Book/Dissertation / PhD Thesis

FZJ-2024-03898

http://join2-wiki.gsi.de/foswiki/pub/Main/Artwork/join2_logo100x88.png Elucidation of Barocaloric Effect in I-4Spin Crossover Compounds

Shahed, H. (Corresponding author)FZJ*

2024
Forschungszentrum Jülich GmbH Zentralbibliothek, Verlag Jülich
ISBN: 978-3-95806-758-5

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Please use a persistent id in citations: doi:10.34734/FZJ-2024-03898

Abstract: The search for new materials for energy-efficient and environmentally friendly refrigerant technologies is a key challenge to replace conventional vapor compression technology. An attractive alternative approach uses the barocaloric refrigeration cycle, which is based on the adiabatic temperature and isothermal entropy change of a material upon tuning an external hydrostatic pressure. Recently, spin crossover (SCO) compounds have been pointed out as promising candidates, which exhibit large barocaloric effects: large isothermal entropy changes have been reported for some of these SCO compounds at fairly low hydrostatic pressures (< 1.2 GPa). In SCO complexes, the central metal ion switches between a low spin (LS) state at low temperature / high pressure and a high spin (HS) state at high temperature/low pressure. The LS to HS transition involves an increase of the spin entropy, but the larger part of the entropy change originates from changes in the intramolecular vibrations. The fundamental understanding of the nature of HS-LS transition and its effect on the physical properties is still being explored fervently, and in particular, the microscopic knowledge of the transition mechanism is essential to tailor new materials. Although the spin crossover is by essence a molecular scale phenomenon, the large diversity in the spin transition characteristics makes it challenging to predict. The spin transition is controlled by cooperativity, which is a parameter that describes to what extent the spin transition occurs collectively across the material rather than in isolated molecules. The exploration of the intricate interplay between cooperativity and structural changes in the SCO material is in the focus of this thesis. The compound Fe(Pm-Bia)2(NCS)2, where Pm-Bia = (N-(21-pyridylmethylene)-4-amino-bi-phenyl), is an ideal candidate for these studies, as it exists in two different polymorphs which exhibit SCO transitions with significantly different characteristics.

Note: Dissertation, RWTH Aachen University, 2024

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Contributing Institute(s):

1. Streumethoden (JCNS-2)
2. JARA-FIT (JARA-FIT)

Research Program(s):

1. 899 - ohne Topic (POF4-899) (POF4-899)

Appears in the scientific report 2024

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