First-princiles Calculation On The Dielectric Properties Of AlN And MgAl₂O₄

First principles calculations on the structural, vibrational and dielectric propertiesfor c-AlN and MgAl₂O₄, which is used pseudopotential plane wave method, have beenaccomplished. This will be conducive to the establishment of the relationship betweenmicrostructure and dielectric property of the materials. Firstly, the structures have beenfully relaxed through first principles molecular dynamical method. Then the zone-centerphonon-mode frequencies are evaluated within the framework of density functionalperturbation theory（DFPT）. Group theory is adopted to identify these phonon modes atthe center of Brillouin zone and the LO/TO（longitudinal optic and transverse optic）splittings of the infrared active optic mode are reported, the thermodynamic stability ofAlN was discussed in vacuum high–temperature environment. Finally, properties relatedto electrical perturbation, including electronic dielectric tensors, static dielectric tensorsand dielectric spectrum, are presented. And dielectric, refractive index, extinctioncoefficient and infrared reflectance spectra of c-AlN and MgAl₂O₄are given. Excellentagreements have been achieved between the theoretical results and the referencedexperimental data.For c-AlN, The calculation indicates that strong IR reflection occurs in the range400^-1000cm^-1by reason of one strong IR-active mode, resulting in a poor transmissionproperty. For MgAl₂O₄, strong infrared reflectivity exists from300to1000cm^-1due tothe three strong infrared active modes, leading to a poor electromagnetic wavetransmission property. In the spectrum range above1000cm^-1or below300cm^-1,however, there is relatively good transmission property because of low reflectivity anddissipation resulting from no infrared active mode or weak modes.