MÖSSBAUER MEASUREMENTS ON Hg-Pt-ALLOYS USING THE 158 keV TRANSITION IN 199Hg

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MÖSSBAUER MEASUREMENTS ON Hg-Pt-ALLOYS

USING THE 158 keV TRANSITION IN 199Hg

W. Wurtinger

To cite this version:

W. Wurtinger. MÖSSBAUER MEASUREMENTS ON Hg-Pt-ALLOYS USING THE 158 keV

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MOSSBAUER

MEASUREMENTS ON ~ g - ~ t - A L L O Y S

USING THE 158 keV TRANSITION IN lg9Hg

W. WURTINGER

Institut fiir Kernphysik, Technische Hochschule Darmstadt

Rbum6.

-

Avec l'aide de la technique de I'inthgration du courant, l'effet Mossbauer de la transition 512-112 E2 2 i 158,4 keV dans 199Hg 6tait observ6.

La recherche des alliages Hg-Pt ayant une concentration infkrieure

ZI

65 % at. Hg montre une attenuation relativement petite de la liaison impuretk-porteur

ZI

basse concentration de mercure. I1 en r6sulte une temp6rature de Debye OD = 211 f 4 K pour des concentrations de mercure

< 1 %. Entre 20 et 65 % at. Hg existent deux composhs de mercure mktalliques, HgPt et HgSt.

Le dernier montre une subdivision quadripolaire dans le spectre MB de

Abstract.

-

By aid of the current integration technique the up-to-date unknown MB effect of the 512-112 E2 transition at 158.4 keV in 199Hg was observed.

Using this transition Hg-Pt alloys in the concentration range up to 65 atomic % mercury at 4.2 K were investigated. For mercury concentrations < 1 % the alloy system show a relatively small weakening of the impurity-to-host binding at low mercury concentrations, resulting in a Debye temperature of OD = 211 f 4 K. Between 20 and 65 at. % Hg there exist two intermetallic mercury compounds, HgPt and HgzPt. The latter shows a quadrupole splitting in the MB spectra of eQ Vzz/4 = (1.83 & 0.03) mm s-1.

1. Introduction. - The MB effect in the 158.4 keV

transition in Ig9Hg has been measured in a number of compounds and alloys. Due to the high energy of the transition and the small binding forces of mercury compounds the resonance absorption is quite small. We were able to detect this absorption with reasonable statistical accuracy only by means of the current integration technique because the half-life of the source lg9Au is 3.14 d.

In this report we will concentrate on one aspect of the results and will discuss the question of bonding of the mercury atom in a platinum host. Due to the high energy of the resonance transition the Debye- Waller-factor (DWF) is extremely sensitive to small changes in bonding.

2. Experimental.

-

The measurements were performed at 4.2 K using a Helium bath cryostat in transmission geometry. As a source, a Pt metal foil of 400 mg, with lg8Pt to 95.83

%

enriched, was activated by the lg8Pt(n, y)lg9Pt reaction to 4 Ci. Figure 1 shows the decay scheme of lg9Au, which is populated by the decay of lg9Pt with a half-life of 30 min.

Using a 30 cm3 Ge(Li) Diode as detector the solid angle of the experimental arrangement was 5

%

and the counting rate lo9

CIS.

This produces an average detector current of 3.4 FA, which was treated by the current integration technique described in detail in

FIG. 1. - Decay scheme of l99Au which is populated by the decay of 199Pt with a half-life of 30 min.

ref. [I] and references cited therein. I n this technique the detector current is converted to a voltage which is brought to the input of a voltage to frequency converter (VFC). The pulses from the VFC are then fed to a multichannel analyzer in the multiscaler mode.

In the experiment the absorber was moved by a recently developed electromechanical drive-system [I]. This includes a position feed-back by which a high

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C6-698 W. WURTINGER

stability and accuracy in the source absorber detector geometry could be achieved.

For the Hg-Pt-absorber-alloys the following components were used : 99.99

%

pure Hg and Pt of natural composition. The samples were prepared reducing both metals from a hydrous solution of HC1 with Mg. The amorphous mixture of Hg and Pt was then heated to 200-450 OC for 24 h in an evacuated glass tube. After preparation the mercury content of the alloys was verified with an atomic absorption spec- trometer and from each alloy powder photographs

were taken. The weight p Ax of the absorbers was about 1 g/cm2, which leads in the case of Hg-Pt-alloys to an effective absorber thickness t in the range of

3. Results.

-

According to Bauer et al. (2) the alloy system Hg-Pt with less than 18 at.

%

Hg consists under normal condition of a solid solution with its lattice constant increasing with increasing mercury content.. Above 18

%

in addition an intermetallic compound HgPt was observed with an AuCu I

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structure. At higher mercury concentrations ,two further compounds were formed : Hg2Pt, with a structure similar to A12Cu, and above 65 at.

%

mercury Hg,Pt, which is cubic. In the concentration range between 50 and 65 at.

%

mercury Hg2Pt was found together with HgPt.

In figure 2 the Mossbauer spectra of Hg17Pt8, and Hg,,Pt3, are shown. In the spectra of HgI7Pt8, a single resonance line near zero velocity is seen, whereas the other one shows a quadrupole splitting superposed by a single line. This could be explained as a mixture of HgPt and Hg,Pt at this concentratio%. The tetragonal structure of Hg2Pt leads to a big quadrupole splitting of

whereas in HgPt no quadrupole interaction could be observed. This is consistent with the small tetragonal distortion from cubic structure in this compound.

From the area under the resonance lines of the spectra the recoilless fraction for the various alloys was determined. The spectra were fitted with Lorent- zian line shapes, no deviation from this form within the statistical error was found.

Figure 3 shows in the upper part the dependence of F = fA

.f,.

6, on the mercury concentration. The

value of F is defined as the product of the DWF's from source f , and absorber fA multiplied by an

experimental factor 6, arising from the integration technique.

FIG. 3.

-

The upper part shows the product F =

f~.fs.a~

of the Debye-Waller factors from source

fs

and absorbers f~

multiplied by the experimental factor 6~ as a function of mercury concentration C H ~ . In the middle and lower part the dependence of the experimental line width TI2 and the measured isomer

shift 61s from the mercury concentration Crrg is shown.

The f a c t o ~ 6, results from the charge distribution f (q) in the Ge(Li) detector and is given by the equation

Here f,(q) is the charge distribution of the resonant part of the y-spectrum. The determination of f, from the pulse height spectrum in the case of Ig9Au gives 6, = 0.42 +_ 0.07. If we extrapolate the values of F in figure 3 to vanishing mercury concentration the mercury atoms find the same surrounding in source and absorber and therefore should have the same DWF's : With the above 6, we derive

In the middle and lower part of figure 3 finally the measured isomer shift and the dependence of the experimental line width r / 2 from the mercury concentration is given. From r / 2 = 0.37

+

0.03 one gets a lower limit for the half-life of the 158.4 keV level in lg9Hg to be TI,, = (2.33

+

0.19) ns. This is in very good agreement with the value TI,, = (2.37 f 0.07) ns found by y-y coincidence [3]. This implies also that the natural line width in the source and in the absorber, at low concentration, was achieved.

4. Discussion.

-

Measurement of the DWF of impurity atoms using the MB effect were performed for various hosts and impurities by several authors [4- 101. There also exists a number of theoretical papers concerning the effect of mass change, changes in the impurity-to-host binding and deviations from har- monic forces on the frequency spectrum of the impurity [ll-171. With the assumption of central forces to the nearest neighbours various approximations have been made.

Marshall [12] assumes that the frequency spectrum is similar to that of the host with some shift in the maximum frequency. Therefore he gets for a Debye model a simple relationship between the Debye temperature of the impurity 86 and that of the host

0, :

where M, M' are the masses of the host and impurity atom and A, 1' are the corresponding force constants. As shown by Dawber and Elliot [13] this relation gives a good approximation at low temperatures.

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C6-700 W. WURTINGER

where

I

~ ( o )

l2

is the relative amplitude of the impu- model for the frequency spectrum g(o) of the host rity in a band mode given by the equation as a function of ;l'/l. Comparing the calculated values

of 8'(- 1) with the experimental Debye temperature

6; according to the relation

with one yields a force constant change of

M A'/,% = 0.80 $. 0.02

p(o) =

-

1 + 2 < ( I - $ ) ,

M' om for C,, < 1

%.

Figure 5 shows the calculated ampli-

tude

I

~ ( o )

l2

as a function of o/oD for this ratio. where on, and g(w) are the maximum frequency and

the frequency spectrum, respectively. One may obtain localized modes for the impurity atom if for any

a,,,

>

o the relation

js satisfied.

With the equation for the DWF in the limit T + 0

using the Debye model, the Debye temperatures of the Hg-Pt-alloys in the region of the solid solution can be determined and they are shown in figure 4.

FIG. 4. - The Debye temperatures for the solid solution derived from the measured F values with 6~ = 0.42 f 0.07 and

fs

= (3.78 & 0.37) 10-3 as a function of mercury con-

centration are plotted.

For the Debye temperature of the impurity for

CHg < 1

%

one derives 8, = 211

+

4

K.

With 9, = 233 K

for the Pt host lattice [6] we obtain from Marshall's formula the ratio of the average binding forces A'il = 0.84

+

0.03.

Using Mannheim's theory eq. (2) gives the temperature 6'(- 1) for the impurity atom if g(o) and

~ ( o ) are known. Therefore the relative amplitude ~ ( o ) was calculated from eq. (3) with a Debye

I I

FIG. 5.

-

The relative amplitude

I

~ ( o ) 12 for these case of a

Hg impurity in Pt calculated with 1'112 = 0.80 from eq. (3) as

a function of w / w ~ , assuming a Debye spectrum for g(o).

For I'll = 1 one gets

1

~ ( o ) 12 = 1 indicating that impurity and host atoms vibrate at the same frequencies.

The good agreement between the two values obtain- ed from Marshall's formula and Mannheim's theory shows that for T -+ 0 eq. (1) gives a good approximation also in the case of mercury in platinum.

With increasing mercury content up to 20 at.

%

Hg the Debye temperature increases whereas at higher concentrations above 20 at.

%

Hg a sharp decrease of F in figure 3 is observed. The broadening of the line width r/2 as a function of mercury concentration and the isomer shift

a,,

in figure 3 show also a change in the behaviour above 20 at.

%

Hg. This indicates that we have also at 4.2 K a solid solution up to

18 at.

%

Hg whereas above intermetallic compounds were formed. The occurrence of these compounds is responsible for the decrease of F in the region

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two components and the line width shows no remar- kable broadening, therefore the part of solid solution must be very small. This indicates that we have only the two components HgPt and HgzPt in this region as in the case under normal condition. With a value of 56

%

Hg2Pt and 8

%

HgPt for the alloy Hg64Pt36 We obtain the Debye-Waller-factors for the two compounds from the analysis of the MB spectrum to fA = (8.44

+

1.85) for Hg,Pt and

for HgPt. Using eq. (5) the resulting Debye temperatures are 8, = (167

+

4) K (Hg2Pt) and

We see that the small F value for the alloy with

x = 0.40 indicates that it should also contain a component of Hg,Pt, whereas the F value for the alloy with x = 0.30 is found between the ones of HgPt and the solid solution.

The broadening of the line width with increasing mercury concentration in the solid solution may be caused by the increase of local field gradients. In the region with x > 0.20, where the part of solid solution decreases the unresolved quadrupole split spectrum of Hg,Pt and the different isomer shift of the several components are responsible for the broadening.

MB isomer shift measurements on Hg,F, and HgF, [18] and isomer shift measurements on muonic atoms 119, 201 yield a positive change of the mean-

square nuclear charge radius in lg9Hg. Therefore the positive isomer shift for all HgxPt,

-,

alloys shows that the s-electron density must increase with increasing mercury concentration. Because the source was lg9Hg in Pt with a mercury content x

<

0.01 the d-band in the alloys should be- filled up even at small mercury concentrations. This is in agreement with knight shift measurements on HgPt and Hg,Pt [21] showing that the d-band is filled in these compounds.

5. Conclusion.

-

The MB measurements on lg9Hg in HgPt alloys show a relatively small weakening of the impurity-to-host binding at low mercury content. Higher mercury concentrations lead to an increase of the bonds in the solid solution. The values for the isomer shift indicate that even at low mercury concentrations the d-band should be filled up. Concern- ing the intermetallic compounds HgPt and Hg,Pt with tetragonal structure only Hg2Pt shows a quadrupole splitting in the Mossbauer spectra, with a Debye temperature 40 K below the value of HgPt.

Acknowledgments.

-

I would like to thank Pr. Dr. E. Kankeleit for his continuous support and helpful discussions, Dr. R. Link for a critical reading of the manuscript and informative comments and Ing. H. Arest6 for preparing the samples.

Supported in part by the Gesellschaft fiir Kern- forschung mbH, Karlsruhe.

References

[I] KANKELEIT, E., Proceedings International Conference on Mossbauer Spectroscopy, Cracow 2 (1975) 43.

[2] BAUER, E., NOWOTNY, H. and STEMPFL, A., Monafsh. Chem. 84 (1953) 692.

[31 GRODZINS, L., BAUER, R. W. and WILSON, H. H., Phys. Rev. 124 (1961) 1897.

[4] STEYERT, A. W. and TAYLOR, R. D., Phys. Rev. 134 (1964) A 716.

[51 NUSSBAUM, R. H., HOWARD, D. G., NEES, W. L. and STEEN, C. F., Phys. Rev. 173 (1968) 653.

[6] KITCHENS, T. A., CRAIG, P. P. and TAYLOR, R. D., MZiSS- bauer Eflect Methodology 5 (1969) 123.

[7] O'CONNOR, D. A., REEKS, M. W. and SKYRME, G., J. Phys. F2 (1972) 1179.

[8] h R 1 , R. K. and CONNO NOR, D . A., Proceedings Interna- tional Conference on Mossbauer Spectroscopy, Bratis- lava 2 (1973).

191 SITBK, J., Proceedings International Conference on Moss- bauer Spectroscopy, Bratislava 2 (1973) 451.

[lo] SITEK, J., CIRAK, J. and LIPKA, J., J. Physique Colloq. 35 (1974) C 6-379.

[Ill VISSCHER, W. M., Phys. Rev. 129 (1963) 28. [12] MARSHALL, W., quoted in ref. [13].

[13] DAWBER, P. G . and ELLIOTT, R. J., Proc. R. Soc. A 273 (1963) 222.

[14] LEHMAN, G. W. and DEWAMES, R. E., Phys. Rev. 131 (1963) 1008.

[15] MARADUDIN, A. A., Rep. Prog. Phys. 28 (1965) 33 1. [16] HOUSLEY, R. M. and HESS, F., Phys. Rev. 146 (1966) 517. [17] MANNHEIM, P. D., Phys. Rev. 165 (1968) 1011.

[18] WURTINGER, W. (to be published).

[I91 WALTER, H. K., Nucl. Phys. A 234 (1974) 504.

1201 BACKE, H., KANKELEIT, E. and WALTER, H. K., MBssbauer Isomer Shifs. Eds. G. K . Shenoy and F. E. Wagner (North Holland, Amsterdam) 1976, chap. 13.