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Positron annihilation studies in the mixed crystals of KCl:KBr
Volume 89A, number 8
PHYSICS LETTERS
7 June 1982
POSITRON ANNIHILATION STUDIES IN THE MIXED CRYSTALS OF KQ:KBr * A. SEN GUPTA, S. MAHAPATRA, SHAHNAWAZ, N. PANIGRAHI1
Institute of Physics, Sachivalaya Marg, Bhubaneswar 751005, India
and P. SEN Saha Institute of Nuclear Physics, Calcutta 700009, India Received 8 March 1982
Positron annihilation line shape studies have been made in five mixed crystals of KC1:KBr with varying concentrations. The annihilation line shape parameter S shows a minimum for the 50 mole% KC1/50 mole% KBr specimen indicating an orderly behaviour of the system. At other concentrations, the mixed crystals appear to have defects.
The alkali halide crystals, KU and KBr, have been well studied [1] by the positron annihilation technique. They have identical crystal structures and are ideally suited for the formation of mixed crystals. The X-ray diffraction study [2] shows that in the KU: KBr mixed crystals the anions are not in an ordered state but distributed randomly. The lattice constant changes linearly with molar composition [3,4]. The macroscopic density [5] is lowest for 80 mole% KU! 20 mole% KBr and amounts to 2% below the X.ray density. Haven [6] found no discrepancy but Ivankina [7] in her extensive study of alkali halide mixed crystals found deviations upto 7% for 50 mole% KU/SO mole% KBr. Most of the investigations indicate that mixed crystals of KU:KBr contain defects. Smakula et a!. [81 observe that the defects are probably either submicroscopic cavities or interstitials and both would account for a strong increase in hardness. Since the positrons are very sensitive to the presence of defects, the present paper is intended to study the behaviour of the mixed crystals of KU:KBr via the positron annihilation technique and look for a correlation between the positron parameter and the Work partly supported by CSIR, DST and ISRO projects, Present address: Physics Department, Case Western Reserve University, Cleveland, OH 44106, USA.
‘~
0 031-9163/82/0000—0000/$02.75 © 1982 North-Holland
nature and concentration of the mixed crystals. The alkali halide crystals investigated were pure KU, pure KBr and five mixed crystals of KU:KBr in concentrations of 20% KC1/80% KBr, 40% KU! 60% KBr, 50% KQ/50% KBr, 6 1.5% KU/38.5% KBr and 83% KU/i 7% KBr. The crystals were grown from melts and all the specimens were annealed at 450°C in vacuum before the measurements started. The annihilation line shape was measured with a HPGE detector having a resolution (fwhm) of 950 eV at the 514 keV gamma line from a 855r source and taken with a gain of 65.5 eV/channel. A weak 22Na positron source sandwitched between a folded nickel foil of 1.0 j.tm thickness was placed in between the crystal specimens. Each spectrum contained approximately 0.6 X 106 counts under the annihilation line. The data were recorded in a ND-600 MCA incorporating a two point digital spectrum stabilizer. Repeated runs on every specimen were taken for consistency checks. The annihilation line shape paraemeter S. represents the ratio of counts in the central part of the line to the total area under the peak. The source— specimen sandwitch was kept under vacuum during the measurement and all measurements were made at room temperature. Fig. 1 shows the variation ofS as a function of the concentration in the mixed crystals of KU:KBr. The 427
Volume 89A, number 8
PHYSICS LETTERS
I
0.60
-
0.59 ~ 0.58
r
~ u.57 °- 0.56
7 June 1982
present findings, i.e. a minimum inS for the same concentration. However, all these data, including the present, contradict the findings of Smakula et a!. [8]. It is quite likely that at 50—SO concentration, the system behaves more orderly, somewhat similar to that
I
of pureby KC1theand pure KBr crystals. ideaofisSsupported observation of similarThis values for the three specimens. At other concentrations of the
~
-
0 I
20I
I 40
60
80I
I 100
KBr concentration of 1< Br(%)
mixed crystals, the increase in the S value indicates the possibility of the presence of microcavities or vacancies as mentioned by others and refered to above. The orderly behaviour of the mixed crystal for the 50 mole% KU/SO mole% KBr demands further inpresent we are trying to make positron lifetime distribution studies on these mixed crystal specimens and look for a correlation between positron lifetime parameters and the mixed crystal behaviour, such as the dependence on the changes in lattice constants and the density changes. vestigations. At
Fig. 1. Variation of the annihilation line shape parameterS, as a function of the concentration in the mixed crystals of KCI:KBr.
errors in the S values are typically ±0.001,i.e. ~O.2% and the size of the points shown in fig. 1 includes this uncertainty. The observation of a minimum in S for the SO mole% KU/SO mole% KBr mixed crystal specimen is quite clear and interesting. Earlier at this concentration of mixed crystal, the microhardness was observed [8] to be maximum, showing that the mixed crystal contains a large concentration of defects. The change of the lattice constant is proportional to the molar composition [3,4]. Consequently the Xray density (computed from the lattice constant) changes linearly with the composition too. The macroscopic density determined by weighing shows [8] marked deviation from linearity. It could be positive or negative. The strong density defect indicates that these mixed crystals may contain either vacancies or interstitials.
The minimum in density defect as observed by Tammann—Krings [5] and Ivankina [71for the SO mole% KCI/50 mole% KBr is consistent with the
428
We wish to thank D.P. Mahapatra, A. Das and S. Banerjee for their assistance during the course of this work. References [1] D.P. Kerr, 5. Dannefaer, G.W. Dean and B.G. Hogg, Can. J. Ph~’s.56 (1978) 1453. [2] L. Vegard and H. Schjelderup, Phys. Z. 18 (1917) 93. [31 L. Vegard, Z. Phys. 5 (1921) 17. [4] F. Oberlies, Ann. Phys. 87 (1928) 238. [5] G. Tammann and W. Krings, Z. Anorg. Ailgem. Chem. 130 (1923) 229. [6] Y. Haven, Report of the Conf. on Defects in Crystalline Solids, University of Bristol, 1954 (The Physical Society, London, 1955) p. 261. [7] M.S. Ivankina, Trans. of the 2nd All-Union Conf. on the Phys. of Dielectrics, Nov. 1958; Phys. Inst. of the Acad. of Sci., Moscow (Acad. of Sci., Moscow, 1960) 423. [8] A. Smakula, N. Maynard and A. Repucci, J. Appi. Phys. 33 (1962) 453.
PHYSICS LETTERS
7 June 1982
POSITRON ANNIHILATION STUDIES IN THE MIXED CRYSTALS OF KQ:KBr * A. SEN GUPTA, S. MAHAPATRA, SHAHNAWAZ, N. PANIGRAHI1
Institute of Physics, Sachivalaya Marg, Bhubaneswar 751005, India
and P. SEN Saha Institute of Nuclear Physics, Calcutta 700009, India Received 8 March 1982
Positron annihilation line shape studies have been made in five mixed crystals of KC1:KBr with varying concentrations. The annihilation line shape parameter S shows a minimum for the 50 mole% KC1/50 mole% KBr specimen indicating an orderly behaviour of the system. At other concentrations, the mixed crystals appear to have defects.
The alkali halide crystals, KU and KBr, have been well studied [1] by the positron annihilation technique. They have identical crystal structures and are ideally suited for the formation of mixed crystals. The X-ray diffraction study [2] shows that in the KU: KBr mixed crystals the anions are not in an ordered state but distributed randomly. The lattice constant changes linearly with molar composition [3,4]. The macroscopic density [5] is lowest for 80 mole% KU! 20 mole% KBr and amounts to 2% below the X.ray density. Haven [6] found no discrepancy but Ivankina [7] in her extensive study of alkali halide mixed crystals found deviations upto 7% for 50 mole% KU/SO mole% KBr. Most of the investigations indicate that mixed crystals of KU:KBr contain defects. Smakula et a!. [81 observe that the defects are probably either submicroscopic cavities or interstitials and both would account for a strong increase in hardness. Since the positrons are very sensitive to the presence of defects, the present paper is intended to study the behaviour of the mixed crystals of KU:KBr via the positron annihilation technique and look for a correlation between the positron parameter and the Work partly supported by CSIR, DST and ISRO projects, Present address: Physics Department, Case Western Reserve University, Cleveland, OH 44106, USA.
‘~
0 031-9163/82/0000—0000/$02.75 © 1982 North-Holland
nature and concentration of the mixed crystals. The alkali halide crystals investigated were pure KU, pure KBr and five mixed crystals of KU:KBr in concentrations of 20% KC1/80% KBr, 40% KU! 60% KBr, 50% KQ/50% KBr, 6 1.5% KU/38.5% KBr and 83% KU/i 7% KBr. The crystals were grown from melts and all the specimens were annealed at 450°C in vacuum before the measurements started. The annihilation line shape was measured with a HPGE detector having a resolution (fwhm) of 950 eV at the 514 keV gamma line from a 855r source and taken with a gain of 65.5 eV/channel. A weak 22Na positron source sandwitched between a folded nickel foil of 1.0 j.tm thickness was placed in between the crystal specimens. Each spectrum contained approximately 0.6 X 106 counts under the annihilation line. The data were recorded in a ND-600 MCA incorporating a two point digital spectrum stabilizer. Repeated runs on every specimen were taken for consistency checks. The annihilation line shape paraemeter S. represents the ratio of counts in the central part of the line to the total area under the peak. The source— specimen sandwitch was kept under vacuum during the measurement and all measurements were made at room temperature. Fig. 1 shows the variation ofS as a function of the concentration in the mixed crystals of KU:KBr. The 427
Volume 89A, number 8
PHYSICS LETTERS
I
0.60
-
0.59 ~ 0.58
r
~ u.57 °- 0.56
7 June 1982
present findings, i.e. a minimum inS for the same concentration. However, all these data, including the present, contradict the findings of Smakula et a!. [8]. It is quite likely that at 50—SO concentration, the system behaves more orderly, somewhat similar to that
I
of pureby KC1theand pure KBr crystals. ideaofisSsupported observation of similarThis values for the three specimens. At other concentrations of the
~
-
0 I
20I
I 40
60
80I
I 100
KBr concentration of 1< Br(%)
mixed crystals, the increase in the S value indicates the possibility of the presence of microcavities or vacancies as mentioned by others and refered to above. The orderly behaviour of the mixed crystal for the 50 mole% KU/SO mole% KBr demands further inpresent we are trying to make positron lifetime distribution studies on these mixed crystal specimens and look for a correlation between positron lifetime parameters and the mixed crystal behaviour, such as the dependence on the changes in lattice constants and the density changes. vestigations. At
Fig. 1. Variation of the annihilation line shape parameterS, as a function of the concentration in the mixed crystals of KCI:KBr.
errors in the S values are typically ±0.001,i.e. ~O.2% and the size of the points shown in fig. 1 includes this uncertainty. The observation of a minimum in S for the SO mole% KU/SO mole% KBr mixed crystal specimen is quite clear and interesting. Earlier at this concentration of mixed crystal, the microhardness was observed [8] to be maximum, showing that the mixed crystal contains a large concentration of defects. The change of the lattice constant is proportional to the molar composition [3,4]. Consequently the Xray density (computed from the lattice constant) changes linearly with the composition too. The macroscopic density determined by weighing shows [8] marked deviation from linearity. It could be positive or negative. The strong density defect indicates that these mixed crystals may contain either vacancies or interstitials.
The minimum in density defect as observed by Tammann—Krings [5] and Ivankina [71for the SO mole% KCI/50 mole% KBr is consistent with the
428
We wish to thank D.P. Mahapatra, A. Das and S. Banerjee for their assistance during the course of this work. References [1] D.P. Kerr, 5. Dannefaer, G.W. Dean and B.G. Hogg, Can. J. Ph~’s.56 (1978) 1453. [2] L. Vegard and H. Schjelderup, Phys. Z. 18 (1917) 93. [31 L. Vegard, Z. Phys. 5 (1921) 17. [4] F. Oberlies, Ann. Phys. 87 (1928) 238. [5] G. Tammann and W. Krings, Z. Anorg. Ailgem. Chem. 130 (1923) 229. [6] Y. Haven, Report of the Conf. on Defects in Crystalline Solids, University of Bristol, 1954 (The Physical Society, London, 1955) p. 261. [7] M.S. Ivankina, Trans. of the 2nd All-Union Conf. on the Phys. of Dielectrics, Nov. 1958; Phys. Inst. of the Acad. of Sci., Moscow (Acad. of Sci., Moscow, 1960) 423. [8] A. Smakula, N. Maynard and A. Repucci, J. Appi. Phys. 33 (1962) 453.