We present novel insight on like-spin domains (LSD) in cooperative spin transition solids by following the photo-transformation and the subsequent relaxation of a [Fe(ptz)₆](BF₄)₂ single crystal in the vicinity of the light-induced instability. Self-organization under light is observed, accompanied by Barkhausen-like noise and jumps which reveal the presence of elastic interactions between LSDs. The light-induced phase separation process is discussed in terms of a dynamic potential providing spinodal instability in the corresponding temperature range. This useful concept is applicable to all types of switchable molecular solids.

The relation between the internal pressure during spin-crossover is compared to the chemical pressure induced by dilution with zinc. Further, the light of a specific LIESST (Light Induced Excited Spin State Trapping) wavelength is used to induce partial stabilisation of high-spin state and thus shift temperature of the spin-crossover towards lower values. The de-coupling of the spin-crossover and structural phase transition is discussed.

The quasi-static nature of a light induced thermal hysteresis was studied on the spin-transition compound [Fe(ptz)₆](BF₄)₂, by means of optical spectroscopy and magnetic measurements in the temperature interval between 10 and 80 K. Various experimental procedures are discussed in relation to the competition between the two processes considered, namely the photoexitation and the high-spin→low-spin relaxation. A detailed discussion of the experimental parameters, which should be considered in order to avoid erroneous interpretations of LITH, is given.

Present paper is an overview of our efforts during the past few years to understand complicated corelations of physical phenomena related to pressure in Fe(I1) solid state spin transition systems. Some principal results concerning p, T, λ-experiments are extracted. In the context of correlation of the crystallographic phase transition with simultaneous HS → LS relaxation and LS → HS photopopulation, we show the latest results: Brillouin and magnetic measurements on the crystal [Fe(pt₆](BF₆)₂.

The propagation of the high-spin (HS) → low-spin (LS) relaxation at 53 K in a single crystal of the iron (II) spin-crossover compound [Fe(ptz)₆](BF₄)₂ was followed by photography, after inducing the local photoexcitation to the metastable HS state at 20 K using the single wavelength (457 nm Ar^± ion laser) irradiation. The photoinduced formation of the HS—LS patterns with a characteristic diameter of some 0.1 mm was observed to occur inhomogeneously at a macroscopic scale already during photoexcitation. The contrast between the HS (transparent) and the LS (purple) regions was amplified during relaxation. The effect is described in terms of a transient instability, for which a microscopic model in the mean-field approximation is proposed. The mechanism for the development of patterns at the macroscopic scale is discussed.

In iron(II) spin-crossover compounds, the transition from the ¹A₁ low-spin state at low temperatures to the ⁵T₂ high-spin state at elevated temperatures is accompanied by a large increase in metal-ligand bond lengths. The resulting elastic interactions may be pictured as an internal pressure which is proportional to the concentration of the low-spin species. Because pressure stabilises the low-spin state relative to the high-spin state this results in a positive feedback. Thermal transition curves in neat iron(II) spin-crossover compounds are thus invariable much steeper than in diluted mixed crystals, and the high-spin→low-spin relaxation following the light-induced population of the high-spin state at low temperatures is self-accelerating. Strong interactions give rise to a thermal hysteresis, and light-induced bistabilities may be observed for compounds with initially a high-spin ground state and the potential for a light-induced population of the low-spin state. For such compounds, the increasing internal pressure may stabilise the low-spin state sufficiently so that it becomes the molecular ground state above some critical light-induced low-spin fraction. Secondary effects of the elastic interactions include crystallographic phase transitions, inhomogeneous distributions of sites, and anomalies such as steps in the transition curve.

A helium gas pressure cell for pressures up to 1 kbar (0.1 GPa) has been developed in conjunction with a closed-cycle He refrigerator allowing variable temperatures between 15 and 300 K. Both cell and refrigerator are equipped with optical windows suitable for photophysical measurements, such as temperature- and pressure-dependent absorption spectroscopy or laser flash photolysis. Examples of measurements on iron(II) spin-crossover systems are given. In these compounds, comparatively small external pressures induce significant changes in the thermodynamic equilibrium as well as in the relaxation dynamics.

The iron(II) spin-crossover compound [Fe(ptz)₆](PF₆)₂ (ptz = 1-propyltetrazole) crystallizes in the triclinic space group P†, with a = 10.6439(4) Ã…, b = 10.8685(4) Ã…, c = 11.7014(4) Ã…, α = 75.644(1)°, β = 71.671(1)°, γ = 60.815(1)°, and Z = 1. In [Fe(ptz)₆](PF₆)₂, the thermal spin transition is extremely steep because of cooperative effects of elastic origin. The transition temperature at ambient pressure is 74(1) K. An external pressure of 1 kbar shifts the transition temperature to 102(1) K, corresponding to a stabilization of the low-spin state, which is smaller in volume. The volume difference between the high-spin and the low-spin state, ΔV°_HL, is 24(2) Ã…³/molecule. The interaction constant Γ, as a measure of cooperativity, is within experimental error independent of external pressure and has a value of 101(5) cm^-1. In contrast to the case of the related compound [Fe(ptz)₆](BF₄)₂ (Decurtins et al. Inorg. Chem. 1985, 24, 2174), there is no hysteresis due to a first-order crystallographic phase transition, nor is there a hysteresis induced by external pressure as in the mixed crystal [Zn_1-xFe_x(ptz)₆](BF₄)₂, x = 0.1 (Jeftić et al. J. Phys. Chem. Solids 1996, 57, 1743). However, in [Fe(ptz)₆](PF₆)₂, the interaction constant Γ is found to be very close to the critical value above which a hysteresis solely due to the cooperative effects is expected. In addition, high-spin → low-spin relaxation measurements were performed under external pressures of up to 1 kbar in the temperature interval between 50 and 60 K. An external pressure of 1 kbar accelerates the high-spin → low-spin relaxation by 1 order of magnitude.

In the iron(II) spin-crossover compound [Fe(ptz)₆](BF₄)₂, the thermal spin transition is accompanied by a crystallographic phase transition showing a hysteresis with T_c^↓ = 128 K and T_c^↑ = 135 K at ambient pressure [Franke, P. L.; Haasnot, J. G.; Zuur, A. P. Inorg. Chim. Acta 1982, 59, 5]. The hysteresis is due to an interplay between the spin-transition and the R³ → P† crystallographic phase transition with a large low-spin fraction stabilizing the P† phase at low temperatures. In the mixed crystal [Zn_1-xFe_x(ptz)₆](BF₄)₂, x = 0.1, with the iron complexes imbedded into the isomorphous zinc lattice, the crystallographic phase transition can be induced by an external pressure [Jeftić, J.; Romstedt, H.; Hauser, A. J. Phys. Chem. Solids 1996, 57, 1743]. Thus the P† phase is additionally stabilized by external pressure. The interaction constant Γ, which describes cooperative effects between the spin-changing complexes, differs for the two crystallographic phases. Values for Γ(P†) of 144(8) cm^-1 and the volume difference ΔV⁰_HL of 29(4) Ã…³ are determined from a simultaneous fit to a series of transition curves for different pressures and iron content x in the P† phase. These values are compared to the corresponding values for the R³ phase, viz. Γ(R³) of 170(9) cm^-1 and ΔV⁰_HL(R3) of 26(3) Ã…³. Surprisingly Γ(R³) is larger than Γ(P†) despite the fact that ΔV⁰_HL(R3) is smaller than ΔV⁰_HL(P1). The high-spin → low-spin relaxation at temperatures above ~80 K is thermally activated, while below ~40 K temperature independent tunnelling takes place. An external pressure of 1 kbar accelerates the high-spin → low-spin relaxation exponentially by 1 order of magnitude in the tunnelling region in both crystallographic phases and regardless of x. In the concentrated material the high-spin → low-spin relaxation is self-accelerating due a buildup of an internal pressure [Hauser, A. Chem. Phys. Lett. 1992, 192, 65]. Both cooperative effects and external pressure result in a shift of the maximum of the ¹A₁ → ¹T₁ absorption band.

At low temperatures an external pressure of 1 kbar accelerates the high-spin → low-spin relaxation in the [Zn_1−xFe_x(ptz)₆](BF₄)₂, X = 0.1, spin-crossover system by one order of magnitude. This is due to the large difference in volume between the high-spin and low-spin states of 26 Ã…³/molecule. The relative vertical and horizontal shifts of the potential wells of the two states as a function of pressure are estimated to be 130 cm^−1/kbar and 10^−3 Å/kbar, respectively.

In the [Fe(ptz)₆](BF₄)₂ (ptz = 1-propyltetrazole) spin-crossover system, the thermal spin transition is accompanied by a first order crystallographic phase transition (T_c^↓ = 128 K and T_c^↑ = 135 K) from R³Â above T_c^↓ to P¹Â at low temperatures (Wiehl L., Acta Cryst. B49, 289 (1993)). The high-symmetry phase can be super-cooled, in which case the spin transition is still complete and quite steep (T_1/2 = 125 ± 2 K) but now without a hysteresis. The corresponding interaction constant Γ is 170 cm^−1. In the diluted system [Zn_1−xFe_x(ptz)₆](BF₄)₂, X = 0.1, the spin transition is gradual with T_1/2 = 95 ± 2 K. From the shift of T_1/2 towards high temperatures with external pressure a value for ΔV_HL⁰ of 26 Ã…³Â molecule^−1 is obtained. Pressures above 250 bar induce a crystallographic phase transition even in the diluted system, as a result of which the spin transition is discontinuous. The interplay between the thermal spin transition and the crystallographic phase transition in the neat and the diluted system is discussed consistently.