Form-controlled synthesis of a polymorphic nonlinear optical crystal CsMgPO4·6H2O†

A facile solution method for the form-controlled synthesis of CsMgPO₄·6H₂O is reported in this work and the key factor was determined to be the pH value of the solution used. A pure hexagonal phase (P6₃mc) was obtained at a pH range of 6.9–8.0, whereas a cubic phase (F₃m) was obtained at 9.5–11.3. Compared with the cubic phase, the hexagonal phase of CsMgPO₄·6H₂O possessed a stronger second harmonic generation response of 0.82 pm V⁻¹ due to the distortion of the PO₄ motif from a regular tetrahedron. The form-controlled synthesis mechanism was identified with the aid of infrared spectroscopy: the protonation process reduced the symmetry of the PO₄ motif when the solution pH decreased, resulting in the crystallization of the hexagonal phase of CsMgPO₄·6H₂O with the distorted PO₄ tetrahedron at a lower pH range.