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We investigate the behaviour of photo-excitable, bistable systems, under permanent light irradiation, in presence of relaxation towards the non-excited state. Cooperativity causes bistability of the steady state, leading to light-induced thermal and optical hysteresis (LITH and LIOH). The light-induced instability is expected to induce demixtion, i.e. the coexistence of domains of the two stable steady states. Such effects are evidenced by magnetic and reflectivity measurements on the spin-crossover solid solution: [Fex Co1−x (btr)2(NCS)2] · H2O, with x = 0.3, 0.5, 0.85. Experimental data are in quantitative agreement with a simple macroscopic model which includes a non-linear relaxation term in the master equation. PACS. 64.60.My Metastable phases -70.60.Es Relaxation effects -75.90.+w Other topics in magnetic properties and materials -78.20.Nv Thermooptical and photothermal effects

In the iron(II) spin-crossover compound [Fe(ptz) 6 ](BF 4 ) 2 , the thermal spin transition is accompanied by a crystallographic phase transition showing a hysteresis with T c V ) 128 K and T c v ) 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 h f P1 h crystallographic phase transition with a large low-spin fraction stabilizing the P1 h phase at low temperatures. In the mixed crystal [Zn 1-x Fe x (ptz) 6 ](BF 4 ) 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 Romstedt, H.; Hauser, A. J. Phys. Chem. Solids 1996, 57, 1743. Thus the P1 h 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(P1 h) of 144(8) cm -1 and the volume difference ∆V°H L of 29(4) Å 3 are determined from a simultaneous fit to a series of transition curves for different pressures and iron content x in the P1 h phase. These values are compared to the corresponding values for the R3 h phase, viz. Γ(R3 h) of 170(9) cm -1 andThe high-spin f 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 f 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 f 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 1 A 1 f 1 T 1 absorption band.

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