Journal of Crystal Growth 404 (2014) 20–25
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Growth and spectroscopic properties of samarium oxalate single crystals G. Vimal a, Kamal P. Mani a, Gijo Jose b, P.R. Biju a, Cyriac Joseph a,n, N.V. Unnikrishnan a, M.A. Ittyachen a a b
School of Pure and Applied Physics, Mahatma Gandhi University, Kottayam 686560, India Department of Physics, S.B. College, Changanassery 686101, India
art ic l e i nf o
a b s t r a c t
Article history: Received 29 March 2014 Received in revised form 22 May 2014 Accepted 20 June 2014 Communicated by: Uda Available online 30 June 2014
Single crystals of samarium oxalate decahydrate were synthesized using single diffusion gel technique and the conditions inﬂuencing the size, shape and quality were optimized. Highly transparent crystals of size 3 2 1 mm3 with a well deﬁned hexagonal morphology were grown during a time period of two weeks. X ray powder diffraction analysis revealed that the grown crystals crystallize in the monoclinic system with space group P21/c and the proposed chemical formula and linkage of water molecules were conﬁrmed using thermogravimetric analysis. The various functional groups of the oxalate ligand and the water of crystallization were identiﬁed by Fourier transform infrared spectroscopy. Spectroscopic investigations such as electric dipole transition probability, magnetic dipole transition probability and branching ratios of all possible transitions from 4G5/2 level of Sm3 þ ions were estimated from the absorption spectra using JO theory. The spectroscopic analysis suggested that the crystal has a strong and efﬁcient orange red emission. This is conﬁrmed from the photoluminescence spectrum with a wavelength peak at 595 nm and hence this promising emission can be effectively used for optical ampliﬁcation. & 2014 Elsevier B.V. All rights reserved.
Keywords: A1. Characterization A2. Single crystal growth B1. Rare earth compounds B2. Phosphors
1. Introduction Extensive researches have been carried out over the decades on the growth of defect free single crystals as the subject is indispensable for fundamental research and holds a vital role in the areas of optoelectronics, solid state lasers, remote sensing and medical diagnostics etc. [1–3]. Rare earth based crystals gained wide recognition in recent years due to their excellent physical and chemical properties which led to many potential applications. Research on rare earth based single crystals has intensiﬁed owing to their high radiative transition rates which increases the quantum efﬁciency. Rare earth oxalates are of special importance among the rare earth compounds because of their broad applicability in the ﬁelds of luminescent and magnetic materials [4,5]. Moreover, due to the interesting electrical properties of metal oxalates, active investigations are ongoing in view of their use as a promising material for anode in lithium ion batteries . Oxalates are also utilized as a precursor for many of the technologically important ferroelectric, magnetic and superconducting materials such as BaTiO3, PbTiO3, Ni–Co–Zn ferrites and YBa2Cu3O7 x etc. [7–10]. In recent years single crystals of mixed rare earth oxalates such as neodymium
Corresponding author: Tel.: þ 91 4812731043. E-mail address: [email protected]
http://dx.doi.org/10.1016/j.jcrysgro.2014.06.041 0022-0248/& 2014 Elsevier B.V. All rights reserved.
praseodymium oxalate, dysprosium gadolinium oxalate and dysprosium praseodymium oxalate have been grown and studied [11–13]. Samarium based compounds in different forms such as single crystals, nanocrystals, thin ﬁlms and glasses were synthesized and reported in the previous years on account of their potential applications due to the relatively high quantum efﬁciency of the 4G5/26 H7/2 transition of Sm3 þ ion. [14–17]. Hence the synthesis and detailed spectroscopic investigations of samarium oxalate single crystals deserve special attention and importance. Here we describe the growth, structural and spectroscopic investigations of samarium oxalate decahydrate single crystals. Gel method is the only viable method to grow these crystals since oxalates are sparingly soluble in water and decompose before melting, which impose constraints on the use of other conventional techniques. The structural characterizations of the grown samarium oxalate crystals were done with X-ray powder diffraction (XRD) analysis and thermogravimetric analysis (TGA). Different functional groups associated with the crystals were identiﬁed by Fourier transform infrared spectroscopy (FTIR). Judd–Ofelt theory was applied to analyze the different f–f transitions of Sm3 þ ions in the crystal in order to evaluate the JO intensity parameters and the radiative properties of the excited states. Emissions from the 4G5/2 level of Sm3 þ ions in the crystal to different 6HJ levels were also studied by using the photoluminescence spectrum.
G. Vimal et al. / Journal of Crystal Growth 404 (2014) 20–25
2. Experimental methods 2.1. Crystal growth Samarium nitrate hexahydrate (Sm(NO3)3d6H2O, 99.9%, CDH), oxalic acid dehydrate (H2C2O4d2H2O, 99%, Merck) and sodium silicate (meta) nonahydrate (Na2SiO3d9H2O, CDH) were used as the starting materials. Single crystals of samarium oxalate were grown by single diffusion gel technique and hydrosilica gel was used as the medium of crystal growth. Gel of pre-determined speciﬁc gravity was prepared by dissolving required amount of sodium meta silicate in double distilled water and then mixed with aqueous solution of oxalic acid in the proper ratio to obtain a solution of desired pH. The resulting solution was transferred into test tubes of internal diameter 15 mm and length 150 mm and kept undisturbed for gelation. Aqueous solution of samarium nitrate acidiﬁed with nitric acid was poured gently over the set gel. The samarium ions slowly diffuses through the narrow pores of the hydrosilica gel and reacts with the oxalate ions present in the gel leading to the formation of samarium oxalate single crystals. Optically transparent, yellowish single crystals of average size 3 2 1 mm3 with well deﬁned hexagonal morphology were grown during a time period of two weeks and are shown in Fig. 1 It was found that the morphology, quality, size and the number of crystals vary with gel density, pH of the gel, concentration of the reactants and acidity of the feed solution. The optimum conditions for getting good quality crystals were Speciﬁc gravity of SMS solution pH of the solution Concentration of oxalic acid solution Concentration of the samarium nitrate Acidity of the feed solution
– – – – –
1.03 gm/cm3 6 1M 0.5 M 50%
The proposed chemical reaction is 2Sm(NO3)3 þ 3H2C2O4-Sm2(C2O4)3dnH2O þ6HNO3 The structure of the grown crystal was identiﬁed by X-ray powder diffraction analysis in the 2θ range 10–401 using PANalytical X'pert Pro X–ray diffractometer with Cu-Kα radiation operating at 30 mA, 40 kV. The thermal decomposition stages of the grown crystal were analyzed using Shimadzu thermal analyzer DT–40. FTIR absorption spectrum of the grown crystal was recorded using Shimadzu 8400S FTIR spectrometer in the range 400–4000 cm 1. The absorption and emission spectra of the crystals were recorded using Varian Cary 5000 UV–vis–NIR spectrophotometer in the wavelength range 250– 1700 nm and Shimadzu RFPC 5301 spectroﬂurophotometer in the range 525–725 nm, respectively.
hua hang et al.  using single crystal x-ray diffraction studies. According to them the structure is composed of two dimensional networks of edge sharing 1:5:3 coordination polyhedra matching the (020) set of planes. Each lanthanum atom is being surrounded by three chelating oxalate groups and three aqua ligands. The intervening space is ﬁlled by lattice water molecules disordered over seven major sites. Ollendorff et al. have found that neodymium oxalate decahydrate and praseodymium oxalate decahydrate also crystallizes in the monoclinic system with space group P21/c and the cell parameters are identical with that of La2(C2O4)3d10H2O . Since the ionic radii of La, Nd, Pr and Sm are comparable, one can expect the identical crystal structure of the samarium oxalate crystals. 3.2. Thermal analysis Thermal decomposition stages of samarium oxalate single crystals were investigated by thermogravimetric analysis and the thermogram is shown in Fig. 3. The thermal decomposition process involves stepwise dehydration followed by the conversion of the anhydrous oxalate to the sesquioxide via dioxycarbonate intermediate, which is in conﬁrmation with the results obtained for Gibson and Gallagher [20,21]. The calculated and experimental weight losses of each decomposition stage are presented in Table 1. It is clear from the table that the calculated weight loss of each stage of decomposition as per the proposed formula is in close agreement with the experimental weight loss. According to the analysis, between the temperature range 30–190 1C seven water molecules were eliminated from the crystal. The rest of the water molecules were evaporated in the second stage between 190 1C and 280 1C leading to the formation of the anhydrous samarium oxalate. After the dehydration steps, the anhydrous oxalates were decomposed to an intermediate stage of dioxycarbonate by liberating two CO2 and three CO molecules between 390–500 1C. This unstable intermediate immediately reduced to samarium oxide by the release of a CO2 molecule at around 570 1C. The decomposition stages of the samarium oxalate crystal are in good agreement with that reported for other rare earth oxalates. Four stage decomposition involved in the thermal analysis ascertained the chemical formula and hydration number of the grown crystals. Two distinct stages in the dehydration process in steps of seven and three water molecules are in good agreement with the structure proposed on the basis of X-ray diffraction studies. The seven water molecules distributed randomly in the intervening lattice spaces are loosely bounded and leave the crystal earlier while the other three water molecules coordinated to the crystal lattice take longer time to leave the crystal. Based on the above analysis the following mechanism is proposed for the decomposition of samarium oxalate single crystal. Sm2(C2O4)3d10H2O-Sm2(C2O4)3d3H2O þ7H2O
3. Results and discussion
3.1. X-ray diffraction analysis
Sm2(C2O4)3-Sm2O2CO3 þ2CO2 þ 3CO
The crystal structure of the samarium oxalate single crystals was studied by powder X-ray diffraction and the diffractogram was depicted as Fig. 2. along with the ICDD data ﬁle for samarium oxalate decahydrate (Card no.201021). The position and intensities of the all diffraction peaks matches well with the ICDD data and hence can be indexed to the monoclinic structure of Sm2(C2O4)3 10H2O with cell dimensions a ¼11.10 Ǻ, b¼ 9.621 Ǻ and c¼10.155 Ǻ and space group P21/c. The lattice parameters of Sm2 (C2O4)3 10H2O are comparable with that reported in the structural studies of La2 (C2O4)3d10H2O by Sheng
Sm2O2CO3-Sm2O3 þCO2 3.3. Fourier transform infrared spectroscopy Fourier transform infrared spectrum of the samarium oxalate single crystal in the range 400–4000 cm 1 is shown in Fig. 4. The spectrum gives information about the different functional groups associated with the crystal. Presence of an intense broad band around 3360 cm 1 is due to the symmetric and asymmetric stretching of OH group which conﬁrms the water of crystallization.
G. Vimal et al. / Journal of Crystal Growth 404 (2014) 20–25
Fig. 1. (a), (b) Samarium oxalate crystals growing in the gel system; and (c) typical hexagonal shaped single crystals of samarium oxalate.
Fig. 2. X-ray diffractogram of the Samarium oxalate single crystals. Fig. 3. TGA thermogram of the Samarium oxalate single crystals.
Strong absorption occurred at 1628 cm 1 is attributed to the combined effect of asymmetric stretching of CQO and the bending of water molecules. Symmetric stretching of CO2 is evidenced by the strong IR absorption at 1316 cm 1 [νs (CO) þ δ (O–CQO)]. The presence of metal–oxygen bond [ν(MO)] was evidenced by the strong absorption around 807 cm 1. Absorption at 490 cm 1 is related to the ring deformation and bending of δ (O–CQO) [22,23]. Hence the results of the analysis conﬁrm the
presence of the functional groups and the metal oxygen bond associated with the crystal. 3.4. Spectroscopic analysis 3.4.1. Absorption spectra and Judd–Ofelt analysis Spectroscopic studies were done with the help of optical absorption spectra by using Judd–Ofelt theory, which is a powerful
G. Vimal et al. / Journal of Crystal Growth 404 (2014) 20–25
Table 1 Thermo analytical data of samarium oxalate single crystal. Stage Decomposition Temperature Range (1C)
Loss of material
1 2 3 4
7H2O 16.8 3H2O 7.5 2CO2 þ 3CO 23.8 CO2 5.3
50–190 190–280 280–500 500–650
Observed mass Calculated loss (%) mass loss (%) 16.7 7.3 23.2 5.9
Fig. 4. FTIR spectrum of the Samarium oxalate single crystals.
tool for calculating radiative transition rates between the energy levels of Sm3 þ ion [24,25]. Absorption transitions of samarium oxalate from the ground state 6H5/2 of Sm3 þ ions to various excited states are shown in Fig. 5. The absorption bands observed at energy levels 31,483, 30,049, 28,980, 27,553, 26,653, 25,578, 24,816, 23,950, 22,640, 21,525, 20,924, 10,556, 9176, 8008, 7094, 6646 and 6338 cm 1 are assigned to 6H5/2-4P3/2, 4G9/2, 4H9/2, 4D3/2, 6P7/2, 4L15/2, 6P5/2, 4P5/ 4 G9/2, 4M15/2, 4I11/2, 6F11/2, 6F9/2, 6F7/2, 6F5/2, 4H15/2 and 6F1/2 2, transitions, respectively. The intensities of the different absorption bands can be expressed in terms of experimental oscillator strength using the relation. Z f exp ¼ 4:32 10 9 εðνÞdν ð1Þ where ‘ε’ denotes the molar extinction coefﬁcient and ‘ν’ denotes the energy in wave number (cm 1). The values of the experimental oscillator strength were used to ﬁnd the calculated oscillator strength and the J–O intensity parameters by the least square ﬁt method. According to J–O theory the oscillator strength of the absorption transition from the ground state to an excited state is given by f ed ¼
8π 2 mcν ðn2 þ 2Þ2 0 ∑ Ω ðΨ JjjU λ jjΨ J 0 Þ2 3hð2J þ1Þ 9n λ ¼ 2;4;6 λ
where m is the mass of the electron, ν is the energy of the transition in cm 1, J is the total angular momentum of the ground state, n is the refractive Dindex of theE2material, Ωλ are the JO 0 intensity parameters and Ψ JjjU λ jjΨ J 0 are the squared doubly reduced matrix elements of unit tensor operators of the rank λ ¼2, 4 and 6, which are calculated from the intermediate coupling 0 approximation of a transition Ψ J-Ψ J 0 . The intensity parameters were determined by a least square ﬁtting approach for
Fig. 5. Optical absorption spectra of Samarium Oxalate single crystals in (a) UV–vis region; (b) NIR region.
Eq. (2) which gives best ﬁt between experimental and calculated oscillator strengths. The energy of the different transitions and their experimental and calculated oscillator strengths were given in Table 2. The quality of the ﬁt can be expressed by the magnitude of the root mean square (RMS) deviation. The low RMS deviation (0.356) indicates the good agreement between the experimentally observed and theoretically predicted values of oscillator strengths and hence the validity of JO intensity parameters. The parameters were found to be Ω2 ¼ 1.17 10 20, Ω4 ¼1.93 10 20 and Ω6 ¼ 1.25 10 20. JO intensity parameter provides the information related to the structural change, nature of the bond between Sm3 þ ion and surrounding ligand and also the symmetry of the environment around the Sm3 þ ion. The transition 6H5/2-6F1/2 is a hypersensitive one obeying the selection rules jΔSj ¼ 0, jΔLjZ 0, jΔJj Z 0 and very sensitive to the environment around the Sm3 þ ion. The value of Ω2 is very much depends on the oscillator strength of this hypersensitive transition and also connected to the asymmetry of the local environment. The parameters Ω4 and Ω6 depend on the bulk properties of the material [27,28]. The lower value of Ω2 in the present system indicates the more symmetric structure of the environment and ionic nature of the bonding between Sm3 þ and
G. Vimal et al. / Journal of Crystal Growth 404 (2014) 20–25
Table 2 Experimental and calculated oscillator strength of samarium oxalate single crystals. Transition from 6H5/2
Energy (cm 1)
fexp ( 10 6)
fcal ( 10 6)
6338 7094 8008 9176 10,556 21,525 22,640 23,950 24,816 25,578 26,653 27,553 28,980 30,049 31,483
0.4270 1.5000 1.6800 1.9000 0.4520 0.6770 0.5960 0.7720 2.0200 0.3090 0.7740 0.7420 0.4740 0.1350 0.2620
0.4579 1.3278 2.0816 1.3632 0.2203 0.2351 0.0400 0.3877 2.5722 0.0611 0.8232 0.4267 0.0116 0.0333 0.2635
F1/2 F5/2 6 F7/2 6 F9/2 6 F11/2 4 M15/2 4 G9/2 4 P5/2 6 P5/2 4 L15/2 6 P7/2 4 D3/2 4 H9/2 4 G9/2 4 P3/2 6
the surrounding ligands. The spectroscopic quality factor χ ¼ Ω4 =Ω6 is used to characterize the stimulated emission in a host matrix and is found to be 1.55, which ensures the efﬁcient stimulated emission. 3.4.2. Fluorescence analysis and radiative properties The prominent excitation at 401 nm corresponding to the transition 6H5/2-6P5/2 has been used to record the ﬂuorescence spectrum. On excitation, the 6P5/2 level of Sm3 þ ion becomes populated and then electrons nonradiatively relaxed to the metastable state 4G5/2 level by multiphonon relaxation. The various peaks obtained in the ﬂuorescence spectrum of the single crystals of samarium oxalate are due to the radiative transition from the excited level 4G5/2 to lower ground levels 6H5/2, 6H7/2, 6H9/2 and 6 H11/2 of Sm3 þ ions. Fig. 6 shows the four dominant emission transitions of the samarium oxalate single crystals and the emission from 4G5/2 to 6H7/2 occurred at 595 nm has maximum intensity. ' The radiative transition probability for a transition Ψ J-Ψ J ' is the sum of the electric and magnetic dipole transition probabilities. 0
AðΨ J; Ψ J 0 Þ ¼ Aed þ Amd
64π 4 υ3 ½n3 Smd 3hc3 ð2J þ 1Þ
The total radiation transition probability (AT) for an excited 0 state is the sum of the AðΨ J; Ψ J 0 Þ terms calculated over all the terminal states. 0
AT ðΨ JÞ ¼ ∑ AðΨ J; Ψ J 0 Þ Ψ 0 J″
The relative amplitude of the ﬂuorescence transitions or ﬂuorescence branching ratio corresponding to the transition from 0 an excited state Ψ J 0 to its lower state Ψ J is given by
βR ðΨ J; Ψ 0 J 0 Þ ¼
Table 3 Emission band position, electric and magnetic dipole line strength, transition probability and branching ratios for the 4G5/2 transition level of Sm3 þ ion. Transition from 4G5/2
Energy (cm 1)
17,724 16,647 15,396 14,025 12,578 11,234 11,091 10,988 10,481 9582 8355 6908
0.14 2.77 2.86 1.28 0.25 0.12 0.02 0.14 1.17 0.36 0.29 0.08
0.58 0.58 0 0 0 0 0 0.85 0.66 0.27 0 0
5.85 93.20 76.08 28.78 3.69 1.23 0.21 1.38 9.83 2.34 1.21 0.19
24.71 20.48 0 0 0 0 0 8.57 5.82 1.79 0 0
30.56 113.68 76.08 25.78 3.69 1.23 0.21 9.96 15.65 4.13 1.21 0.19
0.1082 0.4026 0.2694 0.0913 0.0131 0.0043 0.0008 0.0353 0.0554 0.0146 0.0043 0.0007
H5/2 H7/2 H9/2 6 H11/2 6 H13/2 6 F1/2 6 H15/2 6 F3/2 6 F5/2 6 F7/2 6 F9/2 6 F11/2 6 6
where Aed and Amd are the electric and magnetic radiative transition probabilities respectively and are given by # " 64π 4 υ3 nðn2 þ 2Þ2 S Aed ¼ ð4Þ ed 9 3hc3 ð2J þ 1Þ Amd ¼
Fig. 6. Emission spectrum of the Samarium oxalate single crystals.
AðΨ J; Ψ J 0 Þ AT ðΨ JÞ
The excited 4G5/2 level possesses purely radiative relaxation as this level has sufﬁcient energy gap of 7250 cm 1 with respect to next lower level 6F11/2. The relaxation of an excited state to all its lower levels depends on radiative transition probabilities. The value of electric radiative transition probability depends on
Ωλ and energy gap between a starting and terminal level. While, the value of magnetic radiative transition probability depends on the host independent matrix element jjL þ2Sjj2 . The radiative properties such as electric dipole line strength (Sed), magnetic dipole line strength (Smd), electric and magnetic radiation transition probabilities (Aed ,Amd ), total transition probability (AT) and branching ratio (βR ) corresponding to different emissions from 4 G5/2 level have been calculated using Eqs. (3–7) and are presented in Table 3. The experimental branching ratio (β R ) is obtained by integrating the area under the peaks corresponding to different emission transitions in the luminescence spectrum. Comparison between the predicted branching ratio values from JO theory (βR ) with the experimental branching ratio is given in Table 4. The magnitude of branching ratios describes the lasing power of an emission transition and it is well established that transition with the measured branching ratio greater than 0.50 is more potential for laser emission . From Table 4. it is clear that the 4G5/2-6H7/2 transition of samarium oxalate single crystal has branching ratio close to 0.50 which is desirable for optical ampliﬁcation, optical storage devices, color displays and medical diagnostics. The optical characteristics of samarium oxalate single crystals evaluated based on JO theory is in close agreement with that of nanostructured counterpart . But it is found that the luminescence branching ratio is slightly lower for the samarium oxalate
G. Vimal et al. / Journal of Crystal Growth 404 (2014) 20–25
Table 4 Calculated and experimental branching ratios for the transitions from 4G5/2 level of Sm3 þ ion in samarium oxalate single crystals. Transition from 4G5/2
0.10 0.40 0.27 0.09
0.21 0.49 0.17 0.12
H5/2 H7/2 H9/2 6 H11/2 6 6
Acknowledgments The authors are thankful to UGC (Govt. of India) and DST (Govt. of India) for the ﬁnancial assistance through SAP-DRS (No. F.530/12/DRS/2009 (SAP-1)) and DST-PURSE (SR/S9/Z-23/2010/22 (C,G)) programs, respectively. One of the authors Vimal G is thankful to University Grants Commission, Govt. of India for the award of RFSMS fellowship and Kamal P Mani also expresses his thanks to UGC for the sanction of RGNF fellowship. References
single crystals. This could be attributed to the enhanced transition probability ensuing from the particle size reduction and quantum conﬁnement effect in the nanostructured crystals .
4. Conclusions Single crystals of the samarium oxalate decahydrate were successfully grown by single diffusion gel technique. Well faceted transparent yellowish crystals with an average size of 3 2 1 mm3 were obtained during a period of two weeks at room temperature. The grown samarium oxalates were crystallized in the monoclinic structure with space group P21/c. The proposed chemical formula and structure were further conﬁrmed by four stage thermal decomposition stages of the crystal. Vibrational modes of the water molecules and the various functional groups associated with the oxalate ligand were identiﬁed by Fourier transform infrared spectroscopy. Oscillator strength of various excited state transitions of Sm3 þ and Judd–Ofelt intensity parameters were evaluated with the help of optical absorption spectra. Relatively low value of Ω2 indicates the symmetric nature of the local environment of Sm3 þ ion and the ionic nature of the Sm– ligand bond. The intensity parameters were used to predict the electric and magnetic dipole transition probabilities and branching ratios of all possible transitions from 4G5/2 level of Sm3 þ ion. The transition from 4G5/2 to 6H7/2 of Sm3 þ ions has maximum transition probability and branching ratio, which was obviously conﬁrmed by the presence of the strong emission at 595 nm in the ﬂuorescence spectrum. Based on the strong emission and high branching ratio of this transition, it can be concluded that the material can be utilized as an efﬁcient phosphor material for the development of optical devices.
 I.N. Ogorodnikov, V.A. Pustovarov, A.A. Goloshumova, L.I. Isaenko, A.P. Yelisseyev, V.M. Pashkov, J. Lumin. 143 (2013) 101.  L.R. Nirmala, J.T.J. Prakash, Spectrochim. Acta Part A 115 (2013) 778.  L. Misoguti, A.T. Varela, F.D. Nunes, V.S. Bagnato, E.E.A. Melo, J.M. Filho, S.C. Zilio, Opt. Mater. 6 (1996) 147.  T. Hirai, N. Okamoto, I. Komasawa, Langmuir 14 (1998) 6648.  W. Que, C.H. Kam, Opt. Commun. 206 (2002) 211.  W.A. Ang, N. Gupta, R. Prasanth, S. Madhavi, A.C.S. Appl. Mater. Interfaces 4 (2012) 7011.  W. Jung, B. Min, J. Park, D.H. Yoon, Ceram. Int. 37 (2011) 669.  M.A.E. Gabal, Ind. Eng. Chem. Res. 50 (2011) 13771.  J.S. Ghodake, R.C. Kamble, S.D. Kulkarni, S.R. Sawant, S.S. Suryavanshi, Smart Mater. Struct. 18 (2009) 125009.  G.E. Shter, G.S. Grader, J. Am. Chem. Soc. 77 (1994) 1436.  C. Joseph, M.A. Ittyachen, K.S. Raju, Bull. Mater. Sci. 20 (1997) 37.  A. Elizebeth, V. Thomas, G. Jose, N.V. Unnikrishnan, C. Joseph, M.A. Ittyachen, Cryst. Res. Technol. 39 (2004) 105.  V. Thomas, A. Elizebeth, H. Thomas, G. Jose, N.V. Unnikrishnan, C. Joseph, M.A. Ittyachen, J. Optoelectron. Adv. Mater. 7 (2005) 52687.  G.D. Dzik, J. Alloys Compd. 391 (2005) 26.  T. Yu, J. Joo, Y. Park, T. Hyeon, J. Am. Chem. Soc. 128 (2006) 1786.  E. Matei, M. Enculescu, I. Enculescu, Electrochim. Acta 95 (2013) 170.  R. Praveena, V. Venkatramu, P. Babu, C.K. Jayasankar, Physica B 403 (2008) 3527.  H. Sheng-Hua, G.D. Zhou, T.C.W. Mak, J. Crystallogr, Spectrosc. Res. 21 (1991) 127.  W. Ollendorff, F. Weigel, Inorg. Nucl. Chem. Lett. 5 (1969) 263.  J.K. Gibson, N.A. Stump, Thermochim. Acta. 226 (1993) 301.  P.K. Gallagher, F. Schrey, B. Prescott, Inorg. Chem. 9 (1970) 215.  I. Petrov, B. Soptrajanov, Spectrochim. Acta A 31 (1975) 309.  J. Fujita, A.E. Martell, K. Nakamoto, J. Chem. Phys. 36 (1962) 330.  B.R. Judd, Phys. Rev. 127 (1962) 750.  G.S. Ofelt, J. Chem. Phys. 37 (1962) 511.  W.T. Carnall, P.R. Fields, K. Rajnak, J. Chem. Phys. 49 (1968) 4424.  S. Tanabe, T. Ohayagi, N. Soga, T. Hanada, Phys. Rev. B 46 (1992) 3305.  W.F. Krupke, Phys. Rev. 145 (1966) 325.  J.L. Adam, W.A. Sibley, J. Non-Cryst. Solids 76 (1985) 267.  G. Vimal, Kamal P. Mani, P.R. Biju, C. Joseph, N.V. Unnikrishnan, M.A. Ittyachen, Spectrochim. Acta A. 122 (2014) 624.  B.P. Singh, Bull. Mater. Sci. 29 (2006) 559.