Phase formation, crystal structure, and electrical conductivity of triple phosphates of alkali metals and titanium

Article abstract of DOI:10.1134/S0036023606080043

For triple phosphates of composition A_0.5A'_0.5 Ti₂(PO₄)₃ (A-A'=Li-Na, Na-K, K-Rb), phase formation is studied, the crystal structure is refined, and the electrical conductivity is measured. The compounds are classified with the NaZr ₂(PO₄)₃ structure type (NZP, space group R 3? c). The phosphate frameworks are built of TiO₆ octahedra and PO₄ tetrahedra. Extraframework positions M1 are fully occupied by randomly distributed alkali cations. Positions M2 are vacant. Correlations are found between the structural distortion and electrical conductivity of the phosphates, on one hand, and the alkali cation size, on the other. Pleiades Publishing, Inc. 2006.

Full text of DOI:10.1134/S0036023606080043

ISSN 0036-0236, Russian Journal of Inorganic Chemistry, 2006, Vol. 51, No. 8, pp. 1167–1175. © Pleiades Publishing, Inc., 2006.
Original Russian Text © E.A. Asabina, V.I. Pet’kov, M.V. Boguslavskii, A.P. Malakho, B.I. Lazoryak, 2006, published in Zhurnal Neorganicheskoi Khimii, 2006, Vol. 51, No. 8,
pp. 1252–1260.
SYNTHESIS AND PROPERTIES
OF INORGANIC COMPOUNDS
Phase Formation, Crystal Structure, and Electrical Conductivity
of Triple Phosphates of Alkali Metals and Titanium
a
a
b
E. A. Asabina , V. I. Pet’kov , M. V. Boguslavskii ,
b
b
A. P. Malakho , and B. I. Lazoryak
a
Nizhni Novgorod State University, pr. Gagarina 23, Nizhni Novgorod, 603950 Russia
b
Moscow State University, Vorob’evy gory, Moscow, 119992 Russia
Received January 26, 2006
'
Abstract—For triple phosphates of composition A0.5A0.5 Ti2(PO4)3 (A–A' = Li–Na, Na–K, K–Rb), phase for-
mation is studied, the crystal structure is refined, and the electrical conductivity is measured. The compounds
are classified with the NaZr2(PO4)3 structure type (NZP, space group R3 c). The phosphate frameworks are built
of TiO6 octahedra and PO4 tetrahedra. Extraframework positions M1 are fully occupied by randomly distributed
alkali cations. Positions M2 are vacant. Correlations are found between the structural distortion and electrical
conductivity of the phosphates, on one hand, and the alkali cation size, on the other.
DOI: 10.1134/S0036023606080043
Double alkali metal phosphates of Group IVB ele-
ments of the composition AD2(PO4)3 (A = Li, Na, K,
Rb, Cs; D = Ti, Zr, Hf) are now known (Table 1). Many
of them are structurally studied. Their main structural
feature is a stable framework built of DO6 octahedra
and PO4 tetrahedra; alkali metal atoms populate the
interstices in the framework. Such phosphates are of
interest because of their high chemical and thermal sta-
bility, radiation-damage stability, and several other
valuable physical and chemical properties [17]. In par-
ticular, compounds with the NaZr2(PO4)3-type structure
(NZP, NASICON) containing small extraframework
cations (Li+, Na+) frequently have high ionic conductiv-
ity [18–20].
The goal of this work is to synthesize triple phos-
'
phates A0.5A0.5 Ti2(PO4)3 (A–A' = Li–Na, Na–K, K–
Rb); to refine their crystal structures on the basis of
powder data (the Rietveld method); to discuss how cat-
ionic substitutions influence the structural features of
phosphates with similar chemical formulas, structural
parameters, and symmetry; and to study the electrical
conductivity of ceramic samples of the triple phos-
phates from 298 to 1223 K.
EXPERIMENTAL
The starting chemicals used in the synthesis of
'
A0.5A0.5 Ti2(PO4)3 (A–A' = Li–Na, Na–K, K–Rb) triple
phosphates were aqueous solutions of high-purity
reagents (Li2CO3, NaCl, KCl, RbCl, NH4H2PO4) and a
TiOCl2 solution prepared by oxidizing TiCl3 with a
mixture of nitric and hydrochloric acids. The synthesis
procedure was as follows. Stoichiometric amounts of
the aqueous solutions of alkali metal chlorides and tita-
nium oxychloride were combined at room temperature
under continuous stirring; then, an ammonium dihydro-
gen phosphate solution was added under stirring also in
accordance with the stoichiometry. The resulting gel
was dried at 353 K and, then, heat-treated with free
access to air at 873–1473 K. Since minor crystalline
phases with a low diffusion mobility can form upon
rapid temperature elevation, the annealing temperature
was consecutively elevated in 100-K steps; the anneal-
ing duration was 24–30 h with additional dispersion at
each step.
The NZP structure is distinguished by its high iso-
morphic capacity for different types of cations with dif-
ferent sizes and oxidation numbers: cations, either indi-
vidually or in combinations, can occupy the framework
and extraframework positions. Phase formation in
'
A1 − xAx D2(PO4)3 (D = Ti, Zr) system was studied
in [21, 22]; the concentration and temperature limits of
stability for the NZP structure were determined. The
investigations of mixed-alkali triple phosphates have
been confined to X-ray diffraction experiments; the
alkali cation distribution in the crystal lattice remains
unclear. The structural studies of such phosphates are of
theoretical interest: they would help to understand the
isovalent isomorphism features in framework com-
pounds with the lattice symmetry being conserved.
They are also of applied interest for developing engi-
neering ceramics.
Control over the chemical composition and homo-
geneity of the samples wad performed using a Cam-
1167

1168
ASABINA et al.
Table 1. Crystal data for alkali metal phosphates of Group IVB elements (A = Li, Na, K, Rb, Cs; D = Ti, Zr, Hf)
Unit cell parameters
Compound
Space group
Source
[1]
a, Å
b, Å
c, Å
α, β, γ, deg
V, Å3
*
LiTi2(PO4)3
R3 c
8.51173(4)
20.8524(2)
1308.35
*
NaTi2(PO4)3
R3 c
R3 c
R3 c
8.4502(5)
8.4854(2)
8.367(1)
21.833(9)
21.7994(8)
23.074(3)
1366.74
1359.33
1398.9
[2]
[3]
[4]
*
KTi2(PO4)3
*
RbTi2(PO4)3
R3 c
8.290(1)
23.530(4)
1400.3
[5]
*
LiZr2(PO4)3
R3 c (423 K)
C1
8.85493(3)
15.0718(2)
22.1440(1)
1503.69
1010.52
[6]
[7]
8.8556(1)
9.1234(1) α = 89.660(1)
β = 123.912(1)
γ = 90.429(1)
P21/n
8.81277(4)
8.84303(5)
8.94520(5)
8.94120(6)
12.37540(6)
12.41301(8)
90.801(1)
975.482
981.463
[8]
[8]
Pbna (623 K)
*
NaZr2(PO4)3
R3 c
8.815(1)
22.746(7)
1530.7
[9]
*
KZr2(PO4)3
R3 c
R3 c
R3 c
8.71(1)
23.89(2)
24.38(2)
24.9061(4)
1569.6
1583.4
1586.30
[10]
[11]
[12]
RbZr2(PO4)3
8.66(1)
*
CsZr2(PO4)3
8.5758(2)
*
LiHf2(PO4)3
R3 c (320 K)
P1 (175 K)
8.83070(6)
8.69447(8)
22.0272(2)
1487.58
500.663
[13]
[13]
8.74104(7)
9.07206(7) α = 118.6297(6)
β = 90.6805(8)
γ = 119.1605(7)
NaHf2(PO4)3 R3 c
8.7707(5)
8.6819(4)
22.66(2)
1509.7
[14]
[15]
*
KHf2(PO4)3
RbHf2(PO4)3 R3 c
CsHf2(PO4)3 R3 c
R3 c
23.862(2)
1557.64
8.63(1)
8.54(1)
24.34(2)
24.81(2)
1569.9
1567.0
[16]
[16]
* The crystal structure has been solved.
Scan MV 2300 scanning electron microscope equipped structure solution experiments, X-ray diffraction pat-
with a Link Inca Energy 200C energy-dispersive detec- terns were measured in the range 2θ = 5°–110° in 0.02°
tor. The PAP correction procedure was used in compo-
sition calculations. The error of the composition deter-
minations was within 2.5 wt %.
steps. The X-ray diffraction patterns were processed
and the structure was refined by the Rietveld method
[23] using RIETAN97 software [24]. The peak profiles
were described by a modified pseudo-Voigt function
(Mod-TCH pV [25]).
The X-ray diffraction data were collected at room
temperature on a Thermo ARL X’tra powder diffracto-
meter (CuKα radiation, reflection geometry, Peltier
semiconductor detector). X-ray powder diffraction was
used to determine the phase composition of the samples
during their synthesis after each isothermal annealing
Second harmonic generation (SHG) signals were
measured in fine-grained powders on a high-sensitivity
two-channel setup using the procedure described in
[26]. The radiation source used was a Q-switched
step and to control the single-phase composition of the pulsed laser with a wavelength of 1.064 µm. The refer-
resulting compounds. The 2θ range was 10°–60°. In ence used was crystalline α-quartz powdered to a grain
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PHASE FORMATION, CRYSTAL STRUCTURE, AND ELECTRICAL CONDUCTIVITY
1169
I × 104, pulses
4
2
1
0
2
20
40
60
2θ, deg
Fig. 1. Fragments of (1) the observed X-ray diffraction pattern and (2) the stick diagram for Li Na Ti (PO ) .
0.5 0.5
2
4 3
I × 104, pulses
8
6
4
2
2
1
3
~
~
0
4
20
40
60
2θ, deg
Fig. 2. Fragments of (1) observed, (2) simulated, and (4) difference X-ray diffraction patterns and (3) the stick diagram for
Na Ti (PO ) . The simulated pattern is shifted relative to the observed pattern.
K
0.5 0.5
2
4 3
size of 3–5 µm. The SHG signals generated in the test titanium pyrophosphate is an intermediate; it fully con-
sample and reference were recorded in reflection geo- verts into triple phosphates of titanium and alkali met-
metry.
The ceramics for electrical conductivity measure-
ments shaped as disks 8 mm in diameter and 1–2 mm
height were manufactured by sintering at 1373 K for
6 h of a powder compacted at ~500 MPa.
Platinum electrodes were applied to the end surfaces
of the disks by burning-in colloidal platinum. The con-
ductivity was measured with a two-point probe using an
R-5083 ac bridge at frequencies from 1 Hz to 100 kHz
and an E7-12 ac bridge at 1 MHz. The bridges were
operated and data were processed with a computer.
als only when the temperature is stepwise elevated fur-
'
ther. Single-phase A0.5A0.5 Ti2(PO4)3 products are
formed at 1073–1173 K. After annealing at 1473 K for
6 h, the samples partially decompose. Reflections from
titanium dioxide are observed in the X-ray diffraction
patterns of samples annealed at 1473 K. Other decom-
position products are likely alkali phosphates.
'
A0.5A0.5Ti2(PO4)3 crystals for the structural experi-
ments and electrical conductivity measurements were
obtained at 1273 K. X-ray powder diffraction shows
that the phosphates belong to the NZP type. Micro-
probe measurements show that the samples are homo-
geneous and their compositions are close to those cal-
RESULTS AND DISCUSSION
Synthesis and characterization. X-ray powder dif-
fraction shows complex interactions between the com-
ponents in the reaction mixtures. Reflections of tita-
nium pyrophosphate were in the X-ray diffraction pat-
'
culated for the A0.5A0.5 Ti2(PO4)3 formula.
'
Structures of A0.5A0.5 Ti2(PO4)3 phosphates. The
'
terns of the samples annealed at 873 K. Crystalline choice of space group R3 c for the A0.5A0.5 Ti2(PO4)3
RUSSIAN JOURNAL OF INORGANIC CHEMISTRY Vol. 51 No. 8 2006

1170
ASABINA et al.
'
Table 2. Details of the X-ray diffraction experiment and the results of structure refinement for A0.5A0.5 Ti2(PO4)3 phosphates
Compound
Na0.5K0.5Ti2(PO4)3
K0.5Rb0.5Ti2(PO4)3
Space group
Z
R3 c (No. 167)
6
2θ range, deg
5–110
Unit cell parameters:
a, Å
8.4129(3)
22.5426(8)
1381.74(9)
195
8.3300(1)
23.2723(3)
1398.49(3)
199
c, Å
V, Å3
Number of reflections
Number of refined parameters*
18 + 13
16 + 13
Final values, %**
Rwp; Rp
RI; RF
S
8.37; 6.20
2.45; 0.98
3.36
7.18; 5.01
3.30; 1.97
2.41
Notes: * The first figure includes the background and profile parameters, scaling factor, and unit cell parameters; the second figure includes
the positional and thermal parameters of atoms and site occupancies.
** Calculated using formulas from [25].
Table 3. Coordinates and isotropic thermal parameters of atoms in the structures of Na0.5K0.5Ti2(PO4)3 (I) and
K0.5Rb0.5Ti2(PO4)3 (II)
Compound
Atom
A(A')
Position
x
y
z
B, Å2
I
6b
0
0
0
0
0
0
0
0
0
0
0
0
3.12(1)
3.63(6)
0.46(5)
0.37(4)
1.17(6)
0.92(5)
0.89(9)
0.43(8)
1.41(9)
0.99(8)
II
I
Ti
P
12c
18e
36f
36f
0.1479(1)
0.1505(1)
0.25
II
I
0.7156(2)
0.7184(2)
0.2020(4)
0.2085(4)
0.4753(4)
0.4700(4)
II
I
0.25
O1
O2
0.1589(3)
0.1500(3)
0.3014(4)
0.3032(4)
0.3065(1)
0.3020(1)
0.2387(1)
0.2330(1)
II
I
II
triple phosphates (A–A' = Li–Na, Na–K, K–Rb) is con-
firmed by SHG measurements in fine-grained powders.
The absence of an SHG signal for all samples proves
their centrosymmetry.
space group R3 c. For LiTi2(PO4)3, neutron diffraction
showed lithium cation disorder in interstices [1]. How-
ever,
X-ray
powder
diffraction
data
for
Li0.5Na0.5Ti2(PO4)3 do not reveal the distribution char-
acter of the alkali cations in the framework interstices.
The following unit cell parameters were calculated for
Li0.5Na0.5Ti2(PO4)3: a = 8.4871(9) Å, c = 21.752(2) Å,
Figure 1 displays the X-ray diffraction results
obtained in the angle range 5°–110° with 0.02° steps
for Li0.5Na0.5Ti2(PO4)3. This triple phosphate sample is
a single phase; it has the NZP-type structure, and its
X-ray powder diffraction pattern is indexed in terms of V = 1356.9(2) Å3.
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PHASE FORMATION, CRYSTAL STRUCTURE, AND ELECTRICAL CONDUCTIVITY
1171
I × 104, pulses
6
4
2
2
1
~
3
~
0
4
20
40
60
2θ, deg
Fig. 3. Fragments of (1) observed, (2) simulated, and (4) difference X-ray diffraction patterns and (3) the stick diagram for
Rb Ti (PO ) . The simulated pattern is shifted relative to the observed pattern.
K
0.5 0.5
2
4 3
The starting models used in the structure refinement lated, stick, and difference X-ray diffraction patterns
'
'
for the A0.5A0.5 Ti2(PO4)3 phosphates.
for the A0.5A0.5 Ti2(PO4)3 (A–A' = Na–K, K–Rb) com-
pounds were the atomic coordinates in the structures of
titanium alkali phosphatesATi2(PO4)3, whereA = Na or
There is good fit between the observed and simu-
lated X-ray diffraction patterns for Na0.5K0.5Ti2(PO4)3
and K0.5Rb0.5Ti2(PO4)3 (Figs. 2, 3). The unit cell param-
eters and the final refinement values for these phos-
K (space group R3 c) [3, 4]. Table 2 displays the details
of the X-ray diffraction experiment and structure
refinement. Figures 2 and 3 show the observed, simu- phates are listed in Table 2. The coordinates of atoms
Table 4. Selected interatomic distances and bond angles in TiO6 and PO4 polyhedra in the structures of Na0.5K0.5Ti2(PO4)3 (I)
and K0.5Rb0.5Ti2(PO4)3 (II)
Compound
Bond
d, Å
Compound
Angle
O1PO1'
ω, deg
I
A(A')–O2 (×6)
2.604(3)
2.790(3)
1.859(3)
1.905(3)
1.917(3)
1.923(3)
1.577(3)
1.539(3)
1.560(3)
1.540(3)
I
112.7(3)
110.7(2)
106.3(2)
107.6(1)
109.9(2)
109.1(1)
111.8(3)
112.8(3)
92.5(2)
89.7(1)
89.7(1)
87.5(1)
92.8(1)
96.1(1)
174.3(2)
173.6(1)
84.9(1)
87.0(1)
II
I
II
I
Ti–O1 (×3)
Ti–O2 (×3)
P–O1 (×2)
P–O2 (×2)
O1PO2 (×2)
O1PO2 (×2)
O2PO2'
II
I
II
I
II
I
II
I
II
I
II
I
O1TiO1' (×3)
O1TiO2 (×3)
O1TiO2 (×3)
O1TiO2 (×3)
O2TiO2' (×3)
II
II
I
II
I
II
I
II
I
II
RUSSIAN JOURNAL OF INORGANIC CHEMISTRY Vol. 51 No. 8 2006

1172
ASABINA et al.
V, Å3
1400
TiO6
PO4
1350
1300
c, Å
Na/K
24
22
20
c
~
~
b
a
a, Å
8.5
Fig. 4. Fragment of the structure of Na
K Ti (PO ) .
0.5 0.5 2 4 3
8.4
8.3
and their isotropic thermal parameters are listed in
Table 3. Selected interatomic distances and bond angles
are listed in Table 4.
8.2
0.5
1.0
rA+(A'+), Å
1.5
Figure 4 shows the Na0.5K0.5Ti2(PO4)3 structure as
an example of a test sample. The basis of the structure
is a mixed 3D framework built of corner-sharing TiO6
octahedra and PO4 tetrahedra. Each octahedron is
linked to six tetrahedra, and each tetrahedron is linked
to four octahedra. The main building elements of the
framework are fragments composed of two octahedra
and three tetrahedra; these fragments form columns
running along axis c. The columns are linked with each
other through shared PO4 tetrahedra. The alkali cations
Li
Na
K Rb
Fig. 5. Plots of the unit cell parameters a, c, and V vs. alkali
cation radius in ATi (PO ) double phosphates and
2
4 3
A
A' Ti (PO ) triple phosphates.
2 4 3
0.5
0.5
ions into the M1 interstices (Fig. 6). The data displayed
in Table 4 confirm the correlation between the structure
distortion and the A and A' cation sizes: the bond
lengths in the TiO6 octahedra systematically increase,
the bond lengths in the PO4 tetrahedra decrease, and the
OTiO and OPO bond angles change as the mean radius
of the extraframework alkali cation increases.
The A(A')–O bond length for the phosphates in
question is a linear function of the mean alkali cation
size (Fig. 7). From an analysis of the interatomic dis-
tances in NZP phosphates of titanium and alkali metals,
it is evident that the framework cation–oxygen bond
lengths (Ti–O, P–O) are only weakly affected by the
extraframework cation radius. This weak effect is due
to the fact that the covalent chemical bonds in the
framework are far stronger than the bonds formed by
the extraframework cations.
'
in the A0.5A0.5 Ti2(PO4)3 compounds are randomly dis-
tributed in the fully occupied interstitial octahedral
positions M1 inside the columns. The refinement of the
occupancies of the M2 interstices between the columns
for the alkali cations shows that these positions are
vacant.
The unit cell parameters are monotone functions of
alkali cation size in both the ATi2(PO4)3 double phos-
'
phates and the A0.5A0.5 Ti2(PO4)3 triple phosphates
(Fig. 5). For the mixed-alkali triple phosphates, the unit
cell parameters are additive relative to the unit cell
parameters of the double phosphates; that is, in these
compounds there is a mean radius of the formal alkali
'
cation in the interstices: r(A0.5A0.5 ) = 0.5[r(A) + r(A')].
Our data and previous data obtained in [1–6, 9, 10,
The systematic increase in the parameter c and the 12, 13, 15] show that a {[L2(XO4)3]p–}3∞ three-dimen-
decrease in the parameter a in response to the increas-
ing alkali cation radius are due to the distortion of the
columns built of TiO6 octahedra and PO4 tetrahedra;
this distortion is a result of a correlated turn of the frag- lence, X is the anion-forming element with a tetrahe-
ments of the framework upon the entrance of larger cat- dral rosette of chemical bonds, and p is the framework
sional framework is the base of the NZP-type struc-
tures; here, L is the structure-forming octahedral cation
with the dominance of the metal–oxygen bond cova-
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PHASE FORMATION, CRYSTAL STRUCTURE, AND ELECTRICAL CONDUCTIVITY
1173
Na/K
K/Rb
O(2)
O(2)
84.9°
Ti
87.0°
Ti
92.5°
O(1)
P
O(1)
P
89.7°
O(2)
O(2)
Fig. 6. Distortion of the framework structural fragments as a function of the alkali cation radius in the extraframework positions M1.
charge. Phosphorus is the anion-forming element in and the crystal-chemical reasons for the conductivity
most of the NZP compounds. Similar frameworks are
encountered in other classes of compounds that belong
to the structural types of scandium tungstate
Sc2(WO4)3, langbeinite K2Mg2(SO4)3, and garnet
Ca3Al2(SiO4)3. The crystal-chemical analysis of the
are the same both in previously known NZP phosphates
and in those studied here, there is good reason to think
that the origin of the conductivity in all NZP com-
pounds is the same: the conductivity is due to the alkali
cation mobility. The lower electrical conductivity of the
{[L2(XO4)3]p–}3∞ framework for these four structural
'
A0.5A0.5 Ti2(PO4)3 phosphates compared to the classical
types is carried out in [27, 28].
NASICON ionic conductors [18] is likely due to the
low concentration of mobile ions in the interstices and
the smaller interstice sizes in the titanium-containing
framework than in the zirconium compounds.
Cation mobility in phosphates. The electrical con-
'
ductivity of the A0.5A0.5 Ti2(PO4)3 (A–A' = Li–Na,
Na−K, K–Rb) ceramic samples was measured from
room temperature to 1223 K. Electrical conductivity
versus temperature curves are displayed in Fig. 8. The
'
The electrical conductivity of the A0.5A0.5Ti2(PO4)3
phosphates is significantly affected by the A and A'
alkali cation sizes. The highest σ over a wide tempera-
ture range is observed for Li0.5Na0.5Ti2(PO4)3 (Fig. 8).
Ion transport through the crystal structure of this phos-
phate is the least hindered, which follows from the
comparison of the activation energies listed in Table 5.
log( σT)–(103/T) plots for the phosphates are linear;
that is, the conductivity increases with temperature in
accordance with the law
σT = σ0 exp(–Eσ/kT),
where Eσ is the activation energy of conductivity, and σ0
is the frequency factor. The conductivities at 1223 K
and activation energies are listed in Table 5.
Table 5. Electrical conductivity and activation energy in
A0.5A'0.5Ti2(PO4)3 (A–A' = Li–Na, Na–K, K–Rb) phosphates
The characteristic feature of the NZP compounds is
the geometric possibility of cation exchange between
the extraframework positions [18, 19]. The interstices
account for a significant volume of the framework; they
penetrate the NZP framework, creating favorable con-
ditions for ionic conductivity. Inasmuch as the temper-
ature behavior of the electrical conductivity is similar
Compound
σ
1223 K, S/cm
Eσ, eV
Li0.5Na0.5Ti2(PO4)3
Na0.5K0.5Ti2(PO4)3
K0.5Rb0.5Ti2(PO4)3
8.6 × 10–3
7.1 × 10–6
4.5 × 10–6
0.47
0.56
0.80
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1174
ASABINA et al.
distributed in extraframework positions M1. Crystal-
d
A(A')–O, Å
chemical analysis and comparison with other phos-
phates show their structural relations to the NASICON
ionic conductor. Correlations are found between the
structural distortion, unit cell parameters, and electrical
conductivity of the phosphates, on one hand, and the A
and A' alkali cation sizes, on the other. By analogy with
the known NZP phases, the electrical conductivity in
the new triple phosphates is explained by the mobility
of the alkali cations in the interstices.
2.8
2.6
2.4
2.2
0.5
1.0
rA+(A'+), Å
1.5
ACKNOWLEDGMENTS
This work was supported by the Russian Foundation
for Basic Research (project no. 05-03-32127).
Li
Na
K Rb
Fig. 7. A(A')–O bond length vs. alkali cation radius in
ATi (PO ) double phosphates and A A' Ti (PO ) tri-
2
4 3
0.5
2
4 3
0.5
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