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Compton profile study of beryllium oxide
PII:
Radiat. Phys. Chem. Vol. 51, No. 4±6, p. 519, 1998 # 1998 Elsevier Science Ltd. All rights reserved Printed in Great Britain S0969-806X(97)00190-4 0969-806X/98 $19.00 + 0.00
COMPTON PROFILE STUDY OF BERYLLIUM OXIDE K. B. JOSHI,1 RAJESH JAIN,2 R. K. PANDYA,3 B. L. AHUJA4 and B. K. SHARMA1 Department of Physics, University of Rajasthan, Jaipur 302 004, India, 2Department of Physics, S.S.J. (P.G.) College, Jaipur 302 004, India, 3Department of Physics, L.B.S. (P.G.) College, Jaipur 302 004, India and 4Department of Physics, M.R. Engineering College, Jaipur, 302 017 India
1
al., 1988) employing an annular Am241 source (primary energy 59.54 keV) of 5 Ci activity. Because of the toxic nature of BeO, the powder sample of 3.2 mm thickness was kept in an airtight ampoule with mylar windows. Photons scattered at an angle of 1658(22.58) were detected with an intrinsic Ge detector and standard electronics. About 2 105 per 60 eV channel photons were accumulated at the Compton peak in 30 h of measurement. The raw data were processed by application of corrections for background, detector response function, sample absorption, Compton cross-section and double scattering, etc. (PerkkioÈ and Paakkari, 1987). To compare our measurement with the HFLCAO Compton pro®le (Lichanot et al., 1992) we have added the core contribution to the valence
Beryllium oxide (BeO), the ®rst compound in the series of alkaline earth oxides, has speci®c importance because of its high thermal and low electrical conductivity. A few years ago Lichanot et al. (1992) investigated structural and electronic properties of BeO using the CRYSTAL program which employs a Hartree±Fock LCAO approach. They also computed directional Compton pro®les. The objective of the present study on BeO was to obtain accurate data to test the calculations of Lichanot et al. (1992) and to examine the behaviour of its valence electrons compared with those from another member of this family, MgO, which is generally considered to be an ionic compound. The Compton pro®le measurements were performed with our Compton spectrometer (Sharma et 0.15
0.10
∆J(Theo.–Expt.) in e/a.u.
0.05
0
–0.05
HF – LCAO
–0.10
Be ++ (Ionic) = O –– RFA (WF1) Be ++ (Ionic) = O –– RFA (WF2) –0.15
Expt. Error
–0.20
–0.25 0
1.0
2.0
3.0
4.0
5.0
6.0
p Z (a.u.) Fig. 1. Dierence pro®les between convoluted theory and experiemnt. 519
7.0
520
K. B. Joshi et al.
pro®le to get a total Compton pro®le for BeO. Assuming full ionicity, we have also calculated the Compton pro®les using the renormalised-free-atom (RFA)model approach of Berggren (1972) and Mahapatra (1986). The contribution of the cation (Be++) was taken from the ionic wavefunction of ÿÿ Clementi (1965) while for the anions ( O ) we have employed Watson's +1 well solution (WF1) (Watson, 1958) and Yamashita and Asano wavefunction (WF2) (Yamashita and Asano, 1970). All total theoretical Compton pro®les were convoluted with the residual instrument function of our experiment and properly normalised. We have plotted in Fig. 1, the dierence between convoluted theoretical and experimental Compton pro®les. It is seen that, in the vicinity of the Compton peak, i.e. for pz=0 a.u., both the RFA calculations (using WF1 and WF2) underestimate the momentum density. The overall agreement between experiment and theory is seen to be much better for the HF-LCAO calculation. To compare the nature of bonding in BeO with respect to the ionic solid MgO (Causa et al., 1986), we deduced the equal-valence-electron-density Compton pro®les (plotted on pz/pF scale) for BeO (our data) and MgO (Aikala et al., 1982). It was observed that the J(0) value for BeO was about 5% larger than that for MgO but the pro®le for MgO had somewhat
larger tail. These deviations clearly suggest some dierences in the nature of bonding in the two compounds, i.e. it points to partial covalent character in BeO as proposed by Lichanot et al. (1992). A proper test of covalent bonding in BeO would be the measurement of directional Compton pro®les along and perpendicular to the Be±O bond. It is hoped that further work in this direction will be forthcoming.
REFERENCES
Lichanot, A., Chaillet, M., Larrieu, C., Dovesi, R. and Pisani, C. (1992) Chem. Phys. 164, 383. Sharma, B. K., Gupta, A., Singh, H., Perkkio, S., Kshirsagar, A. and Kanhere, D. G. (1988) Phys. Rev. B37, 6821. PerkkioÈ, S. and Paakkari, T. (1987) Rep. Ser. Phys. HUP-246. University of Helsinki, Finland. Berggren, K.-F. (1972) Phys. Rev. B6, 2156. Mahapatra, D. P. (1986) J. Phys. C: Solid State Phys. 19, 2129. Clementi, E. (1965) IBM J. Res. Develop. 9, 2. Watson, R. E. (1958) Phys. Rev. 111, 1108. Yamashita, J. and Asano, S. (1970) J. Phys. Soc. Jap. 23, 1143. Causa, M., Dovesi, R., Pisani, C. and Roetti, C. (1986) Phys. Rev. B34, 2939. Aikala, O., Paakkari, T. and Manninen, S. (1982) Acta Crysta. A38, 155.
Radiat. Phys. Chem. Vol. 51, No. 4±6, p. 519, 1998 # 1998 Elsevier Science Ltd. All rights reserved Printed in Great Britain S0969-806X(97)00190-4 0969-806X/98 $19.00 + 0.00
COMPTON PROFILE STUDY OF BERYLLIUM OXIDE K. B. JOSHI,1 RAJESH JAIN,2 R. K. PANDYA,3 B. L. AHUJA4 and B. K. SHARMA1 Department of Physics, University of Rajasthan, Jaipur 302 004, India, 2Department of Physics, S.S.J. (P.G.) College, Jaipur 302 004, India, 3Department of Physics, L.B.S. (P.G.) College, Jaipur 302 004, India and 4Department of Physics, M.R. Engineering College, Jaipur, 302 017 India
1
al., 1988) employing an annular Am241 source (primary energy 59.54 keV) of 5 Ci activity. Because of the toxic nature of BeO, the powder sample of 3.2 mm thickness was kept in an airtight ampoule with mylar windows. Photons scattered at an angle of 1658(22.58) were detected with an intrinsic Ge detector and standard electronics. About 2 105 per 60 eV channel photons were accumulated at the Compton peak in 30 h of measurement. The raw data were processed by application of corrections for background, detector response function, sample absorption, Compton cross-section and double scattering, etc. (PerkkioÈ and Paakkari, 1987). To compare our measurement with the HFLCAO Compton pro®le (Lichanot et al., 1992) we have added the core contribution to the valence
Beryllium oxide (BeO), the ®rst compound in the series of alkaline earth oxides, has speci®c importance because of its high thermal and low electrical conductivity. A few years ago Lichanot et al. (1992) investigated structural and electronic properties of BeO using the CRYSTAL program which employs a Hartree±Fock LCAO approach. They also computed directional Compton pro®les. The objective of the present study on BeO was to obtain accurate data to test the calculations of Lichanot et al. (1992) and to examine the behaviour of its valence electrons compared with those from another member of this family, MgO, which is generally considered to be an ionic compound. The Compton pro®le measurements were performed with our Compton spectrometer (Sharma et 0.15
0.10
∆J(Theo.–Expt.) in e/a.u.
0.05
0
–0.05
HF – LCAO
–0.10
Be ++ (Ionic) = O –– RFA (WF1) Be ++ (Ionic) = O –– RFA (WF2) –0.15
Expt. Error
–0.20
–0.25 0
1.0
2.0
3.0
4.0
5.0
6.0
p Z (a.u.) Fig. 1. Dierence pro®les between convoluted theory and experiemnt. 519
7.0
520
K. B. Joshi et al.
pro®le to get a total Compton pro®le for BeO. Assuming full ionicity, we have also calculated the Compton pro®les using the renormalised-free-atom (RFA)model approach of Berggren (1972) and Mahapatra (1986). The contribution of the cation (Be++) was taken from the ionic wavefunction of ÿÿ Clementi (1965) while for the anions ( O ) we have employed Watson's +1 well solution (WF1) (Watson, 1958) and Yamashita and Asano wavefunction (WF2) (Yamashita and Asano, 1970). All total theoretical Compton pro®les were convoluted with the residual instrument function of our experiment and properly normalised. We have plotted in Fig. 1, the dierence between convoluted theoretical and experimental Compton pro®les. It is seen that, in the vicinity of the Compton peak, i.e. for pz=0 a.u., both the RFA calculations (using WF1 and WF2) underestimate the momentum density. The overall agreement between experiment and theory is seen to be much better for the HF-LCAO calculation. To compare the nature of bonding in BeO with respect to the ionic solid MgO (Causa et al., 1986), we deduced the equal-valence-electron-density Compton pro®les (plotted on pz/pF scale) for BeO (our data) and MgO (Aikala et al., 1982). It was observed that the J(0) value for BeO was about 5% larger than that for MgO but the pro®le for MgO had somewhat
larger tail. These deviations clearly suggest some dierences in the nature of bonding in the two compounds, i.e. it points to partial covalent character in BeO as proposed by Lichanot et al. (1992). A proper test of covalent bonding in BeO would be the measurement of directional Compton pro®les along and perpendicular to the Be±O bond. It is hoped that further work in this direction will be forthcoming.
REFERENCES
Lichanot, A., Chaillet, M., Larrieu, C., Dovesi, R. and Pisani, C. (1992) Chem. Phys. 164, 383. Sharma, B. K., Gupta, A., Singh, H., Perkkio, S., Kshirsagar, A. and Kanhere, D. G. (1988) Phys. Rev. B37, 6821. PerkkioÈ, S. and Paakkari, T. (1987) Rep. Ser. Phys. HUP-246. University of Helsinki, Finland. Berggren, K.-F. (1972) Phys. Rev. B6, 2156. Mahapatra, D. P. (1986) J. Phys. C: Solid State Phys. 19, 2129. Clementi, E. (1965) IBM J. Res. Develop. 9, 2. Watson, R. E. (1958) Phys. Rev. 111, 1108. Yamashita, J. and Asano, S. (1970) J. Phys. Soc. Jap. 23, 1143. Causa, M., Dovesi, R., Pisani, C. and Roetti, C. (1986) Phys. Rev. B34, 2939. Aikala, O., Paakkari, T. and Manninen, S. (1982) Acta Crysta. A38, 155.