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diffraction peaks on a low, flat background. However, powder indexing methods
were completely unable to index all the peaks in the pattern, despite numerous and
lengthy attempts. However, when compressed further, it was noticed that some
peaks reduced in intensity, while others coalesced into single peaks. Indexing of
only the latter revealed that their positions were all accounted for by a very simple
body-centred monoclinic unit cell with lattice parameters
a
3.9189(3)
˚
,
¼
4.7334(4)
˚
,
c
3.0617(2)
˚
, and
b
113.525(6) at 8.5 GPa [
232
]. But
this solution still left some 16 peaks unaccounted for, as illustrated in Fig.
7
.
Attempts to fit these reflections as either a different phase, or as superlattice of
the monoclinic cell, were unsuccessful.
By 9 GPa, tellurium has passed through two phase transitions, and the atomic
volume has been reduced to less than 80% of the ambient value [
233
]. Despite this,
we attempted to load and pressurise a single-crystal of Te-I with the aim of
obtaining a single-crystal of Te-III that we could use to obtain more structural
information on the phase. After 29 attempts, we finally succeeded in obtaining
a twinned single-crystal of Te-III, part of the diffraction pattern from which is
shown in Fig.
8
. From the d-spacings of the reflections it was immediately clear that
the strongest Bragg peaks were those accounted for by the body-centred monoclinic
cell, while the weaker satellite reflections were the same as those we were unable to
account for in the powder data. Analysis of the positions of these satellite peaks
revealed they were all located in the same relative position to the main Bragg peaks
at (
h
,
k
¼
¼
b ¼
q
,
l
) with
q
~ 0.3: Te-III is incommensurate [
232
]. It should be noted
that a full single-crystal data set was not collected from the Te-III crystal, nor was it
necessary. The single image in Fig.
8
was sufficient to determine the incommensu-
rate nature of the phase.
D
101
2
∗
121
2
∗
∗
∗
∗
∗
011
1
031
1
G
Fig. 8 Part of the 2D diffraction pattern obtained from a twinned single-crystal of Te-III at
7.4 GPa. The main body-centred monoclinic reflections from the two twin components are shown,
indexed (
hkl
)
1
and (
hkl
)
2
. The satellite reflections are marked with
asterisks
, and a powder line
from the tungsten gasket is marked “G”. “D” marks a reflection from one of the diamond anvils
