k4p2o7 - Phase equilibria in the system cepo4 - страница 1

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ВІСНИК ЛЬВІВ. УН-ТУ

Серія фізична. 2007. Вип.40. С.211-218

VISNYKLVIV UNIV. Ser.Physic. 2007. N40. P.211-218

PACS number(s): 81.30.Dz PHASE EQUILIBRIA IN THE SYSTEM CePO4 - K4P2O7 - KPO3

I. Szczygiel

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 СЄРО4 - K4P2O7 - KPO3 portion of the СЄ2О3 - 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: ternary systems, X-ray diffraction, phase diagram, thermoanalytical methods, IR spectroscopy and photoelectron spectroscopy.

There are many literature reports on alkali metal-lanthanide phosphates, where Ln = La, Nd, Sm, Gd, and some on the connections of cerium phosphate with alkali metals. 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 [1-8]. Earlier we studied double phosphates in partial systems: Ce(PO3b - KPO3 - P2O5 [6], CePO4 - KPO3 - Ce^b [7], CePO4 - K3PO4 -K4P2O7 [8]. It was found that these systems contain four double phosphates: KCe(PO3)4, K2Ce(PO3)s, K3Ce(PO4)2 and K4Ce2P4Oi5. K4Ce2P4Oi5 exists only in the solid phase [8] 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 analytical grade: KH2PO4 K2HPO4, Ce(NO3)3-6H2O, 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 [9]. 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 oxide (IV) 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 3 427 (MOM, Hungary), up to 1 350 °C (heating rate: 10 °K-min-1, reference material: a-alumina, platinum crucibles) under air. Thermal

© Szczygiel I., 2007

analysis during cooling was done through the range 1 300 to 20 °C with a resistance furnace constructed with Pt30Rh winding (3 g mass samples were used). Also, the thermoanalytical investigations were performed by using SetsysTM calorimeter (TG-DSC 1 500; SET ARAM) in a temperature range 20-1 300-20 °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/8-280 °C [11]. It results from the figure that only the y/8 transformation of K4P2O7 exhibits weak thermal effects present on the DTA curves, but only in the composition range 60-100 wt % 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 [11]. 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.

The diagram of the CePO4 - K5P3O10 system 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, K4P2O7 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 - phase rule is obeyed.

Table

Some invariant points in the bounding binary subsystems

Reactions

Reaction temperature [°C]

type

K4P2O7 - KPO3 subsystem [11]

 

 

L~ KPO3

810

melting

L~ K4P2O7

1105

melting

L~ K5P3O10 + KPO3

590

eutectic

K5P3O10 <-> L + K4P2O7

615

peritectic

P-K5P3O10 <-> a-K5P3O10

452

transition

Quasi-binary CePO4 - KPO3 system

 

 

[7]

 

 

L~ KPO3

810

melting

CePO4

2045

melting

L <-> CePO4 + KPO3

790

eutectic

P-CePO4 <-> a-CePO4

620

transition

P-KPO3 <-> a-KPO3

665

transition

P-KPO3 ~ y-KPO3

449

transition

Quasi-binary CePO4 - K4P2O7 system

 

 

[8]

 

 

L~ CePO4

2045

melting

L~ K4P2O7

1105

melting

L~ CePO4 + K4P2O7

900

eutectic

5-K4P2O7 Y-K4P2O7

280

transition

P-K4Ce2P4O15 a-K4Ce2P4O15

527

transition

K4Ce2P4O15 CePO4 + K4P2O7

879

peritectoid

reaction

 

 

Fig. 2. Phase diagram of the system CePO4 - K4P2O7

Fig. 3. Phase diagram of the system CePO4 - KPO3

Fig. 4. Phase diagram of the cross section CePO4 - K5P3O10

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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5. Szczygiel I., Znamierowska T. Prace Naukowe AE we Wroclawiu. 610. (1992) 307.

6. Znamierowska T., Obremski J. J. Therm. Anal. 37. 1991. 285.

7. Szczygiel I. Thermochim. Acta. 402. 2003. 153 p.

8. Szczygiel I. Solid State Sciences. 7. 2005. 189 p.

9. Kijkowska R. I International Congress on Phosphours Compounds. Rabat. October

17-21. 1977.

10. Szczygiel I. Thermochim. Acta. 417. 2004. 75 p.

11. Znamierowska T. Polish J. Chem. 55. 1981. 747 p.

ФАЗОВА РІВНОВАГА В СИСТЕМІ CePO4 - K,P2O7 - KPO3

І. Щигел

Відділ неорганічної хімії, інженерний і економічний факультет, Вроцлав Економічний університет, вул. Командорська 118/120, 53-345 Вроцлав,

Республіка Польща

Калориметричними методами, методами рентгенівської дифракції, інфрачервоної та фотоелектронної спектроскопії досліджено систему CePO4 -K4P2O7 - KPO3, яка є складовою частиною потрійної сполуки Ce2O3 - K2O - P2O5. В ділянці нижче від лінії солідусу виявлено дві бінарні системи CePO4 - K4P2O7 та CePO4 - K5P3O10.

Ключові слова: потрійні системи, рентгенівська дифракція, фазова діаграма, калориметричні методи, інфрачервона спектроскопія, фотоелектронна спектро­скопія.

Стаття надійшла до редколегії 29.05.2006 Прийнята до друку 26.02.2007

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