Manipulating electron redistribution to achieve exotic electronic pyroelectricity in dynamic [FeCo] crystals


 Pyroelectricity plays a crucial role in modern sensors and energy conversion devices. However, obtaining materials with large and nearly constant pyroelectric coefficients over a wide temperature range for the practical uses remains a formidable challenge, because in conventional ferroelectric materials the pyroelectric effect promptly declines upon cooling from the transition temperature. Attempting to discover a solution to this obstacle, we combined molecular design of labile electronic structure with the crystal engineering of the molecular orientation in lattice resulting in the electronic pyroelectricity of purely molecular origin. Here, we report a polar crystal of an [FeCo] dinuclear complex exhibiting a peculiar pyroelectric behavior (a substantial sharp pyroelectric current peak and an unusual continuous pyroelectric current at higher temperatures) which is caused by a combination of Fe spin transition and redistribution of electron density between redox isomers of high-spin Fe through a charge transfer between the Fe atom and redox active ligand. As a result, temperature dependence of the pyroelectric behavior reported here is opposite and originates from the interconversion between three distinct electronic states. The obtained pyroelectric coefficient is comparable to that of polyvinylidene difluoride at room temperature.

system with a total spin quantum number (S) of 2. A gradual decrease to 3.00 cm 3 K mol −1 is then observed upon cooling down to 120 K. On further cooling, the mT value abruptly decreases at approximately 90 K, reaching to 0.15 cm 3 K mol −1 at 5 K, indicating that the majority of the molecules populate a diamagnetic state (S = 0). The ligand field around the Fe center is known to be appropriate for the spin crossover in Fe 3+ ion, therefore, the observed behavior can be inferred as the occurrence of the spin transition between the strongly antiferromagnetically (AF) coupled Fe 3+ Pyroelectric property. The complex 1(PF6)3 crystallizes in the polar space group, enabling the observation of the pyroelectric effect from the peculiar change in the electronic structure upon varying the temperature. To unambiguously determine the direction of the pyroelectric current with respect to the single crystal, a plate-like piece of complex 1(PF6)3 crystal was indexed with silver paste attached to one side of the crystal surface and carbon paste attached to the parallel surface and the faces (010) and (0−10) were distinguished. The sign of the pyroelectric coefficient formally corresponds to the electron transfer from dhbq to Fe in the whole temperature range upon heating. Below 60 K, only a weak current signal is detected upon heating, corresponding to a pyroelectric coefficient of less than 0.3 nC cm −2 K −1 (Fig. 4a). At a temperature approaching the abrupt transition point, the pyroelectric current exhibits a sharp increase, giving a peak as high as 30 nC cm −2 K −1 , and then decreases to ca. 3.8 nC cm −2 K −1 after 120 K. This is clearly associated with the redistribution of electronic density in the [FeCo] complex in a narrow temperature range during the spin transition process. Integration over this temperature domain gives a polarization change of approximately 0.4 μC cm −2 (Fig. 4b). Upon further heating to room temperature, it is surprising to find that the pyroelectric current is still detected, and the pyroelectric coefficient remains at approximately 3.4 nC cm −2 K −1 at 300 K, which is as large as that of polyvinylidene difluoride (PVDF) 38 . However, the pyroelectric property is known to be inherent to materials with polar structures. In the case of 1(PF6)3, the pyroelectric behavior could stem not only from the possible intramolecular electron transfer but also from a change in the strain of the crystal sample. To discriminate between these two contributions and to exclude the possibility that the pyroelectric current above the spin transition originates from a secondary pyroelectric effect due to the thermal deformation of the polar crystal, we measured the pyroelectric property of an isostructural  Table 5) as a reference material without charge transfer behavior. The pyroelectric coefficient of the [ZnCo] complex was found to be much smaller (< 15%) than that observed for 1(PF6)3 in the whole temperature range, supporting that the large pyroelectric coefficient of 1(PF6)3 mainly originates from a continuous electronic redistribution in the compound.

Discussion
Further in-depth spectroscopic and theoretical investigation was carried out in pursuit of the origin of the exotic and unusual pyroelectric behavior in terms of the electronic structure determination at different temperature regime.
The electronic structures of this system in the LT phase were examined through a combination of theoretical and variable-temperature spectroscopic studies 30 . Infrared (IR) spectra recorded below 70 K exhibit no significant change in the peak positions and intensities. The strong absorption band at 1485 To understand the nature of the observed transition behavior and gain insight into the electronic structure at the HT phase, the Fe XANES measurements were performed at 250 K. The energy of the absorption edge threshold is consistent with Fe 3+ HS ion (Fig. 2a). However, the relative intensities of the pre-edge peaks are inconsistent with quadrupole allowed transitions expected for octahedral Fe 3+ HS ion (Fig. 5c). The lower energy peak is much less intense than the higher energy peak, which is inconsistent with the 3:2 peak ratio expected for Fe 3+ HS with a 5 A1g (t2g 3 eg 2 ) ground state and 5 T2 (t2g 4 eg 2 ) and 5 E (t2g 3 eg 3 ) final states. The pre-edge has 8.  (Fig. 5d). The HERFD-XANES measurements performed at 250 K provides almost identical pre-edge structure to those obtained at 135 K, and the origin of the high-temperature pyroelectricity remained unsettled (Supplementary Fig. 8).
The Mössbauer spectrum recorded above the transition temperature (125 K) affords IS and QS values of 0.69 and 1.83 mm s −1 , respectively (Fig. 7, Supplementary Fig. 9 and Supplementary  43 should also contribute to the electronic structure, which is consistent with the results from XANES measurements at the Fe K-edge. Notably, no distinct peaks are resolved by Mössbauer spectroscopy, which suggests that the rates of electron hopping occurring between Fe and the dhbq ligand are faster than the Mössbauer time window (τ ~10 −8 s).
IR measurements evidence a change in the spectra at the transition temperature range (80-100 K), which consists of an abrupt decrease of the absorption bands around 1329 cm −1 and the appearance of new bands at 1514 and 1296 cm −1 (Fig. 6). This means that the change in the spin state of the at the HT phase (Fig. 6). This scenario is supported by theoretical calculations (Supplementary Fig. 7).
Mössbauer measurements up to higher temperatures also show a gradual increase in the QS and IS values upon heating, providing further evidence of the increasing population on the Fe 2+ HS state with larger QS values at higher temperatures (Fig. 7). However, no well-resolved peaks can be observed during this process, suggesting that the rates of electron hopping process are faster than the Mössbauer time window (τ ~10 −8 s) but slower than the IR time window (τ ~10 −12 s). SC-XRD also provides evidence for the electronic dynamics in 1(PF6)3. As summarized in Supplementary Figs. 2 and 3 which has never been reported and is essential for application development. Note that these pyroelectric properties can be repeatedly observed in the absence of an electric field because the molecular orientation is fixed in the 1(PF6)3 crystal. This contrasts with the properties of ferroelectric compounds, for which application of an electric field to align the polar direction is required beforehand.