MICROWAVE-ABSORPTION IN SINGLE-CRYSTALS OF LANTHANUM ALUMINATE

A very sensitive dielectric resonator technique is employed to measure loss tangent tan delta and relative permittivity epsilon(r) of lantha num aluminate (LaAlO3) single crystals at 4-300 K and 4-12 GHz. A vari ety of single crystals grown by different techniques and purchased fro m different suppliers are considered. For T > 150 K the loss tangent t an Sis almost sample independent with Linear frequency dependence and monotonous temperature variation from 8 x 10(-6) at 300 K to 2.5 x 10( -6) at 150 K and 4.1 GHz. In this temperature range the experimental d ata are explained by a model based on lifetime broadened two-phonon di fference processes. The loss tangent below 150 K is characterized by a peak in tan delta(T) at about 70 K. The height of this peak is freque ncy and strongly sample dependent. This leads to a variation of the lo ss tangent from 10(-6) to 1.5 x 10(-5) at 77 K and 8.6 GHz, the lowest values are generally achieved with Verneuil grown crystals and approa ch the intrinsic lower limit predicted by the phonon model. The peak i s explained by defect dipole relaxation (local motions of ions). The a ctivation energy of the relaxation process is determined from the meas ured data to be 31 meV. This low value indicates that the defect dipol es are associated with interstitials, possibly impurities in interstit ial positions. Considering absorption due to phonons and due to defect dipole relaxation the loss tangent is calculated for a wide frequency range. (C) 1997 American Institute of Physics.