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EPR lineshape in ethylenediammonium chloromanganate (II), [C2H10N2] [MnCl4]
Solid State Communications, Vol. 16, PP. 389—391, 1975.
Pergamon Press.
Printed in Great Britain
EPR LINESHAPE IN ETHYLENEDIAMMONIUM CHLOROMANGANATE (II), [C2H10N2I [MnCI4J D.B. Losee,* J.W. Hall and W.E. Hatfield Department of Chemistry, University of North Carolina, Chapel Hill, North Carolina 27514, U.S.A. (Recieved 26 September 1974 by A.G. Chynoweth)
The EPR line shape has been measured on a single crystal of ethylene diammonium chloromanganate (II) as a function of field orientation with respect to the expected two dimensional network of manganese ions. The with amay = 20be and j3 = 4, and the expression lineshape may angular variation of2~ the 1)2 linewidth described by the be ~R= described a + f3(3bycos I(H H 0) F [exp (—At —Bt ln (t/t0)] (where F refers to the Fourier transform of the bracketed function) with B/A = 1.9 ±0.8. —
—
THERE IS CONSIDERABLE current interest the 14inand EPR lineshape of low dimensional systems, this interest has prompted such an investigation of [C 2H10 N2 I [MnC14]at room temperature. Preliminary 5 on this material single crystal susceptibility results display a broad maximum at approximately 80°K, and the results reveal highly developed two dimensional short range order at and above this maximum. The material, prepared from a 1 : 1 mole ratio of the corresponding halide salts in water, grows characteristically as thin plates. X-ray precession photographs reveal a monoclinic or higher space group with cell constants equal to a = 7.17, b = 19.00, c = 7.36. The largest crystallographic axis is perpendicular to the
The linewidths (peak tosignal) peak magnetic field as a separation in the derivative were measured function of angle at room temperature and the results are shown Fig. 1. aAs can be seen in Fig. 1, the linewidth goesinthrough minimum at approximately 55°(± 5°)with relative maxima at 0 and 90°.Although there is some amsotropy in the g.value during this rotation the extrema do not occur at this critical angle of 55°.When the rotation was carried out in the plane parallel to the plate there was no linewidth variation observed as a function of rotation angle. If the derivative intensity is plotted in units [‘.,/H H 0/I’(H0)] which will yield a straight line for a Lorentzian lineshape vs the field deviation squared, any non-Lorentzian observed lineshapes will be revealed in a deviation of the plot from linearity. The results of measurements of the derivative lineshape at rotation angles of 0°(i.e. field normal to plate of crystal or parallel to the long crystallographic axis) and at 550 are shown in Fig. 2 and clearly reveal nonLorentzian behavior at 0°.These measurements extend out to approximately six half-widths. Measurements at the critical angle of 55°,on the other hand, display Lorentzian behavior far into the wings of the derivative signal. 3’4 have recently explained Richards andobserved Salamonin the two dimensional similar behavior —
plate. A sample large enough for single crystal EPR measurements at 9.3 GHz was mounted such that the crystal was rotated from a direction normal to the plate of the crystal to a direction parallel to the plate. With the cell constants in mind, then it is reasonable to assume that, in effect, the rotation carries the crystal from a plane perpendicular to a two dimensional network of Mn(II) ions to a plane parallel to this network.
*
Present address: Philip Morris Research Box 26583, Richmond, Virginia 23261,Center, U.S.A. 389
390
EPRLINESHAPE IN [C2H10N2][MnC14]
4C
Vol. 16, No.4
2O—1)2 (1) where 0 refers to thea+(3(3cos angle between the applied field ~.H= and the normal to the two dimensional plane. Likewise
_______________________________ a D
their theory predicts a lineshape given approximately by:
a
I(H—H 0) ~H
a
a
IC
55
0
FIG. 1. EPR peak to peak derivative line-width vs rotation angle at room temperature for [C2H10N2] [MnC14].Angle 0 is relative to a direction parallel to the longest crystallographic axis.
‘
F[exp(—At —Bt ln(t/t0)]
(2)
where F refers to the Fourier transform of the bracketed function. Although Richards and Salomon’s theory allowsofathe calculation both the angular dependence linewidthofand lineshape with no adjustable parameters a knowledge of the crystal structure is required along with the magnitude of the two dimensional exchange constant. Since the structure of [ç2H10N2][MnCl4] is not known an empirical approach was taken. When the linewidth angular variation data were fit to expression (1) above the best fit values were a = 20, and (3= 4. The results of the fit are shown in Fig. 3 where the peak to peak
40 5C
a
2C0
(I-I —H,
4
(3cos’0~1)’
)2
2O 1)2, with best FIG. 3. Linewidth fit to expression vs rotation a +function (3(3 cos~0 (3 cos 1)2, shown as straight line. —
yield 2. FIG. a straight EPR derivative line for aline Lorentzian shape plotted line shape in units when which plotted vs the field deviation squared. 0 measurement at 0°;x measurement at 55°;(—) Fourier transform of expression 2 with B = 15.0 and A 7.5. —
—
—
—
separation is plotted vs (3 cos20 1)2. As can be seen the fit is excellent. In Fig. 2 the Fourier transform —
Heisenberg magnet, K 2 MnF4, as resulting from diffuse motion for long-time dependence of the correlation functions. Their theory predicts an angular dependence, arising from the anisotropy in the dipolar interaction, roughly of the form
of expression 2 has been plotted with the ratio SB/A I = 1.9 ±0.8 (B 15) vs the derivative intensity. Again the fit is excellent especially since measurements extend to better than 6 half-widths. —.
Vol. 16, No.4
EPR LINESHAPE IN [C2H10N2][MnC14J
In conclusion then, [C2H10N2 J [MnC14]has been shown to follow the recently developed theory of Richards and~Salomon for exchange narrowing in the EPR of two dimensional systems. Further work is in progress to relate the parameters appropriate to [C2H10N2] [MnCl4I and these results will be published when the structural results become available.
391
Acknowledgements We wish to thank Mr. Henry L. Suprenaut for assistance with the Fourier transform computations and especially Dr. P. Richards for very valuable discussions. This research was supported by the Materials Research Center of the University of North Carolina under Grant No. GH-33632 from the National Science Foundation. —
1.
REFERENCES DIETZ R.E., MERRI11~F.R., DINGLE R., HONE D., SILBERNAGEL B.G. and RICHARDS P., Phys. Rev. Lett. 26,(19) 1186 (1971).
2.
HENNESSY M.J., MCELWEE C.D. and RICHARDS P.,Phys. Rev. B7, (3) 930 (1973).
3.
SALAMON M.B. and RICHARDS P., AlP Conf Proc. (18th Annual Conference, Denver) 10, 187 (1973).
4.
RICHARDS P. and SALAMON M.B.,Phys. Rev. B9, 32(1974).
5.
LOSEE D.B., HALL J.W., HATFIELD W.E. (to be published).
Pergamon Press.
Printed in Great Britain
EPR LINESHAPE IN ETHYLENEDIAMMONIUM CHLOROMANGANATE (II), [C2H10N2I [MnCI4J D.B. Losee,* J.W. Hall and W.E. Hatfield Department of Chemistry, University of North Carolina, Chapel Hill, North Carolina 27514, U.S.A. (Recieved 26 September 1974 by A.G. Chynoweth)
The EPR line shape has been measured on a single crystal of ethylene diammonium chloromanganate (II) as a function of field orientation with respect to the expected two dimensional network of manganese ions. The with amay = 20be and j3 = 4, and the expression lineshape may angular variation of2~ the 1)2 linewidth described by the be ~R= described a + f3(3bycos I(H H 0) F [exp (—At —Bt ln (t/t0)] (where F refers to the Fourier transform of the bracketed function) with B/A = 1.9 ±0.8. —
—
THERE IS CONSIDERABLE current interest the 14inand EPR lineshape of low dimensional systems, this interest has prompted such an investigation of [C 2H10 N2 I [MnC14]at room temperature. Preliminary 5 on this material single crystal susceptibility results display a broad maximum at approximately 80°K, and the results reveal highly developed two dimensional short range order at and above this maximum. The material, prepared from a 1 : 1 mole ratio of the corresponding halide salts in water, grows characteristically as thin plates. X-ray precession photographs reveal a monoclinic or higher space group with cell constants equal to a = 7.17, b = 19.00, c = 7.36. The largest crystallographic axis is perpendicular to the
The linewidths (peak tosignal) peak magnetic field as a separation in the derivative were measured function of angle at room temperature and the results are shown Fig. 1. aAs can be seen in Fig. 1, the linewidth goesinthrough minimum at approximately 55°(± 5°)with relative maxima at 0 and 90°.Although there is some amsotropy in the g.value during this rotation the extrema do not occur at this critical angle of 55°.When the rotation was carried out in the plane parallel to the plate there was no linewidth variation observed as a function of rotation angle. If the derivative intensity is plotted in units [‘.,/H H 0/I’(H0)] which will yield a straight line for a Lorentzian lineshape vs the field deviation squared, any non-Lorentzian observed lineshapes will be revealed in a deviation of the plot from linearity. The results of measurements of the derivative lineshape at rotation angles of 0°(i.e. field normal to plate of crystal or parallel to the long crystallographic axis) and at 550 are shown in Fig. 2 and clearly reveal nonLorentzian behavior at 0°.These measurements extend out to approximately six half-widths. Measurements at the critical angle of 55°,on the other hand, display Lorentzian behavior far into the wings of the derivative signal. 3’4 have recently explained Richards andobserved Salamonin the two dimensional similar behavior —
plate. A sample large enough for single crystal EPR measurements at 9.3 GHz was mounted such that the crystal was rotated from a direction normal to the plate of the crystal to a direction parallel to the plate. With the cell constants in mind, then it is reasonable to assume that, in effect, the rotation carries the crystal from a plane perpendicular to a two dimensional network of Mn(II) ions to a plane parallel to this network.
*
Present address: Philip Morris Research Box 26583, Richmond, Virginia 23261,Center, U.S.A. 389
390
EPRLINESHAPE IN [C2H10N2][MnC14]
4C
Vol. 16, No.4
2O—1)2 (1) where 0 refers to thea+(3(3cos angle between the applied field ~.H= and the normal to the two dimensional plane. Likewise
_______________________________ a D
their theory predicts a lineshape given approximately by:
a
I(H—H 0) ~H
a
a
IC
55
0
FIG. 1. EPR peak to peak derivative line-width vs rotation angle at room temperature for [C2H10N2] [MnC14].Angle 0 is relative to a direction parallel to the longest crystallographic axis.
‘
F[exp(—At —Bt ln(t/t0)]
(2)
where F refers to the Fourier transform of the bracketed function. Although Richards and Salomon’s theory allowsofathe calculation both the angular dependence linewidthofand lineshape with no adjustable parameters a knowledge of the crystal structure is required along with the magnitude of the two dimensional exchange constant. Since the structure of [ç2H10N2][MnCl4] is not known an empirical approach was taken. When the linewidth angular variation data were fit to expression (1) above the best fit values were a = 20, and (3= 4. The results of the fit are shown in Fig. 3 where the peak to peak
40 5C
a
2C0
(I-I —H,
4
(3cos’0~1)’
)2
2O 1)2, with best FIG. 3. Linewidth fit to expression vs rotation a +function (3(3 cos~0 (3 cos 1)2, shown as straight line. —
yield 2. FIG. a straight EPR derivative line for aline Lorentzian shape plotted line shape in units when which plotted vs the field deviation squared. 0 measurement at 0°;x measurement at 55°;(—) Fourier transform of expression 2 with B = 15.0 and A 7.5. —
—
—
—
separation is plotted vs (3 cos20 1)2. As can be seen the fit is excellent. In Fig. 2 the Fourier transform —
Heisenberg magnet, K 2 MnF4, as resulting from diffuse motion for long-time dependence of the correlation functions. Their theory predicts an angular dependence, arising from the anisotropy in the dipolar interaction, roughly of the form
of expression 2 has been plotted with the ratio SB/A I = 1.9 ±0.8 (B 15) vs the derivative intensity. Again the fit is excellent especially since measurements extend to better than 6 half-widths. —.
Vol. 16, No.4
EPR LINESHAPE IN [C2H10N2][MnC14J
In conclusion then, [C2H10N2 J [MnC14]has been shown to follow the recently developed theory of Richards and~Salomon for exchange narrowing in the EPR of two dimensional systems. Further work is in progress to relate the parameters appropriate to [C2H10N2] [MnCl4I and these results will be published when the structural results become available.
391
Acknowledgements We wish to thank Mr. Henry L. Suprenaut for assistance with the Fourier transform computations and especially Dr. P. Richards for very valuable discussions. This research was supported by the Materials Research Center of the University of North Carolina under Grant No. GH-33632 from the National Science Foundation. —
1.
REFERENCES DIETZ R.E., MERRI11~F.R., DINGLE R., HONE D., SILBERNAGEL B.G. and RICHARDS P., Phys. Rev. Lett. 26,(19) 1186 (1971).
2.
HENNESSY M.J., MCELWEE C.D. and RICHARDS P.,Phys. Rev. B7, (3) 930 (1973).
3.
SALAMON M.B. and RICHARDS P., AlP Conf Proc. (18th Annual Conference, Denver) 10, 187 (1973).
4.
RICHARDS P. and SALAMON M.B.,Phys. Rev. B9, 32(1974).
5.
LOSEE D.B., HALL J.W., HATFIELD W.E. (to be published).