The obtaining of new single crystals for detection of thermal neutrons on the base of lithium dihydrophosphate LiH₂PO₄ (LDP) doped with Eu³⁺ is topical for neutron physical investigations and radiation monitoring.

However, the growth of LDP crystals from the solutions is bound up with certain difficulties. As established earlier while studying the solubility of the system Li₂O–P₂O₅–H₂O, at temperatures higher than 20°С LDP dissolves incongruently. However, some researches have managed to grow  the crystals from the solutions with excessive content of phosphoric acid. In the present work we studied the influence of the concentration of phosphoric acid in the mother liquor on the solubility and crystallization of LDP.

The temperature dependences of LDP solubility in aqueous solutions of H₃PO₄ with the concentrations 20-; 40-; 50-; 60-; 75- and 85%, were investigated by the method of test seeds.  In the interval  20÷80⁰С the solubility of LDP was found to change insignificantly  (not more than by  5 mass %), irrespective of the percentage of the acid. There were obtained the isotherms of LDP solubility in aqueous  H₃PO₄ solutions with different concentrations of the acid. It was established that with the rise of the percentage of the acid from 20 to 85 mass % the solubility diminishes (at  50⁰С it changes from  53 to 16 mass %). This can be used for creating supersaturation in the solution and for the growth of the crystals.

When the supersaturation is achieved by evaporation of the solvent from the solution,  the volume of the solvent and the concentration of the acid in the solution change simultaneously. As the numerical estimations show, in this case it is possible to raise the output of LDP crystal by five times in comparison with the growth by the method of  temperature lowering.

There were studied conditions of LDP crystallization from aqueous solutions of phosphoric acid of different concentrations. It was found that with the rise of the concentration of  H₃PO₄ acid in the solution the growth of the faces of the pyramid  {11ī} and the dome {01ī} was suppressed, whereas the face of the pedestal {00ī} continued to grow. The full-faced form of the crystal habitus was achieved for the samples obtained from the solutions containing 20 – 40 % H₃PO₄ (0.6 >рН> 0.04).

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