Iurchenko A. Influence of the impurity composition and stoichiometry of the solution on the growth and properties of lithium dihydrogen phosphate single crystals

Thesis for degree of Candidate of Technical Sciences (Ph.D.) in specialty 05.02.01. – Material Science. – Institute for Single Crystals of NAS of Ukraine, Kharkiv, 2017. Thesis is devoted to the development of a method for the growth of LiH2PO4 single crystals from viscous non-stoichiometric solutions, as well as to the study of the properties of pure and doped LDP crystals. The physico-chemical and hydrodynamic properties of viscous non-stoichiometric solutions of LiH2PO4 have been studied, and a method for growing oriented crystals from such solutions has been proposed. The crystals were grown on a seed, oriented to the crystallographic plane (110), by pulling from the solution based on 20% H3PO4, using solvent evaporation method at 40 °C. The mass transfer was achieved by rotating the seed holder at a speed of 70-90 rpm in reverse mode and by location of the growth front of the crystal in the near-surface zone of the excessive saturation of the solution. Pure and doped with salicylic, 5-aminosalicylic acid, and thallium ions, LDP crystals, with the orientation of the growth plane (110) were obtained. The influence of the stoichiometry of the solution, as well as the dopant additive on habit, surface morphology and optical properties of LDP crystals were studied. Its mechanical properties have been analyzed, and the reverse indentation size effect has been observed on the natural facets (110) and (111) of the LDP crystal. It is shown that reverse ISE is associated with tensile surface stress. It is determined that: the hardness of LDP crystals, which is 2337 MPa, is 1.5 times higher than reported for KDP and ADP crystals; the fracture toughness of LDP crystals is 0.07 MPa?m1/2, which is 3-4 times lower than for reported KDP crystals due to the greater length of hydrogen bonds in the lattice of LDP crystals. It is shown that LDP crystals grow by dislocation mechanism. When embedding in the interspace of the crystal, the salicylic acid molecules are located at the ends of the growth steps corresponding to the planes (111) and (011) adjacent to the plane (110). During adsorption on the planes (011) and (111), molecules undergo partial and complete dissociation, correspondingly, which may be due to different local polarity of the faces. It is shown that some of the salicylic acid molecules can form hydrogen bonds with dihydrogen phosphate anions of faces {111} during adsorption. Key words: single crystal, lithium dihydrogen phosphate, crystal growth, crystal growth method, doped crystals, mechanical properties, fluorescence.