## 3 Experimental Results and Discussion  
In this Section, images of defects of different types observed in the ZnGeP\({}_{2}\) crystal that were recorded by the digital holography method are discussed. Physical interpretation of the results obtained is provided, and they are compared with the results obtained by other methods. The synthesis of the ZnGeP\({}_{2}\) crystalline compound by horizontal single-temperature or double-temperature Bridgeman technique [5; 6; 18], albeit supposes different technical arrangement and regimes, but the reactions running during ZnGeP\({}_{2}\) synthesis and their sequence are the same for all realizations. As shown in [5], the synthesis of the ZnGeP\({}_{2}\) compound involved the stage of forming binary phosphides in different temperature ranges. Thus, for temperatures in the range 480-550 \({}^{\circ}\)C, molten zinc reacts with phosphorus vapor forming the compound with a low content of phosphorus [6]:  
\[3\mathrm{Zn(L)}+\frac{1}{2}\mathrm{P_{4}(V)}\to\mathrm{Zn_{3}P_{2}(S)},\]  
where S, L, and V denote solid, liquid, and vapor phases, respectively. As the phosphorus temperature increases to \(\sim\)550-850 \({}^{\circ}\)C together with its pressure, zinc phosphide with low phosphorus content is transformed into the ZnP\({}_{2}\) compound:  
\[\mathrm{Zn_{3}P_{2}(S)}+\mathrm{P_{4}(V)}\rightarrow\ 3\mathrm{ZnP_{2}(S)}.\]  
The rate of phosphorus atoms diffusion into the solid phosphide cannot be high; therefore, during period of synthesis-reactor heating to temperatures at which the reaction of triple phosphide formation is possible, zinc phosphide Zn\({}_{3}\)P\({}_{2}\) with low' phosphorus content is transformed into ZnP\({}_{2}\) only partially. At a temperature of \(\sim\)750 \({}^{\circ}\)C, germanium phosphide can be formed:  
\[\mathrm{Ge(S)}+\frac{1}{4}P_{4}(V)\rightarrow\mathrm{GeP(L)}.\]  
At temperatures exceeding \(\sim\)900 \({}^{\circ}\)C, the ternary compound synthesis begins with two reactions [7]:  
\[\mathrm{ZnP_{2}(S)}+\mathrm{Ge(L)}\to 3\mathrm{ZnGeP_{2}(S,L)},( \mathrm{volumetric})\]  
\[\mathrm{Zn_{3}P_{2}(S)}+3\mathrm{GeP(V)}+\frac{1}{4}P_{4}(V)\rightarrow\ \mathrm{ZnGeP_{2}(S,L)},( \mathrm{surface})\]  
Unlike the volumetric (two-component) reaction, the surface (ternary-component) reaction requires contact of condensed reagents with the vapor phase limited by the condensate-vapor interface itself, so its rate is significantly less than the rate of the volumetric reaction, and the residual (unreacted) condensed phases--zinc phosphide with low phosphor and germanium contents--can be presented in the final product of the ZnGeP\({}_{2}\) synthesis. These residual phases are found predominantly in the part of the synthesized ingot that contacts poorly (or does not contact at all) with the vapor phase. These impurities from the intrinsic components of the compound form volumetric needle-shaped inclusions [19] with typical transverse size of \(\sim\)10 \(\upmu\)m and length up to several millimeters. Figures 3 and 4 vividly illustrate the presence of the above-discussed inclusions in the material. The second phase inclusions--the needle-shaped phosphide crystals up to \(\sim\)500 \(\upmu\)m long and up to \(\sim\)15 \(\upmu\)m in diameter--were also detected in the experiment (Figure 4). Analogous second-phase inclusions of the intrinsic components were observed in [9] by the method of optical transmission  
microscopy of thin ZnGeP\({}_{2}\) plates with thickness no more than 1 mm cut along the growth axis. They were also present on X-ray topograms [9]. The total composition of elementary components in the crystal is assumed stoichiometric, excluding pre-designed deviations for scientific research purposes. The appearance of zinc phosphide inclusions automatically raises a question: where and in what form is the part of Ge batch unreacted with zinc phosphides distributed?