A Fast, Low‐Temperature Synthesis Method for Hexagonal YMnO₃: Kinetics, Purity, Size and Shape as Studied by In Situ X‐ray Diffraction

Abstract

The reaction mechanisms, phase development and kinetics of the hydrothermal synthesis of hexagonal‐YMnO 3 from Y 2 O 3 and Mn 2 O 3 using in situ X‐ray diffraction are reported under different reaction conditions with temperatures ranging from 300 to 350 °C, and using 1, 5 and 10 m KOH, and 5 m NaOH mineraliser. Reactions initiated with Y 2 O 3 hydrating to Y(OH) 3 , which then dehydrated to YO(OH). Higher temperatures and KOH concentrations led to faster, more complete dehydrations. However, 1 m KOH led to YO(OH) forming concurrently with Y(OH) 3 before Y(OH) 3 fully dehydrated but yielded a very low phase purity of hexagonal‐YMnO 3 . Using NaOH mineraliser, no YO(OH) was observed. Dehydration also initiated at a higher temperature in the absence of Mn 2 O 3 . The evolution of Rietveld refined scale factors was used to determine kinetic information and approximate activation energies for the reaction. The described hydrothermal synthesis offers a fast, low‐temperature method for producing anisometric h‐YMnO 3 particles.

Hydrothermal synthesis of hexagonal YMnO3 and YbMnO3 below 250 °C

“…However, h-YMnO 3 was formed under conditions that were too mild to form YO(OH) from Y 2 O 3 in KOH solution. Our in situ study did show that YO(OH) was present under milder conditions in the reaction between Y 2 O 3 and Mn 2 O 3 in KOH solution, compared with only Y 2 O 3 in KOH solution 29.…”

“…Reactions were performed in sapphire capillaries with pressure provided using an HPLC pump connected via steel tubing and Swagelok® parts. 29,41 The capillary was held by a…”

“…Particle morphology h-YMnO 3 formed from Y 2 O 3 and Mn 2 O 3 at 240 °C and > 10 M KOH have a hexagonal prism shape (Figure 4 (a) and (b)), often with hollow or rough interior compared to those made at high temperature (≥ 300 °C) and moderate mineraliser concentration (≤ 5 M), as shown in our previous work where hexagonal plate like morphologies were formed. 29 The crystals prepared at 240 °C and > 10 M KOH from the salt precursors have a polygonal plate-like This journal is © The Royal Society of Chemistry 20xx…”

“…The microstructure of h-YMnO 3 is different in the low temperature (≤ 240 °C), high [KOH] (≥ 10 M) region of parameter space (Figure 4) from the high temperature (≥ 300 °C) moderate [KOH] (≤ 10 M) as we have seen in our prior work, in which we observed thin hexagonal crystals ~ 10 m across. 29 In the case of using the oxide precursors, thick plate-like structures, often with rough or porous interiors are formed, and in the case of using salt precursors, thin plate-like structures often with holes in the centre are formed.…”

Wet to Dry Controls Lanthanide Scandate Synthesis

“…One must also consider the possibility of back-reactions: LnOOH formed during calcination may be reverted to Ln(OH) 3 on cooling if P Hd 2 O is still present. Given the evidence that LnOOH is more reactive and water-soluble than Ln(OH) 3 , and thus may more easily form the mixed-cation perovskite phase, 13,28 hydrosauna synthesis was done using a two-step regime for water vapor: the appropriate DFT-predicted P Hd 2 O was applied during the hightemperature (nucleation) calcination step and then lowered by 12,39−41 In reality, however, the Sm 2 O 3 and Gd 2 O 3 starting material will contain a polyphasic mixture of C-and B-type grains at room temperature. While polymorphism in the rare-earth sesquioxides has been studied extensively, the stability of certain phases and the exact nature of the polymorphic phase transitions C → B and B → A are still debated.…”