Positron annihilation in KH2PO4

Positron annihilation in KH2PO4

Solid State Communications, Vol. 18, pp. 1-4, 1976. Pergamon Press. Printed in Great Britain POSITRON ANNIHILATION IN KH2PO4 K.P Singh* and R.N. We...

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Solid State Communications, Vol. 18, pp. 1-4, 1976.

Pergamon Press.

Printed in Great Britain

POSITRON ANNIHILATION IN KH2PO4 K.P Singh* and R.N. West School of Mathematics and Physics, University of East Anglia, Norwich, U.K. (Received 20 May 1975 by C. W. McCombie)

A positron annihilation study of KH2PO4 provides new evidence of a high temperature phase transition at 447K. The results suggest the formation of a long lived positron or positronium state which could be associated with bond defects.

1. INTRODUCTION

1 .28 MeV (start channel) and 0.51 MeV (stop channel) 22Na radiation. energymeasurement windows appropriate to the Each encompassed approximately 24 hr counting time and provided 3 x l0’~counts at the spectrum peak.

POSITRON the have chemical and physical annihilation properties ofstudies solids of andboth liquids become increasingly important in recent years.1’2 The water—ice system has received a great deal of attention and the results obtained35 provide a good illustration of the complex patterns of positron hehaviour that can obtain in such materials. Crystals of the KH 2PO4 type have many features in common with ice and also exhibit interesting solid—solid phase transitions. The sensitivity of positrons to phase 1’2 tran-In sitions has we been established in several works. this paper report a positron study of the, as yet, far from clearly understood high temperature transition6~2of KH 2PO4 at or around 448 K.

-~

Some of the lifetime measurements were accompanied by simultaneous measurements of the Doppler

*

broadening of the 0.51 MeV annihilation line made with a Ge(Li) detector of 1.6 keV resolution (at 0.5 MeV). Measurements of the angular correlation of the annihilation radiation were made, on a sample cut 2 having angular resolution of from the same crystal as the an lifetime specimens, with a long slit apparatus 1 mrad.

2. EXPERIMENTAL Single crystals of analytic grade KH 2PO4 were grown by slow evaporation from aqueous solution.

6~2of the high temperature Previous transitioninvestigators in KH 2PO4 have recognised the possibilityatthat partial decomposition occur temperatures approaching of 470theK.crystals For thismay reason comparative studies were made with samples under vacuum, in free atmosphere, and in a sealed yessel. Sample temperatures were maintained to within ±1 degree and were recorded with the aid of PtPtRh. thermocouples.

Lifetime samples were fabricated into the usual 2 in which approximately 5 pCisandof wich positron arrangement 22Na source were deposited directly onto the face of one of the sample slices. Lifetime measurements were performed with a conventional fast—slow lifetime spectrometer having a resolution of 330 psec FWHM for 60Co gamma rays, observed with the *

On study leave from the Physics Department, G.B. Pant University of Agriculture and Technology, Pant Nagar, India. 1

2

POSITRON ANNIHILATION IN KH2PO4 I

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Fi;.2. Angular distribution for KH2PO4. The distributions have been folded about their centroids: Data points sample preheated to 460K. (0 -- positive ang1es~• negative angles). Solid line -- untreated sample.

FIG. 1. Positron lifetime parameters vs temperature in

KH2PO4.

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Samples in thermal equilibrium. Room

temperature runs on sample previously heated above the transition temperature: o Immediately following cooling. u — Following one month storage at room temperature. o — Following further cooling to 80K. 3. RESULTS AND DISCUSSION Lifetime measurements were performed at ternperatures spanning the range 295-470 K. Each spectrum was analysed with the aid of a multicomponent stripping programme’3 and the goodness of the fit assessed by the usual x2 test. All the analyses showed the presence of at least two apparently distinct cornponents. Analyses involving three or more components failed to converge although it is clear that a small intensity source component must have been present in all the spectra. The results of the two component fits are shown in Fig. 1. At the higher temperatures where the longer lived component

investigated. The modest increase in r1 from 295 to 440 K is most probably the result of thermal expansion and if so. suggests that at least part of this com.

ponent arises from the annihilation of quasi-free positrons. The transition temperature. 447 ±6 K, suggested by the ‘2 data is consistent with that 6 —8 • 11 reported • 12 by most of the previous investigators. The magnitude of the change in 12 certainly suggests a profound change in the physical state of the sample. The possibility that the effect is due to decomposition seems unlikely since essentially the same results were obtained for samples in vacuum and in a sealed vessel. The constancy of the lifetime parameters over the temperature range 450-470 K also suggests the positron “sees” a stable system. The comparative slowness of the positron annihilation techniques demanded that the samples he held at elevated temperatures for several days. On

(12, i2) has a significant intensity higher precision spectra might have revealed a source and possibly other components. However the known instability of KH 2PO4 at these temperatures suggested the inadvisability of the necessarily increased counting periods,

final removal from the apparatus all the samples showed the milky and cracking noted by 6’7’9whiteness The possibility that this cracking other workers. might be the cause of the change in ‘2 was recognised but would appear to be ruled out by our additional observation on the (i.r.) reversibility of the changes.

The data of Fig. 1 show but two significant changes in the spectra over the temperature range

At the end of the high temperature measurenients

Vol. 18, No. 1

POSITRON ANNIHILATION IN KH2PO4

the sample was cooled to room temperature and the lifetime spectrum rerneasured. Apart from a slight decrease in was identical to those obtamed in r~, the the highspectrum temperature phase. After a period of about one month ‘2 had decreased by 8% (Fig. 1). A further room temperature on theternsame sample after a few hoursmeasurement at liquid nitrogen peratures an additional but smaller (3%) reduction inproduced ‘2’ This partial reversibility of the high temperature change, again noted by others,9”1’3 would appear to exclude the cracking as the source of the lifetime The possibility that partial the samples recovered bychanges. water absorption following decomposition at the higher temperatures cannot be entirely neglected but decomposition would seem to be unlikely in view of our earlier observations. In an attempt to cast further light on the nature of the transition we have also measured the angular distribution of the annihilation radiation at room temperature for (i), an untreated sample and (ii), the same sample after heating at 460 K for 24 hr. The angular distributions were indistinguishable (Fig. 2) as were the analogous Doppler broadened energy line shapes obtained simultaneously with the lifetime measurements. Longhved components in the lifetime spectra of molecular solids are frequently associated with 1’2“pick A off” annihilation of orthopositronium atoms. useful additional check on such a hypothesis can often be obtained from the measurement of the intensity of a narrow component in the angular distribution, which arises from the self-annihilation of a related number of parapositronium atoms. An increase in ‘2, such as that observed in this work, can then result from either a relative increase in positronium formation probability and/or a diffusion of orthopositronium atoms from small free volume to large free volume regions. Both these effects may produce a change in the shape of the angular distribution. The lack of resolvable narrow components and any significant difference between the two angular distributions of Fig. 2 thus indicates that if positronium is formed in KH2PO4, it is either closely confined or closely associated, in the form of compounds or cornplexes, with the molecules of the medium.

3

In seeking an explanation for the present results we find comparatively little to guide us inathe existing 2 suggest tetragonal literature. Nicholson and Soest’ monoclinic transformation for analogous transitions in KD andnoRbH2PO4 butthe X-rays studies byofBhinc 82PO4 showed more than disappearance the eta!. diffraction pattern of KH 2PO4 as the 8temperature suggest thatapproached 473 should K. These workers the transition be latter associated with the onset of disordered—hindered 3-dimensional rotation of the H 2P04 groups although this has been 2 Certainly, the interpretation estimated frequency of questioned.’ such rotations8’12 ~ l0~sec_i) is much too small to be seen by a positron in a lifetime of order l0~sec. Nevertheless, some weakening of the hydrogen bonds is also an essential part of the interpretation of conductivity results by O’Keefe and Perrino6 who suggest a mechanism involving the generation and migration of ions and D-and L-defects above the transition temperature. Positron trapping at L-defects has also been suggested4’5 for the interpretation of lifetime and angular correlation results in ice. The lifetime in L-defects reported by those workers I nsec) is sufficiently close to the r 2 values reported here (~‘-800 psec) to support the analogy. —~

(“-S

4. CONCLUSIONS The present clearly presence of a well defined results transition at indicate 447 K intheKH 2PO4. The precise origins of the positron lifetime spectral component which is sensitive to this transition is not entirely clear but positron localisation in a proton vacancy, possibly followed by the formation of, and annihilation from, a positron—oxygen complex would seem to provide the most plausible interpretation at present. Complementary studies of the analogous transitions in other KDP type crystals would be of value.

Acknowledgements We should like to thank the Science Research Council, London for financial support. Thanks are also due to Mr. R.J. Buckingham for his invaluable technical support. —

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POSITRON ANNIHILATION IN KH

2PO4

Vol. 18, No. 1

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