Tetrahedral site of Fe(III) and Cu(II) in renierite

Transcription

1 Cryst. Res. Technol. 39, No. 3, (2004) / DOI /crat Tetrahedral site of Fe(III) and Cu(II) in renierite R. Rama Subba Reddy 1, Md. Fayazuddin 2, G. Siva Reddy 1, S. Lakshmi Reddy* 3, P. Sambasiva Rao 4, B. J. Reddy 5, and F. Nieto Garcia 5 1 Dept. of Chemistry, S.V.U.P.G.Centre, Kadapa , India 2 Dept. of Physics, S.V. College, Suryapet , India 3 Dept. of Physics, S.V.D.College, Kadapa , India 4 Dept. of Chemistry, Pondicherry University, Pondicherry , India 5 Dept. of Mineralogy and Petrology, University of Granada, Granada 18002, Spain Received 24 January 2003, accepted 3 September 2003 Published online 15 March 2004 Key words renierite, tetrahedral, Cu(II), Fe(III), optical absorption, EPR. PACS x Renierite from Congo is used in the present investigations. It contains only two transition elements iron and copper. EPR results indicate the presence of Fe(III) and Cu(II) with g values 2.03 and 2.40 respectively. Optical absorption spectrum shows several energies in UV-Vis and NIR region. These energies indicate both Fe(III) and Cu(II) in tetrahedral site. IR spectrum confirms the presence of sulphate and hydroxyl groups in the mineral. 1 Introduction A number of copper containing mineral samples have been studies extensively by employing spectroscopic methods such as optical absorption, EPR etc., [1-3]. The Cu(II) ion exhibits a distorted octahedral symmetry in most of the samples. However, it rarely presents in tetrahedral symmetry. Many investigators attempted to elucidate crystal field splittings, site distortions of paramagnetic Fe(III) and Cu(II) ions present simultaneously in the same mineral sample. In the present investigation, optical absorption, EPR and IR studies of renierite mineral are reported. Crystal structure of renierite reveals that it has sphalerite derived structure [4]. Fe atoms are tetrahedrally surrounded by four arsenic atoms. Fe atom is replaced by Zn and Cu atoms. EPMA of the mineral originated from Rajasthan, India contains 11% of copper and 6.5% of FeO by weight [5]. 2 Experimental A brownish black coloured renierite, originated from Kipushi, Congo is used in the present investigations. It contain FeO = 0.78% and Cu = 7.1 wt%. Electron paramagnetic resonance (EPR) studies are carried out both at room temperature (RT) and at liquid nitrogen temperature (LNT) on a Varian E-112 EPR spectrometer operating at X band frequencies (ν = 9.07 GHz) having a 100 khz field modulation and a phase sensitive detection to obtain a first derivative EPR signal. Optical absorption spectrum of the sample in mull form has been recorded at room temperature on a Cary 5E UV-Vis NIR spectrophotometer in the region nm. IR spectrum of the sample is recorded on JASCO FT/IR-5300 spectrophotometer. * Corresponding author: slreddy_in@yahoo.com

2 Cryst. Res. Technol. 39, No. 3 (2004) / Theory Fe(III) has [Ar]d 5 configuration, where Ar stands for closed argon shell. The ground state term symbol is 6 S and the terms include 4 G, 4 P, 4 D, 4 F and a number of doublet states. In both octahedral and tetrahedral crystal fields, 6 S and 4 P transform as 6 A 1 (S) and 4 T 1 respectively, where as 4 G and 4 D split into 4 T 1 (G), 4 T 1 (G), 4 E(G), 4 A(G) and 4 T 2 (D), 4 E(D) respectively. Cu(II) has [Ar]d 9 configuration and the ground state term symbol is 2 D. This term splits into 2 T 2 and 2 E levels. In tetrahedral geometry, only one transition from 2 T 2 2 E is expected. Like other coordination systems, tetrahedral Cu(II) complexes also exhibit distortions. In this field, 2 T 2 splits into 2 B 2 and 2 E and 2 E also splits into 2 B 1 and 2 A 1. Therefore three bands are expected. If the symmetry is further lowered, the degeneracy of 2 E level will also be removed resulting four bands. Fig. 1 EPR spectrum of renerite at RT (υ = 9.07 GHz). 4 Results and discussion 4.1 EPR Studies The room temperature powder EPR spectrum of renierite is given in Fig. 1.The EPR spectrum is highly unsymmetrical in nature, indicating the presence of the overlap of two broad resonant lines. The high field portion has g value 2.03 indicating the presence of Fe(III). The low field feature can be attributed to unresolved Cu(II) in the mineral as the concentration of copper is appreciably high. The calculated g value is around 2.4. When the sample cooled to 77 K, an EPR spectrum similar to RT has been obtained with a slight reduction in line width. Because of large concentration of copper, hyperfine lines could not be resolved due to dipolardipolar broadening. 4.2 Optical absorption studies For easy analysis, optical absorption spectrum of renierite, recorded from nm is divided into two and is shown in Fig. 2(a) and 2(b). The spectrum shows two sets of bands one in small wavelength (UV-Vis) region and another in NIR region. The bands in the UV-Vis region are the characteristic of Fe(III) in tetrahedral site. The relatively intense bands observed at cm -1 (675 nm) and cm -1 (615 nm) are attributed to 6 A 1 (S) 4 T 1 (G) and 6 A 1 (S) 4 T 2 (G) transition respectively. With the help of T-S diagram the other bands observed at cm -1 (540 nm) and cm -1 (469 nm) are attributed to the transitions 6 A 1 (S) 4 E(G) + 4 A 1 (G) and 6 A 1 (S) 4 E(D) respectively. The band observed at cm -1 is independent of the magnitude of Dq and should be sharp. But the spectrum is not showing expected sharpness. This may be due to the presence of Cu(II) ion in the mineral, which abs cures the Fe(III) spectrum [6]. By solving T-S matrices for d 5 with Tree s correction α = 0.90, the following crystal field (Dq) and Racah (B, C) parameters are evaluated

3 242 R. Rama Subba Reddy et al.: Tetrahedral site of Fe(III) and Cu(II) in renierite as Dq = 480 cm -1, B = 520 cm -1 and C = 2500 cm -1. As a comparison, the parameters for other sample are given in Table 1. From the table it is noticed that different energy parameters are in good agreement with the present value. Fig. 2 a) Optical absorption spectrum of renierite; b) NIR spectrum of renierite. Table 1 Comparison for different tetrahedral symmetry for Fe(III) bearing minerals. Sample Dq B C Reference Bornite Renierite [7] Present work Table 2 Band head data with assignments. Observed band positions Calculated wave number (cm -1 ) Wave length (nm) Wave number (cm -1 ) Fe (III) Transition 6 A 1 (S) 4 T 1 (G) 6 A 1 (S) 4 T 2 (G) 6 A 1 (S) 4 E(G) + 4 A 1 (G) Cu(II) 6 A 1 (S) 4 E(D) 2 B 2 2 A 1 2 B 2 2 B 1 2 B 2 2 E Fig. 2(b) shows the bands in the NIR region and are attributed to Cu(II) in tetrahedral environment. The bands observed at 4245, 4295 and 4545 cm -1 are attributed to the transitions 2 B 2 2 E, 2 B 2 2 B 1, and 2 B 2 2 A 1 respectively. Using the following equations -3Ds + 5Dt = 2 B 2 2 E 10Dq = 2 B 2 2 B 10Dq 4Ds 5Dt = 2 B 2 2 A 1

4 Cryst. Res. Technol. 39, No. 3 (2004) / for the above assignments, crystal field parameters (Dq) and tetragonal field parameters (Ds,Dt) are evaluated as Dq = 430 cm -1 Ds = -629 cm -1 and Dt = 472 cm -1. The band headed data and their assignments along with the calculated values are present in Table 2. Fig. 3 IR spectrum of reneirite. 4.3 IR Studies IR spectrum of the renierite is shown in Fig. 3. The spectrum consists of vibrational frequencies of sulphate ion and hydroxide ion. Isolated sulphate ion has fundamental frequencies at 981 cm -1 (υ 1 ), 451 cm -1 (υ 2 ), 1104 cm - 1 (υ 3 ) and 630 cm -1 (υ 4 ) [8]. Among these υ 1 is non-degenerate, υ 2 is doubly degenerate, υ 3 and υ 4 are triply degenerate. Among these, υ 2 only infrared active. In solid sulphates, especially in sulphate minerals, the degeneracies are often removed by the lowering of symmetry imposed by the environment of sulphate ion [9,10]. Three bands observed at 1050, 1110, 980 cm -1 corresponds to υ 3 vibrational mode of sulphate ion. The band observed at 940 cm -1 corresponds to υ 1 vibrational mode. The sharp bands at 590, 620 and the shoulder at 630 cm -1 are attributed to υ 4 mode. The very sharp bands at 450 and 460 cm -1 are assigned to υ 2 mode. Bands around 3400 cm -1 are due to fundamental stretching modes of OH group. Since OH groups appear in different sites in minerals, different bands appear for the same OH stretch. Therefore, the bands observed at 3330, 3430 and 3500 cm -1 are attributed to three OH groups in different sites in the crystal lattice [11]. 5 Conclusion Reneierite is basically a copper mineral contains FeO 9.78 and Cu 7.1 wt% Its EPR spectrum is due to both Fe(II) and Cu(II) in the mineral and their g values are 2.03 and At LNT, EPR spectrum is almost similar to RT EPR spectrum. Optical absorption spectrum is attributed to both the transition metal ions Fe(III) and Cu(II) in tetrahedral coordination. Fe(III) crystal field splitting parameter Dq = 480, Racah parameter B = 520 and C = 2500 cm -1 are noticed. Cu(II) in tetrahedral coordination has given energies in NIR region. Crystal field parameters for Cu(II) are evaluated as Dq = 430, Ds = -629 and Dt = 472 cm -1. The IR spectrum is due to sulphate and hydroxyl groups. Acknowledgements The authors wish to express their sincere thanks to Prof. Dr. Michel Delines, Head, Dept. of Mineralogy, Institut Royal des Sciences, Naturelles, de Belgique, Bruxelles for generously donating the renierite mineral sample. The authors are also grateful to UGC, New Delhi for providing financial assistance to R.R. Reddy and S.L.Reddy.

5 244 R. Rama Subba Reddy et al.: Tetrahedral site of Fe(III) and Cu(II) in renierite One of the authors (B.J.Reddy) is thankful to the Ministry of Education, Culture and Sports, Govt. of Spain for the award of visiting professorship so as to work with Prof. Dr.Fernando Neito, Garcia, Dept. of Mineralogy and Petrology, University of Granada, Granada, Spain. References [1] R. Pappalardo, J. Mol. Spectroscopy 6, 554 (1961). [2] B. J. Hathaway and A. A. G. Tomlinson, Coordination Chem. Rev. 5, 1 (1970). [3] R. Rama Subba Reddy, S. Lakshmi Reddy, G. Siva Reddy, and B. J. Reddy, Cryst. Res. Techol. 37, 484 (2002). [4] R. Bernstein Lawrence, G. Reichel Daniel, and Merlin Stepeho, Am.Min. 74, 1177 (1989). [5] N. N. Mazogona, Yu. Borodaev, A. V. Efimor, S. M. Gandhi, A. Mookerji, Mineral. Petrol. 46, 55 (1992). [6] W. B. White, M. Matsumura, D. G. Linnelan, T. Furukawa, and B. K. Chandra Sekhar, Am. Min. 71, 1415 (1986). [7] M. Venkataramanaiah, R. V. S. S. N. Ravi Kumar, A. V. Chandra Sekhar, B. J. Reddy, Y. P. Reddy, and P. S. Rao, Ferroelectrics 216, 27 (1998). [8] G. Herzberg, Molecular Spectra and Molecular Structure II, Infrared and Raman Spectra of Poly Atomic molecules D.Van Nostrand Co. Inc. New York (1962). [9] H. H. Alder and P. F. Kerr, Am. Miner. 50, 135 (1965). [10] A. Hezel and S. D. Ross, Spectro Chemica Acta 22, 547 (1966). [11] B. Wroewolfeite and S. W. Richard, Mineral Rec.13, 174 (1982).