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Perturbed hole centers in KMgF3
I. Phys. Chem. Solids, 1976, Vol. 37, pp. 346347.
Pergamon Prers.
Printed in Great Britain
PERTURBED
HOLE
CENTERS
IN KMgF,t
J. E. RHOADS& B. H. ROSE§ and L. E. HALLIBURTON Physics Department, Oklahoma State University, Stillwater, OK 74074,U.S.A. (Received 29 April 1975; accepted 25 July 1975)
The intrinsic self-trapped hole center has been well characterized in the cubic perovskite KMgF,[l-31. As with similar defects in the alkali halides[4], the hole is equally shared by two neighboring fluorines and is labeled an [FIJ center to reflect its molecular nature. When compared to the alkali halides, an interesting property of the intrinsic [FZJ center in KMgF, is the lower symmetry about the fluorine-fluorine axis and a resulting “bent bond” configuration[ 11. In the present note we report the results of an ESR (electron spin resonance) investigation of perturbed self-trapped hole centers in KMgF,. The samples were grown by the Stockbarger method in the Crystal Growth Laboratory of Oklahoma State University and the defects were created by irradiation with 1.5MeV electrons from a Van de Graaff accelerator. At 77 K, a short electron irradiation produces predominately the intrinsic [F2J centers and no observable number of perturbed [F2J centers. Upon warming the crystal above approximately 110K, the intrinsic centers become mobile[2,3] and either recombine with electrons or are retrapped adjacent to perturbations in the lattice such as impurities or vacancies. As shown in Fig. 1, two distinct perturbed [FZJ centers grow in as the intrinsic centers are thermally destroyed. The decay temperatures of these two perturbed centers are 15OK and approximately 295K respectively[2,3]. The decay of the one center at 150K is associated with motion of the hole since there is no decrease in the concentration of the other more stable perturbed center at this lower temperature. If electrons were being released at 150K, then both perturbed hole centers would be expected to decrease. There FOLLOWING IRRADIATION AT 77 K
AFTER WARMING TO 140 K (GAIN
x2)
AFTER
WARMING TO 170 K (GAIN X2)
is no evidence to distinguish whether the 295K center decays as a result of electron release or hole release. The two perturbed [F2J centers are very similar in structure to the intrinsic center and their ESR line positions are only slightly shifted from those of the intrinsic center. The spectrum corresponding to the more stable perturbed [FZ-] center has been isolated by preferentially destroying both the intrinsic center and the other perturbed [FZJ center with a thermal anneal to temperatures above 150K. The spectrum of this 295K center for the magnetic field along the [lib] direction is shown in Fig. 2 (comparison should be made with Fig. 1 of Ref. [l]). In addition to the small shifts in line position, the most important difference between the intrinsic center and this perturbed center is a splitting of the inner lines arising from the fluorine molecules making a 60 angle with the magnetic field. Each of the two inner 60”lines for the intrinsic center are split into well resolved doublets in the case of the perturbed center. This splitting of lines reflects a reduction in symmetry for the perturbed center compared to the intrinsic center and indicates the perturbation, if assumed to be a nearest neighbor, must be at a potassium site. If the perturbation were at the nearest neighbor magnesium site, it would only change the bond angle, not the symmetry of the defect. Thus no orientational degeneracies would be lifted and there would be no splitting of ESR lines when compared to the intrinsic center. The perturbation, possibly a vacancy or a smaller impurity ion such as Na’ or Li’, at a nearest neighbor potassium site would allow the fluorine molecule to tilt out of the (001) plane by rotating about the internuclear axis and our calculations have shown that the inner 60” ESR lines would be the most sensitive to such a tilt. The appropriate models for the intrinsic [F,J center and the 295K perturbed center are shown in Fig. 3. The spin Hamiltonian used to describe the perturbed center decaying at 295K is
Since the hole is equally shared between the two fluorines, the magnitudes of the A, and AZ hyperfine tensors are equal even though the principal axes for the two tensors are not equivalent. In Fig. 3, the angle 0 represents the deviations of the z, and z2 hypetfme tensor axes from the fluorine internuclear axis and the angle S represents the amount of tilt of the x axis away from the
Fig. 1.Thermal stability of the intrinsic and perturbed [F,J centers in KMnF,. The spectra represent the high field 0” lines (split into three c&ponents by the superhypertine interaction with the two adjacent fluorine nuclei along the axis of the molecular ion). The magnetic field is along the [ 1101direction and the spectra were taken at 96K after briefly warming the sample to the indicated temperatures. The line positions are given in units of gauss. tWork supported by Research Corporation and by National Science Foundation Grant GH-39108. SPresent address: El Paso Natural Gas Company, El Paso, Texas. BPresent address: Brookhaven National Laboratory, Upton, New York.
[liO] direction for the perturbed center. For the i tensor of both centers, the z axis is along the internuclear axis and the y axis is perpendicular to the plane of the molecule. Using the basis set I&, ml, m2), the resulting 8x 8 matrix form of the spin Hamiltonian was repeatedly diagonalized by computer while the parameters were systematically adjusted to give a “best” fit to the experimental line positions in the [loo] and [IlO] spectra. Additional details concerning the determination of parameters are contained in Ref. [S]. Table 1 contains the spin Hamiltonian parameters for both the intrinsic [F2J center and the perturbed center decaying at 295K. An additional property of this more stable perturbed center is a reversible reduction in amplitude of the ESR signal with increasing temperature that cannot be attributed simply to a Boltzmann effect. A most likely explanation for this broadening of the ESR lines is a rapid motion of the trapped hole about the neighboring perturbation although a very short spin lattice relaxation time cannot be ruled out. 346
341
Technical Notes
T =77K r/ = 9.2lOGHz
I
I II
I
I
0”
160”
II
-
90”
I
I
I
I
2500 G
3OOOG
3500 G
4000 G
Fig. 2. ESR spectrum of the perturbed [FZ-]center stable to 295K when the magnetic field is along the [llO] direction. The “stick” diagrams represent the various orientations of the molecular ions relative to the magnetic field.
/3---. +A-
0
’ / ]’
2,
Table 1. Spin Hamiltonian parameters of the intrinsic [Fz-] center and the perturbed [FZ-] center stable to 295K. The principal +. _ values of A, and AI are the same although the directions of the principal axes are different. The parameters 0 and S when combined with Fig. 3 determine the directions of the principal axes. Units for the A’s are gauss
+
.“’
, ‘ec_/’
i’
I
(, /,yy,;
0
,/ -__. //
0
/ /’
/
+
+
f 0 .I,,j
(a)
e
:
7.0° t o.z”
6 Y
/----x
-c0101
0
1’
._____.’
_‘\ -j
I
0
0
/” c”o’ -
COOlI I
Y
+-Mb* I
K*
K*
COOlI I
-7 x
>+ L-1’
---.: -,: I’
0 A*
I
m2
8.2Of 0.2O 6.2Oi o.?
1Reference1. In summary, we have observed in KMgF, two perturbed [F2-] centers having thermal decay temperatures of 150K and approximately 295K respectively. The center decaying at 295K has a stabilizing perturbation at a nearest neighbor potassium site thus allowing the molecule to tilt out of the (001) plane by approximately 6.2” while maintaining a “bent bond” of 8.2”. REFERENCES
(b)
(c)
Fig. 3. Models of the [F2-] centers in KMgF,. (a) Illustration of the “bent bond” for both the intrinsic and the perturbed center. The potassium ions are above and below the plane (adapted from Fig. 3 of Ref. [l]). (b) “End view” of the intrinsic center. (c) “End view” of the perturbed center illustrating the tilt out of the (001)plane.
1. Hall, T. P. P., Bit. J. Appl. Phys. 17, 1011(1966). 2. Riley C. R. and Sibley W. A., Phys. Rev. B 1, 2789 (1970). 3. Lewis J. T., Kolopus J. L., Sonder E. and Abraham M. M., Phys. Rev. B 7, 810 (1973). 4. Kabler, M. N., Point Defects in Solids, (Edited by J. H. Crawford, Jr. and L. M. Slifkin) Vol. 1. Plenum Press, New York (1972). 5. Rhoads, J. E., Ph.D. Thesis, Oklahoma State University (1974).
Pergamon Prers.
Printed in Great Britain
PERTURBED
HOLE
CENTERS
IN KMgF,t
J. E. RHOADS& B. H. ROSE§ and L. E. HALLIBURTON Physics Department, Oklahoma State University, Stillwater, OK 74074,U.S.A. (Received 29 April 1975; accepted 25 July 1975)
The intrinsic self-trapped hole center has been well characterized in the cubic perovskite KMgF,[l-31. As with similar defects in the alkali halides[4], the hole is equally shared by two neighboring fluorines and is labeled an [FIJ center to reflect its molecular nature. When compared to the alkali halides, an interesting property of the intrinsic [FZJ center in KMgF, is the lower symmetry about the fluorine-fluorine axis and a resulting “bent bond” configuration[ 11. In the present note we report the results of an ESR (electron spin resonance) investigation of perturbed self-trapped hole centers in KMgF,. The samples were grown by the Stockbarger method in the Crystal Growth Laboratory of Oklahoma State University and the defects were created by irradiation with 1.5MeV electrons from a Van de Graaff accelerator. At 77 K, a short electron irradiation produces predominately the intrinsic [F2J centers and no observable number of perturbed [F2J centers. Upon warming the crystal above approximately 110K, the intrinsic centers become mobile[2,3] and either recombine with electrons or are retrapped adjacent to perturbations in the lattice such as impurities or vacancies. As shown in Fig. 1, two distinct perturbed [FZJ centers grow in as the intrinsic centers are thermally destroyed. The decay temperatures of these two perturbed centers are 15OK and approximately 295K respectively[2,3]. The decay of the one center at 150K is associated with motion of the hole since there is no decrease in the concentration of the other more stable perturbed center at this lower temperature. If electrons were being released at 150K, then both perturbed hole centers would be expected to decrease. There FOLLOWING IRRADIATION AT 77 K
AFTER WARMING TO 140 K (GAIN
x2)
AFTER
WARMING TO 170 K (GAIN X2)
is no evidence to distinguish whether the 295K center decays as a result of electron release or hole release. The two perturbed [F2J centers are very similar in structure to the intrinsic center and their ESR line positions are only slightly shifted from those of the intrinsic center. The spectrum corresponding to the more stable perturbed [FZ-] center has been isolated by preferentially destroying both the intrinsic center and the other perturbed [FZJ center with a thermal anneal to temperatures above 150K. The spectrum of this 295K center for the magnetic field along the [lib] direction is shown in Fig. 2 (comparison should be made with Fig. 1 of Ref. [l]). In addition to the small shifts in line position, the most important difference between the intrinsic center and this perturbed center is a splitting of the inner lines arising from the fluorine molecules making a 60 angle with the magnetic field. Each of the two inner 60”lines for the intrinsic center are split into well resolved doublets in the case of the perturbed center. This splitting of lines reflects a reduction in symmetry for the perturbed center compared to the intrinsic center and indicates the perturbation, if assumed to be a nearest neighbor, must be at a potassium site. If the perturbation were at the nearest neighbor magnesium site, it would only change the bond angle, not the symmetry of the defect. Thus no orientational degeneracies would be lifted and there would be no splitting of ESR lines when compared to the intrinsic center. The perturbation, possibly a vacancy or a smaller impurity ion such as Na’ or Li’, at a nearest neighbor potassium site would allow the fluorine molecule to tilt out of the (001) plane by rotating about the internuclear axis and our calculations have shown that the inner 60” ESR lines would be the most sensitive to such a tilt. The appropriate models for the intrinsic [F,J center and the 295K perturbed center are shown in Fig. 3. The spin Hamiltonian used to describe the perturbed center decaying at 295K is
Since the hole is equally shared between the two fluorines, the magnitudes of the A, and AZ hyperfine tensors are equal even though the principal axes for the two tensors are not equivalent. In Fig. 3, the angle 0 represents the deviations of the z, and z2 hypetfme tensor axes from the fluorine internuclear axis and the angle S represents the amount of tilt of the x axis away from the
Fig. 1.Thermal stability of the intrinsic and perturbed [F,J centers in KMnF,. The spectra represent the high field 0” lines (split into three c&ponents by the superhypertine interaction with the two adjacent fluorine nuclei along the axis of the molecular ion). The magnetic field is along the [ 1101direction and the spectra were taken at 96K after briefly warming the sample to the indicated temperatures. The line positions are given in units of gauss. tWork supported by Research Corporation and by National Science Foundation Grant GH-39108. SPresent address: El Paso Natural Gas Company, El Paso, Texas. BPresent address: Brookhaven National Laboratory, Upton, New York.
[liO] direction for the perturbed center. For the i tensor of both centers, the z axis is along the internuclear axis and the y axis is perpendicular to the plane of the molecule. Using the basis set I&, ml, m2), the resulting 8x 8 matrix form of the spin Hamiltonian was repeatedly diagonalized by computer while the parameters were systematically adjusted to give a “best” fit to the experimental line positions in the [loo] and [IlO] spectra. Additional details concerning the determination of parameters are contained in Ref. [S]. Table 1 contains the spin Hamiltonian parameters for both the intrinsic [F2J center and the perturbed center decaying at 295K. An additional property of this more stable perturbed center is a reversible reduction in amplitude of the ESR signal with increasing temperature that cannot be attributed simply to a Boltzmann effect. A most likely explanation for this broadening of the ESR lines is a rapid motion of the trapped hole about the neighboring perturbation although a very short spin lattice relaxation time cannot be ruled out. 346
341
Technical Notes
T =77K r/ = 9.2lOGHz
I
I II
I
I
0”
160”
II
-
90”
I
I
I
I
2500 G
3OOOG
3500 G
4000 G
Fig. 2. ESR spectrum of the perturbed [FZ-]center stable to 295K when the magnetic field is along the [llO] direction. The “stick” diagrams represent the various orientations of the molecular ions relative to the magnetic field.
/3---. +A-
0
’ / ]’
2,
Table 1. Spin Hamiltonian parameters of the intrinsic [Fz-] center and the perturbed [FZ-] center stable to 295K. The principal +. _ values of A, and AI are the same although the directions of the principal axes are different. The parameters 0 and S when combined with Fig. 3 determine the directions of the principal axes. Units for the A’s are gauss
+
.“’
, ‘ec_/’
i’
I
(, /,yy,;
0
,/ -__. //
0
/ /’
/
+
+
f 0 .I,,j
(a)
e
:
7.0° t o.z”
6 Y
/----x
-c0101
0
1’
._____.’
_‘\ -j
I
0
0
/” c”o’ -
COOlI I
Y
+-Mb* I
K*
K*
COOlI I
-7 x
>+ L-1’
---.: -,: I’
0 A*
I
m2
8.2Of 0.2O 6.2Oi o.?
1Reference1. In summary, we have observed in KMgF, two perturbed [F2-] centers having thermal decay temperatures of 150K and approximately 295K respectively. The center decaying at 295K has a stabilizing perturbation at a nearest neighbor potassium site thus allowing the molecule to tilt out of the (001) plane by approximately 6.2” while maintaining a “bent bond” of 8.2”. REFERENCES
(b)
(c)
Fig. 3. Models of the [F2-] centers in KMgF,. (a) Illustration of the “bent bond” for both the intrinsic and the perturbed center. The potassium ions are above and below the plane (adapted from Fig. 3 of Ref. [l]). (b) “End view” of the intrinsic center. (c) “End view” of the perturbed center illustrating the tilt out of the (001)plane.
1. Hall, T. P. P., Bit. J. Appl. Phys. 17, 1011(1966). 2. Riley C. R. and Sibley W. A., Phys. Rev. B 1, 2789 (1970). 3. Lewis J. T., Kolopus J. L., Sonder E. and Abraham M. M., Phys. Rev. B 7, 810 (1973). 4. Kabler, M. N., Point Defects in Solids, (Edited by J. H. Crawford, Jr. and L. M. Slifkin) Vol. 1. Plenum Press, New York (1972). 5. Rhoads, J. E., Ph.D. Thesis, Oklahoma State University (1974).