Study of the crystalline precipitates in NaCl:CoCl2 by optical absorption and Raman spectroscopy

Journal of Molecular

Structure,

143 (1986)71-74

E1sevierSciencePublishersB.V.. Amsterdam-Printed

71 inThe Netherlands

STUDY OF THE CRYSTALLINE PRECIPITATES IN NaC1:CoC12 BY OPTICAL ABSORPTION AND RAMAN SPECTROSCOPY

A. DE ANDRES and J.M. CALLEJA Departamento de Optica y Estructura de la Materia e Instituto de Fisica de1 Estado Solid0 (CSIC). Universidad Autonoma de Madrid, 28049 Madrid, Spain. I. POLLINI and

G. BENEDEK

Consiglio Nazionale delle Ricerche, Dipartimento di Eisica dell'llniversita, Milano, Italy.

ABSTRACT Heat treatments applied to NaCl:CoC12 mixed crystals have shown the presence of three kind of crystalline precipitates. Both absorption and Raman measurements indicate the presence of CoC12 in the as grown crystals, whereas upon quenching from different temperatures a tetrahedrally coordinated phase (probably Na2CoC14) and an octahedrally coordinate one (the Suzuki phase Na6CoC18) appear. From the resulting crystal field parameters and the phonon frequencies obseE ved both in absorption and Raman spectra informationon the structure (particularly the Co-Cl distance) and the dynamics of the different mixed salt precipitates in obtained, in good agreement with previous results. The study is partis lly extended to KCl:Co, NaBr:Co and NaBr:Mn

The different types of aggregates of some transition metal ions introduced in alkali halides as impurities havebeen extensively studied by crystal field (CF) (ref. 1-5) and Raman (ref. 6,;) spectroscopies.This work deals with the nature of the precipitates formed in NaCl, KC1 and NaBr crystals doped with co2+ and Mn2+ ions. The coordination and symmetry of the ions the crystallinity of the precipitates and their formation and dissolution depending on different heat treatments are all aspects studied here by CF and Raman experiments. The absorption measurements were made in a Cary 17 spectrophotometerat 4 K and the Raman spectra were taken with the 4880 A line of an Ar' laser at RT. All the single crystals are gmwn by Cscchralskimethod from suprapur powders of NaCl with about 2000 ppm of CoC12 or MnC12 in an inert atmosphere except for KCl:Co. In NaBr:Co the content of impurity was much weaker than in the other crystals. Some samples were quenched from temperatures ranging from 100X to a value 10% under the melting point in order to change the precipitation state of the impurity. Fig. 1 shows the absorption spectra due ro the different coordination states of co2+ in NaCl and NaBr. 0022-2860/86/$03.50 0 1986 Elsevier Science Publishers B.V.

72

25

20

15

7 2) (103ctrr')

6

5

Fig. 1. Absorption spectrum of NaBr:Co and NaCl:Co crystals as grown (a) and quenched from 350% (bj and 750X (c).

In the case of the NaCl:Co three types of precipitates are detected: in the as grown samples Fig. 1.a the spectra are identical to that of CoC12 with vi-1 bronic progressions of 151 and 250 cm . These phonons are also observed in the Rarnanspectrum (Fig. 2.a). The band around 15000 cm-1 does not belong to the CoC12 structure but corresponds to the tetrahedrally coordinated phase (probably Na2CoC14) already reported by Reynolds et al. (ref. 1). This band reaches it maximum intensity for a quenching from 350% (Fig. 1.b) -1 appearing new progressions of 274 cm that are again observed in the Raman spectrum (Fig. 2.b). This frequency corresponds to the breathing mode of a CoC142_ tetrahedra. For a quenching temperature around 7509C (Fig. 1.~1 a new phase is formed whose absorption spectra is similar to that of CoC12. Vibronic progressions of -1 215 and 178 cm are observed on the 2A1(G) and 2T2(F) bands respectively. The corresponding Raman spectrum (Fig. 2.~) shows phonons at 123, 183 and 206 cm-l. Their selection rules indicate that the two first phonons are of T2g symmetry and the third one is A . These results are in excelent agreement with those g measured for the Suzuki phase (SP) in several other systems (ref. 7). The absorption bands of the three phases are collected in the Table 1 together with the corresponding theoretical values obtained from the best fit to the CF data (ref. 9). The Dq and

B parameters are

also

given. On the other hand, the

13 existence of vibronic progressions indicate that the three observed phases are value obtained for the SP one can evaluate

crystalline (ref. 5). From the D

(ref. 5) the Co-Cl distance, whizh turns to be similar to that of CoC12. This indicates that the halogen relaxation towards the impurity is weaker in this case than for the NaCl:Ni SP (ref. 5).

I ’

I

1

‘183 2061

c)

350 3

250 (cm-l

’ 123

150 1

Fig. 2. Raman spectra of NaCl:Co. a, b and c like in Fig. 1.

The NaBr:Co crystal shows a typical tetrahedral CF spectrum (Pig. 1) similar to previous results (ref. 10). In the 4T1(F) regicntwo peaks at 5699 and 5874 -1 cm appear which are thought to belong to a vibronic progression similar to that of NaCliCo. In fact their difference (175 cm-') agree very well with the value expected from the NaCl:Co progression (274 cm-1 ) and the halogen mass ratio. These precipitates are therefore probably crystalline clusters of a mixed salt (Na2CoBr4 or similar). The NaBr:Mn crystal could only be studied by Raman, because of the weakeness of the absorption bands. The phonon frequencies and symmetries observed (Ag:126 -1 -1 cm ; T2g:115,S0 cm ) indicate unambiguously the presence of SP precipitates 1.1 ke in NaEr:Cd (ref. 7). Finally the KCl:Co crystals showed the well known spectrum of tetrahedrally coordinated Co (ref. 3,4). In this case neither vibronic progressions nor Raman bands were observed. this is interpreted as an indication of non crystalline prg cipitates. No significant changes were observed when quenching these crystals. The values of Dq and B are given in Table I, as-for the NaBr:Co crystals. They substantially agree with previous results (ref. 3.4) except for the B value,

74 which is higher than that of Nasu (ref. 3). The trend Do(NaC1)

Do(KC1) >

> Dq(NaBr) is clearly observed. TABLE 1 Tetrahedral coordination NaC1:Co Dq=320, B=Ell

NaBr:Co Dq=300, B=784 Transition

Vth

Y

exp

V

KC1:Co Dq=327, B=804

th

2/ exp

?"th

?_.) exp

16665

16562

16581 16297

16426 16300

16334 15937

16260 15918

15859 15641

15843 15601

2T2(GJ 15963 15835 2Al(G) 15621 15711 --------------------_--_-_--_-_ 15323 4T1(Pl 15260 15138 15045 15081 -----------~-_----------------2Tl(G) 14725 14526 14300 14368 ----------------------------_-_

15672

15740

15341 14900

15288 14903

15094 14656

15110 14706

2E(Gj 14085 14085 ________-_____----------------5635 5699 4TllF) 5058 5018 4695 __--

14568 6026 5474 5077

14577 6361 5397 4614

14437 6125 5687 5250

14438 _-____-__ _--__

Octahedral coordination NaC1:Co coc12 D9=702, B=789 Transition

th

exp

SP D9=713, B=758

th

P

19960 19802

19210

199;o 19120 18939

2Al(G) 18618 19011 --_-_-__-_--_-_--------_ 17724 17857 4T1(P) 17380 17437 17197 17197 ~--------_-_-__---_-_____

17998

17601

17289 1705‘0 16786

17153 17007 16717

6653

6720

2T1(Pl +

2T2(Fj

19581

6580

6580

TABLE 1 Theoretical and experimental vflues of the CF transitions. have been used. The values C=4B and 5 =450 cm REFERENCES

6 7 8 9 10

M.L. Reynolds, W.E. Hagston and G.F.J.Garlick, Phys.Stat.Sol.2, 97 (1968). H.L. Schlafer and G. Gliemann, "Ligand Field Theory", Wiley, London 1969. T. Nasu, Phys. Stat. Sol. 70, 97 (1975). Ath. Trutia and M. Voda, J.Chem.Phys.64, 2715 (1976). G.Benedek, J.M.Calleja, R.Capelletti and A.Breitschwerdt,J.Phys.Chem.Sol. 45, 741 (1984). J.M.Calleja, A.Ruiz, F.Flores, V.R.Velasco and E.Lilley, J.Phys.Chem.Sol. d, 1367 (1980). A.de Andres and J.M.Calleja,,Sol.Stat.Comm. 48, 949 (19831. I.Pollini, G.Spinolo and g.Benedek, Phys.Rev. 82, 6369 (1980). A.D.Liehr, J.Phys.Chem.67, 1316 (1963). M.E.Hills, J.Phys.Soc.Japan,l9, 760 (1964).