Tin phosphate glasses are the alternative as lead-free glasses with zero photoeleastic constant (PEC) for fibers that are used in sensors of electric current. Since the binary glass with zero PEC is hygroscopic a third component is introduced, here ZnO. The PEC shows a linear dependence for more than 15 mol% ZnO content but it is constant for <5 mol%. The origin of this non-linear change is the topic of the suggested diffraction experiment on glasses xZnO.(66.7-x)SnO.33.3P2O5. The Zn-O and Sn-O first neighbor distances and coordination numbers will be determined. The planned combination with X-ray diffraction will allow resolving both distances. The broad Sn-O peak with about three oxygen neighbors is a difficult subject and needs good resolution and well defined experimental conditions that are possible on GEM. Binary SnO-P2O5 glasses were already measured by us we did not publish yet.