Journal of Crystallization Process and Technology, 2011, 1, 8-12
doi:10.4236/jcpt.2011.11002 Published Online April 2011 (http://www.SciRP.org/journal/jcpt)
1
Etching and Micro-Topographical Studies of
Barium Oxalate Crystals Grown in Agar Gel
Paresh V. Dalal*, Kishor B. Saraf
P. G. Department of Physics, Pratap College, Amalner, India
Received March 27th, 2011; revised April 1st, 2011; accepted April 10th, 2011.
ABSTRACT
In the present study, the selection of an etchant for a particular crystal was purely made on empirical basis. The gel
grown barium oxalate crystals were etched by HCl, HNO3, BaCl2, NH4Cl, and NH4Cl-HCl solutions. Micro-topog-
raphical studies ha ve been made and it was found that elongated trian gular etch pits and pits within pits were formed.
Rectangular growth layers in the form of staircase and leaf like dendrite pattern were seen. Kinetics of etching was
studied. Quantitative estimatio n of dissolved crystals in etch ants was used for the de termination of activation energy of
reaction and pre-exp onent i al factor with the help of Arrhe nius equation.
Keyword s: Etching, Barium Oxalate, Micro-Topographical Studies, Activation Energies
1. Introduction
In recent years, crystal growth in gel has attracted many
investigator s [1,2]. The agar gel technique is an in expen-
sive and simple for growing single crystals of certain
class of materials like alkaline earth metal oxalates [3].
Oxalate crystals are insoluble in water and decompose
before melting [4]. Therefore, single crystals of these
materials cannot be grown by either slow solvent evapo-
ration or melt techniques but they can be suitably grown
by gel method. Barium oxalates is a pyro-nature material
that shows great promise in pyrotechnic and high tem-
perature electronic applications. The high dielectric con-
stant and melting point of barium oxalate is an advantage
to improve hardness of barium titanate in capacitor in-
dustries [5]. Nano-particles of barium oxalate [6] and
barium titanyl oxalate have shown its effect on semicon-
ducting properties [7]. A research program on etching
and micro-topographical studies of barium oxalate crys-
tals is being carried out in this laboratory. Etching has
been the most convenient method for revealing disloca-
tions in many crystals [8,9]. Successful, deliberate at-
tempts to etch the surface at the sites of dislocations were
made by Horn [10], and Patel and Desai [11]. Notable
contributions on the theory and applications of etch
methods were made by Honess [12], Miers [13], HariB-
abu and Subba Rao [14], and Patel and Desai [15]. Etch
figures represent very early stage of crystal dissolution.
The attack of solvent or chemical reagent on a seemingly
uniform solid surface is frequently localized during dis-
solution, and the depressions thus formed, are called
‘etch pits’. There exists a report in the literature about
etching [16,17] and kinetics of etching on barium oxalate
dihydrates crystals [18]. However etch rate for different
etchants was calculated by taking an average of meas-
urements of a number of etch pits at a constant magnifi-
cation [18]. This type of method, which is based on av-
erage measurements of pit as well as based on same mag-
nification, cannot give the accurate measurements, be-
cause each etchants producing pits not necessarily ex-
posed at the same magnification. Therefore weighing
method used in the pr esent work rather than the methods,
based on the average measurement of number of etch pits
for estimating etch rate is more reliable, simple and ac-
curate.
2. Experimental Procedure
The barium oxalate crystals grown by the gel method re-
ported earlier [19] were used for the etching studies. Crys-
tals to be employed for etching studies were carefully
picked up from the gel so that they were not damage dur-
ing mechanical handling. An attempt to understand the
mechanism of etching can be made by choosi ng number of
simple etchants. AR grade HCl, HNO3, BaCl2 and NH4Cl
were used as etchants. Barium oxalate crystals were agi-
tated thoroughly and uniformly to bring all the faces in
contact with th e etchant solution in 1 M HCl, 1 M HNO3,

Etching and Micro-Topographical Studies of Barium Oxalate Crystals Grown in Agar Gel 9
4 M BaCl2, 4 M NH4Cl and 4 M NH4Cl + 1 M HCl{in
70:30 ratio} for 30 second, similarly other crystals were
dipped for 1 minute in HCl, HNO3 , and NH4Cl + HCl
solutions. These crystals were arrested from the etchants,
immediately washed with distilled water, dried in air and
observed under microscope for micro-topographical stud-
ies.
For kinetic studies of etching, damage and inclusion
free 10-20 mg crystals were selected. Each initially
weighed crystal immersed in 0.5 M, 1 M, 1.5 M and 2 M
HNO3, 1 M HCl, 4 M BaCl2, 4 M NH4Cl and 4 M NH4Cl
+ 1 M HCl at a constant temperature 28, 32, 49 and
520˚C for 30 seconds. These crystals were taken out
from the solution, washed immediately with distilled
water, air-dried and weighed again. The amount of dis-
solved crystals was calculated from their loss in weight.
3. Observations, Results and Discussion
Micro-topographical studies are helpful in understanding
growth mechanism of crystals under study. Detailed mi-
cro-topographical examinations of the grown crystals
reveal that striations are common features. There is ex-
hibit inclined striatio ns. Sometimes, these striations were
found to cover the whole surface under examination. In
fact these striations are divided into strips. The width of
the strips is not uniform. There are some irregularly
spaced inclined striations strictly oriented 300 to C axis
of the crystal as shown in Figure 1. In addition to hori-
zontal striations, some faces are observed elongated el-
liptical or rectangular growth hillocks. One such case of
dissolution of rectangular growth hillocks in the form of
staircase is illustrated in Figure 2. Figure 3 shows ran-
domly oriented, crowded, triangular etch hillocks di-
rected in one particular way. Because of truncation and
rounding of a corner of etch pits, they look like elong ated
triangular etch pits. Figure 4 shows a rectangular platelet
with leaf like dendrite pattern on its face. Some portions
of platelet are irregularly eaten away and edges are
formed. These edges act as initiation centers of dendrite
patterns. In the present observation, platelets may be de-
veloped first and then dendrite pattern may be formed as
reported [20]. It is also interesting to see pits within p its,
as in Figure 5, similar to an observation reported [21] in
case of natural CaF2 crystals. The smaller pits, which are
seen near the bottom of the larger enveloping pit sug-
gesting the division of a pit, continuing with time may be
believed to be due to a number of other dissolution lines
meeting and forming nodes, thereby giving rise to a dis-
location network in the body of the crystal.
In order to study the kinetics of etching in barium ox-
alate crystals, the etch rates were determined by calcu-
lating the amount of dissolved crystals from their loss in
weight at different temperature, keeping etching time
Figure 1. Striations making an angle of 30 with C-axis.
Figure 2. Rectangular growth layers in the form of stair-
case.
Figure 3. Randomly oriented, crowded, triangular etch
hillocks.
Figure 4. Rectangular platelet with leaf like dendrite pat-
tern.

10 Etching and Micro-Topographical Studies of Barium Oxalate Crystals Grown in Agar Gel
Figure 5. Elongated triangular pits within pits.
constant through out the experiment. The natural loga-
rithm of the etch rate, lnR; was plotted against the recip-
rocals of the absolute temperature, and the results ob-
tained are depicted in Figure 6 and 7 for the five differ-
ent solutions. Evidently the curves follow the Arrhenius
equation [22]:

exp /,RA EKT
where A is the pre-exponential factor, E the activation en-
ergy of the etching process, K is the Boltzmann constant
and T is the temperature of etchant. From these Arrhenius
graphs, the values of activation energy E and pre-expo-
nential factor A thus calculated are shown in Table 1.
It was observed that the values of activation energy E,
for etching in HCl, NH4Cl and HNO3 solutions are close
to each other, though the rates of dissolution in these
etchants are largely different. BaCl2 and NH4Cl + HCl
etch solutions are characterized by relatively larger and
lower values of activation energies respectively. The
faster etch rate in NH4Cl-HCl may not be due to the en-
ergies of the atoms at the surface or due to their geomet-
ric configuration [23], but may be due to both; the
chemical reaction and diffusion are operating simultane-
ously, that makes the etching or dissolution process less
sensitive to temperature change, and hence a very low
value of activation energy was observed. The etchants
such as HCl, HNO3 reacts with the crystal surface to
form oxalic acid, which in turn dissolves in water. These
dissolutions are therefore reaction rate controlled. On the
other hand, the etch rating in BaCl2 solution is observed
to be low, giving the highest value of activation energy
among the etchants used. And, the dissolution in BaCl2 is,
doubtlessly, a diffusion controlled phenomenon, because
of the common Ba2+ ion between the solute and the sol-
vent. This observation of the highest activation energy
characterizing diffusion controlled dissolution is contrary
to the observation reported chemical reaction controlled
on certain semiconductor materials [24,25]. Similarly,
the observation made by Tuck [25] that the diffusion
controlled mechanism of dissolution is not sensitive to
change in temperature, gets itself contradicted in the
present situation .
It was revealed from the data (Table 1) that the values
of activation energy remain constant for the different
concentration of acid, whereas the pre exponential factors
A were increased, with increasing acid conce ntration.
4. Conclusions
From the studies we observe that:
1) The mechanism of etching barium oxalate in
etchant such as 1 M HNO3 is reaction rate controlled
whereas in the case of 4 M NH4Cl, 4 M BaCl2 it is diffu-
sion rate controlled.
Figure 6. Plot of lnR against reciprocal temperature for HNO3.

Etching and Micro-Topographical Studies of Barium Oxalate Crystals Grown in Agar Gel 11
Figure 7. Plot of lnR against reciprocal temperature of 1 M HCl, 4 M NH4Cl, 4 M BaCl2 and 4 M NH4Cl + 1 M HCl.
Table 1. Activation energies and pre-exponential factors of different etchants calculated from arrhenius plots.
S. No. Etchant Activation energy E (in eV) Arrhenius pre- e xponential factor A (in g·sec–1)
0.5 M 0.39 2.6 × 103
1.0 M 0.39 2.8 × 103
1.5 M 0.39 3.1 × 103
01 HNO3
2.0 M 0.39 4.8 × 103
02 4 M NH4Cl 0.38 2.3 × 103
03 1 M HCl 0.34 4.4 × 102
04 4 M BaCl2 0.45 22.3 × 103
05 4 M NH4Cl + 1M HCl 0.28 0.4 × 102
2) The observation of Tu ck [25] that the activation en-
ergy for reaction rate controlled etching is more than that
of the diffusion rate controlled one, is not found true for
barium oxalate dissolution.
3) The mechanism, whether reaction rate controlled or
diffusion rate controlled could be categorized by the
knowledge of the values of only experimental parameter
as activation energy.
5. Acknowledgements
The corresponding author is thankful to the UGC, New-
Delhi and the Principal Dr S. T. Pawar, Shri V. S. Naik
Arts, Commerce and Science College, Raver (M. S.) for
providing an opportunity to short-term visit in Depart-
ment of Phys ics at S. P. Univ ersity, V. V. Nagar( Gujar at).
The corresponding author is also thankful to Prof. Dr. K.
N. Joshipura, Head, Department of Physics, S. P. Uni-
versity, V. V. Nagar (Gujarat) for providing laboratory
facilities.
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