The key role of vibrational entropy in the phase transitions of dithiazolyl-based bistable magnetic materials

Sergi Vela, Fernando Mota, Mercè Deumal, Rie Suizu, Yoshiaki Shuku, Asato Mizuno, Kunio Awaga, Motoyuki Shiga, Juan J Novoa () and Jordi Ribas-Arino ()
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Sergi Vela: Departament de Química Física and IQTCUB, Universitat de Barcelona, Avinguda Diagonal 645
Fernando Mota: Departament de Química Física and IQTCUB, Universitat de Barcelona, Avinguda Diagonal 645
Mercè Deumal: Departament de Química Física and IQTCUB, Universitat de Barcelona, Avinguda Diagonal 645
Rie Suizu: Chiba University, 1-33, Yayoi-cho, Inage-ku
Yoshiaki Shuku: Research Center for Materials Science, Nagoya University, Furo-cho, Chikusa-ku
Asato Mizuno: Research Center for Materials Science, Nagoya University, Furo-cho, Chikusa-ku
Kunio Awaga: JST-CREST, Nagoya University, Furo-cho, Chikusa-ku
Motoyuki Shiga: Center for Computational Science and E-Systems, Japan Atomic Energy Agency, 5-1-5, Kashiwanoha, Kashiwa
Juan J Novoa: Departament de Química Física and IQTCUB, Universitat de Barcelona, Avinguda Diagonal 645
Jordi Ribas-Arino: Departament de Química Física and IQTCUB, Universitat de Barcelona, Avinguda Diagonal 645

Nature Communications, 2014, vol. 5, issue 1, 1-9

Abstract: Abstract The neutral radical 1,3,5-trithia-2,4,6-triazapentalenyl (TTTA) is a prototype of molecule-based bistable materials. TTTA crystals undergo a first-order phase transition between their low-temperature diamagnetic and high-temperature paramagnetic phases, with a large hysteresis loop that encompasses room temperature. Here, based on ab initio molecular dynamics simulations and new X-ray measurements, we uncover that the regular stacking motif of the high-temperature polymorph is the result of a fast intra-stack pair-exchange dynamics, whereby TTTA radicals continually exchange the adjacent TTTA neighbour (upper or lower) with which they form an eclipsed dimer. Such unique dynamics, observed in the paramagnetic phase within the whole hysteresis loop, is the origin of a significant vibrational entropic gain in the low-temperature to high-temperature transition and thereby it plays a key role in driving the phase transition. This finding provides a new key concept that needs to be explored for the rational design of novel molecule-based bistable magnetic materials.

Date: 2014
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DOI: 10.1038/ncomms5411

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