Magnetic compounds with spinel structure play an important role in engineering of magnetic storage devices based on new principles. Study of these materials is necessary for understanding of strongly correlated magnetic, optical and electronic properties, which manifest exotic phenomena such as colossal magnetoresistance [1,2] and multiferroicity [3,4].  Within this work we report the influence of type of the transport agent and the cation substitution on the relationship between the inter-ion distances, lattice parameters and magnetic behavior of magnetic spinel system Fe1-xCuxCr2S4. According to the theory, spinel structure can be considered as having nearly compact packing, which allows to establish the geometric relationship between the lattice parameter (α0) of a cubic elementary cell and inter-ion distances [5]. Thus, using experimental data for α0 and anionic positional parameter (u), obtained from the X-ray powder diffraction it is possible to calculate the distances between the tetrahedral cations and anions (A-X distances). For example, for initial compound FeCr2S4, the calculated A-X distance is 239.2 pm while sum of ion radius is 260 pm. The difference between the calculated distance and sum of ion radius is an evidence of mixt ionic-covalent chemical bonds in Fe1-xCuxCr2S4 system. We found that the covalence becomes more intense with increasing of the substitution.  A comparison between the properties of the poly- (PC) and single crystalline (SC) samples of Fe1-xCuxCr2S4 obtained previously in Ref. [6], and samples with the same substitution level grown by the chemical transport reactions method using bromines as a transport agent, was done Fig.1 documents the increase of the value of the magnetic transition temperature TC with increasing of the copper concentration x. As it was pointed in Ref. [6], for SC samples, the chlorine transport agent penetrates in Fe1-xCuxCr2S4 compounds into the sulfur positions. For resolving this problem it was proposed to change Cl- (ionic radius 181 pm) by Br- (196 pm). The change of the transport agent of chlorine by bromine allowed to achieve high value of TC, close to that of the polycrystalline samples. Therefore, the difference between the TC for the SC samples, grown by different type of transport agent, can be attributed to the Cl impurity. In comparison with S, chlorine has one more electron which makes modifies the magnetic superexchange interactions in these materials.