New Evidence for the Model and the Process of Formation of Z1 Centers

— Z\ centers have been produced in strontium-doped KC1 by a new method which allows one to send negative ion vacancies and electrons separately through the crystal. This new method of producing Z\ centers confirms that the Z\ center contains an F center as a component and gives direct experimental evidence for the existence of an intermediate product in the Zi center formation. This intermediate product is an ionized Z\ center and gives rise to a localized exciton band peaking al 172.5 my*. In spite of many investigations the model of the Z , a band we observed in. K Q [3]. Pulse anneal ing center has been for a long time a controversial quesmeasurements show tha t it begins to decrease at a b o u t t ion ; only rather recently Hartei and Liity [I] were 200 °K and is totally destroyed by warming u p to able to show that the formation of Z , centers under F 250 °K. In figure 1 we have reported the absorp t ion light i l lumination involves no t only an electronic but also an ionic transfer. The results of Harte i and Liity °-*i——~ [1] rule out the possibility, first suggested by Seitz [2], \ ^7^ A bar,iJ that the Z, center is a divalent impurity with a trapped „ «o °K ! •**' ' electron and give instead strong support to a model , ;so •* /'"""""X including an F center. -^ / \ The usual way for producing Z, centers is to shine F | *'*" / \ light, for example, on strontium doped KC1 containing £ / rT-"*~N F centers and kept at an appropriate temperature. We £ / / \ have produced Z1 ceuters by a new method which, s / / \/* though more complicated than the usual one, allows y / / / " \ \ us to send separately through the crystal negative ion % ° ' 7 5 " / / / ^ \ ^ vacancies and electrons. The new procedure for pro/ _ / / / \ V ducing Zl centers can be divided in four parts : / / ~^-^-^ f \ I) a strontium doped KC1 crystal is X-rayed at ^~^>^\-*S'\ J 11 °K and then warmed up to 300 °K ; after such a ' — — ^ •" ~~ito '—~~ TTM treatement the sample contains F and V centers and x -/< (in general) a small concen t ra t ion of M centers . _ A. . ,. r , . u „ „ j v e iv » / »» v p [ G j — Absorption spectra in the region of the K band II) the sample is cooled down to 150 °K and is 0f a strontium doped KC1 sample containing a centers produced i l luminated with F (or white) light and subsequently by optical bleaching of F centers at 150 °K and warmed up to with F' light ; the il lumination causes the F and V h ' S h e r temperatures. All the curves of this figure have been u j „ .,. j „ _.. , . J 4i ... t>„ A *„ ., A tv *u obtained by subtracting to the measured absorption curve the bands to decrease and the a band to grow. After the . .. J „ , . , , „ , , , r „ „ *•„„ Tl.„ . ,„„, , c absorption curve taken before the F -> a conversion. Toe upper i l luminat ion n o detectable F' band was found. c u r v e is t h c ^m^ a s t n a t obtained after the F ->« conversion T h e a b a n d p r o d u c e d in such a way is the most stable at 150 °K. The temperature of measurement was 77 K, Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1967430 spectra in the region of the a band taken at 77 OK after the sample containing a centers produced by optical bleaching of F centcrs at 150 OK has been warmed up to higher temperatures. From figure 1, it appears that while the ol band is decreasing a new band peaking at 172.5 mp grows. The height of this new band obtained by subtracting the contribution to the absorption of the remaining cl band is reported in figure 2 as a function of the tcrnperature. From this figurc it appears that the 172.5 mp band begins to grow when the a band starts to decrease and reaches its maximum at ahout 250 OK (temperature at which the a band has totally disappeared) ; at higher temperatures the 172.5 mp band decreases and disappears at about 280 OK. FIG. 2. Height of thc 172.5 m p band as a function of temperature. causes also a slight decrease of the F band ; no other relevant change has been observed in the 170-2 000 m p range. IV) The sanlpte is cooled down to 77 OK and is X-rayed for short intervals of time ; under the action of the X-rays a band peaking at 600 rnfl grows and reaches saturatiorl ; in figure 3 we have reported the change ia the visiblc region of the absorption spectrum duc to the action of X-rays. The curve shown in this figure is relative to an X-ray dose which has brought to the s:tturation the growth of the 600 mp band. Three littlc humps in thc appropriate regions seem to indicate that beside the I; and the 600 mp band also the M and R bands grow. The 600 mp band of figure 3 can be identified with thc 2, band ; the followi~ig facts support this identification : it appears onIy in stronlium doped samples, its peak position and jls ha1 f-width are the same as those of the Z, band. Before to comment briefly these results it is worth to ~nention some other experimental data : a) Thc Z1 hand can be produced also in a sample treated as indicated previously with only one difference relative to the last step (the fourth in our numeration) of the experimental procedure : instead of k i n g Xrayed at 77 OK the sample is iliuminated with Flight at l50 OK. This result shows that the role of the X-rays i n the production of the 2, band at 77 OK (see point IV) is merely that of supplying free electrons. h) The X, band does not appear at all if the sample I I I ) The sample is warmcd up to 250OK ; we have containing a centers produccd by optical bleaching of just sccn that such a heating causes the a band to F centers is warmed up to tempcraturcs higher than d i sappar and the 172.5 mp band to grow and reach its 280 OK in order to destroy completely the 172.5 rnp ~nsx i~num ; it must now be added that the warming band. FIG. 3. Change in thc 45&900 rnfl range of the absorption spectrum of a strontium dojxd sample produccd by a short X-irradiation following tht trcatclnonts 1-11-111 reported in thc text. NEW EVIDENCE FOR THE MODEL AND THE PROCESS C 4 177 c) Though the detailed behaviour of the 172.5 mp vacancy pair or a divalent impurity with an associated band under the action of X-rays has not yet been stuvacancy pair. Consequently the ZI center shouId be a died we have found that it decreases under X-irradiavacancy pair with a trapped eIectron or a divalent tion. (At the present no data are available on the impurity-positive ion vacancy complex with an assobehaviour of the 172.5 mp band under F light illumiciated Fcenter (as originally proposed by Kleefstra 141). nation.) The E. N. D. 0. R. measurements of Bushneli 151 and The results summarized above lead us to the following interpretation : the thermal decay of o: centers obtained by optical bleaching of F centers is due to the migration of negative ion vacancies ; a fraction of these vacancies corltribute to the formation of complex defects which give rise to the 172.5 rnp IocaIized exciton the (sIight) dependence of tile 2, band on the type of divalent impurity [ l ] lead us to choose tile model including the divalent impurity. The above results together with other experi~nentaldata we are now obtaining will be discussed in detailin a later paper. band ; finally these complex derects transform into Z ,e n t e r s by capturing an electron. Now the question is :Referenceswhat type of defect gives rise to the 172.5 mp band andt r a n s f o r r n s j n t o a ~ ~ c ~ ~ t ~ r b y c a ~ t u ~ i ~ g a n e ~ e c t ~ ~ ~ ?[1]H;i~~~ (~.)andLiirv(F.),Z.Ph~~ik,1964,182,111.0 1 1 the ground of the results obtained we can say that 121 SE~TZ(E.),b. Rev., 1951, 83, 134.such a defect contains a negative ion vacancy as a [3] G~UL~ANI(G.), PERINATI(A.) and Rwuzzo~r(E.),component and that, as a consequence, the 2, centerL11 National Congress of the Italian Physical includes as a component an F center. As far as theSociety (S. I. F.), Trieste, october 1966.[4] KLEEFSTRA(M-), J. Phys. Chem. Solids, I 963, 24, 1 567.entire structure of this defect is concerned we can [5j BUSHNELL(J. C.) , International Symposium on colorobserve iha t two configurations are a priori possible : aCenters in Alkali Halides, Urbana, october 1965.