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Optical properties of SbSi: Co and SbSeI: Co single crystals
0038-1098/88 $3.00 + .00 Pergamon Press plc
Solid State Communications, Vol. 68, No. 1l, pp. 1043-1046, 1988. Printed in Great Britain.
OPTICAL PROPERTIES OF SbSI:Co A N D SbSeI:Co SINGLE CRYSTALS Soonie Jeon, Gijun Cho and Wha-Tek Kim Solid State Physics Laboratory, Department of Physics, Chonnam National University, Kwangju 500-757, Republic of Korea and Sook-II Kwun Department of Physics, Seoul National University, Seoul 151-742, Republic of Korea
(Received 15 July 1988 by J. Kanamori) Single crystals of SbSI:Co and SbSeI:Co were prepared by the Bridgman technique. The optical absorption spectra of these single crystals were investigated in the wavelength range 500-2500 nm. We determined the optical energy gaps of these compounds, and identified the electric states of cobalt impurity in these single crystals. We observed the impurity absorption peaks at energies of 14 000-4000 cm-i range due to cobalt impurity. It can be explained that the impurity absorption peaks are attributed to the electronic transitions between the energy levels of cobalt ions, which are sited in tetrahedral (Td) symmetry of crystal lattice as Co 3+ and Co 2+ ions. The crystal field parameters Dq are given by 697 cm -I for Co 3+ and 4 5 4 c m - ' f o r Co 2+ ions in SbSI:Co single crystals, and also 847cm -t for Co 3+ and 452 cm-t Co2+ ions in SbSeI : Co single crystals.
1. I N T R O D U C T I O N
2. E X P E R I M E N T A L
THE T E R N A R Y semiconducting compounds of the vA--vIA--vIIA-type have been investigated by many authors because of their ferroelectric and photoelectric properties [1]. Among the V A - V I A - V I Ig temary compounds, photo-ferroelectric semiconductors SbSI and SbSeI have a high photoelectric sensitivity, and have a larger band gap than the other photoferroelectric semiconductors. These desirable properties make these compounds promising optoelectronic materials. Therefore, the ferroelectric [2], optical [3], and electrical properties [4] have been studied in these compounds. However, there are no published results on the optical and electrical properties of SbSI and SbSeI doped with 3d transition metals. In this communication, the optical properties of SbSI:Co and SbSeI:Co single crystals doped from 0.25mo1% to 1 mol% of cobalt metal as impurity have been investigated. The optical energy gaps of these compounds were determined from the optical absorption measurements. We have identified for the first time the origin of the impurity optical absorption peaks observed in the optical absorption spectra of these compounds.
Single crystals of SbSI, SbSI:Co, SbSeI, and SbSeI : Co were grown with high purity elements (6N) by the Bridgman technique. It has been determined from X-ray diffraction analysis that these single crystals have orthorhombic structure with lattice constants a = 8.53A, b = 10.14A, c = 4.08A for SbSI, and a = 8.36A, b = 10.98A, c = 4.16A for SbSeI single crystals. The lattice constants of Co-doped single crystals are similar to that of undoped single crystals. For optical absorption measurements, the single crystals were cut perpendicular to c-axis and polished to obtain plane-parallel platelets with 0.4 mm thickness. The absorption spectra were measured using a UVVIS-NIR spectrophotometer (Hitachi, U-3400) in the wavelength range 500-2500 nm. All the optical absorption measurements were carried out at 291 K. 3. RESULTS A N D DISCUSSION 3.1. Optical energy gaps oJ SbSI: Co and SbSeI: Co single crystals The energy dependence of (cthv)1/2 near the
1043
30[
1044
Vol. 68, No. 11
OPTICAL PROPERTIES OF SbSI : Co A N D SbSeI • Co Co;/rn(
i/
2.5
w
II S b S l : C o
ICo:QSir~ ' SINGLE ,, ~ : CRYSTAJ PURE
A
SbSI:Co
>~
SINGLE CRYSTALS 291K
2.0
2O t
--L '~
.. I0
,
I
1.5
~L6~V
0
(
..,
1.6
1.7
1.8
1.9 ENE~
PHOTON
.J
-"g~L87eV
g:l~eV
,o
2.0
I
I
OO
MOLE ~ R ~ N T
(~)
I
LO
0.5
OF ~ T S
Fig. 1. Plots of (~hv) ~/2 vs the incident photon energy hv in SbSI and SbSI:Co single crystals.
Fig. 3. Optical energy gaps for SbSI : Co single crystals with increasing cobalt.
fundamental absorption edge shows that the optical energy gap of pure SbSI and SbSI : Co single crystals is determined by indirect transitions, as shown in Fig. 1. The indirect energy gap of the pure SbSI single crystals, determined by extrapolation of Fig. 1, is given by 1.87 eV. For the single crystals of SbSi : Co doped with 0.5 and 1 mol% of cobalt, and indirect energy gaps are also given by 1.75 and 1.65eV, respectively. Figure 2 shows the dependence of (~hv) v2 on the incident photon energy in pure SbSeI and SbSeI:Co single crystals. The indirect energy gap in these single crystals, obtained from Fig. 2, is given by 1.62 eV for the pure SbSeI, 1.30 eV for the SbSeI: Co (0.5mo1%) and 1.01 eV for the SbSeI:Co (1 mol%) single crystals. The measured values of 1.87 and 1.62 eV of the indirect energy gaps for the pure SbSI and the pure SbSeI single crystals, which correspond to indirect transitions between F- and S-band in the band
structure [5] of these compounds, is smaller than the theoretical values of 2.11 eV for SbSI and 1.67 eV for SbSeI single crystals. Figures 3 and 4 show the dependence of the optical energy gaps on the amounts of cobalt. As shown in Figs. 3 and 4, we can see that the energy gaps of SbSI:Co and SbSeI:Co single crystals are linearly decreased with increasing mole percents of cobalt. 3.2. Impurity optical absorption peaks of SbSI : Co and SbSeI : Co single crystals To investigate the impurity absorption characteristics of cobalt ions, we subtracted the optical density of pure SbSI and pure SbSeI from that of SbSI:Co and SbSeI:Co. These results are shown in Figs. 5 and 6. As shown in Fig. 5 of SbSI : Co single crystals, we observed the impurity absorption peaks due to the electronic transitions between the energy levels of Co
30 ~NGLE C~STAL
A
/1
2.0
1
/
Co: I mol %
i i
I I
__
SbSel:Co SINGLE CRYSTALS
/ PURE
Co:Q5 mol% ~ ~Sel
291K
I.O
2oI
°°°1
0
°°'
•
°'l W
Eg=IDIeV
I
OO 0.6
1.0 PHOTON
EQ: | 1.3OeV J
I
E~ 1.62eV
/
"'1 ~1 14
ENERGY
1.8
(eV)
Fig. 2. Plots of (~hv) ~/2 vs the incident photon energy hv in SbSeI and SbSeI : Co single crystals.
o.sI
0.0
J
0.5
I0
MOLE PERCENT OF COBALTS
Fig. 4. Optical energy gaps for SbSeI" Co single crystals with increasing cobalt.
CONDUCTION BAND
1.5 I=1 O ,¢
SbSI: Co SINGLE CRYSTAL 670era (14~5cm") C43÷(Td,3T2 )
)I-
(/1 Z
1045
OPTICAL PROPERTIES OF SbSI" Co AND SbSeI: Co
Vol. 68, No. 11
1.0 _ \ I
4"fi(4Pl
291 K
~('~-'~C~T'.
~E)
i- " ' •
sT2 (.~)
4~CONDUCTIO BAN[3, Nc4p
2~O0.m ( ~ a ""~T,( 3HI
.J
.~-
~-
/ o o ~
•'~.m ~ /r (IZ~L~cm") L '/ I
Z
Co2÷(Ta,4Td
Ill h ~
o
0.5
0.0
I~
(9090crn" ,I)Co3+(Ta?T,)
I
600
I
1000
" ~ ) 1 I II I I I'E(%)
I
1500
2000
VALENCE BAND
2500
SbSI : Co
WAVELENGTH ( n m )
Fig. 5. Difference optical density of SbSI and SbSI : Co single crystals, at the range of 600-2500 nm. ions, which are sited Td symmetry of crystal lattice as Co 3÷ and Co 2÷ ions [6]. The four impurity absorption peaks at 670nm(14925cm-~), 886nm(11 286cm-1), 1100 nm(9090 cm-i ) and 1435 nm(6738 cm-t) of Fig. 5 are attributed to the electronic transitions of Co 3+ ions from the ground state 5E(SD) to the excited states 3T2(3H), 3E(3H), 3T1(3H), and 5T2(SD). It can be explained that the other three impurity absorption peaks at 750nm(13 333cm-I), 1620nm(6172cm-I), and 2200nm(4545 cm -~) are attributed to the electronic transitions of Co 2+ ions from the ground state 4/12(aF) to the excited states 4T1(4p), 4TI (4F) ' and 4T2(4F). The crystal field parameters Dq in SbSI : Co single crystal, obtained from Fig. 5, are found to be 697 cm-I for Co 3÷, and 454cm -] for Co 2+ ion, respectively. As for the SbSeI : Co single crystals of Fig. 6, we also observed the impurity absorption peaks due to the electronic transitions between the energy levels of Co 3+ and Co 2÷ ions sited Ta symmetry of SbSeI host lattice as well as for the SbSI:Co 2.0 0 0
S b S e l : C0
SINGLE
CRYSTAL
291K
Fz
/ I (8474cm-')
.J ~
1.0
II
0
I
II
I
I
22,2nm(~zo~-').C~T, :T~
.,484nm (673ecm-')'C02+ (Td'4T' ) I[ 1035ran(96r~'c_m-' }'C~÷(Td ?TI) 773nm( 12936CLT)"1), Co24"(Td,4TI )
ul ul
0.0
i
600
I
~000
I
~500 WAVELENGTH
I
2000 ( nrn )
4~¢,....._rI VALENCE
BAND
SbSe I : CO
Fig. 7. Energy level diagram and electronic transition model due to optical absorption of SbSI:Co and SbSeI : Co single crystals. single crystals. The impurity absorption peaks at 1035 nm(9662 cm- l ) and 1180 nm(8474 cm- J) correspond to the electronic transitions from the groundstate 5E(SD) to the excited states 3Tt (3H) and 5Tz(SD) of Co 3÷ ions. The impurity absorption peaks due to Co 2÷ ions are also observed at 773 nm(12 936 cm-l), 1484 nm(6738 cm-1 ) and 2212 nm (4520 cm-i ), which are attributed to the electronic transitions from the ground state 4A2(4F) to the excited states 4TI(4P), 4Tl(4F), and 4T2(4F ) of Co 2+ ions: The crystal field parameters Dq in SbSeI single crystal are given by 847 cm-i for Co 3+ and 452 cm-i for Co 2+ ion, respectively. Figure 7 shows the energy band profile for the SbSI : Co and SbSeI : Co single crystals obtained from the optical absorption spectra of Figs. 5 and 6. 4. CONCLUSION Single crystals of SbSI : Co and SbSeI : Co grown by the Bridgman technique have orthorhombic structure. The indirect band gaps of these single crystals were decreased with increasing mole percents of cobalt. It is concluded that the impurity absorption peaks observed in the optical absorption spectra of the SbSI : Co and SbSeI : Co single crystals are attributed to the electronic transitions from the ground state to the excited states of Co ions, which are sited in Td symmetry of lattice as Co 3÷ and Co 2+ ions.
Acknowledgement -The present studies were supported by the Basic Science Research Institute Program. Ministry of Education, 1987. REFERENCES
2500
Fig. 6. Difference optical density of SbSeI and SbSeI : Co single crystals, at the range of 600-2500 nm.
1.
E. Kaldis ed., Current Topics in Material Science, Vol. 10, p. 55, North-Holland Pub. Comp., Amsterdam (1982).
1046 2.
.
4.
OPTICAL PROPERTIES OF SbSI : Co AND SbSeI : Co E. Fatuzzo, G. Harbeke, W.J. Merz, R. Nitsche, H. Roetschi & W. Ruppel, Phys. Rev. 127, 2036 (1962). T.A. Pikka & V.M. Fridkin, Soy. Phys.-Solid State 10, 2668 (1969). V.G. Alekseeva & E.G. Landsberg, Soy. Phys.-
.
6.
Vol. 68, No. 11
Solid State 8, 2518 (1969). Y.F. Alward, C.J. Fong, M. E1-Batanny & F. Wooten, Solid State Commun. 25, 307 (1978). T.M. Dunn, D.A. McClure & R.G. Pearson, Crystal Field Theory, p. 82, Haper and Row Pub. Comp., New York (1965).
Solid State Communications, Vol. 68, No. 1l, pp. 1043-1046, 1988. Printed in Great Britain.
OPTICAL PROPERTIES OF SbSI:Co A N D SbSeI:Co SINGLE CRYSTALS Soonie Jeon, Gijun Cho and Wha-Tek Kim Solid State Physics Laboratory, Department of Physics, Chonnam National University, Kwangju 500-757, Republic of Korea and Sook-II Kwun Department of Physics, Seoul National University, Seoul 151-742, Republic of Korea
(Received 15 July 1988 by J. Kanamori) Single crystals of SbSI:Co and SbSeI:Co were prepared by the Bridgman technique. The optical absorption spectra of these single crystals were investigated in the wavelength range 500-2500 nm. We determined the optical energy gaps of these compounds, and identified the electric states of cobalt impurity in these single crystals. We observed the impurity absorption peaks at energies of 14 000-4000 cm-i range due to cobalt impurity. It can be explained that the impurity absorption peaks are attributed to the electronic transitions between the energy levels of cobalt ions, which are sited in tetrahedral (Td) symmetry of crystal lattice as Co 3+ and Co 2+ ions. The crystal field parameters Dq are given by 697 cm -I for Co 3+ and 4 5 4 c m - ' f o r Co 2+ ions in SbSI:Co single crystals, and also 847cm -t for Co 3+ and 452 cm-t Co2+ ions in SbSeI : Co single crystals.
1. I N T R O D U C T I O N
2. E X P E R I M E N T A L
THE T E R N A R Y semiconducting compounds of the vA--vIA--vIIA-type have been investigated by many authors because of their ferroelectric and photoelectric properties [1]. Among the V A - V I A - V I Ig temary compounds, photo-ferroelectric semiconductors SbSI and SbSeI have a high photoelectric sensitivity, and have a larger band gap than the other photoferroelectric semiconductors. These desirable properties make these compounds promising optoelectronic materials. Therefore, the ferroelectric [2], optical [3], and electrical properties [4] have been studied in these compounds. However, there are no published results on the optical and electrical properties of SbSI and SbSeI doped with 3d transition metals. In this communication, the optical properties of SbSI:Co and SbSeI:Co single crystals doped from 0.25mo1% to 1 mol% of cobalt metal as impurity have been investigated. The optical energy gaps of these compounds were determined from the optical absorption measurements. We have identified for the first time the origin of the impurity optical absorption peaks observed in the optical absorption spectra of these compounds.
Single crystals of SbSI, SbSI:Co, SbSeI, and SbSeI : Co were grown with high purity elements (6N) by the Bridgman technique. It has been determined from X-ray diffraction analysis that these single crystals have orthorhombic structure with lattice constants a = 8.53A, b = 10.14A, c = 4.08A for SbSI, and a = 8.36A, b = 10.98A, c = 4.16A for SbSeI single crystals. The lattice constants of Co-doped single crystals are similar to that of undoped single crystals. For optical absorption measurements, the single crystals were cut perpendicular to c-axis and polished to obtain plane-parallel platelets with 0.4 mm thickness. The absorption spectra were measured using a UVVIS-NIR spectrophotometer (Hitachi, U-3400) in the wavelength range 500-2500 nm. All the optical absorption measurements were carried out at 291 K. 3. RESULTS A N D DISCUSSION 3.1. Optical energy gaps oJ SbSI: Co and SbSeI: Co single crystals The energy dependence of (cthv)1/2 near the
1043
30[
1044
Vol. 68, No. 11
OPTICAL PROPERTIES OF SbSI : Co A N D SbSeI • Co Co;/rn(
i/
2.5
w
II S b S l : C o
ICo:QSir~ ' SINGLE ,, ~ : CRYSTAJ PURE
A
SbSI:Co
>~
SINGLE CRYSTALS 291K
2.0
2O t
--L '~
.. I0
,
I
1.5
~L6~V
0
(
..,
1.6
1.7
1.8
1.9 ENE~
PHOTON
.J
-"g~L87eV
g:l~eV
,o
2.0
I
I
OO
MOLE ~ R ~ N T
(~)
I
LO
0.5
OF ~ T S
Fig. 1. Plots of (~hv) ~/2 vs the incident photon energy hv in SbSI and SbSI:Co single crystals.
Fig. 3. Optical energy gaps for SbSI : Co single crystals with increasing cobalt.
fundamental absorption edge shows that the optical energy gap of pure SbSI and SbSI : Co single crystals is determined by indirect transitions, as shown in Fig. 1. The indirect energy gap of the pure SbSI single crystals, determined by extrapolation of Fig. 1, is given by 1.87 eV. For the single crystals of SbSi : Co doped with 0.5 and 1 mol% of cobalt, and indirect energy gaps are also given by 1.75 and 1.65eV, respectively. Figure 2 shows the dependence of (~hv) v2 on the incident photon energy in pure SbSeI and SbSeI:Co single crystals. The indirect energy gap in these single crystals, obtained from Fig. 2, is given by 1.62 eV for the pure SbSeI, 1.30 eV for the SbSeI: Co (0.5mo1%) and 1.01 eV for the SbSeI:Co (1 mol%) single crystals. The measured values of 1.87 and 1.62 eV of the indirect energy gaps for the pure SbSI and the pure SbSeI single crystals, which correspond to indirect transitions between F- and S-band in the band
structure [5] of these compounds, is smaller than the theoretical values of 2.11 eV for SbSI and 1.67 eV for SbSeI single crystals. Figures 3 and 4 show the dependence of the optical energy gaps on the amounts of cobalt. As shown in Figs. 3 and 4, we can see that the energy gaps of SbSI:Co and SbSeI:Co single crystals are linearly decreased with increasing mole percents of cobalt. 3.2. Impurity optical absorption peaks of SbSI : Co and SbSeI : Co single crystals To investigate the impurity absorption characteristics of cobalt ions, we subtracted the optical density of pure SbSI and pure SbSeI from that of SbSI:Co and SbSeI:Co. These results are shown in Figs. 5 and 6. As shown in Fig. 5 of SbSI : Co single crystals, we observed the impurity absorption peaks due to the electronic transitions between the energy levels of Co
30 ~NGLE C~STAL
A
/1
2.0
1
/
Co: I mol %
i i
I I
__
SbSel:Co SINGLE CRYSTALS
/ PURE
Co:Q5 mol% ~ ~Sel
291K
I.O
2oI
°°°1
0
°°'
•
°'l W
Eg=IDIeV
I
OO 0.6
1.0 PHOTON
EQ: | 1.3OeV J
I
E~ 1.62eV
/
"'1 ~1 14
ENERGY
1.8
(eV)
Fig. 2. Plots of (~hv) ~/2 vs the incident photon energy hv in SbSeI and SbSeI : Co single crystals.
o.sI
0.0
J
0.5
I0
MOLE PERCENT OF COBALTS
Fig. 4. Optical energy gaps for SbSeI" Co single crystals with increasing cobalt.
CONDUCTION BAND
1.5 I=1 O ,¢
SbSI: Co SINGLE CRYSTAL 670era (14~5cm") C43÷(Td,3T2 )
)I-
(/1 Z
1045
OPTICAL PROPERTIES OF SbSI" Co AND SbSeI: Co
Vol. 68, No. 11
1.0 _ \ I
4"fi(4Pl
291 K
~('~-'~C~T'.
~E)
i- " ' •
sT2 (.~)
4~CONDUCTIO BAN[3, Nc4p
2~O0.m ( ~ a ""~T,( 3HI
.J
.~-
~-
/ o o ~
•'~.m ~ /r (IZ~L~cm") L '/ I
Z
Co2÷(Ta,4Td
Ill h ~
o
0.5
0.0
I~
(9090crn" ,I)Co3+(Ta?T,)
I
600
I
1000
" ~ ) 1 I II I I I'E(%)
I
1500
2000
VALENCE BAND
2500
SbSI : Co
WAVELENGTH ( n m )
Fig. 5. Difference optical density of SbSI and SbSI : Co single crystals, at the range of 600-2500 nm. ions, which are sited Td symmetry of crystal lattice as Co 3÷ and Co 2÷ ions [6]. The four impurity absorption peaks at 670nm(14925cm-~), 886nm(11 286cm-1), 1100 nm(9090 cm-i ) and 1435 nm(6738 cm-t) of Fig. 5 are attributed to the electronic transitions of Co 3+ ions from the ground state 5E(SD) to the excited states 3T2(3H), 3E(3H), 3T1(3H), and 5T2(SD). It can be explained that the other three impurity absorption peaks at 750nm(13 333cm-I), 1620nm(6172cm-I), and 2200nm(4545 cm -~) are attributed to the electronic transitions of Co 2+ ions from the ground state 4/12(aF) to the excited states 4T1(4p), 4TI (4F) ' and 4T2(4F). The crystal field parameters Dq in SbSI : Co single crystal, obtained from Fig. 5, are found to be 697 cm-I for Co 3÷, and 454cm -] for Co 2+ ion, respectively. As for the SbSeI : Co single crystals of Fig. 6, we also observed the impurity absorption peaks due to the electronic transitions between the energy levels of Co 3+ and Co 2÷ ions sited Ta symmetry of SbSeI host lattice as well as for the SbSI:Co 2.0 0 0
S b S e l : C0
SINGLE
CRYSTAL
291K
Fz
/ I (8474cm-')
.J ~
1.0
II
0
I
II
I
I
22,2nm(~zo~-').C~T, :T~
.,484nm (673ecm-')'C02+ (Td'4T' ) I[ 1035ran(96r~'c_m-' }'C~÷(Td ?TI) 773nm( 12936CLT)"1), Co24"(Td,4TI )
ul ul
0.0
i
600
I
~000
I
~500 WAVELENGTH
I
2000 ( nrn )
4~¢,....._rI VALENCE
BAND
SbSe I : CO
Fig. 7. Energy level diagram and electronic transition model due to optical absorption of SbSI:Co and SbSeI : Co single crystals. single crystals. The impurity absorption peaks at 1035 nm(9662 cm- l ) and 1180 nm(8474 cm- J) correspond to the electronic transitions from the groundstate 5E(SD) to the excited states 3Tt (3H) and 5Tz(SD) of Co 3÷ ions. The impurity absorption peaks due to Co 2÷ ions are also observed at 773 nm(12 936 cm-l), 1484 nm(6738 cm-1 ) and 2212 nm (4520 cm-i ), which are attributed to the electronic transitions from the ground state 4A2(4F) to the excited states 4TI(4P), 4Tl(4F), and 4T2(4F ) of Co 2+ ions: The crystal field parameters Dq in SbSeI single crystal are given by 847 cm-i for Co 3+ and 452 cm-i for Co 2+ ion, respectively. Figure 7 shows the energy band profile for the SbSI : Co and SbSeI : Co single crystals obtained from the optical absorption spectra of Figs. 5 and 6. 4. CONCLUSION Single crystals of SbSI : Co and SbSeI : Co grown by the Bridgman technique have orthorhombic structure. The indirect band gaps of these single crystals were decreased with increasing mole percents of cobalt. It is concluded that the impurity absorption peaks observed in the optical absorption spectra of the SbSI : Co and SbSeI : Co single crystals are attributed to the electronic transitions from the ground state to the excited states of Co ions, which are sited in Td symmetry of lattice as Co 3÷ and Co 2+ ions.
Acknowledgement -The present studies were supported by the Basic Science Research Institute Program. Ministry of Education, 1987. REFERENCES
2500
Fig. 6. Difference optical density of SbSeI and SbSeI : Co single crystals, at the range of 600-2500 nm.
1.
E. Kaldis ed., Current Topics in Material Science, Vol. 10, p. 55, North-Holland Pub. Comp., Amsterdam (1982).
1046 2.
.
4.
OPTICAL PROPERTIES OF SbSI : Co AND SbSeI : Co E. Fatuzzo, G. Harbeke, W.J. Merz, R. Nitsche, H. Roetschi & W. Ruppel, Phys. Rev. 127, 2036 (1962). T.A. Pikka & V.M. Fridkin, Soy. Phys.-Solid State 10, 2668 (1969). V.G. Alekseeva & E.G. Landsberg, Soy. Phys.-
.
6.
Vol. 68, No. 11
Solid State 8, 2518 (1969). Y.F. Alward, C.J. Fong, M. E1-Batanny & F. Wooten, Solid State Commun. 25, 307 (1978). T.M. Dunn, D.A. McClure & R.G. Pearson, Crystal Field Theory, p. 82, Haper and Row Pub. Comp., New York (1965).