Sub-micrometer particle size effects on metastable phases for a photoswitchable Co–Fe Prussian blue analog
Sub-micrometer particle size effects on metastable phases for a photoswitchable Co–Fe Prussian blue analog
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DOI：
10.1063/5.0074165
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发表时间：
2022-02
影响因子：
3.2
通讯作者：
M. Itoi;I. Maurin;K. Boukheddaden;M. J. Andrus;D. Talham;E. Elkaim;Y. Uwatoko
中科院分区：
文献类型：
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作者：
M. Itoi;I. Maurin;K. Boukheddaden;M. J. Andrus;D. Talham;E. Elkaim;Y. Uwatoko
采用同步加速器粉末x射线衍射（PXRD）对光开关分子磁体K0.3Co[Fe(CN)6]0.77⋅nH2O在亚微米粒子中的亚稳相进行了结构研究。K0.3Co[Fe(CN)6]0.77⋅nH2O块体化合物（以平均粒径为500 nm的样品为研究对象）发生电荷转移耦合自旋跃迁（CTCST），自旋构型在顺磁性CoII (S = 3/2) -FeIII （S = 1/2）高温（HT）态和反磁性CoIII (S = 0) -FeII （S = 0）低温（LT）态之间变化。体化合物表现出独特的中间相（IM），对应于依赖于冷却速率的高温和低温自旋态的混合物。几个隐藏的亚稳态HT状态作为热和光刺激的函数出现，即：(1)闪冷由HT状态产生的猝灭(Q)状态，(2)由Q状态产生的热松弛产生的LT状态通过光激发获得的LTPX状态，以及(3)由IM状态通过光照射获得的IMPX状态。研究了一个粒径较小的样品（135 nm），在较大的500 nm样品中，颗粒处于IM相的相干LT域的尺度上。在受控的热激发和/或光激发下的PXRD研究表明，颗粒尺寸的减小深刻地影响了CTCST相关的结构变化。在135nm的粒子中也观察到不同寻常的IM态，但在小颗粒中集体结构转变更容易受到阻碍。体积变化减小到2%-3%，几乎是500纳米颗粒（5%-8%）的一半，尽管较小的颗粒的线性热膨胀系数较大。此外，来自IM态和LT态的光激发不会在较小的颗粒中变成单相，这可能是由于晶格中CoII-FeIII和CoIII-FeII小畴共存所产生的多个界面和/或内应力。由于减小的颗粒尺寸限制了晶格中的协同性和畴生长，小颗粒样品中的CTCST对外部刺激的敏感性降低。
Metastable phases of the photoswitchable molecular magnet K0.3Co[Fe(CN)6]0.77 ⋅ nH2O in sub-micrometer particles have been structurally investigated by synchrotron powder x-ray diffraction (PXRD) measurements. The K0.3Co[Fe(CN)6]0.77 ⋅ nH2O 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 CoII ( S = 3/2) –FeIII ( S = 1/2) high-temperature (HT) state and a diamagnetic CoIII ( S = 0) –FeII ( 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 CoII–FeIII and CoIII–FeII 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.