Chemistry Reference
In-Depth Information
Sc-II 23 GPa
4
2
(4201)
(4001)
(2001)
(6001)
0
2
θ
(deg.)
5
10
15
20
25
Fig. 13 Rietveld refinement of Sc-II at 23 GPa using the C-centred guest structure. Reflections are
indexed using their
hklm
indices. The
upper tick marks
show the calculated peak positions for
(
hkl
0) host reflections and (
hk
00) host/guest reflections, and
lower
ones mark (
hk
0
m
) guest-only
reflections. The difference between the observed and calculated profiles is shown below the
tick
marks
. The
inset
shows an enlarged view of the asymmetric peak at 17.5
, showing that it is
accounted for by the (4400) and (0002) peaks. The
vertical dashed line
is a guide to the eye
diffraction patterns would be completely different, even though the diffraction
peaks from the two phases have very similar d-spacings (Fig.
14
). However, Sc is
a hard metal, and preparing a small single crystal of Sc-I, and then compressing it
to
20 GPa through the Sc-I to Sc-II transition such that the sample remained
a single-crystal, seemed unfeasible. However, we noted that while the diffraction
pattern from a small piece of Sc-I gave a perfect powder pattern, the diffraction
rings from the same sample in the Sc-II phase were slightly “spotty”, suggesting
a degree of recrystallisation of the sample at the transition. We had seen this
previously in InSb [
164
], and particularly in Ba [
244
], where the degree of recrys-
tallisation was such that we were able to grow a single-crystal of the Ba-IV phase.
Compressing a small piece of Sc in a helium pressure transmitting medium, rather
than the mineral oil medium used in our initial study, revealed significant recrys-
tallisation at the Sc-I to Sc-II transition, such that layer lines of reflections were
clearly visible in the Sc-II diffraction patterns (Fig.
15
). These had the very
distinctive pattern of diffraction peaks from a host-guest structure, and analysis
of those reflections from the guest component of the structure revealed them to be
completely consistent with a C-face-centred, rather than a body-centred guest.
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