- Email: [email protected]
A single crystal electron paramagnetic resonance study of manganese(II) doped cadmium maleate dihydrate
1. Phw. Rinled
Chem. Solids Vol. 51. No. 4. pp. 329-332, 1990 in Great Britain.
@x2-3697/90 s3.00 + 0.00 F+ergamonRes¶ pk
A SINGLE CRYSTAL ELECTRON PARAMAGNETIC RESONANCE STUDY OF MANGANESE(I1) DOPED CADMIUM MALEATE DIHYDRATE R. S. BANSAL,~ V. P. SETH~ and F%EM CHAND$ tPhysics Department, M.D. University, Rohtak-124001, Haryana, India SPhysics Department, Indian Institute of Technology, Kanpur-208016, Uttar Pradesh, India (Received
8 June 1989; accepted in revised form
14 September
1989)
paramagnetic resonance of Mn’+ doped into cadmium maleate dihydrate single crystal was studied at X-band frequencies (-9.5 GHz) at room temperature. The spin-Hamiltonian parameters Abstract-Electron
were calculated. It was found that Mn2+ enters the lattice interstitially and only one type of complex was identified. Forbidden hypertine transitions were studied in the central group. The doublet separations are in agreement with the calculated values. Keywords:
EPR study, single crystal, Mn’+ impurity.
INTRODUCTION Electron paramagnetic resonance (EPR) studies of transition metal ions cast light on the magnetic properties of unpaired electrons. They may consequently lead to some understanding about the nature of the bonding of the metal ion with its ligands. EPR studies of paramagnetic ions in carboxylic acids have yielded many interesting results [l-5]. The authors were, therefore, interested in EPR studies on Mn*+ in cadmium maleate dihydrate (C4H6Cd06). In the present study the deviation of the g values from the free spin value in d5 systems is of interest [6-81. The Mn*+ was found to enter the host in the interstitial site. CRYSTAL STRUCTURE The crystal structure of cadium maleate dihydrate (CMDH) is monoclinic with space group P 2, /c. The unit cell has the dimensions a = 8.729 (2), b = 14.285 (6), c = 11.622 (4) 8, and /I = 102.66 (2)” and contains eight formula units [9]. Details of the structure are shown in Fig. I. Each of the cadmium atoms is crystallographically and chemically distinct. Atom Cd (I) is six coordinated through four water molecules and two carboxy-oxygen atoms which also bridge to Cd (2). The geometry around Cd (1) is distorted octahedral. Atom Cd (2), however, is eight coordinated in distorted dodecahedral geometry, through four chelated carboxyl groups from two maleate ligands. EXPERIMENTAL Crystals of CMDH doped with Mn2+ were grown at room temperature (300 K) by slow evaporation
of a solution prepared by addition of an aqueous solution of maleic acid to an excess of finely divided cadmium carbonate in suspension in water followed by heating to near the boiling point, to which a few drops of manganese sulphate aqueous solution were added as dopant. The solution was cooled and filtered and kept at RT for evaporation. The transparent crystals grew as approximately rectangular prisms. The first derivative EPR spectra were recorded at RT (300 K) on the X-band (-9.5 GHz) EPR spectrometer (Varian E-109). The crystals were glued to a quartz rod and fixed to a Varian E-229 goniometer inside a rectangular cavity (Varian E-23 1 multipurpose cavity) operating in the TE,, mode (Q-7000). The magnetic field was measured with a digital fluxmeter (Varian E-500). A magnetic field modulation of 100 kHz with peak-to-peak (p-p) amplitude of 0.1 mT was applied. Polycrystalline DPPH was used as a standard g marker (g = 2.0036 * 0.0002). RESULTS AND DISCUSSION For an arbitrary orientation of the crystal, the EPR spectrum consists of a number of lines corresponding to allowed (AM = + I, Am = 0) and forbidden (AM = &-I, Am P 0) transitions. Angular variation studies of Mnr+ spectra show that there are only thirty allowed lines indicating that Mnr+ occupies only one type of site. The principal axes were found by searching the direction corresponding to the extrema in the spread of the spectrum. The z-axis is defined as the direction of maximal splitting of the EPR spectrum, the y-axis is the direction of the minimal splitting in the plane perpendicular to the r-axis and the x-axis is orthogonal to the y- and z -axes.
329
R. S. 0ms.a
330
0 Cd(2) 00
et ai.
OL
Fig. 1. Schematic representation of the crystal structure of CMDH and the paramagnetic centre.
Figure 2 shows the EPR spectrum recorded at RT along the x-axis in the xz plane. The spectra observed at room temperature were anafysed using the spin-Hamiltonian appropriate to the Mn’+ in orthorhombic (or lower) symmetry [IO]: H=jXP~*B+$~O~+jb;O; + &qJ:o:
+ b:O: + b:q)
+s*K.I-g,B,B.I+Q’[I:-~~fll+
I)]
+ QYC - I:),
where the first term represents the electronic Zeeman interactions; the second and third terms represent the axial and rhombic components of the zero field splittings, respectively, and the fourth and fifth terms represent the cubic field and the hyperfine interactions, respectively. The parameters Q’ and Q” represent the axial and rhombic components of the quadrupoie coupling tensor, respectively. Magnetic field measurements were made for the allowed lines with B parallel to the z- and x-axes. No measurement could be made for B paralki to the y-axis, since the lines got mixed up and consequently the various fine structure transitions could not be
I----
8
*
DPPH (0.3300 l-1
i
loomr
4
Fig. 2. EPR spectrum of Mn *+ doped in CMDH single crystal along the x-axis in the IX plane at 300 K and at X-band (v - 9.5 GHz).
Single crystal EPR study distinguished. Therefore, it is assumed that A,=A, and gx = gv. Using the expressions for the resonance field positions up to second order perturbation [lo] for the above spin-Hamiltonian, the best fitted parameters obtained (in units of IO-‘cm-‘) are b;= -310.5 + 1.0,
b; = 256.6 f 2.0,
b: = 3.7 f 1.O,
b: = 22.7 f. 5.0,
b; = 24.9 f 5.0,
g, = 2.016 & 0.001,
g, = 2.037 + 0.001,
g,, = 2.037 + 0.001,
A, = -89.6 f 1.0,
A, = -88.0 f 1.0,
A,= -88.0 * 1.0. The signs of the parameters are only relative. The relative signs of the parameters b! and Al were obtained by comparing the second order shifts in the separation between the hyperfine components for the various electronic transitions [lo, Ill. The sign of 6: is then obtained by assuming A, to be negative because in all the materials so far studied A has been found to be negative [lo, 12). The sign of b: results from the sign of bi and the choice of the x- and y-axes. It is clear from the crystal structure [9] of CMDH that Cd (1) is in a distorted octahedron and Cd (2) in a dodecahedron of oxygen. Assuming that the Mn2+ ion enters the lattice substitutionally in the Cd (1) position, then according to the crystal structure the four Cd (1) sites per unit cell are not equivalent in the ub and bc planes while they are equivalent in the UCplane. This symmetry requires at least two sets of allowed hyperfine Mn2+ lines in any plane except the UCplane. The same is true for the Cd (2) sites since only a single set of allowed hyperfine lines was observed in all the planes. Thus, the substitutional position is ruled out for the Mn2+ ion. It may be possible that Mn’+ is in a distorted octahedral environment having for its ligands O(4), O(4’). O(2), 0(2’) belonging to two Cd (2) atoms and 0(12), O(12’) belonging to two Cd (1) atoms. For this particular
interstitial site the Mn’+ ion is located approximately at the centre of the unit cell, having coordinates roughly (0.5,0.5,0.5). In the case of V@+ ion doped in CMDH it was found [5] that V’+ goes into this interstitial site. The g values, viz. gx * 2.037, gv = 2.037 and g, = 2.016 show large anisotropy. Similar deviation from the free spin value has been observed in distorted octahedral systems such as manganese doped tris (ethylene diamine) zinc (II) [13], zinc acetate dihydrate [14], lithium ammonium tartrate monohydrate [IS] and zinc maleate tetrahydrate [8]. A significant positive g shift, as observed, may be due to an electron-transfer process through spin-orbit interaction from the ligands to the S-state ion [7j. An estimate of the covalency parameter between Mn2+ and the nearest neighbour ligands can be obtained from the A vs C/n plot given by Simanek and Miiller (161. From this curve we obtain, for an average value of A = -88.5 x lo-‘cm-‘, that ionicity is 93% in the present system. The forbidden transitions which occur in pairs were studied in electronic transitions A4 = +$- -i. Figure 3 shows the forbidden lines in the spectrum at RT in the ?cy plane with fI = 90’, 4 = 7”. The magnitude of the splitting in these pairs of lines depends on the angle 4. For large values of 4 the splitting is negligible and each pair appears as one line. The expression for doublet separation for forbidden hyperfine transitions in the M = + f - - f transition [ 17, 181 is
= 172+2($)B,-(2m 0
b
+ 1)
x
t
B-
331
DPPH (0.3300 T)
t
IOOmT
I
Fig. 3. EPR spectrum of Md+ doped in CMDH single crystal in the xy plane with 0 = 90”. 4 = 7’ at 300 K and at X-band (v w 9.5 GHz).
R. S. EMis~i. ef al.
332
Table 1. Observed and calculated doublet separations (AB) of the forbidden hyperftne transitions (Am # 0) in the xy plane (6 = 90”. 4 variable) in CMDH: Mn’+ single crystals at 300K
REFERENCES
Doubiet separation (AB) (nif) M= +f--i Am=+1 4 = 5’ obs. 1.90
‘&f= +f+-t Am=+1 d, =l”
Calc. 1.20
Obs. 1.25
Calc. 1.31 I .94
1.80
1.84
2.19 3.12
2.48
1.56 1.88
3.12
2.81
3.07
3.75
3.76
3.44
3.65
2.48
where P = f[Q’(3 cos* 8 - 1) + 3Q” sin* 0 cos 241
d = #4(3
Acknowledgements-This work was supported by C.S.I.R., New Delhi, and U.G.C.,. New Delhi.
cost 6 - I) + b: sin* 6 cos 2f$t],
where Q’ and Q” represent the axial and rhombic components of the quadrupole coupling constant of the rsMn nucleus, respectively. Taking (~~~/g~) equal to 0.37 x 10V3 from the NMR tables 1191 the best fitted values of Q’ and Q” were found to be Q’= -6.20 x IO-‘cm-’ Q” = - 14.19 x IO-‘cm-‘. Observed and calculated doublet separations (AB) of the forbidden hypetfine transitions (Am # 0) in the xy plane (0 = 90”, d, variable) in CMDH, Mr?+ single crystals at 300 K are given in Table 1.
1. Bleaney B. and Bowers K. D., Proc. R. $0~. A214,451 (1952). 2. Sakaguchi U.. Arata Y. and Fujwara S., J. magn. Reson. 10, 139 (1973). 3. Lau P. W. and Lin W. C., J. ckem. Pkys. 59,3981,3988 (1973). 4. Mouli V. C. and Sastry G. S., Parmana 25,719 (1985). 5. Bansal R. S.. Seth V. P. and Chand P., J. Pkys. Ckem. Soika!sSO, 8iP (1989). 6. Fidone I. and Stevens K. W. H., Proc. phys. Sot. Land. 73 (1959). 7. Watanabe H., J. Pkys. Chem. Solids 25,1471(1964) and Refs therein. 8. Vithal M. and fagannathan R., 1. them. Sot. Dakon Trans. 983 (1988) 9. Post M. L. and Trotter J., J. ckem. Sot. (Dalton) 674. (1974). 10. Abragam A. and Bleaney 8.. EIectron Paramagneric Resonance of Tra~if~on Ions. Clarendon Press, Oxford (1970). 11. Low W., Paramagnetic Resonance in Sok’cis-Solid State Pkysics Supplement 2. Academic Press, New York (1960). 12. Takeda T., J. pkys. Sot. Japan 23, 1961 (1967). 13. Palmier R. A., Yang M. C. L. and Hampel J. C., Inorg. Chem. 17, 1200 (1978). 14. Misra S. K. and Kahrizi M., Pkys. Reu. B30, 2920 ( 1984). 15. Jain A. K. and Upreti G. C., J. them. Pkys. 75, 1623 (1981). 16. Simanek E. and Miiller K. A., J. Phys. Chem. Solids. 31, 1027 (1969). 17. Upreti G. C., J. magn. Reson. 13, 336 (1974). 18. Takeda T., J. pkys. Sot. Jamzn 23, 1314 (1967). 19. NMR Tables, 4th edn. Varian associates, Pala Alto (1964).
Chem. Solids Vol. 51. No. 4. pp. 329-332, 1990 in Great Britain.
@x2-3697/90 s3.00 + 0.00 F+ergamonRes¶ pk
A SINGLE CRYSTAL ELECTRON PARAMAGNETIC RESONANCE STUDY OF MANGANESE(I1) DOPED CADMIUM MALEATE DIHYDRATE R. S. BANSAL,~ V. P. SETH~ and F%EM CHAND$ tPhysics Department, M.D. University, Rohtak-124001, Haryana, India SPhysics Department, Indian Institute of Technology, Kanpur-208016, Uttar Pradesh, India (Received
8 June 1989; accepted in revised form
14 September
1989)
paramagnetic resonance of Mn’+ doped into cadmium maleate dihydrate single crystal was studied at X-band frequencies (-9.5 GHz) at room temperature. The spin-Hamiltonian parameters Abstract-Electron
were calculated. It was found that Mn2+ enters the lattice interstitially and only one type of complex was identified. Forbidden hypertine transitions were studied in the central group. The doublet separations are in agreement with the calculated values. Keywords:
EPR study, single crystal, Mn’+ impurity.
INTRODUCTION Electron paramagnetic resonance (EPR) studies of transition metal ions cast light on the magnetic properties of unpaired electrons. They may consequently lead to some understanding about the nature of the bonding of the metal ion with its ligands. EPR studies of paramagnetic ions in carboxylic acids have yielded many interesting results [l-5]. The authors were, therefore, interested in EPR studies on Mn*+ in cadmium maleate dihydrate (C4H6Cd06). In the present study the deviation of the g values from the free spin value in d5 systems is of interest [6-81. The Mn*+ was found to enter the host in the interstitial site. CRYSTAL STRUCTURE The crystal structure of cadium maleate dihydrate (CMDH) is monoclinic with space group P 2, /c. The unit cell has the dimensions a = 8.729 (2), b = 14.285 (6), c = 11.622 (4) 8, and /I = 102.66 (2)” and contains eight formula units [9]. Details of the structure are shown in Fig. I. Each of the cadmium atoms is crystallographically and chemically distinct. Atom Cd (I) is six coordinated through four water molecules and two carboxy-oxygen atoms which also bridge to Cd (2). The geometry around Cd (1) is distorted octahedral. Atom Cd (2), however, is eight coordinated in distorted dodecahedral geometry, through four chelated carboxyl groups from two maleate ligands. EXPERIMENTAL Crystals of CMDH doped with Mn2+ were grown at room temperature (300 K) by slow evaporation
of a solution prepared by addition of an aqueous solution of maleic acid to an excess of finely divided cadmium carbonate in suspension in water followed by heating to near the boiling point, to which a few drops of manganese sulphate aqueous solution were added as dopant. The solution was cooled and filtered and kept at RT for evaporation. The transparent crystals grew as approximately rectangular prisms. The first derivative EPR spectra were recorded at RT (300 K) on the X-band (-9.5 GHz) EPR spectrometer (Varian E-109). The crystals were glued to a quartz rod and fixed to a Varian E-229 goniometer inside a rectangular cavity (Varian E-23 1 multipurpose cavity) operating in the TE,, mode (Q-7000). The magnetic field was measured with a digital fluxmeter (Varian E-500). A magnetic field modulation of 100 kHz with peak-to-peak (p-p) amplitude of 0.1 mT was applied. Polycrystalline DPPH was used as a standard g marker (g = 2.0036 * 0.0002). RESULTS AND DISCUSSION For an arbitrary orientation of the crystal, the EPR spectrum consists of a number of lines corresponding to allowed (AM = + I, Am = 0) and forbidden (AM = &-I, Am P 0) transitions. Angular variation studies of Mnr+ spectra show that there are only thirty allowed lines indicating that Mnr+ occupies only one type of site. The principal axes were found by searching the direction corresponding to the extrema in the spread of the spectrum. The z-axis is defined as the direction of maximal splitting of the EPR spectrum, the y-axis is the direction of the minimal splitting in the plane perpendicular to the r-axis and the x-axis is orthogonal to the y- and z -axes.
329
R. S. 0ms.a
330
0 Cd(2) 00
et ai.
OL
Fig. 1. Schematic representation of the crystal structure of CMDH and the paramagnetic centre.
Figure 2 shows the EPR spectrum recorded at RT along the x-axis in the xz plane. The spectra observed at room temperature were anafysed using the spin-Hamiltonian appropriate to the Mn’+ in orthorhombic (or lower) symmetry [IO]: H=jXP~*B+$~O~+jb;O; + &qJ:o:
+ b:O: + b:q)
+s*K.I-g,B,B.I+Q’[I:-~~fll+
I)]
+ QYC - I:),
where the first term represents the electronic Zeeman interactions; the second and third terms represent the axial and rhombic components of the zero field splittings, respectively, and the fourth and fifth terms represent the cubic field and the hyperfine interactions, respectively. The parameters Q’ and Q” represent the axial and rhombic components of the quadrupoie coupling tensor, respectively. Magnetic field measurements were made for the allowed lines with B parallel to the z- and x-axes. No measurement could be made for B paralki to the y-axis, since the lines got mixed up and consequently the various fine structure transitions could not be
I----
8
*
DPPH (0.3300 l-1
i
loomr
4
Fig. 2. EPR spectrum of Mn *+ doped in CMDH single crystal along the x-axis in the IX plane at 300 K and at X-band (v - 9.5 GHz).
Single crystal EPR study distinguished. Therefore, it is assumed that A,=A, and gx = gv. Using the expressions for the resonance field positions up to second order perturbation [lo] for the above spin-Hamiltonian, the best fitted parameters obtained (in units of IO-‘cm-‘) are b;= -310.5 + 1.0,
b; = 256.6 f 2.0,
b: = 3.7 f 1.O,
b: = 22.7 f. 5.0,
b; = 24.9 f 5.0,
g, = 2.016 & 0.001,
g, = 2.037 + 0.001,
g,, = 2.037 + 0.001,
A, = -89.6 f 1.0,
A, = -88.0 f 1.0,
A,= -88.0 * 1.0. The signs of the parameters are only relative. The relative signs of the parameters b! and Al were obtained by comparing the second order shifts in the separation between the hyperfine components for the various electronic transitions [lo, Ill. The sign of 6: is then obtained by assuming A, to be negative because in all the materials so far studied A has been found to be negative [lo, 12). The sign of b: results from the sign of bi and the choice of the x- and y-axes. It is clear from the crystal structure [9] of CMDH that Cd (1) is in a distorted octahedron and Cd (2) in a dodecahedron of oxygen. Assuming that the Mn2+ ion enters the lattice substitutionally in the Cd (1) position, then according to the crystal structure the four Cd (1) sites per unit cell are not equivalent in the ub and bc planes while they are equivalent in the UCplane. This symmetry requires at least two sets of allowed hyperfine Mn2+ lines in any plane except the UCplane. The same is true for the Cd (2) sites since only a single set of allowed hyperfine lines was observed in all the planes. Thus, the substitutional position is ruled out for the Mn2+ ion. It may be possible that Mn’+ is in a distorted octahedral environment having for its ligands O(4), O(4’). O(2), 0(2’) belonging to two Cd (2) atoms and 0(12), O(12’) belonging to two Cd (1) atoms. For this particular
interstitial site the Mn’+ ion is located approximately at the centre of the unit cell, having coordinates roughly (0.5,0.5,0.5). In the case of V@+ ion doped in CMDH it was found [5] that V’+ goes into this interstitial site. The g values, viz. gx * 2.037, gv = 2.037 and g, = 2.016 show large anisotropy. Similar deviation from the free spin value has been observed in distorted octahedral systems such as manganese doped tris (ethylene diamine) zinc (II) [13], zinc acetate dihydrate [14], lithium ammonium tartrate monohydrate [IS] and zinc maleate tetrahydrate [8]. A significant positive g shift, as observed, may be due to an electron-transfer process through spin-orbit interaction from the ligands to the S-state ion [7j. An estimate of the covalency parameter between Mn2+ and the nearest neighbour ligands can be obtained from the A vs C/n plot given by Simanek and Miiller (161. From this curve we obtain, for an average value of A = -88.5 x lo-‘cm-‘, that ionicity is 93% in the present system. The forbidden transitions which occur in pairs were studied in electronic transitions A4 = +$- -i. Figure 3 shows the forbidden lines in the spectrum at RT in the ?cy plane with fI = 90’, 4 = 7”. The magnitude of the splitting in these pairs of lines depends on the angle 4. For large values of 4 the splitting is negligible and each pair appears as one line. The expression for doublet separation for forbidden hyperfine transitions in the M = + f - - f transition [ 17, 181 is
= 172+2($)B,-(2m 0
b
+ 1)
x
t
B-
331
DPPH (0.3300 T)
t
IOOmT
I
Fig. 3. EPR spectrum of Md+ doped in CMDH single crystal in the xy plane with 0 = 90”. 4 = 7’ at 300 K and at X-band (v w 9.5 GHz).
R. S. EMis~i. ef al.
332
Table 1. Observed and calculated doublet separations (AB) of the forbidden hyperftne transitions (Am # 0) in the xy plane (6 = 90”. 4 variable) in CMDH: Mn’+ single crystals at 300K
REFERENCES
Doubiet separation (AB) (nif) M= +f--i Am=+1 4 = 5’ obs. 1.90
‘&f= +f+-t Am=+1 d, =l”
Calc. 1.20
Obs. 1.25
Calc. 1.31 I .94
1.80
1.84
2.19 3.12
2.48
1.56 1.88
3.12
2.81
3.07
3.75
3.76
3.44
3.65
2.48
where P = f[Q’(3 cos* 8 - 1) + 3Q” sin* 0 cos 241
d = #4(3
Acknowledgements-This work was supported by C.S.I.R., New Delhi, and U.G.C.,. New Delhi.
cost 6 - I) + b: sin* 6 cos 2f$t],
where Q’ and Q” represent the axial and rhombic components of the quadrupole coupling constant of the rsMn nucleus, respectively. Taking (~~~/g~) equal to 0.37 x 10V3 from the NMR tables 1191 the best fitted values of Q’ and Q” were found to be Q’= -6.20 x IO-‘cm-’ Q” = - 14.19 x IO-‘cm-‘. Observed and calculated doublet separations (AB) of the forbidden hypetfine transitions (Am # 0) in the xy plane (0 = 90”, d, variable) in CMDH, Mr?+ single crystals at 300 K are given in Table 1.
1. Bleaney B. and Bowers K. D., Proc. R. $0~. A214,451 (1952). 2. Sakaguchi U.. Arata Y. and Fujwara S., J. magn. Reson. 10, 139 (1973). 3. Lau P. W. and Lin W. C., J. ckem. Pkys. 59,3981,3988 (1973). 4. Mouli V. C. and Sastry G. S., Parmana 25,719 (1985). 5. Bansal R. S.. Seth V. P. and Chand P., J. Pkys. Ckem. Soika!sSO, 8iP (1989). 6. Fidone I. and Stevens K. W. H., Proc. phys. Sot. Land. 73 (1959). 7. Watanabe H., J. Pkys. Chem. Solids 25,1471(1964) and Refs therein. 8. Vithal M. and fagannathan R., 1. them. Sot. Dakon Trans. 983 (1988) 9. Post M. L. and Trotter J., J. ckem. Sot. (Dalton) 674. (1974). 10. Abragam A. and Bleaney 8.. EIectron Paramagneric Resonance of Tra~if~on Ions. Clarendon Press, Oxford (1970). 11. Low W., Paramagnetic Resonance in Sok’cis-Solid State Pkysics Supplement 2. Academic Press, New York (1960). 12. Takeda T., J. pkys. Sot. Japan 23, 1961 (1967). 13. Palmier R. A., Yang M. C. L. and Hampel J. C., Inorg. Chem. 17, 1200 (1978). 14. Misra S. K. and Kahrizi M., Pkys. Reu. B30, 2920 ( 1984). 15. Jain A. K. and Upreti G. C., J. them. Pkys. 75, 1623 (1981). 16. Simanek E. and Miiller K. A., J. Phys. Chem. Solids. 31, 1027 (1969). 17. Upreti G. C., J. magn. Reson. 13, 336 (1974). 18. Takeda T., J. pkys. Sot. Jamzn 23, 1314 (1967). 19. NMR Tables, 4th edn. Varian associates, Pala Alto (1964).