First study of the photo-induced metastable (paramagnetic) high-spin state in a [Fe(ptz) 6 ](BF 4 ) 2 single crystal by polarised neutron diffraction

Abstract Some spin-crossover solids can show photo-convertibility at low temperatures, which generally makes this class of compounds interesting for studies towards application as optical switches and memories in numerical displays. The mechanism of the optical switching is the light-induced excited spin state trapping (LIESST) and the reverse-LIESST, as reported before [Inorg. Chem. 24 (1985) 2174; J. Phys. Chem. 94 (1991) 2741]. These processes can be combined with the relaxation in order to cycle the excited state, which involves temperature scanning of the title system [Polyhedron 20 (2001) 1599]. In this paper, we report the spin density map in the [Fe(ptz) 6 ](BF 4 ) 2 single crystal in its undercooled R 3 i space group in the photo-induced metastable high-spin state, which is paramagnetic, at T =2 K. The experiment was carried out on the 5C1 polarised neutron diffractometer at LLB newly equipped with an optical fiber for the light-coupled measurements. The photo-excitation reaches a steady state within an hour and a half. The lifetime of the photo-induced high-spin state at 40 K exceeds 10 h, as in accordance with previous spectroscopic, magnetic and Mossbauer measurements. The first results obtained on [Fe(ptz) 6 ](BF 4 ) 2 can serve as a base to future investigations of other photo-convertible materials.