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Ionic crystals: positron defectoscopy
Materials Science and Engineering, BI3 (1992) 169-170
169
Ionic crystals: positron defectoscopy* E. V. Popov Applied Biophysics Laboratory, UralPolytechnical Institute, Sverdlovsk 620002 (USSR) (Received August 28, 1991)
Abstract The present paper is devoted to the results of investigations on ionic crystals by the electron-positron method. It is shown that the introduction of ion impurities into calcium fluoride and bismuth silicon oxide samples is caused by changes in the electron-positron parameters.
1. Defectoscopy of superionic conductors: calcium fluoride The actual problem of the discovery and correct identification of impurities in samples is conditioned by the complexity of the best-quality ionic crystal synthesis. The electron-positron annihilation method is a highly sensitive method for determining the impurities in ionic crystals, because of small changes in the electronic density of samples [1]. The investigations of the distribution of the annihilation photon angles were carried out using an automatic spectrometer with an allowed angle of 0.8 mrad. The computer calculations of the spectral results were made using the program PAACFIT [2]. The purity of the samples investigated was determined by the mass spectrum method [3]. The electron-positron investigation was made using a superionic conducting crystals calcium fluoride [4]; these samples contained rare earth ion impurities. The experimental spectrum obtained for the distribution of annihilation photon angles is shown in Fig. 1. The introduction of 0.1 wt.%Dy into the calcium fluoride resulted in the appearance of a narrow component in the annihilation photon angle distribution. However, the annihilation spectrums did not change when smaller impurity concentrations (0.01 wt.%Dy and 0.01 wt.%Gd) were introduced into the calcium fluoride samples. Such changes in the shape of the annihilation spectrum mean that each positron was associated with an electron of a local centre. The crystalline defects which were created by substitution of divalent calcium ions *Originally submitted for presentation at the International Conference on Advanced Materials' 91. 0921-5107/92/$5.00
for trivalent dysprosium ions may be the local electron centres. So the electron-positron annihilation method is the defectoscopy method for determining impurities in supersonic conductors.
2. Defectoscopy of piezoelectri crystals: bismuth silicon oxide One of the main materials in electronic devices, bismuth silicon oxide, has been chosenas the object of investigation, because it is an ionic-covalent type of crystal. The experimental results are shown in Fig. 2. The positron annihilation investigation shows a consistent "narrowing" of the photon angle distribution spectra from 11.8 to 11.3 mrad due to an increase in the iron cation impurity concentration from 0.001 to 0.1 wt.% in the crystals.
3. Theoretical description of the positron defectoscopy The theoretical description of the electron-positron annihilation process in bismuth silicon oxide was based on the impulse presentation of a spherical-symmetrical coordinate wave electron function which describes the electron localization at iron substitution centres in the silicon matrix atoms:
ql(r)=(~r+3)l/2 exp
(1)
where r+ is the ion radius. © 1992 - Elsevier Sequoia. All rights reserved
E. V. Popov
170
Ionic" crystals: positron defectoscopy
T h e final correlation between the annihilation intensity I of two photons and angle ~ of photons flying away is:
T ~,0 /. 0,9
32 Ar+ I ( ~ ) = 3 2h(1 + ~2m2c2r+2h_2) 3
r'et,un. 0,8
where A is the experimental constant, m is the electron mass and c is the velocity of light. In the case of partial substitution of matrix ions by impurity ions the effective radius r+ must be
0,7
0"6-6
-4
-2
0
~-
4.
6 r+ = ri
Fig. 1. The annihilation photon angle distribution m calcium fluoride crystals with different concentrations of impurities: *, pure crystal; A, CaF2+0.01 wt.%Gd; n CaF2+O.01 wt.%Dy; +, CaF2+ 0.1 wt.% Dy.
t
I,
o,~4
re#.u,. qs0 0,,~6
(2)
Ciqt Cm
C m - - Ci
r., - -
(3)
Cm
where r~ is the impurity ion radius, rm is the matrix ion radius, c~ is the impurity mass concentration in the crystal and Cm is the matrix ion concentration (mass portion) in the crystal. T h e calculation of model parameters which was standardized for the experimental conditions shows that the lowest level of monovalent impurity sensitivity by the positron annihilation method is 10 ~5 cm-~. So the electron-positron annihilation method is a sensitive defectoscopy method for ionic crystals.
References I
4,8
5,z s,6 xg, mr-=d
6,0
i
6,4
Fig. 2. Part of the annihilation photon angle distribution spectra in bismuth silicon oxide with different iron impurity concentrations: line 1, 0.001 wt.%; line 2, 0.01 wt.%; line 3, 0.05 wt.%; line 4, 0.1 wt.%.
1 P.G. Coleman, S. C. Sherma and L. M. Diana (eds.), Positron Annihilation, North-Holland, Amsterdam, 1982. 2 P. Kirkegaard and O. Mogensen, Abstracts, 3rd Int. Conf. on Positron Annihilation, Finland,, 1973, p. 125. 3 E.V.Popov, Phys. Res., 13(1990) 534. 4 M. B. Salamon (ed.), Physics of Superionic Conductors, Springer, Berlin, 1979.
169
Ionic crystals: positron defectoscopy* E. V. Popov Applied Biophysics Laboratory, UralPolytechnical Institute, Sverdlovsk 620002 (USSR) (Received August 28, 1991)
Abstract The present paper is devoted to the results of investigations on ionic crystals by the electron-positron method. It is shown that the introduction of ion impurities into calcium fluoride and bismuth silicon oxide samples is caused by changes in the electron-positron parameters.
1. Defectoscopy of superionic conductors: calcium fluoride The actual problem of the discovery and correct identification of impurities in samples is conditioned by the complexity of the best-quality ionic crystal synthesis. The electron-positron annihilation method is a highly sensitive method for determining the impurities in ionic crystals, because of small changes in the electronic density of samples [1]. The investigations of the distribution of the annihilation photon angles were carried out using an automatic spectrometer with an allowed angle of 0.8 mrad. The computer calculations of the spectral results were made using the program PAACFIT [2]. The purity of the samples investigated was determined by the mass spectrum method [3]. The electron-positron investigation was made using a superionic conducting crystals calcium fluoride [4]; these samples contained rare earth ion impurities. The experimental spectrum obtained for the distribution of annihilation photon angles is shown in Fig. 1. The introduction of 0.1 wt.%Dy into the calcium fluoride resulted in the appearance of a narrow component in the annihilation photon angle distribution. However, the annihilation spectrums did not change when smaller impurity concentrations (0.01 wt.%Dy and 0.01 wt.%Gd) were introduced into the calcium fluoride samples. Such changes in the shape of the annihilation spectrum mean that each positron was associated with an electron of a local centre. The crystalline defects which were created by substitution of divalent calcium ions *Originally submitted for presentation at the International Conference on Advanced Materials' 91. 0921-5107/92/$5.00
for trivalent dysprosium ions may be the local electron centres. So the electron-positron annihilation method is the defectoscopy method for determining impurities in supersonic conductors.
2. Defectoscopy of piezoelectri crystals: bismuth silicon oxide One of the main materials in electronic devices, bismuth silicon oxide, has been chosenas the object of investigation, because it is an ionic-covalent type of crystal. The experimental results are shown in Fig. 2. The positron annihilation investigation shows a consistent "narrowing" of the photon angle distribution spectra from 11.8 to 11.3 mrad due to an increase in the iron cation impurity concentration from 0.001 to 0.1 wt.% in the crystals.
3. Theoretical description of the positron defectoscopy The theoretical description of the electron-positron annihilation process in bismuth silicon oxide was based on the impulse presentation of a spherical-symmetrical coordinate wave electron function which describes the electron localization at iron substitution centres in the silicon matrix atoms:
ql(r)=(~r+3)l/2 exp
(1)
where r+ is the ion radius. © 1992 - Elsevier Sequoia. All rights reserved
E. V. Popov
170
Ionic" crystals: positron defectoscopy
T h e final correlation between the annihilation intensity I of two photons and angle ~ of photons flying away is:
T ~,0 /. 0,9
32 Ar+ I ( ~ ) = 3 2h(1 + ~2m2c2r+2h_2) 3
r'et,un. 0,8
where A is the experimental constant, m is the electron mass and c is the velocity of light. In the case of partial substitution of matrix ions by impurity ions the effective radius r+ must be
0,7
0"6-6
-4
-2
0
~-
4.
6 r+ = ri
Fig. 1. The annihilation photon angle distribution m calcium fluoride crystals with different concentrations of impurities: *, pure crystal; A, CaF2+0.01 wt.%Gd; n CaF2+O.01 wt.%Dy; +, CaF2+ 0.1 wt.% Dy.
t
I,
o,~4
re#.u,. qs0 0,,~6
(2)
Ciqt Cm
C m - - Ci
r., - -
(3)
Cm
where r~ is the impurity ion radius, rm is the matrix ion radius, c~ is the impurity mass concentration in the crystal and Cm is the matrix ion concentration (mass portion) in the crystal. T h e calculation of model parameters which was standardized for the experimental conditions shows that the lowest level of monovalent impurity sensitivity by the positron annihilation method is 10 ~5 cm-~. So the electron-positron annihilation method is a sensitive defectoscopy method for ionic crystals.
References I
4,8
5,z s,6 xg, mr-=d
6,0
i
6,4
Fig. 2. Part of the annihilation photon angle distribution spectra in bismuth silicon oxide with different iron impurity concentrations: line 1, 0.001 wt.%; line 2, 0.01 wt.%; line 3, 0.05 wt.%; line 4, 0.1 wt.%.
1 P.G. Coleman, S. C. Sherma and L. M. Diana (eds.), Positron Annihilation, North-Holland, Amsterdam, 1982. 2 P. Kirkegaard and O. Mogensen, Abstracts, 3rd Int. Conf. on Positron Annihilation, Finland,, 1973, p. 125. 3 E.V.Popov, Phys. Res., 13(1990) 534. 4 M. B. Salamon (ed.), Physics of Superionic Conductors, Springer, Berlin, 1979.