solid solution behaviour of synthetic monazite and xenotime from

Submitted to “Advances in X-ray Analysis” - Proceedings of the 45th Annual X-ray

Conference, Denver, Colorado, USA.

Solid Solution Behaviour of Synthetic Monazite and Xenotime from Structure

Refinement of Powder Data.

by BOB VAN EMDEN

Special Research Centre for Advanced Mineral and Materials Processing, Chemistry

Department, University of Western Australia, Nedlands, W.A., 6907, Australia.

MIKE R. THORNBER,

Special Research Centre for Advanced Mineral and Materials Processing. CSIRO, Division of

Minerals, Waterford, W.A., 6102, Australia.

JIM GRAHAM AND FRANK J. LINCOLN.

Special Research Centre for Advanced Mineral and Materials Processing, Chemistry

Department, University of Western Australia, Nedlands, W.A., 6907, Australia.

All correspondence to be addressed to :

Dr. F.J. Lincoln

Special Research Centre for Advanced Mineral and Materials Processing

Department of Chemistry, University of Western Australia

Nedlands, W.A., 6907

Australia

Telephone : (619)3803142

Facsimile : (619)3801116

Email : [email protected]

Copyright 0 JCPDS-International Centre for Diffraction Data 1997

Copyright (C) JCPDS-International Centre for Diffraction Data 1997ISSN 1097-0002, Advances in X-ray Analysis, Volume 40

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ABSTRACT

In order to study the complex substitution behaviour observed in natural samples, synthetic structural

analogues of the rare earth element (REE) orthophosphate minerals monazite and xenotime have been

prepared. Compounds of the type: (Nd,Yi_,)POb , (La,Yi.,)PO4 , (Sm,Yi_,)POd and (Nd,Ybi_,)PO4 have

been prepared both by precipitation and solid state methods and then heated at temperatures of 1000,

1200 and 1500 “C. Structure refinement of powder X-ray data, using Rietveld full profile methods, has

been carried out on these mixed light- and heavy-REE phosphates to obtain cell parameters and relative

proportions of co-existing phases. The lever law was used to estimate the extent of heavy-REE

substitution in synthetic light-REE-containing monazite phases and light-REE substitution in heavy-

REE-containing xenotime phases. In general, the larger the difference in radii of the two REEs in each

phase, the lower the extent of REE substitution, A higher level of both heavy-REE substitution in

monazite phases and light-REE substitution in xenotime phases was found in preparations heated to

1500 OC. Xenotime phases, containing Y in combination with smaller light-REEs, which have been

heated at 15OO”C, show some evidence of REE ordering, but this behaviour has not yet been well

characterised.

INTRODUCTION

Monazite, a Light Rare Earth Element (LREE) phosphate mineral, crystallises in the monazite

structure type with a monoclinic unit cell and a space group P2l/n (no. 14) (Ni et al., 1995). The REE

atom is in nine-fold coordination with the oxygen atoms of slightly distorted phosphate tetrahedra. The

Heavy REE (HREE) phosphate mineral, xenotime, crystallises in the more dense zircon structure type

with a tetragonal unit cell and a space group 14l/amd (no. 141) (Kristanovic, 1965). The REE atom is

in eight-fold coordination with the oxygen atoms of phosphate tetrahedra. The effective ionic radius (r)

of the REE atom determines which of the two structure types are formed. The larger radii LREEs

crystallise together with phosphate in the monazite structure, whilst the smaller radii HREEs crystallise

in the zircon structure (Miyawaki and Nakai, 1993). We have prepared synthetic structural analogues

of these minerals containing different combinations of light and heavy REEs to determine the

composition range of the structural boundary, and, decide if any REE ordering occurs within the

phases. The crystal chemical limits of HREE substitution in monazite and of LREE in xenotime have

been determined by use of powder X-ray diffraction techniques. The Rietveld method was used to

obtain the unit cell parameters and relative proportion of each phase within a particular compound.



The synthesis temperature was varied to determine its influence on the level of REE

substitution in each of the resultant phases.

EXPERIMENTAL METHODS

i/Synthesis

The synthetic phases were prepared by both precipitation from solution and solid state

reaction. The precipitation procedure was used to prepare compounds of the type [NdxY(l_x)]PO4 (for

x < 1) by heating the corresponding hydrated compounds formed by precipitation from solution. The

procedure involved adding phosphoric acid to a stirred solution of Nd3+ and Y3+ ions (combined

concentration of 0.05 moles / dm3) in various proportions, so that P / (Nd + Y) = 20. The precipitate

that formed after pH adjustment and constant stirring (2 days at 40°C) was collected, washed with

water and then dried (105’C for 2 days). A portion of each precipitate was then heated to temperatures

up to 1OOO’C for 48 hours.

Synthesis via solid state reaction was used to prepare several compounds of [NdxY(l_x)]P04

and all other LREE - HREE compounds as this method was more convenient. The solid state reaction

involved mixing the correct proportion of light and heavy REE203 with (NH4)2HP04, pelletising and

firing at 9OO’C (16 hours), regrinding, pelletising and then firing at 12OO’C (24 hours) following a

procedure similar to that used by McCarthy et al. (1978). Samples fired to higher temperatures were

again reground and pelletised.

ii/ Powder X-ray diffraction and SEA!

Samples for powder X-ray diffraction (XRD) analysis were hand ground by agate mortar and

pestle for a minimum of 10 minutes and mounted so as to minimise preferred orientation effects. Scans

were conducted at room temperature on a Siemens D5000 diffractometer with Cu Ka radiation at 40

kV and 35 mA. Initial phase identification was performed using the Diffrac program and ppdsm

search-match software. Typical Rietveld scan conditions from which cell data were obtained were, a

Bragg range of 14 - 80° at a step width of O.O4O, with 2.5 seconds count time. These conditions are

generally considered to be sufficient to obtain representative estimated standard deviations in cell

parameters (Hill, 1995).

The Rietveld program used is based on that by Wiles and Young (1981). Most refinements

were limited to the refinement of scale factors, zero point, four background coefficients, cell

parameters, peak widths, peak asymmetry and peak shape coefficients. Scanning electron microscopy



(SEM) was performed on uncoated samples on a JEOL JSM 5800 low vacuum SEM with EDAX

detector and Oxford ISIS image software located at CSIRO, Waterford, WA.

RESULTS AND DISCUSSION

i/ [IVdxY(l_x)]P04 type compounds

A number of compounds containing various amounts of Nd and Y together with phosphate

were prepared by precipitation and heated to 1000°C. The unit cell parameters of each phase formed

were determined by Rietveld analysis of the powder XRD patterns. Those compounds containing less

than 30 mole % Y formed single phase monazite. Compounds containing 40 mole % Y or more formed

either single phase xenotime or both phases. The level of Y 3-t held in the monazite phase heated at

1000°C is then between 30 and 40 mole%. Y is not incorporated in the monazite structure as efficiently

as expected from the average r (r-A) of REE3+, as otherwise up to 6.5 %Y should be held in the

structure.

The unit cell data for a number of monazite phases in the range of 0 to 30% Y are shown in

Table 1. The cell parameters a, b and c for each phase are slightly larger than expected from their

rA{ REE3+}. The unit cell volumes of the substituted phases are significantly larger than that expected

for ideal solid solution behaviour. This difference in unit cell volume is shown in Figure 1 which

compares the unit cell volumes of the substituted phases to those obtained for prepared samples of Nd

and Sm phosphate. It appears that the concept of rA{ REE3+} can not be reliably used to predict the

unit cell parameters of mixed LREE - HREE monazite type phases.

The unit cell data of phases in several compounds of [NdxY(l_x)]P04 for which x is between

0 and 0.15 were determined from Rietveld analysis of powder X-ray data. They indicate that the limit

of Nd3+ substitution in the xenotime structure YP04 is between 10 and 15% as the cell parameters of

the xenotime phase in two phase mixtures are only slightly larger than those of single phase xenotime

containing 10 % Nd. Up to 23% Nd 3+ should be held in the xenotime phase YP04 if it formed a

perfect solid solution.

Table 2 shows the unit cell data of phases in several compounds of [Nd,Y(l_,)]PO4 for which

both a monazite and a xenotime phase formed. The unit cell data of the monazite and xenotime phase in

each of the compounds are nearly identical, indicating that phases of similar composition are formed. A

state of, at least, temporary equilibrium appears to exist at this temperature.



Table 1. Unit cell parameters of monazite type precipitate phases in compounds of the type

[NdxY(l_x)]P04 heated to lOOO’C, with estimated standard deviations (esd’s) given in parenthesis.

Atom % Y

in REE site

0

rAIX{REE3+] (A), Cell Parameters (A) Profile Fit

Cell Vol. (A3)

1.163 a 6.7448(2)

b 6.9552(2) RB = 2.68.

Vc = 292.368 c 6.4140(2) GofF = 3.08.

b 103.669(2)

5

1.1586

Vc = 29 1.347

a 6.7363(5)

b 6.9495(5)

c 6.4058(5)

b 103.701(5)

RB = 7.23.

GofF = 2.63.

10

1.1542

Vc = 291.190

a 6.7345(6)

b 6.9471(6)

c 6.4070(5)

b 103.728(6)

RB = 3.03.

GofF = 1.42.

20

1.1454

Vc = 289.216

a 6.7187(7)

b 6.9290(7)

c 6.3962(6)

b 103.765(6)

RB = 5.65.

GofF = 2.03.

30

1.1366

Vc = 287.228

a 6.7030(g)

b 6.9 109(8)

c 6.3850(7)

b 103.809(7)

RB = 8.32.

GofF = 2.69.

Notes : RB : Bragg R factor (%)

GofF : Goodness of fit

Vc : Unit cell volume.



291

289

287

1.13 1.14 1.15 1.16

rALx (REE 3+J (A)

Figure 1. Unit cell volume of single phase monazite type [(NdxY(l_x)]PO4 phases against the

rag{ REE3+}.

ii/ LaxY(i-x)lPO4 type CO~~OUF~~S

The unit cell data of phases in several compounds of [LaxY(1_x)]P04 were determined. The

extent of Y3+ substitution in LaP04 is considerably less than that in NdP04 and is limited to less than

20 mole % Y3+ at 1200°C. This is far below the 80 mole % Y substitution expected from r-A{ REE3+}

considerations. The large size difference between the La 3+ and Y3+ ion (r”{La3+} = 1.216 ‘4 vs

1.075 A for r”{Y3+}; Shannon, 1976) appears to be difficult for the monazite structure to tolerate.

The cell parameters of the xenotime phase in [LaxY(1_x)]P04 compounds were found to be

only slightly larger than those of YP04, indicating that YP04 can only substitute a few mole % of

La3+. The large size of the La3+ ion (rIX{La3+} = 1.216 8, vs 1.163 A for rIX{Nd3+}) severely

restricts its substitution in the xenotime phase.

iii/ [SmxY(l _X)]P04 type compounds

The unit cell data of phases in several compounds of the type [SmxY(1_x)]P04 were

determined. The cell parameters of the monazite phase in the two phase compositions of (Sm.gY.g)P04

and (Sm.25Y.75)P04 are only slightly smaller than that of the single monazite phase of composition

(Sm.75Y.25)P04. The limit of Y 3+ substitution in SmPO4 at 1200°C is then a little greater than 25



mole %. This REE composition has an I-A{REE~+} close to the r{ Gd3+} (rAIx = 1.118 A vs 1.107 A

for r”{ Gd3+}) - the smallest single REE ion to exist in the monazite structure (Ni et. al., 1995).

Table 2. Unit cell data of several [NdxY(l_x)]P04 compounds synthesised by solid state reaction at

1200°C which contain both monazite and xenotime phases (esd’s in parenthesis).

Compound, Monazite phase cell Xenotime phase cell Relative proportion

rA{ REE3+} (A) parameters (A) parameters (A) (wt.%)

tNdo.2oYo.8oY’O4 FIT = 1.037

lJX = 1.093.

Vc M = 285.391

Wo.25Yo.75PQ rvlI1 = 1.042

,1X = 1.097.

Vc M = 285.411

Wo.3oYo.7oY’O4 rvlI1 = 1.046

,1X = 1.101.

Vc M = 285.7 10

WosoYosoW4 rvlI1 = 1.064

lJX = 1.119.

Vc M = 285.838

a 6.6897(8)

b 6.8926(7)

c 6.3749(6)

b 103.855(11)

a 6.6893(7)

b 6.8934(6)

c 6.3753(5)

b 103.873(9)

a 6.6917(7)

b 6.8956(7)

c 6.3770(6)

b 103.842(9)

a 6.6927( 10)

b 6.8972( 10)

c 6.3779(8)

b 103.860( 10)

a 6.9025(4)

c 6.0370(3)

Vc X = 287.630

a 6.9016(3)

c 6.0365(3)

Vc X = 287.53 1

a 6.9024(4)

c 6.0362(4)

Vc X = 287.583

a 6.9037( 11)

c 6.0380( 12)

Vc X = 287.778

24.4(0.3) M,

74.6(0.6) X.

26.2(0.8) M,

73.8(1.0) X.

36.7(0.4) M,

63.3(0.5) X.

65.4( 1.2) M,

34.6(0.9) X.

Notes : VcM : Unit cell volume of monazite phase

VcX : Unit cell volume of xenotime phase.

The composition of the xenotime phase in two phase [SmxY(l-x)]P04 preparations is

estimated to be (Sm_l5Y.85)P04 from the composition of the monazite phase and the relative

proportion of each phase using the lever rule. This REE composition has an rA{REE3+} close to that



of Tb3+ (rAVnl = 1.028 A vs 1.040 8, for r v111{ Tb3+}) - the largest single REE ion to exist in the

xenotime structure (Ni et. al., 1995).

iv/ [NdxYb(l_x)]P04 type compounds

The presence of two phases for the composition (Nd.75yb.25)PO4, and the closeness of the

xenotime phase cell parameters to those of single phase (Nd.gOYb.lO)P04 (see Table 3) suggests that

the amount of substitution of the small radii HREE ion Yb3+ (rvIn{Yb3+} = 0.985 A vs 1.019 8, for

rV1R{Y3+}) in monazite phase NdP04 is considerably lower than for Y3+. Lever rule estimations on

several of the two phase containing [NdxYb(1_x)]P04 compounds limit the substitution of Yb3+ to 12

mole % at 12OO’C. The large size difference between Nd3+ and Yb3+ restricts Yb3+ entry into the

monazite structure.

The level of Nd3+ substitution in xenotime phase YbP04 is limited to approximately 6 mole %

at 1200°C. This level of substitution is considerably lower than the 12 mole % substitution limit found

for Nd3+ in YP04.

v/ Injluence of temperature

Samples of (Nd.gYb.g)P04 and (La_6Y_4)PO4 were heated to temperatures of 1500°C and

1590°C to determine the change in REE substitution behaviour with temperature. The unit cell lengths

of the monazite phase in each compound decreased when the firing temperature was increased from

1200°C to 1590°C. This change in cell lengths indicates that a greater proportion of HREE occurs in

the monazite phase heated at the higher temperature. The unit cell parameters of monazite and

xenotime phases in (La.6Ye4)P04 heated to 12OO”C, 1500°C and 1590°C are shown in Table 4. The

unit cell lengths of the monazite phase and the proportion of the xenotime phase decrease upon heating

from 1200°C to 1590°C. (La.gY.q)P04 is in fact single phase monazite at 1590°C (see Table 4).

The rAM { REE3+} in (La.6Ya4)P04 is close to the r”{Nd3+} (1.160 A vs. 1.163 8, for

Nd3+). The unit cell lengths (of [Lae6Y_4]P04) a and b are slightly smaller whilst the c - length is

significantly larger than those of NdP04 (see Table 4). A preferential elongation of the alternating

REE polyhedra - phosphate tetrahedra chains in the monazite structure which are aligned parallel to the

c-axis must occur. Each chain is connected to five others through oxygen atoms which (when joined)

describe a pentagonal plane (Mullica et al., 1985). This type of linkage locks the structure and explains

why there is less flexibility between chains than within.

The cell parameters of the xenotime phase in (Nd.gYb.g)PO4 and (La.gY.q)P04 compounds

show no significant change with increasing temperature (see Table 4 for example). The extent of LREE

substitution in the xenotime phase of (Nde5Yb.5)P04 and (La_6Y_4)PO4 compounds is unaffected by

temperature. The higher density and symmetry of the zircon structure type in which xenotime phases



crystallise (Milligan et al., 1983) appears to place greater REE size restrictions than does the monazite

structure.

Table 3. Unit cell data of synthetic monazite and xenotime phases in compounds of the type :

[NdxYb(l_x)]P04 heated to 1200°C, with esd’s in parenthesis.

Compound, Monazite phase cell Xenotime phase cell Relative proportion

r~{ REE3+} (A) parameters (A) parameters (A) (wt. %)

NdPO4

rvlI1 = 1.109

rIx = 1.163.

V, M = 292.198

a 6.7428( 1)

b 6.9574( 1)

c 6.4103( 1)

b 103.675( 1)

100 M.

YbP04

rvlI1 = 0.985

rIx = 1.042.

a 6.8196(3)

c 5.9742(3)

V, X = 277.842

100 x.

(Ndo.9oYbo. 1o)PO4 a 6.7221(6)

rvlI1 = 1.097

rIx = 1.151.

Vc M = 289.816

Wo.75~o.zW4 rvlI1 = 1.078

rIx = 1.133.

Vc M = 289.429

(Ndoso~osoF’O4

rvlI1 = 1.047

rIx = 1.102.

Vc M = 289.354

Wo.z~o.75)PO4 rvlI1 = 1.016

rIx = 1.072.

Vc M = 289.138

b 6.9339(6)

c 6.4007(5)

b 103.728(6)

a 6.7189(4)

b 6.9304(4)

c 6.3987(3)

b 103.738(4)

a 6.7187(5)

b 6.9280(5)

c 6.3995(4)

b 103.740(6)

a 6.7207(12)

b 6.9211(11)

c 6.3999( 10)

b 103.766( 17)

a 6.8361(5)

c 5.9881(6)

Vc X = 279.837

a 6.8356(4)

c 5.9867(4)

V, X = 279.73 1

a 6.8345(4)

c 5.9871(4)

V, X = 279.660

100 M.

77.7(0.6) M,

22.3(0.3) X.

47.7(0.5) M,

52.3(0.4) X.

21.4(0.5) M,

78.6(0.7) X.



Table 4. Unit cell parameters of monazite and xenotime phases in (La.6Yv4)P04 heated to

temperatures of 12OO”C, 1500°C and 159O”C, with esd’s in parenthesis.

Compound Monazite phase cell Xenotime phase cell Relative proportion

parameters (A) parameters (A) (wt.%)

(La.6Y.4) 1200°C

40 hours

Vc M = 299.869

(La.gY.4) 1500°C

40 hours

Vc M = 294.132

(La.gY.4) 1500°C

275 hours

Vc M = 292.633

(La.gY.4) 1590°C

330 hours

Vc M = 291.598

NdP04

Vc M = 292.198

a 6.7902(4)

b 7.0158(5)

c 6.4729(4)

b 103.477(4)

a 6.7476(8)

b 6.9652(7)

c 6.4399(7)

b 103.637(7)

a 6.7365(4)

b 6.9523(5)

c 6.4305(4)

b 103.672(4)

a 6.8881(5)

c 6.0246(5)

V, X = 285.843

a 6.8893( 10)

c 6.0234(20)

Vc X = 285.885

a 6.8919(20)

c 6.0176(28)

Vc X = 285.826

73.4 (0.6) M,

26.6 (0.4) X.

89.3 (1.0) M,

10.7 (0.5) x.

94.9 (0.8) M,

5.1 (0.2) x.

a 6.7273(6)

b 6.9445(6)

c 6.4244(5)

b 103.697(5)

a 6.7428( 1)

b 6.9574( 1)

c 6.4103( 1)

b 103.675(l)

100 M.

100 M.

vi/ High temperature xenotime phase alteration

When samples of (Nd.35Ys65)P04 and (Sm.gY.g)P04 were heated to temperatures of 1500°C

and 1590°C powder XRD peaks in addition to those expected from the monazite and xenotime phases

were observed. Contrary to temperature dependent behaviour of (Nd.5Yb.g)PO4 and (La.gY.q)P04

compounds, the proportion of the monazite phase decreases with increasing temperature. The

compound (Nd_35Y_65)P04 changes from 42 wt % monazite at 1200°C to, what we believe is a single,



distorted xenotime phase, at 1590°C. Preliminary SEM (in combination with EDS) work suggests that

there is only one phase of approximate formula (Nd.35Y.65)P04 within the sample heated to 1590°C.

All powder X-ray lines expected from the zircon structure type xenotime phase of rA{ REE3+}

slightly larger than the r of Tb3+ are present. The additional lines match closely reflections of the type

(h,k,l) and (h,h,l) with the restrictions of h+k+l = 2n and 2h+l = 4n respectively, removed. This

corresponds to a loss of I - centering and of the d - glide from the original space group : 14l/amd. The

single 4 - fold REE site within the zircon structure type could be lowered in symmetry to two 2 - fold

special positions which discriminate between the two different sized REEs.

We attempted to determine the structure of the ordered phase by Rietveld analysis of the

powder XRD pattern, by assuming that the structure ordered continuously as the REEs diffused to give

a preponderance of Nd in one of the two special positions. If this REE site discrimination process does

occur then the disordered structure should be able to be refined in the space group of the ordered phase.

Structural refinement in a number of suitably chosen space groups having either orthorhombic or

monoclinic unit cells was attempted with limited success. Refinement of REE positions took place

satisfactorily, but attempts to identify oxygen positions resulted in an unstable refinement. There still

exists a slight mismatch between the calculated and observed peak positions, suggesting that the space

group is actually triclinic. Single crystal X-ray diffraction or perhaps convergent beam electron

diffraction seem to be the only way to determine the space group of the ordered phase.

CONCLUSIONS

The Rietveld method has proved useful in the determination of the unit cell parameters of

monazite and xenotime phase mixtures. The degree of HREE substitution in monazite phases depends

on the difference in size of the two REEs and upon their rA. Large size differences between the two

REEs leads to only minor HREE substitution. Where the rA{REE3+} of the REEs is significantly

larger than the r{ REE3+} of the border element Gd, an increase in HREE substitution in LREEP04 is

seen with increasing temperature. The level of LREE substitution in xenotime phase HREEP04 also

depends on REE size difference and the rA{ REE3+}. There is however no increase in LREE

substitution with increasing temperature.

A lower symmetry, REE ordered phase, closely related to the zircon structure, forms in

compounds with REEs similar in size and with an rA{REE3+} slightly less than the r{ REE3+} of

Gd3+.

Copyright 0 JCPDS-International Centre for Diffrac ,Jata 1997


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Milligan, W. O., Mullica, D. F., Beall, G. W. and Boatner, L. A. (1983) C39,23.

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Mullica, D. F., Grossie, D. A. and Boatner, L. A. (1985) J. Solid State Chem. 58, 71.

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solid solution behaviour of synthetic monazite and xenotime from

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