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Spectroscopic investigations of descloizite
Spectrochimica Acta Part A 72 (2009) 194–197
Contents lists available at ScienceDirect
Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy journal homepage: www.elsevier.com/locate/saa
Spectroscopic investigations of descloizite S. Lakshmi Reddy a , B. Jagannatha Reddy b , R.L. Frost b,∗ a b
Department of Physics, S.V.D. College, Kadapa 516 003, India Inorganic Materials Research Program, Queensland University of Technology, PO Box 2434 GPO, Brisbane, Queensland 4001, Australia
a r t i c l e
i n f o
Article history: Received 18 February 2008 Received in revised form 8 August 2008 Accepted 23 September 2008 Keywords: Descloizite Vanadate Hydroxyl EPR Optical absorption Near infrared Spectroscopy
a b s t r a c t Descloizite mineral originated from Grootfontein, South West Africa, is used in the present work. The chemical analysis of this mineral reveals the presence of V2 O5 = 20.94 wt%, CuO = 0.45 wt%, FeO in traces. An EPR study of sample confirms the presence of VO(II) and Fe(III). Optical absorption spectrum of descloizite indicates that VO(II) is present in distorted octahedral environment. NIR results are due to water fundamentals. © 2008 Elsevier B.V. All rights reserved.
1. Introduction Descloizite is a zinc–copper–lead hydrous vanadite mineral belonging to descloizite group. Descloizite is the name of the descloizite group of minerals. The group is composed of rather rare members of which descloizite is its most common member. Descloizite is an end member of a series composed of the mineral mottramite. Mottramite is the copper rich end member while descloizite is the zinc rich member. Both minerals usually contain significant percentages of both elements and are rarely pure. Descloizite can form nice well shaped crystals that have a nice luster and striking color. The mineral may contain other elements including iron, gallium and germanium. This group of minerals has the formula Pb(Zn, Cu)(VO4 )(OH). Based on composition, this group of minerals is classified as follows [1]: ZnO content greater than 18% ZnO content between 18% and 10% ZnO content between 10% and 2%
Descloizite Cuprian descloizite Zincian mottramite
In the above group of minerals, vanadium can be replaced by Cr, As, Mo, P and zinc by Cu, Fe and Mn [1,2]. The crystal structure of the descloizite is reported [3–5]. The parameters of the orthorhombic cell, with space group Pnca are a = 0.5328, b = 0.5052
∗ Corresponding author. Tel.: +61 7 3138 2407; fax: +61 7 3138 1804. E-mail address: [email protected] (R.L. Frost). 1386-1425/$ – see front matter © 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.saa.2008.09.020
and c = 1.2003 nm. (Zn,Cu) cation is in a distorted tetragonal bipyramid surrounded O and OH ligands, where as VO4 is in tetrahedral. The distribution of transition metal ions in this group is consistent. The chemical analysis of this group of minerals indicates that descloizite consists of only vanadium is major quantity of the transition metal and iron, manganese are in traces [1,2,5]. Detailed electron paramagnetic resonance (EPR) studies and infrared studies on cuprian descloizite from Namibia have been reported but not on descloizite [1,6]. The study of descloizite originating from Grootfontein, South West Africa, is undertaken in the present work. The chemical analysis of this minerals was reported and reveals the presence of V2 O5 = 20.94 wt%, CuO = 0.45 wt%, FeO, MnO in traces [1]. The authors present here a detailed study on this mineral by electron paramagnetic (EPR), optical absorption and infrared (IR) spectroscopic techniques. 2. Experimental Orange blackish brown coloured descloizite sample, originating from Grootfontein, South West Africa was used in the present investigations. The chemical analysis of the mineral gives a formula Pb(Zn,Cu,Mn,Fe)(VO4 )(OH). The mineral may contain adsorbed water but contains no water of crystallization. EPR spectra of the powdered sample are recorded at room temperature (RT) on JEOL JES-TE100 ESR spectrometer operating at X-band frequency ( = 9.39146 GHz) and having a 100 kHz field modulation to obtain a first derivative EPR spectrum. DPPH with a g value of 2.0036 is used for g factor calculations.
S.L. Reddy et al. / Spectrochimica Acta Part A 72 (2009) 194–197
Varian Cary 3 UV-VIS spectrophotometer, equipped with diffuse reflectance accessory (DRA) was employed to record the electronic spectrum of the sample in the region between 200 and 900 nm. The diffuse reflectance measurements were converted into absorption (arbitrary units) using the Kubelka–Munk function (f(R∝) = (1 − R∝)2/2R∝). Near IR spectra were collected on a Nicolet Nexus FT-IR spectrometer with a Nicolet Near-IR Fibreport accessory. A white light source was used, with a quartz beam splitter and TEC NIR InGaAs detector. Spectra were obtained from 11,000 to 4000 cm−1 by the co-addition of 64 scans at a resolution of 8 cm−1 . A mirror velocity of 1.2659 was used. The spectra were transformed using the Kubelka–Munk algorithm to provide spectra for comparison with absorption spectra. Data manipulation was performed using Microsoft Excel. The details of experimental techniques were followed as reported [7]. Band component analysis was undertaken using the Jandel “PEAKFIT” software package which enabled the type of fitting function to be selected and specific parameters to be fixed or varied accordingly. Band fitting was carried out using a Lorentz–Gauss cross-product function with a minimum number of component bands used for the fitting process. The Lorentz–Gauss ratio was maintained at values greater than 0.7 and fitting was undertaken until reproducible results were obtained with squared correlations of r2 greater than 0.9975. 3. Theory Vanadium in its tetravalent state invariably exists as the molecular VO(II) ion (vanadyl). The VO(II) ion has a single d electron which gives rise to the free ion term 2 D. In a crystal field of octahedral symmetry, this electron occupies the lowest t2g orbital and gives rise to ground state term 2 T2g . When the electron absorbs energy, it is excited to the upper eg orbital and gives rise to the 2 Eg term. Accordingly in octahedral symmetry, only one band corresponding to the transition 2 T2g → 2 E is expected. Because of the non-symmetrical alignment of the V O bond along the symmetry axis, the site symmetry in general is lowered to tetragonal (C4V ) or rhombic (C2V ). In C4V site symmetry 2 T2g splits into 2 B2 and 2 E, where as 2 Eg splits into 2 B1 , 2 A1 . Accordingly three bands are expected. The degeneracy of 2 E is also removed in rhombic symmetry resulting four bands. For VO(II) three d–d transition bands are expected in C4v symmetry in the range 11,000–14,000, 14,800–19,000 and 20,000–31,250 cm−1 [8]. The relative positions of the energy levels along with their corresponding expressions are given below: 2
B2 :
−4Dq + 2Ds − Dt
2
E:
−4Dq − Ds + 4Dt
4
A11 = ˛2 + P −
A⊥ = ˛2 + P
7
2 7
195
˛2 + (g11 − ge ) +
˛2 +
11 14
3 7
(g⊥ − ge )
(g⊥ − ge )
Here g11 and g⊥ are the spectroscopic splitting factors parallel and perpendicular to the magnetic field directions. ge = 2.0023 free electron value. A11 and A⊥ are the hyperfine splitting constants parallel and perpendicular to the magnetic field directions. E1 is the energy of 2 B2 → 2 B1 and E2 is the energy of 2 B2 → 2 E; P is dipolar hyperfine coupling constant; K is the Fermi contact term; is the spin–orbit coupling constant for the free vanadium. 4. Results and discussion 4.1. Optical absorption spectral results Optical absorption spectrum of descloizite recorded at room temperature is shown in Fig. 1a and b. The spectrum shows energies at 9803, 11,875, 13,160, 13,515, 14,285, 15,040 and 19,800 cm−1 . The band at 15,040 cm−1 is relatively weak since the low energy side of the 9803 cm−1 band and high-energy side of the 19,800 cm−1 band were almost wholly due to the transition. The occurrence of these bands suggests tetragonal symmetry for vanadyl ion. The general ordering of energy levels is 2 B2 < 2 E < 2 B1 < 2 A2 [10,11]. The transition 2 B2 → 2 E should be most intense d–d transition, where as 2 B → 2 B is the symmetry forbidden transition should be weakest. 2 1 Accordingly the bands at 9803,15,040, 19,800 cm−1 are attributed to the transitions 2 B2 → 2 E, 2 B2 → 2 B1 and 2 B2 → 2 A1 , respectively [8]. Using the formulae [12]: 2
B2 → 2 E = − 3Ds + 5Dt
2
B2 → 2 B1 = 10Dq
2
B2 → 2 A1 = 10Dq − 4Ds − 5Dt
the octahedral Dq and tetragonal (Ds, Dt) crystal field parameters are evaluated from the observed bands positions. The parameters are as follows: Dq = 1504 cm−1 , Ds = −2080 cm−1 and Dt = 713 cm−1 . Whereas the bands at 11,875, 13,160, 13,515, 14,285 cm−1 are attributed to Fe(III). These are assigned to the 6 A1g → 4 T1g (G) transition. 4.2. EPR spectral analysis
2
B1 :
6Dq + 2Ds − Dt
2
A1 :
6Dq − 2Ds − 6Dt
VO(II) ion has S = 1/2 and I = 7/2. In the EPR spectrum it shows hyperfine pattern of eight equidistant lines. In C4v symmetry two sets of eight lines are expected (16-line pattern), where as in C2v symmetry three sets of eight lines are expected. Using optical absorption and EPR data, molecular orbital coefficients can be evaluated by using the following equations [9]: E1 = −
8 g11 − ge
E2 = −
2 g⊥ − ge
The EPR spectrum of polycrystalline sample of descloizite is shown in Fig. 2. From the figure immediately one can observe six lines. If we thought of manganese is present in the mineral, electron paramagnetic resonance will show its presence by a six-line pattern in the EPR spectrum. Fig. 2 shows the EPR spectrum of polycrystalline descloizite mineral observed at room temperature. It is well known that in the transition metal ion EPR studies, Mn(II) is easily observable at room temperature, even if present in minute levels, compared to other ions. Our previous observations indicate that the g and A values for Mn(II) about 2.0 and 9.0 mT. The observed g and A values in the sample are 2.10 and 13.2 mT, respectively. These values in the present case are indicating that which are not due to Mn(II) but due to some other ion present in the mineral. Which is also further confirmed from the chemical analysis of the sample [1]. The mineral descloizite is vanadium mineral we thought that the EPR spectrum is also due to vanadium. If vanadium is present
196
S.L. Reddy et al. / Spectrochimica Acta Part A 72 (2009) 194–197
Fig. 2. EPR spectrum of descloizite at room temperature ( = 9.39146 GHz).
g11 = ge −
g⊥ = ge −
8 11
2 ⊥
The calculated value of = 155 cm−1 which is less than free ion value of 170 cm−1 [13] indicating 9.1% of covalent bonding in the mineral. By assuming the dipolar coupling constant P = −155 × 10−4 cm−1 and Fermi contact term = 125 × 10−4 cm−1 , the calculated value of spin–orbit splitting constant = 155 cm−1 and ˛ = 0.91 instead of 1.0 for purely ionic bonding and using the expressions [9] given in the theory the A⊥ component is evaluated as 5.4 mT. On low field side the resonance marked with g = 6.5 in Fig. 2 in the range 0–400 mT is due to Fe(III) in the mineral. This is further supporting the observed optical absorption studies. 4.3. NIR spectroscopy
Fig. 1. (a) Optical absorption spectrum of descloizite at room temperature in the range 800–450 nm. (b) Optical absorption spectrum of descloizite at room temperature in the range 12200–9200 cm−1 .
in the mineral it exhibits the characteristic eight-line pattern due to the hyperfine coupling of a single unpaired electron with the most abundant isotopic 51 V (spin = 7/2) nucleus. The central eight-line transition of VO(II) may be masked by Fe(III) and the component lines on left side are also superimposed by Fe(III). In other words, the overlap of Fe(III) signal with two lines of parallel component and all features of perpendicular component of VO(II). From the position of peaks and their separations the following spin Hamiltonian parameters are evaluated: g11 = 1.92,
A11 = 132 mT,
g⊥ = 1.97,
A⊥ not resolved
By correlating the optical and EPR results the spin–orbit coupling constant is evaluated using the following formulae [9]:
The NIR spectrum of the sample recorded at room temperature shows several bands. These bands are due to overtones and combination tones of water fundamentals. These are assigned as follows: Water has C2v symmetry and gives three fundamental modes. They are 1 , 2 and 3 . 1 is the symmetric OH stretch, 2 is H–O–H bending mode and 3 is the asymmetric OH stretch. In vapour phase, these modes occur at 3652 (1 ), 1595 (2 ) and 3156 (3 ) cm−1 , whereas in solid they appear at 3220, 1620 and 3400 cm−1 , respectively [14]. The shifting of 1 and 3 towards the low frequency side and 2 toward higher frequency is due to hydrogen bonding [15]. Fig. 3a shows NIR spectrum of descloizite at room temperature in the range 9000–4500 cm−1 . The band observed at 8175 cm−1 is assigned to the combination 1 + 2 + 3 (3220 + 1620 + 3400 = 8240 cm−1 ) of water fundamentals. The OH stretching mode gives rise to the most common features in nearinfrared region. Hydroxyl exists as part of the structure and the stretching mode appears whenever water is present in any form. The OH overtone (2OH ) gives rise to a band in the NIR spectrum [15]. Accordingly the highest intensity bands observed at 7433 cm−1 (3716 × 2 = 2OH ) and 6823 cm−1 (3411 × 2 = 2OH ) are assigned to the first overtone of OH. The band observed at 6050 cm−1 is attributed to water combination mode of the hydroxyl fundamental of water. Very sharp band observed at 5008 cm−1 with a shoulder at 5125 cm−1 is due to (2 + 3 ). Further the sharpness of the bands indicates that water molecules are located in well defined ordered site. [15]. Fig. 3b shows the NIR spectrum of the mineral in the range 4500–4000 cm−1 . The well defined too intensity band at 4154 cm−1 is identified as the fundamental bending mode combined with the lowest frequency of OH stretching fundamental.
S.L. Reddy et al. / Spectrochimica Acta Part A 72 (2009) 194–197
197
Fig. 3. (a) NIR spectrum of descloizite at room temperature in the range 9000–4500 cm−1 . (b) NIR spectrum of descloizite at room temperature in the range 4500–4000 cm−1 .
5. Conclusions 1. The chemical analysis of the sample indicates that vanadium and copper (V2 O5 = 20.94 wt%) being a major constituent of the mineral with copper, iron and manganese as minor constituents. 2. The optical absorption spectrum of mineral descloizite is due to vanadium and iron. The two metal ions are in distorted octahedron environment. 3. Near infrared spectra is due to OH fundamentals and supports the formula of the mineral. 4. The EPR results confirm the presence of vanadium in +4 state with tetragonally distorted environment in the mineral and iron is in +3 state in distorted octahedron. The EPR results and optical absorption spectral results are well agreeing. Acknowledgements The financial and infra-structure support of the Queensland University of Technology, Inorganic Materials Research Program is
gratefully acknowledged. The Australian Research Council (ARC) is thanked for funding the instrumentation. References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15]
H.V.R. Von Rahden, L.W.R. Dicks, Am. Miner. 52 (1967) 1067–1076. A.P. Millman, Am. Miner. 45 (1960) 763–773. M.M. Qurashi, W.H. Basnes, Can. Miner. 8 (1964) 23–39. F.C. Hawthorne, R. Faggiani, Acta Cryst. B 35 (1979) 717–720. J.W. Anthony, R.A. Bideaux, K.W. Bladh, M.C. Nichols, Hand Book of Mineralogy, vol. IV, Mineral Data Publishing, Tucson, AZ, 2000, p. 478. A.B. Vassilikou-Dova, K. Eftarias, J. Phys. Condens. Matter 4 (1992) 241–248. B.J. Reddy, R.L. Frost, W.N. Martens, Miner. Mag. 69 (2005) 155, 343. R.J.H. Clark, The Chemistry of Titanium and Vanadium, Elsevier, Amsterdam, 1962. A.B. Vassilikopu-Dova, G. Lehmann, Phys. Chem. Miner. 15 (1986) 559 (check year). D. Kivelson, Lee Beai-Keuing, J. Chem. Phys. 41 (1964) 1896. C.J. Ballhausen, M.B. Gray, Inorg. Chem. 1 (1962) 111. J.R. Peruma Reddy, J. Phys. Chem. 71 (1967) 3144. A.J. Freeman, R.C. Watson, Magnetism, vol. II, Academic Press, New York, 1965. K. Nakamoto, Infrared Spectra of Inorganic and Coordination Compounds, Wiley, New York, 1970. G.R. Hunt, J.W. Saliesbury, Mod. Geol. 1 (1970) 283.
Contents lists available at ScienceDirect
Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy journal homepage: www.elsevier.com/locate/saa
Spectroscopic investigations of descloizite S. Lakshmi Reddy a , B. Jagannatha Reddy b , R.L. Frost b,∗ a b
Department of Physics, S.V.D. College, Kadapa 516 003, India Inorganic Materials Research Program, Queensland University of Technology, PO Box 2434 GPO, Brisbane, Queensland 4001, Australia
a r t i c l e
i n f o
Article history: Received 18 February 2008 Received in revised form 8 August 2008 Accepted 23 September 2008 Keywords: Descloizite Vanadate Hydroxyl EPR Optical absorption Near infrared Spectroscopy
a b s t r a c t Descloizite mineral originated from Grootfontein, South West Africa, is used in the present work. The chemical analysis of this mineral reveals the presence of V2 O5 = 20.94 wt%, CuO = 0.45 wt%, FeO in traces. An EPR study of sample confirms the presence of VO(II) and Fe(III). Optical absorption spectrum of descloizite indicates that VO(II) is present in distorted octahedral environment. NIR results are due to water fundamentals. © 2008 Elsevier B.V. All rights reserved.
1. Introduction Descloizite is a zinc–copper–lead hydrous vanadite mineral belonging to descloizite group. Descloizite is the name of the descloizite group of minerals. The group is composed of rather rare members of which descloizite is its most common member. Descloizite is an end member of a series composed of the mineral mottramite. Mottramite is the copper rich end member while descloizite is the zinc rich member. Both minerals usually contain significant percentages of both elements and are rarely pure. Descloizite can form nice well shaped crystals that have a nice luster and striking color. The mineral may contain other elements including iron, gallium and germanium. This group of minerals has the formula Pb(Zn, Cu)(VO4 )(OH). Based on composition, this group of minerals is classified as follows [1]: ZnO content greater than 18% ZnO content between 18% and 10% ZnO content between 10% and 2%
Descloizite Cuprian descloizite Zincian mottramite
In the above group of minerals, vanadium can be replaced by Cr, As, Mo, P and zinc by Cu, Fe and Mn [1,2]. The crystal structure of the descloizite is reported [3–5]. The parameters of the orthorhombic cell, with space group Pnca are a = 0.5328, b = 0.5052
∗ Corresponding author. Tel.: +61 7 3138 2407; fax: +61 7 3138 1804. E-mail address: [email protected] (R.L. Frost). 1386-1425/$ – see front matter © 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.saa.2008.09.020
and c = 1.2003 nm. (Zn,Cu) cation is in a distorted tetragonal bipyramid surrounded O and OH ligands, where as VO4 is in tetrahedral. The distribution of transition metal ions in this group is consistent. The chemical analysis of this group of minerals indicates that descloizite consists of only vanadium is major quantity of the transition metal and iron, manganese are in traces [1,2,5]. Detailed electron paramagnetic resonance (EPR) studies and infrared studies on cuprian descloizite from Namibia have been reported but not on descloizite [1,6]. The study of descloizite originating from Grootfontein, South West Africa, is undertaken in the present work. The chemical analysis of this minerals was reported and reveals the presence of V2 O5 = 20.94 wt%, CuO = 0.45 wt%, FeO, MnO in traces [1]. The authors present here a detailed study on this mineral by electron paramagnetic (EPR), optical absorption and infrared (IR) spectroscopic techniques. 2. Experimental Orange blackish brown coloured descloizite sample, originating from Grootfontein, South West Africa was used in the present investigations. The chemical analysis of the mineral gives a formula Pb(Zn,Cu,Mn,Fe)(VO4 )(OH). The mineral may contain adsorbed water but contains no water of crystallization. EPR spectra of the powdered sample are recorded at room temperature (RT) on JEOL JES-TE100 ESR spectrometer operating at X-band frequency ( = 9.39146 GHz) and having a 100 kHz field modulation to obtain a first derivative EPR spectrum. DPPH with a g value of 2.0036 is used for g factor calculations.
S.L. Reddy et al. / Spectrochimica Acta Part A 72 (2009) 194–197
Varian Cary 3 UV-VIS spectrophotometer, equipped with diffuse reflectance accessory (DRA) was employed to record the electronic spectrum of the sample in the region between 200 and 900 nm. The diffuse reflectance measurements were converted into absorption (arbitrary units) using the Kubelka–Munk function (f(R∝) = (1 − R∝)2/2R∝). Near IR spectra were collected on a Nicolet Nexus FT-IR spectrometer with a Nicolet Near-IR Fibreport accessory. A white light source was used, with a quartz beam splitter and TEC NIR InGaAs detector. Spectra were obtained from 11,000 to 4000 cm−1 by the co-addition of 64 scans at a resolution of 8 cm−1 . A mirror velocity of 1.2659 was used. The spectra were transformed using the Kubelka–Munk algorithm to provide spectra for comparison with absorption spectra. Data manipulation was performed using Microsoft Excel. The details of experimental techniques were followed as reported [7]. Band component analysis was undertaken using the Jandel “PEAKFIT” software package which enabled the type of fitting function to be selected and specific parameters to be fixed or varied accordingly. Band fitting was carried out using a Lorentz–Gauss cross-product function with a minimum number of component bands used for the fitting process. The Lorentz–Gauss ratio was maintained at values greater than 0.7 and fitting was undertaken until reproducible results were obtained with squared correlations of r2 greater than 0.9975. 3. Theory Vanadium in its tetravalent state invariably exists as the molecular VO(II) ion (vanadyl). The VO(II) ion has a single d electron which gives rise to the free ion term 2 D. In a crystal field of octahedral symmetry, this electron occupies the lowest t2g orbital and gives rise to ground state term 2 T2g . When the electron absorbs energy, it is excited to the upper eg orbital and gives rise to the 2 Eg term. Accordingly in octahedral symmetry, only one band corresponding to the transition 2 T2g → 2 E is expected. Because of the non-symmetrical alignment of the V O bond along the symmetry axis, the site symmetry in general is lowered to tetragonal (C4V ) or rhombic (C2V ). In C4V site symmetry 2 T2g splits into 2 B2 and 2 E, where as 2 Eg splits into 2 B1 , 2 A1 . Accordingly three bands are expected. The degeneracy of 2 E is also removed in rhombic symmetry resulting four bands. For VO(II) three d–d transition bands are expected in C4v symmetry in the range 11,000–14,000, 14,800–19,000 and 20,000–31,250 cm−1 [8]. The relative positions of the energy levels along with their corresponding expressions are given below: 2
B2 :
−4Dq + 2Ds − Dt
2
E:
−4Dq − Ds + 4Dt
4
A11 = ˛2 + P −
A⊥ = ˛2 + P
7
2 7
195
˛2 + (g11 − ge ) +
˛2 +
11 14
3 7
(g⊥ − ge )
(g⊥ − ge )
Here g11 and g⊥ are the spectroscopic splitting factors parallel and perpendicular to the magnetic field directions. ge = 2.0023 free electron value. A11 and A⊥ are the hyperfine splitting constants parallel and perpendicular to the magnetic field directions. E1 is the energy of 2 B2 → 2 B1 and E2 is the energy of 2 B2 → 2 E; P is dipolar hyperfine coupling constant; K is the Fermi contact term; is the spin–orbit coupling constant for the free vanadium. 4. Results and discussion 4.1. Optical absorption spectral results Optical absorption spectrum of descloizite recorded at room temperature is shown in Fig. 1a and b. The spectrum shows energies at 9803, 11,875, 13,160, 13,515, 14,285, 15,040 and 19,800 cm−1 . The band at 15,040 cm−1 is relatively weak since the low energy side of the 9803 cm−1 band and high-energy side of the 19,800 cm−1 band were almost wholly due to the transition. The occurrence of these bands suggests tetragonal symmetry for vanadyl ion. The general ordering of energy levels is 2 B2 < 2 E < 2 B1 < 2 A2 [10,11]. The transition 2 B2 → 2 E should be most intense d–d transition, where as 2 B → 2 B is the symmetry forbidden transition should be weakest. 2 1 Accordingly the bands at 9803,15,040, 19,800 cm−1 are attributed to the transitions 2 B2 → 2 E, 2 B2 → 2 B1 and 2 B2 → 2 A1 , respectively [8]. Using the formulae [12]: 2
B2 → 2 E = − 3Ds + 5Dt
2
B2 → 2 B1 = 10Dq
2
B2 → 2 A1 = 10Dq − 4Ds − 5Dt
the octahedral Dq and tetragonal (Ds, Dt) crystal field parameters are evaluated from the observed bands positions. The parameters are as follows: Dq = 1504 cm−1 , Ds = −2080 cm−1 and Dt = 713 cm−1 . Whereas the bands at 11,875, 13,160, 13,515, 14,285 cm−1 are attributed to Fe(III). These are assigned to the 6 A1g → 4 T1g (G) transition. 4.2. EPR spectral analysis
2
B1 :
6Dq + 2Ds − Dt
2
A1 :
6Dq − 2Ds − 6Dt
VO(II) ion has S = 1/2 and I = 7/2. In the EPR spectrum it shows hyperfine pattern of eight equidistant lines. In C4v symmetry two sets of eight lines are expected (16-line pattern), where as in C2v symmetry three sets of eight lines are expected. Using optical absorption and EPR data, molecular orbital coefficients can be evaluated by using the following equations [9]: E1 = −
8 g11 − ge
E2 = −
2 g⊥ − ge
The EPR spectrum of polycrystalline sample of descloizite is shown in Fig. 2. From the figure immediately one can observe six lines. If we thought of manganese is present in the mineral, electron paramagnetic resonance will show its presence by a six-line pattern in the EPR spectrum. Fig. 2 shows the EPR spectrum of polycrystalline descloizite mineral observed at room temperature. It is well known that in the transition metal ion EPR studies, Mn(II) is easily observable at room temperature, even if present in minute levels, compared to other ions. Our previous observations indicate that the g and A values for Mn(II) about 2.0 and 9.0 mT. The observed g and A values in the sample are 2.10 and 13.2 mT, respectively. These values in the present case are indicating that which are not due to Mn(II) but due to some other ion present in the mineral. Which is also further confirmed from the chemical analysis of the sample [1]. The mineral descloizite is vanadium mineral we thought that the EPR spectrum is also due to vanadium. If vanadium is present
196
S.L. Reddy et al. / Spectrochimica Acta Part A 72 (2009) 194–197
Fig. 2. EPR spectrum of descloizite at room temperature ( = 9.39146 GHz).
g11 = ge −
g⊥ = ge −
8 11
2 ⊥
The calculated value of = 155 cm−1 which is less than free ion value of 170 cm−1 [13] indicating 9.1% of covalent bonding in the mineral. By assuming the dipolar coupling constant P = −155 × 10−4 cm−1 and Fermi contact term = 125 × 10−4 cm−1 , the calculated value of spin–orbit splitting constant = 155 cm−1 and ˛ = 0.91 instead of 1.0 for purely ionic bonding and using the expressions [9] given in the theory the A⊥ component is evaluated as 5.4 mT. On low field side the resonance marked with g = 6.5 in Fig. 2 in the range 0–400 mT is due to Fe(III) in the mineral. This is further supporting the observed optical absorption studies. 4.3. NIR spectroscopy
Fig. 1. (a) Optical absorption spectrum of descloizite at room temperature in the range 800–450 nm. (b) Optical absorption spectrum of descloizite at room temperature in the range 12200–9200 cm−1 .
in the mineral it exhibits the characteristic eight-line pattern due to the hyperfine coupling of a single unpaired electron with the most abundant isotopic 51 V (spin = 7/2) nucleus. The central eight-line transition of VO(II) may be masked by Fe(III) and the component lines on left side are also superimposed by Fe(III). In other words, the overlap of Fe(III) signal with two lines of parallel component and all features of perpendicular component of VO(II). From the position of peaks and their separations the following spin Hamiltonian parameters are evaluated: g11 = 1.92,
A11 = 132 mT,
g⊥ = 1.97,
A⊥ not resolved
By correlating the optical and EPR results the spin–orbit coupling constant is evaluated using the following formulae [9]:
The NIR spectrum of the sample recorded at room temperature shows several bands. These bands are due to overtones and combination tones of water fundamentals. These are assigned as follows: Water has C2v symmetry and gives three fundamental modes. They are 1 , 2 and 3 . 1 is the symmetric OH stretch, 2 is H–O–H bending mode and 3 is the asymmetric OH stretch. In vapour phase, these modes occur at 3652 (1 ), 1595 (2 ) and 3156 (3 ) cm−1 , whereas in solid they appear at 3220, 1620 and 3400 cm−1 , respectively [14]. The shifting of 1 and 3 towards the low frequency side and 2 toward higher frequency is due to hydrogen bonding [15]. Fig. 3a shows NIR spectrum of descloizite at room temperature in the range 9000–4500 cm−1 . The band observed at 8175 cm−1 is assigned to the combination 1 + 2 + 3 (3220 + 1620 + 3400 = 8240 cm−1 ) of water fundamentals. The OH stretching mode gives rise to the most common features in nearinfrared region. Hydroxyl exists as part of the structure and the stretching mode appears whenever water is present in any form. The OH overtone (2OH ) gives rise to a band in the NIR spectrum [15]. Accordingly the highest intensity bands observed at 7433 cm−1 (3716 × 2 = 2OH ) and 6823 cm−1 (3411 × 2 = 2OH ) are assigned to the first overtone of OH. The band observed at 6050 cm−1 is attributed to water combination mode of the hydroxyl fundamental of water. Very sharp band observed at 5008 cm−1 with a shoulder at 5125 cm−1 is due to (2 + 3 ). Further the sharpness of the bands indicates that water molecules are located in well defined ordered site. [15]. Fig. 3b shows the NIR spectrum of the mineral in the range 4500–4000 cm−1 . The well defined too intensity band at 4154 cm−1 is identified as the fundamental bending mode combined with the lowest frequency of OH stretching fundamental.
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Fig. 3. (a) NIR spectrum of descloizite at room temperature in the range 9000–4500 cm−1 . (b) NIR spectrum of descloizite at room temperature in the range 4500–4000 cm−1 .
5. Conclusions 1. The chemical analysis of the sample indicates that vanadium and copper (V2 O5 = 20.94 wt%) being a major constituent of the mineral with copper, iron and manganese as minor constituents. 2. The optical absorption spectrum of mineral descloizite is due to vanadium and iron. The two metal ions are in distorted octahedron environment. 3. Near infrared spectra is due to OH fundamentals and supports the formula of the mineral. 4. The EPR results confirm the presence of vanadium in +4 state with tetragonally distorted environment in the mineral and iron is in +3 state in distorted octahedron. The EPR results and optical absorption spectral results are well agreeing. Acknowledgements The financial and infra-structure support of the Queensland University of Technology, Inorganic Materials Research Program is
gratefully acknowledged. The Australian Research Council (ARC) is thanked for funding the instrumentation. References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15]
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