{"status":"ok","message-type":"work","message-version":"1.0.0","message":{"indexed":{"date-parts":[[2026,9,10]],"date-time":"2026-09-10T22:52:50Z","timestamp":1789080770064,"version":"build-2803163510"},"reference-count":46,"publisher":"MDPI AG","issue":"17","license":[{"start":{"date-parts":[[2025,8,25]],"date-time":"2025-08-25T00:00:00Z","timestamp":1756080000000},"content-version":"vor","delay-in-days":0,"URL":"https:\/\/creativecommons.org\/licenses\/by\/4.0\/"}],"funder":[{"name":"European Union\u2014NextGenerationEU","award":["3GPV-4INDUSTRY"],"award-info":[{"award-number":["3GPV-4INDUSTRY"]}]},{"name":"European Union\u2014NextGenerationEU","award":["TAEDR-0537347"],"award-info":[{"award-number":["TAEDR-0537347"]}]},{"name":"European Union\u2014NextGenerationEU","award":["pr017022"],"award-info":[{"award-number":["pr017022"]}]},{"name":"National Infrastructures for Research and Technology S.A.","award":["3GPV-4INDUSTRY"],"award-info":[{"award-number":["3GPV-4INDUSTRY"]}]},{"name":"National Infrastructures for Research and Technology S.A.","award":["TAEDR-0537347"],"award-info":[{"award-number":["TAEDR-0537347"]}]},{"name":"National Infrastructures for Research and Technology S.A.","award":["pr017022"],"award-info":[{"award-number":["pr017022"]}]}],"content-domain":{"domain":[],"crossmark-restriction":false},"short-container-title":["Materials"],"abstract":"<jats:p>Lead-free A2ZrX6 \u201cdefect\u201d perovskites hold significant potential for many optoelectronic applications due to their stability and tunable properties. Extending a previous work, we present a first-principles density functional theory (DFT) study, utilizing PBE and HSE06 functionals, to systematically investigate the impact of A-site cation and X-site halogen substitutions on the structural and electronic properties of these materials. We varied the A-site cation, considering ammonium, methylammonium, dimethylammonium, trimethylammonium, and phosphonium, and the X-site halogen, trying Cl, Br, and I. Our calculations reveal that both these substitutions significantly affect the band gap and the lattice parameters. Increasing A-site cation size generally enlarges the unit cell, while halogen electronegativity directly correlates with the band gap, yielding the lowest values for iodine-containing systems. We predict a broad range of band gaps (from ~4.79 eV for (PH4)2ZrCl6 down to ~2.11 eV for MA2ZrI6 using HSE06). The (PH4)2ZrX6 compounds maintain cubic crystal symmetry, unlike the triclinic of the ammonium-derived systems. Finally, our calculations show that the MA cation yields the smallest band gap among the ones studied, a result that is attributed to its size and the charges of the hydrogen atoms attached to nitrogen. Thus, our findings offer crucial theoretical insights into A2ZrX6 structure\u2013property relationships, demonstrating how A-site cation and halogen tuning enables control over electronic and structural characteristics, thus guiding future experimental efforts for tailored lead-free perovskite design.<\/jats:p>","DOI":"10.3390\/ma18173976","type":"journal-article","created":{"date-parts":[[2025,8,26]],"date-time":"2025-08-26T06:26:14Z","timestamp":1756189574000},"page":"3976","update-policy":"https:\/\/doi.org\/10.3390\/mdpi_crossmark_policy","source":"Crossref","is-referenced-by-count":3,"title":["Tailoring the Electronic and Structural Properties of Lead-Free A2ZrX6 \u201cDefect\u201d Perovskites: A DFT Study on A-Site Cation and Halogen Substitutions"],"prefix":"10.3390","volume":"18","author":[{"ORCID":"https:\/\/orcid.org\/0009-0008-0908-4331","authenticated-orcid":false,"given":"Christina","family":"Kolokytha","sequence":"first","affiliation":[{"name":"Theoretical and Physical Chemistry Institute, National Hellenic Research Foundation, 48 Vassileos Constantinou Ave., GR-11635 Athens, Greece"},{"name":"Laboratory of Physical Chemistry, Department of Chemistry, National and Kapodistrian University of Athens, GR-15784 Zografou, Greece"}],"role":[{"vocabulary":"crossref","role":"author"}]},{"ORCID":"https:\/\/orcid.org\/0000-0003-0899-7282","authenticated-orcid":false,"given":"Demeter","family":"Tzeli","sequence":"additional","affiliation":[{"name":"Theoretical and Physical Chemistry Institute, National Hellenic Research Foundation, 48 Vassileos Constantinou Ave., GR-11635 Athens, Greece"},{"name":"Laboratory of Physical Chemistry, Department of Chemistry, National and Kapodistrian University of Athens, GR-15784 Zografou, Greece"}],"role":[{"vocabulary":"crossref","role":"author"}]},{"ORCID":"https:\/\/orcid.org\/0000-0002-5589-7930","authenticated-orcid":false,"given":"Nektarios N.","family":"Lathiotakis","sequence":"additional","affiliation":[{"name":"Theoretical and Physical Chemistry Institute, National Hellenic Research Foundation, 48 Vassileos Constantinou Ave., GR-11635 Athens, Greece"}],"role":[{"vocabulary":"crossref","role":"author"}]}],"member":"1968","published-online":{"date-parts":[[2025,8,25]]},"reference":[{"key":"ref_1","doi-asserted-by":"crossref","first-page":"391","DOI":"10.1038\/nnano.2015.90","article-title":"Metal-Halide Perovskites for Photovoltaic and Light-Emitting Devices","volume":"10","author":"Stranks","year":"2015","journal-title":"Nat. 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