Complex Magnetic Phases in Nanosized Core@Shell Prussian Blue

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Complex Magnetic Phases in Nano-Sized Core@Shell Prussian Blue Analogue Cubes: Rb Co[Fe(CN)] [(HO)] ·0.34HO@K Ni[Cr(CN)] [(HO)] ·0.11HO 0.48

6

0.75

2

6

0.25

2

0.36

6

0.74

2

6

0.26

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Chi-Hung Lee, Chun-Ming Wu, Erdembayalag Batsaikhan, Hsiao-Chi Li, Carissa H. Li, Marcus K Peprah, Daniel R. Talham, Mark W Meisel, and Wen-Hsien Li J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.5b07274 • Publication Date (Web): 09 Dec 2015 Downloaded from http://pubs.acs.org on December 18, 2015

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The Journal of Physical Chemistry

Complex Magnetic Phases in Nano-Sized Core@Shell Prussian Blue Analogue Cubes: Rb0.48Co[Fe(CN)6]0.75[(H2O)6]0.25·[email protected][Cr(CN)6]0.74[(H2O)6]0.26·0.11H2O Chi-Hung Lee,† Chun-Ming Wu,‡ Erdembayalag Batsaikhan,† Hsiao-Chi Li,† Carissa H. Li,§ Marcus K. Peprah,# Daniel R. Talham,§ Mark W. Meisel,# and Wen-Hsien Li†,* † Department of Physics, National Central University, Jhongli 32001, Taiwan ‡ Neutron Group, National Synchrotron Radiation Research Center, Hsinchu 30076, Taiwan § Department of Chemistry, University of Florida, Gainesville, Florida 32611-7200, USA # Department of Physics and National High Magnetic Field Laboratory, University of Florida, Gainesville, Florida 32611-8440, USA

ABSTRACT: Different magnetic phases have been identified in nano-sized core/shell Prussian

blue

analogue

(PBA)

cubes,

with

a

250

nm

Rb-Co-Fe

phase

(Rb0.48Co[Fe(CN)6]0.75[(H2O)6]0.25·0.34H2O) in the core coated by a 45 nm K-Ni-Cr phase (K0.36Ni[Cr(CN)6]0.74[(H2O)6]0.26·0.11H2O) on the shell. Three separate characteristic temperatures at 86, 69, and 67 K are associated with magnetic phases in the K-Ni-Cr shell. Two magnetic exchange paths are identified. One propagates along the three crystallographic axis directions. The other propagates along the [110] crystallographic direction for the associated Ni-Ni interactions, but Cr-Cr interactions. The severe Cr-deficiency and the appearance of direct Ni-Ni exchange are used to understand the appearance of two separate transitions associated with magnetic ordering. A weak moment develops in the core at low temperature, corresponding to separate ordering of the Co-Fe PBA network.

*Corresponding author: E-mail: [email protected] (W.H.L.)

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1. INTRODUCTION Fruitful magnetic behaviors, such as light-induced magnetism,1-8 metastable ferrimagnetism,9,10 negative magnetization,11 spin crossover12,13 and spin delocalization13-15 have been identified in the hexacyanoferrate polynuclear complexes known as Prussian blue (PB) and its analogues (PBA). The PBA’s are composed of alternately stacked MN6 and M′C6 octahedra along the three crystallographic axes, and have a general chemical formula of AxM[M′(CN)6]y·nH2O (denoted as A-M-M′), where M and M′ can be divalent or trivalent transition metal ions and A is a monovalent alkali ion that is accommodated in the voids enclosed by the octahedral.17-23 The flexibility to accommodate either divalent or trivalent ions at the M and M′ sites has led to a large family of analogues and applications. For example, the structure builds up three-dimensional open channels to accommodate weakly bonded ions able to migrate through the channels24 and the framework has been exploited as electrode materials for secondary batteries, providing housing for ions to leave the framework during charging and reenter during discharging.25 With respect to magnetic behavior, the cyanide ion bridge forming the connection between the MN6 and M′C6 octahedra is an effective mediator of magnetic exchange and plays decisive roles in the magnetism of the compounds. Furthermore, novel magnetic behavior is seen in analogues when electron transfer that can occur between the MN6 and M′C6 octahedra, thereby tuning the spin states of the M and M′ ions.9,26-28 2


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Recent studies have looked at core-shell particles derived from different PBA’s29-32 and the ability of one phase to influence the response of the other when two materials are connected across an interface. For example, in core-shell particles composed of a Rb-Co-Fe core with either a K-Ni-Cr or K-Co-Cr shell, the magnetic response of the ferromagnetic shell can be altered in response to thermal or optically induced charge transfer events in the Rb-Co-Fe core.29,31,32 It has been shown that the changing magnetic response of the shell is due to a magnetomechanical coupling to the core as structural changes associated with charge transfer in the core apply stress to the shell via the interface.31 Herein, the results of detailed magnetometry and neutron diffraction studies of the magnetic phases of nano-sized core/shell PBA cubes, with a Rb-Co-Fe phase in the core coated by K-Ni-Cr phase on the shell, are reported. An important result of the study is that two magnetic transitions are associated with the magnetic ordering of the K-Ni-Cr phase on the shell, behavior which is shown to be intrinsic to that phase. It is proposed that two superexchange paths for the nearest and the next-nearest magnetic-neighbor interactions in K-Ni-Cr can be used to understand the appearance of the multiple magnetic phases.

2. MATERIALS AND METHODS 2.1. Sample Fabrication.

The core/shell PBA heterostructure with the Rb-Co-Fe 3



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phase at the core and the K-Ni-Cr phase on the shell was synthesized by the co-precipitation method, processed at room temperature. The core particles (Rb-Co-Fe) were prepared by adding a 100 ml aqueous solution containing 0.4 mmol CoCl2·6H2O and 0.8 mmol RbCl into 200 mL of nanopure water simultaneously with a 100 ml aqueous solution containing 0.45 mmol K3Fe(CN)6. The mixture was slowly stirred in an open atmosphere for another 18 hours after mixing to facilitate the reaction. The microcrystalline powder was then isolated from the solution by centrifugation followed by rinsing three times with nanopure water. To synthesize core/shell (Rb-Co-Fe/K-Ni-Cr) particles, the previously prepared core particles were dispersed in 400 ml of nanopure water, followed by simultaneously adding 200 ml of aqueous NiCl2·6H2O (0.76 mmol) and 200 ml of aqueous K3Cr(CN)6 (0.84 mmol) into the Rb-Co-Fe dispersion using a peristaltic pump at a rate of 10 ml/h. The solution was slowly stirred for 18 hour after complete addition. The particles were subsequently filtered in vacuum before being washed and re-dispersed in 100 ml of nanopure water for three cycles. The microcrystalline powder was then isolated by dispersing it in a 50/50 solvent mixture of water and acetone and then allowing it to dry at room temperature. A sample of the single phase K0.1Ni[Fe(CN)6]0.7·nH2O was prepared and characterized according to procedures previously developed.33 A 100 ml aqueous solution of NiCl2·6H2O (0.40 mmol, 4.0 mM) and a 100 ml aqueous solution of K3Cr(CN)6 (0.45 mmol, 4.5 mM) 4


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were simultaneously added to 200 ml nano-pure water under vigorous stirring at a rate of nominally 1ml/min via the use of a peristaltic pump. Vigorous stirring was maintained overnight. The KNiCr-PBA particles were isolated via the centrifugation (104 rpm, 10 min) and were washed with nano-pure water, before being redispersed in nano-pure water and dried under a flow of air. 2.2. Instrumentation. Core-shell particles. X-ray and neutron diffraction were used to characterize the sample. The X-ray diffraction measurements were performed on a Bruker D8 ADVANCE diffractometer, employing the standard setup in reflection geometry. No obvious differences were found in the X-ray diffraction patterns taken from different portions of the sample. The neutron diffraction measurements were conducted at the Bragg Institute, ANSTO, Australia, using the high-intensity powder diffractometer Wombat, employing an incident wavelength of λ = 2.41 Å defined by Ge (115) crystals. For these measurements, ~0.1 g of the sample was loaded into a cylindrical vanadium-can which gave rise to no measureable neutron diffraction peak. The sample temperature was controlled using a liquid He refrigerator system. The magnetization and ac magnetic susceptibility measurements were performed on a Physical Property Measurement System, manufactured by Quantum Design, employing the standard setup. In the ac magnetic susceptibility measurements the sample was subjected to a weak driving ac magnetic field. The 5



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responses of the system were detected using two identical sensing coils connected in opposition. We note that the in-phase component χ′ measures the response of the system to the driving field, while the out-of-phase component χ″ reflects the losses of the driving field to the system. The magnetization was measured by detecting the change in the magnetic flux as the sample was removed from the sensing core. For these measurements, ~60 mg of the sample were loosely packed into a thin nonmagnetic cylindrical holder that produced a smooth temperature curve and a background signal that is ~1% of the signal from the sample. The magnetic structure was determined by neutron magnetic diffraction patterns, covering the temperature regimes where the susceptibility showed an anomaly. K-Ni-Cr single phase particles.

A 13.87 mg sample was measured with commercial

magnetometers, Quantum Design MPMS XL-7 (DC) and MPMS-5S (AC). The sample was loaded into gelatin capsules (size No. 4) and placed in a commercial Dixie drinking straw. The straw was then mounted on the standard MPMS transport rod for data acquisition in the magnetometer. The protocol involved cooling in a 100 Oe applied field from 300 to 10 K at a rate of 10 K/min. At the base temperature of 10 K, the sample was re-centered after which data was collected up to 90 K while the warming rate between points was a maximum of 10 K/min. AC susceptibility data were acquired at frequencies of 1, 10, and 100 Hz in an AC field of 1 Oe.

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3. RESULTS AND DISCUSSION 3.1. Crystalline Structure.

It is known that Prussian blue and many analogues

crystallize with cubic ‫݉ܨ‬3ത݉ symmetry.1,22-23 In this study, X-ray diffraction was first used to construct the backbone of the crystalline structure. Neutron diffraction was then used to identify the atomic composition and the number of H2O molecules in the compound, since neutron diffraction provides a better resolution than X-ray diffraction for the identification of the light elements in the Prussian blue.34,35 The high-resolution diffraction patterns were analyzed using the General Structure Analysis System (GSAS) program36 following the Rietveld profile refining method.37 Two separated structural phases of the same cubic ‫݉ܨ‬3ത݉ symmetry can be identified in each diffraction pattern, one belonging to the Rb-Co-Fe phase and the other to the K-Ni-Cr phase. Two possible locations for the appearance of H2O molecules were assumed in the refinement: one on the 8c sites that substitute for the alkali ions, and the other bound to the Co or Ni ions at interstitial sites resulting from Fe(CN)6 or Cr(CN)6 vacancies. There was no constraint on the number of H2O molecules in the refinement. Details of the profile refining processes combining the x-ray and neutron diffraction patterns can be found in the Supporting Information. The observed and fitted diffraction patterns taken at 80 K, after cooling from 300 K, are shown in Figure 1 and the refined structural parameters of the two phases are 7



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summarized in Table 1. The chemical compositions obtained for the two phases are Rb0.48Co[Fe(CN)6]0.75[(H2O)6]0.25·0.34H2O with a cubic lattice constant of a = 10.079(1) Å and K0.36Ni[Cr(CN)6]0.74[(H2O)6]0.26·0.11H2O with a = 10.375(1) Å. The volume ratio of the two phases in the heterostructure is 63% of the K-Ni-Cr phase and 37% of the Rb-Co-Fe phase. The relatively low occupation of K+ and H2O on the 8c sites in the K-Ni-Cr phase on the shell may be a direct result of crystalline imperfections on the surface together with the fact that 8c sites are weakly bonded to their neighboring sites. The relatively high background intensity originates from the incoherent scattering of neutrons by the hydrogen atoms, as expected. Various sizes for the MN6 and M′C6 octahedra in PBA have been reported.22,35,38-41 It is interesting to note that relatively compact CoN6, FeC6, NiN6 and CrC6 octahedra are formed in the present nano-sized core/shell cubes. For the Co-Fe component, the Fe-C bond length (1.821 Å at 80 K) of the present compound is ~4% smaller than the reported38 ones (1.91~1.93 Å at 300 K), and the Co-N bond length (1.929 Å at 80 K) is ~7% smaller (2.07~2.12 Å at 300 K). Nevertheless, much smaller FeC6 octahedra with a Fe-C bond length of ~1.79 Å at 90 K has also been reported.39 The compact CoN6 and FeC6 octahedra formed in the present compound resulting in a relatively larger separation (1.29 Å at 80 K) between the N and C ions, where capability in mediating change transfer between Fe and Co ions can be limited. On the other hand, bond lengths of 1.839 Å for Ni-N and 1.927 Å for Cr-C were obtained at 80 K, 8


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which are 2% and 9% smaller than the reported40 ones at 300 K. Nevertheless, much larger CrC6 octahedra with a Cr-C bond length of 2.42 Å linked to much smaller NiN6 octahedra with a Ni-N bond length of 1.43 Å have recently been reported44 as well. The Prussian-blue analogue structures known for these phases are obtained. The Rb-Co-Fe phase is composed of CoN6 and FeC6 octahedra that are alternatively linked to form Co-N-C-Fe-C-N-Co chains along all three crystallographic axis directions. This structure opens up channels among the chains, accommodating the Rb+ ions or H2O in the voids, as shown in Figure 2. The K-Ni-Cr phase crystallizes into the same structure, but with alternately linked NiN6 and CrC6 octahedra that form Ni-N-C-Cr-C-N-Ni chains accommodating K+ ions or H2O in the voids. According to the refinement, 25% of the [Fe(CN)6] units in the Rb-Co-Fe phase are substituted by interstitial H2O, whereas 26% of the [Cr(CN)6] complexes in the K-Ni-Cr phase are substituted by interstitial H2O. There are no identifiable traces of impurity phases in the neutron or X-ray diffraction patterns. No structural change is detected between 80 and 1.5 K, the lowest temperature achieved in the neutron diffraction study. 3.2. Core/shell configuration.

The core/shell design is revealed in the TEM images

(inset to Figure 3a), where nano-sized core/shell cubes are clearly seen. In these dark-field images, the shells are noticeably darker than those of the cores, indicating the absorption of electron beam by the shell is considerably lower than for the core. Note that the electron 9



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density of the shell is 24 % lower, mainly due to the low occupancy of K ions on the 8c sites. Size analysis, based on the TEM images, reveals a mean edge length of d = 350 nm for the cubes and a mean shell thickness of t = 50 nm (Figure 3a), while 80% of the cubes in the assembly have an edge length in the range of 350 ± 50 nm. The diffraction peaks associated with the K-Ni-Cr phase are significantly broader than their Rb-Co-Fe counterparts (Figure 3b), showing a smaller crystalline size for the K-Ni-Cr phase that crystallizes on the shell. Size analysis based on the Scherrer formula,42 using the widths of the diffraction peaks, gives a mean size of 250 nm for the Rb-Co-Fe phase (Figure 4a) and 46 nm for the K-Ni-Cr phases (Figure 4b), leading to a mean edge length of d = 342 nm with a mean thickness of t = 46 nm for the core/shell cubes in the assembly. These results agree well with the TEM-determined dimension, which together make it clear the heterostructure particles consist of Rb-Co-Fe cubic cores with a mean edge length of 250 nm and K-Ni-Cr shells with a mean thickness of 46 nm. The core-shell particles are hereafter designated as Rb-Co-Fe@K-Ni-Cr with a volume ratio of 36 % for the Rb-Co-Fe phase and 64 % for the K-Ni-Cr phase. These results agree very well with the results obtained from the neutron diffraction measurements, which were 37% for the core Rb-Co-Fe phase and 63 % for the K-Ni-Cr phase. 3.3. Strain.

Bragg peak broadening resulting from accumulated strain,

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e ≡ ( ∆d / d )hkl , can be obtained from the plot of

(δ 2θ ) 2

2

tan θ0

≡

 δ 2θ kλ  2   + 16e , as L  tan θ0 sin θ0 

shown in Figure 4, where δ2θ is the full-width-at-half-maximum of the (hkl) reflection at a scattering angle of 2θ0, k = 0.94 for cubes, λ is the wavelength of the incident beam, and L is the mean size. The Scherrer lines (solid lines in Figure 4) derived from the core and the shell diffraction patterns show significant y-axis-intercepts, reflecting the considerable stress that appears in the structure. Averaged strains of 1.77×10-3 in the core and 2.71×10-3 in the shell can be obtained from the X-ray diffraction profiles. Note that the inhomogeneous strain observed43 in the shell of much smaller Rb-Co-Fe@Rb-Ni-Cr particles was not revealed in the present sample. In addition, the diffraction peak profile from a size-dispersed particle assembly can be used to extract the size distribution by combining the expected diffraction profiles contributed from each individual particle in the powder. The solid curves in Figure 3b indicate the calculated line profiles of the {220} reflections with the size distributions shown in the insets, using an instrumental resolution broadening of 0.06o and incorporating a 0.13o and 0.08o peak broadening from the strain in the shell and core, respectively. The size distributions obtained from the X-ray line profiles, Figure 3b, agree very well with those obtained from the TEM images, Figure 3a. The mean sizes of the core and shell obtained from the X-ray diffraction profiles are utilized hereafter, since the X-ray profiles reflect the results from all particles in the assembly.

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3.4. Magnetic Transition.

The temperature dependent magnetization, M(T), taken

with an applied magnetic field of Ha = 200 Oe in zero-field-cooled (ZFC) and field-cooled (FC) processes are shown in Figure 5a. The divergence of the FC and ZFC data near 40 K reflect the glassy nature known for the non-stoichiometric PBA’s. The ZFC M(T) measured in a weak Ha can be described by Curie-Weiss behavior down to 86 K (inset to Figure 5a). This temperature (marked Tm1 = 86 K) indicates the temperature below which magnetic correlations develop. It is interesting to note that sizable increases of M appear below Tm2 = 69 K, which is considerably lower than Tm1. Tm2 shifts to a noticeably higher temperature when a higher Ha is employed (Figure 5b), showing the ferromagnetic (FM) character for the transition at Tm2, consistent with what has been reported for Ni-Cr-PBA’s, which typically order in the range 60-90 K depending on composition.23 An additional anomaly at Tm4 = 8 K (a feature labelled Tm3 will be introduced, below) is also evident in the M(T) curves taken at Ha = 50 Oe (inset to Figure 5b). Tm4 shifts to a lower temperature when a higher Ha is used. The change of M through Tm4 is significantly smaller than the increase of M through Tm2 (15 times smaller at Ha = 1 kOe), signaling that the ordered moment through Tm2 is significantly larger. Consistent with previous studies, Tm4 corresponds to the magnetic ordering of the residual high-spin Co2+ and Fe3+ ions in the Rb-Co-Fe phase in the core.31 The isothermal M(Ha) curves taken at four representative temperatures are shown in 12


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Figure 6. There is essentially no difference found between the increasing and the decreasing-field branches in the M(Ha) loops taken at 100 K, which is well above Tm1. Magnetic hysteresis is clearly revealed in the low-Ha regime (Ha < 0.3 kOe) of the M(Ha) loops taken at 50 K (inset to Figure 6), signaling the existence of a net ferromagnetic moment at this temperature. The magnetic hysteresis extends to a slightly higher field at lower temperatures (inset to Figure 6) and the magnetization is essentially saturated at Ha ≈ 3 kOe at low temperatures. The saturation magnetization, MS, reaches 50.8 emu/g at 2 K. 3.5. Spin Domain.

Similar to what has been identified in the dc magnetization

curves, features at Tm1, Tm2 and Tm4 are clearly evident in the χ′(T) curves as well (Figure 7). Interestingly, the magnetic response associated with the transition at Tm2 is largely suppressed by an applied field, with Ha as weak as 200 Oe decreasing the intensity of χ′ by as much as one order-of-magnitude. Close examination of the χ′(T) curves taken with Ha > 200 Oe reveals that χ′ again begins to increase below Tm3 = 67 K upon cooling through the peak associated with the transition at Tm2 (Figure 7). This feature is more clearly revealed in the χ′(T) curve taken with a weaker driving field, Hac (Figure 8a). Interestingly, Tm2 is the temperature below which sizable absorption losses of the driving field for the system (represented by χ″) begin to appear (filled circles in Figure 8a). The data in Figures 7 and 8 are for the core-shell sample, but two features associated with the Ni-Cr-PBA ordering are also seen in single-phase particles (Figure 9), suggesting they are intrinsic to the 13



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Ni-Cr-PBA and not a result of interface effects in the core-shell heterostructure. At lower temperature, the degree of magnetic instability associated with Tm4 is evident but less significant (Figure 7). For Ha ≥ 500 Oe, there is essentially no difference observed between the χ′(T) curves taken in ZFC and FC process (Figure 8b). A remarkably strong dependence of the absorption of Hac by the system on the driving frequency can be observed. Sharp increases in the absorption loss, χ″, with increasing driving frequency, f, are revealed in the low frequency regime (f < 300 Hz), but the rate of increase slows at higher frequencies, Figure 10a. In addition, a stronger driving field produces a higher absorption loss to the system. The corresponding χ' response decreases with increasing f, and there is also a sharp change in the rate of decrease at f = 300 Hz (Figure 10a). The spin glass characteristics, typical of Prussian blue analogues,44 are clearly revealed in the χ'(T) curves as the system responds better to a lower f (Figure 10b) and to a weaker Hac (Figure 10c). This behavior appears below Tm2, and persists down to the lowest temperature studied of 1.8 K. The χ' and χ″ responses stabilize at a driving field with a strength stronger than 0.1 Oe or a frequency higher than 3 kHz, but there is a significant reduction in χ' while the absorption loss χ″ is remarkably enhanced. Spin glass behavior is known to originate from the formation of locally correlated magnetic domains. Such spin domains frequently form in the sub-micrometer range, yet it is surprising to find that numerous locally correlated spin domains would form in the 46 14


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nm thick shell and that the local magnetic correlations are strong enough to generate such strong dependence of the magnetic response on the driving strength and frequency. It is the strong local spin correlations that drag the spins following the time-varying direction of the driving ac magnetic field, resulting in retarded responses of the system to high frequency or strong field strength. Although the spin glass characteristics are significantly suppressed at frequencies higher than 300 Hz, they persist up to a driving frequency of 3 kHz. Note that a value of 3 kHz corresponds to a spin relaxation time of 0.33 msec, which is remarkably long for a spin system. 3.6. Spin Arrangement.

It is known that periodic magnetic correlations will

generate additional intensities in the neutron diffraction peaks. Neutron diffraction intensities, taken at selective temperatures while warming after cooling (~200 K/h) from 300 to 1.5 K, were collected to study the variations of magnetic correlations with temperature. At low temperatures, the diffraction intensities associated with the K-Ni-Cr phase progressively decrease as the temperature is raised (Figure 11a), revealing an ordering temperature at Tm2. On the other hand, there is no detectable change found in the diffraction intensities associated with the Rb-Co-Fe phase upon warming through the transition at Tm2 (Figure 11a), reflecting the core is not influenced by the magnetic transition at Tm2. The magnetic diffraction pattern obtained at 1.5 K is shown in Figure 11b, where the diffraction pattern taken at 80 K, serving as the non-magnetic background, has 15



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been subtracted. This difference pattern reveals the additional intensities that developed as the temperature was reduced from 80 to 1.5 K, signifying the magnetic ordering of the nickel and chromium ions in the K-Ni-Cr phase. The magnetic diffraction peaks appear on top of the nuclear ones, indicating the ordering of both the Ni and Cr spins. Note that the Ni sublattice and the Cr sublattice both have the face-centered cubic (fcc) symmetry that generates magnetic diffraction peaks at other specific positions. The magnetic diffraction pattern was analyzed employing the GSAS program, preserving the same spatial symmetry of the crystalline structure as for the magnetic structure. The magnetic diffraction pattern at 1.5 K can be described (solid lines in Figure 11b) by a ferromagnetic arrangement of both the Cr and Ni moments pointing at the [111] crystallographic direction, with low temperature saturated moments of Cr = 2.79(9) µB and Ni = 2.21(9) µB at 1.5 K. No additional intensities can be identified at the positions associated with the Rb-Co-Fe phase. A small but visible reduction of M upon cooling below Tm4 is revealed in the M(T) curves (Figure 5). This feature is understood to be the result of the magnetic ordering of the remaining Co2+ and Fe3+ spins in the Rb-Co-Fe phase, sometimes called primordial spins during photomagnetic studies,1,2 that did not undergo the thermal charge transfer induced spin transition (CTIST) known for this phase. The reduction of M through the Tm4 transition is ~4 % of the increase of M through the Tm2 transition, which is consistent with 16


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the incomplete thermal CTIST. Although this small amount of change in M (0.4 emu/g at Ha = 50 Oe) is beyond the detection of neutron diffraction at current sensitivity, nevertheless a 2% decrease of the diffraction intensities associated with the Rb-Co-Fe phase does appear when the temperature is raised from 1.5 to 15 K. 3.7. Next-nearest Neighbor Interaction.

Tm1 = 86 K indicates the temperature at

which separated (interaction free) nano-sized spin clusters begin to develop in the K-Ni-Cr phase in the shell. The development of magnetic correlations within nano-sized clusters results in small but visible increases in the dc response, M, (inset to Figure 5a) and in the ac response, χ' (Figure 7). As temperature is lowered, Tm2 = 69 K indicates the temperature below which long range FM ordering of the Ni/Cr ions develops (Figure 11a), as the inter-cluster interactions among the correlated spin clusters become significant. The long range ordering not only generates large increases in M, but also produces spin-glass type absorption, χ″, of the driving magnetic field, slowing the increase of χ' (Figure 8a). It is possible that the formation of nano-sized spin domains can be linked to the high atomic deficiency (26%) of the Cr ions that interrupts the propagation of superexchange interactions. There is also an anomaly, marked by Tm3 = 67 K, which is visible in χ'(T) and χ″(T) curves (Figures 7-9) but is barely revealed in M(T) curves. The origin of this anomaly is not yet completely understood, but it may be linked to the obvious difference in the 17



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

interaction strengths between the Ni ions and the Cr ions in the K-Ni-Cr phase. The Ni2+ ions form an fcc sublattice, with Cr3+ ions on the edges between the Ni2+ ions, but with a 26% atomic deficiency. As the energy associated with magnetic dipolar interactions is proportional to the spatial separation r of the two dipoles as r-3, and with separations of r = 5.2 Å for the nearest Ni-Cr pair and r = 5.2×1.414 Å for the next nearest neighbors (Ni-Ni or Cr-Cr pairs), the energy associated with the direct dipolar interaction is estimated to be on the order of no more than 10-2 meV. This energy is far too small to drive magnetic ordering with an ordering temperature as high as 69 K. However, there are two possible wave-function overlaps between the Ni2+ and Cr3+ ions that can be identified: First, the superexchange interaction between the Ni2+ and Cr3+ ions, mediated through the bridging CN- ions. This interaction propagates along the three crystallographic axis directions, but is severely interrupted when 26 % of the [Cr(CN)6]3vacancies are replaced by water molecules, breaking the interaction path through the Ni-N-C-Cr-C-N-Ni chains. Secondly, there is a direct interaction between the next nearest neighbor Ni-Ni and Cr-Cr pairs along the [110] direction. This connection is evidenced by the electronic charge density in the (010) plane shown in Figure 12. The electron density map was obtained by employing the GSAS program, starting with a profile refining the X-ray diffraction pattern, followed by calculation of the inverse Fourier transforms of the structure factors to extract the electron density distribution. The electron density contour 18


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map of a specific plane was then obtained by slicing the electron density in the vicinity, covering 0.025 Å below and above the plane. Noticeable electron density is distributed between the two next nearest neighbor nickel ion pairs (Figure 12), establishing an exchange pathway between them. The K+ ions on the 8c sites are unlikely to be the main source, since they are severely deficient (82%) and located well above (2.6 Å) the metal ion plane. It is the atomic framework that forms the electronic charge distribution, providing a direct exchange path between the next nearest neighbor Ni-Ni contacts. On the other hand, the Cr-Cr next nearest neighbors are not as well electronically connected (Figure 12). These differences likely not directly linked to the Cr-deficiency, but rather to the atomic framework itself. The Ni-N-C-Cr superexchange interaction is stronger than the Ni-Ni direct interaction, but combining the nearest and next-nearest magnetic neighbor interactions, the interaction strength of Ni2+ is obviously stronger than that of the Cr3+ ions. It is likely that the slightly separated Tm2 and Tm3 (by only 2 K) reflect the difference of the next nearest interactions, so that the Ni2+ ions order at Tm2 but the Cr3+ ions become ordered at a slightly lower Tm3.

4. CONCLUSIONS A

nano-sized

Prussian

blue

core/shell

Rb0.48Co[Fe(CN)6]0.75[(H2O)6]0.25·0.34H2O

core

heterostructure coated

19


by

with a

a

250 46

nm nm


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

K0.36Ni[Cr(CN)6]0.74[(H2O)6]0.26·0.11H2O shell was synthesized by a co-precipitation method. Unit cell constants, with a relatively small strain of 0.177%, are consistent a predominately low-spin Co-Fe-PBA network at low temperature, leading a weak ordered moment below 8 K associated with residual high-spin Co2+/Fe3+ sites. On the other hand, for the K0.36Ni[Cr(CN)6]0.74[(H2O)6]0.26·0.11H2O shell, two superexchange paths are identified, one propagating along the three crystallographic axis directions through the Ni-N-C-Cr paths while the other propagates in the [110] direction through next-nearest neighbor Ni-Ni interactions. This direct exchange is not present or very weakly connected for the Cr ions causing the response of the Ni2+ sublattice to differ from the Cr3+ sublattice. The formation of locally correlated spin domains is linked to the severe Cr-deficiency, whereas the appearance of two transitions corresponding to ordering in the K-Ni-Cr network may originate from the obvious difference in the strengths of the interaction between the Ni2+ ions and between the Cr3+ ions.

Supporting Information.

Although the electronic charge density contours of the

K-Ni-Cr shell component of the present core@shell sample are discussed in detail in the main text and in Figure 12 of the manuscript, the results of similar analysis for the core component, Rb-Co-Fe, are not a part of the principal findings that are communicated. For the purpose of completeness, the results for the Rb-Co-Fe core of the core@shell sample 20


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studied in the present work and compares them to findings for single-phase K-Co-Fe (Reference 8 in the main text) are provided in the Supporting Information. This information is available free of charge via the internet at http://pubs.acs.org

AUTHOR INFORMATION Corresponding Author *E-mail: [email protected] Tel: +886 3 921 127692

ACKNOWLEDGEMENTS We thank ANSTO for providing the neutron beam time that made the neutron diffraction measurements possible. This work was supported by the Ministry of Science and Technology of Taiwan under Grant No. MOST 104-2112-M-008-007-MY3. Partial support was also provided by the US National Science Foundation via award DMR-1405439

(DRT)

DMR-1202033

(MWM),

and

DMR-1157490

(NHMFL).

Technical assistance, in the early stages of the AC studies at UF by P. A. Quintero and sample synthesis by Jiamin Liang are gratefully acknowledged.

21



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REFERENCES (1) Quintero, P. A.; Dumont, M. F.; Grant, D. M.; Knowles, E. S.; Jeena, H.; Biswas, A.; Talham, D. R. Films of Photomagnetic CoFe Prussian Blue Analogue on Thin Manganite Substrates: Fabrication and Characterization. Polyhedron 2013, 66, 201–204. (2) Knowles, E. S.; Li, C. H.; Dumont, M. F.; Pepraha, M. K.; Andrus, M. J.; Talham, D. R.; Meisel, M. W. Photoinduced Perturbations of the Magnetic Superexchange in Core@Shell Prussian Blue Analogues. Polyhedron 2013, 66, 153–156. (3) Pajerowski, D. M.; Andrus, M. J.; Gardner, J. E.; Knowles, E. S.; Meisel, M. W.; Talham, D. R. Persistent Photoinduced Magnetism in Heterostructures of Prussian Blue Analogues. J. Am. Chem. Soc. 2010, 132, 4058-4059. (4) Pajerowski, D. M.; Gardner, J. E.; Franz, F. A.; Andrus, M. J.; Dumont, M. F.; Knowles, E. S.; Meisel, M. W.; Talham, D. R. Photoinduced Magnetism in a Series of Prussian Blue Analogue Heterostructures. Chem. Mater. 2011, 23, 3045-3053. (5) Dumont, M. F.; Knowles, E. S.; Guiet, A.; Pajerowski, D. M.; Gomez, A.; Kycia, S. W.; Meisel, M. W.; Talham, D. R. Photoinduced Magnetism in Core/Shell Prussian Blue Analogue Heterostructure of KjNik[Cr(CN)6]l·nH2O with RbaCob[Fe(CN)6]c·mH2O. Inorg. Chem. 2011, 50, 4295-4300. (6) Coronado, E.; Fitta, M.; Prieto-Ruiz, J. P.; Prima-Garc´ıa, H.; Romero, F. M.; Cros, A. 22


Page 22 of 47

Page 23 of 47

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


MOKE

Magnetometry

Electropolymerized

as

Magnetic

a

Probe Thin

of

Films

Surface

Magnetic

of

Prussian

the

Impurities Blue

in

Analogue

Fe3[Cr(CN)6]2·15H2O. J. Mater. Chem. C 2013, 1, 6981-6988. (7) Yoshii , S.; Nojiri , H.; Sato, O. High-Field ESR on Light-Induced Transition of Spin Multiplicity in FeCo Complex. J. Low. Temp. Phys. 2013, 170, 383–388. (8) Pajerowski, D. M.; Garlea, V. O.; Knowles, E. S.; Andrus, Dumont, M. F,; Calm, Y. M.; Nagler, S. E.; Tong, X.; Talham, D. R,; Meisel, M. W. Magnetic Neutron Scattering of Thermally Quenched K-Co-Fe Prussian Blue Analog Photomagnet. Phys. Rev. B 2012, 86, 054431. (9) Champion, G.; Escax, V.; Cartier dit Moulin, C.; Bleuzen, A.; Villain, F.; Baudelet, F.; Dartyge, E.; Verdaguer, M. Photoinduced Ferrimagnetic Systems in Prussian Blue Analogues CxICo4[Fe(CN)6]y (CI = Alkali Cation). 4. Characterization of the Ferrimagnetic of the Photoinduced Metastable State in CxICo4[Fe(CN)6]3.3·13H2O by K Edges X-Ray Magnetic Circular Dichroism. J. Am. Chem. Soc. 2001, 123, 12544-12546. (10) Bhatt, P.; Thakur, N.; Mukadam, M. D.; Meena, S. S.; Yusuf, S. M. Evidence of Oxygen Clustering and Understanding of Structure Disorder in Prussian Blue Analogues Molecular Magnet M1.5[Cr(CN)6]·zH2O (M=Fe and Co): Reverse Monte Carlo Simulation and Neutron Diffraction Study. J. Phys. Chem. C 2013, 117, 23



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 24 of 47

2676−2687. (11) Kahn, O. Condensed-Matter Physics: The Magnetic Turnabout. Nature 1999, 399, 21-22. (12) Gutlich, P.; Hauser, A.; Spiering, H. Thermal and Optical Switching of Iron(II) Complexes. Angew. Chem. 1994, 33, 2024-2054. (13) Mathoniere, C.; Nuttall, C. J.; Carling, S. G.; Day, P. Ferrimagnetic Mixed-Valency and Mixed-Metal Tris(oxalato)iron(III) Compounds: Synthesis, Structure, and Magnetism. Inorg. Chem. 1996, 35, 1201-1206. (14) Middlemiss, D. S.; Wilson, C. C. Ferromagnetism and Spin Transitions in Prussian Blue: A Solid-State Hybrid Functional Study. Phys. Rev. B 2008, 77, 155129. (15) Egan, L.; Kamenev, K.; Papanikolaou, D.; Takabayashi,Y.; Margadonna, S. Pressure-Induced

Sequential

Magnetic

Pole

Inversion

and

Antiferromagnetic-Ferromagnetic Crossover in a Trimetallic Prussian Blue Analogue. J. Am. Chem. Soc. 2006, 128, 6034-6035. (16) Arrio, M. A.; Sainctavit, P.; Cartier dit Moulin, C.; Mallah, T.; Verdaguer, M.; Pellegrin, E.; Chen, C. T. Characterization of Chemical Bonds in Bimetallic Cyanides Using X-ray Absorption Spectroscopy at L2,3 Edges. J. Am. Chem. Soc. 1996, 118, 6422-6427. (17)

Pajerowski,

D.

M.;

Yamamoto,

T.;

Einaga,

24


Y.

Photomagnetic

Page 25 of 47

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


K0.25Ni1-xCox[Fe(CN)6]·nH2O

and

K0.25Co[Fe(CN)6]0.75y[Cr(CN)6]0.75(1-y)·nH2O

Prussian Blue Analogue Solid Solutions. Inorg. Chem. 2012, 51, 3648-3655. (18) Sato, O.; Einaga, Y.; Fujishima, A.; Hashimoto, K. Theoretical Study of the Intercalation of Li into TiO2 Structures. Inorg. Chem. 1999, 38, 20-28. (19) Chapman, K. W.; Chupas, P. J.; Kepert, C. J. Compositional Dependence of Negative Thermal Expansion in the Prussian Blue Analogues MIIPtIV(CN)6 (M = Mn, Fe, Co, Ni, Cu, Zn, Cd). J. Am. Chem. Soc. 2006, 128, 7009-7014. (20) Bleuzen, A.; Lomenech, C.; Escax, V.; Villain, F.; Varret, F.; Cartier dit Moulin, C.; Verdaguer, M. Photoinduced Ferrimagnetic Systems in Prussian Blue Analogues CxICo4[Fe(CN)6]y (CI = Alkali Cation). 1. Conditions to Observe the Phenomenon. J. Am. Chem. Soc. 2000, 122, 6648-6652. (21) Margadonna, S.; Prassides, K.; Fitch, A. N. Zero Thermal Expansion in a Prussian Blue Analogue. J. Am. Chem. Soc. 2004, 126, 15390-15391. (22) Buser, H. J.; Schwarzenbach, D.; Petter, W.; Ludi, A. The Crystal Structure of Prussian Blue: Fe4[Fe(CN)6]3·xH2O. Inorg. Chem. 1977, 16, 2704-2710. (23) Verdaguer, M.; Bleuzen, A.; Marvaud, V.; Vaissermann, J.; Seuleiman, M.; Desplanches, C.; Scuiller, A.; Train, C.; Garde, R.; Gelly, G. et al. Molecules to Build Solids: High TC Molecule-Based Magnets by Design and Recent Revival of Cyano Complexes Chemistry. Coord. Chem. Rev. 1999, 190, 1023-1047. 25



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 26 of 47

(24) Ellis, D.; Eckhoff, M.; Neff, V. D. Electrochromism in the Mixed-Valence Hexacyanides. 1. Voltammetric and Spectral Studies of the Oxidation and Reduction of Thin Films of Prussian Blue. J. Phys. Chem. 1981, 85, 1225-1231. (25) Lu, Y.; Wang, L.; Cheng J.; Goodenough, J. B. Prussian Blue: A New Framework of Electrode Materials for Sodium Batteries. Chem. Commun. 2012, 48, 6544–6546. (26) Ohkoshi, S. I.; Tokoro, H. Photomagnetism in Cyano-Bridged Bimetal Assemblies. Acc. Chem. Res. 2012, 45, 1749-1758. (27) Escax, V.; Bleuzen, A.; Moulin, C. C. D.; Villain, F.; Goujon, A.; Varret, F.; Verdaguer, M. Photoinduced Ferrimagnetic Systems in Prussian Blue Analogues CxICo4[Fe(CN)6]y (CI = Alkali Cation). 3. Control of the Photo- and Thermally Induced Electron Transfer by the [Fe(CN)6] Vacancies in Cesium Derivatives. J. Am. Chem. Soc. 2001, 123, 12536-12543. (28)

Shimamoto,

N.;

Ohkoshi,

S.;

Sato,

O.;

Hashimoto,

K.

Control

of

Charge-Transfer-Induced Spin Transition Temperature on Cobalt-Iron Prussian Blue Analogues. Inorg. Chem. 2002, 41, 678-684. (29) Dia, N.; Lisnard, L.; Prado, Y.; Gloter, A.; Stephan, O.; Brisset, F.; Hafez, H.; Saad, Z.; Mathoniere, C.; Catala, L. et al. Synergy in Photomagnetic/Ferromagnetic Sub-50 nm Core-Multishell Nanoparticles. Inorg. Chem. 2013, 52, 10264-10274. (30) Catala, L.; Brinzei, D.; Prado, Y.; Gloter, A.; Stephan, O.; Rogez, G.; Mallah, T. 26


Page 27 of 47

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Core-Multishell Magnetic Coordination Nanoparticles: Toward Multifunctionality on the Nanoscale. Angew. Chem. Int. Ed. 2009, 48, 183-187. (31) Risset, O. N.; Quintero, P. A.; Brinzari, T. V.; Andrus, M. J.; Lufaso, M. W.; Meisel, M. W.; Talham, D. R. Light-Induced Changes in Magnetism in a Coordination Polymer Heterostructure, Rb0.24Co[Fe(CN)6][email protected][Cr(CN)6]0.70·nH2O and the Role of the Shell Thickness on the Properties of Both Core and Shell. J. Am. Chem. Soc., 2014, 136, 15660–15669. (32) Dumont, M. F.; Knowles, E. S.; Guiet, A.; Pajerowski, D. M.; Gomez, A.; Kycia, S. F.; Meisel, M. W.; Talham, D. R. Photoinduced Magnetism in Core/Shell Prussian Blue Analogue Heterostructure of KjNik[Cr(CN)6]l·nH2O with RbaCob[Fe(CN)6]c·mH2O. Inorg. Chem. 2011, 50, 4295-4300. (33) Olivia N. Risset. Coordination Polymer Structure: Design and Photomagnetic Properties; Ph.D. thesis, University of Florida, 2015. (34) Day, P.; Herren, F.; Ludi, A.; Gudel, H. U.; Hulliger, F.; Givord, D. Valence Delocalization in Prussian Blue Fe4III[FeII(CN)6]3·xD2O by Polarized Neutron Diffraction. Helv. Chim. Acta 1980, 63, 148-153. (35) Herren, F.; Fischer, P.; Ludi, A.; Halg, W. Neutron Diffraction Study of Prussian Blue, Fe4[Fe(CN)6]3·xH2O. Location of Water Molecules and Long-Range Magnetic Order. Inorg. Chem. 1980, 19, 956-959. 27



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 28 of 47

(36) Larson, A. C.; Von Dreele, R. B. General Structure Analysis System (GSAS); LANL Report 2004, LA-UR-86-748. (37) Rietveld, H. M. A Profile Refinement Method for Nuclear and Magnetic Structures. J. Appl. Crystallogr. 1969, 2, 65-71. (38) Yokoyama T.; Ohta T.; Sato O.; Hashimoto K. Characterization of Magnetic CoFe Cyanides by X-Ray-Absorption Fine-Structure Spectroscopy. Phys. Rev. B 1998, 58, 8257-8266. (39) Kim, J.; Tanala H.; Kato K.; Takata M.; Moritomo Y. Extended d-Electron State of Fe(CN)6 Unit in Prussian Blue Analogue. Appl. Phys. Express 2011, 4, 025801. (40) Bhatt P.; Yusuf S.M.; Bhatt R.; Schutz G. Magnetic Properties of Nanoparticles of Prussian Blue-Based Molecular Magnets M3[Cr(CN)6]2·zH2O (M = Fe, Co, and Ni). Appl. Phys. A 2012, 109, 459-469. (41) Bhatt P.; Banerjee S.; Anwar S.; Mukadam M. D.; Meena S. S.; Yusuf S. M. Core-Shell

Prussian

Blue

Analogue

Molecular

Magnet

Mn1.5[Cr(CN)6]·[email protected][Cr(CN)6]·nH2O for Hydrogen Storage. ACS Appl. Mater. Interfaces 2014, 6, 17579-17588. (42) Patterson, A. L. The Scherrer Formula for X-Ray Particle Size Determination. Phys. Rev. 1939, 56, 978-982. (43) Presle, M.; Maurin, I.; Maroun, F.; Cortès, R.; Lu, L.; Hassan, R. S.; Larquet, E.; 28


Page 29 of 47

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Guigner, J.-M.; Rivière, E.; Wright, J. P. et al. Photostrictive/Piezomagnetic Core-Shell Particles Based on Purissan Blue Analogues: Evidence for Confinement Effects? J. Phys. Chem. C 2014, 118, 13186-13195. (44) Pajerowski, D. M.; Frye, F. A.; Talham, D. R.; Meisel, M. W. Size Dependence of the Photoinduced Magnetism and Long-Range Ordering in Prussian Blue Analogue Nanoparticles of Rubidium Cobalt Hexacyanoferrate. New J. Phys. 2007, 9, 222-226.

29



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Table 1.

Page 30 of 47

List of the refined structural parameters of the core/shell PBA at 80K, where

Biso represents the isotropic temperature parameter. Rb0.48Co[Fe(CN)6]0.75[(H2O)6]0.25·0.34H2O Cubic ‫݉ܨ‬3ത݉ ( No. 225, Z = 2 ), T = 80 K, a =10.079(1) Å Atom

x

y

Z

M

Biso( Å2 )

Occupancy

Rb

0.25

0.25

0.25

8c

1.1(3)

0.24(2)

Co

0.5

0.5

0.5

4b

2.3(4)

1

Fe

0

0

0

4a

0.5(1)

0.75(1)

C

0.181(2)

0

0

24e

1.0(1)

0.75†

N

0.309(2)

0

0

24e

2.1(1)

0.75†

O(1)

0.2718

0

0

24e

1.4(2)

0.25(1)

H(1)

0.327

0.055

0.055

96k

1.2(3)

0.125*

O(2)

0.25

0.25

0.25

8c

1.4(3)

0.17(3)

H(2)

0.197

0.197

0.197

32f

10(1)

0.085*

K0.36Ni[Cr(CN)6]0.74[(H2O)6]0.26·0.11H2O Cubic ‫݉ܨ‬3ത݉, ( No. 225, Z = 2 ,) T = 80 K, a =10.375(1) Å Atom

x

y

Z

M

Biso( Å2 )

Occupancy

K

0.25

0.25

0.25

8c

2.1(3)

0.18(3)

Ni

0.5

0.5

0.5

4b

2.6(2)

1

Cr

0

0

0

4a

0.5(1)

0.74(1)

C

0.186(2)

0

0

24e

1.9(1)

0.74†

N

0.323(1)

0

0

24e

1.4(1)

0.74†

O(1)

0.275

0

0

24e

2.1(3)

0.26(1)

H(1)

0.329

0.053

0.053

96k

1.2(2)

0.13*

O(2)

0.25

0.25

0.25

8c

2.4(9)

0.05(2)

H(2)

0.191

0.191

0.191

32f

4.4(9)

0.025*

χ2=1.004, Rp= 0.76%, Rwp= 0.99% †

The occupancies of C and N are constrained to be the same with the associated Fe or Ni. *The occupancy of H is constrained to be half that of the associated O.

30


Page 31 of 47

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Figure captions Figure 1.

Observed (crosses) and fitted (solid lines) high-intensity neutron powder

diffraction pattern taken at 80 K, assuming a cubic symmetry of the space group ‫݉ܨ‬3ത݉ for both the K-Ni-Cr and Rb-Co-Fe structural phases and taking the pseudo-Voigt function for the peak profile. The differences between the calculated and observed patterns are plotted at the bottom. The solid vertical lines mark the calculated positions of the Bragg reflections of the proposed crystalline structures.

Figure 2.

Schematic drawing of the proposed crystalline structure of the Rb-Co-Fe in the

core and the K-Ni-Cr on the shell. This structure can be viewed as consisting of Co-N-C-Fe-C-N-Co chains in the core and Ni-N-C-Cr-C-N-Ni chains on the shell along the three crystallographic axis directions.

Figure 3.

(a) Size distribution obtained from the TEM images, revealing a 350 nm edge

length for the majority of the cubes. The inset shows representative TEM images of the PBA, revealing a core/shell structure of the cubes. (b) A portion of the x-ray diffraction pattern taken at 300 K, showing the {220} Bragg reflection of the K-Ni-Cr phase which is significantly broader than its Rb-Co-Fe counterpart. The solid lines indicate the calculated diffraction profiles using the size distributions shown in the insets. 31



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Figure 4.

Scherrer plots of (a) the Rb-Co-Fe phase and (b) the K-Ni-Cr phase, based on

the widths of the x-ray diffraction peaks. The solid lines indicate the results of the linear fits, giving a mean size of 250 nm with a strain of 1.77×10-3 for the Rb-Co-Fe phase in the core and a mean size of 46 nm with a strain of 2.71×10-3 for the K-Ni-Cr phase on the shell.

Figure 5.

(a) M(T) curves measured with Ha=200 Oe in a ZFC (open squares) and in a

FC (filled circles) processes. The inset shows the temperature dependence of 1/M taken at Ha=50 Oe. It can be seen that M departs from the Curie-Weiss behavior below Tm1 = 86 K and the shape turns at Tm2 = 69 K. (b) M(T) curves measured at five representative Ha in the zero-field-cooled process. The inset displays the low temperature portion of the M(T) curve taken at Ha = 50 Oe, revealing an anomaly at Tm4 = 8 K.

Figure 6.

M(Ha) loops taken at four representative temperatures, revealing sharp

increases of M in the low-Ha regime but becoming saturated at Ha ≈ 3 KOe for the curves taken at 2, 20 and 50 K. The inset shows the M(Ha) curves at 2 and 50 K in an expanded scale, revealing magnetic hysteresis in the low-Ha regime. No difference between the field-increasing and the field-decreasing branches is seen at 100 K.

32


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Page 33 of 47

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Figure 7.

χ'(T) curves taken at six representative Ha, measured using a driving ac

magnetic field with a root-mean-square strength of 1 Oe and a frequency of 97 Hz. The inset shows the temperature dependence of 1/χ' taken without Ha, revealing that χ' departs from the Curie-Weiss behavior below Tm1 = 86 K and the shape turns at Tm2 = 69 K. In addition, Tm3 = 67 K and Tm4 = 8 K are also clearly revealed.

Figure 8.

(a) Portions of the χ'(T) and χ″(T) curves, measured using a driving field with

a root-mean-square strength of 1 Oe and a frequency of 13 Hz. Tm2 = 69 K and Tm3 = 67 K are clearly revealed. (b) Zero-field-cooled and field-cooled χ'(T) curves measured at Ha = 500 Oe, using a driving field with a root-mean-square strength of 1 Oe and a frequency of 97 Hz. Four anomalies at Tm1 = 86 K, Tm2 = 69 K, Tm3 = 67 K and Tm4 = 8 K are seen. No observable difference between the zero-field-cooled and the field-cooled curves is seen.

Figure 9.

(a) M(T) and (b) χ′(T) for K-Ni-Cr single phase material in Ha = 100 Oe. The

insets for both panels show the derivatives of the signals with respect to temperature. Although the bulk ferromagnetic transition, Tm2, is detected in both measurements, the second thermal feature, Tm3, is only observed in the ac study. These data confirm Tm2 and Tm3 are present in K-Ni-Cr, and an expanded data set is available in Ref. 33.

33



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Figure 10.

(a) Variation of χ' (filled squares) and χ″ (open circles) at 3 K with the driving

frequency, measured at a root-mean-square strength of 1 Oe. Sharp decreases of χ' together with sharp increases of χ″ appear below f = 300 Hz. (b) χ'(T) curves measured using a weak driving field with a root-mean-square strength of 0.02 Oe at five representative driving frequencies. (c) χ'(T) curves measured using a driving frequency of 13 Hz at five representative root-mean-square field strengths.

Figure 11.

(a) Variations of the {200} integrated intensities with temperature of the

K-Ni-Cr (solid circles) and Rb-Co-Fe (open triangles) phases, showing an ordering temperature of Tm2 = 69 K for the K-Ni-Cr phase on the shell. (b) Magnetic intensities observed at 1.5 K, where the diffraction intensities observed at 80 K serving as the non-magnetic background have been subtracted. The solid curves indicate the calculated magnetic intensities based on the proposed spin arrangement.

Figure 12.

Electronic charge density contours at a density of 0.05 e/Å3 in the (010) plane,

as inferred from the x-ray diffraction data. The direct connection for the Ni-Ni path is clearly revealed, but not for the direct Cr-Cr path.

34


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Page 35 of 47

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Table of Contents Graphic

35



2.0

4

Intensity (10 counts)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47

Page 36 of 47

Rb0.48Co[Fe(CN)6]0.75[(H2O)6]0.250.34 H2O

Cubic Fm3m Neutron a = 10.079(1) Å K0.36Ni[Cr(CN)6]0.74[(H2O)6]0.260.11 H2O

1.8 1.6

-

1.4

T = 80 K

Cubic Fm3m a = 10.375(1) Å

= 2.41 Å 2

 = 1.004

Rp = 0.76 %

1.2

Rwp = 0.99 %

1.0 0.8 20

40

60

80

100

120

Scattering angle 2 (deg.)


Fig. 1

Page 37 of 47

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47


C Co or Ni Rb or K N Fe or Cr O H


Fig. 2


160

(a)

Number of counts

140 120

d

t

100 80 100 nm

60 40 Total counts = 250 20 0

250

300

350

400

450

Edge length d (nm)

4

1.2

0.6 0.4 0.2 0.0

1.8 1.5

0.8

K-Ni-Cr (shell)

Number fraction (a.u.)

2.1

1.0

1.0

Number fraction (a.u.)

2.4 (b)

Intensity (10 counts)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47

Page 38 of 47

30

35

40

45

50

55

60

0.8

Rb-Co-Fe (Core)

0.6 0.4 0.2 0.0 100

Edge length d (nm)

150

200

250

300

350

400

Edge length d (nm)

T = 300 K Rb-Co-Fe {220}

K-Ni-Cr {220}

0.9 23.0

23.5

24.0

24.5

25.0

25.5

Scattering angle 2 (deg.)


Fig. 3

Page 39 of 47

(a)  2 2 tan 2  0



e  d

-4

2 /tan  (10 )

0.7



d

k L

  2   tan 0 sin  0 

   16 e 2 



hkl

2

Rb-Co-Fe 0.6

2

Mean size = 250 nm -3 Strain e = 1.77  10 0.5

0.02

0.03

0.04

2/tan0sin

(b)

-4

2 /tan  (10 )

4

3

2 2

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47


K-Ni-Cr 2

Mean size = 46 nm -3 Strain e = 2.71  10

1 0.02

0.04

0.06

0.08

0.10

2/tan0sin


Fig. 4

120

FC

1/M (g/emu)

(a)

30 25

ZFC 20

Page 40 of 47

Ha = 50 Oe

ZFC

90

Tm1 = 86 K

60

Tm2 = 69 K

30 0 50

60

70

80

90

100

110

120

Temperature (K)

15

Rb-Co-Fe

Ha= 200 Oe

10

@K-Ni-Cr

5 0

(b) 50

5 kOe 1 kOe

40 500 Oe

30

ZFC

9.6

M (emu/g)

Magnetization M (emu/g)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47

Magnetization (emu/g)


Ha = 50 Oe 9.4

9.2

Tm4 = 8 K

9.0 0

5

10

15

20

25

Temperature (K)

200 Oe

20

Rb-Co-Fe @K-Ni-Cr

Tm2

10

Tm4

0 0

20

50 Oe 40

ZFC 60

80

100

120

Temperature (K) ACS Paragon Plus Environment

Fig. 5

50

2K 20 K

40 30

50 K

20

Rb-Co-Fe @K-Ni-Cr

10


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47



Page 41 of 47

40

2K

30

50 K 20

10

Rb-Co-Fe@K-Ni-Cr

0

100 K 0 0

0.0

0.2

0.4

0.6

0.8

1.0

Applied magnetic field Ha (kOe)

1 2 3 Applied magnetic field Ha (kOe)


4

Fig. 6


1/' ( 10 emu/g Oe)

6

Rb-Co-Fe

5

Curie-Weiss fit Tm1 = 86 K

4

@K-Ni-Cr

3

3

0.12

' (emu/g Oe)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47

0.09

2 1

Tm2 = 69 K

0 50

60

70

80

90

100

110

120

Temperature (K)

Tm4 = 8 K

0.06

Page 42 of 47

Tm3 = 67 K 0.03

0 Oe 50 Oe 200 Oe 500 Oe 1 kOe 5 kOe Hac = 1 Oe f = 97 Hz

0.00 0

20

40

60

80

Temperature (K)


Fig. 7

(a) Rb-Co-Fe @K-Ni-Cr

10

4 3

8 6

2

Tm3

-2

' (10 emu/g Oe)

12

5

4

Hac= 1 Oe

2

1

Tm2

f = 13 Oe

0 75

0 60

65

-3

14

70

Temperature (K)

(b)

Rb-Co-Fe @K-Ni-Cr

1.2

Tm2

Tm4

0.9

Ha= 500 Oe ZFC FC

0.6

-2

' (10 emu/g Oe)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47


'' (10 emu/g Oe)

Page 43 of 47

0.3

Hac= 1 Oe

Tm3

f = 97 Hz

0.0 0

20

Tm1

40

60

80

100

120

Temperature (K)


Fig. 8


(a) 30

Ha= 100 Oe

0

20

dM/dT (emu / g K)

M (emu/g)

K-Ni-Cr

10

-2

-4 50

60

70

80

0 15

(b)

K-Ni-Cr

Ha= 100 Oe Hac= 1 Oe d'/dT (10 emu / g K)

10

-2

' (10 emu/g Oe)

5

-2

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47

Page 44 of 47

0

Tm3

1 Hz 10 Hz 100 Hz

Tm2 -1

-2 50

60

70

80

0 10

20

30

40

50

60

70

80

90

Temperature (K)


Fig. 9

-2

' (10 emu/g Oe)

4.15

300 Hz

6 5

4.10

4

T=3K Hac = 1 Oe

4.05

3 2

4.00 0

1

2

3

4

5

-4

(a)

1

Driving frequency f (kHz)

' (emu/g Oe)

(b)

f =11 Hz f =13 Hz f =19 Hz f =31 Hz f =97 Hz

0.15

0.10

0.05 Hac= 0.02 Oe 0.00

(c)

Hac=0.01 Oe

0.20

' (emu/g Oe)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47


'' (10 emu/g Oe)

Page 45 of 47

Hac=0.02 Oe Hac=0.04 Oe

0.15

Hac=0.1 Oe Hac=1 Oe

0.10 0.05

f = 13 Hz

Rb-Co-Fe @K-Ni-Cr

0.00 20

30

40

50

60

70

80

Temperature (K)


Fig. 10

96

Neutron 196

{200} 92

194

Tm2 Rb-Co-Fe (core)

192

88

190 84 0

40

60

Temperature (K)

80

100

(b) I1.5 K-I80 K Ni = 2.21(9) B Cr = 2.79(9) B Neutron = 2.41 Å

{222}

2

{220}

{200}

4

{400}

3

along [111]

{422}

6

{420}

8

20

Integrate intensity (arb. unit)

Page 46 of 47

(a)

K-Ni-Cr (shell)

198

Net intensity (10 counts)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47

Integrate intensity (arb. unit)


0 -2 -4

20

30

40

50

60

Scattering angle 2 (deg.) ACS Paragon Plus Environment

70

Fig. 11

Page 47 of 47

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47


a b

c

Ni

N

C


Cr

Fig. 12

Complex Magnetic Phases in Nanosized Core@Shell Prussian Blue

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