Cesium and its analogs, rubidium and potassium, in rhombohedral [NaZr 2(PO4)3 type] and cubic (langbeinite type) phosphates: 2. Properties: Behavior on heating, in aqueous solutions, and in salt melts

Article abstract of DOI:10.1007/s11137-005-0079-5

The properties (behavior on heating, in aqueous solutions, and in salt melts) of orthophosphates A₂RM(PO₄)₃, A ₂B_0.5Zr_1.5(PO₄)₃, and ABR₂(PO₄)₃ [A = K, Rb, Cs; B = Mg, Sr, Ba; R = Ga, Fe, Cr, Ln (Ce-Lu)] crystallizing in the structure of langbeinite mineral (cubic system, space group P2₁3, Z = 4) were studied and compared with those of NZP-type phosphates. The thermal transformations of the structure and the influence of temperature on the distortion of the framework-forming polyhedra were examined. The volatilization of cesium, in particular, from the solid phase in the course of its formation, was evaluated. The rates of cesium and barium leaching at 90 and 95°C were determined. 2005 Pleiades Publishing, Inc.

Full text of DOI:10.1007/s11137-005-0079-5

Radiochemistry, Vol. 47, No. 3, 2005, pp. 235 240. Translated from Radiokhimiya, Vol. 47, No. 3, 2005, pp. 213 218.
Original Russian Text Copyright 2005 by A. Orlova, V. Orlova, Buchirin, Korchenkin, Beskrovnyi, Demarin.
Cesium and Its Analogs, Rubidium and Potassium,
in Rhombohedral [NaZr2(PO4)3 Type] and Cubic
(Langbeinite Type) Phosphates: 2. Properties: Behavior
on Heating, in Aqueous Solutions, and in Salt Melts
A. I. Orlova*, V. A. Orlova*, A. V. Buchirin*, K. K. Korchenkin**,
A. I. Beskrovnyi***, and V. T. Demarin****
* Chemical Faculty, Lobachevsky Nizhni Novgorod State University, Nizhni Novgorod, Russia
** RT-1 Radiochemical Plant, Mayak Production Association, Federal State Unitary Enterprise,
Ozersk, Chelyabinsk oblast, Russia
*** Frank Laboratory of Neutron Physics, Joint Institute for Nuclear Research, Dubna, Moscow oblast, Russia
**** Research Institute of Chemistry, Chemical Faculty, Lobachevsky Nizhni Novgorod State University,
Nizhni Novgorod, Russia
Received July 22, 2004
Abstract The properties (behavior on heating, in aqueous solutions, and in salt melts) of orthophosphates
A2RM(PO4)3, A2B0.5Zr1.5(PO4)3, and ABR2(PO4)3 [A = K, Rb, Cs; B = Mg, Sr, Ba; R = Ga, Fe, Cr, Ln
(Ce Lu)] crystallizing in the structure of langbeinite mineral (cubic system, space group P213, Z = 4) were
studied and compared with those of NZP-type phosphates. The thermal transformations of the structure and
the influence of temperature on the distortion of the framework-forming polyhedra were examined. The
volatilization of cesium, in particular, from the solid phase in the course of its formation, was evaluated. The
rates of cesium and barium leaching at 90 and 95 C were determined.
Diversified isomorphism in rhombohedral {of
NaZr (PO ) type [1]} and cubic (of langbeinite type
metals of the formula AM (PO ) are readily formed
2 4 3
by precipitation from aqueous solutions followed by
thermal treatment of the precipitates, from molten
salts (alkali metal chlorides or fluorides) on adding
appropriate precipitates, by solid-phase synthesis, and
in ion-exchange processes [3 6]. Ceramic samples
based on cesium phosphates were prepared [7]. The
phosphates KZr (PO ) , RbZr (PO ) , CsZr (PO ) ,
2
4 3
[2]) phosphates is characteristic of many phosphate
minerals. It provides a basis for substantiated formu-
lation of single-phase ceramic materials for various
purposes, including matrix materials for concentrating
wastes from nuclear and other industries, with the aim
to firmly isolate these wastes from the environment.
2
4 3
2
4 3
2
4 3
and their titanium and hafnium analogs exist in a wide
temperature range and retain the chemical and phase
composition when heated to 1200 C. Their thermal
expansion is negligible. The mean coefficients of ther-
mal expansion of KZr (PO ) , RbZr (PO ) , CsZr
The successful application of such materials de-
pends on their properties that determine the safety of
long-term storage: resistance to heat, high pressure,
and radiation; behavior in water, aqueous solutions,
and in the presence of other chemical agents.
2
4 3
2
4 3
2
(PO ) , KHf (PO ) , RbHf (PO ) , and CsHf (PO )
4 3
2
4 3
2
4 3
2
4 3
Our goal was to study the thermal behavior of new
cesium, rubidium, and potassium phosphates de-
scribed in [2], to evaluate their stability under hydro-
thermal conditions and in alkali metal chloride melts,
and to summarize the relevant data concerning NZP-
and langbeinite-type phosphates.
are close to zero, and the coefficients along particular
6
axes are as follows:
(1.0 22.3) 10 deg [8].
= ( 7.3 0.0) 10 and
=
a
c
6
1
Irradiation of CsTi (PO ) and CsZr (PO ) with a
2
4 3
2
4 3
60
1
Co -ray source at a dose rate D of up to 10 Gy s
6
8
to doses of 1 10 5 10 Gy caused no changes in
the phase and chemical compositions, as judged by
the IR and X-ray diffraction data [9, 10]. Cesium
zirconium phosphate underwent no noticeable changes
under bombardment with accelerated Ar nuclei [11].
The properties of cesium, rubidium, and potassium
phosphates of the rhombohedral NZP-type modifica-
tion are better studied compared to those of the cubic
langbeinite phosphates.
NZP-type phosphates of cesium and other alkali
Rhombohedral CsTi, CsZr, and CsHf phosphates
1066-3622/05/4703-0235 2005 Pleiades Publishing, Inc.

236
ORLOVA et al.
after keeping in molten chlorides ACl at the above-
indicated temperatures and in molten nitrates ANO
at 300 350 C for 10 12 h [13].
3
All the above data on the thermal, radiation, and
chemical stability of framework phosphates of Cs and
n
other alkali elements {framework unit [T (PO ) ] }
2
4 3
concern phosphates of the NaZr (PO ) structural
type. We found no such data on langbeinite-type
phosphates.
2
4 3
The experiments made in this study were aimed to
evaluate the stability of langbeinite-type phosphates
of cesium and other alkali elements. Their synthesis
and crystal chemistry were described previously [2].
Fig. 1. Solubility of AM (PO ) in ACl melts as a function
2
4 3
In addition to the methods described in [2], we
used in this study atomic absorption, X-ray fluores-
cence, atomic emission, and photometric methods.
Elemental analysis was performed with a Perkin
Elmer 603 atomic absorption spectrophotometer (solu-
tions to be analyzed were atomized in the acetylene
air flame of a slit burner), an ERA-03 energy-disper-
sive X-ray fluorescence analyzer (energy resolution
300 eV, identification of elements with Z from 21
to 92), an SAE-01 inductively coupled plasma atom-
ic emission spectrometer (wavelength range 240
800 nm), and a KFK-3 photoelectric photometer
equipped with a microprocessor, with computer data
treatment (wavelength range 400 950 nm, range of
optical densities measured from 0 to 3.0).
of the ionic radius of A (Li Cs): (1) ATi (PO ) , (2) AZr
2
4 3
2
(PO ) , and (3) AHf (PO ) .
4 3
2
4 3
were also subjected to hydrothermal tests, which were
performed in the dynamic and static modes at room
temperature and at 95 and 200 C in distilled water
and, in some experiments, in seawater.
In some experiments, the testing time reached two
years (at T = 200 C and pressure of up to 600 atm).
The chemical degradation was never observed; the
minimal attained rates of cesium leaching did not
5
2
1
exceed 1 10 g cm day .
8
Preliminary irradiation (absorbed dose 3 10 Gy)
had virtually no effect on the leaching rates [12].
The phosphate KZr (PO ) is difficultly soluble in
2
4 3
THERMAL BEHAVIOR
water and dilute mineral acids (HClO , HCl, HNO ,
4
3
H SO ). Its solubility (equilibration time 120 150 h)
2
4
The phosphates with the langbeinite framework
motif, prepared and characterized in [2], crystallize at
T = 800 1000 C. They undergo no chemical degrada-
tion or phase transitions when kept at 1000 C for
24 h. Some samples were heat-treated at 1300 C for
24 h. In so doing, K Mg Zr (PO ) underwent no
depends on pH and anionic composition of the solu-
tion. As the pH is decreased from 6.5 to 0.3, the solu-
26
21
bility product (molar scale) changes from 2 10
to
in
22
23
7
10
in HClO , from 2 10
to 1 10
4
22
20
HCl, from 7 10
to 4 10
in HNO , and from
in H SO . The effect of the
3
2
0.5 1.5
4 3
21
19
1
10
to 10 10
phase or chemical transformations, according to X-ray
phase analysis. K ErZr(PO ) decomposed under the
2 4
anion on the solubility is due to zirconium complexa-
tion and correlates with the complexing power of the
2
4 3
same conditions; its diffraction pattern contained the
reflections of zirconium oxide and simple erbium
orthophosphate.
anion, decreasing in the order H SO > HNO >
2
4
3
95
HCl > HClO . These data were obtained using Zr
and P tracers introduced into KZr (PO ) in the
4
32
To reveal changes occurring in the structure under
heat treatment, we examined the phosphate Cs Mg
Zr (PO ) [14] by high-temperature neutron diffrac-
tion using the method of full-profile analysis (Rietveld
method). The measurement temperature was 15, 150,
300, 450, and 600 C. The results are listed in Table 1.
2
4 3
course of its synthesis.
2
0.5
The solubility of the phosphates AM (PO ) ,
1.5
4 3
2
4 3
where A = K, Rb, Cs and M = Ti, Zr, Hf, was studied
in molten salts ACl: KCl at 800 850 C and CsCl at
650 750 C. It depended on particular cations A and
M, and also on temperature (Fig. 1). Data for A = Li
and Na are also shown in Fig. 1 for comparison. The
equilibration time was 2.5 3 h. The compositions and
structures of the initial phases remained unchanged
Variation of the selected interatomic distances and
bond angles is illustrated by Table 2. The structure
(Fig. 2) is formed by discrete MgO , ZrO , and PO
4
6
6
4 3
polyhedra combined in the [Mg Zr (PO ) ] frame-
0.5 1.5
RADIOCHEMISTRY Vol. 47 No. 3 2005

CESIUM AND ITS ANALOGS, RUBIDIUM AND POTASSIUM... : 2.
237
Table 1. Results of refinement of the crystal structure of Cs2Mg0.5Zr1.5(PO4)3*
Characteristic
15 C
150 C
300 C
450 C
600 C
a,
V,
10.2624(5)
1080.804
10.2657(9)
1081.847
10.2735(9)
1084.315
10.2835(9)
1087.484
10.2886(9)
1089.103
3
Number of reflections
303
1.69
3.75
4.17
322
2.62
4.89
5.65
315
2.58
4.52
5.38
323
2.56
4.45
5.21
317
2.37
4.39
4.89
Rexp
Rwp
Rp
2
6.94
5.78
4.02
3.58
5.03
* Cubic system, space group P2 3, Z 4, white powder, range of d 0.73 3.53 , 39 refined parameters.
1
work with the random distribution of the magnesium
and zirconium cations over two nonequivalent crys-
tallographic positions, and it somewhat deforms on
heating.
The distortion of the framework-forming polyhed-
ra, evaluated by the maximal scattering in the bond
lengths, varies with temperature. In particular, at 15 C
the phosphorus tetrahedra are deformed insignificant-
ly: = 0.05 , and the O P O bond angles are close
to the ideal tetrahedral angle, 109.5 . With increasing
temperature, the bond lengths change, and at 600 C
in the phosphorus tetrahedron reaches 0.21 . The
distortion of the MgO and ZrO octahedra depends
6
6
on temperature to a lesser extent: = 0.23 at 15 C
and 0.35 at 600 C. On heating, the polyhedra turn
relative to each other, as shown in Fig. 3.
The temperature dependences of the unit cell pa-
Table 2. Selected interatomic distances ( ) and bond
angles (deg) in the structure of Cs2Mg0.5Zr1.5(PO4)3 at
T = 15 and 600 C
Bond,
angle
Bond,
angle
15 C 600 C
15 C 600 C
Fig. 2. Fragment of the Cs Mg Zr (PO ) structure.
2
0.5 1.5
4 3
Cs1O9 polyhedron
PO4 tetrahedron
1.48
Cs1 O2
Cs1 O3
Cs1 O4
3.16
3.06
2.83
3.18 P O1
1.63
1.42
1.48
1.56
3.08 P O2
3.36 P O3
P O4
1.45
1.50
1.50
Cs2O9 polyhedron
O1 P O2 113.9
110.3
Cs2 O1
Cs2 O2
Cs2 O4
3.11
3.19
2.83
3.11 O1 P O3 107.7
3.11 O1 P O4 115.9
2.79 O2 P O3 107.8
O2 P O4 102.8
O3 P O4 108.3
1.89
2.19
1.84
2.17
107.0
112.2
113.7
104.7
109.0
MO6 octahedron
Zr O1
1.95
2.18
Zr O2
Mg/Zr O3 2.05
Mg/Zr O4 2.07
Fig. 3. Distortions of the framework-forming polyhedra on
heating.
RADIOCHEMISTRY Vol. 47 No. 3 2005

238
ORLOVA et al.
Figure 5 shows the temperature dependence of the
thermal expansion coefficients. As seen, and
increase with temperature, reaching at T = 600 C
a
V
6
6
1
5.05 10 and 12.10 10 deg , respectively.
As seen, the langbeinite-type and rhombohedral
phosphates differ in the characteristics of thermal
expansion. The expansion of the former compounds
is stronger (for an NZP-type phosphate CsZr (PO ) ,
2
4 3
6
6
1
1
10 and
1
10 deg ) and isotropic
a
c
(the thermal expansion of NZP phosphates is aniso-
tropic, although for the CsZr and CsHf phosphates
the anisotropy is close to zero).
Fig. 4. Temperature dependences of the (a) unit cell param-
eter a and (b) unit cell volume V. Regression equations:
8
2
5
(a) a = 1 10
T
+ 4 10 T + 10.261 and (b) V =
Although the chemical composition of the samples
does not noticeably change under prolonged heat
treatments, volatilization of small amounts of cesium
cannot be ruled out. Therefore, we made special ex-
periments to estimate the volatilization of Cs from
6
2
2
4
10
T
+ 1 10 T + 1080.3.
the phosphates. For this purpose, a sample of Cs Fe
2
Zr(PO ) (final synthesis temperature 800 C) was
4 3
heated at T = 800 and 1000 C for 2 h in each step,
and the cesium content in the gas phase was deter-
mined. The volatilization of Cs from the solid phase
in the course of its formation was also determined for
the phosphate Cs Mg Zr (PO ) . Simultaneously
2
0.5 1.5
4 3
with cesium, we monitored the volatilization of other
components.
Fig. 5. Temperature dependences of the thermal expansion
coefficients (a) and (b)
The experimental procedure was as follows. A cru-
cible with a powdered sample was placed in a quartz
beaker. The gas phase was passed through a thermally
insulated quartz tube, a U-shaped air-cooled steel
condenser, and a bubbler, using a water-jet pump.
The water from the bubbler and the material washed
out from the walls of the unit with a dilute acid were
analyzed. The analysis was performed after each step
of heating: 2 h at 800 C and 2 h at 1000 C for
Cs FeZr(PO ) ; 24 h at 80 C, 24 h at 600 C, 24 h at
.
V
a
rameters a and V (Fig. 4) are described by quadratic
polynomials. The polynomial parameters of the ther-
mal expansion were calculated by Eqs. (1) and (2):
p = p2T2 + p1T + p0,
= (p1 + 2p2T)/p,
(1)
(2)
=
2
4 3
where
(1/p)dp/dT.
is the thermal expansion coefficient;
800 C, and 24 h at 1000 C in the experiment on syn-
thesis of Cs Mg Zr (PO ) , in accordance with [2].
2
0.5 1.5
4 3
The thermal expansion coefficients
given in Table 3.
and
are
V
In all the samples taken, the content of cesium and
zirconium was below the detection limit. Therefore,
the fraction of Cs and Zr that volatilized from the
solid sample into the gas phase can be estimated at
0.0005 and 0.00003, respectively. The volatiliza-
tion of Mg and Fe is somewhat larger, compared to
the stoichiometric ratio. Apparently, the magnesium
and iron compounds formed by thermal degradation
of the phosphate sample have higher vapor pressure
under the experimental conditions than the zirconium
and cesium compounds.
a
Table 3. Thermal expansion coefficients of Cs2Mg0.5Zr1.5
(PO4)3
1
1
T,
C
106, deg
106, deg
V
a
15
3.93
4.18
4.48
4.76
5.05
11.76
11.85
11.93
11.20
12.10
150
300
450
600
The volatilization of cesium from Cs FeZr(PO )
2
4 3
and Cs Mg Zr (PO ) is considerably smaller
2
0.5 1.5
4 3
RADIOCHEMISTRY Vol. 47 No. 3 2005

CESIUM AND ITS ANALOGS, RUBIDIUM AND POTASSIUM... : 2.
Table 4. Volatilization of cesium from solid matrices
Matrix Process
239
Temperature, Volatilization of
References
C
Cs, %
Aluminophosphate glass Evaporation of cesium-containing solutions
with glass-forming additives
100 120
0.4
[15]
Glass founding
1100
100 120
0.4
0.9
[16]
[17]
Borosilicate glass
Synroc-C
Evaporation of cesium-containing solutions
in the presence of silica gel
Glass founding
Calcination
Melting of charge from metal oxides and
nitrates using IMCC*
800 1000
700
1300 1600
1 2
0.1
3 18
[18]
[19]
[20]
Phosphate ceramic of
langbeinite structure
Synthesis of cesium-containing sample
Calcination
80 1000
800 1000
<0.05
<0.05
This work
* Induction melting in a cold crucible.
compared to the known Cs-containing solid matrices
of other compositions and structures (Table 4).
the sample in time t (days); c , specific concentra-
sp
1
tion of an element in the solid phase (g g solid
2
phase); and s , sample surface area (cm ).
surf
On the whole, phosphates of both langbeinite and
NZP structures are resistant to high temperatures (up
to 1200 1300 C), and the arising thermal deforma-
tions are insignificant.
The kinetic curves and the dependences of the
leaching rates on time for the phosphates Cs Mg
2
0.5
Zr (PO ) and CsBaFe (PO ) are shown in Fig. 6.
1.5
4 3
2
4 3
The alkali metal leaching rate was the highest in the
first day, after which it rapidly decreased with time.
HYDROTHERMAL STUDIES
Hydrothermal tests were performed with Cs Mg
2
0.5
4 3
Zr (PO ) , CsBaFe (PO ) , K Mg Zr (PO ) ,
1.5
4 3
2
4 3
2
0.5 1.5
and K FeZr(PO ) . The experiments were performed
2
4 3
in static [K Mg Zr (PO ) , Cs Mg Zr (PO ) ]
and dynamic [CsBaFe (PO ) , K FeZr(PO ) ] modes.
2
0.5 1.5
4 3
4 3
2
0.5 1.5
4 3
2
2
4 3
Samples of phosphates in the form of ceramics were
prepared by pressing the powders prepared by the
sol gel method (final synthesis temperature 800 C),
followed by annealing at 1000 1100 C for 24 h.
Ceramic discs of Cs Mg Zr (PO ) and K
2
2
0.5 1.5
4 3
Mg Zr (PO ) were placed in Teflon test tubes
0.5 1.5
4 3
80% filled with distilled water; the tube were placed
in hermetically sealed steel containers. The experi-
ments were performed in the static mode at T = 90 C
for 28 days; samples were taken on the 3rd, 7th, 14th,
and 28th days.
Samples of CsBaFe (PO ) and K FeZr(PO ) ,
2
4 3
2
4 3
prepared as ceramics, were placed in the glass extract-
or of a Soxhlet apparatus. Tests were performed in the
dynamic mode at T = 90 C for 28 days. Samples were
taken at regular intervals, and the content of elements
was determined.
The leaching rate was calculated by the formula
R = m/(csp Ssurf t),
Fig. 6. Kinetic curves of leaching and variation with time
of the leaching rates: (a) Cs from Cs Mg Zr (PO )
2
0.5 1.5
4 3
(static mode), (b) Cs from CsBaFe (PO ) (dynamic mode),
2
4 3
where m is the weight (g) of an element released from
and (c) Ba from CsBaFe (PO ) (dynamic mode).
2 4 3
RADIOCHEMISTRY Vol. 47 No. 3 2005

240
ORLOVA et al.
The minimal leaching rate of K from K FeZr(PO )
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197.
2
4 3
6
2
1
and K Mg Zr (PO ) was (1 2) 10 gcm day ;
2
0.5 1.5
4 3
that of Cs from Cs Mg Zr (PO ) and CsBaFe
2
0.5 1.5
4 3
2
6
2
1
(PO ) , (2 5) 10 g cm day ; and that of Ba
4 3
6
2
1
from CsBaFe (PO ) , 8 10 g cm day .
2
4 3
STABILITY IN MELTS
Tests for stability in alkali metal chloride melts
were performed with phosphates crystallizing in the
langbeinite structure: K FeZr(PO ) and K ErZr
2
4 3
2
(PO ) . The experimental procedure was as follows.
4 3
An intimate mixture of a phosphate sample ( 0.5 g)
and an alkali metal chloride (molar ratio 1 : 10) was
placed in a platinum crucible and heated for several
days at 830 C. Then the melt was cooled, and the
soluble fraction was washed off with hot water acidi-
fied with HNO . The insoluble residue was dried at
3
100 C and analyzed by X-ray diffraction.
The phosphates tested underwent no changes on
contact with a KCl melt. The crystallinity of the
samples did not change either.
To sum up the results obtained in this study and
reported in [2], we can note that, among framework
phosphates of cesium, potassium, and rubidium, com-
pounds of the langbeinite structure, along with phos-
phates of the NaZr (PO ) type, deserve attention as
14. Orlova, A.I., Orlova, V.A., Beskrovnyi, A.I., et al.,
Kristallografiya (in press).
2
4 3
matrices for concentrating large alkali metal (prima-
rily cesium) ions. In some parameters, langbeinite-type
phosphates are even preferable. In particular, they in-
corporate more alkali metals: Cs up to 38, Rb up to 28,
and K up to 15 wt %. Compounds with the langbein-
ite structural motif are prepared by relatively simple
procedures and are stable thermally and chemically.
The structural features of langbeinite and the revealed
crystal-chemical relationships allow incorporation into
these matrices of d elements, lanthanides, and, ap-
parently, also actinides. Phosphates with NZP and
langbeinite frameworks show isomorphism with many
cations and in systems of different compositions and
complexities. Such crystalline matrices are undoubted-
ly promising for immobilization of radioactive waste.
15. Aloy, A.S., Trofimenko, A.V., and Faddeev, I.S.,
Technical Information on the IGITs Theme, Docu-
ment of the Khlopin Radium Inst., Research and Pro-
duction Association, July 18, 1990, no. 150-03/2315.
16. Sizov, P.V. and Yakovlev, N.G., Volatilization of
Cesium in the Course of Founding of Cesium Alumi-
nophosphate Glasses, Report of the Mayak Production
Association, 1992, no. TsL/2618.
17. Korchenkin, K.K., Mashkin, A.N., Dzekun, E.G.,
et al., Abstracts of Papers, Tret’ya rossiiskaya konfe-
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ACKNOWLEDGMENTS
The study was financially supported by the Russian
Foundation for Basic Research (project nos. 02-03-
32181 and 03-03-32538) and the grant for leading
scientific schools (no. VNSh-1514.2003.2).
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