V. Ziraps

Ionic charge transport, thermoactivated delocalization and interaction with radiation-induced defects in LiBaF3 crystals

Abstract The ionic and ion-diffusion controlled thermally stimulated relaxation (TSR) processes in CaF2, BaF2 and LiBaF3 crystals have been investigated at 290–650 K by means of the ionic conductivity, ionic thermally stimulated depolarization current (TSDC) and X-ray induced optical absorption spectra TS bleaching (TSB) techniques. In the region of the extrinsic ionic conductivity a number of the ionic TSDC peaks (anion vacancy and interstitial detrapping) and correlated TSB stages have been detected for the first time. The TSR and TSB kinetics above RT in fluoride crystals (X-ray irradiated at RT) are initiated and controlled by the anion interaction with the radiation-induced defects, i.e., the anion diffusion controlled processes take place.

Thermally Stimulated Ionic and Electronic Processes and Radiation-Induced Defect Annealing in LiBaF3 Crystals

The electronic, ionic and ion-diffusion controlled thermally stimulated relaxation (TSR) processes in X-ray irradiated (at 80 K or 290 K) nominally pure LiBaF3 fluoroperovskite crystals have been investigated in the 90–550 K range by means of the ionic conductivity, ionic thermally stimulated depolarization current (TSDC), as well as the thermally stimulated current (TSC), thermally stimulated luminescence (TSL) and the X-ray induced optical absorption spectra thermal bleaching techniques. The role of the thermoactivated ionic and ionic-electronic processes in the TSR (thermal bleaching, TSC and TSL) of X-ray irradiated crystals is studied above 250 K. The TSL efficiency (ratio TSL/TSC) versus T evidences that holes are released at ~130 K (migration of the self-trapped holes), at ~170 K and at ~220 K. The main TSL peaks seem to arise from the hole-diffusion controlled radiative charge transfer (electron tunneling) within close trapped electron and hole pairs.

Annealing of radiation defects in x-irradiated LiBaF3

Results of the glow rate technique application for analysis of the activation energy of thermostimulated annealing of X-ray created F-type color centers in pure and containing oxygen centers LiBaF3 crystals are presented. It is shown that depending on the impurity composition two alternative mechanisms could be involved in the annealing of color centers. It is proposed that either the anion vacancy governed migration of F-centers resulting in recombination with complementary defects, or the thermal delocalization of radiation created fluorine (Fi) interstitials captured by anti-structure defects followed by recombination with all kinds of complementary F-type centers are responsible for the recombination of radiation defects above RT.

Ion diffusion-controlled thermally stimulated processes in x-ray irradiated halide crystals

The ionic and ion diffusion-controlled thermally stimulated relaxation (TSR) processes in CaF2, BaF2, LiBaF3 and KBr crystals were investigated above 290 K by means of the ionic conductivity, ionic thermally stimulated depolarisation current (TSDC) and thermal bleaching techniques. Under a DC field the halide crystals store large ionic space charge. We were able to detect in CaF2, BaF2, LiBaF3 and KBr in the extrinsic ionic conductivity region a series of the ionic defect (the interstitial anion and/or anion vacancies - in fluorides; the cation vacancies - in KBr) release stages: 3-6 wide and overlapping ionic TSDC peaks. The correlated data of the ionic TSDC and the F band thermal show that above 290 K the TSR processes are initiated and controlled by the ionic defect thermal detrapping, migration and interaction with the localised electronic and ionic charges and colour centres. The ion diffusion-controlled TSR processes take place in the above halide crystals.

Radiation-induced electronic and ionic charge storage and release in sapphire

Radiation-induced thermally stimulated relaxation (TSR) processes in the reduced f -Al 2 O 3 (sapphire) crystal were investigated at 290-650 v K by means of the TS current (TSC), ionic depolarisation current (TSDC) and electron emission (TSEE) techniques. After thermal (ionic) polarisation of sapphire wide (∼75 v K) and asymmetric ionic dipolar TSDC peak at T max , 590 v K (disorientation of the anion vacancy-related dipoles) was detected. This peak correlates with the wide TSEE peak at T max , 615 v K, the radiation-induced electrical degradation (RIED) yield rise above 550 v K ( T max , 745 v K) and the chromium emission line broadening in ruby. Above 450-500 v K the anion vacancy hopping (migration) starts. This can lead to lattice dynamic disordering and anion vacancy diffusion-controlled processes in sapphire (especially in vacuo near the sample surface, grain boundaries, dislocations) in various TSR (TSC, TSDC, TS heat release and bleaching) and RIED phenomena. Surface structure and impurity content, surrounding atmosphere (vacuum or air) and electric fields determine these phenomena.

Ion diffusion-controlled processes in fluoride crystals

Ionic and ion diffusion-controlled thermally stimulated relaxation (TSR) processes in CaF 2 , BaF 2 and LiBaF 3 crystals (X-ray irradiated at 290 K) have been investigated by means of ionic conductivity and the correlated ionic thermally stimulated depolarisation current (TSDC), and radiation-induced optical absorption band thermal bleaching techniques at 290-650 K. It was found that under a DC field fluorides store large ionic space-charge. In CaF 2 , BaF 2 and LiBaF 3 , by using the ionic TSDC technique we were able to detect a series of the interstitial anion and/or anion vacancy delocalisation stages in the extrinsic ionic conductivity region. At least 4-6 wide and overlapping ionic TSDC peaks were observed. The activation energy values of the ionic conductivity and ionic TSDC coincide, or are very close: 1.313 ± 0.002 and 1.27 ± 0.02eV for CaF 2 , 1.040 ± 0.002 and 1.00 ± 0.02 eV for BaF 2 and 0.91 ± 0.04 and 0.780 ± 0.013 eV for LiBaF 3 , respectively. The correlated data of the ionic TSDC and thermal bleaching (of the F band and other radiation-induced absorption bands) suggest that above RT ion diffusion-controlled TSR processes take place in CaF 2 , BaF 2 and LiBaF 3 crystals. These processes are initiated and controlled by the anion defect thermal detrapping, migration and interaction with the trapped charges.

Thermostimulated electronic and ionic processes in irradiated sapphire

Electronic and ionic thermostimulated (TS) relaxation (TSR) processes in nominally pure sapphire (α-Al2O3 grown with oxygen deficiency) have been investigated at 290 - 650 K by means of the TS current (TSC), ionic depolarization current (TSDC) and electron emission (TSEE) techniques. After thermal (ionic) polarization of the reduced sapphire wide (approximately 75 K) and asymmetric ionic dipolar TSDC peak at 590 K (disorientation of the anion vacancy-related dipoles) was detected. Above 450 - 500 K the anion vacancy hopping (migration) starts and their interaction with defects take place. This can lead to lattice dynamic disordering and anion vacancy diffusion-controlled processes in sapphire (especially -- in vacuum near the sample surface, grain boundaries, dislocations) in various TSR (TSC, TSDC, TSEE, TS heat release and bleaching) phenomena. The ionic TSDS peak at 590 K correlates with the wide TSEE peak at 615 K, with the rise stage of the radiation-induced electrical degradation (RIED) above 550 K (maximum at 745 K) and the chromium emission line broadening in ruby. The REID effect (observed above 550 K by E. R. Hodgson et al.), colloid formation and structure change of sapphire are caused by oxygen exchange at the grain boundaries, surfaces, dislocations and impurity-rich regions. Surface structure, impurity content, surrounding atmosphere (vacuum or air) and electric fields determine these phenomena.

Ionic, electronic and ion-diffusion controlled relaxation processes in CaF2, BaF2 and LiBaF3 crystals

Abstract The ionic, electronic and anion-diffusion controlled thermally stimulated relaxation (TSR) processes at 80—700 K in CaF2 BaF2 and LiBaF3 crystals (X-ray irradiated or non-irradiated) have been investigated by means of ionic conductivity, ionic thermally stimulated (TS) depolarization current (TSDC); as well as current (TSC), luminescence (TSL) and bleaching (TSB) techniques. Above 250—290 K broad and overlapping anion TSDC peaks and correlated TSB stages are detected. The TSB kinetics is initiated and controlled by anion detrapping and interaction with the localized charges, i.e., the anion-diffusion controlled TSR processes take place in fluorides. The TSL and TSC data for LiBaF3 indicate that the lifetime and drift of electrons at 80—250 K is very small because of deep retrapping. The main TSL peaks at 132K, 170K and 220 K are caused by Vk center detrapping and hole-diffusion controlled tunnel recombination within pairs like {Dn e−…Vk }.

Ion and electron trapping: release and relaxation processes in fluoride crystals

The thermally stimulated relaxation (TSR) processes in CaF2, BaF2 and LiBaF3 crystals (X-ray irradiated at LNT or RT) have been investigated by means of the ionic conductivity, thermally stimulated (TS) ionic depolarization current (TSDC), TS current (TSC), TS luminescence (TSL) and thermal bleaching techniques. The ionic TSDC measurements evidence that under DC field fluorides accumulate large ionic space-charge (thermoelectric state is formed) as a result of the migrating anion interstitial and/or vacancy capture on defects. In the ionic conductivity region (290 - 650 K) the thermoelectric state anneals, and several wide and overlapping anionic TSDC peaks are detected. The ionic TSDC stages correlate with the X-ray induced absorption band (F-type and other) thermal bleaching stages. These data evidence that the TSR processes are initiated and controlled by the anion defect thermal detrapping and interaction with the color centers and other localized charges. The anion diffusion-controlled TSR processes take place in fluorides. The TSL, TSC and TSL efficiency (TSL/TSC) data evidence that holes and, probably, the interstitials are detrapped: in CaF2 -- at 125 - 190 K, 260 - 320 K; in LiBaF3 -- at 132 K, 170 K and 220 K. The hole or interstitial diffusion-controlled radiative recombination (TSL) takes place above LNT.© (2001) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only.

On nature of the transient IR-absorption STE-like bands at 0.15-0.36 eV in alkali halides

New evidence is given that two classes (A and B) of the transient IR-absorption bands: (A) with max. at 0.15-0.36 eV (in NaCl : I, NaBr, NaI, KCl : I, KBr : I, RbCl : I, RbBr : I), due to on-centre self-trapped exciton and (B) with max. at 0.27-0.36 eV (in NaCI, KCl, KBr, RbCl), due to shallow trapped electrons or bound polarons, are caused by the same defect-shallow trapped electron (e-) at the substitutional (cation: c-site) alkali impurity cation (M + ): [M - ] 0 c e - . The A- and B-class IR bands have the same location, similar shape, half-width (exactly coincide for KCl : I and KCl at 80 or 10 K with the same vibration structure). It is established that the same Mollwo-Ivey plot curves E 0 = a/d n (d--nn anion-cation distance, n-exponent, a-constant) are fulfilled for both IR band classes, if we plot instead the IR band peak energy values the more definite values of the IR band zero-phonon line energy E 0 (for NaCI, KCl, KBr, RbCl and KCl : I) and/or the IR band low-energy edge energy E 0 (±0.03 eV) values (for NaBr, NaI, NaCl : I, KBr : I, RbCl : I and RbBr : I). These data are significant additional evidence that the A-class IR bands are caused by the same type defects as the B-class IR bands-by the shallow electron traps, i.e., centres [M + ] 0 c e - Two types of the [M + ] 0 c e - centres are predominant: (i) [Li + ] 0 c e - in NaX host crystals with E 0 29.4/d 4.72 and (ii) [Na + ] 0 c e - in KX and RbX host crystals with the relation E 0 6.15/d 2.74 .