Crystalline cerium(IV) phosphates—I Preparation and characterization of crystalline compounds

J. inorg, nucl. Chem., 1975, Vol. 37, pp. 1697-1704. Pergamon Press. Printed in Great Britain

CRYSTALLINE CERIUM(IV) PHOSPHATES--I P R E P A R A T I O N A N D C H A R A C T E R I Z A T I O N OF CRYSTALLINE COMPOUNDS* R. G. HERMAN+ and A. CLEARFIELD Departmentof Chemistry,ClippingerGraduate Research Laboratories, Ohio University,Athens,OH 45701,U.S.A. Abstract--The preparation of seven cerium(IV) phosphates, six of which are new compounds, is described. The compounds were characterized by means of analytical, thermal, i.r. and X-ray methods. Five of the compounds belong to a series whose general formula is Ce(OH)~(PO~)x(HPO4),, 2,. yH20. Two of these can be characterized as being end members of the series, i.e. Ce(HPO4)2and Ce(OH)o.7(PO,),., although the latter compound does not quite conform to the required stoichiometry.The remainingtwo compounds contain dihydrogenphosphate groups as well as orthophosphate and may be members of additional series.

INTRODUCTION

CERIUM(IV) phosphates are of interest because some of them exhibit ion exchange behavior. Cilley and Larsen[l,2] carried out an extensive study of the preparation and properties of amorphous cerium(iv) phosphate gels. Precipitates having a phosphate-cerium ratio of from 0.55 to 1-55 were obtained by varying the mole ratio of the reactant mix, by changing the pH and by aging of the Ce(IV) solutions for different lengths of time before use. They assigned the stoichiometric formula Ce3(OH)8(H2PO4)4 to one of the products which had a P-Ce ratio of 1.4812]. Subsequently, K6nig and Meyn prepared amorphous cerium(IV) phosphates with phosphate to cerium ratios of from 1.03 to 1.9713]. They also obtained a crystalline product from more concentrated H3PO4, to which they assigned the formula Ce20(HPO4)3. H20. These crystals did not exhibit macroscopic ion exchange behavior but did exchange trace amounts of cations [4]. KiSnig and Meyn also observed that the products prepared in sulfate containing solutions incorporated appreciable amounts of sulfate ion[4]. Subsequently, K6nig and Eckstein were able to synthesize a series of solids of general composition Ce20(HPO4)3 , (SO4)x.4H20 where x has values between zero and one[5]. These solids were crystalline and their ion exchange capacities were roughly in keeping with those calculated on the basis of their HPO42- content[5,6]. Another series of compounds in which x exceeded one was prepared but these did not exhibit ion exchange behavior [5]. It was also found that cerium(IV) phosphates could be reduced to cerium(III) phosphates and that these products exchanged Ce +3 for other lanthanide ions [7]. AIberti et al. examined the effect of temperature, *This work was supported in part by National Science Foundation Grants GP-10150 and 26050. ~Portions of this paper were taken from the Ph.D. Dissertation of R. G. Herman presented to the Chemistry Department (Dec. t972).

digestion time, PO,-Ce ratio and order of mixing of reactants upon the composition and crystallinity of precipitated cerium(IV) phosphates[8]. They obtained four different products: (1) amorphous cerium phosphates, (2) a microcrystalline cerium phosphate with PO4/Ce-~ 1.15 containing about 7 per cent sulfate ion, (3) a second microcrystalline product with PO4/Ce = 1.5 (4) and a fibrous crystalline solid of idealized formula Ce(HPO4),_. H:O. Product (2) is undoubtedly a phosphate-sulfate similar to those reported by K6nig et al. [4, 5]. Product (3) did not exhibit ion exchange behavior while the fibrous crystalline solid (4) had an exchange capacity close to the expected value based upon the assumption that the protons of the monohydrogen phosphate groups are exchangeable. Subsequently, AIberti et al. prepared a microcrystalline cerium phosphate of composition Ce(HPO4)2.1.33 H20[9]. In other preparations of cerium(IV) phosphates, solids of composition Ce4H2(PO4)~.25H20[10], Ce2H(PO4h. 25H20 [11], and Ce(HPO4)(H2POD:. 2H20 [12] have been reported. From the above summary, it is evident that cerium(IV) phosphates constitute a complicated system of compounds of unknown structure and interesting behavior. The present study was undertaken to attempt to clarify and extend our knowledge of these compounds. The ion exchange properties of these crystalline compounds will be reported in a subsequent paper. EXPERIMENTAL

Method of preparation. All preparations were carried out in

3-neck round bottom flasks fitted with a mechanical stirrer, reflux condenser and addition funnel. A cerium(IV) solution whose acidity had been adjusted to a desired value with nitric {and in some cases sulfuric) acid, was heated with stirring in the flask. Then phosphoric acid was added dropwise until a specifiedratio of phosphate to Ce(1V) in the reactant mix was attained. After the addition was completed, the addition funnel was replaced by a thermometer and the mixture refluxed for times of up to 21 days. Samples were withdrawn periodically, and an X-ray diffraction pattern of the recovered solid recorded. At the end of a specified

1697

1698

R.G. HERMANand A. CLEARF1ELD

time of refluxing, the mixture was allowed to cool to 40--50°, filtered and the solid washed with water. X-ray diffractionpatterns of both the wet and dessicator dried (over CaSO,) solids were obtained. This was done to insure that no phase change occurred on drying. In some instances the order of addition of reagents was reversed. Unless otherwise specifiedthe source of cerium(IV)was (NH4)2Ce(NO3)6. Analytical Approximately 0.5 g samples were fused with 10 times their weight of sodium carbonate in platinum crucibles. The crucible and fusion product were then submerged in 100ml of hot water and allowed to remain in contact until the fusion cake crumbled into a fine powder. The crucible was then removed and thoroughly washed with hot water. The washings were combined with the main solution and filtered to recover the cerium oxide. This was then calcined at ~I000°C to constant weight and the cerium weighed as CeO2. Phosphate was then determined on the recovered filtrate and washings by a double precipitation as magnesium ammonium phosphate followed by calcination to MgP2OT. Instrumental X-ray powder patterns were obtained with a Norelco wide angle goniometer with nickel filtered copper radiation (h = 1.5418]k). Accuracy and precision of the instrument was checked periodically with a standard silicon sample. Weight losses at elevated temperatures were determined by thermogravimetric analysis using a Tem-Pres TG-2A unit. The heating rate was 25°Cper hr up to 200°Cand 430 per hr at higher temperatures. The accuracy of the unit was checked by frequently running a standard sample of Ca(C20~). H20. I.R. spectra were recorded with a Perkin-Elmer model 621 spectrometer. The samples were prepared as KBr disks under a pressure of 20,000lbs/in2. Reagents. The sources of Ce(IV) used were as follows: (NH,)2Ce(NO3)~, Fisher certified and Matheson, Coleman and BelI-ACS Reagent; (NtL),Ce(SO,),. 2H20 and Ce(HSO,),, G. F. Smith Chem. Co.--Reagent Grade. All other materials were reagent grade quality. Distilled, deionized water was used throughout. RESULTS

Seven crystalline compounds were isolated and characterized. The preparative conditions are summarized in Table 1. Each compound is identified by a capital letter (A-G) assigned in the order of their preparation. Analytical data are given in Table 2 and X-ray d-spacings in Table 3. Some additional remarks concerning the individual preparations are in order. For convenience the preparations may be divided into two groups, those formed in

Table 2. Analyticaldata for cerium(IV)phosphates Percent Comoosition

!~

l~nition

~3.98

l.!sS

-.?~

±7.92

53.02

59.P2

1.09

O

9.9

£3.71

51.2£

1.3

0

i0.~9

Ce

A

43.66

B C D

£6.7 ~

48.9)~

].55

O

O.!~

E

40.~0 a 41.5S

~5.31 5p.13

2.00 1. °6

2.17 2.17

18.O5 9,85

&5.81 a 45.86

50.£4 50.71

1.62 1.6h

0.47 0.57

9.25 .....

L6.50

49.76

1.5 S

2.55

~.22

PO~__

a ~¢o different preparations.

more concentrated acid media (C, D, E, F) and those obtained from low or moderately acid solutions (A, B, G). We shall discuss the latter compounds first. Compound A. This phase formed most readily when the total acidity was kept below I M and the phosphate-cerium ratio was low. For most runs, the product was found to have incorporated 1.6-2.2 per cent of ammonium ion. However, in some instances it was possible to decrease the amount of ammonium ion below 1.6 per cent by increasing the nitric acid conc. above 1 M and reducing the H3PO4 cone. to 0.1 M. Compound B. The conditions for the preparation of this compound are somewhat similar to those for compound A; namely, low acid concentration and a low phosphate--cerium ratio. However, compound B is favored by lower acidities and longer reflux times. This phase was never found to contain ammonium ion. Compound C. This compound was prepared by dissolving a cerium phosphate gel in hot concentrated HaPO4 followed by refluxing. Then water was added dropwise until the acid concentration reached approx. 6M. A whitish gel gradually formed and precipitation continued for about I hr. Refluxing was continued until X-ray patterns showed that compound C had formed (never more than 24 hr of re fluxing). Compound D. Compound D formed in fairly strong acid solutions (3-8 M). The acidity was achieved with H3PO4 alone or in combination with HNO3. Refluxing dispersions of the other cerium phosphates in 3-8M

Table I. Preparative conditionsfor the synthesisofcrystallinecerium(IV)phosphates* compound

*

Loss c ~

~/:e

Co~ound

Conc. of Reactants (M.] ...... Ce(IV) ~FO~ HNCa

po4/Ce in Sol'n

A

O.04-O.14

O.i - 0.7

0-i

1.9-5

B

O. O1-0.04

O. O15 -O. 13

0-0. i

C. 6-I0

C

0. 029-0. 056

5.95-6.60

---

118-215

D

O. 003-O. 08

O. 3-8

0 -O. 6

E

O. 025 -O. 5

3 -10

F

O. 012-0.05

3-6

G

O. 02-0.08

O. 125-1

Reflux time (days) 1.2-7.6 3 -2! i

3-5OO

4.5 -13

- --

2C -216

O. 25-8

---

120-250

2-9

---

5-111

For details of the 85 individual preparations of these pure compounds refer to the Ph.D. Dissertation of R. G. H. (University Microfilms, No. 73-12, 634).

2.3-13

Crystalline cerium(IV)phosphates--I

1699

Table 3. The X-ray d-spacingsfor the cerium(IV)phosphates Compound A

Compound B

Compound C

Compound D

Compound E

"or:lpoun~ Y

7.7S

i00

6.09

94

25.2

80

i0. !

14. ~

11.,

6.93

9

5.02

6

12 7

IOC

6.~6

ii

6,6~

9

4.52

47

6.33

21

5. L6

5

5. oo

42

h..2g

63

5.10

42

5.~6 l[l'

2:;

L4

• 5~

3.15

if

f-3'

U

~, =5

6~

?

-' g

v

7.1~

"

[ •

5./i

) . 15

-,.09

72

4.56

1~

a.i7

:~

~, ~,t"

~

-.

t"

,1 t

13

3.63

35

3.95

23

L i~

,

4.45

'

-:.',o

L

< "

4.3'

ii

3.47

iOO

3,62

26

4.

~

.:z

4.03

6

3.27

24

3.h5

31

4. (

3

1.b,

~5

5.46

3.3 ~

13

3. O~

15

3.25

23

~.

3

4

'.

3.19

~7

3.02

52

2.96

39

~. ;

6

i:-t(

3

1.1,

3.13

3~

2.75

1-

2.32

14

3. )=

7

.16

~

=:='#':

}. 09

~7

2.70

~-8

2.56

i1

3.

~'~

.9£

?

: .;;

z. 96

6

2.67

65

2.54

ii

7.

z

2.={0

6

2.52

36

2.~7

11

~-5

5

/- . 29

14

2. i ~

9

3. :

L

iv

2,23

13

2.2?

~-'1

[,1'

"

i . 46

9

2.15

15

2.J,2

i3

3.1

1C

• r'L

2.3~

3

2.11

22

1.88

14

•

11

;.(,~

2.2

7

2.5

14

!.5~

11

2.22!

5

2,04

16

1.7~

16

i.98

2~

.61

10 lucre %0 ~.66

~'

10 more LO

1.66

'- 37

.

.%:1+

< . . . .

~

1"

e

.

i~

~. [~" .

1'

,

L

": ["

~'. x

:

,L!

1

.

",

,

~

, 5'

)

2.

:

.=I

[

:.

;: L

['

2.:;t

32

l.:~

"

;~ '

-

[

]

:.,

~

.5

2.7J

i0

i. 5

]

1.6

.~:

l,

2.c"

"

.

L;

~6

1. )~

!

'.

1

2.~

5

1.32

2

/.76

1

.1:'

2. [: ,"

14

.4~

19 '.:ore zc

i

1.66

phosphoric acid usually converted them to compound D. Comparison of the d-spacings (Table 3) and analytical data for this compound with that given by Alberti for microcrystalline CeP indicates that they are the same compound [8]. Compound E. Conditions which favored the formation of compound E include a strong (3-10 M) phosphoric acid solution of Ce(IV) in which ammonium ion was present, and a high ratio of phosphate to cerium. Compound F. This compound was found to form on refluxing solutions in which the phosphate-cerium ratio was high and the HcPO4 concentration was maintained at 3-6 M. Compound G. The best conditions for the preparation of this compound seem to be 1M HcPO~ and P/Ce = 30-40. This solid is a much brighter yellow than are the other compounds. In summary it can be stated that the conditions for the preparation of individual cerium(IV) phosphates are rather ill-defined and in many instances mixtures of phases were obtained. Furthermore, longer refluxing usually converted one crystalline phase into another. Therefore, it was expedient to monitor the reaction by obtaining X-ray patterns of samples removed at different time intervals. Thermogravimetric and i.r. analysis. Weight loss as a function of temperature was determined for each crystalline cerium(IV) phosphate by thermogravimetric analysis (results are shown in Figs. 1 and 2). These weight loss results, taken in conjunction with analytical and i.r. data, collected in Tables 2 and 4, respectively, permit formulas to be derived for each of the cerium phosphates.

1

'

;,

:

. 5 : - '5

v[

L.~

~. 53

4.7

JINC VoL 37, No. 7/8--J

i z

~ompound ~ '3

i

.4 =

$

1':

~

mor< to

2

9 more " c i . ,d

! . (6

The details are given below in order of increasing difficulty and uncertainty. (1) Compound E Reference to Fig. 2 shows that no loss in weight occurred on heating compound E until about 300°C where, between this temperature and 350°C, the loss was 7-30 per cent. A second weight loss of 0.76 per cent occurred at

o;

ooo

4 0 ~i 60 ~ 80 ,oo

L-

800 I 600 400

/

~

O' A

: zoo SAMPLE ~

B

1303q [

0

40 ~

~, 8 0 0

so ~ 80 L

i i

I00

600 400

m

200

~D

.J

0

......

c

20 ~

C.9

i,o0o

"°i

m

0 C

SAMPLE W~ =i044eg,

~°° Zo

60 L

~ 600

I

200 0

20 40' 60! 80 ~ I00 ~

D

SAMPLE WX'OBgS~ ~ I 0 0 0

~ .

.

.

.

.

.

.

800 ~ 600 ];400 i 200 ' 0

Fig. 1. Thermogravimetricanalysis curves of compounds A, B, C and D.

1700

R.G. HSP,MAN and A. CLSARFISLD 0

F-

S A M P L E W'~ •

0.9301~

20

IOOO 800

40

600

60 80

J-

400

k

2o0 -I 0 r11

I00 0

~0 (/) I-b,J

20 40

r~

I000

60

800 6 0 0 --t

80

400

I00

20O

0

C m

SAMPLE WT • 09610q =

G

IOOO

~O 40

'°k .o[_/

800

~

4

~oo

i

200 0

l,oo

I00 i

hydration since the bending mode at 1600-1650 cm-~ is absent. A strong band at 1406 cm -~ can be attributed to the bending mode for ammonium ion confirming its presence in the solid. Another strong sharp band at 1234cm -a indicates the presence of monohydrogen phosphate groups. Thus the broad band at ~3500-2300cm-' is attributed to N-H and O-H stretch from NH4+ and HPO42- groups.

From these observations it is concluded that compound E should be represented as Ce(NH4)o.4nHr.56(PO4h. For. Wt. = 339.58. This formula requires 41.26 per cent Ce, 55.93 per cent PO4 and 2.2 per cent NH3. The weight losses observed at elevated temperatures can be attributed to the following reactions: 300~100 °

Ce(NH4)o.,4H,.56(PO4)z

) CeP207 + 0'44NH3 + H20

Fig. 2. Thermogravimetricanalysis curves of compounds E, F and G. 400--440°C. The solid at this point was identified as CeP207 from its X-ray pattern. When the heating was stopped at 350°C, the solid still contained 0.2-3 per cent NH3. Apparently some of the ammonia is more tightly held than the bulk of it and so splits out at the higher temperature (400-440°C). Another weight loss amounting to 2.06 per cent took place at 800°C. An X-ray pattern of the solid obtained at the completion of the TGA run showed that it was monoclinic cerium(HI) phosphate. The i.r. pattern of compound E exhibits a broad absorption band from 3500 to 2300 cm -~. However, sharp maxima occur in this band at 3309, 3254, 3050 and 2844cm -~. None of these bands are due to water of

(1) CeP2Ov 800° , CePO, + 0.5 (P20~)~ + 1 02. X

Weight losses calculated from reactions (1) and (2) are 7.51 and 2.36 per cent, respectively compared to 7.99 and 2.06 per cent actually observed. The excess phosphate liberated in reaction (2) must form a glassy polymer since it neither volatilizes at 800 ° nor shows up in the X-ray pattern. Thus it is designated as (P2OD~. The small weight loss at 400--440°C is probably due to the evolution of a portion of the ammonia which was tightly held. (2) CompoundB. The i.r. spectrum of compound B exhibits a very sharp strong absorption band at 3502 cm-' but superimposed

Table4. I'R'abs°rpti°nbandsf°rcerium(IV)ph°sphatespresentedaswavenumbers(cm ') Compound A

Compound B

Compound C

Compound D

3497 M

3502 S (Shp)

3474 W (Sh)

3495 M (t)

3200 W (b)

3180 M (b)

33o9 ~ (b) 3209 M (Sh)

2799 1613 M

1609 W

Z098 S (Sh)

1114 S 1039 S (8hp)

Compound E

Compound Y

Compound G

35o2 x

35o5 w (s~)

3309 :~ (sh) 3254 s

3226 VW(b)

3244

3050 VW

3O5O w (Sh)

3o6o vw (Sh)

2~44 W

2830 VW

2320 V~

2327 VW

1606 w

1613 w 1406 M (b)

1409 W

1233 S

1229 S

1234 S (shp)

z234 S (shp)

1095 S

1098 S (Sh)

1083 S

14oo M

1059 S

1027 S 1006 S (b)

2326 VW

234~ "T¢~

9332 VW

16o8 w ZL15 S (S~)

1058 S

1064 S (Sh)

i018 S

ioo3 s (sh)

zo18 s

io66 s (b) lO28 s

1oZ.6 s (sh)

994 S

1010 S (Sh)

988 s

97o M (Shp)

957 S (Shp)

911 S

935 S (Sh)

944 s ( s h p )

925 S (Sh)

936 S

996 S (Sh)

834 s (Shp)

8O6 M (Shp)

915 S

923 s

913 S

921 S

698 W (b)

736 VW (b)

975 S (Shp) 938 s (Sh)

637 M (Shp)

628 S (Shp)

608 M (Shp)

62~ w (sh)

610 S (Shp)

588 w

549 W

553 M

611 M (Shp)

626 M (Sh) 6 l l S (Shp)

633 M (Sh)

563 W

534 M

508 VW

575 VW (sh)

57o w

578 ~

(sh)

61y S (Shp)

524 M

504 W

557 W 51Y M (sh)

551 W 520 S (Shp)

555 W 530 VN (Sh)

584 w (Shp) 574 W (Shp)

518 S (S~p)

548 S (Shp)

509 W (Sh)

929 S

539 S (Sh) 525 s (shp) S M W 4~

= =

stror~ medi~m weak very weak

Sh = sho-~der b = broad Shp = sharp

(2)

t4

Crystallinecerium(IV)phosphates~I upon it is a weak broad band. This broad band together with a weak one at 1609 cm -~ indicates the presences of lattice water while the strong, sharp one is attributed to the O-H stretch of a hydroxyl group. The absence of a band at 1230cm -a is taken as evidence that monohydrogen phosphate groups are not present. Thus, the proposed formula for compound B is (OI-I)o.7 (PO4)v~. ½I-I20 (For. Wt. = 265.5) which requires 52'78% Ce, 39.35% PO4 and 3.39% HsO. The sequence of reactions which takes place when compound B is heated can be explained from the proposed formula by the following scheme: The first weight loss starts at about 100°C, is complete at ~200°C and amounts to 4.2 per cent. This change is attributed to loss of lattice water, which according to reaction (3) requires a 3.39 per cent weight loss.

Ce(OH)oT(P04),., .½HsO . , Ce(0H)o.7(P04)l, +½HsO. (3l An X-ray pattern of a sample of compound B which had been heated at 200°C did not indicate the presence of CeOs, CePO4 or CeP207 and is apparently that of the anhydrous phosphate. In the i.r. pattern of this sample the OH stretching band had decreased in intensity. Thus the higher observed weight loss may be due to some condensation of hydroxyl groups in addition to the removal of lattice water. Above 20&C there is observed (Fig. 2) a gradual loss of weight followed by another sharp one which commences at 600°. These changes are attributed to the reactions depicted by equations (4) and (5). 250-600 °

Ce(OH)o.v(PO4)~ ~

~ CeOo.3.s(PO4)J J + 0.35H20 (4)

6~650 o

CeOo.35(PO4h ~

~ CePO4 +

0"05 (P20.O~ + 1 X

(4

02. (5)

Reactions (4) and (5) require weight losses of 2.37 and 3'01 per cent, respectively, and a total for compound B of 8.77 per cent. This compares to observed values of 2.1 and 3.2 per cent and a total of 8.9 per cent. An i.r. pattern taken after compound B had been heated at 400°C for 8 hr showed that the band at 3502 cm 1 was no longer present. Similarly, heating compound B for several hr at 600°C converted it to CePO4. These observations lend further validity to reactions (4) and (5). (3) Compounds C, D, F These three compounds yield i.r. spectra and TGA curves which have many features in common. For example, the presence of a weak absorption band at 1606--1613 cm 1 (Table 4) shows that all of them contain some lattice water. In addition, the absorption band at 1409cm -~ confirms the presence of ammonium ion in compound F. All three solids have a sharp band at 1233-4 cm -~ indicating that they contain monohydrogen

1701

phosphate groups. However, this does not exclude the possibility that other types of phosphate groups are contained within the structure. In fact, in order to achieve charge balance and to explain certain features of the TGA curves it is necessary to propose that the compound also contains POfl- groups. Finally, the medium sized absorption band in the vicinity of 3500cm-' is taken as supporting evidence that the compounds contain hydroxyl groups. All of the above cited facts are incorporated into the general formula Ce(OH)~(PO,)x(HPO,):-sx. yH:O. The proper choice of x and y are obtainable from the weight loss curves. Reference to Figs. 1 and 2 shows that each of the compounds exhibits a small sharp weight loss at 600-650 ° and a somewhat larger one at 700--750°C. This latter temperature is that at which the conversion of CeP:O7 to CePO4 occurs. However the magnitudes of the observed weight losses at 700-750° are less than required for this reaction. But, the sum of the two weight losses at 600-650 and 700-750 ° is the correct magnitude for this conversion. Thus, it is proposed that only the monohydrogen phosphate groups split out water to form a proportionate amount of CeP207 which in turn is reduced to CePO4 at 700-750 °. On the other hand, the orthophosphate groups are thought to form intermediates which revert to CePO4 at 600-750°C. On this basis specific formulas for each of the compounds can be derived. The details are presented below. Compound C. The proposed formula is Ce(OH)o.27 (PO4)o.27(HPO4)v46.0'55H20 (For. Wt.=320-39) which requires 43.74 per cent Ce and 51.28 per cent PO4. The TGA curve shows a gradual loss of weight up to 200°C followed by a small, sharper loss with an inflection point at 210°C. The total weight loss to just beyond this inflection point amounts to 3.43 per cent and is attributed mainly to the split out of lattice water. The calculated value is 3.09 per cent. At higher temperatures there is a steady weight loss culminating in a plateau starting at 550°C. These weight losses are attributed to the condensation of hydroxyl and phosphate groups as shown in reactions (6) and (7). Ce(OH)o.2;(PO4)o.:7(HPO,)l.46 [CeOo.~35(PO4)o.27(HPO4)v46]+ O.135H,_O

(6)

CeO0 ,,(PO4)027(HPO4)1.,6 ~ 0.73CeP_,O~ + Ceo 27Oolss(PO4)0.?7 + 0.73H_,O.

(7)

The calculated weight losses are 0.76 per cent for reaction (6) and 4.10 per cent for reaction (7). This compares to a total observed weight loss of 4.35 per cent. It is quite likely that the discrepency is due to some condensation of hydroxyl or phosphate group at lower temperatures, i.e. along with the lattice water dehydration. The two final weight losses are then attributed to reactions (8) and (9) 600-650 ° Ceo.27Oo.135(PO4)o

27

~,

0.27CEPO4 + 0-06750: (8)

0-73CeP~_O7

v°°-75°° ~ ~ 0.73CePO4 +

(

P:O~)x

+ 0.182502.

(9)

1702

R.G. HERMANand A. CLEARFIELD

which calculate to losses of 0.67 and 1.82 per cent. The observed weight losses are 0.9 and 2.06 per cent. Actually these two combined observed weight losses (2.96 per cent) are somewhat larger than can be accounted for by just oxygen split out and may involve some last traces of condensation. The total observed and calculated weight losses, 10.89 and 10.44 per cent, are in good agreement. An X-ray powder pattern obtained after heating compound C at 325°C could not be matched with that recorded for known cerium compounds and was therefore attributed to hypothetical intermediate [CeOoq35(PO4)o.27 (HPO4)1.46].After heating at 550°C the product was mainly CeP207 and at 800°C CePO4, Compound D. The formula proposed for this compound is Ce(OH)0.45(PO4)0.45(HPODN.1]3FI20, (For. Wt. = 302.48), which requires 46.32 per cent Ce, 48.66 per cent PO4 and 9.23% loss or~ ignition, The thermal decomposition scheme is represented by Eqns (10-13). 150-200°

Ce(OH)o.45(PO4)o.45(HP04)vi. 1/3H20 CeOo.225(PO,)o.4s(HPO4)H+ 0-559H20 (3.33, 3-36) 2O0-606*

) 0.55CEP207

CeOo.225(PO4)o.4s(HP04)|.l

+ Ceo.45Oo.225(PO4)o.45+ 0.55H20 (3.28, 3.42)

(11)

600-650° Ceo.45Oo.225(PO4)o.45

) 0.45CEPO4

+0"112502(1"19, 1"15) 0.55CEP207

700-750°

) 0.55CePO4 +

~X 5

(12)

(P2Os)x

+0.137502(1.45, 1.25).

(13)

The calculated and observed percent weight losses for each reaction are given in parentheses. Compound F. The formula proposed for this compound is Ce(OH)o.37~(PO,)o.~7_~ [(NH4)o.ogHI.16(PO4)I.25]• ]H20, (For. Wt.=308.12), which requires 45.48% Ce, 49.93% PO4, and 0.50% NH3 and 9.30% loss on ignition. The termal decomposition scheme for compound F is given below in Eqns (14-17). Ce(OH)o 37,(PO4)o37,[(NH,)oogH116(PO4),25]. ~H20 200-280°

' ~CeOo.187s(PO4)o.375[(NtG)o.09HI.16(P04),.25]

(14)

+ 0.4375H20 (2.56, 2.81) 280-600°

CeOo.,s75(PO4)o.375[(Nl-~)o.ogHi.16(PO4)t.25]

)

0.625CeP207 + Ceo.375Oo.ms(PO4)o.375+ 0.09NH3 + 0.6251-120(4"15, 4'02)

0.625CEP04

0.625 0.625 +T (P20,)~ ~ 02 (1.62, 1.42).

(15)

(16)

(17)

(4) CompoundA This compound contains varying amounts of ammonia and samples with as little as 1.5 per cent and as much as 2.1 per cent were obtained. The preparation examined in detail contained 1.79% NH3 and this was very much in evidence in the i.r. pattern (Table 4). Compound A also contained lattice water as shown by the strong absorption band at 1615cm-~. The absence of a peak at 1230cm-~ was indicative that monohydrogen phosphate groups are not present. However, from the weight loss pattern it was evident that the bulk of the phosphate must then be present as dihydrogen phosphase groups. Therefore, compound A is formulated as Ce(OH)j .62(NH4HPO4)o.3~(H2PO,)o.6s(PO4)o.45.0.6H20 (For. Wt. × 327.07) which requires 42.84 per cent Ce, 42.97 per cent PO4, 1'8 per cent NH3. There are four sharp breaks in the TGA curve. The first is the largest and amounts to 10.66 per cent. The weight loss begins at about 100°C and levels out at about 210°C. An i.r. spectrum, of a sample which had been heated to 275°C, showed the absence of the water band at 1615 cm-~ and an increase in intensity and sharpness of the absorption bands due to ammonia. Analysis of the solid for ammonia gave 2.03 per cent, showing that none was lost during this first weight loss. Furthermore, the i.r. spectrum now contained a strong sharp band at 1207cm-~ indicative of the presence of monohydrogen phosphate groups. Thus, this first weight loss at elevated temperature is formulated as a condensation of water between hydroxyl and H2PO4- to leave HPO42- (Eqn 18). Lattice water also splits out at this temperature so that the combined reaction is Ce(OH)l.62[(NH4)HPO4]o.35(H2PO,)o.6s(P04)o.,5.0.6H20 100-250°

:' CeOo.295(NH4PO4)o.35(1,1PO4)o.es(PO4)o.4~

+ 1.9251-I20.

(18)

Reaction (18) entails a weight loss of 10.6 per cent. At higher temperatures, there is a gradual loss of weight followed by a sharper one with an inflection at 400°C and ending in a plateau. The total weight loss for this change is 4.92 per cent, and the product remaining is CeP207 plus an unidentified phase (or phases). The reaction is thus represented as C cOo 295(NH4POa)o.35(HPO4)o.6s(PO4)o.,5 [Ceo.485Oo.295(PO4)o.45]

+ 0.515H20 + 0.35NH3

> 0'375CEPO4

0'1875 +T 02 (0.97, 1.0)

700-750°

25~-5ooo~, 0.515CEP207 +

600-650° Ceo.37~Ooq 875(PO4)o.375

0.625CePz07

(19)

which gives a calculated weight loss of 4.68 per cent. The cerium pyrophosphate is formed by condensation of monohydrogen phosphate while the (P04) 3- groups are depicted as forming an unidentified intermediate (in brackets).

Crystallinecerium(IV)phosphates--I The final two weight losses result in formation of cerium(III) phosphate and therefore must be due to liberation of oxygen. The reactions are represented by Eqns (20) and (21). ~O0-~50 o

) 0.45CePO, + 0.035CEO.,

Ceo 48500.295(POa)o.4,

700-750

0.515CeP207

+0.1125H20

(20)

0-515 (P20,)~ + ~ 02.

(21)

,0'515CePO4 +0,515

The respective calculated and observed weight losses for reactions (20) and (21) are 1.10, 1-26 and 1.07, 1.27 per cent. (5) C o m p o u n d G This compound contains considerable ammonium ion as shown by analysis and the i.r. bands at 3244, 3060 and 1415cm ~. However, there is no absorption near 1600 cm-t indicating the absence of lattice water. Also, the absence of an absorption band near 1230cm' is taken to indicate that the phosphate groups are not monohydrogen phosphate. Thus, compound G is formulated as CeOoa~(PO~)(NH4HPO4)o.45(H2PO&~3 (For. Wt. = 302"36). This formula requires 46.34% Ce, 49.62% PO4 and 2.53% NH3. No hydroxyl groups were attributed to compound G because its TGA curve shows the first weight loss to occur at 400°. In compound A, the split-out of water from interaction of hydroxyl with H2PO] groups was proposed to occur at a much lower temperature. Thus, the large weight loss (6.56 per cent) which occurs in the temperature range 400-500ois ascribed to the condensation of water and loss of ammonia from the dihydrogen phosphate groups. An X-ray pattern of the solid taken after heating at 500°C, could not be correlated with those of known cerium compounds such as CeP207 or CePO4. Thus the weight loss is formulated as 400-500 ~

CeOo.21(NH4)o.45HII.71(PO4)158

) 0.45NH3

+ 0.58H20 + [CeOo.2~(PO3)o.:s(PO4)]

(22)

which amounts to 5.99 per cent. Since the resultant cerium compounds(s) could not be identified, the formula is placed in brackets to indicate a hypothetical intermediate. This intermediate decomposes over a broad range of temperature (500-800°C), the total weight loss amounting to 1.82 per cent. The final product was monoclinic cerium(III) phosphate so the reaction is formulated as 5 O 0 , - 8 0 O°

[CeOo ,_I(PO3)I~~(PO~)]

~ CePO4 + 0.250: + 0'58 (P.~Os)~. 2x

(23)

This reaction requires a weight loss of 2.64 per cent which is considerably higher than the observed value. However,

1703

the combined calculated weight loss is 8.63 per cent compared to 8.22 per cent observed. Thus some decomposition of the intermediate may have occurred below 500°C. DISCUSSION It seems clear from this study and those that preceded it that the cerium(IV) phosphate system is a very complicated one. Not only is the list of compounds increasing but rarely is the stoichiometry of the synthesized product rational. This of course implies (barring the possibility of mixtures) that the compounds are polymeric in nature. Tending to support this premise was our observation that in some instances the cerium(IV)-phosphoric acid mixture was observed to gel on standing at room temperature and fibers could be drawn from the gel. Certainly a family of compounds of formula Ce(OH)dPO4)~(HPO4h .~, implies that different degrees of hydrolytic polymerization can occur during the synthesis of these compounds. At one end of the series, with x = 0, is the compound Ce(HPO&. Such a compound has been synthesized by us (compound E) and by Alberti et all8, 9]. However, our preparation contained appreciable amounts of ammonia and was anhydrous, while AIberti's contained water of hydration. Thus, the X-ray powder patterns of the two preparations are different and in fact do not permit us to determine whether they are different forms of the same compound or different structurally. At the other end of the series should be a compound with x - I or Ce(OH)(PO4). Our compound B is of this type but somewhat high in phosphate content. Compounds C, D, F belong to this series with X values of 0.27, 0.45 and 0-3"7,5, respectively. Our final two compounds, A and G, contain both orthophosphate and dihydrogen phosphate groups, and Pajakoff[12] claimed to have prepared an amorphous compound containing both monohydrogen and dihydrogen phosphate groups. Thus, it is possible that these compounds may be members of additional families of cerium(IV/phosphates. In fact if one also considers the two families of phosphate-sulfate compounds[4, 5], it would appear that the grouping into related series is the norm for the cerium(IV) phosphate system. This raises the possibility that not only may it be possible to prepare other members of a family but that new families containing other tetrahedral species such as vanadate or silicate may exist. REFERENCES

1. W. A. Cilley,Ph.D. Thesis, Universityof Wisconsin,Madison (1963). 2. E. M. 1,arsen and W. A. Cilley.J. inoro~, nucl. Chem. 30, 287 (1968). 3. K. H. K6nigand E. Meyn, J. inorg, nucl. Chem. 29, 1153 (1067). 4. K. H. K6nigand E. Meyn, J. inorg, nucl. Chem. 29, 1~19 (1967). 5. K. H. K6nigand G. Eckstein, J. inor,e. nucL Chem. 31, 1179 (1969). 6. K. H. KiSnigand G. Eckstein, J. inore, nucl. Chem. 34, 3771 (1972).

1704

R.G. HERMANand A. CLEARFIELD

7. K. H. Ki~nig and G. Eckstein, J. inorg, nucl. Chem. 35, 1359 (1973). 8. G. Alberti, U. Constantino, F. DiGregorio, P. Galli and E. Torracca, J. inorg, nucl. Chem. 30, 295 (1968). 9. G. Alberti, U. Constantino and L. Zsinka, J. inorg, nucl. Chem. 34, 35,19 (1972).

I0. W. N. Hartley, J. chem. Soc. 41, 202 (1882). 11. B. M. Shukla and R. S. Tripathi, Proc. 1st Chem. Syrup, Vol. 2, p. 95, Chem. and Metallurgy Comm., Dept. Atomic Energy: Bombay, India (1970). 12. S. Pajakoff, Monatsh. Chem., 99, 1400 (1%8).