Recent studies have identified halide vacancies as critical defects affecting catalytic activity in analogues systems. However, in our previous work, we have noticed that the Cs2AgBiCl6 perovskite alone produces a very low quantity of NH3 while Cs2SnBr6 results to be ⁓5 time more active. In contrast to Cs2AgBiCl6, Cs2SnBr6 does not show effective self-trapping exciton (STE) state (see SI), and in addition shows a different defect energetics. The enhanced performance could be attributed to the better alignment of the band edges, particularly with respect to the transition ionization level (+/0) associated with the bromide vacancy found to be -0.067 V (NHE) very close and slightly above to the redox potential for N2/NH3 reported as dashed line in Figure . Due to the structure of the vacancy ordered perovskite, the electron is found to be localized in the superficial octahedra from where the bromide was removed to generate the defect. Our calculations predict this (+/0) transition level at 1.98 eV above the valence band maximum. The analysis of the charge density for this defect shows the presence of a localized electron that could in principle be transferred to the nitrogen starting the mechanism of photofixation. As we mentioned before, another important feature of Cs2SnBr6 is the lack of self-trapped electrons, visible both from photoluminescence spectra (Figure ) as well as from theoretical calculations reported in the SI which shows the absence of emission from the distorted octahedral. This absence of STE removes extra non-radiative recombination channels, thus increasing the charge separation and the possible transfer to the defects active in the plausible mechanism of the ammonia generation. When we move to analyze the g-C3N4 material, we refer to already reported data by our group. From the energy alignment reported in Figure we can see that the g-C3N4/Cs2SnBr6 heterointerface provides the possibility of the electron injection upon photoexcitation from the perovskite conduction band (CB) to the nitrogen vacancy (VN) of g-C3N4 promoting the electron trapping responsible of the initial N2 reduction. At the same time, the electron can be trapped also by the bromide vacancy introducing additional reactive sites. To further verify the electronic structure of the heterointerface, we simulate the interacting g-C3N4/Cs2SnBr6 system reported in Figure , and we found that the interaction between perovskite and the carbon nitride takes place by the formation of N-Sn bond with a formation energy of 91 eVnm -2 confirming a very favorable interface formation tendency. In term of the electronic properties the g-C3N4/Cs2SnBr6 interface at PBE0 (see SI), shows the same qualitative behavior of the energy level alignment reported in Figure at HSE06-SOC level confirming the proposed electron transfer mechanism. The evolution of the NH3 production with respect to the system composition results similar to that we found for Cs2AgBiCl6 but, in general, is all up-shifted in terms of absolute NH3 production. This in such way confirms that the perovskite alone already is active in producing NH3 due to the favorable defect energy alignment, and then, when g-C3N4 is added, the Cs2SnBr6 is going to play a double role: (i) as reactive site and (ii) as light adsorber and charge generation. This aspect in conjunction with absence of STE state and the reduction of non-radiative decay lead to a higher ammonia production rate. Once the carbon nitride is coupled with Cs2SnBr6 the ammonia generation increase is attributed to the additional charge generation upon photoexcitation that enables a better charge transfer to the nitrogen vacancy of the carbon nitride further promoting catalytic activity. On the other hand, when the quantity of perovskite is very low or absent, the carbon nitride itself does not generate a relevant amount of ammonia. As a matter of fact, the NH3 production mechanism promoted by Cs2SnBr6/g-C3N4 heterojunction results similar to that of Cs2AgBiCl6/g-C3N4 but the strategy employed here to boost the ammonia production in this novel MHP-based heterojunction was confirmed and, from a microscopic point of view, is due, mainly, to a combination of different factors such as the absence of STE state in the perovskite that reduce charge losses, and to the active role of perovskite itself in offering efficient reactive site centered at the interfacial bromide vacancy.