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Indirect electronic transitions in alkali halate crystals-II (sodium bromate and iodate) crystals
J. Phys. Chon. Solids. 1973. Vol. 34. pp. 48 l-486.
INDIRECT (SODIUM
Pergamon Press.
Printed in Great Britain
ELECTRONIC HALATE BROMATE
S. B. S. SASTRY, Department
of Physics,
TRANSITIONS CRYSTALS-II AND IODATE)
R.
B. TRIPATHI
Indian
Institute
(Received
and
CRYSTALS
C. RAMASASTRY
ofTechnology,
25 April
IN ALKALI
Madras-36,
India
1972)
AbstractIn the case of NaCIO, and KCIO:, crystals, analysis of the long wavelength tail of their fundamental absorption revealed the active participation of the internal vibrations of the chlorate ion (Part I). In order to test the validity of the above interpretation the absorption spectra of two more halates with different anions namely sodium bromate and sodium iodate are analysed in a manner similar to that given in Part 1. It is found that the principle internal vibrations of bromate and iodate ions are involved in the indirect transitions. The variation of indirect band gap with temperature is found to be -2.5 X IO+eV/K and -2.9 X IO-“eV/K for sodium bromate and sodium iodate respectively.
1. INTRODUCTION
2. EXPERIMENTAL
THE LONG wavelength tail of the ultraviolet absorption spectrum in sodium chlorate and potassium chlorate [ 1,2] crystals showed evidence of indirect band to band transitions in these crystals in which phonons specially the internal vibrations in the chlorate ion actively participate. The analysis of these spectra taken at various temperatures is based on the theory first given by Bardeen et al. [3] in the case of semiconductors like silicon[4] and germanium [5] and later employed successfully in the case of ionic crystals like silver halides [6,7]. The threshold energy intervals which are measures of the energies of the participating phonons come out to be the same in the case of both NaCIO, and KCIO, crystals. If this is true, these threshold energy intervals, obtained from such an analysis should be dependent on the nature of the complex molecular ions in these crystals. With this view an analysis of the absorption spectra of NaBrOts and NaIO, crystals is made and the results are reported.
Sodium bromate crystals are grown from aqueous solution of pure material. They grow as transparent triangular pyramids developing (111) faces (NaBrO, has f.c.c. structure)[8]. Suitable pieces are cut from these pyramids with a wet thread, polished and used for absorption measurements. Sodium iodate crystals are difficult to grow by this slow evaporation method. Evaporation of saturated aqueous solution at 100°C yielded only small crystals [9] which are not suitable for absorption measurements. We could get these crystals in the form of platelets suitable for our work by adding 1 per cent by weight of HIO, in the solution and evaporating it at around 85°C. Such a method was employed by Kasatkin et al. [ 101 to grow single crystal seeds of potassium iodate. The absorption spectra are taken as described in Part I of this paper. The range of study is from 150 to 370 K in the case of NaBrO, and 153 to 320 K in the case of NaIO, crystals. NaIO, is known to decompose at 481
INDIRECT
ELECTRONIC
TRANSITIONS
IN ALKALI
HALATE
CRYSTALS-II
483
4.20-
Temperature,
“K
I 4.1
e2
Photon energy,
3.9
eV
Fig. 2. Absorption spectrum of NaIOJ crystal at different temperatures as indicated after reflection loss correction as shown in Part I. The temperature variation of the indirect band gap E; is shown as inset.
tion and phonon emission. Insets in Figs. 3 and 4 show a typical analysis for the room temperature spectra i.e. for curve 4 and 5 in Figs. 3 and 4 respectively. The threshold energies are indicated by the arrows at points A, B, C and D. The average threshold energy intervals are tabulated in the Table 1. The indirect band gap energy E,: is also calculated at various temperatures and given in the same table. We again assume that the first two straight line regions from long
wavelength side arise due to transitions involving absorption of phonons and the other two are due to transitions involving emission of phonons. The threshold energy intervals A-D and B-C which according to the theory described earlier [5,6] will give twice the phonon energies involved in these transitions. It is interesting to note that the threshold intervals remain the same within experimental error. The spectrum itself, however gets shifted
s. B. s. SASTRY,
R. B. TRIPATHI
and c. RAMASASTRY
44
4.6 5-
Photon
\
energy,
4.2
4
eV
4-
3-
2-
I-
4.7
4.6
4.5
4.4
Photon
energy,
4.3
4.2
eV
Fig. 3. (@“* vs photon energy plots for the spectra of NaBr0, Fig. 1. The four straight line regions and the knee points A, B, C and D are indicated by arrows. Inset shows the analysis of the plot 4 into individual phonons.
Table 1. The threshold energy intervals obtained from (0~)~‘~us photon energy plots of NaBrOS and NalO, crystals (Figs. 3 and 4) at various temperatures. The last column gives the indirect band gap energy at different temperatures
crystal
Temp. (“K)
A-B
Thresholds energy intervals in eV A-C A-D B-C B-D
C-D
Average band gap energy EL eV
NaBr0,
153 211 226 281 305 346 366
0.104 0.094 0.098 0.094 049 0.08 049
0.214 0.22 0.20 0.20 0.20 0.19 0.19
0.354 0.35 0.35 0.358 0.37 0.35 0.35
0.11 0.106 0.104 0.11 0.11 0.1 I 0.10
0.25 0.265 0.25 0.266 0.28 0.27 0.26
0.14 0.15 0.148 0.154 0.17 0.16 0.16
4.3% 4.389 4.386 4.370 4.365 4.355 4.345
153 211 241 281 305 320
0.07 0.07 0.07 0,08 0.07 0.07
0.14 0.156 0.16 0.16 0.16 0.15
0.30 0.31 0.32 0.32 0.31 0.31
049 0.086 049 0.08 049 0.08
0.23 0.24 0.25 0.24 0.24 0.24
0.14 0.134 0.14 0.16 0.15 0.16
4.160 4.146 4.140 4.13 4.115 4.107
NaIO,
INDIRECT
ELECTRONIC
TRANSITIONS
Photon
IN ALKALI
HALATE
Photon
eV
energy.
energy.
CRYSTALS-II
485
eV
Fig. 4. (@ vs photon energy plots for the spectra of NaIO, Fig. 2. The four straight line regions and the knee points A, B, C and D are indicated by arrows. Inset shows the analysis of the plot 5 into individual phonons.
towards long wave lengths as the temperature increases. In the case of NaBrO, the phonon energies that are involved come out to be 0.18+-0.02eV and 0*055+0.01 eV. These energies are too large to be considered as lattice phonon energies but they seem to be in good agreement with the internal vibrations of BrO,- ion (Table 2). The most intense vibrations [12] are 795 cm-l (O-098 eV) and 822 cm-’ (0.102eV) which are u1 (symmetric stretch) and v3 (asymmetric stretch) respectively. The phonon energy of 0.18 +- 0.02 eV fit in as the overtone frequency 2vl or the sum of the two vibrations (vl + ~‘3)(0.20 eV). Similarly the other phonon energy O-055 + 0.01 also cannot be just a lattice vibration (= 0.01 eV). This can be tentatively thought of as a combination of a lattice phonon and one of the other two internal vibration vq (asymmetrical bending) or up (symmetrical bending).
In the case of NaIO,, the internal vibrational frequencies published earlier [ 13, 141 differ from each other. We made both Raman and infrared studies on single crystals of NaIO, and completely assigned the spectra [ 151. Our analysis gave the most intense I-O vibrations as indicated in Table 2. The phonons involved in the indirect transition in these crystals are 0.15 k 0.02 and O@l+O*Ol eV which are deduced from the threshold energy intervals A-D and B-C given in Table 1. The 0.15 eV energy can again be interpreted as combination of y1 and +, or as the first overtone of vl. The latter can be a combination of v4,, or v4bwith a lattice phonon. The variation of Ei with temperature is shown as inset in Fig. 1 for NaBrO, and in Fig. 2 for NaIO,. The slopes are found to be -2.5 X 10d4 eV/K and -2.9 X 10v4 eV/K for the two crystals respectively.
486
S. B. S. SASTRY,
R. B. TRIPATHI
and
C. RAMASASTRY
Table 2. Internal vibrational frequencies of BrO,- ion in NaBrO,, and IO,- ion in NalO, crystals with their energies and assignment (from Raman Spectra)
Internal modes of vibration
NaBrO, and Venkateswaran [ 121 [I61 eV cm-’ eV
cm-’
842
0.104
-
-
776
0.095
774
0.095
822
0.101
774
0.095
0.094
797 452
0.098 0.056
754 373 355
0.093 0.046 0.044
0.097 0.093 0.091 -
764
0.098 0.055
789 755 741 -
746 371 352
0.092 0.046 0.043
0.046 0444
378 357
0.046 0444 332
0.041
-
325
0440
Couture Mathieu cm-’
Durig
NalO,, Sherwood and Turner [ I4]t cm-’ eV
et al.* [131 eV
Present
authorsS
[Iv cm-’
eV
0.101
*Data tData SData
are from the spectra on polycrystalline material are from the spectra on polycrystalline material on single crystal of NaIO, with Helium-Neon
4. CONCLUSION
The phonons involved in the indirect band to band transitions in alkali halate crystals are predominantly the internal vibrations of the halate (X03-, X = Cl, Br, I) ions unlike other simple crystals like silicon, germanium and silver halides, where lattice phonons are predominant for obvious reasons.
1. SASTRY SASTRY 2. SASTRY SASTRY 3. BARDEEN Proceeding
REFERENCES S. B. S., TRIPATHI C., J. Non-Metals
R. B. and RAMA1,93
(1972).
S. B. S., TRIPATHI R. B. and RAMAC., J. Phys. Chem. Solids 34,473 (1973). J., HALL L. H. and BLATT F. J., in of the Conference
Atlantic City. 1954 et al) p. 146, Wiley, 4. MACFARLANE QUARRINGTON Rev. 111, 1245 (1958).
on Photoconductivity
(Edited by R. G. Breckenridge New York (1956). G. G., MCLEAN T. P., J. E. and ROBERTS V., Phys.
with with
mercury source. Helium-Neon laser laser source.
source.
5. MACFARLANE G. G., MCLEAN T. P., QUARRINGTON J. E. and ROBERTS V., Phys. Rev. 108, 1377 (1957). 6. JOESTEN B. L. and BROWN F. C., Phys. Rev. 148. 919 (1966). 7. BROWN F. C., MASUMI T. and TIPPINS, J. Phys. Chem. Solids 22, 101 (1962). 8. RAMASASTRY C. and MURTI Y. V. G. S., .I. Phys.
Chem.
9. NA’RAY them.
Sot.
Solids
24. 1384 ( 1963).
I. S. and NEUGEBAUER J., J. Am. 69, 1280 (1947). A. P., PETROV T. G. and E. B.. Soviet Phvs._ Crvstallow. _ -. 7. 770
10. KASATKIN TREIVUS (1962-1963). 11. URBACH F., Phys. Rev. 92, 1324 (1952). 12. COUTURE L. and MATHIEU J. P., Ann. Phys. 3, 521 (1948). 13. DURIG J. R., BONNER 0. D. and BREAZEALC W. H., J. Phys. Chem. 69,3886 (1965). 14. SHERWOOD P. M. A. and TURNER J. J., Spectrochim. Acta 26A. 1975 (1970). 15. TRIPATHI R. B., SASTRY’S. B. S. and RAMASASTRY C., To be published. 16. VENKATESWARAN C. S., Proc. Ind. Amd. Sci. 7A, 144 (1938).
INDIRECT (SODIUM
Pergamon Press.
Printed in Great Britain
ELECTRONIC HALATE BROMATE
S. B. S. SASTRY, Department
of Physics,
TRANSITIONS CRYSTALS-II AND IODATE)
R.
B. TRIPATHI
Indian
Institute
(Received
and
CRYSTALS
C. RAMASASTRY
ofTechnology,
25 April
IN ALKALI
Madras-36,
India
1972)
AbstractIn the case of NaCIO, and KCIO:, crystals, analysis of the long wavelength tail of their fundamental absorption revealed the active participation of the internal vibrations of the chlorate ion (Part I). In order to test the validity of the above interpretation the absorption spectra of two more halates with different anions namely sodium bromate and sodium iodate are analysed in a manner similar to that given in Part 1. It is found that the principle internal vibrations of bromate and iodate ions are involved in the indirect transitions. The variation of indirect band gap with temperature is found to be -2.5 X IO+eV/K and -2.9 X IO-“eV/K for sodium bromate and sodium iodate respectively.
1. INTRODUCTION
2. EXPERIMENTAL
THE LONG wavelength tail of the ultraviolet absorption spectrum in sodium chlorate and potassium chlorate [ 1,2] crystals showed evidence of indirect band to band transitions in these crystals in which phonons specially the internal vibrations in the chlorate ion actively participate. The analysis of these spectra taken at various temperatures is based on the theory first given by Bardeen et al. [3] in the case of semiconductors like silicon[4] and germanium [5] and later employed successfully in the case of ionic crystals like silver halides [6,7]. The threshold energy intervals which are measures of the energies of the participating phonons come out to be the same in the case of both NaCIO, and KCIO, crystals. If this is true, these threshold energy intervals, obtained from such an analysis should be dependent on the nature of the complex molecular ions in these crystals. With this view an analysis of the absorption spectra of NaBrOts and NaIO, crystals is made and the results are reported.
Sodium bromate crystals are grown from aqueous solution of pure material. They grow as transparent triangular pyramids developing (111) faces (NaBrO, has f.c.c. structure)[8]. Suitable pieces are cut from these pyramids with a wet thread, polished and used for absorption measurements. Sodium iodate crystals are difficult to grow by this slow evaporation method. Evaporation of saturated aqueous solution at 100°C yielded only small crystals [9] which are not suitable for absorption measurements. We could get these crystals in the form of platelets suitable for our work by adding 1 per cent by weight of HIO, in the solution and evaporating it at around 85°C. Such a method was employed by Kasatkin et al. [ 101 to grow single crystal seeds of potassium iodate. The absorption spectra are taken as described in Part I of this paper. The range of study is from 150 to 370 K in the case of NaBrO, and 153 to 320 K in the case of NaIO, crystals. NaIO, is known to decompose at 481
INDIRECT
ELECTRONIC
TRANSITIONS
IN ALKALI
HALATE
CRYSTALS-II
483
4.20-
Temperature,
“K
I 4.1
e2
Photon energy,
3.9
eV
Fig. 2. Absorption spectrum of NaIOJ crystal at different temperatures as indicated after reflection loss correction as shown in Part I. The temperature variation of the indirect band gap E; is shown as inset.
tion and phonon emission. Insets in Figs. 3 and 4 show a typical analysis for the room temperature spectra i.e. for curve 4 and 5 in Figs. 3 and 4 respectively. The threshold energies are indicated by the arrows at points A, B, C and D. The average threshold energy intervals are tabulated in the Table 1. The indirect band gap energy E,: is also calculated at various temperatures and given in the same table. We again assume that the first two straight line regions from long
wavelength side arise due to transitions involving absorption of phonons and the other two are due to transitions involving emission of phonons. The threshold energy intervals A-D and B-C which according to the theory described earlier [5,6] will give twice the phonon energies involved in these transitions. It is interesting to note that the threshold intervals remain the same within experimental error. The spectrum itself, however gets shifted
s. B. s. SASTRY,
R. B. TRIPATHI
and c. RAMASASTRY
44
4.6 5-
Photon
\
energy,
4.2
4
eV
4-
3-
2-
I-
4.7
4.6
4.5
4.4
Photon
energy,
4.3
4.2
eV
Fig. 3. (@“* vs photon energy plots for the spectra of NaBr0, Fig. 1. The four straight line regions and the knee points A, B, C and D are indicated by arrows. Inset shows the analysis of the plot 4 into individual phonons.
Table 1. The threshold energy intervals obtained from (0~)~‘~us photon energy plots of NaBrOS and NalO, crystals (Figs. 3 and 4) at various temperatures. The last column gives the indirect band gap energy at different temperatures
crystal
Temp. (“K)
A-B
Thresholds energy intervals in eV A-C A-D B-C B-D
C-D
Average band gap energy EL eV
NaBr0,
153 211 226 281 305 346 366
0.104 0.094 0.098 0.094 049 0.08 049
0.214 0.22 0.20 0.20 0.20 0.19 0.19
0.354 0.35 0.35 0.358 0.37 0.35 0.35
0.11 0.106 0.104 0.11 0.11 0.1 I 0.10
0.25 0.265 0.25 0.266 0.28 0.27 0.26
0.14 0.15 0.148 0.154 0.17 0.16 0.16
4.3% 4.389 4.386 4.370 4.365 4.355 4.345
153 211 241 281 305 320
0.07 0.07 0.07 0,08 0.07 0.07
0.14 0.156 0.16 0.16 0.16 0.15
0.30 0.31 0.32 0.32 0.31 0.31
049 0.086 049 0.08 049 0.08
0.23 0.24 0.25 0.24 0.24 0.24
0.14 0.134 0.14 0.16 0.15 0.16
4.160 4.146 4.140 4.13 4.115 4.107
NaIO,
INDIRECT
ELECTRONIC
TRANSITIONS
Photon
IN ALKALI
HALATE
Photon
eV
energy.
energy.
CRYSTALS-II
485
eV
Fig. 4. (@ vs photon energy plots for the spectra of NaIO, Fig. 2. The four straight line regions and the knee points A, B, C and D are indicated by arrows. Inset shows the analysis of the plot 5 into individual phonons.
towards long wave lengths as the temperature increases. In the case of NaBrO, the phonon energies that are involved come out to be 0.18+-0.02eV and 0*055+0.01 eV. These energies are too large to be considered as lattice phonon energies but they seem to be in good agreement with the internal vibrations of BrO,- ion (Table 2). The most intense vibrations [12] are 795 cm-l (O-098 eV) and 822 cm-’ (0.102eV) which are u1 (symmetric stretch) and v3 (asymmetric stretch) respectively. The phonon energy of 0.18 +- 0.02 eV fit in as the overtone frequency 2vl or the sum of the two vibrations (vl + ~‘3)(0.20 eV). Similarly the other phonon energy O-055 + 0.01 also cannot be just a lattice vibration (= 0.01 eV). This can be tentatively thought of as a combination of a lattice phonon and one of the other two internal vibration vq (asymmetrical bending) or up (symmetrical bending).
In the case of NaIO,, the internal vibrational frequencies published earlier [ 13, 141 differ from each other. We made both Raman and infrared studies on single crystals of NaIO, and completely assigned the spectra [ 151. Our analysis gave the most intense I-O vibrations as indicated in Table 2. The phonons involved in the indirect transition in these crystals are 0.15 k 0.02 and O@l+O*Ol eV which are deduced from the threshold energy intervals A-D and B-C given in Table 1. The 0.15 eV energy can again be interpreted as combination of y1 and +, or as the first overtone of vl. The latter can be a combination of v4,, or v4bwith a lattice phonon. The variation of Ei with temperature is shown as inset in Fig. 1 for NaBrO, and in Fig. 2 for NaIO,. The slopes are found to be -2.5 X 10d4 eV/K and -2.9 X 10v4 eV/K for the two crystals respectively.
486
S. B. S. SASTRY,
R. B. TRIPATHI
and
C. RAMASASTRY
Table 2. Internal vibrational frequencies of BrO,- ion in NaBrO,, and IO,- ion in NalO, crystals with their energies and assignment (from Raman Spectra)
Internal modes of vibration
NaBrO, and Venkateswaran [ 121 [I61 eV cm-’ eV
cm-’
842
0.104
-
-
776
0.095
774
0.095
822
0.101
774
0.095
0.094
797 452
0.098 0.056
754 373 355
0.093 0.046 0.044
0.097 0.093 0.091 -
764
0.098 0.055
789 755 741 -
746 371 352
0.092 0.046 0.043
0.046 0444
378 357
0.046 0444 332
0.041
-
325
0440
Couture Mathieu cm-’
Durig
NalO,, Sherwood and Turner [ I4]t cm-’ eV
et al.* [131 eV
Present
authorsS
[Iv cm-’
eV
0.101
*Data tData SData
are from the spectra on polycrystalline material are from the spectra on polycrystalline material on single crystal of NaIO, with Helium-Neon
4. CONCLUSION
The phonons involved in the indirect band to band transitions in alkali halate crystals are predominantly the internal vibrations of the halate (X03-, X = Cl, Br, I) ions unlike other simple crystals like silicon, germanium and silver halides, where lattice phonons are predominant for obvious reasons.
1. SASTRY SASTRY 2. SASTRY SASTRY 3. BARDEEN Proceeding
REFERENCES S. B. S., TRIPATHI C., J. Non-Metals
R. B. and RAMA1,93
(1972).
S. B. S., TRIPATHI R. B. and RAMAC., J. Phys. Chem. Solids 34,473 (1973). J., HALL L. H. and BLATT F. J., in of the Conference
Atlantic City. 1954 et al) p. 146, Wiley, 4. MACFARLANE QUARRINGTON Rev. 111, 1245 (1958).
on Photoconductivity
(Edited by R. G. Breckenridge New York (1956). G. G., MCLEAN T. P., J. E. and ROBERTS V., Phys.
with with
mercury source. Helium-Neon laser laser source.
source.
5. MACFARLANE G. G., MCLEAN T. P., QUARRINGTON J. E. and ROBERTS V., Phys. Rev. 108, 1377 (1957). 6. JOESTEN B. L. and BROWN F. C., Phys. Rev. 148. 919 (1966). 7. BROWN F. C., MASUMI T. and TIPPINS, J. Phys. Chem. Solids 22, 101 (1962). 8. RAMASASTRY C. and MURTI Y. V. G. S., .I. Phys.
Chem.
9. NA’RAY them.
Sot.
Solids
24. 1384 ( 1963).
I. S. and NEUGEBAUER J., J. Am. 69, 1280 (1947). A. P., PETROV T. G. and E. B.. Soviet Phvs._ Crvstallow. _ -. 7. 770
10. KASATKIN TREIVUS (1962-1963). 11. URBACH F., Phys. Rev. 92, 1324 (1952). 12. COUTURE L. and MATHIEU J. P., Ann. Phys. 3, 521 (1948). 13. DURIG J. R., BONNER 0. D. and BREAZEALC W. H., J. Phys. Chem. 69,3886 (1965). 14. SHERWOOD P. M. A. and TURNER J. J., Spectrochim. Acta 26A. 1975 (1970). 15. TRIPATHI R. B., SASTRY’S. B. S. and RAMASASTRY C., To be published. 16. VENKATESWARAN C. S., Proc. Ind. Amd. Sci. 7A, 144 (1938).