222
M. Onoda et al. / Journal of Solid State Chemistry 219 (2014) 220–227
the successful solution and refinement of the structure, the space
group and the lattice constants are determined as listed in Table 1.
Various parameters used for the structure refinements are also
summarized.
almost agree with those expected from the ionic crystal. The
apparent valence reduction for Li1 and Li4 indicates that they
may oscillate markedly, which is consistent with their large
thermal parameters.
On the basis of the atomic parameters of Li9V3P8O29 [4] and the
difference Fourier synthesis, the atomic coordinates were refined
by means of full-matrix least-squares calculations with anisotropic
displacement parameters and isotropic thermal parameter [7].
Here, atomic scattering factors were taken from Ref. [8], and
anomalous dispersion effects were included using the values from
Ref. [9]. In this refinement, there exists the difference density of
ꢀ2.5 e Åꢀ3 at around V ions by somewhat poor crystallinity owing
to the soft-chemical reaction. All of the calculations were per-
formed using the CrystalStructure crystallographic software pack-
age [10].1
The atomic coordinates, equivalent isotropic thermal para-
meters, and isotropic thermal parameter for Li4 of Li10V3P8O29 at
295 K are listed in Table 2, and the anisotropic displacement
parameters are in Table 3 in terms of the same atom-numbering
scheme as that for Li9V3P8O29 [4]. Different from the Li9V3P8O29
structure, the Li4 atom that exists in the 2a site with a full
occupancy is added from the difference Fourier synthesis as well
as from the consideration of the probability as a variable, and thus
the final chemical composition is expressed as Li10V3P8O29. The
existence of Li4 atom is also confirmed by Hamilton's significance
test on the R-factor [11]. Selected interatomic distances are listed
in Table 4. Fig. 2(a) and (b) shows the crystal structures of
Li10V3P8O29 projected along the c- and aþb-axes, respectively,
where the ellipsoids are drawn at the 90% probability level. Except
for the existence of Li4 atom, this structure is basically isomor-
phous to that of Li9V3P8O29 with (VP22O7)3(P1O4)2 layers in the
ab-plane built up of VO6 octahedra sharing corners with P1O4
tetrahedra and P22O7 groups [4]. The O3, O4, and O5 atoms by
which the V atom is surrounded octahedrally have relatively small
thermal parameters compared with those of the O1, O2, and O6
atoms only bound to the P and Li atoms. The anisotropic displace-
ment parameters are in a normal range.
As pointed out above, four crystallographically independent Li
sites labelled as Li1–Li4 exist in the present structure. The Li1, Li2,
and Li3 atoms exist between the (VP22O7)3(P1O4)2 layers, where
the Li1 atom is surrounded octahedrally and the Li2 and Li3 atoms
are coordinated tetrahedrally. The Li1 atom exists in the hexagonal
tunnels along the cꢀaxis formed by the stacking of the layers. On
the other hand, the Li4 atom with a significantly large thermal
parameter resides in the tunnels at the z level similar to that of
layers and in between the Li1 atoms as indicated in Fig. 2(c). This
atom is coordinated in a planar triangular arrangement by the
symmetry-related O2 atoms. Thus, Li10V3P8O29 has a characteristic
linear chain of Li ions parallel to the direction of diffusion path-
ways with the Li1–Li4 distance of c/4, that is 3.4040(5) Å. Here, it
should be noted that the difference Fourier map without Li1 and
Li4 atoms does not show any site disorder.
2.2.2. Li–Ag substituted phase
The X-ray powder diffraction patterns for the Li9ꢀyAgyV3P8O29
polycrystals show the single phase for the range of 0ryr1. The
composition dependence of trigonal lattice constants with hex-
agonal basis is indicated in Fig. 3 including the single-crystal data
in Table 1. The reason why the behaviors of a- and c-dimensions
do not simply follow Vegard's law is that the size of the hexagonal
tunnel is too wide for the Li occupation in the first place as
discussed later.
For the small single crystal with nominal composition
Li8AgV3P8O29, X-ray diffraction measurements were done using a
four-circle diffractometer described above. The crystal data, and
summaries of intensity measurements and refinements with
corrections for Lorentz polarization, absorption, and secondary
extinction effects are given in Table 1. The atomic coordinates and
the equivalent isotropic thermal parameters are provided in
Table 5. Considering the Li–Ag solution model, almost all Li atoms
in the 2b site are found to be substituted by the Ag atoms, and
the precise composition is determined to be Li8.06Ag0.94V3P8O29
.
Table 6 indicates the selected interatomic distances and the
effective valences calculated with Eq. (1). Although this structure
is basically isomorphous to that of Li9V3P8O29, the symmetry of
anisotropic displacement parameters for the Ag atom, where
1
U11 ¼ U22 ¼ 0:0120ð2Þ, U33 ¼ 0:0452ð4Þ, and U12 ¼ 0:00601ð7Þ , is
clearly different from that for the Li1 atom. This is because the size
of the hexagonal tunnel extending along the cꢀaxis is suitable for
the occupation of Ag ion [13], while it is wide for Li ion. Thus, the
Li1 ion has a degree of freedom not only for the c-axis but also for
the ab-plane. This characteristic may give rise to a rupture of
Vegard's law in Fig. 3. In this structure, the effective valence of Ag
ion agrees nearly with that from the ionic crystal.
The delithiated LixV3P8O29 system with xo9 has two kinds of
V–O–P–O–V superexchange pathways in the ab-plane, regarded as
being the distorted kagome lattice, due to the existence of some
spin density on the P ions [6]. Here, since the electronic admixture
for V–O3 bond is expected to be large, the V–O3–P1–O3–V path-
way may be more effective. Table 7 shows the composition
dependence of nearest-neighbor V–V distance and interatomic
angles of V–O–P–O–V pathways for Li9V3P8O29 [6], Li10V3P8O29
,
and Li8.06Ag0.94V3P8O29. There is little difference between the
pathways of Li9V3P8O29 and Li10V3P8O29. On the other hand, in
Li8.06Ag0.94V3P8O29, except for the P–O–P bond angles in the rigid
units of P1O4 and P22O7, the values of V–V and V–O3–P1 increase,
and those of V–O4–P2 and V–O5–P2 decrease. This tendency is
similar to the behavior of the V–O–P–O–V pathways against the
increase of V valence in the delithiated phase, where a ferromag-
netic coupling is induced gradually [6].
Using results in Table 4, the effective ionic valences v for all
atoms estimated in terms of the bond length r versus bond
strength s relation [12]:
2.3. Electrochemical characteristics
ꢀ
ꢁ
r0 ꢀr
s ¼ exp
;
ð1Þ
The electrochemical Li extraction–insertion characteristics for
Li9V3P8O29 and Li8AgV3P8O29 were obtained in the voltage range
between 1 V and 5.1 V using a cylindrical stainless steel cell [14]
with a constant current of C-rate 1/100 as in the case for
Li1þxV3O8, where metallic Li foils were used as a counter elec-
trode. The concentration for extracted and inserted Li ions was
calculated on the basis of the quantity of the electricity passed
through the working electrode.
B0
where r0 values for the V–O, P–O and Li–O bonds are 1.743, 1.617,
and 1.466 Å, respectively, and B0¼0.37 Å, are described in the
same table. Except for the Li1 and Li4 ions, the effective valences
1
Further details of the crystal structure investigations are obtained from the
Fachinformationszentrum Karlsruhe, 76344 Eggenstein-Leopoldshafen, Germany
(fax: þ49 7247-808-666; e-mail: crysdata@fiz-karlsruhe.de on quoting the deposi-
tory numbers CSD-47844 and CSD-47845.
The voltages
ϕ as a function of the electric capacity Q for
Li9V3P8O29 and Li8AgV3P8O29 are shown in Fig. 4(a) by the full and