Formation, cationic site exchange and surface structure of mechanosynthesized EuCrO₃ nanocrystalline particles

Abstract

Nanocrystalline EuCrO3 particles (∼25 nm) have been prepared by pre-milling a 1 : 1 molar mixture of Eu2O3 and Cr2O3 for 60 h followed by sintering at 700 °C (12 h). This temperature is ∼500–600 °C lower than those at which the material, in bulk form, is conventionally prepared. Rietveld analysis of the x-ray powder diffraction pattern of the EuCrO3 nanoparticles favours a structural model involving a slight degree of cationic exchange where ∼11% of the Eu3+ and Cr3+ ions exchange their normal dodecahedral A- and octahedral B-sites, respectively, in the perovskite-related structure. This cationic site exchange, which is unusual in a perovskite structure, has been well supported by the corresponding room-temperature 151Eu Mössbauer spectrum of the nanoparticles that in addition to displaying a distribution in the principal component of the EFG tensor (V zz ) at the usual A-sites of the 151Eu nuclei, also revealed the presence of a subcomponent with ∼11% area fraction and a considerably increased |V zz | value that was associated with Eu3+ ions at octahedral B-sites. X-ray photoelectron and Auger electron spectroscopic techniques reveal a complex surface structure where extremely thin layers of un-reacted Eu2O3 and Cr2O3 cover most of the EuCrO3 nanoparticles' surfaces together with some traces of elemental Cr. The binding energies associated with Eu3+ 3d5/2, Eu3+ 4d3/2, Cr3+ 2p3/2 and O2− 1s core-level electrons in EuCrO3 are estimated from the x-ray photoelectron data for the first time.

Structural and 57Fe Mössbauer study of EuCr1 − x Fe x O3 nanocrystalline particles

“…Previous studies on the EuCrO 3 / EuFeO 3 solid solutions were mainly performed on the EuCr 1−x Fe x O 3 microcrystalline particles [2,5]. Recently we have shown an unusual cationic site exchange to exist in EuCrO 3 nanocrystalline particles [1]. In the present study we report our preliminary studies on nanocrystalline EuCr 1−x Fe x O 3 particles.…”

“…This unit cell expansion is expected as more of the larger Fe 3+ ions (0.785 Å) replace the smaller Cr 3+ ones (0.755 Å) [6]. Referring to our Rietveld refinement of the XRD pattern of the sample with x = 0.0 (EuCrO 3 ) [1], where the Eu 3+ and Cr 3+ ions were shown to partially exchange the A-and B-sites, a more pronounced site-exchange of Eu 3+ and Fe 3+ is anticipated in EuCr 1−x Fe x O 3 nanoparticles on the basis of the higher ionic size of Fe 3+ . Indeed initial analysis of 151 Eu data (not shown) suggests that a considerable amount of Eu 3+ ions in the spectra of all samples exists at the transition metal B-sites.…”

“…Nanocrystalline EuCr 1−x Fe x O 3 particles were prepared using mechanosynthesis starting from appropriate amounts of high purity Eu 2 O 3 , Cr 2 O 3 and α-Fe 2 O 3 in the 1: 1-x:x molar ratio (x = 0.0, 0.1, 0.3, 0.5, 0.7, 0.9, 1.0) using an identical milling and sintering regime to that reported by us recently [1]. X-ray powder diffraction (XRD) patterns were collected with a Philips PW1710 diffractometer using CuKα radiation (λ = 1.5406 Å).…”

“…communications [1][2][3]. The compounds crystallize in the space group Pbnm with four distorted perovskite units in the true crystallographic orthorhombic cell [1][2][3][4] where Eu 3+ and the transition metal ions occupy the dodecahedral A-and octahedral Bsites, respectively.…”

“…The compounds crystallize in the space group Pbnm with four distorted perovskite units in the true crystallographic orthorhombic cell [1][2][3][4] where Eu 3+ and the transition metal ions occupy the dodecahedral A-and octahedral Bsites, respectively. Both solids are anti-ferromagnetic with only weak ferromagnetism due to spin canting below ∼181 K and ∼662 K respectively [3,5].…”