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Electron paramagnetic resonance studies of VO2+ in single crystals of alkali halides
JOURNAL
OF MAGNETIC
RESONANCE
16,1%2&t
(1974)
Electron ParamagneticResonanceStudiesof VOz+ in Single Crystals of Alkali Halides S. RADHAKRISHNA,
B. V. R. CHOWDARI, AND A. KASI VISWANATH
Department of Physics, Indian Institute of Technology, Madras 600036, India Presented at the Fifth International Symposium on Magnetic Resonance, Bombay, India, January, 1974 Results of electron paramagnetic resonance studies of VOz+ in KBr and KI are reported over a wide temperature range. At room temperature there is an isotropic eight-line pattern attributed to a freely rotating VOZ+ molecular ion in the crystal. At low temperature an anisotropic spectrum attributed to the hindered rotation of VOZ+ ion is obtained. The spin-Hamiltonian parameters at room and liquidnitrogen temperatures are determined, and study of their dependence on the lattice constant shows that as the lattice constant increases, the hypertlne parameter (A,) increases while g,, and linewidth decrease. INTRODUCTION
Although several paramagnetic species in single crystals have been studied in detail, relatively few molecular impurities have been studied. The study of the EPR of molecular impurities like V02+ in single crystals will give information about the effects of the host lattice on the rotation and vibration of the molecule in addition to the information about the associated defects. So far V02’ has been studied in Tutton salts (I), GeO, (2), KNO,, CsNO, (3), alums (d-6), and double sulphates (7, 8). In all these cases, oxygen is a primary constituent. Experiments on these crystals have shown that the V4+ ion of the V02+ is surrounded by a distorted octahedron of oxygens. The octahedron is composed of five oxygens of the lattice and one oxygen of the VO molecule. In all these cases except KNO, and CsNO,, there are preferred orientations of the VO molecule. In KNOJ and CsNO,, V02+ does not have any preferred orientations and behaves as it does in the solution of vanadyl etioporphyrin (VEPI) in benzene solution (9). VO’+ has also been studied in single crystals in which chlorine is a primary constituent. In lattices like NH&l (IO), NaCI, KCl, and RbCl (II), V02+ does not have any preferred orientation. All these studies described so far suggest that V02+ ion has preferred orientations in some oxygen ligands while it is freely rotating in other oxygen and chlorine ligands and is similar to a liquid solution. The purpose of the present investigations is to see whether V02+ molecules will rotate freely or take preferential orientations in lattices where bromineand iodine are the primary constituents. We have studied the EPR spectra of V02+ in KBr and KI, and by comparing our results with the results obtained in V02+-doped NaCl and KCl, we propose to correlate the spin-Hamiltonian parameters with the lattice constants of the host crystal. Co y&&t QJ 1974 by Academic Press, Inc. At P.rights of reproduction in any form reserved. Printed in Great Britain
199
200
RADHAKRISHNA,
CHOWDARI
EXPERIMENTAL
AND
VISWANATH
PROCEDURE
By using Analar grade KBr and KI powders and vanadyl chloride, single crystals of KBr and KI doped with V02+ ions have been grown by slow evaporation of the saturated solutions. EPR spectra have been recorded by using a Varian E-4 X-band spectrometer with lOO-kHz field modulation. Spectra are recorded over the temperature range +25”C to -160°C by using aVarian E-257 variable temperature accessory. For recording the spectra at LNT, a quartz dewar was used. DPPH, for which the g value is taken as 2.0036, is used as field marker. RESULTS
AND
DISCUSSION
Figure 1A shows the EPR spectrum of V02+ in KBr crystal at room temperature. It can be seen from the spectrum that there are eight lines. The intrinsic symmetry of the V02+ molecule reduces the ground state of tetravalent vanadium ion (38) to an orbital
DPPH 3232.5
50G h+++ 5oc B~/r-------
bb
bb
b
FIG. 1. EPR spectraof VO*+ in KBr at various temperatures.Curves A-D are the spectrarecorded at room temperature and at -40, -160, and -196”C, respectively. Lines marked “a” are parallel components, and lines marked “b” are perpendicular components.
singlet. The EPR spectrum ofthe 3d’ electron consists of one fine structure line which will split into eight lines because of the hyperfine interaction of this electron and the 9 nucleus (I= 7/2). This eight-line pattern is representative of a freely rotating V02+ molecular impurity consistent with the known spectrum of VEPI in benzene solution
VANADYL
ION
IN ALKALI
201
HALIDES
(9). As the sample is cooled to -40°C (Fig. lB), all the lines are broadened and the intensity decreases. As the temperature is lowered, further new lines emerge as can be seen in Fig. 1C. At this temperature (-16O”C), we can see two groups of lines (marked “a” and “b”). The spectrum taken at LNT (Fig. 1D) in a separate dewar enables the two groups of lines to be resolved better. This spectrum is similar to the spectrum obtained for VEPI in highly viscous oil (9) and VO*+ in polycrystalline Tutton salts (I). This similarity shows that KBr essentially behaves like a viscous liquid at low temperatures as far as the rotation of VO*+ ion is concerned. Hence all orientations of VO are possible in the crystal just as in the case of liquid, and readjustment of the VO molecule from one orientation to the other takes place. This results in the random distribution of the VO molecule in the lattice. At room temperature, such readjustment is very fast (correlation time is small), and hence we observe an eight-line isotropic pattern instead of two groups of lines. One of the groups (marked “a”) pertains to the VO*+ molecule oriented in the direction of the magnetic field while the other group (marked “b”) represents the orientation of the molecule in a direction perpendicular to the magnetic field. Both these groups of lines are angular independent. The spectra are analyzed using the spin Hamiltonian (9) :
[II
~=g,,B~,~,+g,B(~,~,+~,~,)+AS,~,+B(~,~,+Z,~,)
corresponding to VOZf in tetragonal symmetry. The various terms in the above equation have their usual meaning. Making use of the above equation, the spin-Hamiltonian parameters are obtained and given in Table 1. The parameters for VO*+ in NaCl and KC1 (II) are also given for comparison. The Hamiltonian given in Eq. [I] reduces to an isotropic Hamiltonian (9, 12) when TABLE
I
SPIN-HAMILTONIAN~ARAMETERSFOR VO*+ INALKALIHALIDES
Crystal system NaCl (II)
Lattice constant (4 5.62
T,,, - Ta,,rso Room-temperature spectrum (“Cl -160
go = 1.971 fc 0.001 Ao” = 104.0 + 1.0
KC1 (11)
6.28
-160
g, = 1.969 + 0.002 A,, = 106.8 + 2.0
KBr
6.58
-35
go = 1.965 f 0.001 Ao = 108.8 + 1.O
KI
7.06
-40
go = 1.932 + 0.001 At, = 109.5 + 1 .O
’ The values of Ao, A, and Bare in the units of IO-” cm-‘.
Liquid-nitrogentemperature spectrum g ,= g, = A” = B” = g,! = g, = A = B= g,, = g, = A= B= g!, = g, = A= B=
1.925 f 0.002 1.996 + 0.002 176.8 + 2.0 64.2 + 2.0 I .932 + 0.002 1.988 & 0.002 186.0 + 3.0 70.4 + 5.0 1.933 + 0.002 1.986 + 0.002 190.2 + 1.0 61.8 + 2.0 1.932 + 0.002 1.983 + 0.002 194.5 + 1.0 60.2 Z!Y2.0
Linewidth for mI = 312 (gauss) 20
16.9
16.3
10
202
RADHAKRISHNA,
CHOWDARI
AND
VISWANATH
the correlation time is short. Assuming that the correlation time of VO in KBr is short, the room-temperature spectra have been analyzed and the isotropic spin-Hamiltonian parameters (A, and g,) are also given in Table 1. It can be seen from Table 1 that the relations g,, = $(g,, + 2gJ and A, = +(A + 2B) hold well, which can be taken as a support to the assumption that V02+ ion is freely rotating at room temperature and executes hindered rotations at temperatures below -35°C. Similar experiments have been performed for V02+ in KI single crystals, and a representative spectrum observed at room temperature is given in Fig. 2. The results obtained are similar to KBr both at room temperature and low temperatures, and the spin-Hamiltonian parameters are tabulated in Table 1.
1
FIG.
2. EPR
spectrum of
VOz+
in KI at room
temperature.
The room-temperature spectrum completely disappears at -35°C in KBr and -40°C in KI whereas the corresponding temperatures are found to be -160°C for NaCl and KC1 (II), -150°C for KNOB, and -70°C in CsNO, (3). This shows that the temperature where the motion becomes restricted is strongly dependent on the lattice. It appears that KBr and KI are more effective than the other alkali halides in restricting the free motion of V02+ ion in the lattice. It is known that there is a dependence of the linewidth on the nuclear spin quantum number (mJ. In the spectra observed for KBr and KI at room temperature, a similar result has been observed and Fig. 3 shows the dependence of the linewidth on m, for these lattices. This result is an independent check for the fact that the molecule exhibits free rotation at room temperature. According to Kivelson’s (13) theory of liquids, the linewidths are expected to have an asymmetrical (linear and quadratic) dependence on m, since “g” and “A” are anisotropic. This is an experimentally observed fact in the present case and therefore suggests that the molecule is in a liquid medium for purposes of EPR spectra. Since Kivelson’s theory of liquids is found to be valid for V02+ ion in KBr and KI, the linewidth may be represented by an equation of the type (14) & = d5
(a, + a, m, + a3 m’;),
203
VANADYL ION IN ALKALI HALIDES
FIG. 3. Variation of linewidth versus the nuclear spin quantum number m, for VO’+ KRr, and KI. The data for KC1 are taken from Ref. (11)).
in KC1
where a,, a,, and a3 are constants. Using the experimentally observed linewidth, the best set of values for a,, a,, and a, are obtained by least-squares fitting. A HewlettPackard Calculator (9100A) has been used for this purpose. The constants are found to be a, = 17.5, u* = 1.195, and uJ = 0.52 for KBr and a, = 11.35,
a, = 1.99,
and
u3 = 0.967 for KT.
From Table 1 it can be seen that as the lattice constant increases, the hyperhne constant A0 increases, indicating that the electron “sees” more of the nucleus when the halogen ion is taken away from VO*+ ion. The electron therefore has a stronger interaction with the nucleus. The linewidth for a particular m, value is found to decrease as the lattice constant increases. This is as expected since the linewidth is a result of the unresolved superhyperfine structure which arises because of the interaction of the halogen ion with the vanadium nucleus. As the halogen ion is farther away, this interaction is less. A study of the VO*+ ion in other lattices will enable us to draw more definite conclusions. ACKNOWLEDGMENTS The authors thank Prof. C. Ramasastry and the Structural Chemistry Group for their interest in the work. One of theauthors (AKV) thanks the Councilof ScientificandIndustrial Research,NewDeIhi, for awarding the research fellowship. REFERENCES I. 2. 3. 4.
R. I. K. A.
H. B~RCHERTS AND C. KIKUCHI, J. Chem. Phys. 40,227O (1964). SIEGEL, Phys. Rev. 134, Al93 (1964). V. S. RAO, M. D. SASTRY, AND P. VENKATESWARLU, J. Chem. Phys. MANOOGIAN AND J. A. MACKINNIN, Can. J. Phys. 45,2769 (1969).
49,1714 (1968).
204
RADHAKRISHNA,
CHOWDARI
AND
VISWANATH
5. K. V. S. RAO, M. D. SASTRY, AND P. VENKATESWARLU, J. Chem. Phys. 49,4984 (1968). 6. K. V. S. RAO, M. D. SASTRY, AND P. VENKATESWARLU, J. Chem. Phys. 51,812 (1969). 7. B. V. R. CHOWDARI, J. Phys. Sot. Japan 27,1135 (1969). 8. B. V. R. CHOWDARI, J. Phys. Sot. Japan 29,105 (1970). 9. D. E. O’REILLY, J. Chem. Phys. 29,1188 (1958). 10. M. D. SASTRY AND P. VENKATESWARLU, Mol. Phys. 13,161 (1967). 11. A. V. JAGANNADHAM AND P. VENKATESWARLU. Proc. Znd. Acad. Sci. 69,307 (1969). 12. H. M. MCCONNELL, J. Chem. Phys. 25,709 (1956). 13. D. KIVELSON, J. Chem. Phys. 33,1094 (1960). 14. R. N. ROGERS AND G. E. PAKE, J. Chem. Phys. 33,1107 (1960).
OF MAGNETIC
RESONANCE
16,1%2&t
(1974)
Electron ParamagneticResonanceStudiesof VOz+ in Single Crystals of Alkali Halides S. RADHAKRISHNA,
B. V. R. CHOWDARI, AND A. KASI VISWANATH
Department of Physics, Indian Institute of Technology, Madras 600036, India Presented at the Fifth International Symposium on Magnetic Resonance, Bombay, India, January, 1974 Results of electron paramagnetic resonance studies of VOz+ in KBr and KI are reported over a wide temperature range. At room temperature there is an isotropic eight-line pattern attributed to a freely rotating VOZ+ molecular ion in the crystal. At low temperature an anisotropic spectrum attributed to the hindered rotation of VOZ+ ion is obtained. The spin-Hamiltonian parameters at room and liquidnitrogen temperatures are determined, and study of their dependence on the lattice constant shows that as the lattice constant increases, the hypertlne parameter (A,) increases while g,, and linewidth decrease. INTRODUCTION
Although several paramagnetic species in single crystals have been studied in detail, relatively few molecular impurities have been studied. The study of the EPR of molecular impurities like V02+ in single crystals will give information about the effects of the host lattice on the rotation and vibration of the molecule in addition to the information about the associated defects. So far V02’ has been studied in Tutton salts (I), GeO, (2), KNO,, CsNO, (3), alums (d-6), and double sulphates (7, 8). In all these cases, oxygen is a primary constituent. Experiments on these crystals have shown that the V4+ ion of the V02+ is surrounded by a distorted octahedron of oxygens. The octahedron is composed of five oxygens of the lattice and one oxygen of the VO molecule. In all these cases except KNO, and CsNO,, there are preferred orientations of the VO molecule. In KNOJ and CsNO,, V02+ does not have any preferred orientations and behaves as it does in the solution of vanadyl etioporphyrin (VEPI) in benzene solution (9). VO’+ has also been studied in single crystals in which chlorine is a primary constituent. In lattices like NH&l (IO), NaCI, KCl, and RbCl (II), V02+ does not have any preferred orientation. All these studies described so far suggest that V02+ ion has preferred orientations in some oxygen ligands while it is freely rotating in other oxygen and chlorine ligands and is similar to a liquid solution. The purpose of the present investigations is to see whether V02+ molecules will rotate freely or take preferential orientations in lattices where bromineand iodine are the primary constituents. We have studied the EPR spectra of V02+ in KBr and KI, and by comparing our results with the results obtained in V02+-doped NaCl and KCl, we propose to correlate the spin-Hamiltonian parameters with the lattice constants of the host crystal. Co y&&t QJ 1974 by Academic Press, Inc. At P.rights of reproduction in any form reserved. Printed in Great Britain
199
200
RADHAKRISHNA,
CHOWDARI
EXPERIMENTAL
AND
VISWANATH
PROCEDURE
By using Analar grade KBr and KI powders and vanadyl chloride, single crystals of KBr and KI doped with V02+ ions have been grown by slow evaporation of the saturated solutions. EPR spectra have been recorded by using a Varian E-4 X-band spectrometer with lOO-kHz field modulation. Spectra are recorded over the temperature range +25”C to -160°C by using aVarian E-257 variable temperature accessory. For recording the spectra at LNT, a quartz dewar was used. DPPH, for which the g value is taken as 2.0036, is used as field marker. RESULTS
AND
DISCUSSION
Figure 1A shows the EPR spectrum of V02+ in KBr crystal at room temperature. It can be seen from the spectrum that there are eight lines. The intrinsic symmetry of the V02+ molecule reduces the ground state of tetravalent vanadium ion (38) to an orbital
DPPH 3232.5
50G h+++ 5oc B~/r-------
bb
bb
b
FIG. 1. EPR spectraof VO*+ in KBr at various temperatures.Curves A-D are the spectrarecorded at room temperature and at -40, -160, and -196”C, respectively. Lines marked “a” are parallel components, and lines marked “b” are perpendicular components.
singlet. The EPR spectrum ofthe 3d’ electron consists of one fine structure line which will split into eight lines because of the hyperfine interaction of this electron and the 9 nucleus (I= 7/2). This eight-line pattern is representative of a freely rotating V02+ molecular impurity consistent with the known spectrum of VEPI in benzene solution
VANADYL
ION
IN ALKALI
201
HALIDES
(9). As the sample is cooled to -40°C (Fig. lB), all the lines are broadened and the intensity decreases. As the temperature is lowered, further new lines emerge as can be seen in Fig. 1C. At this temperature (-16O”C), we can see two groups of lines (marked “a” and “b”). The spectrum taken at LNT (Fig. 1D) in a separate dewar enables the two groups of lines to be resolved better. This spectrum is similar to the spectrum obtained for VEPI in highly viscous oil (9) and VO*+ in polycrystalline Tutton salts (I). This similarity shows that KBr essentially behaves like a viscous liquid at low temperatures as far as the rotation of VO*+ ion is concerned. Hence all orientations of VO are possible in the crystal just as in the case of liquid, and readjustment of the VO molecule from one orientation to the other takes place. This results in the random distribution of the VO molecule in the lattice. At room temperature, such readjustment is very fast (correlation time is small), and hence we observe an eight-line isotropic pattern instead of two groups of lines. One of the groups (marked “a”) pertains to the VO*+ molecule oriented in the direction of the magnetic field while the other group (marked “b”) represents the orientation of the molecule in a direction perpendicular to the magnetic field. Both these groups of lines are angular independent. The spectra are analyzed using the spin Hamiltonian (9) :
[II
~=g,,B~,~,+g,B(~,~,+~,~,)+AS,~,+B(~,~,+Z,~,)
corresponding to VOZf in tetragonal symmetry. The various terms in the above equation have their usual meaning. Making use of the above equation, the spin-Hamiltonian parameters are obtained and given in Table 1. The parameters for VO*+ in NaCl and KC1 (II) are also given for comparison. The Hamiltonian given in Eq. [I] reduces to an isotropic Hamiltonian (9, 12) when TABLE
I
SPIN-HAMILTONIAN~ARAMETERSFOR VO*+ INALKALIHALIDES
Crystal system NaCl (II)
Lattice constant (4 5.62
T,,, - Ta,,rso Room-temperature spectrum (“Cl -160
go = 1.971 fc 0.001 Ao” = 104.0 + 1.0
KC1 (11)
6.28
-160
g, = 1.969 + 0.002 A,, = 106.8 + 2.0
KBr
6.58
-35
go = 1.965 f 0.001 Ao = 108.8 + 1.O
KI
7.06
-40
go = 1.932 + 0.001 At, = 109.5 + 1 .O
’ The values of Ao, A, and Bare in the units of IO-” cm-‘.
Liquid-nitrogentemperature spectrum g ,= g, = A” = B” = g,! = g, = A = B= g,, = g, = A= B= g!, = g, = A= B=
1.925 f 0.002 1.996 + 0.002 176.8 + 2.0 64.2 + 2.0 I .932 + 0.002 1.988 & 0.002 186.0 + 3.0 70.4 + 5.0 1.933 + 0.002 1.986 + 0.002 190.2 + 1.0 61.8 + 2.0 1.932 + 0.002 1.983 + 0.002 194.5 + 1.0 60.2 Z!Y2.0
Linewidth for mI = 312 (gauss) 20
16.9
16.3
10
202
RADHAKRISHNA,
CHOWDARI
AND
VISWANATH
the correlation time is short. Assuming that the correlation time of VO in KBr is short, the room-temperature spectra have been analyzed and the isotropic spin-Hamiltonian parameters (A, and g,) are also given in Table 1. It can be seen from Table 1 that the relations g,, = $(g,, + 2gJ and A, = +(A + 2B) hold well, which can be taken as a support to the assumption that V02+ ion is freely rotating at room temperature and executes hindered rotations at temperatures below -35°C. Similar experiments have been performed for V02+ in KI single crystals, and a representative spectrum observed at room temperature is given in Fig. 2. The results obtained are similar to KBr both at room temperature and low temperatures, and the spin-Hamiltonian parameters are tabulated in Table 1.
1
FIG.
2. EPR
spectrum of
VOz+
in KI at room
temperature.
The room-temperature spectrum completely disappears at -35°C in KBr and -40°C in KI whereas the corresponding temperatures are found to be -160°C for NaCl and KC1 (II), -150°C for KNOB, and -70°C in CsNO, (3). This shows that the temperature where the motion becomes restricted is strongly dependent on the lattice. It appears that KBr and KI are more effective than the other alkali halides in restricting the free motion of V02+ ion in the lattice. It is known that there is a dependence of the linewidth on the nuclear spin quantum number (mJ. In the spectra observed for KBr and KI at room temperature, a similar result has been observed and Fig. 3 shows the dependence of the linewidth on m, for these lattices. This result is an independent check for the fact that the molecule exhibits free rotation at room temperature. According to Kivelson’s (13) theory of liquids, the linewidths are expected to have an asymmetrical (linear and quadratic) dependence on m, since “g” and “A” are anisotropic. This is an experimentally observed fact in the present case and therefore suggests that the molecule is in a liquid medium for purposes of EPR spectra. Since Kivelson’s theory of liquids is found to be valid for V02+ ion in KBr and KI, the linewidth may be represented by an equation of the type (14) & = d5
(a, + a, m, + a3 m’;),
203
VANADYL ION IN ALKALI HALIDES
FIG. 3. Variation of linewidth versus the nuclear spin quantum number m, for VO’+ KRr, and KI. The data for KC1 are taken from Ref. (11)).
in KC1
where a,, a,, and a3 are constants. Using the experimentally observed linewidth, the best set of values for a,, a,, and a, are obtained by least-squares fitting. A HewlettPackard Calculator (9100A) has been used for this purpose. The constants are found to be a, = 17.5, u* = 1.195, and uJ = 0.52 for KBr and a, = 11.35,
a, = 1.99,
and
u3 = 0.967 for KT.
From Table 1 it can be seen that as the lattice constant increases, the hyperhne constant A0 increases, indicating that the electron “sees” more of the nucleus when the halogen ion is taken away from VO*+ ion. The electron therefore has a stronger interaction with the nucleus. The linewidth for a particular m, value is found to decrease as the lattice constant increases. This is as expected since the linewidth is a result of the unresolved superhyperfine structure which arises because of the interaction of the halogen ion with the vanadium nucleus. As the halogen ion is farther away, this interaction is less. A study of the VO*+ ion in other lattices will enable us to draw more definite conclusions. ACKNOWLEDGMENTS The authors thank Prof. C. Ramasastry and the Structural Chemistry Group for their interest in the work. One of theauthors (AKV) thanks the Councilof ScientificandIndustrial Research,NewDeIhi, for awarding the research fellowship. REFERENCES I. 2. 3. 4.
R. I. K. A.
H. B~RCHERTS AND C. KIKUCHI, J. Chem. Phys. 40,227O (1964). SIEGEL, Phys. Rev. 134, Al93 (1964). V. S. RAO, M. D. SASTRY, AND P. VENKATESWARLU, J. Chem. Phys. MANOOGIAN AND J. A. MACKINNIN, Can. J. Phys. 45,2769 (1969).
49,1714 (1968).
204
RADHAKRISHNA,
CHOWDARI
AND
VISWANATH
5. K. V. S. RAO, M. D. SASTRY, AND P. VENKATESWARLU, J. Chem. Phys. 49,4984 (1968). 6. K. V. S. RAO, M. D. SASTRY, AND P. VENKATESWARLU, J. Chem. Phys. 51,812 (1969). 7. B. V. R. CHOWDARI, J. Phys. Sot. Japan 27,1135 (1969). 8. B. V. R. CHOWDARI, J. Phys. Sot. Japan 29,105 (1970). 9. D. E. O’REILLY, J. Chem. Phys. 29,1188 (1958). 10. M. D. SASTRY AND P. VENKATESWARLU, Mol. Phys. 13,161 (1967). 11. A. V. JAGANNADHAM AND P. VENKATESWARLU. Proc. Znd. Acad. Sci. 69,307 (1969). 12. H. M. MCCONNELL, J. Chem. Phys. 25,709 (1956). 13. D. KIVELSON, J. Chem. Phys. 33,1094 (1960). 14. R. N. ROGERS AND G. E. PAKE, J. Chem. Phys. 33,1107 (1960).