## 1 Introduction  
ZnGeP\({}_{2}\) single crystals are unique nonlinear media for parametric frequency conversion of optical radiation in the middle IR range, possessing high nonlinear optical (NLO) properties [1]. In addition, interest in the application of the ZnGeP\({}_{2}\) crystals is additionally stimulated by high potential for their application as tunable terahertz (THz) radiation sources that has not yet been realized [2; 3; 4]. Among the most dangerous defects of nonlinear optical materials that make them unsuitable for their intended purpose, the volumetric inclusions, whose properties differ strongly from the matrix properties, should be mentioned. They destroy the synchronism of the interacting optical waves and hence, decrease the efficiency of optical parametric conversion of pumping radiation into radiation of the signal (or idler) wave practically to zero. At high radiation densities, when the nonlinear effects are becoming significant, even smeared defect boundaries (transition layers) can decrease the conversion efficiency (due to wave phase mismatching) or can become a source of primary electrons forming the electron avalanche that results in optical damage of materials. Given that the technological requirements of semiconductor manufacture are met, the incorporation of inclusions in the form of particles of foreign crystal phases into the crystal volume can be considered unlikely, but the appearance of inclusions in the form of particles of secondary phases formed from the major crystal matrix components seems to be highly likely. Due to the presence of two easily volatile components (Zn and P) that can form stable binary phases in the ZnGeP\({}_{2}\) compound, the initial stage of compound synthesis involves the formation of intermediate binary phases [5; 6] whose subsequent reaction results in the formation of ternary compounds. Since the synthesis proceeds during limited (finite) time of intermediate phase interaction, some intermediate phases have no time to be transformed into the ternary compound, thereby leading to uncontrollable deviations of the composition of the synthesized material from the stoichiometric one. During subsequent single crystal growth from the melt of the preliminary synthesized compound, variations in the composition of the polycrystalline batch and additional losses of volatile components during non-isothermal growth lead to uncontrolled doping of ZnGeP\({}_{2}\) crystals by the intrinsic non-volatile component--mostly byGe and/or by some amount of binary compounds of metal components with phosphorus--GeP and/or Zn\({}_{3}\)P\({}_{2}\). Such doping may cause the formation of defects with sizes up to \(\sim\)10-50 \(\upmu\)m that decrease the optical quality of the material and hence, its potential efficiency. For the examined materials with high refractive indices, and especially for crystals intended for IR radiation nontransparent in the visible range, the determination of the defect parameters turns into a difficult technical problem. Therefore, the development of technical means of control, that is, detection, visualization, measurement, and classification of the volumetric single crystal defects in one measuring experiment is an urgent technical problem. It is important not only from the viewpoint of investigation of intrinsic material defects and physical mechanisms leading to them, but also from the viewpoint of technological control and quality assurance certification of single crystals. Knowledge of the volumetric defect distribution in crystals intended for IR range acquires special significance for production of NLO elements when it is required to choose the crystal region with preset orientation, sizes, and minimal optical absorption. At present, there are different methods of qualitative and quantitative analysis of defects, such as the polarization-optical method, X-ray topography, IR microscopy, shadow method, and so on [7; 8; 9].