High-density (HDA) and low-density amorphous ices (LDA)
are believed
to be counterparts of the high- and low-density liquid phases of water,
respectively. In order to better understand how the vibrational modes
change during the transition between the two solid states, we present
infrared spectroscopy measurements, following the change of the decoupled
OD-stretch (
v
OD
) (∼2460
cm
–1
) and OH-combinational mode (
v
OH
+
v
2
,
v
OH
+ 2
v
R
) (∼5000 cm
–1
).
We observe a redshift from HDA to LDA, accompanied with a drastic
decrease of the bandwidth. The hydrogen bonds are stronger in LDA,
which is caused by a change in the coordination number and number
of water molecules interstitial between the first and second hydration
shell. The unusually broad uncoupled OD band also clearly distinguishes
HDA from other crystalline high-pressure phases, while the shape and
position of the in situ prepared LDA are comparable to those of vapor-deposited
amorphous ice.
High-energy X-ray
diffraction (XRD) and Fourier transform infrared
spectroscopy (FTIR) of amorphous solid water (ASW) were studied during
vapor deposition and the heating process. From the diffraction patterns,
the oxygen–oxygen pair distribution functions (PDFs) were calculated
up to the eighth coordination shell and an
r
= 23
Å
. The PDF of ASW obtained both during vapor deposition
at 80 K as well as the subsequent heating are consistent with that
of low-density amorphous ice. The formation and temperature-induced
collapse of micropores were observed in the XRD data and in the FTIR
measurements, more specifically, in the OH stretch and the dangling
mode. Above 140 K, ASW crystallizes into a stacking disordered ice,
I
sd
. It is observed that the fourth, fifth, and sixth peaks
in the PDF, corresponding to structural arrangements between 8 and
12 Å, are the most sensitive to the onset of crystallization.
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