Anti-Stokes luminescence of up-converting nanocrystals SrF:Yb,Er can be used as a high pressure optical sensor alternative to the ruby fluorescence-scale. In nanocrystalline SrF:Yb,Er, high pressure reversibly shortens the emission lifetimes nearly linearly up to 5.29 GPa at least. Its advantage is the use of NIR (≈980 nm) radiation, highly penetrable for many materials. The shortening of up-conversion lifetimes has been attributed mainly to the changes in energy transfer rates, caused by decreased interatomic distances and increased overlap integrals between 4f electrons and the valence shells of ligand ions. The origin of high-pressure effects on the luminescence intensity, band ratio and their spectral position has been explained by the increased interactions and distortions of the crystal-field symmetry around the emitting ions in the compressed structure.
The isothermal compression
and isobaric expansion of crystalline o-xylene (1,2-dimethylbenzene)
are inconsistent with the
rule of reverse relationship between effects of pressure and temperature
attributed to most crystals in general. On isobaric cooling at ambient
pressure, the o-xylene crystal shrinks, with the
strongest contraction of the unit-cell dimensions a and c, while during isothermal compression at ambient
temperature these are the least compressed directions. This direct
relationship (as opposed to the “inverse relationship”
rule) between the compression and expansion of o-xylene
has been associated with weak directional CH···π
interactions arranging the molecules into a two-dimensional framework
and with its distinct mechanisms of distortions occurring at high
pressure and low temperature. Single crystals of o-xylene were grown in situ in isochoric and isothermal conditions
in a diamond-anvil cell and their structure determined by X-ray diffraction.
At normal conditions, mandelic acid
(MA) is exempt from Wallach’s
rule, as enantiomers l-MA and d-MA are denser than
racemate dl-MA. However, the enantiomers are less compressible
than the racemate, dl-MA, which additionally is anomalously
compressed at the phase transition at 0.65 GPa and becomes more dense
than the enantiomers. Our results exclude the possibility of resolving
racemic dl-MA into enantiomers due to their higher density
in a spontaneous crystallization induced by pressure up to 2.6 GPa
at least. Also, the l-MA crystal undergoes an isostructural
phase transition at 1.52 GPa, allowing a tighter packing of molecules
mimicking a racemic arrangement. It is a general thermodynamic rule
that pressure-induced phase transitions in racemates and enantiomers
have the opposite effect for their density relations and act toward
or against Wallach’s rule, respectively.
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