kp2o7 - Phase equilibria in the system cepo4 - страница 1
ВІСНИК ЛЬВІВ. УН-ТУ
Серія хім. 2007. Bun. 48. Ч. 1. С. 8-13
VISNYK LVIV UNIV. Ser. Khim. 2007. No 48. Part 1. P. 8-13
PACS 81.30.Bx; 81.70.Pg; 61.10.Nz
PHASE EQUILIBRIA IN THE SYSTEM CePO4 - K,P2O7 - KPO3
Department of Inorganic Chemistry, Faculty of Engineering and Economics, Wroclaw University of Economics, ul. Komandorska, 118/120, 53-345 Wroclaw, Poland e-mail: irena. szczygiel @ ae.wroc.pl
The CePO4 - K4P2O/ - KPO3 region of the Се20з - K2O - P2O5 ternary system has been investigated using thermoanalytical methods (DTA, DSC), powder X-ray diffraction, IR spectroscopy and photoelectron spectroscopy (XPS). Two systems: CePO4 - K4P2O7 and CePO4 -K5P3O10, which are binary only in subsolidus region, have been found.
Key words: phase equilibria, potassium-cerium phosphates, ternary system.
There are many literature reports on alkali metal-lanthanide phosphates, where Ln = La, Nd, Gd and Y and some on the connections of cerium phosphate with alkali metals [1-8]. The published data usually deal with the methods of their synthesis, crystal and spectroscopic characteristic. Studies on phase equilibria are less popular.
The investigations of cerium-potassium phosphates have been carried out in our laboratory for over ten years [6-8]. Earlier we studied double phosphates in partial systems: Ce(PO3>3 - KPO3 - P2O5 , CePO4 - KPO3 - Ce(PO3>3 , CePO4 - K3PO4 - K4P2O7 . It was found that these systems contain four double phosphates: KCe(PO3)4, K2Ce(PO3)5, K3Ce(PO4)2 and K4Ce2P4O15. K4Ce2P4O15 exists only in the solid phase  and the remaining phosphates crystallize from the liquid phase. The present paper is a next part of the investigations on the potassium-cerium phosphates in the Ce2O3 - K2O - P2O5 ternary system within the CePO4 - K4P2O7 - KPO3 composition range.
Initial materials for the phase equilibria studies were of analytical grade: KH2PO4, K2HPO4, Ce(NO3)36H2O, NH4H2PO4 and CeO2. From these commercial reagents the phosphates CePO4, K4P2O7, K5P3O10, KPO3, K4Ce2P4O15. were synthesized. Cerium orthophosphate CePO4 was synthesized by the method provided in . Potassium diphosphate K4P2O7 was prepared from K2HPO4 by heating at 300 °C for 2 h and at 500 °C for 5 h. Potassium tripolyphosphate K5P3O10 was obtained by sintering the mixture of KH2PO4 and K2HPO4 in the molar ratio 1 : 2 at 200 °C for 1 h and at 550 °C for 5 h. Metaphosphate KPO3 was obtained from KH2PO4 by heating at 350 °C for 2 h. The binary phosphate K4Ce2P4O15 was produced by solid state reaction of cerium (IV) oxide CeO2 with potassium phosphate KPO3.
The samples were investigated using differential thermal analysis, both heating and cooling, X-ray powder diffraction, IRS and XPS. The DTA-heating was carried out with a derivatograph type 3427 (MOM, Hungary), up to 1350 °C (heating rate: 10 °K-min-1, reference material: a-alumina, platinum crucibles) under air. Thermal analysis during cooling was done through the range 1300 °C to 20 °C with a resistance furnace constructed
© Szczygiel I., 2007
with Pt30Rh winding (3 g mass samples were used). The thermoanalytical investigations were performed by using SetsysTM calorimeter (TG-DSC 1500; SETARAM) in a temperature range 20-1300-20 °C (scanning rate 10 K-min-1, reference material: a-alumina, platinum crucibles) in argon atmosphere. Temperature was measured by means of a Pt/PtRh10 thermocouple. The initial reagents were mixed in appropriate ratios, ground, pelletized, placed in platinum crucibles and sintered at 500-900 °C. The quenching techniques was also used for phase determinations.
The phase purity of the reagents and phase structure of products were controlled and identified by powder X-ray diffraction at room temperature on a Siemens D 5000 diffractometer with Co/Fe-radiation. IR spectra were measured in the range 1400-400 cm-1 with a Specord M-80 spectrophotometer (Carl Zeiss Jena). XPS were used to study the chemical state of cerium in cerium phosphates and cerium-potassium phosphates.
The part of the Ce2O3 - K2O - P2O5 ternary system contained within the area limited by phosphates CePO4, K4P2O7 and KPO3 (Fig. 1) has been examined using thermoanalytical methods (DTA, DSC), powder X-ray diffraction, IR spectroscopy and photoelectron spectroscopy (XPS). Phase relations within ternary systems are inherently related in nature of phase equilibra in the surrounding subsystems. It was found that the CePO4 - K4P2O7 system, shown in Fig. 2, is quasi-binary only in the subsolidus region, i.e. below 900 °C. Above 900 °C it is only binary in the composition range 87-100 wt.% K4P2O7. In the other part of the composition range it is a polyphase. The system CePO4 - K4P2O7 contains one binary phosphate: K4Ce2P4O15 (molar ratio CePO4 : K4P2O7 = 2:1). This compound exists only in the solid state. K4Ce2P4O15 decomposes irreversibly at 879 °C [8, 10]. It exhibits a polymorphic transition at 527 °C. Cerium orthophosphate CePO4 occurs in two polymorphic modifications with a transition temperature of a/p-620 °C, whereas K4P2O7 is known to appear in a four polymorphic modifications with transition temperatures of a/p -1080 °C; p/y - 486 °C; y/5 - 280 °C . It results from the figure that only the y/5 transformation of K4P2O7 exhibits weak thermal effects present on the DTA curves, but only in the composition range 60-100 wt % K4P2O7.
Fig. 1. The examined region in the Ce2O3 - K2O - P2O5 ternary system
Some invariant points in the bounding binary subsystems
K4P2O7 - KPO3 subsystem 
L~ K5P3O10 + KPO3
K5P3O10 <-> L + K4P2O7
P-K5P3O10 <-> a-K5P3Ow
Quasi-binary CePO4 - KPO3 system
L ^ CePO4 + KPO3
P-CePO4 <-> a-CePO4
P-KPO3 <-> a-KPO3
P-KPO3 <-> y-KPO3
Quasi-binary CePO4 - K4P2O7 system
L~ CePO4 + K4P2O7
8-K4P2O7 — y-K*P2O7
P-KtCe2P4O15 — a-K4Ce2P4O15
K4Ce2P4O15 — CePO4 + K4P2O7
The other subsystem, K4P2O7 - KPO3, contains intermediate compound K5P3O10. This phosphate melts incongruently at 615 °C according to the scheme: K5P3O10 — K4P2O7 + liquid (rich in KPO3), and occurs in two polymorphic modifications . Its transformation point is 452 °C.
In the last subsystem CePO4 - KPO3 no intermediate compounds are formed; it is a simple eutectic system with the eutectic composition 85 wt.% KPO3 (15 wt.% CePO4) at 790 °C. Phase diagram of the system CePO4 - KPO3 is given in Fig. 3.
Phase equilibria in three subsystems that surround the CePO4 - K4P2O7 - KPO3 system (i.e. CePO4 - K4P2O7, K4P2O7 - KPO3, and CePO4 - KPO3) place constraints on the ternary equilibria. Invariant points in these subsystems are given in Table. Polythermal cross section of ternary system in Fig. 1 shows another system, built of two real compounds: CePO4 and K5P3O10.
CeP04 20 40 60 80 К,Р3О,0
(cp) weight %K5P3-► (K5P3)
Fig. 4. Phase diagram of the cross section CePO4 - K5P3O10
The CePO4 - K5P3O10 phase diagram is given in Fig. 4. CePO4 - K5P3O10 section does not have all the features of the real equilibrium system. In other words it does not obey the Gibbs phase rule. The result is one of components of the system is an incongruently melting compound (K5P3O10). Therefore, this phase diagram may only be correctly interpreted if considered as a part of the CePO4 - K4P2O7 - KPO3 ternary system. Above the temperature of 560 °C it has a polyphase character with its four phases: a liquid L, CePO4, K4P2O^ and K5P3O10. As a result of a peritectic reaction the liquid L and the phosphate K4P2O7 undergo consumption to produce K5P3O10 crystals. Below 560 °C only two compounds exist, which are CePO4 and K5P3O10. The section has a quasi-binary character in the subsolidus region only - the phase rule is obeyed.
It has been found that a ternary peritectic and ternary eutectic occur in the CePO4 - K4P2O7 - KPO3 system. The peritectic reaction proceeds according to the equation: L(P) + K4P2O7 ^ CePO4 + K5P3O10 at 560 °C (L(P) stands for a liquid of the composition corresponding to point P). Ternary eutectic (CePO4+K5P3O10+KPO3) is yielded at a constant temperature of 540 °C.
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ФАЗОВІ РІВНОВАГИ У СИСТЕМІ CePO4 - K,P2O7 - KPO3
Кафедра неорганічної хімії, факулътет інженерії і економіки, Вроцлавеъкии економічний університет, вул. Командореъка, 118/120, 53-345 Вроцлав, Полъща e-mail: irena. szczygiel @ae.wroc.pl
Потрійну систему CePO4 - K4P2O7 - KPO3 досліджено в області Ce2O3 - K2O - P2O5 методами термоаналітичного (flTA, ДСК), рентгенофазового аналізів та 14 і фотоелектронної спектроскопії. Виявлено дві бінарні системи у сабсолідусній області CePO4 - K4P2O7 і CePO4 -
Ключові слова: фазові рівноваги, калій-церій фосфати, потрійна система.
Стаття надійшла до редколегії 28.05.2006 Прийнята до друку 05.10.2006