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ESR and optical spectra of Cu2+:NH4Br
VoIume 56, number 1
CHEMICAL PHYSICS LETTERS
15 May 1978
ESR AND OPTICAL SPECTRA OF Cu2+: NH+ N.J. TRAPPENIERS,
F.S. STIBBE and J.L.. RAO
Vi der Waak-Zaboraton-urn, Uniuersiteit vanAmsterdam.Amsterdam,T?ieNetherlands (243th publication of the van der Waals Fund) Received 24 February 1978
ESR and opt&I studies of Cu*+ ions doped in N&Br singlecrystals have been carried out at room temperature. A new centre has been identitiedfor the crystals grown from acidic medium.The room temperaturedata revealthat Cu*+ ions go to interstitialsites havingsquareplanarcoordination of four Br- ions.
I_ Introduction The investigation of the ESR spectrum of CuZt ions in single crystals of ammonium chloride has revealed many interesting features. -Amongst other things, these studies indicate that different copper complexes can be introduced into the crystal depending on the growth conditions. Trappeniers and Hagen [1,2] showed that the nature of these centres could be controlled by the pH of the aqueous solutions from which the crystals were grown. They also proposed models for two types of centres that were found in the crystals of Cu2+: -Cl which were grown respectively from the HCl acidic, the NH40H basic, and the neutral solutions (without auy addition of either HCl or NH,+OH)_ Sastry and Venkateswarlu [3] and Chirkin et al. [43 have studied the ESR spectra of Cu2+: WBr crystals grown from neutral solution. Since different copper complexes could be introduced into the NH4Cl crystal depending on the growth conditions, we have made an attempt to investigate more specifically the ESR spectrum of Cu2* ions incorporated into single crystals of NH4Br grown from the HBr acidic medium. If the configuration of these centres can be elucidated, they could then be used as probes for investigating the NH4Br phase diagram as a function of temperature and pressure.
10
2. Experimental NqBr crystals doped with Cu2+ ions are grown from a saturated solution of ammonium bromide in water to which a few mole percent of CuBr2 is added. Urea is added to stimulate the cubic growth of the crystal. To the growth solution, two drops of concentrated hydrobromic acid are added. The crystals are yellow in colour and of a good cubic shape. ESR spectra are recorded at room temperature on a K-band spectrometer with 100 kHz field modulation. DPPH is used as a field marker. The optical absorption spectrum is recorded on a Cary model 14 spectrophoton eter.
3. ESR measurements The free Cu2+ ion has a 3dg configuration with a ground state 2D. When subjected to a cubic crystalline field this five-fold orbital degenerate state splits into a doublet (2E) and a triplet (2T) and the orbital degeneracy is further removed in the presence of a tetragonal or an orthorhombic component in the crystalline field leaving an orbital singlet as the ground state. The nuclear spin of copper is 312 and the hyperfine interaction gives a four-line spectrum. Fig. 1 shows the ESR spectrum taken when the direction of the magnetic field is parallel to a (100) axis of the crystal. The angular variation of the spectrum, as shown in fig. 2, reveals
15 May 1978
CEIEhfICAL PHYSICS LElTERS
VoIume 56, number 1
that the symmetry of the centres is axial with the syrnmetry axis pointing in the direction of the cubic axis. The main features of the spectra can be described with the spin-hamiltonian
4L;v J
.
fMzSz
dH
315
I
3L.0
360
300 MHz
g,, = 2.035,
Fig_ 1. ESR spectrum of Cu*+: NJ&Br-
gL = 2.204,
A = 70 X 10m4 cm-l
DPPH
The optical absorption spectrum has been recorded in the wavelength region from 0.3 p to 1 S P at 300 K. The absorption spectrum is shown in fig. 3. It is found that Cu2+ ions give rise to two main absorption bands; one at about 28900 cm-1 with relatively high intensity and the other at about 13380 cm-t with au accompanying shoulder at 9160 cm-1 _ In octahedral symmetry, the ground state electronic configuration of t$e3 for Cu2+ gives rise to a 2Eg state. When one of the electrons iu a t2 orbital is promoted to an e orbital the excited electron configuration t$e”
350
w I
Fig_ 2.. hgular
Table 1 Spin-ham&o&n
I
variation of the ESR
I
spectrum.
constants for Cu*+- _NH.+Br reported by different authors
Authors
C3irkin et al. Sastry et al. present work
B=OX104cm-1.
4. Optical measurements
-
II 34.0 I-
and
The spin-hamiltonian constants reported by different authors are given in table 1. The g values as given by Sastry and Venkateswarlu and Chirkin et al. roughly agree with those of our complex but there is a large difference in the hyperfine value. This indicates that a different centre is incorporated into the crystal grown from the acidic solution.
IH
360
(1)
withS = l/2 and nuclear spin I= 3/2. Superhyperfhre effects are not included iu tbis expression. The parameters in the spin-hamiltonian, as calculated from the spectra are:
DP-PH
I
+IySy).
+BU,S,
300 300 300
&l
g1
A X 104(cmW1)
f3 X lo4 (cm-‘)
2.029 f <0.003) 2.0036 1(0.005) 2.035
2.190 * (0.005) 2.217 2.204
170 182.6 70
0 0 0
11
CHEMICAL
Volume 56. number 1
15 May 1978
PHYSICS LETTERS
*
e
I dg( ‘ol
.**’ ‘. *. \
%
lP
% /=.
2a lP f I!3
-6Oq-zos*ot -6Dq+2Os*fXlt
compressed
octahedral
free ton
Leq-ZDs*Dt ZDq* OS -.xX
[cl
lb1
to1
29
‘E!3
tetragonal
Fig. 4. Energy level splitting of Cuz+ in octahedral and tetra-
gonal crystal fields_
L
/
1.5l.L
03s wavelength
Fig. 3. Optical
absorptionspectrum of Cu*+:N&Br.
rise to a *Tz8 uppe r state. Thus, only one single transition is expected for Cu2+ in 4, symmetry. Since the ground state 2Eg is split under the Jahn-Teller effect, it is never possible to have a regular octahedrally coordinated CuZt complex (Bailhausen) [S] . An Eg state can be split only in a tetragonal, but not in z trigonal field, so that the copper doped crystals normally exhibit tetragonal or lower symmetries. in a tetragonal field, the ground state ‘Eg would be split into two levels 2Blg and 2Alg and similariy the upper 2T2g into 2B2, and 2Eg levels. One of the levels 2A,, or 2BI, forms the ground state accordingly as g,, is equal to or different from 2.0023. Since our ESR results give go = 2.035 andgL >glI, we assume that the ground state is 2Alg_ The same conclusion was reached previously with respect to the ground state of Cu2+: NH,Cl [2] _Thus, the bands at 9160 cm-l and 13380 cm-l have been assigned to 2Alg + 2B1, and 2Alg + 2Eg transitions respectively. The most intense band at 28900 cm-l has been assigned to the 3Alg + 2B2, transition. It should be pointed out that Kuroda [6] observed a band above 2000 cm-l for Cu2+: NH4Br (basic) and from ils intensivity attributed this to a charge transfer band. However, Maki and McGarvey [7] and Krishnan et al. [8] reported a band at about 25000 cm-l for some Ctt2+ complexes and they attributed it to a ligand tieId band. We shall also assume that the baud at 28900 cm-l belongs to a d-d transition, i.e. the 2A~g + 2B2g transition. The splittings of the energy levels in octahedral and tetragonal fields are shown in fig. 4. Energy values of gives
12
tetragonal field levels are also shown in the same figure in terms of the crystal (m) and tetragonal(Ds, Dr) field parameters. If, in addition to a tetragonal field, spin-orbit interaction is taken into account, the ground 2Eg and the upper 2T2g states would be split in two (r, and r7) and three levels (r6, r7 and r,) respectively. The tetragonal field parameters (Ds and Dt) have been evaluated from the observed band positions as follows (see fig. 4): -4Ds
- SDr=9160,
1ODq - Ds -
1ODt = 13380,
1ODq - 4Ds -
5Dt = 28900,
Dq = 1974 cm-l Dt = 988 cm-l
,
Ds = -3525
cm-1
,
.
c-9
Using these values of Ds, Dt, Dq and the free-ion spinorbit coupling constant X = -830 cm-l, the energy matrices [9] for r6 and r7 have been diagonalised. The observed and calculated band positions are given in table 2. The good agreement between theoretical and observed results justifies the interpretation of our Table 2 Assignments for the opticai absorption bands Transition
Cu**: N&Br(acidic). present work obsetved
*A +*B *Alg’2$ tg
2AlE-,2B2g
9160
13380 28900
Co*+: NI&Br (basic) Kuroda
calculated 9118 13100 13948
29032
9357 13790
Volume56. number 1
CHENICAL PHYSICS LETTERS
results. In the present work, we obtained a higher 04 value (1974 cm-l) than that normally obtained (1120 cm-l) for Cu2+ complexes. ‘Ihis is discussed below in section 5. The optical data observed for Cu2% NQBr (basic) by Kuroda as well as our present data are given in table 2. It is interesting to observe that for the acidic crystal the bands are shifted towards the lower energy side. A similar shift of the bands towards lower energy side is also observed by Baglio and Vaughan [lo] for the absorption spectrum of Cu(NH&+ ion in Cu(NH,),Cu(Br2)2 and in Cu(NH3)4(CuC12),- H20. They found that the bands of the latter compound are significantly lower in energy than those in the former compound, consistent with “semi-coordination” of the water molecules_
5. Discussion When the Cu2+ ion is incorporated in NH4Br it can go substitutionally to the m site and the charge neutrality can be achieved by producing a cation vacancy. If it substitutes for an NG ion and is associated with a first neighbour vacancy, the CL?+ ion will have a tetragonal site symmetry with (100) axis as the site symmetry axis. If the cation vacancy is at a second nearest neighbour site, the site symmetry of the magnetic ion will be orthorhombic with the symmetry axis along a ( 1 LO) direction of the crystal. As the angular variation of the spectrum (fig. 2) shows that the local symmetry about Cu2+ is tetragonal with (100) axis as symmetry axis, the latter type of chars compensation is to be given up in favour of the first. However, there is a second possible way of incorporating Cu2+ ion in the lattice, the local site symmetry still being tetragonal as observed. The Cu2- ion can go at an interstitial position i.e. in the plane of four Br- ions while for the compensation of extra positive charge, the two first neighbour NI$ ions lying along the fourfold axis get removed, thereby giving rise to a tetragonal symmetry. Since both the interstitial and substitutional models lead to ESR spectra with the same symmetry properties it is not possible to say defmitely what is the position of the paramagnetic ion unless one can make use of some additional features in the spectrum. The presence of superhyperfine (shf) structure from
15 Nay 197
the neighbouring nuclei gives supplementary inform2 tion concerning the position of the paramagnetic ion In the present work, we observed 16 shf lines on the isotropic line (gl); the four hyperfine lines being ove lapped so as to give rise to a single broad line. As alI the bromines are equivalent, the effective nuclear spi of the ligand nuclei becomes 6 for interstitial positio of Cu2+ and 12 for substitutiona! position of Cu2+_ This gives rise to a shf interaction resulting in the spl ting of each of the Cu2+ hyperfine lines into 13 and lines for interstitial and substitutional positions of C respectively. Since the hyperfine lines of the isotropi line are overlapped, we expect at least 13 or 25 shf lines for interstitial and substitutional positions of C respectively. The actually observed 16 shf lines on ti broad line suggest that the Cu2+ ion goes into the in. terstitial position_ This would correspond to the mot el for Cu2* in NH,CI as proposed by Hagen and Trappeniers [2]. In an interstitial complex, there are three possible coordinations for the metal ion site viz. (1) four bramine ions and two vacancies, (2) four bromme ions and two Ns ions and (3) four bromine ions and two water molecules. All the three coordinations give rise to tetragonal symmetry at the metal ion site. The fol Br- ions would be in a plane of the octahedron with the two vacancies or two H20 molecules or two NH: ions at the first neighbour sites along ( 100) direction situated at the opposite vertices of the octahedron. The formation of *he cation vacancies is due to the necessity of the charge neutrality. If it is assumed th; the metal ion is associated with two first neighbour NH$ ions along the
13
CHEMICAL PHYSICS LETI-JZRS
Volume 56, number 1 Table 3 Calculated values of P and PK P (cm-‘)
Cu*+: N&Br(present work) Cu*+: ND4Cl(I) (Trappeniers and Hagen) g,=
2.0023
0.0288 0.021
and gl =2.0023 -6Xct/AE,
PK
(cm-‘) 0.0086 0.0067
0)
where Af? = E(2Eg) - E(2Alg)_ Assuming the free-ion spin-orbit splitting constant X = -830 cm-l, the orbital reduction factor o2 is calculated and is found to be 0.54. Thisvalue indicates a large covalency as is supported by the observed superhyperfine structure_ For a 2A I- ground state, the hyperfme constants
15
May
1978
NH4Br with the results obtained in a previous study of ‘he ESR spectrum of Cu2*: N&Cl (centre 1) shows that there is a great similarity between the two centres. The 0.1~~ ion in the NH4Br (acidic) crystal is surrounded by a square consisting of four Br- ions and two water molecules; i.e. the paramagnetic ion is in the centre of a common face of two unit cells of NH4Br. The charge of the copper ion is compensated by the formation of two Ns vacancies which are filled by two water molecules.
Acknowledgement The authors thank Dr. Peter van der Valk for many valuable discussions.
A and B occuking in the spin-hamiltonian are given by A=P(--K+++$X&AE).
(4) B=P(--K-;-Y
XCY~/AE-),
with P = gegna& re3_In this expression ge and gn are the electron and nuclear g factors and p and &, are
Bohr and nuclear magnetons- K is a dimensionless quantity which is a measure of the contribution of selectrons to the hyperfme interaction and is generally found to have a value of about 0.3. Since our ESR results give approximately a zero value for B and assuming the value 0.3 for K, we calculated P from expression (4) and found it to be 0.0288 cm-l giving a Fermi contact term&k to be 0.00864 cm-l_ The calculated parameters are given in table 3. In conclusion, it can be stated that a comparison of the constants of the spin-hamiltonian for Cu2+:
14
References 111 N.J. Tnppeniers and S.A. Hagen. Physica 31(1965) 122 [21 S.H. Hagen and N.J. Trappeniers. Physica 47 (1970) 165 131 M.D. SasUy and P. Venkateswarlu, Rot. Indian AcadSci- 66 (1967) 208. [41 G-M- Larin. I-V. Miroshnichenko and G-K. Chirkin. Soviet Phys. Solid State 9 (1967) 529. 151 CJ. Bat&amen, Introduction to @and field theory (McGraw-Hrll. New York, 1962) p_ 108. [61 N_ Kuroda, J_ Phys_ Sot. Japan 29 (1970) 802. [71 A.H. M&i and B.R MccarVey, J_ Chem. Phys. 29 (1958) 31.
VI V-G_ KIMIII~II, S.G. Sathyanarayan and G_ Sivarama Sastry, J_ Chem. Phys. 66 (1977)
17:5_
191 A-D. fiehr, J. Phys. Chem. 64 (1960)43_ WI J.A. Bag&o and P.A. Vaughan, J. Inorg. Nucl. Chem. 32 (1970) 803.
CHEMICAL PHYSICS LETTERS
15 May 1978
ESR AND OPTICAL SPECTRA OF Cu2+: NH+ N.J. TRAPPENIERS,
F.S. STIBBE and J.L.. RAO
Vi der Waak-Zaboraton-urn, Uniuersiteit vanAmsterdam.Amsterdam,T?ieNetherlands (243th publication of the van der Waals Fund) Received 24 February 1978
ESR and opt&I studies of Cu*+ ions doped in N&Br singlecrystals have been carried out at room temperature. A new centre has been identitiedfor the crystals grown from acidic medium.The room temperaturedata revealthat Cu*+ ions go to interstitialsites havingsquareplanarcoordination of four Br- ions.
I_ Introduction The investigation of the ESR spectrum of CuZt ions in single crystals of ammonium chloride has revealed many interesting features. -Amongst other things, these studies indicate that different copper complexes can be introduced into the crystal depending on the growth conditions. Trappeniers and Hagen [1,2] showed that the nature of these centres could be controlled by the pH of the aqueous solutions from which the crystals were grown. They also proposed models for two types of centres that were found in the crystals of Cu2+: -Cl which were grown respectively from the HCl acidic, the NH40H basic, and the neutral solutions (without auy addition of either HCl or NH,+OH)_ Sastry and Venkateswarlu [3] and Chirkin et al. [43 have studied the ESR spectra of Cu2+: WBr crystals grown from neutral solution. Since different copper complexes could be introduced into the NH4Cl crystal depending on the growth conditions, we have made an attempt to investigate more specifically the ESR spectrum of Cu2* ions incorporated into single crystals of NH4Br grown from the HBr acidic medium. If the configuration of these centres can be elucidated, they could then be used as probes for investigating the NH4Br phase diagram as a function of temperature and pressure.
10
2. Experimental NqBr crystals doped with Cu2+ ions are grown from a saturated solution of ammonium bromide in water to which a few mole percent of CuBr2 is added. Urea is added to stimulate the cubic growth of the crystal. To the growth solution, two drops of concentrated hydrobromic acid are added. The crystals are yellow in colour and of a good cubic shape. ESR spectra are recorded at room temperature on a K-band spectrometer with 100 kHz field modulation. DPPH is used as a field marker. The optical absorption spectrum is recorded on a Cary model 14 spectrophoton eter.
3. ESR measurements The free Cu2+ ion has a 3dg configuration with a ground state 2D. When subjected to a cubic crystalline field this five-fold orbital degenerate state splits into a doublet (2E) and a triplet (2T) and the orbital degeneracy is further removed in the presence of a tetragonal or an orthorhombic component in the crystalline field leaving an orbital singlet as the ground state. The nuclear spin of copper is 312 and the hyperfine interaction gives a four-line spectrum. Fig. 1 shows the ESR spectrum taken when the direction of the magnetic field is parallel to a (100) axis of the crystal. The angular variation of the spectrum, as shown in fig. 2, reveals
15 May 1978
CEIEhfICAL PHYSICS LElTERS
VoIume 56, number 1
that the symmetry of the centres is axial with the syrnmetry axis pointing in the direction of the cubic axis. The main features of the spectra can be described with the spin-hamiltonian
4L;v J
.
fMzSz
dH
315
I
3L.0
360
300 MHz
g,, = 2.035,
Fig_ 1. ESR spectrum of Cu*+: NJ&Br-
gL = 2.204,
A = 70 X 10m4 cm-l
DPPH
The optical absorption spectrum has been recorded in the wavelength region from 0.3 p to 1 S P at 300 K. The absorption spectrum is shown in fig. 3. It is found that Cu2+ ions give rise to two main absorption bands; one at about 28900 cm-1 with relatively high intensity and the other at about 13380 cm-t with au accompanying shoulder at 9160 cm-1 _ In octahedral symmetry, the ground state electronic configuration of t$e3 for Cu2+ gives rise to a 2Eg state. When one of the electrons iu a t2 orbital is promoted to an e orbital the excited electron configuration t$e”
350
w I
Fig_ 2.. hgular
Table 1 Spin-ham&o&n
I
variation of the ESR
I
spectrum.
constants for Cu*+- _NH.+Br reported by different authors
Authors
C3irkin et al. Sastry et al. present work
B=OX104cm-1.
4. Optical measurements
-
II 34.0 I-
and
The spin-hamiltonian constants reported by different authors are given in table 1. The g values as given by Sastry and Venkateswarlu and Chirkin et al. roughly agree with those of our complex but there is a large difference in the hyperfine value. This indicates that a different centre is incorporated into the crystal grown from the acidic solution.
IH
360
(1)
withS = l/2 and nuclear spin I= 3/2. Superhyperfhre effects are not included iu tbis expression. The parameters in the spin-hamiltonian, as calculated from the spectra are:
DP-PH
I
+IySy).
+BU,S,
300 300 300
&l
g1
A X 104(cmW1)
f3 X lo4 (cm-‘)
2.029 f <0.003) 2.0036 1(0.005) 2.035
2.190 * (0.005) 2.217 2.204
170 182.6 70
0 0 0
11
CHEMICAL
Volume 56. number 1
15 May 1978
PHYSICS LETTERS
*
e
I dg( ‘ol
.**’ ‘. *. \
%
lP
% /=.
2a lP f I!3
-6Oq-zos*ot -6Dq+2Os*fXlt
compressed
octahedral
free ton
Leq-ZDs*Dt ZDq* OS -.xX
[cl
lb1
to1
29
‘E!3
tetragonal
Fig. 4. Energy level splitting of Cuz+ in octahedral and tetra-
gonal crystal fields_
L
/
1.5l.L
03s wavelength
Fig. 3. Optical
absorptionspectrum of Cu*+:N&Br.
rise to a *Tz8 uppe r state. Thus, only one single transition is expected for Cu2+ in 4, symmetry. Since the ground state 2Eg is split under the Jahn-Teller effect, it is never possible to have a regular octahedrally coordinated CuZt complex (Bailhausen) [S] . An Eg state can be split only in a tetragonal, but not in z trigonal field, so that the copper doped crystals normally exhibit tetragonal or lower symmetries. in a tetragonal field, the ground state ‘Eg would be split into two levels 2Blg and 2Alg and similariy the upper 2T2g into 2B2, and 2Eg levels. One of the levels 2A,, or 2BI, forms the ground state accordingly as g,, is equal to or different from 2.0023. Since our ESR results give go = 2.035 andgL >glI, we assume that the ground state is 2Alg_ The same conclusion was reached previously with respect to the ground state of Cu2+: NH,Cl [2] _Thus, the bands at 9160 cm-l and 13380 cm-l have been assigned to 2Alg + 2B1, and 2Alg + 2Eg transitions respectively. The most intense band at 28900 cm-l has been assigned to the 3Alg + 2B2, transition. It should be pointed out that Kuroda [6] observed a band above 2000 cm-l for Cu2+: NH4Br (basic) and from ils intensivity attributed this to a charge transfer band. However, Maki and McGarvey [7] and Krishnan et al. [8] reported a band at about 25000 cm-l for some Ctt2+ complexes and they attributed it to a ligand tieId band. We shall also assume that the baud at 28900 cm-l belongs to a d-d transition, i.e. the 2A~g + 2B2g transition. The splittings of the energy levels in octahedral and tetragonal fields are shown in fig. 4. Energy values of gives
12
tetragonal field levels are also shown in the same figure in terms of the crystal (m) and tetragonal(Ds, Dr) field parameters. If, in addition to a tetragonal field, spin-orbit interaction is taken into account, the ground 2Eg and the upper 2T2g states would be split in two (r, and r7) and three levels (r6, r7 and r,) respectively. The tetragonal field parameters (Ds and Dt) have been evaluated from the observed band positions as follows (see fig. 4): -4Ds
- SDr=9160,
1ODq - Ds -
1ODt = 13380,
1ODq - 4Ds -
5Dt = 28900,
Dq = 1974 cm-l Dt = 988 cm-l
,
Ds = -3525
cm-1
,
.
c-9
Using these values of Ds, Dt, Dq and the free-ion spinorbit coupling constant X = -830 cm-l, the energy matrices [9] for r6 and r7 have been diagonalised. The observed and calculated band positions are given in table 2. The good agreement between theoretical and observed results justifies the interpretation of our Table 2 Assignments for the opticai absorption bands Transition
Cu**: N&Br(acidic). present work obsetved
*A +*B *Alg’2$ tg
2AlE-,2B2g
9160
13380 28900
Co*+: NI&Br (basic) Kuroda
calculated 9118 13100 13948
29032
9357 13790
Volume56. number 1
CHENICAL PHYSICS LETTERS
results. In the present work, we obtained a higher 04 value (1974 cm-l) than that normally obtained (1120 cm-l) for Cu2+ complexes. ‘Ihis is discussed below in section 5. The optical data observed for Cu2% NQBr (basic) by Kuroda as well as our present data are given in table 2. It is interesting to observe that for the acidic crystal the bands are shifted towards the lower energy side. A similar shift of the bands towards lower energy side is also observed by Baglio and Vaughan [lo] for the absorption spectrum of Cu(NH&+ ion in Cu(NH,),Cu(Br2)2 and in Cu(NH3)4(CuC12),- H20. They found that the bands of the latter compound are significantly lower in energy than those in the former compound, consistent with “semi-coordination” of the water molecules_
5. Discussion When the Cu2+ ion is incorporated in NH4Br it can go substitutionally to the m site and the charge neutrality can be achieved by producing a cation vacancy. If it substitutes for an NG ion and is associated with a first neighbour vacancy, the CL?+ ion will have a tetragonal site symmetry with (100) axis as the site symmetry axis. If the cation vacancy is at a second nearest neighbour site, the site symmetry of the magnetic ion will be orthorhombic with the symmetry axis along a ( 1 LO) direction of the crystal. As the angular variation of the spectrum (fig. 2) shows that the local symmetry about Cu2+ is tetragonal with (100) axis as symmetry axis, the latter type of chars compensation is to be given up in favour of the first. However, there is a second possible way of incorporating Cu2+ ion in the lattice, the local site symmetry still being tetragonal as observed. The Cu2- ion can go at an interstitial position i.e. in the plane of four Br- ions while for the compensation of extra positive charge, the two first neighbour NI$ ions lying along the fourfold axis get removed, thereby giving rise to a tetragonal symmetry. Since both the interstitial and substitutional models lead to ESR spectra with the same symmetry properties it is not possible to say defmitely what is the position of the paramagnetic ion unless one can make use of some additional features in the spectrum. The presence of superhyperfine (shf) structure from
15 Nay 197
the neighbouring nuclei gives supplementary inform2 tion concerning the position of the paramagnetic ion In the present work, we observed 16 shf lines on the isotropic line (gl); the four hyperfine lines being ove lapped so as to give rise to a single broad line. As alI the bromines are equivalent, the effective nuclear spi of the ligand nuclei becomes 6 for interstitial positio of Cu2+ and 12 for substitutiona! position of Cu2+_ This gives rise to a shf interaction resulting in the spl ting of each of the Cu2+ hyperfine lines into 13 and lines for interstitial and substitutional positions of C respectively. Since the hyperfine lines of the isotropi line are overlapped, we expect at least 13 or 25 shf lines for interstitial and substitutional positions of C respectively. The actually observed 16 shf lines on ti broad line suggest that the Cu2+ ion goes into the in. terstitial position_ This would correspond to the mot el for Cu2* in NH,CI as proposed by Hagen and Trappeniers [2]. In an interstitial complex, there are three possible coordinations for the metal ion site viz. (1) four bramine ions and two vacancies, (2) four bromme ions and two Ns ions and (3) four bromine ions and two water molecules. All the three coordinations give rise to tetragonal symmetry at the metal ion site. The fol Br- ions would be in a plane of the octahedron with the two vacancies or two H20 molecules or two NH: ions at the first neighbour sites along ( 100) direction situated at the opposite vertices of the octahedron. The formation of *he cation vacancies is due to the necessity of the charge neutrality. If it is assumed th; the metal ion is associated with two first neighbour NH$ ions along the
13
CHEMICAL PHYSICS LETI-JZRS
Volume 56, number 1 Table 3 Calculated values of P and PK P (cm-‘)
Cu*+: N&Br(present work) Cu*+: ND4Cl(I) (Trappeniers and Hagen) g,=
2.0023
0.0288 0.021
and gl =2.0023 -6Xct/AE,
PK
(cm-‘) 0.0086 0.0067
0)
where Af? = E(2Eg) - E(2Alg)_ Assuming the free-ion spin-orbit splitting constant X = -830 cm-l, the orbital reduction factor o2 is calculated and is found to be 0.54. Thisvalue indicates a large covalency as is supported by the observed superhyperfine structure_ For a 2A I- ground state, the hyperfme constants
15
May
1978
NH4Br with the results obtained in a previous study of ‘he ESR spectrum of Cu2*: N&Cl (centre 1) shows that there is a great similarity between the two centres. The 0.1~~ ion in the NH4Br (acidic) crystal is surrounded by a square consisting of four Br- ions and two water molecules; i.e. the paramagnetic ion is in the centre of a common face of two unit cells of NH4Br. The charge of the copper ion is compensated by the formation of two Ns vacancies which are filled by two water molecules.
Acknowledgement The authors thank Dr. Peter van der Valk for many valuable discussions.
A and B occuking in the spin-hamiltonian are given by A=P(--K+++$X&AE).
(4) B=P(--K-;-Y
XCY~/AE-),
with P = gegna& re3_In this expression ge and gn are the electron and nuclear g factors and p and &, are
Bohr and nuclear magnetons- K is a dimensionless quantity which is a measure of the contribution of selectrons to the hyperfme interaction and is generally found to have a value of about 0.3. Since our ESR results give approximately a zero value for B and assuming the value 0.3 for K, we calculated P from expression (4) and found it to be 0.0288 cm-l giving a Fermi contact term&k to be 0.00864 cm-l_ The calculated parameters are given in table 3. In conclusion, it can be stated that a comparison of the constants of the spin-hamiltonian for Cu2+:
14
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