Electron Paramagnetic Resonance (EPR) and optical absorption spectra of VO2+ ions in K2SO4Na2SO4ZnSO4 glasses

October 1995

ELSEVIER

Optical Materials 4 ( 1995 ) 723-728

Electron Paramagnetic Resonance (EPR) and optical absorption spectra of VO 2 ÷ ions in K2SO4-Na2SO4-ZnSO4glasses R. Rama Kumar, A.S. Rao, B.C. Venkata Reddy Department of Physics, S. V. Universi~, Tirupati - 517 502, India Received 24 July 1994; revised 16 March 1995; accepted 26 April 1995

Abstract Electron Paramagnetic Resonance (EPR) spectra of K2SO4-Na2SO4-ZnSO 4 glasses containing 3 mol.% of vanadium sulphate have been studied. Spin-Hamiltonian parameters (gll, g ±, AII,A ± ), the dipolar hyperfine coupling parameter (P), the Fermi contact interaction parameter (K) and the tetragonality of the V4÷ site (A gll/ A g • ) have been calculated. The optical absorption spectra of VO2÷ ions doped these glasses show three bands. By correlating the EPR and optical spectral data. the molecular orbital parameters have been evaluated. Results agree with similar studies made earlier.

1. Introduction Interest in glasses has rapidly increased in recent yearsbecause of their diverse applications in electronics, nuclear and solar energy technologies and acoustooptic devices. The use of electron paramagnetic resonance (EPR) to study magnetic ions in glasses has grown steadily since the appearance of the first paper by Sands [ 1 ]. EPR studies take on an additional importance in determining the structure of the glass, which is not easily elucidated by ordinary physical methods such as X-ray diffraction. Optical absorption and EPR spectra of vanadium (IV) in different glasses have been studied by various workers [2-14]. The ternary sulphate glass system K e S O 4 - N a 2 S O 4 - Z n S O 4 has been the subject of many investigations for the past few years [ 15-19]. Jain et al. [ 19] studied the EPR spectra of VO 2÷, Mn 2÷ and Cu 2+ ions in these glasses at 290 K only and made a tentative analysis. This paper deals with the EPR of VO 2 + ions in K2SO4-Na2SO4-ZnSO4 glasses measured in the extended range between room and the liquid 0925 -3467 / 95 / $09.50 © 1995 Elsevier Science B.V. All rights reserved SSD10925-3467(95)00021-6

nitrogen temperatures and also the optical absorption spectra at room temperature.

2. Experimental Potassium sulphate-sodium sulphate-zinc sulphate glasses of composition (i) I 0 K 2 S O 4 - 40 Na2SO4 - 50 ZnSO4 (RI), (ii) 25 K 2 S O 4 - 25 N a 2 S O 4 - 5 0 Z n S O 4 (R2), (iii) 3 0 K 2 S O 4 - 2 0 N a ~ S O 4 - 5 0 Z n S O 4 ( R 3 ) , a n d (iv) 4 0 K 2 S O 4 - 10 N a 2 S O 4 - 5 0 Z n S O 4 ( R 4 ) , doped with VOSO4. H20 were prepared, the composition being expressed in mol.%. The Analar grade reagents of Z n S O 4. 7 H 2 0 , N a z S O 4 and K2SO4 were used as raw materials, 3 mol.% of VOSO4' H20 was added as an impurity to the batches. Batches were melted in crucibles using an electric muffle furnance at 900°C. The melt was then quenched to room temperature in air to form glasses. The quenched glass was immediately immersed and stored in liquid paraffin to avoid moisture attack. No attempts were made to anneal the glasses prior to recording the spectra in the light of

R.R. Kumar et al. / Optical Materials 4 (1995) 723-728

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earlier reports [20,21 ] where it has been shown that annealing does not materially affect the EPR spectral features. The glasses were crushed into pieces for EPR measurements. EPR spectra were recorded in the extended range between room and the liquid nitrogen temperatures on a JEOL JES-FE 3X Spectrometer operating at X-band ( u ~ 9.215 GHz). A magnetic field modulation of 100 kHz was applied. DPPH with g = 2.0036 was used as a standard field marker. The optical spectra were recorded at 300 K using a Cary-17D Spectrophotometer in the range 300-900 nm, in which the standards used were glasses of the same composition without vanadium.

The EPR spectra obtained were very similar to those reported by other workers, and can be described by means of an axial spin-Hamilton•an of the form: = gltfll-t:S: + g • fl(H~Sx + H~Sy) +AIIIzSz+A±(I:,S~+g&),

where the symbols have their usual meaning and quadrupole and nuclear Zeeman interaction terms are ignored [22,23]. The solutions of the spin-Hamilton•an ( 1 ) are given in Eqs. (2) and (3) for the parallel and perpendicular hf lines, respectively. HII(m)=HII(O) - mAll--(-~ - m2) 2HII A2( 0 ) ' 2

Since the EPR spectra recorded at room temperature for all four different compositions of glasses namely RI, R2, R3, and R4 are similar, that for glass R3 is given as an example in Fig. 1. The spectrum shows a structure which is characteristic of hyperfine interaction of a single unpaired electron with a 5Iv (99.8% abundant, spin I = 7/2) nucleus. The EPR spectra at LNT were very similar to those at RT except that the individual hyperfine lines were sharper and more intense which is ascribed to a spin-lattice relaxation effect. Further, there was no appreciable change in the spectra on temperature variation.

•

~

DPPH

A± = - P [ K - 2 / 7 I

r

2250

I

I, 3250

I

I

I

I z,25~

F i g . I. T h e E P R s p e c t r u m o f V O 2+ i o n s i n 3 0 K 2 S O 4 - 2 0 N a 2 S O 4 5 0 Z n S O 4 g l a s s at 3 0 0 K.

)

2

4H± (0)

.

(3)

Here m is the magnetic quantum number of the vanadium nucleus; H , ( O ) = h v / g t f f 3 and H ± ( O ) = h v / g±/3. Measurements for the H u position were taken which corresponds to a maximum in the first derivative curve of the parallel hfs component for a given 'm' value, whereas the H± position is enclosed between the first derivative perpendicular peak and its "zero" [5]. The spin-Hamilton•an parameters for various compositions calculated from Eqs. (2) and (3) are given in Table 1. An iterative procedure [ 20] has been used in the numerical analysis. The uncertainty in the value ofg is _+0.002 and in the value of A is + 1 X 10- 4 cm --1 From Table 1, it can be seen that there is a slight difference among the gll' All, g± and A± values for different compositions of glasses. It should be noticed that as the relative concentration of K2SO4 increases, the gll value increases whereas g ±, All and A ± decrease. For all calculations the spectra recorded at 300 K have been taken. Kivelson and Lee [2] gave the following relations: All = - - P [ K + 4 / 7 - - A g l l - - 3 / 7

I

(2)

. 2 ~ A • +All

H±(m)=H±(O)-mA±-(~-m

3. R e s u l t s a n d a n a l y s i s

(1)

Ag±]

11/14 Ag±]

(4) (5)

where P = 2 y f l / ~ ( r - 3 > is the dipolar hyperfine coupling parameter and K is proportional to the amount of isotropic Fermi contact interaction, Agll = g l l - ge and Ag± = g ± --ge. Using Eqs. (4) and (5), the values of P and K calculated from the spin-Hamilton•an para-

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R.R. Kumar et al. / Optical Materials 4 (1995) 723-728

Table 1 Spin-Hamiltonian parameters, dipolar hyperfine coupling, P, and Fermi contact term, K, of VO 2÷ in K2SO4-Na2SOa-ZnSO4 glasses Composition

gll

g±

[All] ( 1 0 - 4 c m -1)

[A±] ( 1 0 - 4 c m

Rt R2 R3

1.927 1.928 1.929 1.930

1.977 1.977 1.976 1.975

181 181 180 180

71 7l 70 70

R4

Table 2 All, A~, PK and Agll/A g

±

t)

P (10-4cm -I)

K

119 119 119 120

0.86 0.86 0.85 0.85

of VO 2÷ in KzSO4 -- Na2SO4-ZnSO 4 glasses

Composition

[All ] ( 10-4 c m - 1)

[A~ ] ( 10 -4 cm 1)

PK ( 10 -4 cm t )

Agll/Ag l

RI R2 R3

78 78 78 78

32 32 32 32

103 103 102 102

2.976 2.937 2.787 2.648

R4

meters are included in Table 1. From Table 1 it can be seen that as the concentration of K 2 S O 4 increases the value of P increases while the value of K decreases. A similar behaviour of the variation of the spin-Hamiltonian parameters, P and K is observed in vanadyl ions d o p e d K 2 S O a - Z n S O 4 glasses [24] for the increased concentration of K 2 S O 4. Molecular orbital theory shows that the components All and A± consist of the contributions A' n and A'c of the 3dxy electron to the hyperfine structure and whereas however the PK term

/ B2g-'~ 2Eg

0.5

0.4 ZB29"~" 2B 0.3

0"2

0.1

o.o 640

I

I

700

800

900

arises due to the anomalous contribution of the s-electrons. Eqs. (4) and (5) can be rewritten in the following component parts: All =All - P K

(6)

A_c =A'~ - P K

(7)

where A l l = - - P [ 4 / 7 - - A g l I - - 3 / 7 Ag±], and A'± =

P[2/7 + l l / 1 4 Ag±]. The calculated values of All, A'~ and PK are given in Table 2. It is seen that there is no significant change in the value of All and A'c in the glasses studied here. The optical absorption spectrum of VO 2+ ions in R 3 glass at room temperature is shown in Fig. 2. The observed spectrum consists of one intense band at 11873 c m - i and two weak bands at 13657 and 14789 cm -1. These bands are typical for the VO 2+ ion and can be assigned to the d-d transitions, viz., 2B2g-->2Eg, 2B2g--->2Big, a n d 2B2g--> 2Alg, respectively. A shift in the band maxima has been observed in the spectra with the replacement of sodium sulphate by potassium sulphate. A similar trend has been observed by Ahmed et al. [25] in their absorption spectra of mixed alkali borate glasses containing copper.

4. Discussion

Wavelength (nm)

Fig. 2. The optical absorption spectrum of the VO 2+ ion in 30 K2SO4 -20 Na2SO4 - 50 ZnSO4 glass at 300 K.

Hecht and Johnston [ 5 ] indicated that threefold or fourfold symmetries are possibilities to describe the

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R.R. Kumar et al. / Optical Materials 4 (1995) 723-728

Table 3 EPR parameters of VO 2÷ in different glasses System

gll

g±

A| (10 4cm ])

A~ ( 1 0 - 4 c m

Na20-B203 Li20-B203 Na20-B203 K=O-B203 Cs20-B203 K2SO4-ZnSO 4 ZnO-B203 PbO-B203 Li20-CdO-B203 Li20-BaO-B203 Li20-CaO-B203 Li20-MgO-B203 LiEO-Na20-B203 KzSO4-N~SO4-ZnSO4

1.943 1.935 1.932 1.933 1.934 1.9216 1.929 1.919 1.9354 1.9365 1.9365 1.9392 1.931 1.927

1.980 1.987 1.989 1.987 1.988 1.9697 1.961 1.960 1.9749 1.9708 1.9715 1.9697 1.956 1.977

168 174 175 174 173 180.32 165.2 166.3 171 167.43 169.06 166.58 171 181

61 64 63 62 62 68.05 58.6 56.7 65.5 58.9 60.75 60.69 5l 71

crystal field of the V 4 + ion in glass. An octahderal site with a tetragonal compression would give values of g, < g i < g~ (2.0023). It is found that the values measured in this work coincide with this relationship and are close to those of the vanadyl complexes which are listed in Table 3. It is therefore confirmed that V 4÷ ions in our samples exist as VO 2÷ ions in octahedral coordination with a tetragonal compression and belong to C4v symmetry. The value of A g , / A g ± which measures the tetragonality of the V 4 ÷ site is included in Table 2. The decrease in the value of A g , / A g ± shows the improvement of the octahedral symmetry of the V 4 + site. They are slightly high when compared to that of the vanadyl ion in borate [26,27] and phosphate [28,29] glasses. This shows that the vanadyl ion in the present glasses are in more tetragonally distorted octahedral sites. The electron paramagnetic resonance and optical spectral data were correlated to obtain the molecular orbital (MO) parameters for the vanadyl ion from the following relations [2,30] (neglecting small correction terms)

1)

Ref.

[51 [6] [6] [6] [6] [12] [14] [14] [261 [27] [271 [27] [28] present work

[

4Aa2fl 2 ] g , = g e [ 1 E(EB-~_---~ZB2g)j

r

A72f12

(8)

]

(9)

g ± = ge L 1 -- E ( 2 E g ) - E ( 282g )

where A is the spin-orbit coupling constant which, for the free V 4+ ion, is 170 cm-1 [2]. The bonding parameters t~2, 72,2and f12 ( = 1.00) characterize respectively the in-plane it-bonding, in-plane ~'-bonding, and out-of-plane ~r-bonding of the VO 2+ complex. The weaker the covalent character of the bonding, the closer the values of the parameters a 2 and 72 to unity [20]. The expressions ( 1 - a 2) and ( 1 - y2) are the covalency rates, the former giving an indication of the influence of the tr-bonding between the vanadium atom and the equitorial ligands and the latter indicating the influence of the ~'-bonding with the vanadyl oxygen. The band positions for different compositions of glasses and the corresponding values for the molecular orbital parameters ( a 2 and 72) for A= 170 cm -] are given in Table 4, where it may be seen that all the

Table 4 Band positions and their assignments, molecular orbital (MO) parameters and covalency rates for VO 2+ ions in K2SO4-Na2SO4-ZnSO 4 glasses Composition

2B2g---~2Eg(cm -1)

2B2g_.,2Blg (c m i)

2B2g...+2Atg(c m i)

or2

,y2

( l _ o t 2)

(1_3,2)

R1

11958 11902 11873 11831

13789 13695 13657 13602

14921 14833 14789 14702

0.732 0.737 0.745 0.752

0.959 0.919 0.882 0.879

0.268 0.263 0.255 0.248

0.041 0.081 0.118 0.121

R2

R3 R4

R.R. Kumar et al. / Optical Materials 4 (1995) 723-728

bands are shifted towards longer wavelengths as the concentration of potassium in the glass increases. The values of the MO parameters a 2 and 32 obtained for different compositions of glasses in the present work indicate that there is a moderate covalency for the inplane tr-bonding; whereas the in-plane 7r-bonding is significantly ionic. The parameters, ( 1 - a 2) and ( 1 - 32) have been calculated and are included in Table 4. As expected, the degree of covalency in the in-plane V-O ~r-bond decreases and simultaneously that of ~-bonding with vanadyl oxygen increases on substitution of K 2 S O 4 for N a 2 S O 4 in the ternary sulphate glass system. A similar situation has been reported by Paul and Assabghy in Na20:CaO:P205 glasses [ 8] on substitution of CaO for Na20 in the system. The value of h = 170 cm i is within the range of 160-190 cm - j as given by Bogomolova et al. [31] showing that the covalent character of the bonding is weak, which is consistent with the statement [32] that sulphate glasses are typically ionic in nature. It is also confirmed from the molecular orbital data. In materials containing vanadyl, the V 4÷ ion is usually surrounded by a distorted octahedron of oxygen ions. One of these is much closer than the others and this defines the axial direction (parallel-direction) for the vanadyl ion and the remaining oxygens provide the C4 point symmetry of this group. In K 2 S O a - N a 2 S O 4Z n S O 4 glasses, potassium is coordinated with twelve oxygens, sodium is coordinated with eight oxygens and zinc is octahedrally coordinated with six oxygens [ 16]. The zinc site in triple sulphate glasses provides a very suitable surrounding for the VO z ÷ ions. Thus the VO 2÷ substitutes for the zinc site in the host and shows an axial spectrum characteristic of glasses.

5. Conclusions 1. VO 2 ÷ ions in the K 2 S O 4 - N a 2 S O 4 - Z n S O 4 glasses are octahedrally coordinated with a tetragonal compression. 2. The molecular orbital parameters a,2 and 3,2 obtained for different compositions of glasses in the present work indicate that there is a moderate covalency for the in-plane o'-bonding and whereas, the in-plane zr-bonding is significantly ionic. Also, the degree of covalency in the in-plane V-O tr-bond decreases along the simultaneous increase of zr-bonding with

727

the vanadyl oxygen as the concentration of potassium in the glass increases. 3. EPR and optical absorption studies reveal that the vanadyl ions are incorporated substitutionally at the zinc ion sites.

Acknowledgements The authors are thankful to Dr. K.V. Reddy, University of Hyderabad, India, for providing the recording facilities of EPR and optical spectra at CIL. One of the authors (RRK) is indebted to CSIR, New Delhi for SRF.

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[28] D. Suresh Babu, M.V. Ramana, S.G. Satyanarayan and G.S. Sastry, Phys. Chem. Glasses 31 (1990) 80. [29] A. Yadav and V.P. Seth, J. Mater. Sci. 22 (1987) 239. [30] R.H. Borcherts and C. Kikuchi, J. Chem. Phys. 40 (1964) 2270. [311 L.D. Bogomolova, T.F. Dologolenko, V.N. Lazukin, E.N. Nozdrina and N.V. Petrovykh, Soviet Phys. Dokl. 15 ( 1971 ) 238. [32] P.S.L. Narasimham and K.J. Rao, Proc. Ind. Acad. Sci. 87 A (1978) 275.