Form-controlled synthesis of a polymorphic nonlinear optical crystal CsMgPO4<middle dot>6H2O

A facile solution method for the form-controlled synthesis of CsMgPO4<middle dot>6H(2)O is reported in this work and the key factor was determined to be the pH value of the solution used. A pure hexagonal phase (P6(3)mc) was obtained at a pH range of 6.9-8.0, whereas a cubic phase (F(3)m) was obtained at 9.5-11.3. Compared with the cubic phase, the hexagonal phase of CsMgPO4<middle dot>6H(2)O 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<middle dot>6H(2)O with the distorted PO4 tetrahedron at a lower pH range.