Probing the effect of R-cation radii on structural, vibrational, optical, and dielectric properties of rare earth (R=La, Pr, Nd) aluminates

The present study correlates the effect of R-cation radii on structural, vibrational, optical, and dielectric properties of rhombohedral rare earth aluminates RAlO 3 (R = La, Pr, Nd). The polycrystalline samples of RAlO 3 have been synthesized using sol-gel synthesis technique. Pure rhombohedral phase of RAlO 3 samples has been confirmed with X-ray diffraction. Systematic decrements in the lattice parameter, bond length, and bond angle have been observed, giving rise to structural distortion due to decrease in ionic radii of R-cation. The phononic properties of RAlO 3 have been investigated through Raman spectroscopy, where the degree of distortion of AlO 6 octahedra can be analyzed with the peak position of E g and A 1g modes. An increase in the energy bandgap with decreasing R-cation radii shows an interconnection with the decrease in Al–O bond length. Interestingly, the decreasing dielectric constant with decreasing ionic radii of R-cation has been correlated with the difference in electronegativity of cation(R 3+ )-anion(O 2− ) pair. Also, a positive linear relationship between dielectric constant and energy bandgap has been investigated using Penn model.