Large, optically transparent crystals of CdSiP2 (CSP) have been grown for the first time from a stoichiometric melt. The material is a high temperature analog to ZnGeP2 with promising characteristics for IR frequency conversion. Crystals are birefringent and are transparent from 0.5 to 9 mu m. Polycrystalline charges were successfully synthesized from high purity elemental starting materials by two-temperature vapor transport despite the very high equilibrium vapor pressure (similar to 22 atm) at the melting point of CdSiP2 (1133 degrees C). Single crystals were grown using the horizontal gradient freeze (HGF) technique in high-temperature transparent furnaces. Over the course of several growth runs, the material proved to be prone to cracking and to twinning along (1 1 2) planes. Twinning was eliminated by seeded growth along directions normal to the 112 planar boundaries. Further modifications to growth conditions resulted in high optical quality, crack- and twin-free single crystals 70 x 25 x 8 mm(3). The largest CdSiP2 single crystals previously reported in the literature were grown through either halogen assisted vapor transport or from a molten Sn flux and measured 2 x 2 x 0.2 mm(3). The HGF growth of large CdSiP2 crystals has allowed several bulk properties to be measured for the first time, including the thermal expansion coefficients, thermal conductivity, and wavelength dependent birefringence and dispersion. Measurements of the optical and thermal properties reveal this to be an extremely promising material for 1-, 1.5-, and 2 mu m-pumped mid-IR lasers. (C) 2009 Elsevier B.V. All rights reserved.