Study On The Dielelctric Properties Of Fluoride Single Crystals

The fluorides have attracted intensive technological and commercial interests owing to their superior optical properties. chemical stability and mechanical properties. It is well-know that even trace concentrations of defects can have significant influence on material’s properties. The study of optical properties revealed that there were various intrinsic in fluorides. It is well-known that dielectric spectroscopy technique is a powerful and non-destructive tool to characterize the behavior of defects. Thus, we can make good use of it to have a thorough understanding of the dynamic behavior of the defects in fluorides. Meanswhile, as one of the fundamental properties, the study of dielectric properties has an important significance on material applications. In this paper, we have performed systematically investigation on the dielectric properties of (MFx, M=Ca, La, Mg, Li; x=1,2,3) by means of various dielectric spectra. The research results are listed as following:(1)The dielectric properties of CaF2 single crystals were investigated in the temperature range from room temperature to 773 K. A Debey-like relaxation and a relaxor-like dielectric anomaly were observed. Raman spectrum and the differential scanning calorimeter measurement revealed the coexistence of native defects of fluorine vacancies (V·F) and interstitial fluorine ions (F’i) and external defect of oxygen ions. Impedance analysis showed that the Debey-like relaxation results from the mobility of V·F, while the relaxor-like behavior is associated with the relaxation caused by complexes of O"-V·F.(2) The dielectric properties of LaF3 single crystals were investigated in the wide temperature range from 110 to 773 K and the frequency range from 100 Hz to 10 MHz. Two thermally activated relaxations (R1 and R2) and a dielectric anomaly (A) were observed. The lower temperature relaxation (R1) was ascribed to a polaronic relaxations due to fluorine ions diffusion within the F1 sublattice and fluorine ions hopping in F1 sublattice. The higher temperature relaxation (R2) are Maxwell-Wagner relaxation due to the blocking of electrodes associated with the ionic exchange between F1 and F2.3 sublattices and among the three non-equivalent sublattices. The anomaly appearing in the highest temperature range is related to the inductive effect arising from the coupled electron-ionic inductive response.(3) The electrical and dielectric properties of MgF2 single crystals were investigated in the temperature range of 284-1073 K and the frequency range of 20 Hz-10 MHz using impedance and modulus spectroscopy. Impedance analysis revealed that MgF2 shows intrinsic dielectric properties below～773 K. A thermally activated relaxation at higher temperatures was observed, which was ascribed to be a Debye-type relaxation caused by the mobility of fluorine interstitials. The result indicates that MgF2 holds great promising applications as gate dielectric, substrate materials, and buffer layer in silicon technology.(4) The dielectric properties of LiF single crystals were investigated as functions of temperature in the range of 303-1073 K. Two thermally activated relaxations, Rl and R2, were observed. The relaxation R1 showing activation energy around 0.8 eV was found to be related to the Li-ion diffusion in the crystal. The relaxation R2 contains three Arrhenius segments, the low-, mid-, and high-T segments of R2 separated by boundary temperatures of 598 K and 698 K. These segments in the order of ascending temperature were found to be caused by F3, F3+centers, F2 centers, and F centers, respectively.A series of fluoride single crystal samples were investigated by means of dielectric properties measurements and analysis of relaxations, indicating that macroscopic dielectric properties were associated with the microscopic movements of intrinsic defects.