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Identification of Z2- and associated Z-centres in LiF
through a correlation of electrical conductivity, ITC and
optical absorption measurements
S. Murali Dhara Rao
To cite this version:
JOURNAL D E PHYSIQUE Colloque C6, supplément au n° 7, Tome 41, Juillet 1980, page C6-123
Identification of Z2- and associated Z-centres in LiF through a correlation
of electrical conductivity, ITC and optical absorption measurements
S. Murali Dhara Rao
Multidisciplinary Research Scheme, Bhabha Atomic Research Centre, Bombay 400 085, India
Résumé. — La conductivité électrique et le thermo-courant de dépolarisation (ITC) ont été mesurés dans des échantillons de LiF : Mg trempés à la température ordinaire après recuit à différentes températures entre 50-350 °C ; ces mesures montrent que les dipôles impureté-lacune (I-V) sont dominants après recuit à 50 °C, les précipités à 150 °C et les dipôles (I-V) et Mg+ + isolé à et au-dessus de 250 °C. Des études d'absorption optique sur des
échantillons soumis à de tels traitements et irradiés avec une dose y de 104 R, montrent que l'intensité des bandes
à 225 nm (5,5 eV), 310 nm (4 eV) et 380 nm (3,3 eV) augmente après recuit à 50 °C, diminue à 150 °C et augmente jusqu'à saturation au-delà de 250 °C. En corrélant ces résultats, on conclut que la bande à 4 eV est due aux centres Z2 qui consistent en un centre F ' adjacent à un ion Mg2 + . La disparition des dipôles (I-V) et de la bande à 3,5 eV
dans les échantillons recuits à 350 °C est identique et se fait selon une cinétique du troisième ordre. On conclut que la bande à 3,3 eV est due à un centre Z associé constitué d'un centre F adjacent à un dipôle (I-V).
Abstract. — Electrical conductivity and ionic thermo-conductivity (ITC) measurements on LiF : Mg samples
quenched to room temperature after annealing at different temperature in the range 50-350 °C, indicate that impurity-vacancy (I-V) dipoles are predominant after the annealing at 50 °C; precipitates at 150 ° C ; and I-V dipoles and free Mg+ + at and above 250 °C. Optical absorption studies on samples subjected to similar treatments
and irradiated with a y-dose of 104 R, show that the 225 (5.5 eV), 310 (4 eV) and 380 (3.3 eV) nm bands exhibit an
increase in intensity when annealed at 50 °C, minimum at 150 °C and an increase and saturation beyond 250 °C. By a correlation of these results, it has been concluded that the 4 eV band results from a Z2-centre, consisting
of an F'-centre adjacent to an M g2 + ion. The decay of the I-V dipoles, and the 3.5 eV band in samples annealed at
350 °C is identical and exhibits a third order kinetics. It is concluded that the 3.3 eV band results from an associated Z-centre consisting of an F-centre adjacent to an I-V dipole.
1. Introduction. — Mort [1] used Mollow-Ivey rela-tion for Z3-centres in alkali halides to identify the
226 nm band with Z3-centres in LiF. He also proposed
that the 310 nm band is also due to a Z-type of centre. Christy et al. [2] suggested that the 380 nm band is due to I-V dipoles. Jackson and Harris [3] concluded through pre- and post-irradiation annealing studies that the I-V dipole aggregation alters the 310 nm and 380 nm absorption bands. The ambiguity in the iden-tification has been due to the sensitivity of the Z2
-centre to divalent cations [1] which makes the accurate fitting of Mollow-Ivey relation difficult. In the present study therefore a correlation of the electrical conduc-tivity and ITC data with the optical absorption is employed to identify the Z2- and associated Z-centres
in LiF : Mg.
.2. Experimental.— 2.1 SAMPLE PREPARATION. —
Commercial LiF was purified by vacuum distillation [4] to minimize the background impurities. This material was used to grow single crystals by batch crystalliza-tion employing Bridgman technique in a vacuum furnace [4]. The ingots measured 8 mm diameter and 40 mm lona. 6 mm x 6 mm x 1 mm samples
were cleaved out of these ingots for the studies. Samples employed in electrical conductivity and ITC were coated with Aquadag colloidal suspension in alcohol to enhance the electrical contact.
2.2 RESULTS AND DISCUSSION. — a) Electrical
conductivity measurements. — The conductivity cell
used for these studies is described elsewhere [5]. The conductivity was measured using a Keithley 416 A picoammeter in series with the sample and a 6 V d.c. cell. The current was recorded while the temperature of the sample is increased at 3 °C/min. The samples were annealed at different temperatures in the range 50-350 °C for 16 hrs and quenched to room tempe-rature. Conductivity measurements were made in the range 50 to 400 °C after each quench.
The conductivity measured at 50 °C and 100 °C is plotted against annealing temperature in the case of pure LiF and LiF doped with 0.004, 0.01 and 0.04 w t . % Mg in figure 1. The conductivity of pure LiF is almost constant, whereas it increases initially when the sample is annealed at 50 ° C ; decreases up to 150 °C and registers an increase again to saturate beyond 250 °C. The magnitude of the variation of a is observed to depend on the dopant concentration.
C6-124 S. MURAL1 DHARA RAO
-15
I
1
IW m YXI 400
Annealing temperature OC
Fig. 1. - Variation of conductivity with annealing temperatures in LiF : (1) pure, (2) 0.004, (3) 0.01, (4) 0.04 wt.% Mg.
The present result is similar to the observation of Dabes and Frohlich [6] on KC1 : Sr. They concluded through a correlation of density and conductivity measurements that the initial increase is due to the dissociation of the aggregates ; following decrease to the precipitation of the dopant; while the redissolu- tion of the dopant increases the conductivity which saturates when all the dopant is dissolved.
b) ITC measurements. - The experimental set up
is described by Rao [5]. The ITC measurements were made using a Keithly 610C electrometer amplifier. The samples were poled at RT with a d.c. field of 900 V/cm for 15 min. and cooled to 100 K. The variation of the ITC peak height with annealing temperature in the case of a sample doped with 0.02 wt.
%
Mg is shown in figure 2. It is observed that the peak height exhibits a maximum when the sample is annealed near 50 OC ;a minimum near 150 OC and increase and saturation of further increase in annealing temperature. The maxi- mum can be explained to be a result of the dissocia-
ANNEALING TEMPERATURE Z
Fig. 2. - Variation of the ITC peak height with annealing tempe- rature in LiF : Mg (0.02 wt.%).
tion of the aggregated dipoles; the minimum due to the formation of neutral aggregates or the precipita- tion of the impurity; while the redissolution of the impurity and the accompanying I-V dipoles would increase the peak height. This result confirms the conductivity observations.
c) Optical absorption spectra. - A Beckman DS
spectrophotometer was used for recording the absorp tion spectra in the range 200-600 nm. A 0.012 wt
.
%
Mg doped sample was annealed for 6 hrs at different temperatures in the range 50-350 OC and quenched to room temperature. After each quench the sample was irradiated with a y-dose of 1.3 x lo5 R to record the absorption spectra. Before the annealing, the sample was bleached by heating at 400 OC for 5 min. Figure 3 shows the spectra recorded after quenching from 50, 150 and 350 OC besides the untreated sample. The variation of the 250,380 and 310 nm band height is shown as a function of annealing temperature in figure 4. The identical nature of this figure and figures 1 and 2 can be easily observed. By correlating the varia- tion of the 310 and 380 nm bands with the conduc- tivity and ITC results it can be readily concluded that they are associated with the divalent cation impurity. Figure 5 shows the absorption spectra recorded of a sample quenched from 350 OC recorded after 30 min.,Fig. 3. - Influence of thermal annealing on the optical absorption spectra of LiF : Mg (0.012 wt.;,,).
0 I I I
I
o IW ZOO 3 00 4 0 3
4NNCALlNG T E M P E I A l Y R f *C
IDENTIFICATION OF Z,- AND ASSOCIATED Z-CENTRES IN LiF C6-125
Fig. 5. -Optical absorption spectra of LiF : Mg quenched to room temperature after annealing for 6 hrs at 350 OC, recorded after different periods after quenching.
2 hrs and 196 hrs of irradiation. It is observed that while the 380 nm band decays considerably, the 310 nm band enhances in intensity. Change in the 250 nm band is relatively small. This figure brings about the difTe- rence between the-310 and 380
nm
bands.i) The 3.3 eV band. - It is observed that the 310 nm band and the ITC peak both follow a 3rd- order kinetics. Capelletti and Benedetti [7] have shown that the decay of the I-V dipoles in NaCl : Cd follows a second-order kinetics in the initial stage and a third-order kinetics in the latter stage. They conclud- ed that the I-V dipoles aggregate to form trimers from dimers. It can therefore be concluded from the decay characteristics that the 3.3 eV nm band is associated with I-V dipoles.
ii) The 4 eV nm band. - The increase in the 4 eV
band height with increasing Mg concentration [5] and its vaiiation with annealing temperature indicates that it is associated with Mgz+ in its free state. This is in accordance with the conclusion drawn by Mayhugh
et'al. [8] by optical bleaching studies of the 5 eV and 4 eV bands.
iii) Identification of the centres.
-
The foregoing results and its position relative to the (5 eV) F-band, strongly indicate that the 4 eV band is a Zz-centre proposed by Ohkura [9] consisting of an F'-centre adjacent to a MgZ+ ion as shown in [5, 9, 111.On the basis of the present results and the optical [8] and thermal bleaching studies [lo, 21 the 3.3 eV band
is proposed to be an F-centre adjacent to an I-V dipole. This is further supported by the fact that the 4 eV band enhances while the 3.3 eV band decays. This could be possible if the electron released during the aggregation is trapped at a Z3-centre, which is very likely as the intensity of the 5.5 eV band decreases during this time as a result of the Z3 to Zz-conversion. This Z,-centre may be called an associated Z3-centre because it consists of a Z3-centre associated with a vacancy.
Acknowledgments. -The author is grateful to Shri S. D. Soman and Dr. P. R. K. Rao for their encouragement. He thanks Dr. B. B. Singh for allow- ing the use of the Beckman DB Spectrophotometer and Shri S. S. Rama Murty for the placing of the Keithley 416 A Picoammeter at the disposal of the author.
DISCUSSION
Question. - H . J . PAUS. Reply. - V . V . RATNAM.
I would just like to add the comment, that from Yes. In alkali halides like KC1, NaCl, etc..
.
Z centres simple energy considerations Mg-impurities should are formed with divalent cation dopants which have really give Z-centres in LiF but never in KC1, NaC1, low second ionization potentials like Ca, Sr, Ba but KBr, RbC1, etc. not with Mg.LiF is an exception in these respects. This needs investigating.
References
[I] MORT, J., Phys. Lett. 21 (1966) 124. [7] CAPELLETTI, R. and DE BENEDETTI, E., Phys. Rev. 165 (1968) [2] CHRISTY, R. W., JOHNSON, N. M., WILBRAG, R. R., J. Appl. 981.
Phys. 38 (1970) 2968. [8] MAYHUGH, M. R., CHRISTY, R. W. and JOHNSON, N. M., [3] JOCKSON, J. H. and HARRIS, A. M., J. Phys. C 3 (1970) 1967. J. Appl. Phys. 38 (1967) 2099.
[4] MURALIDHAR RAO, S. and BHIDE, G. K., Ind. J . Pure Appl. [9] OHKURA, H., Phys. Rev. 136 (1964) 46. Phys. 12 (1974) 358. [lo] MAYHUGH, M. R., J. Appl. Phys. 41 (1970) 4776. [5] MURALIDHAR RAO, S., Ph. D. Thesis (1976). [ l l ] SEITZ, F., Phys. Rev. 83 (1951) 134.
