Article
P4/nmm Phase. A transition from the cubic to a new
high-pressure phase starts at 3 GPa and completes at 3.4
Inorganic Chemistry, Vol. 49, No. 11, 2010 5287
I-42m Phase. First traces of yet another phase appear at
∼17 GPa. The transition is not fully completed even at the
highest pressure achieved in our experiment. However,
the six most pronounced peaks were unambiguously
indexed in a body-centered tetragonal cell, suggesting
23
GPa. First 16 diffraction peaks were indexed by Dicvol in
a primitive tetragonal cell with a volume slightly over one-
half that of the cubic structure. Analysis of systematic
24
absences using the program ChekCell suggested two
possible space groups: P4/n and P4/nmm. A database search
suggested that this phase may be an antitype (positions of
four RbBH units per unit cell. Metrically, this tetragonal
4
structure is closely related to the C222 one, with the a and
b parameters of the latter becoming equal and the c
parameter doubled. Similar to the C222 phase, the new
structure also roughly resembles the CsCl-type, but a
closer analysis was required to reveal a more detailed
picture. The superstructure peaks 101 and 103 can be
clearly observed, and they cannot be modeled in the space
2
5
cation and anion are interchanged) of NH Br. A good fit
4
to the data confirmed this suggestion. Thus, the position
of the hydrogen atom was assumed by the similarity to the
NH Br structure. A small PO along [001] was modeled by
4
one refined parameter, which converged to 0.9635(14) at
5
.5 GPa; its value is systematically smaller than 1 on
groups containing 4 or 4 axes: these peaks indicate a
2
compression and larger than 1 on decompression. In
total, 38 reflections were fitted with only four intensity-
dependent parameters, including one coordinate para-
meter. The refinement converged at R = 4.0%, R =
corrugated distortion of the CsCl-type, which involves a
displacement of the heavy atom (Rb) along the c axis. Such
displacement can be accommodated in the space group I-4
and its supergroups. A good fit to the experimental data was
accomplished in the space groups I-4m2 and I-42m, where
B
F
3
background), and R =7.8%, R =5.5% (conventional).
.9%, R = 0.36%, R = 0.51% (not corrected for
p wp
the latter allows longer Rb H distances for the ordered
BH group. To our knowledge, this structure, like the C222
p
wp
3 3 3
C222 Phase. Above 10.4 GPa the P4/nmm phase starts
to transform into a new phase, and this transformation
completes at ∼12.2 GPa. Thirteen peak positions were
extracted from a data set collected at 16.1 GPa and indexed
4
one, represents a new structure type derived from the CsCl
type. A I-42m structural model well approximating the
2
6
experimental data was first created in FOX and then
refined by the Rietveld method. A PO along [100], very
similar to the one in the C222 phase, was modeled by one
refined parameter, which converged to 1.576(14). In total, 36
reflections were fitted with four intensity-dependent para-
meters, including one coordinate parameter. For the two-
phase samples containing C222 and I-42m phases the atomic
displacement factors for the identical atom types were
constrained to be the same, thus reducing the number of
parameters intrinsic to the I-42m phase to two. For the
sample containing ∼50% of the I-42m phase (23.5 GPa), the
refinement converged at R = 8.3%, R = 6.3%, R =
2
3
by Dicvol in a monoclinic cell. Although its unit cell
volume allowed only one RbBH unit per unit cell, the
4
structure could not be solved in any of the possible mono-
clinic space groups. Therefore, similar to the case of the
6
high-pressure structure of LiBH , we attempted a structure
4
solution in the space group P1. The structure has been
solved in P1 by a global optimization in direct space using
the program FOX. Starting with this model, a C-centered
26
orthorhombic structure with 2 times larger volume was
2
7
uncovered by Platon software. Only after taking into
account a pronounced PO along [010] was the structure
successfully refined in the space group Cmmm. Rb and B
atoms were reliably located in the special positions. The
resulting atomic arrangement represents an orthorhombi-
cally distorted CsCl-type structure, and to the best of our
knowledge it has no analogues, at least none that may
suggest possible positions of hydrogen atoms. Our analysis
B
F
p
0.57%, Rwp = 0.80% (not corrected for background), and
R = 12.0%, R = 9.9% (conventional).
p
wp
B. Pressure Evolution of the Unit Cell Dimensions. We
find three phase transitions at 3.0, 10.4, and 18 GPa, all of
them of first order. Two decompression experiments showed
that the first two transitions are reversible. The decompres-
sion experiment was not made for the highest pressure I-42m
phase. However, since the last transition is not reconstruc-
tive, we expect it also to be reversible. The ambient-pressure
Fm-3m polymorph transforms into the new P4/nmm phase
with a remarkable volume collapse of 8.8% and with a small
hysteresis of ∼0.4 GPa. Interestingly, in a narrow pressure
range, starting just below the cubic-to-tetragonal transition,
we observed additional diffraction lines which do not origi-
nate from the Fm-3mand P4/nmm phases. These peaks show
a maximum intensity at ∼2.75 GPa (d-spacings: 4.015, 3.877,
shows that a fully ordered BH group can be accommodated
4
in two subgroups of Cmmm: in Cmm2, where nearly linear
B-H Rb fragments lead to unlikely short H Rb
3
3 3 3 3 3
contacts; and in C222, where Rb HandH Hdistances
3
3 3
3 3 3
can be kept sufficiently long. Thus, we finally assumed the
ordered C222 model for this phase. In the final refinement, a
strong PO along [010] was modeled by one refined para-
meter, which converged to 1.542(10). In total, 33 reflections
were fitted with five intensity-dependent parameters, includ-
ing three anisotropic displacement factors for the Rb atom,
yielding a reasonable displacement ellipsoid (B =2.42(12),
˚
2.327, and 1.996 A). However, they always coexist with
11
2
˚
B = 4.7(5), B =1.76(11) A ). The refinement converged
33
peaks of the Fm-3m or the P4/nmm phases or from both
phases. Taking into account that the space group symmetry
change Fm-3m(number of formula units per primitive cell
Z = 1) f P4/nmm(Z = 2) breaks the group-subgroup
22
at R = 5.8%, R = 5.2%, R = 0.67%, R = 1.05%
B
F
p
wp
(
not corrected for background), and R =13.7%, Rwp =
p
1
0.9% (conventional).
P
P
relationship, this observation might be explained by the
onset of an intermediate (possibly metastable) phase, which
emerges during this reconstructive transition accompanied
by the big volume drop.
The experimental unit cell volumes were fitted with
the Murnaghan equation of state (EOS), yielding mole-
cular volumes at zero pressure and the bulk moduli
(
(
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