Positron lifetimes in rare earth oxides

Positron lifetimes in rare earth oxides

Volume 34A, number 7 PHYSICS POSITRON LIFETIMES LETTERS IN RARE 19 April 1971 EARTH OXIDES K. P. SINGH, R. M. SINGRU Department of Physics, ...

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Volume 34A, number 7

PHYSICS

POSITRON

LIFETIMES

LETTERS

IN RARE

19 April 1971

EARTH

OXIDES

K. P. SINGH, R. M. SINGRU Department

of Physics,

Indian

Instituteof Technology,

Kanpur,

India

V. G. KULKARNl, R. G. LAGU and G. CBANDRA Tata Institute

of Fundamental Research,

Bombay-5,

India

Received 10 March 1971 The data of positron lifetimes with their intensities in La 03, Nd905 Smj03, Dy909and Yb9O3are presented. A possible origin for the long components ~9 an2 ~4 is suggeste . Recently considerable interest has been shown in the annihilation of positrons in ionic crystals, particularly halides and oxides. The life-times of positrons in various oxides have been studied by many workers [l-5]. Most of the oxides exhibit two longer life-times besides 71 N 0.4nsec. These longer life-times are, 72 _ 1-5nsec and 74 N 20-80nsec. This longer component is denoted as 73 by’ many workers. We prefer the nomenclature 74 to avoid confusion with the three photon life-time ~3. The origin of 74 is not properly understood yet. Sen and Patro [2] have shown that 74 observed in Al203 arises out of two photon decay. They have observed 74 by two different methods, with the standard slow-fast coincidence set-up and also with triple slow double fast coincidence set-up designed to detect only two photon events and have obtained the same value (63 nsec) for 74 in Al2O3. From this result they conclude that 74 is associated with two photon decay and not with three photon decay. They have, however, not established any definite mechanism for the origin of 74. Paulin and Ambrosino [l] suggest some surface phenomena for the origin of 74. On the other hand Kulkarni et al. [3] have suggested that ortho-positronium atoms quenched in the intercrystallite regions in As406 could give rise to 74. A systematic study of a series of oxides with a common factor was therefore undertaken with a view to examine whether the positron lifetimes and their intensities show any dependence on the rare-earth ion properties. With this aim positron lifetimes in five rare earth oxides, La203, Nd203, Sm203, By203 and Yb2O3 have been studied and the data presented in table 1. Of these oxides only La203 was studied earlier [2].

All the samples were prepared in the form of pellets, under a pressure of 1 ton/inch2, from analar grade powders. The positron source, 5~ Ci of 22NaC1, sealed between two thin nickel foils (0.1 mil), was sandwiched between the pellets and the lifetimes spectra were recorded by using the standard slow-fast coincidence technique described elsewhere [3]. The detectors used were 2” x l:“, Pilot B, plastic scintillators coupled to 56 AVP photomultiplier tubes. The apparatus was arranged to ive a prompt resolution curve obtained with 6%Co with a full width at half maximum of 1.2nsec. The samples were studied at room temperature in vacuum by continuously evacuating the glass tube containing the source sample assembly. Several spectra were taken to reduce the statistical error and the lifetimes and intensities were obtained by a computer fit. The results given in table 1 show that all the oxides exhibit three lifetimes, the inner component ~1 fi: 0.4nsecs, 72 and 74. It is seen that except for Yb203, the values of T and r4 do not show any systematic variation Prom lanthanum to dysprosium, even though the ion size varies considerably [6]. Yb203 exhibits a much smaller 72 with a larger intensity, while the value of 74 is essentially the same as for other oxides in this series. Recently Bartolaccini et al. [4] have resolved the inner lifetime component of positrons annihilating in some oxides into two components, Ti - 0.25 nsec and r; - 0.5 nsec. These lifetimes are explained by them in terms of a model involving positron states in ionic media and they are able to correlate the systematic variation in these lifetimes with the ionic properties of the solids. Our results seem to suggest that the longer lifetimes 72 and 74 in these 377

PHYSICS

Volume 34A. number 7

-__-_ Oxide

Positron 7 l(nsec) -____--

lifetime

7 2(nsec) _... ..__~

~____.

I2 (o/c)

LETTERS

Table 1 data in some rare-earth -___ r4(nsec) ‘4 (o/c)

19 April

oxides Lattice

-_.____ constants

[6] 6.1299 d Hexagonal

La203

0.4

4.4 f 0.8

2.6 f 0.8

12

1

1.0 f 0.3

U = 3.9373 A,

Nd203

0.4

3.5

0.7

2.5 + 0.5

14 f 1

1.0 5 0.3

a = 3.829 A,

SmzCs

0.4

4.0 i 0.8

2.2 f 0.6

15 f 1

1.0 f 0.3

a = 3.905 A, c = 6.215

Dy2°2

0.4

5.0

1.2

2.0 f 0.6

14 f 1

1.0 f 0.3

a = 3.82

Yb203

0.4

1.6 i 0.2

7.0 * 3.5

12 f 1

0.8 f 0.3

a = 13.73

l

l

l

C =

c =

6.002 h Hexagonal

A Hexagonal

A, c = 6.115 A Hexagonal A, b = 3.425 A, c = 8.452 A, ______

oxides have an origin different from that suggested by Bertolaccini et al. [4]. It is known [6] that La803, Nd303, Sm303 and Dy303 havesimilar i.e. hexagonal crystal structure with almost the same lattice constants (see table l), while Yb303 formed in B type oxide has a different crystal structure with different lattice constants [6]. All the samples under study were in polycrystalline powder forms. The X-ray Debye diffraction pictures taken by US clearly showed their polycrystalline character and the fact that Yb303 has a crystal structure different from the rest. The data therefore seem to be consistent with the hypothesis of Kulkarni et al. [3] that 73 arises out of ortho-positronium atoms quenched within the crystallite while ortho-Ps atoms quenched in inter-crystallite regions give rise to 74. Both these components (73 and 74) in these samples, therefore, seem to be related to the state of aggregation rather than ionic properties of the rare ea,rths. P”erhaps, the inner lifetime components ~1 and ~1, if studied carefully, could show a correlation with the ionic properties of the solids [4]. It has been shown that for molecularly simple materials the Z3 - 73 correlation is represented by three distinct curves corresponding to fully *****

378

1971

‘B’ type.

amorphous solids and liquids, semi-crystalline polymers, and crystals [7,8]. The 13 - 73 points for all these oxides under study lie close to the curve representing crystals, lending additional support to the origin of 73 suggested earlier [3]. The authors are grateful to Professor C. N. R. Rao for suggesting the problem and to Professor B. V. Thosar for his interest in the work. Two of us (KPS and RMS) wish to thank the Tata Institute of F’undamental Research, for the hospitality extended to us.

References [l] R.Paulin and G.Ambrosino, J.Phys. 29 (1966) 263. [2] P. Sen and A. P. Patro, Nuovo Cimento 64B (1969) 324. [3] V.G.Kulkarni, R.G.Lagu, Girish Chandra and B.V. Thosar, Proc.1nd.Acad.W. 70A (1969) 107. [4] M. Bertolaccini, A.Bisi, G.Gambarini and L. Zappa, J. Phys. C (Proc. Phys. Sot. London) to be published. [5] P.Sen and A.P.Patro, Phys.Letters, 28A (1968) 414. [6] G. Brauer, Progress in the Science and Technology of the Rare Earths, Ed. LeRoy Eyring. Vol. 3 (Pergamon Press, 1968) 434. [7] R.G.Lagu, V.G.KuIkarni, B.V.Thosar andGirish Chandra, Proc.1nd.Acad.W. 69A (1969) 48. [8] B.V.Thosar, V.G.Kulkarni, R.G.Lagu and Girish Chandra, Phys.Letters, 28A (1969) 760.