Three fluorotoluene isomers, all liquids under ambient conditions, have been crystallized using two distinct in situ methods: cryo-crystallization (by decreasing the temperature below the freezing point) and through the application of high-pressure. In all three isomers, crystallization from highpressure results in different polymorphic forms to those observed at low-temperature: cryo-crystallization and compression of the pure liquid results in the formation of two distinct polymorphs of both 3-and 4-fluorotoluene, but only one polymorph of 2-fluorotoluene. However, compression of a 1:1 mixture of 2-and 3-fluorotoluenes results in a different polymorph of 2-fluorotoluene to that seen when crystallizing the pure liquid. The effects of temperature and pressure on the crystallization process in systems that have only weak C−H•••F−C hydrogen bonds are confirmed to be very different.
Crystallization of 2-fluorophenylacetylene
at room temperature
using high pressure yields three different polymorphs. These have
distinct supramolecular assemblies despite the only variation in the
crystallization protocol employed being the rate of compression.
Single-crystal X-ray diffraction has been used to elucidate the structure of two polymorphs of isopropyl alcohol, one grown through in situ cryo-crystallisation, the other through high-pressure crystallisation.
Polycrystalline Y 2 CoGe 4 O 12 has been prepared by standard ceramic methods. The crystal structure (space group P4/nbm; a = 9.8465(2), c = 4.92986(9) Å) consists of metal-rich layers separated from each other by Ge 4 O 12 groups comprised of four corner-sharing GeO 4 tetrahedra. Two cation sites lie within the layers; an eight-coordinate site occupied by yttrium and a six-coordinate site occupied by a 1:1 disordered distribution of yttrium and cobalt.Neutron diffraction revealed two-fold disorder on the oxide sublattice; this has been elucidated using Co K-edge EXAFS spectroscopy. The availability of two sites allows each oxide ion to accommodate the coordination preferences of its single Co/Y neighbour; the GeO 4 tetrahedra distort to absorb any consequent strain. The octahedron of anions around each Co 2+ cation shows a pseudo-tetragonal distortion with a strain (Co -O) eq -(Co -O) ax /(Co -O) eq = -0.173. This results in an unusually large effective magnetic moment of 6.05 µ B per Co 2+ cation.
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