The influence of uniaxial mechanical stresses applied along the crystal-physical axes on the principal values of the birefringence, ∆ ∆ ∆ ∆n i , was studied for TGS crystals with 5% L-valine admixture. It was found that ∆ ∆ ∆ ∆n i are quite sensitive to stress and adopt different values and opposite signs, when compared to pure TGS. The baric coefficients ∂ ∂ ∂ ∂Т c /∂ ∂ ∂ ∂σ m for the phase transition (PT) temperature appear to be less than for pure TGS. The temperature dependence of the

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... ed piezooptic constants 0 im π was calculated. The values of the jumplike changes occurring for all the piezooptic constants at the PT are notably lower for the doped crystals than for pure TGS.