"PHOTOPLASTIC EFFECT IN PURE AND DOPED ALKALI HALIDES"

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”PHOTOPLASTIC EFFECT IN PURE AND DOPED ALKALI HALIDES”

J. Cabrera, F. Agulló-López

To cite this version:

J. Cabrera, F. Agulló-López. ”PHOTOPLASTIC EFFECT IN PURE AND DOPED ALKALI HALIDES”. Journal de Physique Colloques, 1973, 34 (C9), pp.C9-253-C9-260.

�10.1051/jphyscol:1973944�. �jpa-00215419�

JOURNAL DE PHYSIQUE Colloque C9, supplkmenl au t P 1 1-1 2, Tome 34, Nouembre-DPcembl.e 1973, page C9-253

<( PHOTOPLASTIC EFFECT IN PURE

AND DOPED ALKALI HALIDES ^B

J. M. C A B R E R A and F. AGULLO-LOPEZ

Departamento d e Fisica, Universidad Aut6noma de Madrid, Cantoblanco, Madrid, Spain

Resume. - Nous presentons des resultats detailles sur la structure et les proprietes de I'effet photoplastique dans les halures alcalins dopes. Des cristaux dopes avec des H - et des cations divalents ont eti: etudies. Les I-esultats sont conipares avec ceux qu'on observe avec des cristaux purs. Nous presentons un schema possible pour expliquer qualitativement le comportement observC.

Abstract.

Detailed data on the structure and properties of the photoplastic effect in doped alkali halides are presented. H and divalent cation doped crystals have been investigated. The data are conipsred to those observed for pure samples. A possible scheme to qualitatively under- stand the observed behavior is discussed.

1 . Introduction.

Dislocations in ionic crystals, particularly alkali halides, are electrically charged [I], [I I]. Extensive measurements of that charge have been reported for pure as well as anion and cation doped crystals. This charge is supposed to influence a variety of mechanical, electrical, optical and diffu- sive properties of the crystals.

Recently, a detailed investigation of the photo- plastic effect [I21 in pure colored alkali halides lias been performed in our laboratory and it is being reported elsewhere [13]. It lias been shown tliat this effect cnn be bnsic;illy understood in terms of the interaction between positively charged dislocations and electrons photoionized from F-centers. A number of properties of tlie c f i c t , s i ~ c h as its dependence on light-wa\!elength, light intensity and strain rate can be semiqt1a11tit~ltively explained.

In this paper. the el'fect of impurity doping on tlie photoplnstic elTect is described. I-ipdrogen ions and a nuriiber of divalent cation impurities 1i:ive been investigated. I t is known that the magnitude of tlie electrical charge is strongly modilied by doping and therefore drastic changes in tlie structure and pro- perties of the elfect arc foreseen. I n fact, tlie photo- plastic belia\~ior of' dopcd crystals difTers markedly from that obscrvcd for pure crystals. Its structure lias revealcd quite c0111pl~.i and the results reported here are not s~llli~ierlt to provirlc ;I complete picture.

It has clearly appcn~.cd t l i t i t the ideas adv;inced to explain tlie eil'ect in pure crystals are not adequate.

However a qualiti~live scheme which I-ationalizes some of the tlatii on dopetl crystals. is tentali\lely proposed.

It is felt that t11c photoplastic efli'ct constitutes a n unique way to o b s c r ~ c iind measure the electrostatic interaction between dislocatic,ns anti point defects in ionic crystals. Co~iscc~ucntly i t shoiild provide

an adequate test to current models dealing with such an interaction.

In a first part of this paper, the most significative features of the effect in pure crystals are briefly reported together with a short account of the model proposed.

The emphasis is on the data which are relevant in connection with the work on doped crystals. This provides an adequate background for a comparative analysis of tlie data.

2. Experimental. - Nominally pure alkali halide crystals have been purch;ised from the Harshaw Chemic:il Co. Doped crystals have been supplied by Dl-. F. Rosenberg of Univessity of Utah. Pure samples were colored either ndditively, or by y-irradiation of by hydrogen doping followed by y-irradiation.

Additive coloring was performed with the method advanced by Van Doorn. Hydrogen doping was achieved by additive coloring followed by treatment of the samples in high pressure ( 5 70 kg/crn2) hydro- gen atmosphere at hil~_h ten~pcratures (500-600 OC).

Irradiations of pure or H--doped crystals were carried out in :I Co"' swini~ning-pool type y-source.

Cation doped samples were colored by X-ray irra- diation at room temperature in a Siemens apparatus operating at 50 kV, 40 m A .

Colored samples - ^{3 x 4 x} 12 mm3 in size were then stl-airied in compression in :in Instros~ TT testing macliine. At a cestnin point of the stress-strain curve they were sub.jectet1 to light of a Xe-arc, Hg arc o r tungsten filament (quartz-iodine) lamp either directly or through a Bauscli-Lomb high intensity monochl-o- mator. In view of the small magnitude of the pholo- pl;rstic elrect. tlie niosl sensitive stress scale (0- I kg) has been used combined with a displacement of the zcro. Cliiingcs in lo~id :IS small as 10 g can be detected.

For t Iic low (ernpcra[ure tvor-k. a specially designed

Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1973944

cryostat was used. It has a set of aluminium-coated mirrors to drive the liglit into the sample compartment during straining. Tlie experiments at 77 K were usually performed by immersing the samples into a liquid nitrogen bath inside a transparent cryostat.

This system provided a higher illumination efficiency and consequently led to a larger size of tlie effect.

3. Photoplastic effect in pure crystals. - In nomi- nally pure alkali Iialide crystals. which have been colored, a positive photoplastic effect is always observed as indicated in figure 1, for NaCl and KCI.

It corresponds to room-temperature testing. When F-light is shone on the crystal a fast increase in flow stress is observed. Then a yield-point behavior appears leading to a region of steady flow. In some crystals, particularly at high illuminations intensities, an abrupt yield-drop is often found. On cutting the illumination, the flow stress essentially recovers the pre-illumination growth rate.

4 - ON OFF ,

1

3 -

/ /

2

, , ,

K CL

0 0.1 0.2 0.3

INCREMENTAL STRAIN, A € ,

FIG. 1. - Typical room-temperature photoplastic effect in NaCl and KCI. The start and completion of the illumination

are marked on the figure.

The magnitude of the effect, as measured by the fast increase in stress, depends on a number of varia- bles. We will now briefly review some of these depen- dences [13] :

3 . 1 The effect is zero in the elastic region and increases strongly at the yield point. It remains practically constant for strain percentages in the range 2-8 %, except for NaCI where it increases monotonously with deformation. However the detailed sliape of the dependence varies somewhat with the F-center concentration.

3 . 2 Tlie effect is enhanced on lowering tlie strain rate or increasing the light intensity. A saturation value is obtained for high intensities.

3 . 3 The effect depends markedly on testing teni- perature. At all temperatures in the range 77-300 K the effect is positive.

3 . 4 The excitation spectrum of the effect closely follows the shape of tlie F-band at room as well as low temperatures, for lightly colored crystals.

For heavily colored samples the excitation spectrum is broader and a marked dip appears at the F-band peak. This behavior is illustrated in figure 2, corres- ponding to KC1 tested at room temperature. Tlie shape of the spectrum for high colorations has been attributed to the non-uniform absorption of liglit in tlie sample. At tlie F-band peak most liglit is absorbed in a small fraction of the sample tickness.

PHOTON E N E R G Y , e V

FIG. 2.

Spectral sensitivity of the photoplastic kfl'cct in KC1 for high (2 x

cm-1) F-center concentrations (uppcr frame) and low (1.1 .<

cni

F-ccntcr concentl.ations

(lower frame).

3 . 5 The effect increases monotonously with F-ce~iter- concentration. The theoretical model which has been proposed [I31 to explain the photoplastic elycct in pure crystals is schematically illustrated in figure 3.

When F-light is shone on the crystal, electrons are

photoionized from the F-centers ant1 attracted to

the charged dislocations (Fig. 3u). An electron c l o ~ ~ d .

detertilined by Boltzmann's law should, then, be

formed around the dislocation core a n d be trapped

at impurities or defects (Fig. 3/91. Consequently the

potential barrier generated by the electron cloud

((

PHOTOPLASTIC EFFECT IN PURE AND DOPED ALKALI HALIDES

(39-255

FIG. 3. - Schematic illustration of the proposed mechanism for the photoplastic effect. Electrons are photoionized from the F-centers and attracted towards the dislocation line a).

A clo~td of trapped electrons is formed and opposes dislocation motion b).

opposes dislocation motion and induces hardening.

This model accounts for a number of qualitative features of the effect and leads to the formula

(AT being tlie magnitude of the pliotoplastic effect,

u the average dislocation velocity and I the light intensity) which provides a quantitative explanation for the role of strain rate and light intensity. C, and C, are constants.

It is worth mentioning that electric-field induced photo-ionization of electrons has to be invoked to account for the strong magnitude of the effect a 77 K for NaCI. This is not surprising if one considers the high electric fields to be expected in the neighbour- hood of charged dislocation [14].

4. Photoplastic effect in doped crystals. - It is expected that impurity doping should introduce deep modifications in the structure and properties of the photoplastic effect. Both the plastic properties and color center behavior are altered by doping. I n particular, anion as well as cation impurities change the magnitude (and even the sign) of the electrical charge on dislocations.

4.1 PHOTOI~LASTIC EFFECT IN CRYSTALS DOPI:D WITH U-CENTERS.

The substitutional H - ion.

U-center, is electronically similar to the F-center and has, also, a well-delilled absorption band in tlie U V region of the spectrum. The observation of the photoplastic ell'ect in crystals doped with U-ccnters is of much intercst since thc rcsults can be correlated with those observed for crystals with F-centers. and

~ h o u l d help to conlirm a n adequate model.

For KCI, KHr and KI wherc thc U-band peaks at 214. 218 and 370 mp respectively [15]. :\ photo- plastic erect. similar to that cicscribed for k--ccntc~s, has been observetl at I-ooni Lcmper:lture. A tipical erect, as i t i~ppears 1-or KC]. is depicted in ligure 4u.

In some c:~ses a sudden decrease in Ilow-stress occurs on starting illumination, prior to (lie

l l O l ' l l l ~ /

positive efi'cct. For Na('l wllosc bi~rid I'alls ^fill. in

W A V E L E N G T H , m p

FIG. 4.

Structure of the room-temperature photoplastic effect in U-doped KC1 a). Spectral sensitivity of the effect

corresponding to highly U-doped KBr b).

the UV, the observation of the effect appears more difficult and has not been successful.

A detailed study of the characteristics of the effect has not been possible because of the low light intensity available in the U-band spectral region. However, in highly U-doped (4 x 1017 ~ 1 1 1 - ~ ) KBr, the spectral dependence has been obtained figure 46. The effect is prominent in the U-band region although it presents some of the features found for heavily colored (F- centers) crystals. The marked decrease in the magni- tude of the effect at the U-band peak has to be, again, related to the small penetration of the exciting light.

Similar results have been obtained for KI.

I t is worth mentioning that for nominally pure KI crystals, a very small photoplastic effect has been observed for light of wavelengtli close to the funda- mental absorption edge [l6]. I t might be due to some impurities, especially OH-, \vhich is known to be a common impurity in Harshaw crystals.

4 . 2 PHOTOPLASTIC EI:I-ECT I N CATION DOPED CRYS- TALS. - The investigation has been concentrated on divnlent cation impurities. The following systems have been studied : NaCI : Z n + + (5 x rnol), NaCI : Cd" ( 8 x lo-' rnol), NaCI : N i + + (5 x a n d 2 x lo--' niol) NaCI : Mn'

(3 x rnol), KC1 : Bn" ( 7 x lo-' rnol), KC1 : Pbf

and

10- " mol) and K.CI ^: Silt ⁽ 10-"01).

At variance with the case of U-dooed crystals the

photopI;istic effect is now mnrkcdly different from

that appe~~ring in pure cl-ystnls. Now, we will describe

solnc features of the eff'ect.

4 . 2 . 1 Structure of' tlie effect.

Although marked differences are apparent among the various crystal systems, all impurities yield a photoplastic effect corresponding to one of the three typical patterns to be described next. In the first type, figure 5a, the flow stress experiences a fast decrease on illu- mination and then levels off to keep a small positive slope. On cutting the light off the stress rate is not appreciably modified. In the second type, figure 5b a mixed behavior is observed. Initially, there is a fast decrease in flow stress, as in the previous case, but afterwards the stress rate increases above the value corresponding to the

(lark >> curve. Finally, on cutting the light off, the flow stress decreases to reach the preillumination level as in the case of pure crystals. The third type is the standard positive effect, figure I , and is observed in some doped crystals at very low or rather high strain levels (see later).

Exceptionally, a small positive effect has been always observed in NaCl : Mnf +. All these effects refer to room temperature testing. The illumination has been performed at the F-band peak.

o !

0 0 4 0 2 0.3

INCREMENTAL S T R A I N E ,

FIG. 5. - Typical photoplastic effects in cation-doped alkali halides. Pure negative effect a) and mixed-type effect /I).

The difference between the structure of tlie effect in pure and doped crystals is also readily apparent if the experiments are performed in tlie plastic rela- xation after keeping the strain constant, figure 6.

Effects depicted in figure 5 have some peculiar properties which are essentially different from those

1 _{K C L : ~ a + +}

q u e n c h e d

8 -

m

5 ^ti-

\

m ON OFF

'f 4 - wl

1 1

W

rr 2 - t-

V) J

2 O -

CONSTANT S T R A I N

I ^{ON N a C L}

TIME ,

FIG. 6.

Typical photoplastic effects appearing on the rela- xation curves after keeping the strain constant. Doped and

pure crystals are compared.

observed in pure crystals. If, after completing an illumination period, a new illumination is started, the new photoplastic effect depends very markedly on the strain elapsed between the successive illumi- nations, figure ^{7 .} In fact, if that strain 1s "< 0,2 "b,

there is not second effect if the first one was pure negative (Fig. 7a). If the first effect was of a mixed type, the second one is

standard

positive (Fig. 7 h ) . It is to be stressed that the parameter is the strain and not the time elapsed between the illuminations.

This behavior is at variance with that found in pure crystals where tlie effect is clearly repetitive (Fig. 7(,).

4 . 2 . 2 Itifluence of' c o i ~ c n ~ t r c r t i o i ~ ail(/ tl~o.iiru/ tr r u t - )?wilts.

The effect of irnpurity concentration has been investigated in two systems, KC1 : Pb'

(concen- trations : lo-' and ¹¹¹⁰¹⁾ and NaCI ^: Ni '

(concentrations : 5 x and 2, x mol). In both cases the effects are small and clear systcrnatic differences have not been detected, for equal dose irradiations.

On the other hand quenching is known to :il~er considerably the state of aggregation of the impurity.

The influence of a quenching treatnient has bcen

studied in several systems. Crystals were kept at

400 OC for one I I O L I ~ and then rapidly cooled on a

metal plate. After this treatnient they were X-irra-

diated and tested at room temperature. Significative

results have been obtained for NwCI : Mn''.. NaCI :

((

PHOTOPLASTIC EFFECT IN PURE AND DOPED ALKALI HALIDES

O N OFF OFF I I 1-

' \ / ^{K C [ : quenched B a * +}

KC1 B a + + u n t r e a t e d

ON OFF ON OFF

I N C R E M E N T A L STRAIN, %

FIG. 7. - Repetitivity of the effect in doped and pure crystals.

Shortly after the completion of an illumination period a very small photoplastic effect is observed in quenched KCI : Ba++ a).

If the effect was of a mixed type a small positive effect is observed on reillumination b). Positive effect in pure crystals is clearly

repetitive.

Errrrttt~n : The values of the three abscisses must be read .I .2 . 3 . . . instead of 1 2 3...

Zn

and K.C1 : Ba

+. In these cases the negative effect is clearly enhanced by tlie treatment and in the case of NaCI : M n i f where the effect is positive i n untreated samples, becomes negative in the treated ones. The greater changes are observed in KC1 : Baf

where the effect increases almost one order of magni- tude. The effect keeps its magnitude after about one week in the dark.

4 . 2 . 3 D(yc.iic1cwc.c 011 . r t r a i i ~ levc4. - - As a general rule the size of the negative photoplastic effect increases with strain nt low strains, then levels off and finally decreases and changes to n mixed type for higher strains. In some systems the effect becomes positive as in pure crystnls. In figure 8 data for NaCl : N i + + , NaCI : Z n + + ond KC1 : S n + ' are given. In KC1 : Ba", which has been quenched prior to irradiation, the ert'ect is essentially constant in a wide strain range.

This allows for studying the dependence of the erect on some vitrinbles (strain-rate, light-intensity and wavelength) on the samc sample.

4 . 2 . 4 D c ~ l ) c ~ r t / ~ i ~ c ~ r oil I;-c,c~il/(,i. c.oi~c.c~ilfrulioi~.

The magnitude of tlie clkcl incrcascs with irr~tdiation dose o r F-center conccntr:~tinn. I)a[a for ^LI reitsonitble range of colicelltr;~tic>ns I i a ~ e been obtained 1.01-

S T R A I N ,

FIG. 8. - Dependence of the photoplastic effect on plastics strain for various doped systems. Crystals X-irradiated for

30 rnin.

FIG. 9. - Dependence of the photoplastic effect on F-center concentration for untreated NaCI : Zn++.

NaCI : Z n f +, figure 9. Each point corresponds to an average of two o r three experiments on different samples. The dispersion of the data with regard to that average is as high a s ^-, 50 %.

4 . 2 . 5 Dc~penrkence 011 light intensity. - The depen-

dence on li,oht intensity is similar t o that for pure

crystals. The range 1-50 x l O I 4 pliot/cm2.s has

been obtained by using neutral screen-type optical

filters on the higher intensity light beam. On increasing

intensity the magnitutle of the negative effect grows

but ^:I tendency to saturation is observed. Significative

data have been obtained for clucnched KC1 : Baf

and are displayed in figure 10.

C9-258 J. M. CABRERA AND F. A G U L L ~ L ~ P E Z

FIG. 10. - Dependence of the photoplastic effect on light intensity for quenched KC1 : BaC+. Samples were X-irradiated

for 30 min.

4 . 2 . 6 Dependence on strain rate. - The general trend of this dependence is, again, similar t o that found in pure crystals, figure 11. The data correspond t o quenched KC1 : B a + + . The effect is greatly enhanced for low strain rates. Monochromatic light corres- ponding t o the F-band peak has been used t o excite the effect.

C R O S S - H E A D S P E E D .

m r n / m i n

FIG. 11. - Dependence of the photoplastic effcct on strain rate for quenched KC1 : Ba++-. Samples were X-irradiated for

30 min.

4.2.7 Dependenc~e on light n~ureletlgtlr.

Figure 17 shows the data obtained for quenched KC1 : B a t ' . The effect is prominent at the F-band region, although the photoplastic excitation spectrum appears broader and a small valley is observed just at the F-band peak. This behavior has to be compared to that found in pure crystals (Fig. 1) and should be, again, attributed to non-uniform absorption of the liglil throughout the sample thickness. One should take into account that F-center concentration is - ^{10' ' cril-'}

for this experiment.

P H O T O N E N E R G Y , e V

FIG. 12. - Spectral sensitivity of the photoplastic effect in quenched KC1 : Ba++. The position of the F-band peak is marked with an arrow. Two different samples (X-irradiated for 30 min)

have been used ^{( 0} and 0).

It is, then, clear that for doped as well as pure alkali halides, F-band excitation is primarily res- ponsible for the photoplastic effect.

4.2.8 Dependet~ce on testing tentyeruttire. - There are preliminary data [I71 indicating that dislocations might change their sign to positive when plastic straining is performed at low temperature (liquid nitrogen or better liquid helium temperature). This behavior suggests that the structure and magnitude of the photoplastic effect might change at low tem- peratures. The role of testing temperature was inves- tigated by making experiments at liquid nitrogen temperature on KC1 : B a t ' and NaCI : Zni + , both quenched a i d untreated. Samples were X-ir-ra- diated for 30 min and illumination was performed at the F-band peak corresponding to 77 K. Main results are the following :

i) I11 quenched KC1 : Bu", the effect is posltlve at 77 K for very low strain levels. On increasing the strain the elrect becomes negatiie but much smaller ( - 115) than at roo111 temperature. In untreated saniples, the etTect ic al\f2nys zero or small positive.

ii) I n NaCI : Zn '+, quenched 3 s well as untreated, llie efTect is a1wa)s positive at 77 K.

11 can. then. be concluded that tcsting temperature

has a marked influence on the sign and n ~ n g n i t ~ ~ d e

of the pliotoplast~c erect. I t becomes less negatihe

or even positi\e on going d o ~ v n to liquid nitrogen

temperature.

((

PHOTOPLASTIC EFFECT IN P U R E A N D DOPED ALKALI HALIDES

C9-259

5. Discussion of the effect on doped crystals. -

In principle, the data on the photoplastic effect in U-doped crystals seem to confirm the model proposed for pure crystals. The effect is related to excitation of the U-centers. Although U-band absorption does not lead to photoconductivity, except at its high energy tail, one can invoke field-induced ionization of the excited U-center as for the low temperature effect in pure crystals. However, it is not completely clear why dislocations should be positively charged in this case. One has to conclude that U-band illu- mination or the combined effect of a dislocation moving near an excited U-center ejects the hydrogen ion and leaves an empty vacancy swept along with the dislocation.

On the other hand, the model proposed for the photoplastic effect in pure crystals does not appear suitable for divalent cation doped crystals. In fact, this type of doping increases the negative charge of the dislocations prior to irradiation. It might be, then, reasonably expected that irradiation cannot reverse the sign of the charge to positive. In that case electrons liberated by the light are pushed away from the dislocations core. A cloud of positive charge formed by the empty vacancies will be produced around the dislocations. The scheme discussed in section 3, would again predict an increase in flow stress due to the electric field opposed by the positive vacancy cloud. This is at variance with experiment.

Let us think of the negatively charged dislocation as dragging an excess of CI- ions at the core, figure 13.

FIG. 13. - Schematic illustration of the processes occurring during F-light illumination of a divalent cation doped crystal.

The excess of C1- ions on the dislocation line recombine with the empty anion vacancies generated by the light and electrical

charge is neutralized.

It appears quite reasonable that these ions might easily recombine with empty anion vacancies and regenerate the perfect crystal. This would provide a simple mechanism to remove the negative charge of the dislocations. In fact this is the mechanism invoked to account for the reversal of sign in colored samples. The role of illumination in the pliotoplastic

effect, should be to increase the concentration of empty vacancies and therefore enhance their rate of capture by the moving dislocations.

The removal of the dislocation charge should be responsible for the decrease in flow-stress. Once the dislocation is uncharged, no further change in flow stress is expected on cutting the light off. This is in agreement with the pure negative effect shown in figure 5a. Furthermore, if a new illumination is started no effect will be observed unless enough straining in the dark has taken place as to recharge again the dislocation line figure 7a.

If the illumination is proceeded enough time, the above mechanism can reverse the sign of the charge to positive and then the model proposed for pure crystals should be operative. Hardening sets in and cutting the light leads to a decrease in flow stress.

This is the type of effect depicted in figure 76.

The evolution in the magnitude of the effect with strain, and particularly the change in sign for high strains is not readily understood. It is probably related to changes in dislocation charge during straining.

The role of quenching treatments is, in principle, consistent with the scheme proposed. The dissolution of impurity aggregates carried by the treatment should favor the negative charging of the dislocations.

In fact, Turner and Whitworth [18] have detected (in NaCl : Mn") a large increase in negative charge after quenching, which is qualitatively in agreement with our findings. The change in the sign of the photo- plastic effect at low temperatures appears, to be consistent with the change in the sign of the dislo- cation charge when the straining is performed at low temperature [I 71.

Finally, the data on the influence of light-intensity and strain rate can be qualitatively understood in terms of our scheme. Higher light intensity implies a higher concentration of empty anion vacancies and consequently a higher positive charge to be acquired by the dislocation. Furthermore, it is rea- sonable that the processes for removal or loss of charge be more operative on increasing dislocation charge and it leads to a saturation of the effect.

On the other hand, lower strain-rates, i. e. lower dislocation velocities, imply a higher dislocation charge after the dynamical equilibrium between the rates of capture and loss of charge have been esta- blished.

Although, a mathematical model for this scheme can be developed, more detailed data on the properties of the effect are needed before the model can be successfully tested.

Acknowledgments.

Authors are grateful to the Division de Metalurgia. J E N for providing expe- rimental facilities and technical a,;istance. Partial support from Institute de Estudios Nucleares, J E N , is also gratefully acknowledged.

I X

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