2 research outputs found
Pressure-Induced Crystallization from Amorphous Calcium Carbonate
This report describes a newly observed phenomenon: pressure-induced
crystallization from amorphous calcium carbonate (ACC). Synthetic
ACC samples were pressurized up to 800 MPa at room temperature. Then
crystallization of vaterite and calcite was observed from X-ray diffraction
patterns. The crystallization pressure depends on the H<sub>2</sub>O contents of ACCs. The ACC samples with high-H<sub>2</sub>O content
(21 wt %), middle-H<sub>2</sub>O content (17 wt %), and low-H<sub>2</sub>O content (10 wt %) underwent crystallization at pressures
higher than 240 MPa, 400 MPa, and 640 MPa, respectively. These results
indicate that H<sub>2</sub>O in the ACC serves an important role in
the crystallization process and that we should treat ACC carefully
in preparation for analyses such as infrared spectroscopy to obtain
the intrinsic information related to amorphous materials
High-Pressure–High-Temperature Study of Benzene: Refined Crystal Structure and New Phase Diagram up to 8 GPa and 923 K
The high-temperature structural properties
of solid benzene were
studied at 1.5–8.2 GPa up to melting or decomposition using
multianvil apparatus and <i>in situ</i> neutron and X-ray
diffraction. The crystal structure of deuterated benzene phase II
(<i>P</i>2<sub>1</sub>/<i>c</i> unit cell) was
refined at 3.6–8.2 GPa and 473–873 K. Our data show
a minor temperature effect on the change in the unit cell parameters
of deuterated benzene at 7.8–8.2 GPa. At 3.6–4.0 GPa,
we observed the deviation of deuterium atoms from the benzene ring
plane and minor zigzag deformation of the benzene ring, enhancing
with the temperature increase caused by the displacement of benzene
molecules and decrease of van der Waals bond length between the π-conjuncted
carbon skeleton and the deuterium atom of adjacent molecule. Deformation
of benzene molecule at 723–773 K and 3.9–4.0 GPa could
be related to the benzene oligomerization at the same conditions.
In the pressure range of 1.5–8.2 GPa, benzene decomposition
was defined between 773–923 K. Melting was identified at 2.2
GPa and 573 K. Quenched products analyzed by Raman spectroscopy consist
of carbonaceous material. The defined benzene phase diagram appears
to be consistent with those of naphthalene, pyrene, and coronene at
1.5–8 GPa