Elementary building blocks for the growth of KDP crystals were established. The solution of potassium dihydrogen phosphate (KH2PO4–KDP) has been experimentally studied by the small-angle X-ray scattering (SAXS) method. The analysis of SAXS data in the temperature range of 2.5–90 °C using a set of models of 3D fragments of the crystal structure showed that the saturated solution contains above K⁺, H₂PO₄⁻ and KH₂PO₄ monomers, as well as mainly octamers. The 3D model of the octamer isolated from the crystal structure has dimensions of 17.443 Å along the [001] axis and 5.963 Å along the [100] and [010] axes. As the temperature is decreased, starting from the saturation temperature of the solution, the volume fraction of octamers sharply increases while the volume fraction of monomers decreases. The results indicate that the monomers and octamers represent major components in the solution with the presence of minor populations of other oligomers. The significant dominance of octamers in the supersaturated solution indicates that they are elementary building blocks for the growth of KDP crystals of tetragonal modification.

The structure of an aqueous solution of potassium dihydrogen phosphate (KH₂PO₄, KDP) was studied by small-angle X-ray scattering (SAXS) and molecular dynamics (MD). According to SAXS data, the octameric species (KH₂PO₄)₈ are formed in solution in addition to K⁺, (H₂PO₄)⁻, and KH₂PO₄, while the presence of other types of oligomers is not observed. When the temperature drops below the saturation temperature (~60 °C), the volume fraction of octamers increases sharply, reaching 50% at 4 °C. The results of MD calculations of the temporal stability relationships of dimers (KH₂PO₄)₂, tetramers (KH₂PO₄)₄, and octamers (KH₂PO₄)₈ show that the dimers and tetramers disintegrate rapidly (50–100 ps), while the octamers remain stable. A comparative analysis of the bonds between the octamers and the KDP crystal lattice was carried out when the octamer was inserted during crystal growth in the directions [001] and [100]. The possible relationship of the obtained results with the changes in the anisotropy of growth rates (habitus) of KDP crystals at different degrees of supersaturation is discussed.

A sample cell with increased efficiency compared to standard quartz capillaries for studying the structure of solutions by small-angle X-ray scattering (SAXS) has been developed and tested. The circular cross-section of the standard quartz capillary leads to a decrease in the effective aperture and the appearance of additional parasitic scattering. A special feature of the cell design is the presence of plane–parallel X-ray transparent windows, which provide a significant improvement in the signal-to-noise ratio of SAXS data compared to data obtained using standard imported capillaries. The cell design includes at least two identical microliter cavities for samples, which allows comparing the measured object with a reference solution in one experiment or using solutions with different chemical compositions (including concentrations). Test measurements for a standard capillary and the proposed cell were carried out, showing a significantly more isotropic scattering pattern when using the cell. Its advantages are the reusable cell design and replacement of imported products (quartz capillaries). The cell has been successfully tested for studying crystallization solutions of potassium dihydrophosphate and lysozyme protein at different temperatures.