STUDIES OF DEFECT PROPERTIESAn X-ray Study of Defective Ionic Single Crystals

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STUDIES OF DEFECT PROPERTIESAn X-ray Study of Defective Ionic Single Crystals

Z. Morlin, Á. Péter, I. Földvári, A. Mecseki

To cite this version:

Z. Morlin, Á. Péter, I. Földvári, A. Mecseki. STUDIES OF DEFECT PROPERTIESAn X-ray Study of Defective Ionic Single Crystals. Journal de Physique Colloques, 1980, 41 (C6), pp.C6-513-C6-515.

�10.1051/jphyscol:19806134�. �jpa-00220043�

STUDIES OF DEFECT PROPERTIES.

An X-ray Study of Defective Ionic Single Crystals

Z. Morlin, A. Peter, I. Foldvari and A. Mecseki

Institute of Crystal Physics of the Hungarian Academy of Sciences, 1502 Budapest Pf 132, Hungary

Abstract. — The line profiles of the (200) and (220) reflections of large ( 2 x 2 x 1 cm3) pure and doped i.e.

deformed alkali-halide single crystals were investigated with CuKe radiation. The defective state of the crystals presents itself in the formation of a diffuse background and the shift of the Bragg angle. A negative correlation was found between the lattice parameter and residual stress values. Heavily doped crystals develop complex line profiles which can be deconvoluted in the vicinity of the Bragg peak. NaCl. CaCI2 and NaCl. NiCl2 systems yielded a number of well resolved peaks already in the as grown state. This effect might be due either to a small angle misorientation of mosaic blocks or some paracrystalline state as considered by Suzuki and Hosemann respectively.

An X-ray line profile analysis of large ( 2 x 2 x 1 cm³)

alkali-halide single crystals has been carried out to study the effect of point defects and dislocations introduced into the crystals by doping, thermal shock and plastic deformation. Although excellent theore- tical works discuss the possible effects [1, 2, 3] only few experimental results are published [e.g. 4, 5, 6].

Theoretically the reflections of crystals containing point defects and/or dislocations can be separated into Bragg and diffuse components due to blurring in the reciprocal space.

The experiments presented were carried out with a manually operated diffractometer (SECASIPZ gonio- meter) constructed for single crystal orientation and operated at 30 kV, 13 mA. The results, intensity versus angle of diffraction (9°) curves were related to the (1011) reflection of a nominally pure and defect-free quartz standard without any appreciable diffuse background and an intensity half-width of 0.05°.

Monochromatic CUKJJ (A = 1.392 A) was used, and the (200) or (220) reflections of the crystals were investigated. The line-profiles were analysed by the variance method of probability theory [7, 8].

The defective state of the crystals presented itself in the broadening of the profile diffuse background and a shift of the Bragg angle i.e. lattice parameter.

Results obtained with N a F . ZnF² and KC1. Tl quench-

Fig. 1. — Line profiles of quenched 10 3 mol/mol NaF. ZnF2

and 10~5 mol/mol KC1.T1C1 crystals; (200) reflections. 1.

NaF. ZnF2 quenched from 300 °C, 2. NaF. ZnF2 quenched from 400 °C, 3. N a F . Z n F2 quenched from 600 °C, 4. KC1.T1C1 quen- ched from 600 °C. (90 — Bragg peak, 6' = centre of the diffuse scattering.)

JOURNAL D E PHYSIQUE Colloque C 6 , supplément au n° 7, Tome 4 1 , Juillet 1980, page C6-513

Résumé. — Les profils des réflexions (200) et (220) de la raie de diffraction CuK^ sur les monocristaux d'halogénures alcalins de dimensions ( 2 x 2 x 1 cms) sont examinés. Nos mesures ont porté sur des échantillons purs et dopés, parfait et déformés. La diffraction des rayons X sur des échantillons défectueux fait apparaître expérimentalement un déplacement de la position angulaire et une déformation de la forme des raies. La variation des paramètres réticulaires de la maille cristalline et des contraintes résiduelles se rapporte inversement. Pour des cristaux for- tement dopés les formes de profils sont complexes et peuvent être décomposées à proximité de l'angle de Bragg.

Les systèmes NaCl.CaCl2 et NaCl.NiCl2 donnent des profils distincts déjà à l'état natif. Ces phénomènes peu- vent être interprétés par une réflexion sur des grains désorientés de la matrice ou sur les phases secondaires du type Suzuki et Hosemann.

Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:19806134

ed crystals are presented in figure 1. Quenching from moderate temperatures ( t < 4000) results in a shift of the lattice parameter and a diffuse background as measured at the intensity I = 0.05, whose peak coin- cides with the Bragg peak. This corresponds to a correlated displacement of ions as should be expected from theory [l, 61.

With mechanically deformed or heavily doped crystals beside line broadening the profile becomes structured and eventually breaks up in two or more peaks in the vicinity of the Bragg angle.

Since quenching from moderate temperatures below 400 OC affects only the B r a g angle i.e. the lattice parameter without any appreciable change of the half-width intensity as measured at the beginning of the experiment, and since on the other hand, any influencing of the original dislocation density by deformation increases the half-width or even results in splitting of the rocking curve, one may come to the conclusion that lattice parameter changes which are not accompanied by any broadening of the half-width of the as grown crystals are due to point defects.

Figure 2 shows the change of the rocking curve due to plastic deformation.

Fig. 2. - Line profile of the (220) reflection of a nominally pure NaC1. 1. As grown crystal. 2. After 1.3 % plastic deformation.

Fig. 3. - The trend of the variation of lattice parameter-residual stress of a nominally pure LiF crystal quenched from 400 OC in 15 min. mtervals. Upper diagram : lattice parameter ver.$uJ time, below : residual stress versus time.

Fig. 4. - Intensity versus 2 0 curve of mol/mol NaCI. CaCI Z.

The /I values (200 reflection) of the numbered peaks, 1 : 1.88, 2 : 1.93, 3 : 1.95, 4 : 1.97, 5 : 1.98, 6 : 2.0, 7 : 2.02, 8 : 2.09, 9 : 2.13.

Residual stress determination (Adld, d = lattice are in the vicinity of the Bragg angle (Fig. 4). Similar spacing) shows a negative correlation (r z 0.85) results were obtained with NaCl .NiC12 samples. Two with the lattice parameter. This trend is depicted in possible explanations may account for this result : figure 3.

Crystals quenched from 400 OC or higher tempera- ture especially if doped develop a complex line profile which can be deconvoluted into a series of maxima in the vicinity of the Bragg peak.

With NaCl crystals doped with l o w 3 mol/mol Ca2+

no deconvolution was needed since these samples

I . The secondary peaks originate from mosaic blocks slightly disoriented relative to each other and separated by small angle grain boundaries, or

2. a paracrystalline state as considered by Hose- mann [3] is superimposed on the matrix. Here again two possibilities may be conceivable :

yielded many well resolved peaks already in the as a) The paracrystalline short range order consists grown state. The Bragg peak shows in this case no of a collection of various chemical compositions as diffuse background (intensity half-width : 0.050 cor- was found with NaCl.CaC1, systems by Miyke and responding to the quartz standard). The other peaks Suzuki [9, 101 and Capelletti [ll] or more recently

AN X-RAY STUDY OF DEFECTIVE IONIC SINGLE CRYSTALS C6-515

with CaF, by Buhl and Preisinger in SIMS experi- ments 1121. Similarly we have found in our laboratory the precipitation of some composite structures in NaCl

.

b) On the other hand a paracrystalline state of the mother matrix, NaCI, might be created by the dopants, vacancy clusters, point defects. In this latter case the electron density distribution of zones c n'sisting of highly displaced Na' and C1- ions with

/

way by the Fourier transforms of the structure factor equation.

If the various compositions (case 2.a)) are preci- pitated the matrix should be relaxed (no distorted zones exist in the crystal). This may account for the relatively low intensity half-width values of the peaks

in figure 4 and the small diffuse background.

The authors wish to thank Mrs. E. Torok for her valuable help in preparing the samples and in carry- ing out the experiments.

DISCUSSION

Question. - E. LILLEY.

What is the quenching procedure and the size of crystals used ?

Reply. - Z. MORLIN.

The crystals were taken from the furnace and put on a large copper plate. On the plate they were moved so that the crystal was always on a cold part. Crystals size : 2 x 2 x 1 cm3.

Question. - A. B. LIDIARD.

You showed results which demonstrated the good quality of natural NaCl compared to deformed NaCl crystals. Would you comment on the perfection of

natural crystals generally, since Levy (this conference) reported that the radiation damage characteristics of natural NaCl crystals from Kansas were very similar to those of heavily deformed synthetic crystals ?

Reply. - Z . MORLIN.

Not all natural NaCl are good quality crystals. We tried crystals of various deposits and found only one (obtained from the State Institute of Mineralogy), namely a large crystal from Vielicka (Poland) which proved to be good. It was investigated by its phospho- rescence, and this was the only one which did not show any phosphorescence indicating a very low concentration of impurities. According to X-rays the line profile of this crystal was rather good.

References

[I] KRIVOGLAZ, M., Theory of X-ray and Thermal Neutron Scatte- ring by Real Crystals. Translation from Russian ed.

S. C. Moss. (Plenum, New York) I-XIX, 1-405, 1969.

[2] WILKENS, M., Phys. Status Solidi (a) 2 (1967) 359.

[3] HOSEMANN, R., Krist. Techn. 11 (1976) 1139.

[4] WILKENS, M., ECKERT, K., 2. Naturforseh. 19a (1964) 459.

[5] OETTEL, H., Phys. Stat. Sol. (a) 6 (1971) 265.

[6] HOWARD, C. J., SABINE, T. M., J. Phys. C : Solid State Phys.

7 (1974) 3453

( i l KI G , H. P., ALEXANDER. L. E

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2nd ed. (Wiley J . New York) I-XX1V 1-966, 1974.

[8] DE, M., SEN GUPTA, P., J. Phys. D : Apl. Phys. 3 (1969) 33.

191 MIYAKE, S., SUZUKI, P., J. Phys. Soc. Japan 9 (1954) 702.

[lo] SUZUKI, K., Phys. Soc. Japan 10 (1955) 794 ; 11 (1957) 179.

[I 11 CAPELLETTI, R . , Proc. Symp. on Thermal and Photostimulated Currcrzts in Insulators. Ed. by Smyth D. M. The Electro- chem. Soc. 1-31, 1976.

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[I31 FOLDVARI, I., VOSZKA, R., MORLIN, Z . , Phys. Status Solidi (b) (1978) 235.