Search results for "spin crossover"

The influence of hydrostatic pressure on hysteresis phase transition in spin crossover compounds

1999

Abstract The effect of hydrostatic pressure on the transition temperature and the hysteresis widths of first order spin crossover phase transitions has been studied. A decrease as well as an increase of the hysteresis width with increasing pressure was reported in the literature. The increase of width with increasing pressure contradicts the expectation derived from mean field theory. We remeasured the pressure dependence of the temperature hysteresis of the compound [Fe(phy)2](BF4)2 (phy=1, 10-phenanthroline-2-carbaldehydephenylhydrazone). The spin transition temperatures at ambient temperature are T ↑ 1/2 =289±1 K, T ↓ 1/2 =283±1 K . An increase of the hysteresis width of d Δ T 1/2 / d p=…

Nature of the phase transition in spin crossover compounds

1990

Abstract Starting from the phenomenological free energy describing the spin equilibrium of continuous or gradual high spin (HS) ⇌ low spin (LS) transitions a reduced equation of state has been derived which is of the type known in mean field theories. The continuous HS ⇌ LS transitions of [Fe(2-pic-ND 2 ) 3 ]Cl 2 ·EtOD (2-pic = 2-picolylamine) at ambient pressure and p = 1200 bar and of [Fe(2-pic) 3 ]Cl 2 -MeOH can be classified as isobars above the critical point of the system. Around and below the critical point a complex behaviour is expected for thermodynamic reasons combined with the consequences of an elastic interaction mechanism between the HS and LS complex molecules in the crystal…

Bidirectional photo-switching of the spin state of iron(II) ions in a triazol based spin crossover complex within the thermal hysteresis loop

2009

Abstract We have investigated the effect of short laser pulses (532 nm, 4 ns, −2 ) on the spin state of iron(II) ions in the spin crossover compound {[Fe II (Htrz) 2 (trz)](BF 4 )} within the hysteresis region of the high-spin (HS) to low-spin (LS) first-order thermal phase transition. Using Raman spectroscopy we have evidenced quasi-complete HS → LS as well as LS → HS photo-conversions, which can be induced by a single laser shot in the descending (351 K) and ascending (378 K) branches of the hysteresis loop, respectively. No effect has been observed, however, close to the center of the hysteresis loop even for repeated exposures.

Analytical solution of 1D Ising-like systems modified by weak long range interaction

1999

It is well-known that 1D systems with only nearest neighbour interaction exhibit no phase transition. It is shown that the presence of a small long range interaction treated by the mean field approximation in addition to strong nearest neighbour interaction gives rise to hysteresis curves of large width. This situation is believed to exist in spin crossover systems where by the deformation of the spin changing molecules, an elastic coupling leads to a long range interaction, and strong bonding between the molecules in a chain compound leads to large values for nearest neighbour interaction constants. For this interaction scheme an analytical solution has been derived and the interplay betwe…

Thermochromic Meltable Materials with Reverse Spin Transition Controlled by Chemical Design

2020

International audience; We report a series of meltable FeII complexes, which, depending on the length of aliphatic chains, display abrupt forward low‐spin to high‐spin transition or unprecedented melting‐triggered reverse high‐spin to low‐spin transition on temperature rise. The reverse spin transition is perfectly reproducible on thermal cycling and the obtained materials are easily processable in the form of thin film owing to their soft‐matter nature. We found that the discovered approach represents a potentially generalizable new avenue to control both the location in temperature and the direction of the spin transition in meltable compounds.

Pressure-induced structural evaluation and insulator-metal transition in the mixed spinel ferrite Zn0.2Mg0.8Fe2O4

2017

The effect of pressure on the electronic properties and crystal structure in a mixed spinel ferrite $\mathrm{Z}{\mathrm{n}}_{0.2}\mathrm{M}{\mathrm{g}}_{0.8}\mathrm{F}{\mathrm{e}}_{2}{\mathrm{O}}_{4}$ was studied for the first time up to 48 GPa at room temperature using x-ray diffraction, Raman spectroscopy, and electrical transport measurements. The sample was cubic (spinel-type $Fd\overline{3}m$) at ambient pressure and underwent a pressure-induced structural transition to an orthorhombic phase $(\mathrm{CaT}{\mathrm{i}}_{2}{\mathrm{O}}_{4}\ensuremath{-}\mathrm{type}\phantom{\rule{0.16em}{0ex}}Bbmm)$ at 21 GPa. This structural transformation corresponded to a first-order phase transition …

Cover Picture: One Shot Laser Pulse Induced Reversible Spin Transition in the Spin-Crossover Complex [Fe(C4H4N2){Pt(CN)4}] at Room Temperature (Angew…

2005

Guest Modulation of Spin-Crossover Transition Temperature in a Porous Iron(II) Metal Organic Framework: Experimental and Periodic DFT Studies

2014

The synthesis, structure, and magnetic properties of three clathrate derivatives of the spin-crossover porous coordination polymer {Fe(pyrazine)[Pt(CN)(4)]} (1) with five-membered aromatic molecules furan, pyrrole, and thiophene is reported. The three derivatives have a cooperative spin-crossover transition with hysteresis loops 14-29 K wide and average critical temperatures T-c=201 K (1.fur), 167 K (1.pyr), and 114.6 K (1.thio) well below that of the parent compound 1 (T-c=295 K), confirming stabilization of the HS state. The transition is complete and takes place in two steps for 1.fur, while 1.pyr and 1.thio show 50% spin transition. For 1.fur the transformation between the HS and IS (mi…

Spin- and phase transition in the spin crossover complex [Fe(ptz)6](BF4)2 studied by nuclear inelastic scattering of synchrotron radiation and by DFT…

2006

Abstract Nuclear inelastic scattering (NIS) spectra of [Fe(ptz) 6 ](BF 4 ) 2 (ptz = 1- n -propyl-tetrazole) have been measured for five phases differing in spin state and crystallographic structure. Different spectral patterns have been found for the low-spin and high-spin phases and are described in terms of normal coordinate analysis of the complex molecule. For both low-spin and high-spin phases the conversion from ordered to disordered phase results in splitting of the observed NIS bands. Packing becomes visible in the NIS spectra via coupling of the Fe–N stretching vibrations with those of the terminal n -propyl groups. The DFT-based normal coordinate analysis also reveals the characte…

High-Spin → Low-Spin Relaxation in [Fe(bpp)2](CF3SO3)2 H2O after LIESST and Thermal Spin-State Trapping—Dynamics of Spin Transition Versus Dynamics o…

1996

The iron(II) complex [Fe(bpp)2]-(CF3SO3)2 H2O (bpp = 2,6-bis(pyrazolyl-3-yl)pyridine) shows a thermal spin transition associated with a hysteresis of approximately 140 K width. The transition temperatures T1/2 (where the fraction of HS species γHS = 0.5) are 147 K and ≈285 K in the cooling and heating directions, respectively. The compound shows the LIESST and reverse-LIESST effects at low temperatures. The relaxation of the metastable HS states generated by LIESST was observed quantitatively at temperatures between 77.5 and 85 K by Mossbauer spectroscopy. Metastable HS states can also be generated by rapid cooling of the sample. The relaxation of the metastable HS states formed by thermal …