Confined growth of Eu₂O₃ nanocrystals in a new polymorph in amorphous mesoporous Al₂O₃

Abstract

Eu₂O₃ in divided groups in pores in a mesoporous Al₂O₃ (amorphous) has a confined growth in nanocrystals (average, 30 nm diameter) in a new polymorph of R3c hexagonal crystal structure with lattice parameters a = 0.5468 nm and c = 1.6950 nm. This occurs on reacting dispersed Eu³⁺ cations (in water) with a mesoporous AlO(OH) · α₂O powder. A pore incorporates Eu³⁺ cations in a confined group depending on its size and governs controlled Eu₂O₃ nucleation and growth in a self-confined dimension in a nanocrystal. This new lattice involves a 5.60, 2.99 and 1.03 times larger volume V ₀, enclosed in a reduced S ₀ = 5.41 nm² surface per unit volume relative to 9.27, 9.28 and 5.52 nm², in the bulk hexagonal Eu₂O₃, monoclinic Eu₂O₃ and cubic Eu₂O₃ polymorphs respectively. From the pressure–volume isotherm, the large V ₀ implies that it grows quickly under the influence of a reduced effective pressure in the pores so that it balances the ▵P → 0 pressure gradient as early as possible. The results are discussed with a proposed model of nucleation and growth in a self-confined dimension under the influence of a reduced pressure.