Phase equilibria in a portion of the system La2O3 — K2O — P2O5 rich in P2O5

Materials Chemistry and Physics, 24 (1990) 481-494

PHASE RICH

EQUILIBRIA

IN A PORTION

487

OF THE

SYSTEM

-KO-PO 2

La203

2 5

IN P205

W. JUNGOWSKA Department Economics, Received

and T. ZNAMIEROWSKA of Inorganic Academy

August

53345

of Economics,

10,

Faculty

Chemistry,

1989;

accepted

of Engineering

Wrociaw

September

and

(Poland)

13,

1989

ABSTRACT In the ternary La(P03)3

- KP03

system

- K20 - P205

La203

- P205 has been

and IR-absorption

and

its phase

examined diagram

the partial

by thermal, was

system

X-ray

analyses

suggested.

INTRODUCTION Both

the phosphates

condensed

phosphates

interesting

materials

of rare

earth

elements

of rare

earths

and alkali

mainly

because

of their

as well

as double

metals

are known

luminescence

and

to be laser

properties. There

are many

papers

try charcteristics results

concerning

of the above

of the examinations

the synthesis

mentioned

of phase

and crystallochemis-

compounds

equilibrium

but more

relations

and more

are being

published. In this paper,

the results

of the

gram

for a portion

of the ternary

P205

are reported.

In this

La(P0)3

composition

[i] and KP03

qetaphosphates

Double which

- P205

are formed

investigations

system

of the

formulae:

peritectically

range

-

the phase

[2] systems

- Lap309

of the phase

- K20

La203

KLa(P03)4

P205,

diagrams

are already

in

of the

known.

and K2La(P03)5

at 13SO" and 770°C

dia-

rich

respectively,

, are

also known. According

to paper

(NH4)2La(P03)5 Pl. The

[2], compound

. They belong

compound

0254-0584/90/$3.50

KLa(P03)4

K2La(P03)4

to the triclinic

has the monoclinic

is isotypic system

with

structure

with space

group

P21.

0 ElsevierSequoia/Printedin The Netherlands

EXPERIMENTAL PROCEDURE The following reagents were used: lanthanum oxide (99.99%), K2C03 analytical grade, Kli2PO4 analytical grade, NH4H2P04 analytical grade and 85% phosphoric acid analytical grade. Lanthanum retaphosphate La(PO3)3 was produced from La203 and H3P04 by sintering the mixture of these substances in a porcelain crucible at 800°C for 3 days. Then the sinter was ground, washed with hot, distilled water, dried and sintered at 950°C for several hours. Lanthanum ultraphosphate Lap5014 was prepared from La203 and H3P04. The starting components mixed at a molar ratio P/La = 20 were slowly heated in a porcelain evaporating dish to obtain a gluey, transparent mass. Then the mass was sintered in a porcelain crucible at 6OO'C for 1 day and at 700°C for 2 days. The obtained product was ground, washed with hot, distilled water, dried at 200°C and powdered carefully. Potassium qetaphosphate KP03 was produced as the result of the full dehydration of KH2P04 at 350°C for 1 h. Potassium - lanthanum phosphate KLa(P03)4 was prepared from KP03 and La(P0 )3 by sintering % the stoichiometric mixture of these compounds at 800 C.for several daya. Potassium - lanthanum phosphate K2La(P03)5 was produced by sintering the mixture of 1 mole of La(P03)3 and 2 moles of KP03 at 700°C for several days. The investigations were carried out by didferential thermal (DTA)

and X-ray

analyses

and

IR spectroscopy. Test samples were presynthesized by the reaction in the solid phase. Initial substances, mixed in desirable composition , were pressed into pellets and sintered at different temperatures for different times, according to the composition. Gold and porcelain crucibles were used. The differential thermal analysis was performed by means of a derivatograph type 3427 (MOM, Hungary)

within the temperature range

20 to 13OOOC. The operating conditions were as follows: sensitivity TG - 500 qg, DTA - l/10, DTG - l/10, heating rate lOO/min, platinum crucible. The standard substance used was AL203. The temperature was measured with the Pt/PtRhlO thermocouple which was standardized using the melting points of NaCl, K2S04 and the polymorphic transition temperature of K2S04

(583OC). The thermal analysis of heating was used

most often because the samples crystalliee with difficulty and usually form glazes. At higher temperatures they decompose irreversibly changing their initial composition. The visual observation of the samples during heating was performed, additionally. The visual observation consisted in taking notes of the temperature when the first traces of liquid were seen, and of the temperature at which the sample liquefied completely and became transparent.

489

To define phase equilibria in the discussed system, the quenching technique was also used. Samples equilibrated in an electric furnace were quenched in water. The obtained phases were identified by X-rays and IR absorption. The powder X-ray analysis at room temperature was performed in an HZG - 4 diffractometer with a nickel filter and cu - KG radiation. A Specord IR - 75 spectrophotometer was used for the IR absorption spectroscopy using pellets formed by mixing the specimens with KBr.

RESULTS AND DISCUSSION Phase equilibria in a region of the ternary system La203 - K20 P203,rich in phosphorous pentoxide, i.e. within the composition range La(P03)3 - KP03 - P203 were examined. There are two known systems in this field: La(P03)3 - P203 [l] and KP03 - Lap309 [2]. Both systems were examined in our laboratory. With the first one, the results were the same as Park and Kreidler

[l]. However, there are some divergen-

cies with the KP03 - Lap309 system. The occurence of two peritectic compounds KLa(P03)4 and K2La(P03)3 was confirmed. It was found that KLa(PO3)4 decomposes at 84O'C (act. to [2] at 88OOC). The system was discovered to be non binary in the part rich in lanthanum metaphosphate, within the composition range 88 - 100 wtP of La(P03)3. Results obtained from DTA measurements and several quenching data showed that LaP04 occurs above 119O'C. This compound is formed as a result of La(PO3)3 decomposition into LaP04 and a liquid. It agrees with that reported by Durif [3] that RP30g (R = La, Ce, Pr,...) decomposes into RP04 and a liquid. The verified phase diagram of the KP03 - La(P03)3 system is presented in Fig. 1. To define phase equilibria in a portion of the ternary system - K20 - P203 rich in phosphorous pentoxide, differentiated La203 thermal treatment (sintering and quenching from different temperatures) was performed on the ternary samples (the ternary compositions). The differential thermal analysis of heating (DTA) was also used. The phase composition of the obtained products was each time controlled by the powder X-ray analysis. Figure 2 shows the DTA curves of the at random chosen compositions containing: (A) 8.0 rt%.of K201 25.2 w-t% of La203, 66.8 WtX of p203; (B) 20.0 wt% of K20, 64.3 wt2 of P205; (C) 29.9 wtX of K20,

7.9

nt%

15.7 rtX of La203, of

La203, 62.2 wt% of

respectively. Figures 3 and 4 show the change of the phase com-

P205' position of the randomly chosen ternary samples, rich in P20 , that d resulted from sintering over the temperature range 530 - 650 C

490

(Fig. position

(Fig. 4). The open circles mean the initial com3) and melting of samples. Numbers at some of the open circles show the

randomly

chosen

temperatures

at

which

the

samples

were

sintered

and

sub-

sequently quenched or cooled to room temperature. It can be concluded from Figs. 3 and 4 that as a result of both melting and sintering, the initial phase composition of samples undergoes different, considerable changes. The final phase composition was found to depend not only on the initial composition of the sample but on the performing thermal treatment as well, e.g. the sample of the composition: 8.0 wt% of K20, 25.2 wtI of La203, 66.8 wtX of Pa05 after melting and (a) quenching from 850°C is a mixture of La(P03)3 and LaP5014 (Fig. 4), (b) quenching from 750°C is a mixture of KLa(P03)4 and LaP5014. The thermal effects on the DTA curves for the ternary samples were interpreted on the basis of data shown in Figs. 3 and 4. Hence, effects at 770°C and 780°C shown on the DTAl and DTA2 curves (Fig. 2) are connected with the peritectic decomposition of KLa(P03)4. Effects on the DTA3 curve (Fig. 2) are attributed to the

1

i280~

.l2350

KY+ K2i

600 t

KPO; WI

=”

~Lo(PO$~KL~(P~~) Lab’O& (K2LI$J n
(Lp3)

weight% LoPO&-

Fig. 1. Phase diagram of the system KPO3 - La(PO3)3; LaP04 - LP.

491

635

Fig.

2. DTA

(Cl

curves

of the sample

25.2 wt% of La203, of La203,

containing:

66.8 rtX of P205;

64.3 wt% of P205;

(A) 8.0 nt% of K20,

(B) 20.0 wt% of K20,

(C) 29.9 wtX of K20,

15.7 wt%

7.9 wtX of La203,

62.2 wtP of P205.

wt%p205Fig.

3. Sintering

data

temperature

range

for ternary

samples

0, initial following

rich

4. Melting

nary

samples

0, initial

in P205;

transitions:

P/y - KP03,

composition eutectic

550 - 6SOOC)

Fig.

data

rich

for ter-

in P205j

composition.

composition. phase

transition

(over the

of K2La(P03)5,

temperature

(a) effect

(b) effect

from

(c) effect system

at 450°C

at 635OC

KP03

- to polymorphic

- to the peritectic

at 670°C

- to the

- K2La(P03)5.

lowered

de-

492

CONCLUSION The results discussed above lead to the conclusion that the temperature ranges of double qetaphosphates: KLa(P05)4 and K2La(P05)5 stabilities, in ternary samples rich in P205, are lowered by approximately 100°C and 150° - 250°C respectively, in comparison to stability ranges shown in Fig. 1. On the basis of the results presented in this paper, it was found that the pseudobinary section KLa(P03)4 - LaP5014 occurs in the discussed range of compositions. Fig. 5 shows the phase diagram of this section examined on the basis of DTA heating. The section KLa(P05)4 - LaP5014 has a ternary nature in its top part, resulting from the peritectic reaction taking place. In the high temperature . liquid C and compounds part, up to 750%, four phases occur, La(PO2)2, LaP5014, and KLa(P03)4 . As a result of the peritectic reaction liquid C and La(P02)2 become used up to yield KLa(P05)4 crystals. Below 750°C this section has a binary nature and only KLa(P02),, and LaP5014 exist.

KL?

+ LpS

Fig. 3. Phase diagram of the system KLa(PO5),, - LaP5014; La(PO5)2 = LPS.

493

The phase diagram with liquidua isothermal lines for the system - K20 - P203 in a P203 - rich portion, obtained from the present La203 experiments is suggested in Figure 6. In the composition range under consideration six primary crystallization fields of binary and ternary compounds appear. These fields are separated by suitable eutectic or peritectic curves. The p,pe curve corresponds to solidification of a double peritectic point according to the reaction: C(p,p,) + Lap04 -

LafP03)3. The p3PI

curve corresponds to solidification of a double peritectic point KLa(P0 ) During 3 4' solidification of the melts, corresponding to the points of the

according to the reaction: C(p3PL) + La(P03)3 +

La(PO313 - KLa(PO3)4 - Pt - LaP5014 field (triple peritectic quadrangle), a triple peritectic reaction takes place: C(PL) + La(P03)3 -+

KLa(P03)4 + LaP5014 at 750°C (C(P,) - the

liquid whose composition corresponds to point P,).

lEifP LOP0

weigM%P

d+r---

109505 14 Fig.

6.

Phase diagram of the system La203 - K20 - P2011i KPO3 - KP,

La(P03)3 = LP3, LaPSOL LaPOh = LP.

- LP5, KLa(P0314 = KLP4, K2LaW03)5

= K2LPg,

494

ACKNOWLEDGMENT This paper was financially supported by the Ministry of National Education. REPBRENCBS

1

H.D. Park and E.R. Kreidler, J. Am. Ceram. Sci., 67(l)

2

M. Ferid, N.K. Ariguib and M. Trabelsi, Mater. Chem. Phys., g

3

A. Durif, Bull. Sot. Pr. Mineral. Cristallogr., E

(1984) 23. (1984) 175. (1971) 314.