Mild Hydrothermal Crystallization of Heavy Rare-Earth Chromite RECrO₃ (RE = Er, Tm, Yb, Lu) Perovskites and Magnetic Properties

Abstract

Crystallization of perovskite structure chromites (ACrO3) in aqueous conditions is difficult owing to the amphoteric nature of the Cr3+ in the alkaline-mediated reaction conditions. This is especially true for the small metal cations at the A-site with large distortion angle of CrO6 octahedral and small Goldschmidt tolerance factors. Here, we performed a progressive dehydration crystallization strategy to synthesize four RECrO3 with the smallest radii of rare earth elements (Er, Tm, Yb, Lu) in mild hydrothermal conditions. Profile refinement of the high-resolution powder X-ray diffraction results indicated slightly longer unit cell parameters of a and c in our samples with a higher distorted angle of CrO6 octahedral units along ⟨010⟩ direction. All of the samples show rounded rectangle plate morphology with uniform distribution of particle sizes. These four RECrO3 crystals can only form in a very narrow mineralization temperature range, i.e., 260–280 °C and 4.45–6.24 M of KOH. HRTEM results indicated that the normal crystallographic direction is ⟨001⟩, and the lattice of steps at the edge of elliptic rounded crystal is consistent with the bulk, which demonstrated single crystalline nature of the as-obtained crystals. Room-temperature Raman and FT-IR spectra reveal a continuous symmetry mode shift-dependent on the radii of A-site rare-earth cations. Temperature-dependent magnetization curves of RECrO3 show typical antiferromagnetism to paramagnetism transition with Néel temperature of 93, 90, 86, and 83 K for ErCrO3, TmCrO3, YbCrO3, and LuCrO3, respectively. Samples of YbCrO3 and LuCrO3 show clear magnetization reversal and exchange bias phenomena below their Néel points. This paper indicates that the coupling of magnetic exchange in perovskite structure oxides could be tailorable in mild hydrothermal condition, towards the exploration of new magnetic and other physical properties.

Effect of Ce substitution on the local magnetic ordering and phonon instabilities in antiferromagnetic DyCrO3 perovskites

“…These results induce the tilting angle Φ shift from the 'three-fold-axes' which reduces from 19.20 • to 17.39 • upon increasing Ce content from x = 0.1 to x = 0.5. Overall our results demonstrate that the anomalous reduction of the tilt angles is associated with the relaxed CrO 6 octahedral distortion with increasing the Ce substitution and these results are consistent with the previous reports [38,44].…”

“…b , φ =cos −1 a b , and the corresponding relation between θ, φ and Φ is given by cos Φ = cos θ cos φ [44,45]. Usually, the value of θ estimated from atomic positions of O atoms along the [101] pseudo axes varying from 10.36 • (for x = 0.1) to 12.25 • (for x = 0.5).…”

Insight into the Magnetization Reversal and Exchange Bias in RFe0.5Cr0.5O3 Ceramics

“…Rare-earth orthoferrites (RFeO 3 ) and orthochromites (RCrO 3 ) (R is a rare-earth element of the lanthanide series) have been intensively studied for decades due to their compelling magnetic properties, − which are mainly induced by the complex 3d–4f interactions between the Fe 3+ /Cr 3+ and R 3+ ions. − Both RFeO 3 and RCrO 3 families show a weak ferromagnetic (WFM) state ascribed to the canted antiferromagnetic (AFM) interaction in the Fe 3+ and Cr 3+ sublattices, respectively. The AFM Néel temperature T N1 (620–740 K) , originated from the Fe 3+ –O 2– –Fe 3+ super-exchange interaction is much higher than that for Cr 3+ ions (110–300 K) .…”

Pourbaix-Guided Mineralization and Site-Selective Photoluminescence Properties of Rare Earth Substituted B-Type Carbonated Hydroxyapatite Nanocrystals

“…Especially, hydroxyapatite has two crystallographic sites for Ca, and therefore two choices for the doped RE 3+ ions. Generally, RE 3+ ions with the larger ionic radii are prone to dehydrate at lower pH, especially in hydrothermal condition, i.e., dehydration is easier that assisted by pressure [74,75]. Therefore, it should be delicately control the pH and reaction temperature for RE 3+ involved hydrothermal synthesis, especially for the competition phase could be prone to form, such as hydroxides or (oxy)hydroxides.…”