This thesis covers the nonlinear process of parametric down-conversion in the x-ray regime. It provides an extensive review of x-ray frequency conversion phenomena - covering theoretical and experimental studies. It focuses specifically on the investigation of parametric down-conversion of x-rays into visible photons. With the aim to identify and investigate the effect’s characteristic scattering signature, an energy-resolved diffraction setup is implemented at different synchrotron sources. The non-linear process is experimentally investigated by a systematic mapping of its parameter space, yet the effect’s anticipated signature is not observed in the measured scattering patterns. Instead, the intensity distributions therein are attributed to regular elastic scattering. This identification is achieved on the basis of instrumental function considerations as known from high-resolution x-ray diffractometry and comparable reciprocal space map studies. Nevertheless, the achieved resolution of the experimental setup allows a new determination of an upper bound of the effect’s conversion efficiency. The results are fully compatible with novel theoretical approaches to x-ray optical wave-mixing based on quantum electrodynamics. These results are in stark contrast to earlier experimental studies that identify similar scattering signals as parametrically generated photons. As such, the methodology developed in this thesis will serve to revisit these reports with increased accuracy and - beyond that - provide the basis for future studies and applications of x-ray nonlinearities. In particular, its conclusions provide a clear path of optimizations to achieve the outstanding proof of parametric down-conversion of x-rays into visible photons.