Superlattice and Dielectric Properties of BaO-R2O3-TiO2 (R=La, Nd and Sm) Microwave Dielectric Compounds

Dielectric crystals having composition in the vicinity of Ba3.75 R 9.5Ti18O54 and/or BaR 2Ti4O12 (R=La, Nd and Sm) were obtained from the melts with BaR 2Ti5O14 composition. A superlattice with two times the fundamental lattice spacing along the c-axis has been found in the three kinds of crystals. Crystal data of the superlattice are as follows: orthorhombic, P b n21 (No. 33) or P b n m (No. 62); a=12.267(5), b=22.410(9), c=7.732(6) Å for Ba3.75La9.5Ti18O54; a=12.189(3), b=22.319(6), c=7.677(4) Å for Ba3.75Nd9.5Ti18O54; a=12.142(4), b=22.300(6), c=7.653(4) Å for Ba3.75Sm9.5Ti18O54. Space group of the fundamental lattice is P b a2 (No. 32) or P b a m (No. 55). The dielectric constants, εr, of Ba3.75 R 9.5Ti18O54 (R=Nd and Sm) are in the high range from 78 to 86, and unloaded Q values are in the range of 2400 to 3000. The excellent values of single phase are better than those of BaR 2Ti5O14 composition including secondary phases.