In this work we present a detailed study showing the importance of the Kubelka-Munk (KM) correction in the analysis of diffuse reflectivity measurements to characterize spin crossover compounds. Combined reflectance and magnetic susceptibility measurements are carried out as a function of temperature or time to highlight the conditions under which this correction becomes critical. In particular, we investigate the influence of the color contrast between the two spin states on the reflectance measurements. Interestingly, the samples’ contrast seems to play an important role on the spin-like domain structure as suggested by the symmetry of the FORC diagrams. These latest results are discussed within the framework of Classical Preisach model (CPM).

The structural properties of the iron mixed-valence complex ( n-C3H7)4N[FeIIFeIII(dto)3] (dto = dithiooxalato, C2O2S2) have been investigated by single-crystal X-ray diffraction (SCXRD) at low temperatures. ( n-C3H7)4N[FeIIFeIII(dto)3] has two-dimensional (2D) honeycomb layers consisting of alternating FeII and FeIII arrays bonded by bis-bidentate dithiooxalato ligands. Upon cooling, a superlattice structure with q = (1/3, 1/3, 0) was observed below 260 K, which corresponds to an order-disorder transition of the ( n-C3H7)4N+ ions between the honeycomb layers. The charge-transfer phase transition (CTPT) occurs at TC↑1/2 ∼ 120 K and TC↓1/2 ∼ 90 K upon heating and cooling, respectively, with an electron transfer between the FeII and FeIII ions, accompanied by a spin-state change, FeII ( S = 2; HS)-O2C2S2-FeIII ( S = 1/2; LS) ↔ FeIII ( S = 5/2; HS)-O2C2S2-FeII ( S = 0; LS). During the CTPT, the intersheet [FeIIFeIII(dto)3] distance decreased monotonously upon cooling, and an abrupt structural contraction was observed in the hexagonal 2D network. The volume contraction during the CTPT was quite small (∼0.7%), and differences in the structural distortions at the FeS6 and FeO6 sites were not found in the vicinity of the CTPT. We also calculated the orbital energies and the occupied spin states for the [Fe(O2C2S2)3] and [Fe(S2C2O2)3] octahedra in the vicinity of the CTPT by density functional theory (DFT). Because the local symmetry around the two coordinating iron ions is already lowered to trigonal symmetry, the CTPT did not cause any further deformation. This symmetry invariance resulted in an absence of orbital contributions to the total entropy change (Δ S) in the CTPT, which is in agreement with the previous heat capacity measurements. [Nakamoto, T; Miyazaki, Y; Itoi, M; Ono, Y; Kojima, N; Sorai, M. Heat Capacity of the Mixed-Valence Complex {[( n-C3H7)4N][FeIIFeIII(dto)3]}∞, Phase Transition because of Electron Transfer, and a Change in Spin-State of the Whole System. Angew. Chem., Int. Ed. 2001, 40, 4716-4719.].

We have used reflectance measurements to investigate the effect of a hydrostatic pressure on the molecular 1D spin crossover coordination polymer [Fe(hyptrz)]A2·H2O (hyptrz=4-(3′-hydroxypropyl)-1,2,4-triazole and A=4-chloro-benzenesulfonate) Rev. Sci. Instrum. 80 (2009) 123901. Both thermal and pressure hysteresis have been recorded at different pressures and temperatures, respectively, in order to obtain valuable information about the optimal conditions of their use as wireless temperature and pressure sensors. The experimental analysis has been completed with a theoretical study and potential applications in terms of temperature and pressure wireless detection are discussed.

The thermal properties of the perovskite slab alkylammonium lead iodide (FC6H4C2H4NH3)2[PbI4] are investigated using spectroscopic ellipsometry, differential scanning calorimetry, photoluminescence, and Raman spectroscopy. The spectroscopic ellipsometry, performed in the heating mode, clearly evidenced the presence of a singularity at 375 K. This is corroborated by the temperature dependence of the photoluminescence, which pointed out a first-order order-disorder phase transition at ∼375 K, with a hysteresis loop of 40 K width. Raman spectroscopy data suggest that this transition arises from a dynamic rotational disordering of the ammonium headgroups of the alkylammonium chain. In contrast, differential scanning calorimetry measurements on a pellet sample led to an entropy change value ΔS ≈0.39 J/K/mol at the transition, suggesting the existence of a residual short-range order of the NH3+ on cooling from the high temperature phase.

Iron(II) spin crossover molecular materials are made of coordination centres switchable between two states by temperature, pressure or a visible light irradiation. The relevant macroscopic parameter which monitors the magnetic state of a given solid is the high-spin (HS) fraction denoted n(HS), i.e., the relative population of HS molecules. Each spin crossover material is distinguished by a transition temperature T(1/2) where 50% of active molecules have switched to the low-spin (LS) state. In strongly interacting systems, the thermal spin switching occurs abruptly at T(1/2). Applying pressure induces a shift from HS to LS states, which is the direct consequence of the lower volume for the LS molecule. Each material has thus a well defined pressure value P(1/2). In both cases the spin state change is easily detectable by optical means thanks to a thermo/piezochromic effect that is often encountered in these materials. In this contribution, we discuss potential use of spin crossover molecular materials as temperature and pressure sensors with optical detection. The ones presenting smooth transitions behaviour, which have not been seriously considered for any application, are spotlighted as potential sensors which should stimulate a large interest on this well investigated class of materials.

In this work we present results obtained by the first-order reversal-curve (FORC) method for finding the effect of pressure on a spin transition. The FORC data were recorded by diffuse reflectivity measurements for the spin-transition compounds [Fe(btr)(2)(NCS)(2)]center dot H2O and [Fe0.6Zn0.4(btr)(2)(NCS)(2)]center dot H2O (btr = 4,4'-bis-1,2,4-triazole)under constant pressure in the range 1-1600 bars. The joint distributions in coercivity-bias coordinates, obtained by the FORC method, were expressed in energy gap Delta and interaction parameter J coordinates for a complete discussion of the pressure effect in terms of physical parameters, including their average values, distribution widths, and Delta-J correlation parameter. Pressure increases both Delta and J, as expected. Pressure has a negligible effect on the distribution widths but sizably decreases the correlation parameter value of the diluted system, an unexpected feature which suggests that the diluted system has multiple-domain behavior with pressure-dependent domain size. This deduction is supported by inspection of the samples using optical microscopy at room temperature. Simulations of the metal distribution in a two-dimensional lattice are performed in order to estimate the pressure-induced change in domain size.

The thermodynamic properties of the photomagnetic Prussian blue analog K0.3Co[Fe(CN)(6)](0.77)center dot 3.6H(2)O, which exhibits a charge-transfer-induced spin transition (CTIST), are reported. According to the thermal history of the sample, different low-temperature states are obtained: a quenched high-spin state (Q), a low-spin state (LT) achieved through the decay of the Q state, and an intermediate state (IM) combining low-spin (LS) and high-spin (HS) local states. The IM state is obtained by slow cooling and displays a slight kinetic dependence. The long lifetime of the IM state, which persists up to the vicinity of the CTIST temperature, is unusual and differs from all metastable states reported so far for photomagnetic Prussian blue analogs. The properties of the different states and of their photoexcited counterparts are investigated by magnetic and photomagnetic measurements, calorimetry, diffuse reflectance, and x-ray diffraction. A key feature of the IM state revealed by x-ray diffraction is the onset of phase separation between a LT-type fraction and a mixed HS-LS fraction (approximately 50 : 50). X-ray patterns of the IM state obtained during successive cooling and heating stages document irreversible transformations. The formation and properties of the IM state suggest a self-organization process between low-and high-spin sites in response to internal structural stresses, and this effect is hindered by irradiation with visible light. The relaxation kinetics of the Q and photoexcited states are analyzed by using a two-variable model, which accounts for the onset of correlations due to short-range elastic interactions. A quantitative comparison to the analogous sodium compound Na0.32Co[Fe(CN)(6)](0.74)center dot 3.4H(2)O confirms the nonstandard behavior of the title potassium compound and supports the structural origin of the self-organization processes.

We use spectroscopic ellipsometry to study the elastic and thermodynamic properties of the structural first-order transition of the cooperative Jahn–Teller solid, RbMn[Fe(CN)6]. While the analysis of the thermal dependence of the dielectric constant revealed a remarkable energy shift in the metal-to-ligand charge transfer band, that of the refractive index allowed to evaluate the volumetric thermal expansion coefficient in the two phases through the Gladstone–Dale relation. The access to the elastic properties of the solid is obtained with the reflectivity under pressure measurements, from which we estimated the bulk modulus values in the low- and high-temperature phases as BLT=30(±3) GPa and BHT=23(±2) GPa, respectively. Assuming the system is isotropic, the corresponding Debye temperature values have been found to be θDLT(SE)≃360(±25) K and θDHT(SE)≃290(±20) K, in good agreement with those derived from independent Mössbauer spectrometry investigations which led to θDLT(Moss)≃332(±7) K and θDHT(Moss)≃280(±11) K. Moreover, we have been able to extract from the SE data, the entropy change at the transition, ΔS≈64 J K−1 mol−1, which has been also found in good agreement with the value, ΔS≈60±5 J K−1 mol−1, derived from calorimetric experiments performed in this work.

The thermodynamic properties of the photomagnetic Prussian blue analog K0.3Co[Fe(CN)6]0.77 · 3.6H2O, which exhibits a charge-transfer-induced spin transition (CTIST), are reported. According to the thermal history of the sample, different low-temperature states are obtained: a quenched high-spin state (Q), a low-spin state (LT) achieved through the decay of the Q state, and an intermediate state (IM) combining low-spin (LS) and high-spin (HS) local states. The IM state is obtained by slow cooling and displays a slight kinetic dependence. The long lifetime of the IM state, which persists up to the vicinity of the CTIST temperature, is unusual and differs from all metastable states reported so far for photomagnetic Prussian blue analogs. The properties of the different states and of their photoexcited counterparts are investigated by magnetic and photomagnetic measurements, calorimetry, diffuse reflectance, and x-ray diffraction. A key feature of the IM state revealed by x-ray diffraction is the onset of phase separation between a LT-type fraction and a mixed HS-LS fraction (approximately 50 : 50). X-ray patterns of the IM state obtained during successive cooling and heating stages document irreversible transformations. The formation and properties of the IM state suggest a self-organization process between lowand high-spin sites in response to internal structural stresses, and this effect is hindered by irradiation with visible light. The relaxation kinetics of the Q and photoexcited states are analyzed by using a two-variable model, which accounts for the onset of correlations due to short-range elastic interactions. A quantitative comparison to the analogous sodium compound Na0.32Co[Fe(CN)6]0.74 · 3.4H2O confirms the nonstandard behavior of the title potassium compound and supports the structural origin of the self-organization processes.

Imaging nano-domains in spin crossover (SCO) compounds remains so far an unreached goal. However, researchers in France and Belgium bring us a step closer to this objective by reporting for the fi rst time an Atomic Force Microscopy (AFM) tapping-mode investigation of SCO single crystals. The techique images the local variations of the wetting properties of the crystal surface with respect to the coating fl uid (in this case atmospheric water) and can assign them to short-range changes in the physico-chemical properties of the compound. Such coating effects are suggested to be of general application for visualizing the surface properties of any material at the nano-scale, provided that a fl uid with suitable characteristics can be found. The study suggests the use of ferrofl uids coating layers to fi nally achieve imaging of like-spin domains structures in SCO compounds.

The photoreactivity of two iron(II)−styrylpyridine frameworks Fe(stpy)4(NCSe)2 (stpy = 4-styrylpyridine) has been investigated for the very first time in a crystalline solid. A quantitative cis-to-trans isomerization of stilbenoids is shown to occur in the confined environment of the inorganic solid. The photochromic reaction was driven by a visible excitation into the metal-to-ligand charge transfer absorption of the high-spin all-cis complex. The solid-state transformation is accompanied by a unit-cell volume increase and an amorphization. Interestingly, the photoproduct formed by irradiating the high-spin all-cis reactant undergoes a spin conversion when the temperature is decreased. This observation is related to the "ligand-driven light-induced spin change" effect in a constrained environment.

We investigated by diffuse reflectivity the effect of hydrostatic pressure (1–1800 bar) on the thermal spin transition of Fe(PM−BiA)2(NCS)2 polymorph I, where PM=N-2-pyridylmethylene and BiA=4-aminobiphenyl. We evidenced the onset of a progressive transformation into a phase of higher cooperativity which cannot be assigned to polymorph II. This result is discussed with respect to previous pressure investigations.

An iron(II) complex [Fe(L5)2][ClO4]2 of a new nonplanar tridentate ligand [L5 = 2-[3-(2'-pyridyl)pyrazol-1-ylmethyl](1-methylimidazole)] has been synthesized and its spin-transition properties have been investigated. X-ray structural analysis of the solvated compound [Fe(L5)2][ClO4]2.CH3CN at 100 K and at 298 K revealed a mononuclear Fe(II)N6 pseudooctahedron with the tridentate ligand in the meridional coordination mode. Magnetic susceptibility measurements revealed LS (1A1) <--> HS (5T2) transition starting above room temperature and remained incomplete even up to 350 K, however providing evidence for the hysteretic behaviour of the transition. Photo-magnetic measurements using a 532 nm laser revealed incomplete light-induced excited spin-state trapping (LIESST). Relaxation curves in the dark exhibited a stretched exponential shape, typical for broad distributions of relaxation times. The complete photo-magnetic data were modelled using a non-cooperative master equation including a tunnelling process with distributed parameters and accounting for the bulk absorption of light. We also measured the diffuse reflectance properties, with detailed investigation of the absorption spectra and discussed the obtained features with respect to structural properties, including the suggested presence of disorder.

One-dimensional (1D) coordination polymers of formula [Fe(NH(2)trz)(3)]A.nH(2)O, {A = TiF(6)(2-), n = 0.5 (1) and n = 1 (2); A = ZrF(6)(2-), n = 0.5 (3) and n = 0 (4); A = SnF(6)(2-), n = 0.5 (5) and n = 1 (6); A = TaF(7)(2-), n = 3 (7) and n = 2.5 (8); A = GeF(6)(2-), n = 1 (9) and n = 0.5 (10), NH(2)trz = 4-amino-1,2,4-triazole} have been synthesized, fully characterized, and their spin crossover behavior carefully studied by SQUID magnetometry, Mossbauer spectroscopy, and differential scanning calorimetry. These materials display an abrupt and hysteretic spin transition around 200 K on cooling, as well as a reversible thermochromic effect. Accurate spin transition curves were derived by (57)Fe Mossbauer spectroscopy considering the corrected f factors for the high-spin and low-spin states determined employing the Debye model. The unusual hysteresis width of 3 (28 K), was attributed to a dense hydrogen bonding network involving the ZrF(6)(2-) counteranion and the 1D chains, an organization which is also revealed in [Cu(NH(2)trz)(3)]ZrF(6).H(2)O (11). Trinuclear spin crossover compounds of formula [Fe(3)(NH(2)trz)(10)(H(2)O)(2)](SbF(6))(6).S {S = 1.5CH(3)OH (12), 0.5C(2)H(5)OH (13)} were also obtained. A structural property relationship was derived between the volume of the inserted counteranion and the transition temperature T(1/2) of the 1D chains. Two linear size regimes were identified for monovalent anions (0.04 or= 0.11 nm(3)) with saturation around T(1/2) = 200 K. These characteristics allowed us to derive an anion based database that is of interest for the prediction of the transition temperature of such functional switchable materials. Diffuse reflectivity measurements under hydrostatic pressure for 3,4 combined with calorimetric data allow an estimation of the electrostatic pressure between cationic chains and counteranions in the crystal lattice of these materials. The chain length distribution that ranges between 1 and 4 nm was also derived.

Imaging nano-domains in spin crossover (SCO) compounds remains so far an unreached goal. We report on the first AFM tapping-mode investigation of SCO single crystals, performed at room temperature with the well known mononuclear compound [Fe(ptz)(6)](BF4)(2) (ptz = 1-propyl-tetrazole) and the trinuclear supramolecular compound [Fe-3(hyetrz)(6)(H2O)(6)](CF3SO3)(6) (hyetrz = 4-(2'-hydroxyethyl)-1,2,4-triazole) which shows a gradual spin conversion centred at room temperature. The natural surface of the former crystal revealed a volatile coating of the scanned area attributed to the transport of adsorbed water under the effect of interaction with the AFM tip. The second one showed astonishing leopard-skin patterns assigned to the effect of atmospheric humidity on this hygroscopic compound. Their origin is discussed. We suggest the use of fluid coating layers as a general method for revealing the nano-patterning of physical properties (e.g. like-spin domains) at the surface of dielectric materials.[GRAPHICS]AFM-tapping images of [Fe-3(hyetrz)(6)(H2O)(6)](CF3SO3)(6) room temperature and ambient atmosphere. (C) 2010 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim