The existence of a high temperature phase transition in KD 2 PO 4 was confirmed by thermal analysis, dielectric analysis and Raman spectroscopy. The results showed that KD 2 PO 4 presented only one high temperature phase transition at about T tr = 430 K, which is much lower than the thermal decomposition temperature. The absence of Raman shift attributable to the dehydration product indicated that the HTPT had nothing to do with thermal decomposition. The structural change during the HTPT was discussed. The strong similarity between the HT-phase and tetragonal phase decreases the temperature of the HTPT in the KD 2 PO 4 crystal. The hydrogen-deuterium exchange can be suppressed under a protective ambient environment, which helps improve the annealing temperature. A new annealing method was developed by using silicone oil as a protective ambient environment under higher temperature up to 403 K, thus improving the quality of KD 2 PO 4 crystals.

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The neutron diffraction method was adopted to study the three-dimensional residual stress distribution in SiC bulk crystals for the first time.

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Refractive indices of partially deuterated potassium dihydrogen phosphate (DKDP) crystals with 55%, 70% and 80% deuterium contents were measured by auto-collimation method at 293 K between 0.254 to 1.529 μm. Dependence of refractive indices of DKDP on deuterium content show different trend in the infrared region as in uv-visible region. Dependence of n2 (the square of refractive index) on the mole fraction of deuterium shows a difference between pure KDP and partially deuterated KDP. The Sellmeier equations were obtained by the least square method. The non-critical phase matching angles calculated from the fitted formula were in good agreement with laser experiment results, by which the reliability of these Sellmeier equations was confirmed.