Many properties of solid materials are intimately related to their crystal structures, and changes in properties are often accompanied by phase transitions. Here we show that a deeper understanding of the various phases of spin-crossover compounds, their phase transitions and properties, calls for multiple structure determinations as a function of external variables; the usual "one phase-one crystal structure" protocol is insufficient.The spin state of several first-row transition-metal complexes may be switched between high and low under the influence of an external perturbation such as a change in temperature or pressure or irradiation with light. The characteristics of the bi-or multistable compounds, for example, transition temperatures, hysteresis properties, thermochromism, photochromism, and photomagnetism as well as thermodynamic and kinetic quantities, depend on the interplay between the ligand-field strength at the transitionmetal ion and the interactions between the metal complex, the counterions, and solvate molecules, as governed by the crystal packing. The properties of spin-crossover complexes, especially of iron(ii) complexes, have been widely studied over the last several decades. Many publications stress the potential-unproven so far-for applications of such compounds in nanotechnology, for example, in information storage, as sensors, or as molecular switches. [1][2][3][4] In octahedral d 6 iron(ii) complexes the conversion is between a low-spin (LS) and a high-spin (HS) state of nearly equal energies (LS, S = 0, t 2g 6 e g 0 ; HS, S = 2, t 2g 4 e g 2 ). Spin

Illuminating disorder: Many highly efficient light emitters are members of the family β‐NaLnF4 (Ln=Y, La–Lu). Diffuse X‐ray scattering is combined with single‐crystal absorption spectroscopy at cryogenic temperatures to unambiguously identify and characterize two crystallographically inequivalent spectroscopic sites (A and B). The high light yield is correlated with the existence and nature of these sites.

"Structure determines properties", this assertion is a central paradigm in materials research as well as in molecular biology. It is a prime reason for the popularity of the notion of crystal engineering, whose goal it is to develop novel materials with predefined and tuned properties.[1] Such engineering necessarily proceeds in two steps: 1) establishing empirical or theoretical relationships between solid-state structure and desired properties, 2) tailoring of the structure by a synthesis process.[2] The property of main interest in spincrossover compounds is the thermal evolution of magnetic behavior. Herein we report on a serendipitous observation derived from magnetic and structural studies on six different alcohol solvates of iron(ii) tris(2-picolylamine) dichloride (1), in which the size of the solvating molecules ranges from methanol to tert-butyl alcohol. The six crystal structures are either isostructural or polytypic. In spite of evident structural similarity, the spin-transition curves are very different.

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Crystal structures, magnetic and thermodynamic properties of the spin-crossover compound tris(2-picolylamine)iron(II) dichloride (with 2-propanol solvent molecules) have been measured in the temperature range from 15 to 293 K. X-ray diffraction, SQUID, and calorimetric experiments all showed two first-order phase transitions with hysteresis loops, a narrow one at T(1) approximately 196 K and a broad, triangular one covering the range 153