In order to obtain a new type of organic-inorganic hybrid material for nonl
inear optics, two pyridinium-CdI2 crystals, i.e., bis(4-cyano-1-methylpyrid
inium) tetraiodocadmate (CNP/CdI4) and bis(4-dimethylamino-1-methylpyridini
um) tetraiodocadmeate (DAP/CdI4) were prepared. From X-ray crystal structur
e analyses, the inorganic parts of both of the crystals are found to be the
tetrahedral [CdI4](2-) structure. Although the CNP/CdI4 crystals have a ce
ntrosymmetric layered structure, the DAP/CdI4 crystals do not have layered
structures resulting in non-centrosymmetric space group Cc and second-order
nonlinear optical activity. Thus, the second-harmonic generation (SHG) coe
fficients (d) of the DAP/CdI4 crystal were estimated using the oriented gas
model. In spite of the shorter cutoff wavelength of DAP/CdI4 compared with
that of the well-known 2-methyl-4-nitroaniline (MNA) crystal, the maximum
d value was expected to reach one-third of that of the MNA in the diagonal
component.