A Complex Spin-Transition Thermal Hysteresis Unlocked by Two Competing Symmetry-Breakings
Abstract
The spin-crossover material [Fe(NH2trz)3](NO3)2 belongs to the class of 1D triazole-based polymeric SCO complexes. It exhibits a thermal phase transition, with a 28 K wide thermal hysteresis, characterized by phase transition temperatures T↑ = 346 K upon warming and T↓ = 318 K upon cooling. This hysteretic thermal phase transition is unusual because it is associated with two competing symmetry-breaking phenomena: a ferroelastic distortion upon warming from the low-spin (LS) trigonal phase to the high-spin (HS) triclinic phase, and a unit cell tripling upon cooling from the HS phase to the LS phase. Magnetic, spectroscopic and X-ray diffraction measurements reveal a clear coupling between the spin transition and the two competing symmetry changes. Since there is no group-subgroup relationship between the LS and HS phases, the phase transition can only be discontinuous. However, the detailed analysis shows that the situation is more complex, since both LS and HS phases are two daughter phases of a parent high-symmetry phase. The phase transitions are then rationalized within the frame of our theoretical model based on the Landau theory of phase transitions, which considers the coupling of the spin transition to both types of competing symmetry-breakings between the LS and HS phases. The theoretical approach reproduces the experimental findings and explains the origin of this hysteretic spin transition.
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BibTeX
@article{alashoor2026complex,
title = {{A Complex Spin-Transition Thermal Hysteresis Unlocked by Two Competing Symmetry-Breakings}},
author = {Maryam Alashoor and Ricardo Guillermo Torres Ramírez and Olaf Stefanczyk and Kazuki Nakamura and Francisco Javier Valverde-Muñoz and Elzbieta Trzop and Ivan da Silva and Shin-Ichi Ohkoshi and Arnaud Grosjean and Nathalie Daro and Philippe Guionneau and Laurent Guérin and Eric Collet},
journal = {Inorganic Chemistry},
year = {2026},
month = aug,
doi = {10.1021/acs.inorgchem.6c02968},
url = {https://hal.science/hal-05743362v1},
note = {CC BY-NC-ND 4.0},
}
RIS
TY - JOUR
TI - A Complex Spin-Transition Thermal Hysteresis Unlocked by Two Competing Symmetry-Breakings
AU - Maryam Alashoor
AU - Ricardo Guillermo Torres Ramírez
AU - Olaf Stefanczyk
AU - Kazuki Nakamura
AU - Francisco Javier Valverde-Muñoz
AU - Elzbieta Trzop
AU - Ivan da Silva
AU - Shin-Ichi Ohkoshi
AU - Arnaud Grosjean
AU - Nathalie Daro
AU - Philippe Guionneau
AU - Laurent Guérin
AU - Eric Collet
PY - 2026
DA - 2026/08/10
JO - Inorganic Chemistry
DO - 10.1021/acs.inorgchem.6c02968
UR - https://hal.science/hal-05743362v1
AB - The spin-crossover material [Fe(NH2trz)3](NO3)2 belongs to the class of 1D triazole-based polymeric SCO complexes. It exhibits a thermal phase transition, with a 28 K wide thermal hysteresis, characterized by phase transition temperatures T↑ = 346 K upon warming and T↓ = 318 K upon cooling. This hysteretic thermal phase transition is unusual because it is associated with two competing symmetry-breaking phenomena: a ferroelastic distortion upon warming from the low-spin (LS) trigonal phase to the high-spin (HS) triclinic phase, and a unit cell tripling upon cooling from the HS phase to the LS phase. Magnetic, spectroscopic and X-ray diffraction measurements reveal a clear coupling between the spin transition and the two competing symmetry changes. Since there is no group-subgroup relationship between the LS and HS phases, the phase transition can only be discontinuous. However, the detailed analysis shows that the situation is more complex, since both LS and HS phases are two daughter phases of a parent high-symmetry phase. The phase transitions are then rationalized within the frame of our theoretical model based on the Landau theory of phase transitions, which considers the coupling of the spin transition to both types of competing symmetry-breakings between the LS and HS phases. The theoretical approach reproduces the experimental findings and explains the origin of this hysteretic spin transition.
SN - 0020-1669
SN - 1520-510X
ER -