RELAXATION MEASUREMENTS IN ZnS : Fe2+ FROM ABSORBER AND SOURCE EXPERIMENTS

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RELAXATION MEASUREMENTS IN ZnS : Fe2+

FROM ABSORBER AND SOURCE EXPERIMENTS

P. Bonville, C. Garcin, P. Imbert, G. Jéhanno

To cite this version:

P. Bonville, C. Garcin, P. Imbert, G. Jéhanno. RELAXATION MEASUREMENTS IN ZnS : Fe2+

FROM ABSORBER AND SOURCE EXPERIMENTS. Journal de Physique Colloques, 1980, 41 (C1),

pp.C1-237-C1-238. �10.1051/jphyscol:1980178�. �jpa-00219747�

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JOURNAL DE PHYSIQUE Colloque C1, suppldment au

n

O 1, Tome 41, janvier 1980, page

C1-237

R E W T I O N MEASUREMENTS I N

ZnS :

Fe 2+ FROM ABSORBER

AND

SOURCE EXPERIMENTS

P. Bonville, C. Garcin, P. Imbert and G. JBhanno

DPh. G/PSRM, C. E. N. SacZay, B. P. n02, 92290 Gif-sur-Yvette (France)

I. INTRODUCTION. Striking differences exist between the MEssbauer spectra of the ~ e ion in ~ + dilute Z ~ S : 5 7 ~ e absorbers and in Z ~ S : 5 7 ~ o sources [Iy2]. Namely in low temperature source experiments (TC5K)two excited electronic levels of Fe 2+ in ZnS remain populated out of thermal equilibrium after the decay of the radioactive parent and contribute two slow relaxation quadrupole doublets to the ~Sssbauer spectrum. Detailed information about the vibronic properties of Fe 2+

12]

have been obtained from the study of these extra contributions which do not appear in the absorber spectra.

The present paper deals with electronic relaxation measurements on Fe2+ in ZnS, obtained from the absorber and source results by two different methods, which relate respectively to two distinct

temperature ranges.

If. ABSORBER EXPERIMENTS. Substitutional Fe 2+

impurities in ZnS present a ground orbital doublet E which splits into five almost equidistant spin-

g

orbit levels : TI (I), r4(3), r3(2), r5(3),

r2(1).

Isolated Fe2+ impurities give a single absorption line at all temperatures, but the linewidth goes through a sharp maximum at near 8K [I1. This line broadening is due to relaxation effects involving the first excited spin-orbit level, the triplet T4, whose energy is K=15 cm [31. Whereas the ground singlet TI corresponds to a zero electric field gradient (EFG), the triplet T4 should give a quadrupole splitting E

( r

⁾when relaxing slowly

Q

4

between its strain separated sublevels

r

4xy r4y and 14z[41. However, contrary to the case of

CaF2 : Fe

'+

^[51,this quadrupole doublet does not appear in the Zns:Fe2+ absorption spectra because the triplet T4 is much too depopulated at temperatures where the relaxation is slow.

In order to interpret the line broadening, we used a model of a randomly fluctuating E F G based on a stochastic relaxation theory of Blume and Tjon 161

,

as for CaF2:Fe

'+

[51. Two different types of relaxation processes can affect the lineshape in a different way :

1. The relaxation processes acting within the excited

triplet

r4 ,

which correspond to the transition rate W from any sublevel to another.

2. The relaxation processes between the triplet

T4

and the ground singlet TI, which correspond to the transition rate W' from any sublevel of

r4

^{to rl,or}

inversely to the rate wlexp{-K/~~T} from TI to T4.

The only adjustable parameters of the relaxation lineshape are the transition rates W and W'.

The r e s u l t s are as foZZows :

i) Fig. 1 shows two attempts to fit the eelaxation spectrum at 8K, successively in terms of W(with W1=O) and of Wt(with W=O). The result unambiguously shows that the dominant relaxation rate i s W ' , contrary to the case of CaF2:Fe 2+,

.

Further fits using the two parameters W and W' showed that the value of W' is at least 10 times larger than W.

ii) The W' values fitted from the absorber experiments are represented by black circles on fig. 3 at four different temperatures. It is seen that these vaZues are roughly compatibZe with a

T

7 variation:

W'

(s-')=~I

.35 T~(K) (straight dashed line). This behaviour is characteristic of Raman processes when the temperature is not much lower than the separation K betweep the relevant levels.

iii) Extrapolating this law towards lower temperatures shows that the transition rate W' from the

r

₄

sublevels to

r

becomes inferior to the inverse

1 6 -1

nuclear life time T=l/~=7.09 10 s below about 5K.

An interesting consequence follows for the source experiments : the thermalization of the level T4 of Fe2+ after the decay of 5 7 ~ o in a Z ~ S : ~ ~ C O source requires a time longer than the emission time T when T 3 5 K . This result unambiguously proves that the external doublet which appears at about 5 K in the Z ~ S : ~ ~ C O source spectra does come from the level i-

111

⁴

out of thermal equilibrium

.

11. SOURCE EXPERIMENTS. Near zero Kelvin the NEssbauer spectrum emitted by Z ~ S : 5 7 ~ o contains

3 distinct contributions from substitutional Fe 2+

ions ['"]: a central line due to the ground singlet TI, an external quadrupole doublet (E

(r

) =

-

¹

^Q

⁴

2.99k0.04 m . s ) due to the excited triplet

r4

and an internal quadrupole doublet (E

(r

^)=I .30f0.04mm.s-~)

Q 5

Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1980178

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C1-238 .JOURNAL DE PHYSIQUE

due t~ the second spin-orbit triplet

r

whose energy

'-1 131 5

is E(r5)=45. cm : -

-

The relative intensities P(r4) and P(r5) of the doublets are given versus temperature in fig. 2. The maximum variation occurs around 4 K for P(r4) and around 2.5K for P(rS).

Let PO(r4) be the initial population of the level

r4

just after the decay of the radioactive parent, and P

(r

) the Boltzmann population of this level. It

B 4

can be easily shown that the transition rate W1 can be expressed as [I

I

I IT , p0(r4)-p(r4)

W'= 1+3 exp (-~/k~l'T P(r4)

-

^PB(r4) ( 1

The value P

(r

)".13.7% is given by the limi- 0 4

ting value of P(T4) which is observed below 2 K (fig. 2) where the condition W'<<l/T is fulfilled.

The values of W' (open circles, fig. 3) which are calculated by the relation ( I ) from the P(rL) measurements between 3.1 and 4:2K are compatibli!

with the absorber measurements made at higher temperatures ( 7 s T ~ 1 2 ~ ) . However the thermal,

-

variation of W' departs from the T' law towards the low temperatures where T < < K , and it can be approxima- tely represented by the law W1(sc1)=21467 T between 4

3.1 and 4.2KwhichY using the relation (I), gives in turn the P(T4) curve on fig, 2.

A similar treatment can be applied to the population of the triplet

r5.

As T<<E(T5), the relation equivalent to ( 1 ) becomes simply :

wW=(l

/T)CP~(~~)

-

^p(r5)1 ⁽²⁾

where W" is the depopulation rate of 'each sublevel of

r5

towards the lower.leve1

rl

(or possibly

r3)

It is to be observed that the transitions

r

₅

+r

₄

are not active because P(r4) does not increase between 1.3 to 2.5K when P(T5) considerably decreases (fig. 2). From the P(r ) measurements,

5

using the limit value PO(T5)2.31 %

,

we deduced the values of W1' between 1.7 and 4.2K (fig. 3). These values again agree with a T~ type law

6 4

(w"(s-')=o. 15 10 T ) which, in turn, gives the P(F ) curve drawn on fig. 2.

5

The two quadrupole doublets in the source spectra thus show very similar thermal behaviours, leading to similar W' and W" variations, which is additional evidence that they originate from the two closely related spin-orbit triplets T4 and T5.

REFERENCES

(I) C. GARCIN, P. IMBERT and G. JEHANNO, Solid St.

Corn. 21, 545 (1977).

(2) P. IMXTRT, C. GARCIN, G. JEHANNO and A. GERARD, Int. Conf. Miissbauer Spectr., Bucharest 1977, Proceed. 11, 123.

(39 J. VALLIN, G. A. SLACK and C. C. BRADLEY, Phys.

Rev.

2,

4406 (1970).

(4) F. S. HAM, J . de Phys. 35, Colloq. C6-121 (1974).

(5) P. BONVILLE, J.

CHAPPERF

C. GARCIN, P. IMBERT and J. R. REGNARD, this Conference.

(6) 3. A. TJON and M. BLUME, Phys. Rev.

165,

456

(1 968). /

Fig. 1

-

Fitted relaxation lineshapes at 8K.

a) W=O, W'#O b) W#O, W1=O. Doublet _:~ e pairs ~ *

Fig. 2

-

Relative intensity measurements in source spectra. P(r4) : ext. doublet ; Pus) : int. doublet

Fig. 3

-

Transition rates versus tem erature.

6

t

Wr:r

+rl.

W1':r5+TI. Full lines : T variations Stralght dashed line : T~ variation.