MIXED CRYSTALS - Shodhganga : a reservoir of Indian...
Transcript of MIXED CRYSTALS - Shodhganga : a reservoir of Indian...
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.