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, I4mmm, 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 AlO 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 CaYAlO4κ′a = 21.44 ± .02tan δa = 0.0008κ′c = 16.12 ± .04tan δc = 0.0008CaNdAlO4κ′a = 19.65 ± .1tan δa = 0.0002κ′c = 17.65 ± .1tan δc = 0.0004SrLaAlO4κ′a = 16.81 ± .1tan δa = 0.0006κ′c = 20.02 ± .1tan δc = 0.0008. The deviations of measured dielectric polarizabilities, as determined from the Clausius-Mosotti equation and those calculated from the sum of oxide polarizabilities according to αD(MM′AlO4) = αD(MO) + 0.5 αD(M′2O3) + 0.5 αD(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.