Chapter 10

OBSERVATIONS ON THE GROWTH AND CHARACTERIZATION

OF PRASEODYMIUM COPPER OXALATE

MIXED CRYSTALS

10.1 Introduction

Mixed crystals of praseodymium copper oxalate do not

have natural occurrence. So far there has been no

reports on the growth of these crystals in silica gel.

This chapter deals with the growth of the crystals, the

influence of the concentration of the nutrients, the pH

of the gel, morphology of these crystals and the

characterization by different methods.

10.2 Growth procedure

For the preparation of hydro silica gel, sodium meta

silicate powder ( L R ) was dissolved in distilled water and

3 this solution (density 1.03 g/cm ) was titrated against

IM oxalic acid (AR). The resulting solution was kept in

glass tubes (i.d 2.5cm and length 15cm) at different pH

values and allowed to gel. After attaining proper

gelation, a mixed solution of praseodymium nitrate and

cupric nitrate (AR) along with nitric acid was taken as

the outer electrolyte and poured drop by drop over the

gel. At the contact of the outer electrolyte,a band of

precipitate appeared at the gel-solution interface.

Afterwards, this greenish blue colloidal precipitate

extended downwards. On attainment of a certain thickness

a partial dissolution started and slowly made the whole

precipitate partially transparent. During this process

nucleation also developed inside and below the

precipitate. These nuclei continued to grow forming well

defined, faceted crystals of size 3mm x 2mm x 2mm within

25-30 days. The optimum condition for the growth of well

faceted crystals was at a pH 5 of the gel and 1M outer

electrolyte.

10.3 Observations

10.3.1 Effect of variation in the concentration of

outer electrolyte

Experiments were done with various molar concentra-

tions of the outer electrolyte. Experimental details are

shown in the table (10.1). Figs. (10.1 & 10.2) show the

effect of the variation of the outer electrolyte, at a pH

of 8 and 7 of the gel, on the precipitation and

crystallization region. The depth of precipitation and

crystallization region increased directly with the

increase in the concentration of the outer electrolyte.

10.3.2 Effect of pH variation of the gels

The pH of the gel medium was found to be a critical

parameter which influenced the depth of precipitation and

dissolution, leading to the formation of crystals.

Table 10.1

EXPERIMENTAL CONDITIONS FOR CRYSTALLIZATION OF PRESEODYMIUM COPPER OXALATE MIXED CRYSTALS

[Variation of outer electrolyte with pH values] 0

Mean temperature of growth = 28 C, ~ensity of gel solution = 1 . 0 3 g/cm3, Age of gel = one day, Growth period = 25-30 days. Growth apparatus = a single tube of size 2.5cm

x 15 cm. Inner electrolyte = oxalic acid Outer electrolyte = Praseodymium nitrate

+ Cupric nitrate + HNO 3

pH value Outer electrolyte

Results

bluish green precipitate, partial dissolution, a lot of clustered crystals, 1 or 2 single crystals at bottom - do - rate of crystals decreased - do - rate of crystals further reduced translucent precipitate, few nucleation precipitate, dissolution, lot of clustered crystals, single crystals at the bottom - do - clustered, and single crystals - do - few single crystals thin layer precipitate, few crystals thick precipitate, dissolution, most single crystals, few clustered - do - - do - thin clustered crystals thin layer precipitate, few minute crystals precipitate, dissolution, a lot of single crystals - do - single, clustered crystals - do - small clustered crystals no precipitate, no crystal precipitate, dissolution, lot of small single crystals no precipitate, no crystal

crystallization region S I I= Outer electrolyte

outer electrolyte on depth of precipitation and cry- stallization region (pH=8)

- crystallization region ..... precipitation front

b 5 Outer electrolyte I

0 5 10 1 5 20 25 30 35 I I I I I I t I

Time (in days) I Fig. 10.2 Effect of variation of the

outer electrolyte on depth of precipitation and cry- - stallization region (pH=7)

Fig.(l0.3) shows the effect of pH on the precipitation and

crystallization front movements. With the decrease in

the pH value of the gel from 8 to 6, the depth of the

precipitation and crystallization region increased and

thereafter decreased. The nucleation rate(density)of the

crystals in the gel decreased with the decrease in pH

value of the gel. But good well faceted single crystals

were obtained towards lower pH values.

10.3.3 Morphology of the crystals

The growth conditions of the praseodymium copper

oxalate mixed crystals inside the gel is shown in

figure (10.4). The crystals formed are orthorhombic with

lattice parameters a = 13.80, b = 14.86 and c = 7.66.

They crystallized in different morphologies. At greater

depths, well faceted single crystals were formed. A

typical pattern is shown in fig. (10.5). Spherical dark

patches seen on the faces are spherulites incorporated

during the growth of the crystal. Fig. (10.6) shows the

development of cavities along the length of the crystal.

Spherulitic inclusion can also be seen here. In the

middle of the crystallization region,twinned crystals were

observed as in fig. (10.7). Near to the interface, ie.,at

the top layer where diffusion rate is greater, cluster

type crystals were formed as shown in fig. (10.8).

The crystals were observed under SEM to examine the

microstructures on the faces. The faces of the crystals

were found to be smooth, devoid of any visible growth

layers as in fig. (10.9). The crystals could not be

examined for longer period due to the heating of the

specimen. Heating will cause decomposition in oxalate

crystals. The initiation of the decomposition is evident

from the figure. SEM photograph, fig. (10.10), shows the

thermal etch hillocks produced on the faces of the

crystal. It is interesting to note that all these etch

hillocks are oriented in one direction.

At a pH between 4 and 7. and at a medium

concentration of the outer electrolyte, Liesegang rings

were formed. These were not so distinct as in molybdates,

since the HN03 used in the outer electrolyte enhance the

dissolution. Nearer to the interface,where the diffusion

rate is greater, the crystals formed were clustered in

type. Spherulitic crystallization was also observed in

this region. Fig. (10.11) shows SEM photograph of a

typical case of spherulite formed. These spherulites were

found to take a form from perfect sphere to oval shape,

having crystallites emerging outwards from the centre. The

spherulitic growth seems to have occurred through a

lengthening of the plates with accompanied plate

branching, causing it to "fill-in" the interior of the

FIGURE CAPTIONS

Fig. 10.4. Growth process of praseodymium copper oxalate mixed crystals is shown

Pig. 10.5. A typical single platelet crystal along with spherulitic incorporation x 300

Fig. 10.6. Cavity formation in the crystal along the length x 200

Fig. 10.7. A typical twinned crystal x 250

Fig. 10.8. A typical clustered crystal x 200

Pig. 10.9. A crystal with smooth faces devoid of any visible growth layers x 500.

Fig. 10.10. SEM photograph showing etch hillocks which are oriented in one direction x 600

Fig. 10.11. A typical spherulite in oval shape x 200

Fig. 10.12. A typical pattern of the crystal showing cleaved faces having lengthwise cavity formation x 50

spherulite. It can be considered as a three dimensional

spherulite[326-3271.

Etching was done using the same etchants as in

chapter 8 and the same type of etch patterns were

obtained. The crystals were cleaved. One typical pattern

of a cleaved crystal is shown in fig. (10.12. It has a

lengthwise cavity formation.

10.4 Characterization

10.4.1 Chemical analysis

Praseodymium copper oxalate mixed crystals grown in

gel are light green crystals of orthorhombic system, but

crystallize in different morphology. On heating, these

crystals become polycrystalline. These crystals dissolve

in almost all the acids.

The presence of praseodymium and copper were

confirmed by chemical analyses.

10.4.2 XRD analysis

The powder X-ray diffraction was taken (Fig. 10.13)

in a Philips model XRD PW 1050/70 with C u k ~ radiation

( X = 1.5418 A ) . The 'd' (obs.) and 'd' (cal.) computed

for the assigned (hkl) values are given in table. (10.2).

Table 10.2

XRD DATA OF PRASEODYMIUM COPPER OXALATE MIXED CRYSTAL

S1.No. hkl d (obs.) d (cal.) I/Imax.

It was found that praseodymium copper oxalate is

crystalline, belonging to the orthorhombic system with the

lattice parameters a = 13.80 (51, b = 14.86 (4) and

c = 7.66 (3) AO respectively.

10.4.3. IR analysis

IR spectrum (Fig. 10.14) was recorded using PE 580 in

KBr matrix. The presence of water of crystallization in

the crystal is proved by the broad peak at 3600-3200 cm - 1

(relating to asymmetric and symmetric OH

stretching) [312,314] and at 1630cm-I (relating to HOH

bending). The crystal has copper linked to oxalate ion

which is supported by the revelati'on of well pronounced

peaks at 1720cm-I [pa (C = O)], 795cm-I [ s(0 - C = 0)

+ J (Mo)], 590cm-l, 485crn-I [ring deformation +S(O - C =

O)l, 380 cm-I [ d(0 - C = 0) +lJ(cc)l (perhaps crystal

water) and 320cm-I ( 7i ). These peaks confirm the presence

of oxalato complexes of copper and probably praseodymium,

with the presence of water of crystallization agreeing

quite well with the reported values in the

literature[317]. The general crystal structure can be

represented schematically [318] as

where M = Pr + Cu.

10.4.4 Laser Raman spectra analysis

The C.M.-82 L.R. spectrometer was used to record the

spectrogram (Figs. 10.15 & 10.16) of the crystals. The

observed data on acid oxalates[319-3211, K2C204[322] and

metal oxalates[313] were compared to obtain the

assignment. The stretching frequencies observed were at

3300 (OH), 3020 (OH), 2875, 2390, 1620 (symmetric), 1470

(antisymmetric), 1440 (syrmnetric) , 1340 and 910cm-~. The

band at 200crn-~ can be assigned to the liberation of

oxalate ion and the band at 100cm-~ is assigned to the

c-c torsional mode.

10.4.5 uv-visible-near infrared analysis

uv-visible -near infrared spectrum was taken using a

spectrophotometer 11-3410 (Hitachi) in nujol. The spectrum

obtained is shown in fig. (10.17. The peaks obtained at

444.2 and 491.4 nm show the presence of praseodymium in

the crystal and the peaks at 202.4, 218.1 and 249.2 nm

indicate the presence of copperI227-2291.

10.4.6 Energy dispersive x-ray analysis (EDAX)

The surfaces of the crystals were examined in the

EDAX analyser No.711. The EDAX plot so recorded is shown

in fig.(lO.l8a & 10.18b).

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Fig.lO.17 uv-visible-near i n f r a red spectrum of praseodymium coppcr oxa l a L e mixed crystal (Range: 200-500)

The three dominant peaks positioned at 5.033, 5.488

and 5.849 Kev correspond quite well with the Lot, L P and

L r energies of praseodymium, while a small hemp at 8.04

Kev corresponds to Kd line of copper (shown magnified

Fig.lO.18b) (reported in the EDAX international chart),

giving a clue that praseodymium is dominant over copper in

the crystal.

The integrated counts of X-ray photoelectrons

relating to praseodymium (Ld) collected for 150 seconds

were found to be 68, 107 while for copper (koc ) , the

integrated counts were 4,206.

From the details of the integrated counts, the

percentages of praseodymium and copper were seen to be

94.184 and 5.816 ie.,hthe ratio 7:l. So the proposed

formula for the crystal is Pr14 Cu2 (C204)23. 70 H20.

10.4.7 Inductively coupled plasma (ICP) analysis

The atomic emmission spectrum of the crystal by

inductively coupled argon plasma was studied by dissolving

50-55 mg of the crystal in HC1 and making the solution

upto 25ml. The quantitative analysis of praseodymium and

copper was carried out with the calibration standard of

Pr6 011 and Cu So4 -5 H20 in-the range of 0, 10, 20, 50

79 5030 EV L 259 PR VS: 5000 AS: 20 EV/CH

4.0 6.0 8.0 10.0 I 1 1 I ENERGY (KEW)

Fig. 10.18(a). EDAX pattern of praseodymium copper oxalate mixed crystal

79 8040 EV k 229 CU VS: 1000 HS: 20 EV/CH -

- - - - . -

- 10.0 ,

ENERGY (KEW) F i g . 10.18(b). 10.18(a) of range

7-10 KEV magnified.

and 100 ppm. The concentration of praseodymium and copper

was found to be 1084.6225 and 5.8720 ppm respectively.

These computations are in good agreement with the findings

of EDAX, confirming the dominance of praseodymium over

copper in the crystal.

10.4.8 XRF analysis

On utilising LiF220 crystals as X-ray crystals the

peaks obtained were at 119.73~ and 65.56O as 20 showing

the presence of praseodymium and copper. The plot shows

(Fig.lO.19) the dominance of praseodymium over copper.

10.4.9 Thermal analyses

The thermogravimetric plot of the crystal was taken

(Fig. 10.20) using a TGA Dupont 2000 in N2 (10LAR) 50

ML/MIN. This plot showed that in praseodymium copper

oxalate crystal, dehydration process took place in the

first stage and decomposition in the second stage.

The crystal began to lose water molecules at 40°c and

dehydration was completed at 2 9 0 ~ ~ by loosing 70 molecules

of water. In this stage 23.5% of its weight was lost. In

the second stage 33% of the weight was lost by

decomposition. The temperature change during this stage

- 10.19. XRF spectrum of praseodymium

copper oxalate mixed crystal.

was from 2 9 0 ~ ~ to 8 0 0 ~ ~ . In this stage a side reaction,

was observed in the DTG curve along with the main

decomposition reaction, which was not identified. The

residue obtained in the form of oxide was only 43.5%.

These stages are evident from the derivative

thermogravimetric (DTG) plot drawn along with the TG plot.

The DTA curve was plotted using Dupont 2000 (Fig.

10.21). Two endothermic peaks obtained at 135Oc and 200'~

are related to dehydration of water. On the basis of

these studies the following tentative mechanisms have been

proposed for the thermal conversion.

Prl4CU2 (C204)23. 70 H20 Stage I Pr14 Cu2 (C204)23

Stage I1

10.5 Conclusion

It is concluded that crystallization takes place by

dissolution from the colloidal precipitate. The gel grown

praseodymium copper oxalate mixed crystals are

crystalline, belong to the orthorhombic system, have

oxalato complexes and water of crystallization.