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2VOCl 3 1
8Y
ð
OH
Þ 3 !
Y 8 V 2 O 17 1
6HCl
9H 2 O
1
2VOCl 3 1
2Y
ð
NO 3 Þ 3 !
2YVO 3 1
3N 2 O 5 1
3Cl 2 m
YCl 3 1
H 3 VO 4 !
YVO 4 1
3HCl
VOCl 3 1
Y
ð
OH
Þ 3 !
YVO 4 1
3HCl
2H 3 VO 4 1
2Y
ð
NO 3 Þ 3 !
2YVO 4 1
3H 2 O
3N 2 O 5 m
1
H 3 VO 4 1
Y
ð
NO 3 Þ 3 !
YVO 4 1
3HNO 3
H 3 VO 4 1
Y
ð
OH
Þ 3 !
YVO 4 1
3H 2 O
Underlined formulae represent the solid precipitation. The viscosity of nitric
acid and sulfuric acid greater than that of HCl to HNO 3 would form a more effi-
cient solvent for the growth of R:YVO 4 . The dissolution of rare earth vanadates in
HCl and HNO 3 is a process of complex formation between R 3 1 ,V 5 1 , and the
ligands present in the solution OH 2 ,H 2 O, Cl 2 ,NO 3 2 . With a rise in temperature,
the association of Cl 2 with H 1 increases the yield of HCl and destabilizes the R 3 1
complex. Under these circumstances, nitrates replace chloride ligands. Finally, the
nucleation takes place by the polymerization of tetrahedral R 3 1 complexes. The
yttrium orthovanadate, YVO 4 , crystals were doped with active ions like Nd or Eu
and obtained crystals of Nd:YVO 4 and Eu:YVO 4 . The rare earth concentrations are
in the range 5
10%. After the experimental run, the autoclaves were quenched ini-
tially using an air jet followed by water, and the products were washed thoroughly
in double distilled water in an ultrasonic cleaner to remove all excess acid media.
The crystals obtained were well-faceted single crystals of 2
4 mm in size.
Solubility. Successful growth of any crystal insists on the availability of solubil-
ity data. In fact, the earlier workers did not attempt the hydrothermal growth of
rare earth vanadate crystals due to the lack of the solubility data on these com-
pounds. Mineralizers like HCl, HNO 3 , HCOOH, H 2 SO 4 , NaOH, NaF, and mixed
acid mineralizers like HCl
HCOOH have been
tried. The solubility measurements were carried out by the weight loss method.
Here a crystal is kept in equilibrium with an appropriate solution at the desired
pressure and temperature conditions for a period of time greater than that is neces-
sary to establish equilibrium. All the solubility measurements were carried out in
Morey-type autoclaves. Figure 7.75 shows the solubility of R:YVO 4 (in wt%) as a
function of temperature and at a pressure of 100 bar in 1.5 M HCl
HNO 3 , HCl
H 2 SO 4 , HCl
1
1
1
3.0 M HNO 3 .
R:YVO 4 shows a negative temperature coefficient of solubility under hydrothermal
conditions. The enthalpy of dissolution calculated from an Arrhenius plot log S
versus 10 3 /T was found to be 4.250 kJ/mole ( Figure 7.76 ). The experimental tem-
perature to study the solubility was up to 350 C. It was found that above 300 C,
1
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