Synthesis and characterization of a hexagonal polymorph of GdAlO3

Abstract

The structure and basic properties of a hexagonal polymorph of GdAlO3 aluminate (GAH) were studied using a set of experimental methods (XRD, FTIR, Raman spectroscopy, diffuse reflectance spectroscopy, luminescence spectroscopy, magnetic susceptibility, and TEM). The GAH crystallized at 900 °C from an amorphous precursor and decomposed into perovskite GdAlO3 (GAP) at 1000 °C. XRD pattern of polycrystalline GAH could be indexed in P63/mmc space group with unit cell parameters a = 0.3734 nm, c = 1.0535 nm, following literature data. The electron diffraction revealed, however, the presence of well-defined superstructure reflections due to defects ordering. Contrary to a recent report suggesting the presence of CO3 groups in the structure of similar hexagonal YAlO3, we did not observe such groups in the FTIR spectra of GAH. On the other hand, the stoichiometry of GAH differs from ideal GdAlO3, with Al/Gd atomic ratio close to 1.2, similar to reported for hexagonal YAlO3. GAH exhibits paramagnetic behavior over 1.8–300 K temperature range, with the effective Gd magnetic moment μeff = 7.90 μB, close to the theoretical value for free Gd3+ ion. No paramagnetic – antiferromagnetic phase transition at ~4 K, characteristic for perovskite-type GdAlO3, was observed. The reason is probably a less "dense" structure of GAH, with longer Gd-Gd and Gd-O-Gd interatomic distances. Emission spectra of Dy doped (~1 at%) GAH and GAP revealed dramatic differences in Dy3+ emission bands, despite similar mean crystallite size of the host material. An almost structureless 4F9/2→6H13/2 and 4F9/2→6H15/2 emission bands, observed for GAH, indicate, following TEM data, that Dy ions occupy numerous nonequivalent sites due to the presence of a large concentration of crystal defects.