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Chapter IV Thermal Studies of …..
structure at room temperature. The observed change of symmetry of the
crystal suggested a phase transition like phenomenon called HTPT at
characteristic temperature Tp = 430 K. The observed weight loss from TG
curve suggested that Tp marked the onset of partial polymerization taking
place on the surface of the sample such that the phase above 430 K was a
mixed phase consisting of tetragonal ADP in the bulk of the sample and
ammonium polyphosphates (NH4)2H2P2O7 at the surface of the crystal.
Several other authors studied thermogravimetry of pure KDP crystals
[12, 13] and various doped KDP crystals [14-17]. It has been found that
glycine doped KDP crystals starts decomposing at 204.93 o C and KDP
crystal at 207.91 o C [16] this also further supports that amino acid doping
slightly increases thermal destability and reduces the phase transition
temperature.
Moreover, thermal studies on amino acids (L-glutamic acid, L-histidine
and L-valine) doped KDP crystals are reported by Kumaresan [17] using TGA
and DTA. The author [17] observed that L-glutamic acid doped KDP melted at
a 125 o C and undergone irreversible endothermic transition around 210 o C
followed by another endothermic transition around 290-675 o C. The TGA
exhibited a weight loss of about 13 % in the temperature range 190-350 o C
due to liberation of volatile substances. However, the prolonged heating up to
900 o C did not produce any endothermic peaks in the DTA curve and TGA
exhibited a total weight loss of nearly 13 % at 900 o C. Interestingly, the DTA
curve of L-histidine doped KDP revealed two more events at 560 o C and 798
o C reported which did not accompany by weight loss. It has been argued that
this might be due to high temperature phase transition of KDP. Apart from
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