OPTICAL-DETECTION OF THE SPIN TRANSITION BY REFLECTIVITY - APPLICATION TO [FEXCO1-X(BTR)(2)(NCS)(2)]CENTER-DOT-H2O

Apparatus for measuring a reflected light signal on crystalline or pow der samples of thermochromic materials, such as 'spin-crossover' mater ials, has been developed in conjunction with a helium cryostat enablin g measurements from cryogenic to ambient temperatures. Depending on th e intensity and wavelength of the incident light and optical propertie s of the sample, reflectivity measurements provide information about a relatively thin surface layer of the investigated compound. The therm al spin transition or photoexcitation from the low-spin state, which i s usually coloured, to the high-spin state, which is white or transpar ent, is detected by a change in the relative intensity as shown on exa mples of [FexCo1-x(btr)(2)(NCS)(2)]. H2O crystalline powders (btr = bi s-triazole). To compare the situation in the bulk, simultaneous magnet ic measurements were performed.