Evidence for Point Transformations in Photoactive Molecular Crystals by the Photoinduced Creation of Diffuse Diffraction Patterns

Time-resolved diffuse X-ray scattering is one powerful method for monitoring the progression from the creation of local structural changes inside a crystalline material up to the transformation of the whole crystalline bulk. In this work, we study the mechanism of phototransformation of a molecular crystal by time-resolved diffuse X-ray scattering. Here, an optical excitation source, like a pulsed laser, initiates structural transformations which are monitored by X-ray scattering techniques. We have studied the dimerization process of the molecular switch α-styrylpyrylium (trifluoromethanesulfonate) TFMS, in particular for understanding whether cooperative effects influence the changes of the structure in the bulk and its periodicity. Upon illumination with optical light, α-styrylpyrylium TFMS instantaneously photoswitches. Depending on the optical fluence, X-ray diffuse planes are observed prior to phototransformation of the bulk. In the early stages of transformation, the analysis reveals systems of randomly distributed islands of product clusters with gradual growth in size and amount. The degree of transformation follows the optical excitation profile, i.e., the spatial absorption of the laser beam. In the present studies, no influence of cooperativity on the photodimerization process has been found

Materials Properties of Ultra-Incompressible Re₂P

In situ high-pressure X-ray powder diffraction measurements on Re₂P up to 37.0 GPa at ambient temperature in diamond-anvil cells were carried out at two different synchrotron facilities (ESRF and DESY). The compressibility of Re₂P (<i>Pnma</i>, no. 62, <i>a</i> = 5.5464(17), <i>b</i> = 2.9421(8), <i>c</i> = 10.0483(35) Å, <i>V</i> = 163.97(9) ų, <i>Z</i> = 4, <i>R</i>_p = 0.1008, <i>wR</i>_p = 0.1341 at ambient conditions) was investigated and resulted in a bulk modulus of <i>B</i>₀ = 320(10) GPa after fitting the experimental <i>p</i>–<i>V</i> data to a second- and third-order Birch–Murnaghan equation of state. In addition, the determined bulk modulus is compared to values obtained from an Eulerian strain versus normalized stress plot with values ranging form 315(7) to 321(15) GPa. These experimental findings are confirmed by density functional theory (DFT)-calculations ranking Re₂P among ultra-incompressible materials. However, the Vickers hardness of a high-pressure sintered Re₂P–Re_<i>x</i>C_<i>y</i> composite material in the asymptotic hardness region was found to be of only 13(2) GPa. Electrical conductivity measurements indicate that metallic Re₂P exhibits Pauli-paramagnetism. Analysis of temperature-dependent in situ X-ray diffractometry reveals an approximately isotropic expansion of the lattice parameters with a thermal expansion coefficient of (α­(<i>V</i>) = 28.5–32.8(2)·10^–6 K^–1)

High-Pressure Synthesis of β‑Ir₄B₅ and Determination of the Compressibility of Various Iridium Borides

A new iridium boride, β-Ir₄B₅, was synthesized under high-pressure/high-temperature conditions of 10.5 GPa and 1500 °C in a multianvil press with a Walker-type module. The new modification β-Ir₄B₅ crystallizes in a new structure type in the orthorhombic space group <i>Pnma</i> (no. 62) with the lattice parameters <i>a</i> = 10.772(2) Å, <i>b</i> = 2.844(1) Å, and <i>c</i> = 6.052(2) Å with <i>R</i>1 = 0.0286, <i>wR</i>2 = 0.0642 (all data), and <i>Z</i> = 2. The structure was determined by single-crystal X-ray and neutron powder diffraction on samples enriched in ¹¹B. The compound is built up by an alternating stacking of boron and iridium layers with the sequence ABA′B′. Additionally, microcalorimetry, hardness, and compressibility measurements of the binary iridium borides α-Ir₄B₅, β-Ir₄B₅, Ir₅B₄, hexagonal Ir₄B_3–<i>x</i> and orthorhombic Ir₄B_3–<i>x</i> were carried out and theoretical investigations based on density function theory (DFT) were employed to complement a comprehensive evaluation of structure–property relations. The incorporation of boron into the structures does not enhance the compressibility but leads to a significant reduction of the bulk moduli and elastic constants in comparison to elemental iridium