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

Abstract The structures of CaYAlO 4 , CaNdAlO 4 , and SrLaAlO 4 have been refined using single-crystal X-ray diffraction data. These compounds possess the full symmetry of the K 2 NiF 4 structure type, I4 mmm , with lattice parameters a = 3.6451(1) and c = 11.8743(6)A for CaYAlO 4 , a = 3.6847(3) and c = 12.124(2)A for CaNdAlO 4 , and a = 3.7564(1) and c = 12.6357(5)A for SrLaAlO 4 . Inspection of the interatomic distances reveals stretching of the AlO bond, from an average of 1.878 to 1.901 to 1.935 A, 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 CaYAlO 4 , CaNdAlO 4 , and SrLaAlO 4 were measured at 1 MHz using a two-terminal method with empirically determined edge corrections. The results are CaYAlO 4 κ′ a = 21.44 ± .02 tan δ a = 0.0008 κ′ c = 16.12 ± .04 tan δ c = 0.0008 CaNdAlO 4 κ′ a = 19.65 ± .1 tan δ a = 0.0002 κ′ c = 17.65 ± .1 tan δ c = 0.0004 SrLaAlO 4 κ′ a = 16.81 ± .1 tan δ a = 0.0006 κ′ c = 20.02 ± .1 tan δ c = 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 α D ( MM ′AlO 4 ) = α D ( M O) + 0.5 α D ( M ′ 2 O 3 ) + 0.5 α D (Al 2 O 3 ), are + 1.7% for CaYAlO 4 , −2.0% for CaNdAlO 4 , and −5.9% for SrLaAlO 4 . The deviations from additivity are believed to result from K 2 NiF 4 structural constraints leading to “rattling” M and M ′ ions in CaYAlO 4 and “compressed” M and M ′ ions in CaNdAlO 4 and SrLaAlO 4 .

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