Growth of pure crystalline carbon nitride (c-CN) with crystal sizes large enough to enable measurement of its properties has not been achieved so far. We report here that incorporation of silicon in the growth of CN can promote formation of large, well faceted crystallites. Crystalline thin films of SiCN have been grown by microwave plasma-enhanced chemical vapor deposition using CH4, N2, and SiH4 gases. Auger electron spectroscopy, scanning electron microscopies, and X-ray diffraction spectroscopy have been employed to characterize the composition, the morphology and the structure of the films. The new crystalline ternary compound (C; Si)xNy exhibits hexagonal structure and consists of a network wherein the Si and C are believed to be substitutional elements. While the N content of the compound is about 35%, the extent of Si substitution varies from crystal to crystal. In some crystals, the Si content can be as low as 10%. Optical properties of the SiCN compounds have been studied by photoluminescence (PL) and piezoreflectance (PzR) spectroscopies. From the PzR measurement, we determine the band gap of the new crystals to be around 3.8 eV at room temperature. From the PL measurement, it is found that the compounds have a strong subband-gap emission centered around 2.8 eV at room temperature, which can be attributed to the effect of defects containing in the crystals.