Journal of Minera ls & Materials Ch ar ac teri zatio n & Engineeri ng, Vol. 9, No.12, pp.1081-1086, 2010 Printed in the US A . All rights reserved
Growth and Structural Characterization of Gadolinium Neodymium Oxalate
Crystals Grown in Hydro-Silica Gel
Ignatius Korah1, Cyriac Joseph2*, M.A. Ittyachen2
1 Department of Physics, St. Georges College, Aruvithura, Kerala, India - 686122
2 School of Pure and Applied Physics, Mahatma Gandhi University, Kottayam, India-686560
*Corresponding author :
Gadolinium Neodymium Oxalate (GNO) single crystals were grown in hydrosilica gel by the
diffusion of a mixture of aqueous solutions of the respective rare earth nitrates into a gel,
impregnated with oxalic acid, in a test tube. Pink, transparent, hexagonal GNO crystals were
obtained on optimization of growth parameters. The crystals are found to be monoclinic by the
XRD analysis. The presence of water of hydration and carboxylic group in the grown crystals
was confirmed by IR analysis. EDAX analysis con firm ed th e p resence of G d and Nd in the gro w n
samples. The thermal decomposition behavior of the grown crystals were studied by
thermogravimetric analysis (TGA) and differential thermal analysis (DTA). The thermal
analysis results concur with the proposed crystal structure.
Key words : Rare earths, Oxalate Crystals, Gel growth
The compounds of rare earths, especially oxalates are technologically important on account of
their luminescent, ferroelectric and ferroelastic behavior [1,2]. Superconducting compounds
have been synthesized by the controlled precipitation of rare earth oxalates followed by
calcination [3]. Czochralski technique has been employed for the growth of rare earth oxalates at
elevated temperature [4]. These crystals are found to be defective due to thermal stresses
introduced during growth. To avoid these defects, methods of growth at room temperature are
preferred. The gel technique [5] has been successfully employed for the growth of several rare
earth oxalate crystals [6-9]. The authors report here the growth and structural characterization of
gadolinium neodymium oxalate crystals in hydro-silica gel, and analysis of the thermal
decomposition behaviour of the grown crystals.
1082 Ignatius Korah, Cyri a c Joseph, M . A. Ittyachen Vol.9, No.12
Aqueous solution of sodium meta silicate of density 1.03 g/cm3 is mixed with 1M oxalic acid to
get desired pH. The resulting solution is allowed to set in test tubes of length 150mm and internal
diameter 15mm. At a pH of 6 gelling is completed within two days. Over the fully set gel,
supernatant solution comprising of a mixture of equal volumes of 0.5M gadolinium nitrate and
0.5M neodymium nitrate solutions acidified with nitric acid was slowly added. The rare earth
ions diffuse slowly through the gel and react with the oxalic acid already incorporated in the
medium in a controlled manner. Well defined pink coloured crystals of gadolinium neodymium
oxalate of size 3.5 x 2.5 x 1mm3 are obtained in a period of three weeks.
The proposed reaction is
Gd(NO3)3 + Nd(NO3)3 + 3H2C2O4 Æ GdNd(C2O4)3.nH2O + 6HNO3
The gel medium prevents turbulence and being chemically inert, it provides a three dimensional
structure which permits controlled diffusion of reagents. The morphology and size of the crystals
are found to depend on various growth parameters such as pH of the gel, density of the gel,
concentration of the feed solution and acidity of the feed solution.
Figure 1. Growth system of GNO crystals
The lattice parameters of the grown crystals were determined by single crystal X-ray diffraction
analysis using an ENRAF (Bruker) Nonius CAD4 single crystal X-ray diffractometer. IR
absorption spectrum was recorded in the range 400 cm-1 to 4000cm-1 using a Shimadzu IR 470
spectrophotometer. The thermal characteristics of the grown crystals were studied using a
Shimadzu thermal analyser DT-40 in the temperature range 30-8000C at a scan rate of 100C per
Vol.9, No.12 Growth and Structural Characterization 1083
3.1. X-ray Diffraction Studies
X-ray diffraction studies confirmed that gadolinium neodymium oxalate crystal belongs to the
monoclinic system with space group P 2 1/C with Z = 2. The unit cell dimensions are a=11.19 Å,
b=9.89 Å and c=10.24 Å. The crystal structure is expected to be identical with that proposed by
Sheng Hua Huang et al.[10] for lanthanum oxalate, as evidenced by the similarity of the lattice
parameters of the double rare earth oxalate crystal as that of the single component ones. It may
be concluded that GNO crystals are formed by the substitutional exchange of the two rare earth
ions and has the same structure as that of lanthanum oxalate crystal, La2 (C2O4)3. 10H2O.
3.2. Infrared Absorption Studies
FTIR studies reveals the presence of water of crystallization in the grown samples which is
evidenced by a broad envelope extending from 3000 cm-1 to 3600 cm-1, due to the stretching
modes of water molecules. The broad band appearing in the IR spectrum around 1620 cm-1 may
be identified as due to the asymmetric stretching mode of an oxalate ion [11]. The CO2 group
may be identified by a strong peak around 1316 cm-1.
3.3. Energy Dispersive X-ray Analysis
Figure 2. EDAX spectrum of GNO crystals
The elemental incorporation of Gd and Nd in GNO crystal is confirmed using the EDAX
analysis. The presence of Gd and Nd is established from their characteristic peaks in the EDAX
spectrum. The peak heights or areas are a measure of the respective elements incorporated in the
1084 Ignatius Korah, Cyri a c Joseph, M . A. Ittyachen Vol.9, No.12
3.4. Thermal Analysis
Figure 3. TGA and DTA curves of GNO crystal
Extensive research on the mechanism and chemical intermediates associated with the thermal
decomposition of oxalates have been conducted by many researchers [12-15]. Various reports
are available in literature regarding the thermal studies of lanthanide oxalates prepared either by
mechanical mixing or by co-precipitation methods [16, 17]. Based on thermogravimetric and
differential thermal analysis, Glasner et al. [18, 19] suggested a decomposition mechanism for
anhydrous rare earth oxalate via a divalent intermediate. Gallagher and co-workers have reported
thermal studies on the decomposition of europium oxalate [20].
The thermal decomposition stages of gadolinium neodymium oxalate decahydrate crystal are
studied from the TGA and DTA curves shown in Figure 3. These stages are analyzed from the
proportionate mass at each stage taking the initial weight as standard. The three steps involved in
the decomposition process of GdNd(C2O4)3.10H2O are dehydration, decomposition of oxalate to
carbonate and decomposition of carbonate to oxide. The liberation of ten molecules of water of
crystallization in GNO crystal takes place in two stages in the temperature ranges 60-1600C and
160-3500C. In the first stage, seven water molecules which are distributed randomly in the
intervening lattice space are eliminated. In the second stage, the remaining three water molecules
are liberated. These two different stages in the dehydration process appear as two distinct
endothermic peaks in the DTA curve.
The next stage of decomposition occurs as a continuous process of dehydration. The oxalate
decomposition of GNO crystals can be assigned to two stages. During the first stage in the
temperature range 350-5200C, two CO2 molecules and three CO molecules are lost resulting in
the formation of an unstable intermediate, dioxycarbonate. The reduction of this intermediate
compound into the oxide form occurs in the range 520-6500C with the release of one CO2
molecule. These decomposition reactions are represented by the two exotherms in the DTA
Vol.9, No.12 Growth and Structural Characterization 1085
curve. The thermal studies on GNO crystals are in good agreement with the proposed chemical
formula and hydration number for these crystals.
The mechanism of thermal decomposition of GNO crystals may be described as follows.
GdNd(C2O4)3.10H2O 2
-7H O
60-160 C
⎯⎯⎯⎯→ GdNd(C2O4)3.3H2O
Gd Nd (C2O4)3.3H2O 2
-3H O
160-350 C
⎯⎯⎯⎯⎯ GdNd(C2O4)3
GdNd(C2O4)3 2
-(2CO +3CO)
350-520 C
GdNdO2CO3 2
-C O
520-650 C
⎯⎯⎯⎯⎯ GdNdO3
The thermo analytical data for GNO crystals is given in Table 1.
Stage Decomposition
Temperature range (oC) Loss of
material Observed mass
loss (%) Calculated mass
loss (%) Nature of reaction
60 – 160 7H2O 16.08 16.91 Endo dehydration
160 –350 3H2O 7.39 7.25 Endo dehydration
350 – 520 2CO2 + 3CO 23.95 23.07 Exo decomposition
520 – 650 CO2 6.14 5.90 Exo decomposition
Table-1. Thermo analytical data of GdNd(C2O4)3.10H2O
The gel grown gadolinium neodymium sample is crystalline as evidenced by XRD analysis and
has identical structure as that of single rare earth oxalate crystals. Presence of water molecules in
the sample is confirmed by the broad envelope in the IR spectrum extending from 300cm-1 to
3600cm-1 and that of carboxylic groups is evidenced by the peak at 1620 cm-1. The presence of Gd
and Nd in the grown sample is established from their characteristic peaks in the EDAX spectrum.
The proposed chemical formula and crystal structure for the grown sample are in good
1086 Ignatius Korah, Cyri a c Joseph, M . A. Ittyachen Vol.9, No.12
agreement with the results obtained from TGA and DTA studies. The observed mass loss and
calculated mass loss agrees in all stages of dehydration and decomposition.
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