SECTION
A:
CRITIQUE
OF
THE
ONE-CENTRED
DESCRIPTION
OF
THE
EXCITATION.
It has been suggested1,2 that excitations in alkali halide crystals do not involve charge transfer
but that in these processes the halide ion remains in an essentially one-centred but excited state.
These one-centred halide ion states were then compared with those of the iso-electronic noble gas so
as to obtain a useful description of the coupling between the angular momentum of the np5 halide
core with that of the excited electron, especially if this is not considered to be in a halide s orbital.
However this model is unable to offer any useful insights in to the energy needed to reach the first
excited level which would be described as containing a halide ion in an np5(n+1)s excited state. The
fluorides provide the most clear cut example of the failure of this approach. Here the anion excited
state is compared directly with that of the 2p53s level of a neon atom. This neon excitation energy of
16.6eV is 6eV larger than experiment for any of the fluorides. Furthermore, if this state is taken to be
essentially localized on the fluoride, polarization can hardly be invoked to explain the discrepancy. It
is also hard to believe that the magnitude of this polarization energy can be as large as 6eV. This
shows that the difference between the nuclear charges of fluorine and neon can not be neglected.
However, since the ionization potential of the fluoride ion is 3.4eV, the excitation energy to the 2p53s
level of a free F– ion would be even lower. This shows that the one-centred description must
introduce the electrostatic potential generated by the surrounding lattice. Since, however, this
potential is constant in the spatial region of the fluorine nucleus, introduction of this factor can only
affect the predictions if it recognized that the electron, considered as being in the 3s orbital of the
fluoride ion, has a significant electron density in the vicinity of the cores of the surrounding cations.
In these spatial regions, it experiences both the purely atomic like potential generated by each such
core, as experienced by a valence s electron of the metal, as well as experiencing the repulsive
electrostatic potential (M-1)/Re of the surrounding lattice. This repulsion, taken in conjunction with
the attractive potential –M/Re experienced by a halide electron, shows that the change in the
Madelung energy must contribute to Ect. However introduction of these factors introduces all the
energetic contributions predicted by the charge transfer description whilst simultaneously removing
any purely halogen property other than the electron affinity AH. These considerations show that the
halogen centred excitation model in its simplest form yields no insight into the excitation energies
whilst refinements introduced to rectify this deficiency lead essentially to the charge transfer
description in which the term (2M–1)/Re is the largest contributor to Ect.
1
It has also been pointed out3 that the one-centred np5(n+1)s exciton description of the first
excited state does not predict the correct intensities of the two lowest energy transitions. Thus the
charge transfer model predicts that the lowest energy transition will appear as a doublet because, after
the excitation, the halogen can be left in either the 2P1/2 or 2P3/2 states. In contrast, the lowest three
levels of the 2p53s configuration of Ne, having J=0, 1 or 2, belong in the first approximation to the 3P
term and are separated by about 400cm-1. The second J=1 level is essentially a singlet lying higher in
energy by around 1000cm-1. In the absence of spin-orbit coupling, which will mix a small quantity of
the 1P1 state into the 3P1 level, there is only one allowed transition. Since spin-orbit coupling mixes a
small quantity of the 1P1 state into the 3P1 level, a second, but much weaker transition will also be
observed. As the halogen nuclear charge increases, the second peak would be predicted to increase in
intensity until, for the 5p56s configuration of Xe, the spin-orbit coupling is significantly larger than
the 5p-6s electrostatic repulsion thereby predicting that the two peaks would have comparable
intensities. However the absence of any observed increase in intensity of the second peak, which
appears with a much greater intensity in the spectra of the alkali fluorides than would be predicted by
comparison with the spectrum of neon, must constitute evidence against the one-centred description.
In contrast the charge transfer model predicts, in agreement with experiment, that the relative
intensities of the two transitions are essentially independent of the nuclear charge although this latter
does govern the energy difference.
It should also be noted, as reported previously3-5, that the splitting between the two peaks in
each of the spectra of the rock-salt structured materials is very close to that in the spectrum of the
corresponding free halogen. This is not inconsistent with the charge transfer description after
recognizing that any small discrepancies could well be explained by in-crystal modifications of the
electronic structures of the halogens. Consideration of the spectra of the salts having the cesium
chloride structure yields3 the same conclusion.
SECTION B: COMPARISION OF THE POLARIZATION RESPONSES IN FOUR
DIFFERENT CHARGE TRANSFER DESCRIPTONS OF THE EXCITATION.
Section IIB outlined the principles and presented the conclusions deduced from approximate
calculations of the polarization response of the lattice to the electric fields created by four of the
possible types of defect created by the electronic excitation. The lattice, which consists of all the ions
still carrying the same charges as in the electronic ground state, was divided into those lying at a
distance Re from any defect species plus the remainder (for the forbidden state, the anions located a
2
distance
2 Re from the neutral halogen were also treated as close neighbours). The field created by
the defect at each of the former ions was calculated exactly by summing the fields generated by each
species in the defect. The remainder of the lattice was treated as a continuum with the defect treated
as a point multipole.
The (=x, y or z) component, Fs, of the electric field at any ion s located at a distance Re
from any defect species was calculated by summing over all the contributions created by each defect
species, the latter being labelled t.
Fs   qt Rts Rts3
(B1)
t
Here qt is the charge of species t relative to that in the unexcited lattice, so that qt is +1 for the
halogen created by the excitation.In (B1) Rts is the  component of the vector from species t to ion s.
The resulting fields at each type of lattice ion, as illustrated in Figure 2, are presented in Table SI for
each of the four possible defects. The quantities reported as (1/2)|F(S)|2 in the rows labelled ‘Total’
were calculated as the sum of the products of the individual anion (1/2)|F(S)|2 multiplied by the
degeneracies listed in the last column. Each of these latter is the number of ions related by symmetry
to that listed in the second column. These total (1/2)|F(S)|2 appear in the first column of Table III
where they yield the polarization energy of the near neighbour anions as –A(1/2)|FA2| in the
approximation in which the interaction between the induced dipoles is neglected. For the cations in
the dipolar and symmetric defects, the mean square fields and degeneracies are not reported in Table
SI as symmetry demands that they are the same as the corresponding quantities for the anions. For the
quadrupolar and forbidden defects, the coefficient of each cation contribution to the polarization
energies reported in Table III was derived as the product of the cation (1/2)|F(S)| with the degeneracy.
In the continuum description of the ions located at distances greater than Re from any species
pol
in the defect, their contribution E crdis
to the lattice polarization energy is given in the approximation
of neglect of interactions between the induced dipoles by

1
pol
E crdis
    | F(R) |2 dV
2
(B2)
Here |F(R)|2 is the square modulus of the electric field created at position R by the defect and  is the

density of polarizable material
given by cr/Vm which reduces to cr/(2 R e3 ) for rock-salt structured
crystals. The origin is taken at the centre of the defect so that the volume integration in spherical
polar coordinates is over both the angles  and  with the radial integration extending from some

3
minimum Rm to infinity. Each defect is treated as a point multipole with its components presented in
the first two lines of Table SII. The three resulting components F of the electric fields presented in
lines 5 to 7 of this table were derived from the expression appearing in the third line by using the
relations (41) and (36) of the review6. The polarization energies presented in the ninth line of Table
SII were derived from equation (B2) after performing the integrations described above. The lower
limit Rm of each radial integration was taken to be the distance from the origin to the closest ion not
considered amongst the defect nearest neighbours whose contributions appear in Table SI. In the last
pol
line of Table SII, the final polarization energies E crdis
arising from the ions treated as a continuum
were calculated by using the Rm values appearing in the previous line. The numerical contributions to
pol
pol
E crdis
appear in decimal form in Table III. For the defect labelled forbidden, the value of E crdis
is

twice that for the quadrupolar defect because the unique quadrupole moment component of the
former is twice that of the latter, both quadrupoles being traceless with only two different non-zero

numerical values.

SECTION C: POTENTIAL PARAMETERS AND CALCULATIONAL DETAILS
1. Polarizabilities and dispersion energies
The dispersive attraction between mononuclear species X and Y was evaluated as a function of
their internuclear separation rXY through
disp
6
8
QD
8
VXY
(rXY )   6 (dX ,dY ;rXY )C6 (XY)rXY
  8DQ (dX ,dY ;rXY )C8DQ (XY)rXY
  QD
8 (dX ,dY ;rXY )C8 (XY)rXY
(C1)

Here C6(XY), C8DQ (XY) and C8QD (XY) are respectively dipole-dipole, dipole-quadrupole and
quadrupole-dipole dispersion coefficients. The parameters dX and dY derived from the properties of
the individual species X and Y define the function n(dX,dY;rXY) which reduces the magnitude of the


undamped (n(dX,dY;rXY) =1) attraction at rXY values at which the overlap of the electron densities of
the interacting pair is not negligible. The Axilrod-Teller triple dispersion interactions make only very
small (c.a. 5kJ mole-1) energy contributions7 and were not included as their consideration would very
significantly increase the computational requirements.
The dipole-dipole C6(XY) coefficients were calculated from the polarizabilities via the SlaterKirkwood formula8
4
C6 (XY) 
3
 X Y
1/ 2
2 ( X / PX )  (Y / PY )1/ 2
(C2)
For each cation and anion, the electron number PX was derived9 by demanding that this formula
reproduced the exactly known
C6(XX) for the iso-electronic noble gas from the exact polarizability of

that gas. The required noble gas data are assembled in Table SIII. Each alkali atom electron number
PM presented in this table, derived using (C2) from accurately known values of C6(MM) and M,
differs very slightly from that derived previously9 because the present calculations used newer and
very slightly different values for the alkali atom polarizabilities M. The testing9 of (C2) by
comparing the best currently available values with its predictions for the coefficients of both unlike
alkali atom pairs and alkali atom noble gas pairs was repeated using the present M and PM. This
comparison does not need to be presented here in detail because the present Slater-Kirkwood
predictions differ from the results appearing in Table A2.29 by at most 1%.
The dipole-quadrupole dispersion coefficients were calculated by scaling the predictions of
Starkschall-Gordon formula10
3
C8DQ (XY)  C6 (XY) r 4
2
Y
/ r2
Y
(C3)
Here r2Y and r4Y are the electronic expectation values calculated by considering just the most
Y as shown to yield the most reliable predictions9,11. Thus only the six
loosely bound electrons of ion
p electrons in species of s2p6 outermost electronic configuration are included whilst for an alkali atom
only the valence s electron is considered. However, for the interactions of two species having
outermost p6 electronic configurations, even this direct use of (C3) predicts values that are between
20% and 30% too small11. Each prediction of (C3) was therefore multiplied by a scaling factor
derived from a closely related pair for which a more accurate result12,13 is available. For interactions
involving solely the ions, each scaling factor K(CC), K(AA), KDQ(CA) and KQD(CA), in an obvious
notation, of 1.403, 1.285, 1.352 and 1.394 was derived from NaCl as the best ab initio quantum
chemistry result divided by the corresponding prediction from (C3). Current evidence indicates that
these scaling factors for species of p6 outermost electronic configuration are insensitive to the nuclear
charges11-14. For the interactions between an alkali metal and a noble gas, the accuracy of the
Starkschall-Gordon predictions (C3) was tested (Table SIV) by the comparing these with the
currently most reliable values. The results show that these predictions are as accurate as those for
those of the interactions between two species of p6 outermost electronic configurations. This justifies
5
using for each metal-ion interaction the scaling factor (Table SV) derived as the ratio of the best to
the Starkschall-Gordon prediction for the interaction of that metal with the noble gas iso-electronic
with the ion.
The mathematical forms9,11,15 of the dispersion damping functions in (C1) have been
conveniently summarized7. The damping parameters (dX) derived using the previously presented
methods11, have already been presented for all the anions7,16, excepting those in KCl, KBr and RbBr,
all the cations7,16,17 and the nanotube carbon atoms18. For the three latter crystals, the parameters will
be reported elsewhere. For the neutral metal atoms the damping parameters (dM, Table SVI) were
calculated from the result of Lassetre theory19 also used for the cations and carbon atom
dM  2
I
M
 IM  IM

(C4)
Here all quantities are expressed in atomic units with IM being the energy of the longest wavelength

dipole allowed transition. The required
energies were taken from Moore20.
All the dispersion coefficients involving purely inter-ionic7,16,17 or ion-carbon18 interactions
have already been presented excepting those involving the anions in KCl, KBr and RbBr which will
be reported elsewhere. The coefficients for the interaction of each metal with both its cation and a
carbon atom are presented in Table SVI whilst Table SVII reports those for the interaction of each
metal with its in-crystal anion.
In the computations using the GULP program, the shell model used to describe the
polarization responses requires for each species X, a shell charge (YX) and a quadratic force constant
kX, which determine the polarizability X as YX2 / k X . For the cations, metal and nanotube carbon
atoms, the natural choice for YX (–PX) of the negative of the electron number for each shell charge
allows kA to determined by demanding that the polarizability is reproduced (see Table I, Table SIII,

and footnote b to Table SVI). For the in-crystal anions, the parameters YA and kA were determined, as
described elsewhere16 for RbCl, by demanding that the experimental values of both the high
frequency and low frequency dielectric constants of the bulk crystals are simultaneously reproduced.
The details for both the neutral halogens and the other crystal ions will be reported elsewhere.
2. Short range interactions
The short range inter-ionic potentials needed for the GULP computations were derived using
the RELCRION program as described elsewhere16. Since the resulting potentials do not describe any
effects arising from electron correlation, each such potential was augmented9,11,16 by an electron
6
correlation contribution of short range computed from the electron densities of the ions using the
density functional theory of an electron gas of uniform density21. The model, the OHSMFS model11,
used to describe the significant modifications of the anion wavefunctions by their environment incrystal causes the total crystal energy to contain, in addition to the usual two-body interactions, the
rearrangement energy9,22 Ere(R) needed to convert a free anion into its form optimal for the crystal
having geometry defined the closest cation-anion distance R. Since, however, the GULP program
does not handle compressible ion models and rearrangement energies, it was necessary to introduce
eff
(R) ) defined by
for each of the cubic phases, the short range cation-anion effective pair potential ( VsCA
eff
VsCA
(R)  VsCA (R) 
1
E re (R)
nCA

(C5)
where nCA is the coordination number. This definition ensures17,23 that the expression (2.3) of
reference 21 for the total crystal
 cohesion UL(R) is reproduced if (C5) is taken to be the short range
cation-anion interaction in standard expressions for the crystal energy which do not contain a
rearrangement term. The GULP program can handle two-body potentials in both numerical and a
range of analytic forms. Since these potentials have already been computed for both RbCl16 and all
three of the iodides7 considered here, the only interactions that needed to be computed were those
involving the anions in KCl, KBr and RbBr. For each of these three crystals, the computed potentials
and their representation by simple analytic functions will be reported elsewhere.
For the encapsulated crystals of KI, the potentials describing the interaction between each ion
and the carbon atoms of the nanotube wall had to be computed for the previous study18 of ground
state encapsulated crystal geometries. The short range interactions were computed from the HartreeFock electron densities21,24 using the density functional theory of an electron gas of uniform density
and finite extent25,26 in the variant27 as fully discussed elsewhere22.
3. Computational techniques
Since the ions in an encapsulated crystal do not lie on centres of symmetry, these species will
be polarized in even the ground state of the entire system of tube plus crystal. If the computation is
performed for a geometry, which does not minimize the total energy in the ground state, there will be
non-zero forces acting on the shells which will therefore acquire additional displacements and energy
contributions which are spurious in that they do not arise at the equilibrium geometry predicted by
the computations.
7
The above difficulty was handled by first calculating the energy of the ground state at the
optimal nuclear positions with all ions carrying their charges of unit magnitude and neutral carbon
atoms. For the case of square planes, these nuclear positions were generated from the experimental
values of a and b whilst, for the diamond shaped planes predicted by the computations, each nuclear
geometry was taken from the latter. The omission of any shell model shells ensured that all the
species were unpolarizable. This gives the electrostatic, short range and dispersive contributions to
the excitation energy. Then the shells are added to the ions still retaining their charges of unit
magnitude with still neutral carbons and the shells in the ground state relaxed to equilibrium at the
same nuclear geometry. The excitation energy to reach the excited state is then computed with the
shells retaining their displacements previously found to be optimal for the ground state. This
calculation is then repeated after relaxing the shells to their positions optimal for the excited state.
The difference in the excitation energies predicted by the two latter calculations is the required
polarization energy. This polarization energy, considered as the best approximation, is added to the
electrostatic, short range and dispersion contributions predicted at the experimental geometry to yield
the final prediction for the excitation energy.
A computation of the excitation energy, at a nuclear geometry not yielding the minimum
energy predicted from the potential model used, will predict an excited state polarization energy
containing a spurious contribution from forces arising from the inconsistency between the potential
model and the fixed nuclear geometry. The fully ionic computation for the ground states of some of
the crystals, including all of the iodides, predicted7 internuclear separations that were greater than
experiment by around 0.15a.u. The need to perform the excitation energy computations using
potentials predicting the experimental nuclear geometry to a much greater precision necessitated
introducing the small additional attractive cation-anion interaction defined by equation (3.1) of
reference 7. For each crystal, the two parameters defining this potential were derived by demanding
that both the experimental equilibrium separation and bulk compressibility were reproduced. The
need to introduce this small additional potential is shown by the results, derived without its
consideration, of 8.40eV, 7.97eV, 7.52eV and 7.35eV for the excitation energies of KCl, RbCl, KBr
and RbBr respectively computed, for the reasons presented above, at the equilibrium geometries
predicted using the fully ionic model. These results are significantly greater than both experiment and
the predictions presented in Table IV. These energies are greater than experiment because the
positive purely coulombic ([2M–1]/Re) electrostatic contribution decreases with increasing
internuclear distance more rapidly than the magnitudes of the negative dispersion and polarization
contributions. However it should be noted that the small additional potential, being taken to be the
8
same in the ground and excited states, does not very directly affect the predicted excitation energy.
The only role played by this potential in the determination of Ect is that of making a minor
contribution to the changes in the inter-shell interaction energies arising when the shells in the excited
state are displaced from their ground state equilibrium positions.
9
TABLE SI. Electric fields created at nearest neighbour ions by four types of defect a-c
Defect
dipolar
ion
A1`
A2
total
quadrupolar
Fx(S )
–3/4
1/(2 2 )

Fy(S )
Fz(S )
(1/2)|F(S)|2
0
0
0.2813
1
0
0.2715
4
–[1–1/(2 2 )]

1.367
A1
–0.1172
A2
0
0
0.0069
4
0.0365
0.0365
0
0.0013
4
A3
0.2126
0
0.0227
8
C1
0
0
0.2089
2
–0.0150
1–1/(2 2 )
total
symmeterical
0.2144
A1
–0.0544
A2
–0.0780
0
–0.0780
0
0.0015
6
0
0.0061
12
total
forbidden
deg
A1
0.0822
–11/36
0
–0.1912
0
0.0467
2
0
0.0531
8
A2
0.2641
A3
0
0.1611
0.1611
0.0260
4
C1
0
1–1/(2 2 )
0
0.2089
4
total
0.6224
a
absolute values of fields F(=x,y or z) given by multiplying the scaled F (S ) values by 1/Re2
b
absolute values of mean square fields |F|2 given by multiplying the |F(S)|2 results by 1/Re4
c
the quantities labelled “total” are defined in the text.



10
TABLE SII. Continuum descriptions of polarization energies of distant ions.
dipolar

quadrupolar
 y =  z =0
 = 0 (≠)
 x = – R e/2
–2xx =–2xx = zz = R e2 /2
F(D ) = ∑ T
F(Q ) = ∑ ∑T
T = 3R-5( – R2)
-7
T = –3R
[5 –R2)(++)]
Fx(D ) = xR-5(3x2–R2)
Fx(Q ) =(3/2)zzR-7x(5z2–R2)

Fy(D ) =3 xR-5xy
Fy(Q ) =(3/2)zzR-7y(5z2–R2)

Fz(D)=3 xR-5xz

Fz(Q) =(3/2)zzR-7z(5z2–3R2)

|F(D)|2 = x2 R-6(3sin2cos2 +1)

|F(Q)|2 =(9/4)  2zz R-8(5cos4–2cos2 +1)

pol
E crdis
= –(1/2)cr4/(3Re Rm3 )
pol
E crdis
= –(1/2)cr3 /(10Re Rm5 )

Rm=( 13 /2)Re


Rm= 3 Re
pol
E crdis
= –cr Re4[16 /(39 13 )]
pol
E crdis
= –cr Re4 [
 /(60 3 )]




TABLE SIII. Polarizabilities, C6(XX) coefficients and electron numbers for alkali atoms and noble
gases (a.u.) a,b,c,d
X
Li
Na
K
Rb
Cs
Ne
Ar
Kr
Xe
X
164.0
162.7
292.9
319.2
402.2
2.676 11.096 16.753 27.318
C6(XX)
1390
1580
3820
4600
7380
6.93
PX
0.7787
1.030
1.032
1.157
1.488
4.455
68.5
6.106
139
301
7.305
7.901
a
X = alkali metal M or noble gas N.
b
Na polarizability from atomic interferometary experiment28, other alkali polarizabilities from E-H
balance experiments29
c
see footnotes to Table A2.1 of reference 9 for the primary sources of noble gas data
d
alkali atom C6(XX) from reference 30.
11
TABLE SIV. Comparison of best and Starkschall-Gordon predictions for dipole-quadrupole
dispersion coeffcients (a.u.) a,b
C8DQ (NM)
C8DQ (NM)
NM
USG
TNC
USG
NM
USG
NeLi

NeNa
2062.14
205.781
263
2647.06
1910

2380
233.473
285
ArLi 8234.21

ArNa 10534.09
NeK
5141.27
2380
233.473
285
ArK
NeRb
6258.57
6460
351.881
NeCs
9291.26
9360
KrLi
C8QD (NM)
TNC
USG
7360

9200
1630.73
1910
1843.93
2061
20649.87 21000
2538.49
3210
554
ArRb 25119.19 25300
2802.85
3880
447.85
675
ArCs 37274.02 36800
3565.63
4700
12301.08 10800
3040.85
3070
XeLi 19640.58 16500
6342.84
4930
KrNa
15714.52 13500
3433.52
3320
XeNa 25014.40 20600
7140.14
5330
KrK
30901.57
3100
4741.64
5150
XeK 49454.85 47800
9913.68
8270
KrRb
37585.37 37400
5234.85
6170
XeRb 60139.99 57600
10942.75
9900
KrCs
55757.50 54500
6657.70
7470
XeCs 89208.03 84400
13915.62 12000
a
TNC
C8QD (NM)
TNC
USG, predictions of Starkschall-Gordon formula (14) derived using the Slater-Kirkwood results for
the C6(NM) coefficients and radial expectation values taken from reference 31.
b
TNC values taken from Table IV of reference 32 derived from Pade approximant analyses of
experimental oscillator strength data.
TABLE SV. Starkschall-Gordon scaling factors for metal-noble gas (MN) interactions
KAr
KKr
KXe
RbAr
RbKr
RbXe
CsXe
KDQ(MN)
1.265
1.086
0.834
1.384
1.179
0.905
0.862
KQD(MN)
1.017
1.003
0.967
1.007
0.995
0.958
0.946
12
TABLE SVI. Metal dependent dispersion coefficients and damping parameters (a.u.)a-d
dM
C6(MC)
C8DQ (MC)
C8QD (MC)
C6(MW)
C8DQ (MW)
C8QD (MW)
K
1.0125
131.194
1075.8
10340.9
226.79
3073.3
17481.2
Rb
0.9910
0.9604
2449.9

5143.8
20658.5
Cs
244.494

516.979

317.92

4308.2
31491.4
a
48443.6
Cn(MW) is the Cn(XY) involving the interaction of a neutral metal atom (M) with a nanotube carbon
(W).
b
C6(MX) calculated from the Slater-Kirkwod formula (13) from the polarizabilities in Tables I and II
and electron numbers in Table SIII using18 W=10.26a.u. and PW = 1.118.
c
C8DQ (MX) and C8QD (MX) calculated from Starkschall-Gordon formula (14) using the scaling
factors presented in Table SV and metal valence orbital radial expectation values from reference 30.

C8DQ (MW) and C8QD (MW) calculated from C6(MW) using (14) without scaling factor and the radial

expectation values from references 18 and 7 respectively for carbon and the cations.
d


TABLE SVII. Metal-anion dispersion coefficients (a.u.)a,b
KCl
C6(MA)
534.660
KBr
KI
RbCl
RbBr
RbI
CsI(8:8)
712.230 1025.130
603.637
807.316 1153.431 1489.986
C8DQ (MA) 8804.3
11812.3
16504.8
10870.8
14407.6
20012.2
C8QD (MA) 10870.8
55063.7
76410.0
51619.4
68214.1
93834.9 139619.3
24491.0


a
see notes b and to Table SVI.
b
all values for the Rock-salt structure (6:6) except CsI(8:8) for the 8-fold coordinated CsCl structure.
13
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14
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15