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Effect of phase transition on ESR spectra of Cu2+: NH4Br
1. Phys. Chem. Solids. Vol. 42, pp. 617-619. 1981 Printed in Great Britain.
OOZZ-3697/811070617~33SO2.@3/0 Pergamon Press Ltd.
TECHNICAL NOTE EFFECT OF PHASE TRANSITION ON ESR SPECTRA OF CL?+: NH4Brt N. J. TRAPPENIERS,F. S. STIBBEand J. L. RAOS Van der Waals-Laboratorium, Universiteit Van Amsterdam, Amsterdam, The Netherlands (Received 20 August 1980; accepted 16 September 1980) In recent years, there has been considerable interest in the investigation of phase transitions in ionic crystals by using transition metal ions as probes. The best known system is Cu*+:NH&I which is of great interest because of the wellknown A-transition at -30.6”C. Manv workers114 have studied the ESR spectrum of the Cu*+ion in this crystal and they have found striking changes in the g-factor as well as in the hypertine constant at the transition. In the present paper results are reported of a study of Cu*+ doped in NH4Br. The CsCl-type NHIBr crystal undergoes two successive phase transitions associated with the ordering of the orientation of NH,’ ions[5]. The lowest temperature phase is characterized by the “parallel” arrangement of NH,+ ions, while the intermediate phase is characterized by an “anti-parallel” arrangement of NH,+ ions and the highest temperature phase is the disorder phase, where each NH.,+ ion is randomly oriented in “up” or “down” configurations. The known phases and transition points of NH4+Brare as follows[6,7] t27lth publication of the Van der Waals Fund. DPresent address: Department of Physics, S.V. University, Tirupati, India.
31
33
Fig. 1. ESR spectrum of Ct?:NH,Br
NH.,Br Phase I Phase II Phase III
(NaCI)
137°C. Phase II - 38°C. Phase III -16S”C. Phase IV
Sastry and Venkateswarlu[8] studied the ESR spectrum of Cu’+:NH,Br on a X-band spectrometer in crystals grown from neutral solution. They identified different centres from the spectrum recorded at low temperature. We have studied the ESR spectrum on a K-band spectrometer and identified only one centre, which is different from those found by Sastry and Venkateswarlu. The results obtained are discussed here. Single crystals of NH,Br doped with CuZ+ions are grown from a saturated solution of ammonium bromide in water to which a little CuBrr is added. Urea is added to stimulate the cubic growth of the crystals. The crystals are yellowish green in colour and of good cubic shape. Spectra are recorded at different temperatures on a K-band spectrometer with 100kc/s field modulation. DPPH is used as a field marker. Figure I. represents the spectrum recorded at room tem-
36.5 M Hz 35 H(neutral) at room temperature with H parallel to (100)axis 617
(CSCI) (Tetragonal) order-disorder (CSCI).
N. J. TRAPPENIERSefal.
618
I
I
I
I
I
I
I
38 MHz
36
3L
32
H-
Fig. 2. ESR spectrum of Cu*+: NH,Br (neutral) at -85°C with H parallel to (100)axis. perature by keeping the (100) axis of the crystal parallel to the direction of the magnetic field. It shows the characteristic four line hyperfine structure (m,: 3/2, l/2, -l/2, -3/2) of Cu*+. The angular variation of the spectrum reveals that the local symmetry about Cu*+ion is tetragonal with (100)axis as the symmetry axis. The spectra can be described with the spin-Hamiltonian
with S = l/2 and nuclear spin I = 3/2. Superhypertine effects are not included in this expression. The parameters in the spinHamiltonian, as calculated from the spectra are given in Table 1. The spectra are recorded down to -85°C by keeping (100)axis of the crystal parallel to the direction of the magnetic field and the spectrum recorded at -85°C is shown in Fig. 2. Though the symmetry of the crystal below the phase transition temperature is lowered to orthorhombic, the spectrum is still being described assuming the axial symmetry. Thus the spectrum is analysed using the spin-Hamiltonian equations and the constants obtained are also given in Table 1. The large number of lines visible at low temperature made it impossible to study the angular dependence of the spectra. The spectra have further been studied at different temperatures and the values of hyperfine constant A are plotted
against temperature as shown in Fig. 3. It is interesting to note that at -42°C there is a striking change in the spectra and also in the magnitude of hyperfine constant A, which indicates a phase change. As mentioned before, ammonium bromide is said to have a A-point transition at -38°C below which it goes to an ordered tetragonal phase. In the present study, the transition temperature is found to be -42°C and not -38°C; the change is due to the presence of the copper concentration[9] in the crystals used. Unlike in ammonium chloride[l] the hyperfine constant A decreases with the lowering of temperature. However, the gfactors change very little, i.e. not more than about 0.002 in the range from 20°C to -85°C. The room temperature spectra have revealed axial symmetry of the centres with the symmetry axis pointing in the direction of the cubic axis Therefore, we can conclude that the environment of the Cu*+ions has tetragonal symmetry. Since our ESR spectra and the spin-Hamiltonian parameters are very similar to those (neutral) we assume that the Cu*’ ions reported for Cu2+.NH,Cl . are incorporated interstitially between four bromide ions with water molecules replacing two neighbouring NH,+ ions in the (100) direction. Acknowledgement-The authors thank Dr. Peter van der Valk for many valuable discussions.
Table 1. Spin-Hamiltonian parameters for Cu*+: NH4Br (neutral solution) T-P
9,,
91
1 x 10+4
_-l
B x lo+' cm-l
20%
2.031
2.189
116.4
0
-85.C
2.033
2.181
153.8
0
Effect of phase transition on ESR spectra of Cu*+:NH,Br
1501
’
I
-85
-70
I
-50
I
I
I
-30
-10
0
619
1 +30°c
Fig. 3. Variation of hypertine constant A with temperature.
REFERENCES
1. Trappeniers N. J. and Hagen S. H., Physica 31,251 (1965). 2. Pilbrow J. R. and Spaeth I. M. Phys. Stutus Solidi 20, 225 (1%7). 3. Kuroda N. and Kawamori A. J. Phys. Chem. Solids 32, 1233 (1971). 4. Hagen S. H. and Trappeniers N. J. Physicu 66, 166(1973). 5. Yamada Y., Mori M. and Noda Y. J. Phys. Sot. Japan 32, 1565(1972).
6. Levy H. A. and Peterson S. W. I. Am. Chem. Sot. 75, 1536 (1953). 7. Sorai hf., Suga H. and Seki S. Bull. Chem. Sot. Japan 38,1125 (1%5). 8. Sastry M. D. and Venkateswarlu P. Proc. Ind. Acud. Sci. 66,208 (1967). 9. Kuroda N., Kawamori A. and Mito E. J. Phys. Sot. Japan 26, 868 (1%9).
OOZZ-3697/811070617~33SO2.@3/0 Pergamon Press Ltd.
TECHNICAL NOTE EFFECT OF PHASE TRANSITION ON ESR SPECTRA OF CL?+: NH4Brt N. J. TRAPPENIERS,F. S. STIBBEand J. L. RAOS Van der Waals-Laboratorium, Universiteit Van Amsterdam, Amsterdam, The Netherlands (Received 20 August 1980; accepted 16 September 1980) In recent years, there has been considerable interest in the investigation of phase transitions in ionic crystals by using transition metal ions as probes. The best known system is Cu*+:NH&I which is of great interest because of the wellknown A-transition at -30.6”C. Manv workers114 have studied the ESR spectrum of the Cu*+ion in this crystal and they have found striking changes in the g-factor as well as in the hypertine constant at the transition. In the present paper results are reported of a study of Cu*+ doped in NH4Br. The CsCl-type NHIBr crystal undergoes two successive phase transitions associated with the ordering of the orientation of NH,’ ions[5]. The lowest temperature phase is characterized by the “parallel” arrangement of NH,+ ions, while the intermediate phase is characterized by an “anti-parallel” arrangement of NH,+ ions and the highest temperature phase is the disorder phase, where each NH.,+ ion is randomly oriented in “up” or “down” configurations. The known phases and transition points of NH4+Brare as follows[6,7] t27lth publication of the Van der Waals Fund. DPresent address: Department of Physics, S.V. University, Tirupati, India.
31
33
Fig. 1. ESR spectrum of Ct?:NH,Br
NH.,Br Phase I Phase II Phase III
(NaCI)
137°C. Phase II - 38°C. Phase III -16S”C. Phase IV
Sastry and Venkateswarlu[8] studied the ESR spectrum of Cu’+:NH,Br on a X-band spectrometer in crystals grown from neutral solution. They identified different centres from the spectrum recorded at low temperature. We have studied the ESR spectrum on a K-band spectrometer and identified only one centre, which is different from those found by Sastry and Venkateswarlu. The results obtained are discussed here. Single crystals of NH,Br doped with CuZ+ions are grown from a saturated solution of ammonium bromide in water to which a little CuBrr is added. Urea is added to stimulate the cubic growth of the crystals. The crystals are yellowish green in colour and of good cubic shape. Spectra are recorded at different temperatures on a K-band spectrometer with 100kc/s field modulation. DPPH is used as a field marker. Figure I. represents the spectrum recorded at room tem-
36.5 M Hz 35 H(neutral) at room temperature with H parallel to (100)axis 617
(CSCI) (Tetragonal) order-disorder (CSCI).
N. J. TRAPPENIERSefal.
618
I
I
I
I
I
I
I
38 MHz
36
3L
32
H-
Fig. 2. ESR spectrum of Cu*+: NH,Br (neutral) at -85°C with H parallel to (100)axis. perature by keeping the (100) axis of the crystal parallel to the direction of the magnetic field. It shows the characteristic four line hyperfine structure (m,: 3/2, l/2, -l/2, -3/2) of Cu*+. The angular variation of the spectrum reveals that the local symmetry about Cu*+ion is tetragonal with (100)axis as the symmetry axis. The spectra can be described with the spin-Hamiltonian
with S = l/2 and nuclear spin I = 3/2. Superhypertine effects are not included in this expression. The parameters in the spinHamiltonian, as calculated from the spectra are given in Table 1. The spectra are recorded down to -85°C by keeping (100)axis of the crystal parallel to the direction of the magnetic field and the spectrum recorded at -85°C is shown in Fig. 2. Though the symmetry of the crystal below the phase transition temperature is lowered to orthorhombic, the spectrum is still being described assuming the axial symmetry. Thus the spectrum is analysed using the spin-Hamiltonian equations and the constants obtained are also given in Table 1. The large number of lines visible at low temperature made it impossible to study the angular dependence of the spectra. The spectra have further been studied at different temperatures and the values of hyperfine constant A are plotted
against temperature as shown in Fig. 3. It is interesting to note that at -42°C there is a striking change in the spectra and also in the magnitude of hyperfine constant A, which indicates a phase change. As mentioned before, ammonium bromide is said to have a A-point transition at -38°C below which it goes to an ordered tetragonal phase. In the present study, the transition temperature is found to be -42°C and not -38°C; the change is due to the presence of the copper concentration[9] in the crystals used. Unlike in ammonium chloride[l] the hyperfine constant A decreases with the lowering of temperature. However, the gfactors change very little, i.e. not more than about 0.002 in the range from 20°C to -85°C. The room temperature spectra have revealed axial symmetry of the centres with the symmetry axis pointing in the direction of the cubic axis Therefore, we can conclude that the environment of the Cu*+ions has tetragonal symmetry. Since our ESR spectra and the spin-Hamiltonian parameters are very similar to those (neutral) we assume that the Cu*’ ions reported for Cu2+.NH,Cl . are incorporated interstitially between four bromide ions with water molecules replacing two neighbouring NH,+ ions in the (100) direction. Acknowledgement-The authors thank Dr. Peter van der Valk for many valuable discussions.
Table 1. Spin-Hamiltonian parameters for Cu*+: NH4Br (neutral solution) T-P
9,,
91
1 x 10+4
_-l
B x lo+' cm-l
20%
2.031
2.189
116.4
0
-85.C
2.033
2.181
153.8
0
Effect of phase transition on ESR spectra of Cu*+:NH,Br
1501
’
I
-85
-70
I
-50
I
I
I
-30
-10
0
619
1 +30°c
Fig. 3. Variation of hypertine constant A with temperature.
REFERENCES
1. Trappeniers N. J. and Hagen S. H., Physica 31,251 (1965). 2. Pilbrow J. R. and Spaeth I. M. Phys. Stutus Solidi 20, 225 (1%7). 3. Kuroda N. and Kawamori A. J. Phys. Chem. Solids 32, 1233 (1971). 4. Hagen S. H. and Trappeniers N. J. Physicu 66, 166(1973). 5. Yamada Y., Mori M. and Noda Y. J. Phys. Sot. Japan 32, 1565(1972).
6. Levy H. A. and Peterson S. W. I. Am. Chem. Sot. 75, 1536 (1953). 7. Sorai hf., Suga H. and Seki S. Bull. Chem. Sot. Japan 38,1125 (1%5). 8. Sastry M. D. and Venkateswarlu P. Proc. Ind. Acud. Sci. 66,208 (1967). 9. Kuroda N., Kawamori A. and Mito E. J. Phys. Sot. Japan 26, 868 (1%9).