Local A‐Site Layering in Rare‐Earth Orthochromite Perovskites by Solution Synthesis

Abstract Cation size effects were examined in the mixed A‐site perovskites La0.5Sm0.5CrO3 and La0.5Tb0.5CrO3 prepared through both hydrothermal and solid‐state methods. Atomically resolved electron energy loss spectroscopy (EELS) in the transmission electron microscope shows that while the La and Sm cations are randomly distributed, increased cation‐radius variance in La0.5Tb0.5CrO3 results in regions of localised La and Tb layers, an atomic arrangement exclusive to the hydrothermally prepared material. Solid‐state preparation gives lower homogeneity resulting in separate nanoscale regions rich in La3+ and Tb3+. The A‐site layering in hydrothermal La0.5Tb0.5CrO3 is randomised upon annealing at high temperature, resulting in magnetic behaviour that is dependent on synthesis route.


S1 Further Synthesis Details
The hydrothermal synthesis of LaxSm1-xCrO3 solid solutions is detailed elsewhere, [1] and the production of La0.5Tb0.5CrO3 uses the same high-temperature hydrothermal treatment of an amorphous mixed-metal precursor. The amorphous precursor for La0.5Tb0.5CrO3 was produced by dissolving the required molar ratios of lanthanum(III) nitrate hexahydrate (99.999%, Aldrich), terbium(III) nitrate hexahydrate (99.9%, Alfa Aesar), and chromium nitrate nonahydrate (99%, Aldrich) into deionised water. Addition of 2 M KOH solution caused the precipitation of a mixed-metal amorphous hydroxide gel, which was filtered and washed copiously with water, before being dried and ground into a fine powder. Approximately 350 mg of powdered precursor was then placed into an Inconel high-pressure vessel along with 20 ml of deionised water. The vessels were sealed and heated to 410 °C for 12 hours generating autogeneous pressures of 200+ bar, producing the bright green perovskite powders.

S2 Further Characterisation Details
Raman spectra were recorded in backscattering geometry using a Renishaw inVia Raman Microscope with spectral cutoff at ~120 cm -1 and equipped with a 632.8 nm He-Ne laser. A <1 mW laser power was used focused into a ~1 μm 2 spot to avoid local laser heating of the polycrystalline samples. Powders were placed in a Linkam THMS 600 sample stage and cooled to 123 K under liquid N2.
Differential scanning calorimetry (DSC) was performed using platinum crucibles under a constant flow of N2 (50 mL min -1 ) on a Mettler Toledo Systems TGA/DSC1-1600 instrument. Data were recorded from room temperature up to 1400 °C at a rate of 20 °C min -1 . Table S1. Crystallographic data for the LaxTb1-xCrO3 with x = 1, x = 0.5 and x = 0. Crystallographic data for LaxSm1-xCrO3 were reported previously. [1]

S4 Raman Scattering
Raman scattering shows that for hydrothermal La0.5Tb0.5CrO3, although measured at -150 °C to minimise thermal broadening, increased mode broadness is observed compared to both end members, arising from the variance disorder present as a result of La 3+ and Tb 3+ both occupying the A site. Despite the broadness, it is possible to discern the behaviour of some modes through the series. For example, the B3g(3) mode is associated with the stretching vibrations of the CrO6 octahedra, and is therefore indirectly affected by structural distortion. The increase in Cr-O distances towards TbCrO3 mean that the B3g(3) mode undergoes a softening with decreasing x. All other observed modes exhibit a greater dependence on structural distortion, and so soften towards the less-distorted LaCrO3. The same mode broadening and dependence upon structural distortion were observed in the LaxSm1-xCrO3 solid solutions reported previously. [1] (2) (3) S5 Synthesis of La0.5Tb0.5CrO3 Figure S4. Ex situ PXRD patterns (λ = 1.54056 Å) of the solid state synthesis of La0.5Tb0.5CrO3 showing that long firings at 1400 °C are required to produce a well reacted phase.

S6 Attempted syntheses of other mixed rare-earth chromites
Attempts at synthesising solid solutions with greater variance proved unsuccessful, as pure, crystalline samples could not be produced even at increased hydrothermal temperatures. The attempted synthesis of La0.5Ho0.5CrO3 (σ 2 = 5.18×10 -3 Å 2 ) produced a perovskite phase with broad reflections as well as a small amount of hydroxide impurity. It is possible that the reaction resulted in a mixture of lower Asite radius variance solid solutions, for example, an equimolar ratio of La0.25Ho0.75CrO3 and La0.75Ho0.25CrO3. However, this information is lost within the broad peaks.
An attempt at incorporating two lanthanides from opposite ends of the lanthanide series also proved impossible. La0.5Yb0.5CrO3 (σ 2 = 7.57×10 -3 Å 2 ) does not form, and instead a powder mixture of the end members LaCrO3 and YbCrO3 results. This suggests that the increased A-site radius variance of such systems led to these failed synthesis attempts.   Figure S6. HAADF-STEM image (left) showing crystallite aligned along the [101] zone axis and the area selected for EELS (yellow box) on a thin section (approx. 40 nm) close to the crystal edge. On the right are separate integrated EELS maps for chromium (red), lanthanum (green), and samarium (blue), and all three combined into a single map. The white scale bar represents 1 nm. Figure S7. EELS maps formed from integrated La M4,5, Sm M4,5, and Cr L2,3 edge spectra recorded from hydrothermal La0.5Sm0.5CrO3 crystallites aligned along the [101] direction. Colours: lanthanum in green, samarium in blue, and chromium in red. White scale bars in each image represent 1 nm. Figure S8. EELS maps formed from integrated La M4,5, Sm M4,5, and Cr L2,3 edge spectra recorded from solid state La0.5Sm0.5CrO3 crystallites aligned along the [101] direction. Colours: lanthanum in green, samarium in blue, and chromium in red. White scale bars in each image represent 1 nm. Figure S9. EELS maps formed from integrated La M4,5, Tb M4,5, and Cr L2,3 edge spectra recorded from hydrothermal La0.5Tb0.5CrO3 crystallites aligned along the [101] direction. Colours: lanthanum in green, terbium in blue, and chromium in red. White scale bars in each image represent 1 nm. Figure S10. EELS maps formed from integrated La M4,5, Tb M4,5, and Cr L2,3 edge spectra recorded from solid state La0.5Tb0.5CrO3 crystallites aligned along the [101] direction. Colours: lanthanum in green, terbium in blue, and chromium in red. White scale bars in each image represent 1 nm. Figure S11. Large-scale a) and atomic-scale b) EELS maps of La-rich and Tb-rich domains observed in La0.5Tb0.5CrO3 made from solid state synthesis. The map b) is a magnified section (yellow box) of a). HAADF-STEM images were recorded from both the Tb-rich c) and La-rich d) regions of this crystallite, and the structural overlays (lanthanide -green, chromium -red) show the increased octahedral distortion in c). Images e) and f) are HAADF and BF micrographs, respectively, across the boundary between the two domains. BF imaging is more sensitive to electron scatter from light elements (i.e. oxygen) and so the boundary is more apparent in f) as the structural distortions between LaCrO3 and TbCrO3 manifest themselves mostly through anion positions.

S8 Differential Scanning Calorimetry (DSC)
Differential scanning calorimetry (DSC) was performed on the La0.5Tb0.5CrO3 materials following different synthetic treatments in order to investigate the possiblity of thermal randomisation of the A site cation order observed in hydrothermal La0.5Tb0.5CrO3. The DSC traces over the temperature range 1050 -1350 °C show clear differences with an exothermic destabilisation of the local A site layers observed in the hydrothermal sample at approximately 1200 °C. This supports the magnetic susceptibility measurements discussed in the main text, where changes are observed between hydrothermal and solid state La0.5Tb0.5CrO3, and with each subsequent annealing.

S9 Magnetic susceptibility
Field-cooled cooling (FCC) magnetic susceptibility data were recorded on materials in the LaxTb1-xCrO3 series prepared hydrothermally. FCC data for the LaxSm1-xCrO3 were reported previously. [1]