Optical properties of gold centers in alkali halides

TECHNICAL NOTES

of precipitated MnS the width is about 120 G. The large jump of the line width from sample L to sample N characterizes large differences in the structural properties of the unincorporated MnS fraction of the two groups of samples. If one assumes that the line width of 300 G represents a compact crystalline MnS system, then the width of 120 G obviously means that up to the critical region unincorporated MnS does not exist in such a good crystalline form. In this region 'MnS is either not crystalline at all or the crystaUites are very small and thus the relevant spin-spin interaction does not represent the bulk property of the MnS phase of the precipitate. Although only from the EPR measurements one cannot definitely solve the unusual behaviour of the CdS-MnS precipitates, electron paramagnetic resonance is proved to be a useful technique for studying both CdS : Mn and MnS phases of these systems. Institute "Rudjer BaJkovib" and Institute of Physics of the University, Zagreb, Croatia, Yugoslavia

J. N. H.ERAK M. P A I C

REFERENCES 1. P A I C M.,Croat. Chem. Acta 43, 169 (1971). 2. PAIC M. and D E S P O T O V I C Z., Croat Chem. Acta 43, 175 (1971). 3. PA1C M., K R A N J C K. and P A I C V.;Fizika 4, 247 (1971). 4. D O R A I N P. B., Phys. Rev. 112, 1058 (1958). 5. L A M P E J. and K I K U C H I C., Phys. Rev. 119, 1256 (I 960).

J. Phy$.Chem.Solids Vol.33, pp. 1161-1166.

Optical properties Ofgold centers in alkali halides (Received 31 March 1971 ; in revised form 10 September 1971) 1. INTRODUCTION

OPTICAL properties of uncolored and colored alkali halide crystals doped with silver and

1161

copper have been studied extensively[I-10]. Some work has also been done on the gold doped additively colored alkali halide crystals [11,12]. Detailed spectra of uncolored, irradiated and electrolytically colored alkali halides doped with gold are not available [13, 14]. In this paper we discuss experimental results on optical absorption and Raman scattering of the gold doped uncolored and colored NaCI, KCI, KBr and KI crystals. 2. EXPERIMENTAL

Single crystals of alkali chlorides doped with gold were grown by Kyropoulos method in chlorine atmosphere from the melt containing the alkali chloride with chloroauric acid. In the case of alkali bromides and iodides, gold was incorporated in the crystals by diffusion from the respective gold halides. For this purpose single crystals (1 cm x 1 cm × 0.3 cm) of pure KBr and KI were sealed along with the respective gold halides in evacuated quartz ampoules and the diffusion was carried out at temperatures about 100°C below the respective melting points of the alkali halides. The crystals were uniformly doped by this method in about 48 hr. The absorption measurements on these crystals were made using Beckman DK-2A recording spectrophotometer and Carl Zeiss VSU 2B spectrophotometer. The Raman spectra were recorded using the exciting radiation (6328 A) from a Model 125 Spectra-Physics helium-neon gas laser and a Coderg recording Raman spectrometer incorporating a double grating monochromator and photoelectric recording system. The electrolytic coloration was done at 560°C from a platinum pointed electrode. Additive coloration was carried out at 560°C with the crystals sealed in pyrex glass ampoules with potassium metal. For X-irradiation, Philips PW 1009/30 X-ray diffraction unit with copper target working at 35 kV and 10 mA was used. Normally crystals were cooled at therate of l°C/min after they were grown by Kyropoulos method. For rapid quenching, the

1 I62

TECHNICAL NOTES

crystals (1 cm × 1 cm x 0.1 cm) were heated close to their melting point for about half an hour and then quickly dropped on a large cool copper plate. The E P R measurements were made with V-4502-12 type Varian spectrometer. 3. RESULTS AND DISCUSSION

(a) Optical and raman studies o f uncolored crystals Single crystals of KCI grown from the melt were pink at the top and were bluish pink at the bottom where the concentration of gold was large. The color of the NaCI crystals was light blue at the top, deep blue in the center and a dirty blue (with a brownish hue) in the lower portion of the crystal which was heavily doped. Thin slices (1 mm thick) of the heavily doped KCI and NaCI crystals looked transparent to the naked eye and were bluish pink and blue in color respectively. Thin slices of heavily doped KBr crystals were bluish green and KI crystals were yellow. All crystals except KI became pink on being rapidly quenched from 700°C. The optical absorption spectra of unquenched and quenched gold doped alkali halides measured at room temperature are shown in Fig. 1. The peak positions are summarised in Table 1, and the bands are designated as G 1, G2, ----, G8 in order of increasing wavelength. At liquid nitrogen temperature, the peaks become somewhat sharper but the total areas under the peaks do not change. The background absorption decreases appreciably on quenching the NaCI, KCI and KBr crystals. The absorption peaks G3 and G6 (at 239 and 315 nm in NaC1, at - 238 and 320 nm in KCI and at 264 and 405 nm in KBr) disappear completely on rapidly quenching the crystals. Also the absorption under the G8 band (at 575 nm in NaCI, 570 nm in KCI and 6 1 0 n m in KBr) decreases appreciably in intensity on quenching the crystals, and a new band appears at - 5 3 0 nm in these crystals. The peaks sensitive to heat treatment are marked with an asterisk in Table 1. New

9~9

12'0 271

Zl2 ~ ' ~ 2 4 7 /

e-o

NoCt-Au

12'( r~238

8"0

....

~ , ~

0

KCt-Au

-

T 0-

ti~p64

. . . . . . . . . . . . . .KBr-Au .........

........... ~

~20.0 "1

........

J

l°

6.0 3106

4"0

150

I 250

~

KI-Au

I 350

i

I 45o

~

Wavelength,

I 55o

i

I 650

i

I 750

nm

Fig. 1. Optical absorption spectra of the gold doped NaC1, KC1, KBr and KI crystals measured at room temperature. Full lines show the spectra for the unquenched crystals and the dotted lines for the quenched crystals.

peaks (designated as G4 and G5) at 247 and 271 nm in NaCI, at 249 and 272 nm in KCI, and at 238 and 268 nm in KBr, appear in the quenched crystals. There also seems to be a weak band at 224 nm in the quenched KBr crystals. The change in spectra on quenching the KI crystals is not significant. The A u C L - complex in solution gives[15, 16] two charge transfer bands at 228 and 312 nm. We have also measured the spectra of the AuCI4- and AuBr4- complexes in aqueous solution. Our results for AuCI4- agree with the results reported earlier[15, 16]. In AuBr4- we get two bands at 255 and 380 nm. In solution~ containing AuCI4- complexes, these bands are assigned to the 1Azo ~ ZEu and 1A1o ~ IAz~, transitions [16] respectively. These complexes are known to be square planar molecules with a D4h point group symmetry. The peak positions of the G3 and G6 bands, in the three alkali halides, (which disappear on quenching the crystals and reappear on slowly cooling the crystals from higb temperatures) agree closely with the peak

1163

TECHNICAL NOTES

Table 1. Peak positions o f the optical absorption bands observed at room temperature in gold doped alkali halide crystals GI G2 Au + Au + J

Peak positions in nm for the centers G3 G4 G5 G6 G7 AuYv Au + Au + AuY4Gold colloidal

System NaCI-Au KCI-Au KBr-Au KI-Au

Unquenched Quenched Unquenched Quenched Unquenched Quenched Unquenched Quenched

194 194 197 197

239* 212 ' 212

315' 247

271

249

272

238

268 266 266

~238"

570--680* 520-560*

320*

264* 224

G8 Alkali auride band

540-575* 530-550*

405*

600-640* 530-580*

310~ 305

*The bands are sensitive to heat treatment. Y stands for C 1, Br or I. aThe band does not disappear on quenching presumably due to the low solubility of the gold iodide in KI.

positions of the charge transfer bands of the AuCI4- or AuBr4- complexes in solutions. The nature of the centers responsible for the G8 absorption peak is not very clear. The peak position of this band agrees with that of the gold colloids, but its stability at high temperatures is very poor as compared to those of the gold colloids (discussed later) in the crystals. It is possible that a small concentration of non-metallic alkali metal auride is formed which gives rise to optical absorption [17] in the region of the G8 band. The G7 band which is observed in the gold doped crystals quenched from high temperatures seems due to colloidal gold centers (discussed later). We tentatively attribute the other absorption peaks G 1, G2, G4 and G5 listed in Table I to Au + centers. Some metallic gold was'seen floating in the melt when the amount of chlorine in the ambient atmosphere during crystal growth was reduced. This suggests the gold to be present in the melt and in the crystal in the lower oxidation state. Therefore it is unlikely that the gold is present in the form in the crystals. The formation of A u centers (discussed below) on additive or electrolytic coloration and particularly on X-

irradiation also suggests that Au + and not Au 3+ centers are involved. We have also measured E P R spectra of these crystals at room temperature but the results were negative. This supports the view that the Au + centers which are diamagnetic in nature are present in the crystal. H o w e v e r low temperature E P R measurements should be made to confirm these results. The Raman spectra of gold doped KBr crystals prior to and after quenching the crystals are shown in Fig. 2. The results of the Raman scattering measurements for the gold doped crystals are summarised in Table 2. The Raman peaks are designated as G R 1 , G R 2 , ---, and GR8. The peaks GR3, G R 4 , GR5, GR6, G R 7 and G R 8 marked with an asterisk in Table 2 are observed only in the slowly cooled crystals. We also measured Raman spectra of AuCI4- and AuBr4- complexes in solution and our results agree with those reported earlier[18]. The peaks designated G R 3 , G R 4 and G R 5 (Table 2) are those which are also observed in the solution containing the AuCi4- or AuBr4- complexes. The GR3, G R 4 and G R 5 peaks are known to be due to the planar bending (u3, B~.o), antisymmetric stretching (us, B~.~) and symmetiSc

1164

TECHNICAL NOTES

216

12cm-I

KBr- Gold Slit

(b) Optical studies of gold doped colored crystals

II~tt/

r

e JGI 0

[14

.

I. I1

:

a}

._¢ 401~,J

605

,ooo

I

ego

'

doo

Frequency

'

,'oo

'

2.

~;o

'

o

shift, cm-I

Fig. 2. Raman spectra (Stoke's side o n l y ) f o r the gold doped unquenched (curve 1) and quenched (curve 2) KBr crystals.

On additive coloration (for about 2 hr at 560°C) or electrolytic coloration the gold doped KCI crystals show, in addition to the F band, a set of four absorption bands designated as A, B, C and D bands at 306,286,230 and 202 nm respectively. If additive coloration was carried out for about 6 hr, the F band completely disappears and only the A, B, C and D bands are observed. The results obtained with additively colored KBr crystals (quenched prior to coloration) were essentially the same. Since the A, B, C and D bands in the additively colored gold doped crystals have been discussed in detail by Fischer[11], and since our results agree with his work, we

Table 2. Frequency shift from 6328/1 for Raman peak in gold doped KCI, NaCI and KBr crystals System

GRI

GR2

NaCI KCI KBr

63 58 61

143 141 141

Frequency shift (cm -~) for Raman peaks GR3 GR4 GR5 GR6 GR7 184 *c 180 *c 114 *c

326 *cb 324 *cb 194 *c

344 *c 344 *~ 216 *c

535 *b 560 *b 264 *b

685 *u 790 *b 402 *b

GR8 855 *b 980 *a 605 *b

*Disappear in the quenched samples. bBroad bands; peak positions are approximate. CBands also observed in the spectra of AuCI4- or AuBr4- in solution.

stretching (va,Alg) modes of vibrations in the complexes. The other peaks marked with an asterisk (but not with c) appear to be the overtone or combination peaks involving the above mentioned fundamental modes of vibrations of the complexes. The observations, that the slowly cooled alkali halide crystals doped with gold show electronic bands (G3 and G6, Table 1) and Raman peaks (GR3, GR4, GR5, GR6, GR7 a n d GR8, Table 2) characteristic of AuY4(Y stands for CI or Br) complexes and that these bands disappear in the crystals, suggest that on slowly cooling the crystals gold precipitates as MAuY4 (M is for Na and K) in alkali halides a t low temperatures.

will not discuss further the results of the additively colored crystals. The A, B, C and D bands decay as a result of heating the colored crystals at 600°C for different durations. The decay of these bands is accompanied with the growth of a new band at 550 nm. T h e peak position of the 550 nm band shifts to longer wavelengths on prolonged heating of these crystals. The behaviour of 550 nm band is the same as that of Ag colloid band[9] in alkali halide crystals. The peak position of this band observed experimentally in various alkali halides agree with the values calculated theoretically[19] using Savostianova's theory[20]. The band position also agrees approximately with the

TECHNICAL NOTES absorption band due to colloidal gold particles in glass[21]. The band is therefore attributed to gold colloidal centers. The peak positions along with the halfwidth of the colloidal band in various alkali halides are summarised in Table 3.

45°~L~21° 40"0

1~236 35"0 ToE 50-025-0 -

Table 3. Peak positions (nm) and halfwidths (eV) of absorption bands due to gold colloidal center's Crystal doped with gold

Peak position (nm)

Halfwidth (eV)

NaCI KCI KBr

530-545 520-530 540-570

0.55 - 0.44 0"71

1165

:~

258 [ A"'d240

4~c, "f"~

570 _.L

15.o

o

200

I 300

I 400 Wavelength,

I 500 nrn

I 600

I 700

Fig. 3. Optical absorption spectra of the electrolyticaily

If the gold doped KBr crystals are electrolytically colored at 560°C, the results (Fig. 3) are somewhat different from those obtained with additively colored KBr crystals. In addition to the A, B, C and D bands at 312, 294,236 and 210 nm, three new bands at 264, 430 and 5 7 0 n m are observed. The halfwidths of the C and D bands in these crystals are nearly double than for those in additively colored crystals discussed above. The behaviour of the three new bands as a function of heat treatment at 600°C is also shown in Fig. 3. The 4 3 0 n m band continuously decreases and disappears in about 8 hr. The apparent shift of the peak position of this band is due to the overlapping absorption of the 570 nm band. The absorption under 570 nm band becomes maximum after about 4 hr and then starts decreasing in intensity. The 264 nm band increases initially on heating the crystals and then starts decreasing after" 4 hr of heating. The A, B, C and D bands also disappear as a result of prolonged heat treatment and only a strong band at the position of D band (210 nm) persists. It is known that during the process of electrolytic coloration halogen gases are evolved. In the case of KBr crystals, the gold reacts s t r o n g l y with bromine gas to form the gold complexes and the 430 nm band appears to be

JPCS Vol.33 No. 5-N

colored gold doped KBr crystals. Curve I is for crystals colored electrolyticallyat 560°(2. Curves 2-4 are for the colored crystals heat treated at 600*(2 for 1 hr, 4 hr, and 8 hr, respectively. due to the AuBr4- complexes. The position of this band is somewhat different from that (405 nm) observed in the slowly cooled crystals due to AuBr4- complexes. However the 405 nm band is very weak and broad and the difference in peak positions perhaps is not significant. The 264 nm band is the same as observed in uncolored crystals and is due to the Au + centers. The other bands due to Au ÷ centers are masked by the strong absorption due to the C and D bands. The 570 nm band due to gold colloids has already been discussed. On heating the crystals for long times gold diffuses out of the crystal and all bands due to gold impurity decrease in intensity. The band at 210 nm is possibly due to the O H centers which diffuse in the crystal during heat treatment [22]. In gold doped NaCI crystals, colored electrolytically the A, B, C and D bands are masked by the strong absorption due to a highly composite and broad band. On heating the crystals at 600°C for about 10rain and quenching them rapidly, the broad band decreases in intensity and weak A, B, C and D bands are observed. The behaviour of gold

1166

TECHNICAL NOTES

colloid b a n d at 530 n m is identical to the gold colloid b a n d in KBr. O n X-irradiation, the gold d o p e d K C I c r y s tals s h o w the F b a n d and A, C and D b a n d s at 2 0 1 , 2 2 8 a n d 304 n m due to A u - centers (Fig. 4 c u r v e 1). T h e B b a n d w a s v e r y w e a k and t h e r e f o r e c o u l d not b e o b s e r v e d . T h e s e p e a k s are s u p e r i m p o s e d on a b r o a d V b a n d [22]. O n bleaching the crystals b y F light, the V b a n d d e c a y s along with the F b a n d and the A, C 55 0o

45 7

=" u

40

35

30

.8

25

r ~

20

201

~228

7. KOJIMA K., et al., J. phys. Soc. Japan 28, 1227 (1970). 8. MURADOV S. M., MURADOVA M. Kh. and ELANGO M. A., Soviet Phys.-solid State 11, 2553 (1970). 9. KLEEMANN W.,Z. Phys.215, 113 (1968). 10. TAKEUCHI N., J. phys. Soc. Japan 26, 872 (1969). 11. FISCHER F., Z. Phys. 231,293 (1970). 12. MABUCHI T., YOSHIKAWA A. and ONAKA R., J. phys. Soc. Japan 28, 805 (1970). 13. LUSHCHIK N. E. and LUSHCHIK CH. B., Opt. Spectrosc. 8, 441 (1960). 14. TAKEUCHI N., NISHIE S. and KONISHI Y., Japan. J. appl. Phys. 8, 814 (I 969). 15. CSASZAR J., BALOG J. and LEHOTAI L.,Acta Univ. Szegediensis, Acta Phys. et Chem. N.S. 2, 56 (1956). 16. GRAY H. B., and BALLHAUSEN C. J., J. Am. Chem. Soc. 85,260 (1963). 17. SOMMER A., Nature, Lond. 152,215 (1943). 18. STAMMREICH H. and FORNERIS R., Spectrochim. Acta 16,363 (1960). 19. JAIN S. C. and SAI K. S. K., unpublished work. 20. S A V O S T I A N O V A M., Z. Phys. 64,262 (1930). 21. DOREMUS R. H., J. chem. Phys. 40, 2389 (1964). 22. SCHULMAN J. H. and COMPTON W. D., Color Centers in Solids, Pergamon Press, New York (1962).

Js

o

200

I

300

I

400 Wovelength,

~

500

I

600

I

700

nm

Fig. 4. Optical absorption spectra of X-irradiated gold doped KC1 crystals. Curve 1 is for crystals irradiated for 2 hr with X-rays from a Cu target operated at 35 kV and 10 mA. Curve 2 shows the effect of optical bleaching for 1 hr of the irradiated crystals with F light. and D p e a k s b e c o m e m o r e p r o m i n e n t . T h e p e a k height ratio o f the C a n d D b a n d s in the X - r a y e d crystals after c o m p l e t e bleaching o f F b a n d b e c o m e s a p p r o x i m a t e l y the s a m e as in the additively or electrolytically colored crystals. REFERENCES 1. FUSSGAENGER K., MARTIENSSEN W. and BILZ H.,Phys. Status Solidil2, 383 (1965). 2. KRATZIG E., TIMUSK T., and MARTIENSSEN W., Phys. Status Solidi 10, 709 (1965). 3. ONAKA R., et al., Japan. J. appl. Phys. 4, Suppl. 1, 631 (1965). 4. FUSSGAENGER K., Phys. Status Solidi 34, 157 (1969). 5. WILSON W. D., et al., Phys. Rev. 184,844 (1969). 6. KLEEMANN W.,Z. Phys. 214, 285 (1968).

Solid State Physics Laboratory, Lucknow Road, Delhi-7, India

S. C. JAIN

Physics Department, Indian Institute of Technology, New Delhi-29, India

H. K. SEHGAL

J, Phys. Chem. Solids Vol. 33, pp. 1166-1169.

On rigid ion models of ionic solids ( R e c e i v e d 14 J u l y 1 9 7 1 )

RECENTLY N a m j o s h i e t al.[1] h a v e published results o f a lattice d y n a m i c a l study on N a F , N a C I and M g O b a s e d on a rigid ion m o d e l in which the C o u l o m b interaction has b e e n modified b y introducing an effective c h a r g e p a r a m e t e r and short range central and noncentral f o r c e s h a v e b e e n a s s u m e d b e t w e e n n e i g h b o u r s out to the third. T h e results arrived a t h a v e led t h e s e authors to c o n c l u d e that modified rigid ion m o d e l ( M R I ) so obtained has a general applicability to the s y s t e m o f solids studied and has definite c o m p u t a tional a d v a n t a g e s o v e r c u r r e n t models. T h e