EPR and optical absorption studies on VO2+ ions doped in cobalt maleate tetrahydrate single crystals

Physica B 307 (2001) 117–124

EPR and optical absorption studies on VO2+ ions doped in cobalt maleate tetrahydrate single crystals N.O. Gopal, K.V. Narasimhulu, J.Lakshmana Rao* Department of Physics, Sri Venkateswara University, Tirupati 517 502, India Received 27 July 2001

Abstract Electron paramagnetic resonance (EPR) studies have been carried out on VO2+ ions doped in cobalt maleate tetrahydrate single crystals in the temperature range 103–373 K on X-band frequency. The angular variation EPR studies show three vanadyl complexes with differing intensities. The two intense vanadyl complexes referred as I and II are found to be magnetically inequivalent, whereas the third complex is not followed due to its weaker intensity and overlapping with the other two. The spin-Hamiltonian parameters have been evaluated from single crystal and powder EPR data. The line broadening of VO2+ spectra on cooling the crystal is explained on the basis of host spin-lattice relaxation narrowing mechanism. From the linewidth of the impurity VO2+ ions, the spin-lattice relaxation time (T1 ) of Co2+ ions is estimated at various temperatures. From the temperature dependence of T1 ; it is suggested that a twophonon Raman process may be the main mechanism for spin-lattice relaxation. The optical absorption spectrum shows two bands characteristic of VO2+ ion. By correlating EPR and optical data, the molecular orbital bonding coefficients have been evaluated and discussed. r 2001 Elsevier Science B.V. All rights reserved. Keywords: Electron paramagnetic resonance; Single crystals; VO2+ ions; Cobalt maleate tetrahydrate; Spin-Hamiltonian parameters

1. Introduction Electron paramagnetic resonance (EPR) studies are usually carried out on paramagnetic impurities doped in diamagnetic hosts. Similar studies can, in principle, be carried out by incorporating paramagnetic ions in paramagnetic hosts. The EPR study of paramagnetic impurity ions in paramagnetic hosts is less common because of the broadening of impurity resonance lines due to the host *Corresponding author. Tel.: +91-8574-49666 Ext. 272; fax: +91-8574-48499. E-mail address: [email protected] (J.L. Rao).

impurity interaction. However, reasonably sharp EPR spectra have been observed in many paramagnetic hosts [1–8]. The vanadyl ion (VO2+) is the most stable cation among a few molecular paramagnetic transition metal ions which is used extensively as an impurity probe for EPR studies. Since the EPR spectrum is very sensitive to the crystalline environment, extensive EPR studies on VO2+ ions in a variety of lattices [9–14] have been reported. Recently, Narasimhulu and Rao [5] have studied the EPR spectra of Mn2+ ions in cobalt maleate tetrahydrate single crystal. They found that the Mn2+ ion is in a highly distorted orthorhombic symmetry. This paper reports the

0921-4526/01/$ - see front matter r 2001 Elsevier Science B.V. All rights reserved. PII: S 0 9 2 1 - 4 5 2 6 ( 0 1 ) 0 1 0 1 7 - 1

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extension of EPR and optical absorption studies of paramagnetic impurity (VO2+) in paramagnetic host (cobalt maleate tetrahydrate) single crystal. 2. Crystal structure Cobalt maleate tetrahydrate (hereafter to be referred as CoMTH) belongs to a group of compounds with general formula H 2M (C4H2O4)2 4H2O where M is a cation such as Zn, Co, Mn or Ni [15,16]. A detailed structure of CoMTH is not available. However, the unit cell dimensions were reported [15] for isomorphous nickel maleate tetrahydrate. The unit cell dimensions for nickel maleate tetrahydrate given are a ¼ 0:520 nm, b ¼ 0:729 nm, c ¼ 0:973 nm, a ¼ 601120 , b ¼ 1041540 , g ¼ 1021240 and Z ¼ 1: The metal ions are octahedrally coordinated by six oxygen atoms, four from water molecules and two from maleate ligands. 3. Experimental Single crystals of CoMTH were grown by slow evaporation method. To this compound, 0.3 mol% of VO2+ was added as dopant. Reddish-coloured transparent crystals with well-developed faces were separated out on concentrating the solution. The EPR experiments were performed on a JEOL FE

1X ESR spectrometer operating at X-band microwave frequency equipped with TE011 cylindrical cavity with 100 kHz field modulation. The resonance line of DPPH was used as a standard field marker. The angular variation spectra were recorded for every 51 interval. The temperature variation studies were performed using a JES UCT 2AX variable temperature controller. Optical absorption spectrum was recorded on a JASCO UV–VIS–NIR spectrophotometer (Model V-570) at room temperature, taking the pure CoMTH crystal as the reference in the UV–VIS region. 4. Results and discussion The EPR studies on paramagnetic ions in carboxylic acids have yielded many interesting results [17–20]. Inspite of the fact that there should be line broadening due to interaction of paramagnetic host and impurity ions, reasonably sharp EPR spectra were observed in VO2+-doped CoMTH single crystals. EPR spectra were recorded in three mutually perpendicular planes, defined by X; Y and Z axes. The Z-axis is chosen to be parallel to the direction of the magnetic field for which the absolute maximum separation of the hyperfine lines occur. Fig. 1 shows the EPR spectrum of VO2+ ions-doped CoMTH single crystals at room temperature for BjjZ axis in the

Fig. 1. EPR spectrum of VO2+ : CoMTH single crystals at room temperature for B||Z axis in ZX 0 plane.

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ZX 0 plane. Two magnetic VO2+ molecular ions in the unit cell of CoMTH giving rise to two sets of eight lines with differing intensities were observed. This octet spectrum is due to the interaction of electron spin (S ¼ 12) with 51V nucleus (I ¼ 72). These two VO2+ ions are denoted as I and II in Fig. 1. In addition, another set of much weaker EPR lines was also observed and is denoted as III in Fig. 1. The ratios of the intensities of the EPR lines corresponding to the VO2+ complexes I, II and III were found to be 12 : 6 : 1. This reveals the presence of three vanadyl complexes with different populations. The EPR spectra corresponding to complex III were not studied in detail due to their much weaker intensities. The octet patterns of these complexes coincide along the crystallographic axes. Fig. 2 shows the spectrum of the two complexes I and II coinciding along the crystallographic axis in ZX 0 plane. The angular variation spectra were studied corresponding to two sets of intense vanadyl lines in all the three crystallographic planes. The analysis has been carried out only for two intense signals I and II, since the weak lines were quite difficult to follow. The angular variation plot of two intense vanadyl complexes in the X 0 Y 0 plane is shown in Fig. 3. The angular variation studies reveal that the sites of complexes I and II are

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substitutional. This can also be judged well by the ( and Co2+ comparable ionic radii of V4+ (0.63 A) ( (0.72 A). From the angular variation studies, these complexes are found to be physically equivalent, but magnetically inequivalent. The two vanadyl complexes may therefore orient themselves along different directions in the unit cell, i.e. along one of the Co–H2O bond directions. Thus, the orientation of the two VO2+ sites substituting the Co2+ ions in the unit cell are different. Therefore, it appears that the site of Co2+ ion is the most probable site for substitution by VO2+ ion. 4.1. Spin-Hamiltonian parameters As the vanadyl ion has a single unpaired electron (S ¼ 12) interacting with its nucleus (51V, 99.8% abundant, I ¼ 72), the following spin-Hamiltonian can be used to analyse the EPR spectra [21]: H ¼ bðgx Bx Sx þ gy By Sy þ gz Bz Sz Þ þ Ax Sx Ix þ Ay Sy Iy þ Az Sz Iz ;

ð1Þ

where the terms have their usual meaning. The g and A values were calculated at each interval from the angular variation spectra in the three mutually perpendicular planes. The

Fig. 2. EPR spectrum of VO2+ : CoMTH single crystals for two complexes I and II coinciding along the X-axis in ZX 0 plane. The spectrum observed at 103 K is also shown in the same figure.

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N.O. Gopal et al. / Physica B 307 (2001) 117–124 Table 1 Spin-Hamiltonian parameters and direction cosines for g and A tensors of VO2+: CoMTH single crystals, A and P values are in units of 104 cm1 a Site

Single crystal complexFI

Single crystal complexFII

Powder spectrum Fig. 3. Angular variation spectrum of VO2+ : CoMTH single crystals for two intense vanadyl complexes (I and II) in X0 Y0 plane.

diagonalization procedure was carried out following the standard procedure given by Schonland [22] in the analysis of EPR spectra, for obtaining the principal g and A tensors and the direction cosines of g2 and g2 A2 tensors corresponding to the three principal directions. The g and A values and the direction cosines of g2 and g2 A2 tensors for VO2+ ions-doped CoMTH single crystals are given in Table 1. The spin-Hamiltonian parameters indicate that the symmetry of VO2+ ions in this lattice is orthorhombic. The higher symmetry (C2v) of vanadyl sites as indicated by EPR data suggests a local relaxation to create the observed rhombicity around VO2+ in this host, and hence, one can safely confirm that by the introduction of VO2+ into the CoMTH lattice, the environment around the VO2+ ions rearranges itself to provide the observed rhombic symmetry at Co2+ sites. The spin-Hamiltonian parameters can be determined more easily by recording polycrystalline

g; A; P and k parameters

Direction cosines l

m

n

gz ¼ 1:935

0.7943

0.5858

0.1612

gy ¼ 1:975 gx ¼ 2:016 Az ¼ 188 Ay ¼ 79 Ax ¼ 71 P ¼ 122 k ¼ 0:89 gz ¼ 1:932

0.5014 0.4552 0.9841 0.1617 0.2998

0.2913 0.7469 0.0864 0.3728 0.8785

0.8564 0.4847 0.1554 0.9137 0.3719

0.4553

0.0964

0.8851

gy ¼ 1:985 gx ¼ 2:005 Az ¼ 188 Ay ¼ 77 Ax ¼ 67 P ¼ 125 k ¼ 0:86 gjj ¼ 1:95 g> ¼ 1:99 Ajj ¼ 187 A> ¼ 73 P ¼ 126 k ¼ 0:85

0.7054 0.6928 0.2740 0.7496 0.9146

0.4883 0.7077 0.2424 0.5416 0.3904

0.5138 0.1387 0.9307 0.3804 0.1055

The errors in g and A values are 70.002 and 73  104 cm1, respectively. a

sample (the crystals are crushed to a fine powder). The polycrystalline spectrum can be used as a cross-check for the single crystal data. Fig. 4 shows the EPR spectra of a polycrystalline sample at room temperature and at 123 K, respectively. The spin-Hamiltonian parameters obtained from the polycrystalline sample are also presented in Table 1. 4.2. Optical absorption studies The optical absorption spectrum of VO2+doped CoMTH consists of two bands centred at 13,325 and 16,060 cm1, these bands were assigned to d–d transitions 2B2g-2Eg and 2B2g-2B1g,

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Fig. 4. The EPR spectra of polycrystalline sample of VO2+ : CoMTH at room temperature and at 123 K.

respectively [23]. By correlating EPR and optical absorption data, the molecular orbital coefficients (b21 and e2 ) are calculated for vanadyl ion. 4.3. Calculation of molecular orbital bonding coefficients, the Fermi contact term (k) and the dipolar hyperfine coupling parameter (P) The molecular orbital bonding coefficients (b21 and e2 ), the Fermi contact term k and the dipolar hyperfine coupling constant P are calculated from the following formulae given by Kivelson and Lee [24] gjj ¼ ge  8ðlb21 =E2 Þ; g> ¼ ge  2ðle2 =E1 Þ

ð2Þ

and A8 ¼ P½ð47 þ kÞ  8ðlb21 =E2 Þ  67ðle2 =E1 Þ ; 2 A> ¼ P½ð27  kÞ  11 7 ðle =E1 Þ ;

b21

2

ð3Þ

where the terms and e are bonding coefficients of jx2  y2 S and (jxzS; jyzS) orbitals, which represent the in-plane s-bonding and out-of plane p-bonding, respectively. l is the spin–orbit coupling coefficient, which is assumed to be 170 cm1

for VO2+ ion [24]. E1 and E2 are energy separations from the ground state 2B2g to the two nearest higher states 2Eg and 2B1g, respectively. ge is the free electron g value (ge ¼ 2:0023 ), P represents the dipole–dipole interaction of the electronic and nuclear moments (P ¼ ge gN be bN /r3 S), k is the Fermi contact parameter, which is related to the unpaired electron density at the vanadium nucleus and the other symbols have their usual meaning. Using the EPR and optical data, Eqs. (2) and (3) can be solved to get b21 ; e2 ; P and k: The values of molecular orbital coefficients obtained in the present work are b21 ¼ 0:62 and e2 ¼ 0:48; respectively. The parameters (1  b21 ) and (1  e2 ) are the measures of the covalency rates [3,10,9], the former gives an indication of the influence of sbonding between vanadium atom and equitorial ligands while the latter indicates the influence of pbonding with the vanadyl oxygen. The calculated values of b21 and e2 indicate that the in-plane sbonding is moderately ionic and out-of plane pbonding is significantly covalent. The values of P and k can be calculated using the above expressions. The calculated values of P and k are listed in Table 1. The parameter k indicates extreme sensitivity to the deformations of

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the electron orbitals of the central vanadium ion. The large value of k indicates a large contribution to the hyperfine constant by the unpaired selectron and also probably a contribution from spin polarization. The standard value of P for a free ion is 160  104 cm1. The calculated value of P in the present system is 126  104 cm1, which is considerably reduced (78%), which indicates a significant amount of covalent bonding in the complex. The values of P and k are close to the values reported for VO2+ ions in other lattices [10,11,18,19]. 4.4. EPR spectra at different temperatures EPR spectra of VO2+ ions doped in CoMTH have been studied at various temperatures from 103 to 373 K. The linewidth variation studies were carried out choosing the EPR spectrum of VO2+ : CoMTH in ZX 0 plane when the magnetic field is along the X-axis. An appreciable linewidth variation of hyperfine lines of VO2+ : CoMTH single crystals was observed in contrast with diamagnetic host lattices. For diamagnetic hosts, smaller linewidths (DB) independent of temperature were reported whereas for paramagnetic hosts, DB values were found to be about twice as large as those in the diamagnetic hosts at room temperature. Thus, linewidths of VO2+ ions reported for diamagnetic hosts K2 Mg(SeO4)2 6H2O [1] and Zn(malate)2.3H2O [25] were 8 and 8 G, respectively. In the present case, the observed linewidth of VO2+ at room temperature is 16 G. Such a line broadening is well comparable with other cobalt host lattices and is expected mostly due to dipolar interactions. As the temperature is lowered, the width of the resonance lines increase and at the same time, intensity decreases and a broadened spectrum was observed at 103 K as shown in Fig. 2. A plot of logarithm of intensity vs. 1=T is shown in Fig. 5 for the hyperfine line MI ¼ 5=2: The activation energy has been calculated from the slope of the log I vs. 1=T graph and it is found to be 0.02 eV. On raising the temperature again, the well-resolved spectrum reappears indicating that the smearing out of the spectrum is due to the increased linewidth on lowering the temperature. The

Fig. 5. A plot of inverse of temperature vs. logarithmic intensity for the hyperfine line MI ¼ 5=2:

observation of narrow signals at room temperature and the broadening of the lines at low temperatures can be explained on the basis of a model proposed by Mitsuma [26]. If the motional narrowing is due to exchange between the host ions, the impurity linewidths will be temperature independent until the temperature is very much lowered and a change in g-value with temperature is also expected. In the present work, an appreciable change in linewidth with temperature has been observed and there is no change in g-values. This can be explained on the basis of spin-lattice relaxation, where the fast spin-lattice relaxation of the paramagnetic host provides sufficient modulation of the dipolar interactions between the host and the impurity ions [26,27], which is mainly responsible for these linewidth variations of hyperfine lines of VO2+ ions in Co2+ lattices. 4.5. Estimation of spin-lattice relaxation time The linewidth variation of VO2+ hyperfine lines in CoMTH can be understood on the basis of host

N.O. Gopal et al. / Physica B 307 (2001) 117–124

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spin-lattice relaxation mechanism. The fast spin-lattice relaxation of the host ions can randomly modulate the dipolar interaction between paramagnetic host and impurity ions, resulting in what is called host spin-lattice relaxation narrowing [26]. When the host spin-lattice relaxation narrowing mechanism is effective, the host spin-lattice relaxation time (T1 ) is given by [26,27] 3 T1 ¼ ð10 Þðh=2gh bÞðDBimp =B2d Þ

ð4Þ

with B2d ¼ 5:1ðgh bnÞ2 Sh ðSh þ 1Þ In the above equation, h is Planck’s constant, b is Bohr magneton, gh is the host g value, Sh is the effective host spin and is taken to be 12; n is the number of host spins per unit volume which can be calculated from crystallographic data of the crystal lattice and Bimp is the impurity linewidth. As the crystal structure of CoMTH is not available, the crystal structure data of isomorphous H2Ni(C4H2O4)2.4H2O has been used in the calculation of n: The g value for Co2+ ions in Co2+ : (NH4)2 Zn(SO4)2.6H2O has been taken to be the gh in the present work. The detailed linewidth studies in VO2+ : CoMTH were carried out for MI ¼ 52 hyperfine line at different temperatures. From the observed linewidth of VO2+ in CoMTH, the order of spin-lattice relaxation time (T1 ) obtained for Co2+ ions at room temperature is 2.51  1012 s. The T1 values calculated for this system along with the other VO2+ doped systems are given in Table 2. The T1 values have been calculated at different temperatures.

Table 2 Host spin-lattice relaxation time (T1 ) of Co2+ in some high-spin cobalt complexes at 300 K

Fig. 6. A logarithmic plot showing the spin-lattice relaxation time T1 of Co2+ in CoMTH single crystal as a function of temperature.

The temperature dependence of T1 can be approximately given by the relation [21] 1=T1 ¼ AT þ BT n :

ð5Þ

Fig. 6 shows a logarithmic graph of the estimated T1 as a function of temperature for the Co2+ host ion when the magnetic field is parallel to Z-axis. It is found from the slope that T1 pT n where n ¼ 1:0: T1 is found to be of the same order as observed for other cobalt salts [1,5,7,8,10,28]. From the nature of the graph, it is postulated that two-phonon Raman process may be the main mechanism for the spin-lattice relaxation.

System

T1 (1012 s)

Reference

5. Conclusions

VO2+ : Cs2Co(SO4)2 6H2O Mn2+ : CoH6CeMo12O42 12H2O Mn2+ : Co(H2O)6 (C6H2N3SO 9 ) 4H2O VO2+ : Rb2Co(SeO4)2 6H2O VO2+ : H2Co(C4H2O4)2 4H2O

1.9 2.6 0.54 2.0 2.51

10 7 8 28 Present work

1. EPR spectra of VO2+ ions in cobalt maleate tetrahydrate (CoMTH) have been studied in the temperature range 103–373 K. Three vanadyl complexes have been identified; two are magnetically inequivalent and one set of weaker lines possess intensity much distinguishable

124

2. 3.

4.

5.

N.O. Gopal et al. / Physica B 307 (2001) 117–124

than the other two. The detailed EPR analysis indicates that the magnetically inequivalent complexes occupy the Co2+ substitutional site in the lattice. The spin-Hamiltonian parameters are found to be in orthorhombic symmetry. The widths of VO2+ resonance lines increase with decreasing temperature. This has been explained due to host spin-lattice relaxation narrowing mechanism. The host spin-lattice relaxation time (T1 ) calculated from the linewidth of VO2+ is comparable to the values reported for other cobalt systems. From the temperature-dependent T1 ; two-phonon Raman process is found to be the main mechanism for spin-lattice relaxation, as was observed in similar cases in Co2+ hosts. The optical absorption spectrum at room temperature shows two absorption bands (2B1g and 2Eg) characteristic of VO2+ ions. By correlating EPR and optical data, the molecular orbital coefficients have been evaluated.

Acknowledgements One of the authors KVN is greatly thankful to CSIR, New Delhi for the award of Research Associateship.

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