Non‐Centrosymmetric Ammonium Rare Earth Nitrates (NH 4 ) 2 Ln(NO 3 ) 5 ·4H 2 O – Crystal Structure, Crystal Growth and Optical Properties

For crystals of (NH 4 ) 2 Ln(NO 3 ) 5 · 4H 2 O (Ln = La, Ce, Pr, Nd), the piezoelectric and the pyroelectric effect was demonstrated unambiguously. X‐ray structure determination shows that all four compounds are isomorphic, with monoclinic, non‐centrosymmetric space group Cc , and are further isomorphic to the related rubidium compounds Rb 2 Ln(NO 3 ) 5 · 4H 2 O. Large single crystals of the colourless compounds (NH 4 ) 2 La(NO 3 ) 5 · 4H 2 O and (NH 4 ) 2 Ce(NO 3 ) 5 · 4H 2 O were grown. On the basis of determined precise refractive indices and their dispersion in the wavelength region 0.365–1.083 μm, phase‐matching conditions for collinear second harmonic generation were analyzed. In both crystals, type I phase matching can be realized for wavelengths from 0.893 μm (La) or 0.892 μm (Ce) to the near IR, where first absorption bands appear.