- Email: [email protected]
E.P.R. Study of γ-irradiated single crystals of K2Cr2O7
J. Phgs. Chrm. SO/;&
1974. Vol. 35. pp. 606-608.
E.P.R. STUDY
Perpmon
Division,
Printed in Greal Britain
OF y-IRRADI’ATED K,Cr,07 M.
Radiochemistry
Press.
Bhabha
V.
SINGLE
CRYSTALS
OF
KRISHNAMURTHY
Atomic (Rcceicer/
Research I8 April
Centre,
Trombay,
Bombay-85,
India
1973)
I
The pentavalent state of chromium is not common and E.P.R. studies on CI-5’ have been made only in a few cases like chromium doped crystals of Ca,PO,CI[I], CaWO., 121 and Be.SiO,l31. Recentlv it has been reported that C>+ state -occursin irradiated crystals of’ potassium chromate[4, 51. The present paper deals with the study of irradiated single crystals of potassium dichromate. Single crystals of potassium dichromate were grown from saturated solution by slow evaporation. Analar grade materials were used. The orange crystals correspond to Groth’s description [6] and (010) plane can easily be identified. The crystals belong to the triclinic system, the space group being P][7]. The cell dimensions are (I= 7.52, b = 1340, c = 7.40 A, OL= 98”. p = 90’50’. y = 96”lO’ and there are four molecules per unit-cell [7]. Adopting the system of axes chosen by Groth. two new axes, b*- and cl*-axes. are defined such that h*-axis is perpendicular to (0 IO) plane and n*-axis is perpendicular to both b*- and c-axes. n*. h* and c-axes form an orthogonal system of crystal axes for the purpose of the present study. The crystals were irradiated with YJo y-rays for 65 hr at room temperature at the rate of 0.3 M rad/hr and the total dose is approximately I9 M rad. The E.P.R. spectra were recorded using a Varian V4502 X-band E.P.R. spectrometer equipped with a I2 in. rotating V-3900 magnet. The crystal orientation studies were made using a Varian V-4533 rotating cavity. 100 kHz field modulation and detection were employed. The microwave frequency was measured with a HewlettPackard frequency meter, Model X 532 B. A sample of DPPH was used for the calibration of g-values. The error in orienting the crystals is about & 3”. The irradiated crystals showed strong E.P.R. absorption at room temperature. For general orientations of the crystal, the spectrum consists of two closely spaced strong lines and several weak lines. The spectra obtained with the magnetic field oriented along b*-. (I*- and c-axes are shown in Fig. I. The strong lines are marked ‘A’ and the weak lines are marked B, C and D. The spectra were recorded for different orientations in cl*b*, b*c and (I*< planes. The two ‘A’ tines merge into a single line for certain orientations only. The weak lines marked ‘E’ do not show any relationship with the A lines and they often overlap. For different orientations, the line-width of A and B lines varies between I.2 and 2 G and that of C line between 6 and 8 G. At liquid nitrogen temperature, the A and B lines became stronger and the lines marked C and D are not observed. The present paper deals with the study of the strong A lines.
t’,=8990GHz
y. = 8995GHz
50 G
-
3283.0
Fig. I. E.P.R. spectrum of an irradiated single crystal of K,Cr?O; with the magnetic field along b*-, N*- and c-axes. The strong lines marked ‘A’ are identified to be due to CrO,“ion formed as a result of irradiation. Lines marked B, C and D are the weak lines not identified in this study. The paramagnetic centre responsible for the A lines obviously has S = l/2 and the spectra are analysed using the spin-Hamiltonian formalism. The principal values of the R-tensor and the direction cosines are determined using the method of Schonland [8] and shown in Table I. The two A lines correspond to centres which are not crystallographically related. As the a-values are very close to each other. it is concluded that the centre is same except that it occupies sites I and 2 which are not crystallographically related. The angular variation of ‘.e’ for tr*b* and tic planes is shown in Fig. 2. It can easily be seen that the experimental points lie close to the curves representating the calculated angular variation for the two sites. The departure of the principal g-values from the free-electron g-value of 2.0023 is considerable and the centre is possibly associated with chromium in KzCrz07 lattice. It is reported that Cr04”occurs in doped chlorospodiosite, Ca,PO,CI[ I] and irradiated crystals of potassium chromate[4, 51 and the g-values for Cr0,3606
Technical Notes
607
Table 1. Principal values of g-tensor for CrO.,R- ion $-tensor with respect to a*- b*- c-axes
Crystal
Site I 3.8404 -0.0147 -0.0129 Site 2 3.8462 -0.0201 -0.0155
K,CML 300°K
Principal g-values
-0.0129 0.0055 3.8603
g,, = I.9814 &r = I.9577 g,, = I .9658
-0.1856 - 0.3753 0.908 I
0.9755 -0.1817 0.1243
0.1184 0.9089 0.3998
Present work
-0~0201 3.9209 0.0058
-0.0155 0.0058 3.8572
szz = g,, = guu = g,, = g, = g,, = i?,, = guv = g,, = g,.r = g”” = g,, = g.r.T= gull =
- 0.2630 -0.4618 0.847 I
0.9550 -0.2495 0.1605
0.1372 0.8512 0.5066
Present work
108°K Be&O, 77°K
l-965
f I.960 ?z &
Y) 0
30
Reference
-0.0147 3.9224 0.0055
C+PO,CI 77°K K,CrO, 10°K
1.g55 b
Direction cosines with respect to a*- b*- c-axes
60
90
120
150
I80
(Deg)
Fig. 2. Angular variation of g in (I*C and u*b* planes. The curves respresent the calculated angular variation and circles and crosses represent the experimental points. CrO.,J- ion occupies sites I and 2 which are crystallographically independent.
I .98 I7 I.9578 I.9657 I .9936 I .9498 I .9776 I.9281 I.9539 I .9762 1.9337 I.9414 I .9929 I.9505 I.9581
[II [41
[51 [31
are close to the present g-values (see Table I). So it is concluded that the centre is CrOJ3- derived from Cr?O,*ion as a result of irradiation. Efforts to observe the expected hyperhne structure due to %‘Cr (9.55 per cent natural abundance) did not suceed. In the case of irradiated K,CrO,, ,Wr hyperfine lines were not detected in natural abundance[4, 51. The two B lines occurring close to A lines have intensities of about IO-15 per cent of the A lines and are not due to %Cr species as the expected four hyperfine lines should have intensity of 2.4 per cent of the strong line. There are four Cr20r2- ions in the unit-cell and fall into two sets of two which are related by inversion[7]. The two ions are not crystallographically related. Hence one expects to observe four non-equivalent sites occupied by CrO.,- centre. Experimentally only two sites are observed corresponding to the two A lines. Possibly the centres occupying the other two sites have less intensities and some of the weak lines observed close to A lines might be due to these. It is known from crystal structure studies171 that the four CrzO,?- ions in the unit-cell fall into two sets of two ions related by inversion. The two Cr,O,?- ions are not crystallographically related. The four chromium atoms in these two Cr,O,‘- ions are crystallographically independent and each chromium is surrounded by four oxygen atoms forming a tetrahedron. The two Cr-tetrahedra are linked through a common oxygen leading to the formation of Cr?Or diorthogroup. 0-Cr-0 angles occurring in Cr,O, diorthogroups vary from 105” to 113.5”. The six of the eight Cr-0 bond distances in each Cr,O, diorthogroup vary from I.51 to I .68 A. The remaining two Cr-0 bond distances involving the common oxygen are considerably elongated. In one of the Cr,Or ions these bond lengths are Cr,-OS = I.725 8, and Cr,-0, = I.856 A and LCr,-OSCr, = 127” where Cr, and Cr, are the two chromiums and On is the comma? oxygen atom. In the otherCr,O, group, Cr,-0, = I.842 A; Crl-Os = I.745 A and LCr.&&-
608
Technical Notes
0, = 122” where Cr, and Crr are the chromiums and 0, is the common oxygen. From these details it can be easily seen that the Cr- tetrahedra are considerably distorted. In the case of Ca,CrO,Cl, the two of the four Cr-0 bond distances are 1.7 13 A and other two bond distances are 1.685 A and the two 0-Cr-0 angles are 119.1” and the other two are 105.1” and 104.6”; so in this case CrO, tetrahedron is as if it is squashed along the bisector of 119. I” angle and the symmetry is &,,[ I, 91. In the case of CrOJ2- ion in alkali chromates the Cr-0 bond distances vary from I .59 to I .60 b; [ IO]. From these details it is clear that CrOs3- ion derived from Cr,O,*- ion is/very much distorted as compared to CrOJ3- formed in doped Ca,PO,CI (if it is assumed that the bond angles and bond distances occurring in CrOa3- ion are same or nearly same as those of the Cr,O,z- ion). Calculations showed that the principal axes of the g-tensor do not coincide with the bisectors of 0-Cr-0 angles obtained in the Cr-tetrahedra of the Cr,0-2- ions. Cr5+ occ&ring in CrO,“- ion has d’ configuration. The resulting ?D state is split into orbital doublet and triplet states if crystal field has pure tetrahedral symmetry, the doublet being the lowest in energy. With axial distortion into D?,, symmetry, the doublet is further split into d, and d,,+ orbital states. McGarvery has shown that for d,, ground state g,, = 2.0023 and g,, > gl. In the present case g,, (= 1.9814) is greater than grr (= 1.9577) and guu (= 1.9658). Hence d,, is the ground state as in the case of doped Ca,POICI [I]. But McGarvey’s calculations predict g,,= 2.0023. In his later .work, McGarvey [ I I] has argued that the departure of gl,from 2.0023 can arise from further distortion to C,,, or lower symmetry. In C,,. symmetry the electronic states can be written as
Table 2.
From second order perturbation are given as
REFERENCES I. Banks E., Greenblatt M. and McGarvey B. R., J. chern. Phys. 47, 3772 (I 967). 2. Lingam K. V., Nair P. G. and Venkataraman B., Proc. Ind. Acad. Sci. A70.29 (1969). 3. Tsukioka M., Yamamoto A. and Kojima H., J. phys. Sot. Japan 33.68 I (I 972). 4. Lister D. H. and Symons M. C. R., J. Chem. Sot. Section A, 782 (1970). 5. Debuyst R., Aspers D. J. and Capron P. C., J. inorg. nud. Chem. 34, 154 I ( 1972). 6. Groth P., Chemische Kryitallographie, Vol. II, p. 586(1906-19). 7. Kaz’min E. A., Iljukhin V. V., Kharitonov Yu. A. and Belov N. V.. Kristall Technik 4,441 (1969). 8. Schonland D. S., Proc. Phgs. Sot. A73,788 (1959). 9. Greenblatt M., Banks E. and Post B., Acta. crystallogr. 23, 166 ( 1967). IO. Structures Reports, Vol. 8, p. I58 (1940-41). International Union of Crystallography, N. V. A. Oosthock’s Uitgevers Mij, Utrecht (1956). Il. McGarvey B. R., Electron Spin Resonance of Metal Complexes, (Technical Editor, Fu. Yen) p. I. Adam Hilger Ltd., London (1969).
g,, = 2.0023 -= g,,=
theory, the g-components
8b*t
2.0023-2t(~;+b)’ “I
g,, = 2.0023 -
2.$(fla-b)2
AE
.I*
where AE,, is the energy separation between d,, and d+,, orbitals, .$ is the spin-orbit coupling. Employing these relations b*, [/AE,,, t/AE,, were calculated assuming AE,, = AE,, (calculations for CrOd3- in Ca,PO,CI showed that BE,, and AE,, are nearly equal [ I I]). The hyperfine parameters, P and K, are not calculated as “Cr hyperfine structure was not observed in the present case. The calculated parameters for CrO.,- in irradiated K2Cr207, doped C&POICI and irradiated KZCrOI are shown in Table 2. The values of s/AE,, for irradiated K&r,O, and doped C%PO,CI are comparable and the distortions from tetrahedral symmetry are small and
b2.
YAE,,
and [/AE,., parameters for CrO.,“- ion
Crystal
b’
KzCr207 Ca,PO,CI K,CrOl
0.0075 0.0053 0.0026
Slf=m
0.35 0.21 I .25
5/L=,,
0.0068* 0.0096 0~0108*~t
*Calculations are made assuming AE,, = AE,,. tData of Debuyst, Apers and Capton [5] is used. comparable in both these cases. b* parameter indicates that the contribution of d,,-,, to the ground state is finite. But in the case of irradiated K?CrO,, b* parameter is small while .$AE,, is rather large. In the case of doped Ca2P0,CI resonance was not detected at room temperature but was observed at liquid nitrogen temperature [ I] possibly because the distortions are not large and d,, and dzz+ states are close energetically making spin-lattice relaxation time T, short at room temperature. Lister and Symons[4] reported that the spectrum due to CrO,“- in irradiated K&rO., was observed at 10°K while Debuyst et c1/.[5] have reported that the same was observable at 108°K. In the present case of irradiated KzCrzOi the resonance could be detected at room temperature thereby implying that the distortions are sufficiently strong and/or other spin-lattice relaxation mechanisms are operative making T, long at room temperature. AcknolI~ledge,nen,s-The author wishes to express his grateful thanks to Dr. M. V. Ramaniah, Head, Radiochemistry Division for his encouragement during the course of this work. He is thankful to Mr. P. B. Ruikar for his assistance and to Dr. B. D. Joshi, D. M. Chackraburtty and Dr. P. R. Natarajan for their cooperation.
1974. Vol. 35. pp. 606-608.
E.P.R. STUDY
Perpmon
Division,
Printed in Greal Britain
OF y-IRRADI’ATED K,Cr,07 M.
Radiochemistry
Press.
Bhabha
V.
SINGLE
CRYSTALS
OF
KRISHNAMURTHY
Atomic (Rcceicer/
Research I8 April
Centre,
Trombay,
Bombay-85,
India
1973)
I
The pentavalent state of chromium is not common and E.P.R. studies on CI-5’ have been made only in a few cases like chromium doped crystals of Ca,PO,CI[I], CaWO., 121 and Be.SiO,l31. Recentlv it has been reported that C>+ state -occursin irradiated crystals of’ potassium chromate[4, 51. The present paper deals with the study of irradiated single crystals of potassium dichromate. Single crystals of potassium dichromate were grown from saturated solution by slow evaporation. Analar grade materials were used. The orange crystals correspond to Groth’s description [6] and (010) plane can easily be identified. The crystals belong to the triclinic system, the space group being P][7]. The cell dimensions are (I= 7.52, b = 1340, c = 7.40 A, OL= 98”. p = 90’50’. y = 96”lO’ and there are four molecules per unit-cell [7]. Adopting the system of axes chosen by Groth. two new axes, b*- and cl*-axes. are defined such that h*-axis is perpendicular to (0 IO) plane and n*-axis is perpendicular to both b*- and c-axes. n*. h* and c-axes form an orthogonal system of crystal axes for the purpose of the present study. The crystals were irradiated with YJo y-rays for 65 hr at room temperature at the rate of 0.3 M rad/hr and the total dose is approximately I9 M rad. The E.P.R. spectra were recorded using a Varian V4502 X-band E.P.R. spectrometer equipped with a I2 in. rotating V-3900 magnet. The crystal orientation studies were made using a Varian V-4533 rotating cavity. 100 kHz field modulation and detection were employed. The microwave frequency was measured with a HewlettPackard frequency meter, Model X 532 B. A sample of DPPH was used for the calibration of g-values. The error in orienting the crystals is about & 3”. The irradiated crystals showed strong E.P.R. absorption at room temperature. For general orientations of the crystal, the spectrum consists of two closely spaced strong lines and several weak lines. The spectra obtained with the magnetic field oriented along b*-. (I*- and c-axes are shown in Fig. I. The strong lines are marked ‘A’ and the weak lines are marked B, C and D. The spectra were recorded for different orientations in cl*b*, b*c and (I*< planes. The two ‘A’ tines merge into a single line for certain orientations only. The weak lines marked ‘E’ do not show any relationship with the A lines and they often overlap. For different orientations, the line-width of A and B lines varies between I.2 and 2 G and that of C line between 6 and 8 G. At liquid nitrogen temperature, the A and B lines became stronger and the lines marked C and D are not observed. The present paper deals with the study of the strong A lines.
t’,=8990GHz
y. = 8995GHz
50 G
-
3283.0
Fig. I. E.P.R. spectrum of an irradiated single crystal of K,Cr?O; with the magnetic field along b*-, N*- and c-axes. The strong lines marked ‘A’ are identified to be due to CrO,“ion formed as a result of irradiation. Lines marked B, C and D are the weak lines not identified in this study. The paramagnetic centre responsible for the A lines obviously has S = l/2 and the spectra are analysed using the spin-Hamiltonian formalism. The principal values of the R-tensor and the direction cosines are determined using the method of Schonland [8] and shown in Table I. The two A lines correspond to centres which are not crystallographically related. As the a-values are very close to each other. it is concluded that the centre is same except that it occupies sites I and 2 which are not crystallographically related. The angular variation of ‘.e’ for tr*b* and tic planes is shown in Fig. 2. It can easily be seen that the experimental points lie close to the curves representating the calculated angular variation for the two sites. The departure of the principal g-values from the free-electron g-value of 2.0023 is considerable and the centre is possibly associated with chromium in KzCrz07 lattice. It is reported that Cr04”occurs in doped chlorospodiosite, Ca,PO,CI[ I] and irradiated crystals of potassium chromate[4, 51 and the g-values for Cr0,3606
Technical Notes
607
Table 1. Principal values of g-tensor for CrO.,R- ion $-tensor with respect to a*- b*- c-axes
Crystal
Site I 3.8404 -0.0147 -0.0129 Site 2 3.8462 -0.0201 -0.0155
K,CML 300°K
Principal g-values
-0.0129 0.0055 3.8603
g,, = I.9814 &r = I.9577 g,, = I .9658
-0.1856 - 0.3753 0.908 I
0.9755 -0.1817 0.1243
0.1184 0.9089 0.3998
Present work
-0~0201 3.9209 0.0058
-0.0155 0.0058 3.8572
szz = g,, = guu = g,, = g, = g,, = i?,, = guv = g,, = g,.r = g”” = g,, = g.r.T= gull =
- 0.2630 -0.4618 0.847 I
0.9550 -0.2495 0.1605
0.1372 0.8512 0.5066
Present work
108°K Be&O, 77°K
l-965
f I.960 ?z &
Y) 0
30
Reference
-0.0147 3.9224 0.0055
C+PO,CI 77°K K,CrO, 10°K
1.g55 b
Direction cosines with respect to a*- b*- c-axes
60
90
120
150
I80
(Deg)
Fig. 2. Angular variation of g in (I*C and u*b* planes. The curves respresent the calculated angular variation and circles and crosses represent the experimental points. CrO.,J- ion occupies sites I and 2 which are crystallographically independent.
I .98 I7 I.9578 I.9657 I .9936 I .9498 I .9776 I.9281 I.9539 I .9762 1.9337 I.9414 I .9929 I.9505 I.9581
[II [41
[51 [31
are close to the present g-values (see Table I). So it is concluded that the centre is CrOJ3- derived from Cr?O,*ion as a result of irradiation. Efforts to observe the expected hyperhne structure due to %‘Cr (9.55 per cent natural abundance) did not suceed. In the case of irradiated K,CrO,, ,Wr hyperfine lines were not detected in natural abundance[4, 51. The two B lines occurring close to A lines have intensities of about IO-15 per cent of the A lines and are not due to %Cr species as the expected four hyperfine lines should have intensity of 2.4 per cent of the strong line. There are four Cr20r2- ions in the unit-cell and fall into two sets of two which are related by inversion[7]. The two ions are not crystallographically related. Hence one expects to observe four non-equivalent sites occupied by CrO.,- centre. Experimentally only two sites are observed corresponding to the two A lines. Possibly the centres occupying the other two sites have less intensities and some of the weak lines observed close to A lines might be due to these. It is known from crystal structure studies171 that the four CrzO,?- ions in the unit-cell fall into two sets of two ions related by inversion. The two Cr,O,?- ions are not crystallographically related. The four chromium atoms in these two Cr,O,‘- ions are crystallographically independent and each chromium is surrounded by four oxygen atoms forming a tetrahedron. The two Cr-tetrahedra are linked through a common oxygen leading to the formation of Cr?Or diorthogroup. 0-Cr-0 angles occurring in Cr,O, diorthogroups vary from 105” to 113.5”. The six of the eight Cr-0 bond distances in each Cr,O, diorthogroup vary from I.51 to I .68 A. The remaining two Cr-0 bond distances involving the common oxygen are considerably elongated. In one of the Cr,Or ions these bond lengths are Cr,-OS = I.725 8, and Cr,-0, = I.856 A and LCr,-OSCr, = 127” where Cr, and Cr, are the two chromiums and On is the comma? oxygen atom. In the otherCr,O, group, Cr,-0, = I.842 A; Crl-Os = I.745 A and LCr.&&-
608
Technical Notes
0, = 122” where Cr, and Crr are the chromiums and 0, is the common oxygen. From these details it can be easily seen that the Cr- tetrahedra are considerably distorted. In the case of Ca,CrO,Cl, the two of the four Cr-0 bond distances are 1.7 13 A and other two bond distances are 1.685 A and the two 0-Cr-0 angles are 119.1” and the other two are 105.1” and 104.6”; so in this case CrO, tetrahedron is as if it is squashed along the bisector of 119. I” angle and the symmetry is &,,[ I, 91. In the case of CrOJ2- ion in alkali chromates the Cr-0 bond distances vary from I .59 to I .60 b; [ IO]. From these details it is clear that CrOs3- ion derived from Cr,O,*- ion is/very much distorted as compared to CrOJ3- formed in doped Ca,PO,CI (if it is assumed that the bond angles and bond distances occurring in CrOa3- ion are same or nearly same as those of the Cr,O,z- ion). Calculations showed that the principal axes of the g-tensor do not coincide with the bisectors of 0-Cr-0 angles obtained in the Cr-tetrahedra of the Cr,0-2- ions. Cr5+ occ&ring in CrO,“- ion has d’ configuration. The resulting ?D state is split into orbital doublet and triplet states if crystal field has pure tetrahedral symmetry, the doublet being the lowest in energy. With axial distortion into D?,, symmetry, the doublet is further split into d, and d,,+ orbital states. McGarvery has shown that for d,, ground state g,, = 2.0023 and g,, > gl. In the present case g,, (= 1.9814) is greater than grr (= 1.9577) and guu (= 1.9658). Hence d,, is the ground state as in the case of doped Ca,POICI [I]. But McGarvey’s calculations predict g,,= 2.0023. In his later .work, McGarvey [ I I] has argued that the departure of gl,from 2.0023 can arise from further distortion to C,,, or lower symmetry. In C,,. symmetry the electronic states can be written as
Table 2.
From second order perturbation are given as
REFERENCES I. Banks E., Greenblatt M. and McGarvey B. R., J. chern. Phys. 47, 3772 (I 967). 2. Lingam K. V., Nair P. G. and Venkataraman B., Proc. Ind. Acad. Sci. A70.29 (1969). 3. Tsukioka M., Yamamoto A. and Kojima H., J. phys. Sot. Japan 33.68 I (I 972). 4. Lister D. H. and Symons M. C. R., J. Chem. Sot. Section A, 782 (1970). 5. Debuyst R., Aspers D. J. and Capron P. C., J. inorg. nud. Chem. 34, 154 I ( 1972). 6. Groth P., Chemische Kryitallographie, Vol. II, p. 586(1906-19). 7. Kaz’min E. A., Iljukhin V. V., Kharitonov Yu. A. and Belov N. V.. Kristall Technik 4,441 (1969). 8. Schonland D. S., Proc. Phgs. Sot. A73,788 (1959). 9. Greenblatt M., Banks E. and Post B., Acta. crystallogr. 23, 166 ( 1967). IO. Structures Reports, Vol. 8, p. I58 (1940-41). International Union of Crystallography, N. V. A. Oosthock’s Uitgevers Mij, Utrecht (1956). Il. McGarvey B. R., Electron Spin Resonance of Metal Complexes, (Technical Editor, Fu. Yen) p. I. Adam Hilger Ltd., London (1969).
g,, = 2.0023 -= g,,=
theory, the g-components
8b*t
2.0023-2t(~;+b)’ “I
g,, = 2.0023 -
2.$(fla-b)2
AE
.I*
where AE,, is the energy separation between d,, and d+,, orbitals, .$ is the spin-orbit coupling. Employing these relations b*, [/AE,,, t/AE,, were calculated assuming AE,, = AE,, (calculations for CrOd3- in Ca,PO,CI showed that BE,, and AE,, are nearly equal [ I I]). The hyperfine parameters, P and K, are not calculated as “Cr hyperfine structure was not observed in the present case. The calculated parameters for CrO.,- in irradiated K2Cr207, doped C&POICI and irradiated KZCrOI are shown in Table 2. The values of s/AE,, for irradiated K&r,O, and doped C%PO,CI are comparable and the distortions from tetrahedral symmetry are small and
b2.
YAE,,
and [/AE,., parameters for CrO.,“- ion
Crystal
b’
KzCr207 Ca,PO,CI K,CrOl
0.0075 0.0053 0.0026
Slf=m
0.35 0.21 I .25
5/L=,,
0.0068* 0.0096 0~0108*~t
*Calculations are made assuming AE,, = AE,,. tData of Debuyst, Apers and Capton [5] is used. comparable in both these cases. b* parameter indicates that the contribution of d,,-,, to the ground state is finite. But in the case of irradiated K?CrO,, b* parameter is small while .$AE,, is rather large. In the case of doped Ca2P0,CI resonance was not detected at room temperature but was observed at liquid nitrogen temperature [ I] possibly because the distortions are not large and d,, and dzz+ states are close energetically making spin-lattice relaxation time T, short at room temperature. Lister and Symons[4] reported that the spectrum due to CrO,“- in irradiated K&rO., was observed at 10°K while Debuyst et c1/.[5] have reported that the same was observable at 108°K. In the present case of irradiated KzCrzOi the resonance could be detected at room temperature thereby implying that the distortions are sufficiently strong and/or other spin-lattice relaxation mechanisms are operative making T, long at room temperature. AcknolI~ledge,nen,s-The author wishes to express his grateful thanks to Dr. M. V. Ramaniah, Head, Radiochemistry Division for his encouragement during the course of this work. He is thankful to Mr. P. B. Ruikar for his assistance and to Dr. B. D. Joshi, D. M. Chackraburtty and Dr. P. R. Natarajan for their cooperation.