5 Mn 0.5 O 3 changes magnetism of the system [11].
In recent years, ABO 3 perovskites have been extensively expanded to smaller A cations, such as, Mn 2+ , Sc 3+ , and In 3+ [12] with expectations to find new magneto-structural coupling behaviours because of large structural distortions. Unusual physical properties were indeed found, for example, in MnVO 3 (incommensurate magnetic ordering and metallic conductivity) [13], In 2 NiMnO 6 (spin-induced ferroelectricity) [12] and ScVO 3 (distinct magnetic, orbital and structural properties from other members of the AVO 3 (A 3+ = Y and La-Lu) family [14]). Because the difference in the sizes of the A and B cations decreases the probability of the A/B inter-site mixing is increased in such perovskites. Noticeable cation mixing or, more precisely, shifts in the composition were found in ( + − +  [16]; in such perovskites, small divalent transition metals are located at the A site.

In Mn
In this work, we investigated ScCoO 3 perovskite and its solid solutions ScCo 1−x Fe x O 3 and ScCo 1−x Cr x O 3 . We found noticeable shifts in the composition of such perovskites from ScCoO 3 to (Sc 1−x Co x )CoO 3 and the appearance of significant amounts of small trivalent cations (Co 3+ and Fe 3+ ) at the A site. To the best of our knowledge, the presence of Fe 3+ at the A site was detected for the first time in ABO 3 perovskites.

Experimental details
Samples were prepared from stoichiometric mixtures of Sc 2 O 3 (99.9%), Co 3 O 4 (99.9%), Cr 2 O 3 (99.9%), Fe 2 O 3 (99.999%) and KClO 4 (as the source of oxygen). The mixtures were prepared in a glove box, placed in Au capsules (in the amount of about 0.5 g for each sample) and treated at 6 GPa in a belt-type high-pressure apparatus at 1570 K for 2 h (heating rate to the desired temperature was 10 min). After the heat treatment, the samples were quenched to room temperature (RT), and the pressure was slowly released. The samples were washed in water to remove KCl obtained after the decomposition of KClO 4 .
X-ray powder diffraction (XRPD) data were collected at RT on a RIGAKU Ultima III diffractometer using CuKα radiation (2θ range of 10-100°, a step width of 0.02°, and a counting time of 2s/step). Synchrotron XRPD data were measured at 293 K on a large Debye-Scherrer camera at the BL15XU beam line of SPring-8 [17]. The intensity data were collected between 1°and 61.5°at 0.003°intervals in 2θ; the incident beam was monochromatized at λ = 0.65298 Å. The samples were packed into Lindenmann glass capillaries (inner diameter: 0.1 mm), which were rotated during measurements. Absorption coefficients were also measured, and Rietveld analysis was performed using the RIETAN-2000 program [18].
Electron probe microanalysis (EPMA) was performed using a JEOL JXA-8500F instrument. The surface of the pellets was polished on a fine alumina (0.3 μm) coated film before the EPMA measurements; and Sc 2 O 3 and Co 3 O 4 were used as standard samples for Sc and Co, respectively. DC magnetic susceptibilities (χ = M/H) were measured using SQUID magnetometers (Quantum Design, MPMS-XL and 1T) between 2 and 400 K in different applied magnetic fields under both zero-field-cooled (ZFC) and field-cooled (FC) conditions. FC measurements were performed on cooling (FCC) from high temperatures to 2 K after the ZFC measurements. In all ZFC measurements, samples were rapidly (within 3-5 min) inserted into a magnetometer, which was kept at 10 K; then, temperature was set to 2 K, and finally a measurement magnetic field was applied. Isothermal magnetization measurements (M versus H) were performed between −70 and 70 kOe at 2 K and 300 K. Specific heat, C p , was recorded between 2 and 300 K on cooling at 0 and 90 kOe by a pulse relaxation method using a commercial calorimeter (Quantum Design PPMS). 57 Fe Mössbauer spectra were recorded at 300 K using a conventional constantacceleration spectrometer MS-1104Em in the transmission geometry. The radiation source 57 Co(Rh) was kept at RT. All isomer shifts are referred to α-Fe at 300 K. The experimental spectra were processed and analysed using methods of spectral simulations implemented in the SpectrRelax program [19]. Differential scanning calorimetry (DSC) curves of (Sc 0.95 Co 0.05 )CoO 3 powder were recorded on a Mettler Toledo DSC1 STAR e system at a heating/cooling rate of 10 K min −1 between 290 K and 873 K in open Al capsules; no DSC anomalies were detected, and the sample remained single-phase after the DSC experiment.

Results and discussion
The stoichiometric ScCoO 3 sample contained Sc 2 O 3 impurity (figure 1(a)) suggesting that the composition of the main perovskite phase is shifted according to the scheme: The amount of Sc 2 O 3 was estimated to be 3-5 weight % from the Rietveld fitting of different laboratory XRPD data. The Rietveld refinement of the synchrotron XRPD data (figure 1(a)) gave about 7.7 weight % of Sc 2 O 3 . A sample with the total chemical composition of Sc 0.9 CoO 2.85 (≈(Sc 0.95 Co 0.05 )CoO 3 ) was prepared without impurities. The EPMA showed that the Sc:Co ratio was 0.901(11):1 in Sc 0.9 CoO 2.85 , in very good agreement with the target chemical composition. No other chemical elements (such as, K and Cl) were detected in Sc 0.9 CoO 2.85 .
The structural analysis showed that all cation and oxygen sites are fully occupied in Sc 0.9 CoO 2.85 suggesting the ( − Sc Co The lattice parameters and unit cell volume of (Sc 0.95 Co 0.05 ) CoO 3 follow the general trends observed in the ACoO 3 (A 3+ = Y and Pr-Lu) family with low-spin Co 3+ ions [12]. However, the unit cell volume of (Sc 0.95 Co 0.05 )CoO 3 (V = 185.141 Å 3 ) with the Co 3+ radius of 0.545 Å [21] is smaller than that of ScAlO 3 (V = 185.915 Å 3 ) with the Al 3+ radius of 0.535 Å probably because of the shift from the stoichiometric composition [12].
The Mössbauer spectrum of (Sc 0  [22]. Taking into account that an increase in the average 〈Fe-O〉 distances leads generally to an increase in δ values [23], the Fe2 doublet with the larger isomer shift of δ Fe2 = 0.45(1) mm s −1 should correspond to 57 Fe 3+ ions located at the larger A site, and the larger quadrupole splitting To verify the correctness of the doublet assignment to the A and B positions, we calculated a lattice contribution (V lat ) to the electric field gradient (EFG) tensor at 57 Fe at the A and B positions, using the experimental crystallographic data of (Sc 0.95 Co 0.05 )CoO 3 (table 1). After diagonalization, the main EFG tensor components (|V ZZ | ⩾ |V XX | ⩾ |V YY |) were used to estimate the theoretical quadrupole splitting Δ theor = eQV ZZ /2 (1 + η 2 /3) 1/2 , where η ≡ (V XX -V YY )/V ZZ is the parameter of asymmetry of EFG. The best agreement between the theoretical (Δ B theor = 0.32 mm s −1 and Δ A theor = 0.56 mm s −1 ) and experimental values (table 3) of quadrupole splitting (Δ i exp ) was obtained for the oxygen dipole polarizability of α О ≈ 0.6 Å 3 (for nominal charges of Z O = −2, Z Sc = +3 and Z Co = +3, and the quadrupole moment of 57 Fe nuclei of Q = 0.21 barns [24]). The obtained high value of α О agrees well with the data for other oxides [25]. The main factors, which can be responsible for the observed discrepancy between the Δ i theor and Δ i exp values, are the uncertainty in choosing the effective charges on the ions (Sc, Co and O) and the nucleus quadrupole moment eQ for 57 Fe nuclei [24]. However, our calculations qualitatively correctly predict the ratio of the    We observed no difference between the ZFC and FCC curves measured at low magnetic fields (e.g., 0.1 kOe) and high magnetic fields (e.g., 70 kOe) ( figure 3(a)). At high temperatures, almost no difference was found in magnetic susceptibilities measured at 0.1 and 70 kOe; however, at low temperatures, magnetic susceptibilities were suppressed by high magnetic fields in agreement with the isothermal M versus H curves ( figure 4(a) (1) 12 (2) a These are the average 〈δ Fe1 〉 and 〈Δ Fe1 〉 values obtained from the distribution functions p(δ Fe1 ) and p(Δ Fe1 ). δ is an isomer shift, Δ is quadrupole splitting, W is linewidth, and I is a relative intensity. The right-hand axes give inverse FCC curves (χ −1 versus T) at 70 kOe. Parameters (μ eff and θ) of the Curie-Weiss fits (bold lines) between 300 and 400 K are given. The thin lines show the same FCC χ −1 versus T curves at 70 kOe corrected for contributions from diamagnetic sample holders and core diamagnetism. Pr-Lu) family [6,26]; the temperature of the spin-state (LS-to-HS) transition increases sharply with decreasing the size of the A type cation [6]. Therefore, a large effective magnetic moment should originate from the high-spin Co 3+ ions located at the A site. The expected calculated effective magnetic moment is 1.124μ B (for 0.0526Co 3+ ), which is close to the experimentally obtained value. Large effective magnetic moments and Curie-Weiss temperatures were also observed in LaCo 1−x M x O 3 (M = Rh and Ir) [27]; μ eff for the impurityrelated magnetism is usually one order of magnitude smaller [28]. were very similar with those of (Sc 0.95 Co 0.05 )CoO 3 (figure 3(b)), with a slightly larger μ eff = 2.050(4)μ B /f.u. because of the presence of Fe 3+ ions (the expected μ eff is about 1.63μ B ). Note that the intrinsic magnetic moment of (Sc 0.95 Co 0.05 )CoO 3 is quite small at high temperatures; therefore, diamagnetic contributions (from sample holders and core diamagnetism) have a significant influence on the μ eff and θ values ( showed no hysteresis and passed through the origin (figure 4); no saturation behaviour was also observed at 2 K, in contrast with the expected property for free ions, that is, the Brillouin function behaviour. The M versus H curve of (Sc 0.95 Co 0.05 ) CoO 3 was linear at 300 K up to 70 kOe. Deviations from the Brillouin function behaviour was observed in some doped LaCoO 3 samples [29]. Specific heat of (Sc 0.95 Co 0.05 )CoO 3 is given on figure 5; between 9 and 31 K, the data follow the equation C p /T = γ + β 1 T 2 with γ = 7.86(8) mJmol −1 K −2 and β 1 = 0.05452(17) mJmol −1 K −4 (the line in the inset of figure 5). Taking into account the fact that (Sc 0.95 Co 0.05 )CoO 3 is an insulator, the upturn of the C p /T values below 9 K and the apparent electronic contribution γ could originate from Schottky-type contributions or single-ion excitations. The β 1 value of (Sc 0.95 Co 0.05 )CoO 3 was close to that of ScRhO 3 (β 1 = 0.0589 mJmol −1 K −4 ) [28].
The presence of Co 3+ ions in the LS and HS states is in qualitative agreement with the energy diagrams of Co 3+ in crystal fields with local symmetries of O h (for the B position, in the first approximation) and D 4h (for the A position, in the first approximation) (figure 6) [30]. In the case of the same average bond distances 〈Co-O〉, the crystal field splitting, 5/ 3α 4 (where α 4 ∼ 1/[〈Co-O〉] 5 is a radial integral), of Co 3+ orbitals for the O h octahedral site is higher than the crystal field splitting for the D 4h site (16/27α′ 4 ). Moreover, the average 〈Co-O〉 bond distances are longer in the A position in comparison with the B position (table 2) . Considering that α′ 4 should be smaller than α 4 (and with the same J H for Co 3+ ), the above conditions result in the HS state of Co 3+ at the D 4h site. Note that the HS state of Co 3+ was experimentally found in BiCoO 3 [31], where Co 3+ ions are located in a square pyramidal coordination.
By the analogy with Sc 0.9 CoO 2.85 , we prepared solid solutions with the total composition of Sc 0.9 Co 1−  Specific heat data of (Sc 0.95 Co 0.05 )CoO 3 (powder was washed from KCl and then pressed into pellets at 3 GPa) at a zero magnetic field (white circles) and 90 kOe (blue diamonds) plotted as C p /T versus T. The inset shows the details below 40 K; the red line is the fit with the equation C p /T = γ + β 1 T 2 between 9 and 31 K. the most intense doublet Fe1 had broadened and asymmetrical components that could be caused by the existence of different configurations {(6-m)Co 3+ , mFe 3+ } in the local surrounding of the Fe 3+ ions within the B sublattice. We fitted the experimental spectrum as a superposition of a discrete doublet Fe2 and a distribution p(Δ Fe1 ) of the quadrupole splittings (Δ Fe1 ), assuming a linear relation between Δ Fe1 and δ Fe1 [33].  (table 3); note that the Mössbauer parameters for this sample in two models (the first model is two discrete doublets, and the second one with a distribution for Fe1) were almost identical. The obtained p (Δ Fe1 ) distributions are shown in figure 2(c), and the best-fit hyperfine parameters (average 〈δ Fe1 〉 and 〈Δ Fe1 〉 values for the Fe1 subspectra) and relative intensities (I i ) of the partial spectra are listed in table 3. A comparison of these data shows that changing the iron content in the samples does not significantly affect hyperfine parameters of the Fe1 and Fe2 doublets, while their relative intensities undergo some changes. According to the experimental intensity ratio of the partial spectra, I Fe1 /I Fe2 (table 3), in the case of (Sc 0.89 M 0.11 ) MO 3 (M = Co 0.6 Fe 0.4 ), Fe 3+ ions were distributed almost statistically between the A and B sites (statistical distribution would give 10% of Fe 3+ at the A site, and the experimental doublet area is 12(2)%). The resulting distribution p(Δ Fe1 ) for (Sc 0.95 M 0.05 )MO 3 ( = M Co Fe ) 0.95 57 0.05 is narrow and has symmetrical profile, thus, indicating a uniform nearest surrounding of Fe 3+ ions.
The location of small Fe 3+ ions at the A site of classical ABO 3 perovskites is quite unusual, especially their strong preference to occupy the A site at small doping levels. To the best of our knowledge, (Sc 1−x M x )MO 3 compounds are the first example of such behaviour. It should be noted that Fe 3+ ions were found by the Mössbauer spectroscopy at the A′ site of A-site ordered perovskites with the general composition of AA′ 3 B 4 O 12 , for example, in CaCu 3 Fe 4 O 12 [34] and CaMn 3 Mn 4 O 12 [35]. However, Fe 3+ ions substitute for Cu 2+ or Mn 3+ ions-other transition metals-in a special A′ position, whose coordination environment (square-coordinated A′ O 4 ) is quite different from a typical coordination of the A site in perovskites (AO 8 -AO 12 ).