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will also be generated. Variations in the refractive index will result in nonideal
phase matching throughout the volume of the nonlinear interaction. As the nonlin-
ear interaction is quite sensitive to the variations in phase matching, the heating of
the crystal can cause a significant degradation in the efficiency of the nonlinear
interaction
[154]
.
Several authors have measured the ionic conductivity in pure and doped KTP
crystals
[155,156]
. The ionic conduction property of these crystals allows ion-
exchanged waveguides to be formed readily in them.
5.6 Potassium Titanyl Arsenate
It has been shown that the nonlinear optic and electro-optic coefficients of potas-
sium titanyl arsenate (KTA) crystals are significantly superior to those of KTP
crystals
[141]
. Similarly, the arsenate isomorphs, like KTA, RTA, and CTA, unlike
the phosphate isomorphs, are prone to ferroelectric multidomain formation which
renders these crystals useless in practical applications. KTA belongs to the class of
40 compounds which are all structurally characterized by corner-linked octahe-
drally coordinated titanium chains connected with tetrahedrally coordinated phos-
phorus or arsenic bridges.
KTA can be prepared by methods analogous to those used for KTP. Durand
and El-Brahimi (1986)
[157]
have reported the structure of KTA crystals. The
reputed effective nonlinear coefficient (d
eff
) of KTA is 1.6 times that of KTP.
This favorable property has encouraged several groups to carry out the hydrother-
mal growth of KTA at temperatures less than 600
C. Belt and Ings (1993)
[158]
have hydrothermally grown KTA crystals using KH
2
AsO
4
and KOH mineralizers.
Flux-grown KTA crystals were used as the seed crystals by these workers. These
authors have also studied the PVT relationship, solubility, and the growth rates
and growth morphology in detail; with the pressure-balancing method, growth
rates of 0.2
0.4 mm/side/day can be achieved on {011} seeds. The experiments
have been carried out in large size autoclaves of 4
2
5 l capacities.
Figure 5.44
shows the characteristic photographs of KTA single crystals obtained by Belt and
Ings under hydrothermal conditions. The ideal morphology of KTA crystals is
shown schematically in
Figure 5.45
. The experimental conditions are given in
Table 5.15
.
Some authors have studied the ionic conductivity in KTA crystals and usually
the ionic conductivity is one order of magnitude lower than KTP crystals. The ionic
conductivity is found to be one dimensional, i.e., along the Z-direction only.
The structure of the KTP family has the typical formula {K
1
} [Ti
4
1
]O (P
5
1
)O
4
,
where the flower brackets indicate nine- or eightfold coordination, the square
brackets sixfold octahedral coordination, and the parentheses fourfold tetrahedral
coordination. Different isovalent and aliovalent substitutions in a given structure
are quite interesting, and could, perhaps, result in different structural and optical
properties. Cheng et al.
[159]
have found that stability of KTA crystals is higher
