Open AccessSub-micrometer particle size effects on metastable phases for a photoswitchable Co–Fe Prussian blue analog
Author(s) -
M Itoi,
Isabelle Maurin,
Kamel Boukheddaden,
Matthew J. Andrus,
Daniel R. Talham,
Erik Elkaı̈m,
Yoshiya Uwatoko
Publication year - 2022
Publication title -
journal of applied physics
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 0.699
H-Index - 319
eISSN - 1089-7550
pISSN - 0021-8979
DOI - 10.1063/5.0074165
Subject(s) - powder diffraction , paramagnetism , metastability , particle size , spin crossover , materials science , analytical chemistry (journal) , diamagnetism , chemistry , crystallography , phase (matter) , condensed matter physics , physics , organic chemistry , quantum mechanics , magnetic field , chromatography
Metastable phases of the photoswitchable molecular magnet K 0.3 Co[Fe(CN) 6 ] 0.77 ⋅ nH 2 O in sub-micrometer particles have been structurally investigated by synchrotron powder x-ray diffraction (PXRD) measurements. The K 0.3 Co[Fe(CN) 6 ] 0.77 ⋅ nH 2 O bulk compound (studied here with a sample having average particle size of 500 nm) undergoes a charge transfer coupled spin transition (CTCST), where spin configurations change between a paramagnetic Co II ( S = 3/2) –Fe III ( S = 1/2) high-temperature (HT) state and a diamagnetic Co III ( S = 0) –Fe II ( S = 0) low-temperature (LT) state. The bulk compound exhibits a unique intermediate (IM) phase, which corresponds to a mixture of HT and LT spin states that depend on the cooling rate. Several hidden metastable HT states emerge as a function of thermal and photo stimuli, namely: (1) a quench (Q) state generated from the HT state by flash cooling, (2) a LTPX state obtained by photoexcitation from the LT state derived by thermal relaxation from the Q state, and (3) an IMPX state accessed by photo-irradiation from the IM state. A sample with a smaller particle size, 135 nm, is investigated for which the particles are on the scale of the coherent LT domains in the IM phase within the larger 500 nm sample. PXRD studies under controlled thermal and/or optical excitations have clarified that the reduction of the particle size profoundly affects the structural changes associated with the CTCST. The unusual IM state is also observed as segregated domains in the 135 nm particle, but the collective structural transformations are more hindered in small particles. The volume change decreases to 2%–3%, almost half the value found for 500 nm particles (5%–8%), even though the linear thermal expansion coefficients are larger for the smaller particles. Furthermore, photoexcitation from the IM and LT states does not turn into single phases in the smaller particles, presumably because of the multiple interfaces and/or internal stress generated by the coexistence of small Co II –Fe III and Co III –Fe II domains in the lattice. Since the reduced particle size limits cooperativity and domain growth in the lattice, CTCST in the small particle sample becomes less sensitive to external stimuli.