Dielectric constants and crystal structures of CaYAlO₄, CaNdAlO₄, and SrLaAlO₄, and deviations from the oxide additivity rule

The structures of CaYAlO₄, CaNdAlO₄, and SrLaAlO₄ have been refined using single-crystal X-ray diffraction data. These compounds possess the full symmetry of the K₂NiF₄ structure type, I4 mmm, with lattice parameters a = 3.6451(1) and c = 11.8743(6)Å for CaYAlO₄, a = 3.6847(3) and c = 12.124(2)Å for CaNdAlO₄, and a = 3.7564(1) and c = 12.6357(5)Å for SrLaAlO₄. Inspection of the interatomic distances reveals stretching of the AlO bond, from an average of 1.878 to 1.901 to 1.935 Å, as smaller cations (CaY) are replaced by larger cations (CaNd) and (SrLa) in the MM′ sites between the two dimensional aluminate sheets. The dielectric constants (κ′) and dielectric loss values of CaYAlO₄, CaNdAlO₄, and SrLaAlO₄ were measured at 1 MHz using a two-terminal method with empirically determined edge corrections. The results are {A table is presented}. The deviations of measured dielectric polarizabilities, as determined from the Clausius-Mosotti equation and those calculated from the sum of oxide polarizabilities according to α_D(MM′AlO₄) = α_D(MO) + 0.5 α_D(M′₂O₃) + 0.5 α_D(Al₂O₃), are + 1.7% for CaYAlO₄, -2.0% for CaNdAlO₄, and -5.9% for SrLaAlO₄. The deviations from additivity are believed to result from K₂NiF₄ structural constraints leading to "rattling" M and M′ ions in CaYAlO₄ and "compressed" M and M′ ions in CaNdAlO₄ and SrLaAlO₄.