Ordered Phases and Nonstoichiometry in the Rare Earth Oxide System

4 Ordered Phases and Nonstoichiometry in the Rare Earth Oxide System LeROY EYRING and BO Department of Chemistry,

HOLMBERG Arizona State University,

Tempe, Ariz.

Downloaded by UNIV ILLINOIS URBANA on May 18, 2013 | http://pubs.acs.org Publication Date: January 1, 1963 | doi: 10.1021/ba-1964-0039.ch004

Stable phases in the rare earth oxide systems are tabulated and discussed.

New data on the struc-

ture of sesquioxides quenched from the melt are reported.

The structural interrelations between

the A, B, and C type sesquioxides and the fluorite dioxides are pointed out.

The sequences of sev-

eral intermediate oxides in the CeO , PrO , and x

TbO

x

x

systems are observed to be related to the

fluorite structure and the C form sesquioxide with respect to the metal atom positions.

A hypo-

thetical homologous series of the general formula MO , n

2n-1

related to the fluorite structure and the

A form sesquioxide with a more or less fixed oxygen lattice, is suggested.

T h e rare earth oxides constitute a richly intricate sequence of phases w i t h many subtle variations. A thorough and accurate description of these materials w i l l provide an exacting test for any theory having predictive value. Table I tabulates stable phases of limited composition range w h i c h have been observed. T h e description of the precise phase diagram i n each system showing the ranges of composition of each phase a n d the extent of its nonstoichiometry is for some future time, although the general outline is emerging i n some cases. O n l y the most painstaking efforts w i l l provide useful data w h i c h w i l l clarify rather than confuse the phase relationships. A thermodynamically complete and satisfying picture of the higher oxides w i l l be difficult to obtain, since the ordering process w h i c h accompanies equilibration at lower temperatures may be very slow. In fact, slow reactions, metastable states, and hysteresis are observed more or less generally i n these oxide systems. The apparent equilibrium oxygen pressure at some given composition and temperature depends upon whether one is oxidizing or reducing. This phenomenon, termed hysteresis, is not fully understood and is probably complex. T h e observed phenomenon could involve such factors as the slow ordering of the oxygen i n the crystal. U n d e r the conditions of preparation of most of the phases tabulated here the oxygen was extremely mobile but the metal atoms were probably immobile. Oxygen uptake is rapid, achieving nearly constant composition i n a matter of minutes at 400° C . M o b i l i t y i n the cation lattice is not great 46 In Nonstoichiometric Compounds; Ward, R.; Advances in Chemistry; American Chemical Society: Washington, DC, 1963.

4. ΕΥ RING AND HOLMBERG

47

Rare Earth Oxides

enough to be practically useful for reactions requiring transport below about 1200° C , w h i c h is approximately the T a m m a n temperature for these materials. However, electronic interchange between the cation w o u l d be r a p i d and w o u l d obviate the need of ion transport i n certain cases. In view of this difference i n mobility, transitions involving metal movement through the lattice w o u l d be expected to be slow and transitions involving only oxygen movement to be rapid above about 400° C . A l l the structural data re ported here were obtained from quenched samples.

Table I.

Some Properties of Stable Phases in Rare Earth Oxide Systems


Cation Radius, Oxide

A.(D

Lattice Parameter, A.

Lattice"Type

Color

0

LaOi. oo

1.14

White

Hex (A)

a c

GcOi.522

1.07

Mustard

Hex (A)

GeOi.651 GeOi.688 GeOi.7i7

Black Black B l u e black

b.c.c. b.c.c. Rhomb.

a c a a

GeOi.775

D a r k blue

Rhomb.

GeOi.8i2

D a r k blue

Rhomb.

5

Ge02.ooo PrOi.5oo PrOi.soo

1.06

PrOi.ee PrOL i4 7

Pale yellow

f.c.c.(F)

Yellow L i g h t green

b.c.c.(G) Hex (A)

Black Black

b.c.c. Rhomb. Pseudo cell

PrOi.78

Black

Rhomb.

PrOi.so Pr0 i PrOi.

Black Black Black Black

(f.c.c.) (f.c.c.) (f.c.c.) (f.c.c.)(F)

L i g h t blue

Hex (A)

1 8

8 3

Pr0 .oo 2

NdOi.Boo

1.04

NdOi.BOO SmOo.5 SmOi SmOi.5oo

Black 1.00

Pale yellow

ah Ch ah Ch a Ch a h

a a c a a a a a a a a a a a

b.c.c.(G)

a c a

Zincblende R o c k salt Monoclinic(B)

a a a

b c SmOi.goo EuOi EuOl.500

0.98

Dark red White

β

b.c.c.(G)

a

R o c k salt Monoclinic(B)

a a

b c

β EuOl.500 GdOi.5oo GdOi.ôoo

0.97

White White

b.c.c.(G)

a

b.c.c.(G) Monoclinic(B)

a a

b c

(Continued)

β

= = = = = = = = = = = =

= = = = =

= = = = = = = =

=

=

3.93 6.12 3.889 =fc 0 . 0 0 2 6 . 0 5 4 =b 0 . 0 0 2 1 1 . 1 2 6 =b 0 . 0 0 1 11.107 ± 0.001 3.921 ± 0.002 9.637 ± 0.002 3.910 ± 0.002 9.502 ± 0.002 3.890 ± 0.002 9.538 ± 0.002 5.409 ± 0 . 0 0 1 11.152 ± 0.002 3.859 ± 0.003 6.008 ± 0 . 0 0 3 11.070 6.750 99°23" 5.516 89°42' 5.487 =b 0 . 0 0 2 90°17' 5.482 ± 0 . 0 0 3 5.478 ± 0.004 5.469 db 0 . 0 0 1 5.393 ± 0.001 3.82 5.98 11.080

Ref. U8) (3) (3) (3) (3) (3) (3) (3) (70) (10) (10) (10) (10) (10) (10) (10) (10) (18) (18)

=

5.376 ± 0 . 0 0 1 4.9883 ± 0.0003 14.177 3.633 8.847 99.96° 10.934

=

5.1439 =fc 0 . 0 0 0 5 1 4 . 1 2 3 =b 0 . 0 0 5 3.605 ± 0 . 0 0 1 8 . 8 1 3 =b 0 . 0 0 3 100.13° ± 0.02° 10.860

(9) (17)

10.8122 14.06 3.572 8.75 100.10°

(18) (18)

= = = = = = = = = = = = = = =

American Chemical Society Library

In Nonstoichiometric Compounds; Ward, R.; Advances in Chemistry; American Chemical Society: Washington, DC, 1963.

(9) (9) (8)

(18)

(18)

48

ADVANCES IN CHEMISTRY SERIES Table 1. Continued

Oxide TbOi.5oo TbOi. oo

Cation Radius, A. (7)

0

0

0.93

White White

5


a a b c β

TbOl.65

Brown Brown

TbOl.715

Rhomb. Pseudo cell

TbOi.si


Lattice Parameter, A.

Lattice Type

Color

Dark brown

Triclinic

a a a a a a 7 a

Dark brown

f.c.c.(F)

DyOi.soo DyOi.5oo

0.92

White White


a a b c

HoOl.500 YOl.500 ΕγΟι.βοο TmOi.5oo YbOi.Boo LuOl.500

0.91 0.91 0.89 0.87 0.86 0.85

White White White White White White

b.c.c.(C) b.c.c.(G) b.c.c.(G) b.c.c.(G) b.c.c.(G) b.c.c.(G)

a a a a a a

TbOl. 5 9

β

= = = = =

= = = = = =

= = = = = = = = = = = =

10.7281 13.92 3.536 8.646 100.2°

± 0.0005

6.509 ± 0. 002 9 9 ° 2 1 ' ± 0Ι . 5 ' 5.319 ± 0. 001 89°41' b — c — 5.286 =b 0. 001 β = 89°25' 90° 5 . 2 2 0 =1= 0. 001

Réf. (2)

(2)

(2)

(2)

10.6647 13.87 3.518 8.589 100.2°

(18)

10.6065 10.6021 10.5473 10.4866 10.4334 10.3907

(18) (18) (18) (18) (18) (18)

° A , B , a n d G refer to the three k n o w n forms of the sesquioxide. structure.

F designates the

fluorite

Oxide Systems Dioxides. Table I indicates that cerium, praseodymium, and terbium form dioxides w h i c h crystallize i n the fluorite lattice. T h e thermal decomposition pres sure of oxygen is vastly different for each oxide at a given temperature. F o r ex ample, i n air at 1000° C . their compositions w o u l d be approximately C e 0 , P r O i 70, a n d T b 0 . P r 0 has a decomposition pressure of 1 atm. at about 310° C . , whhV T b 0 has been prepared only b y the action of atomic oxygen on TbO.. T h e fluorite structure has been described i n terms of a face-centered-cubic unit cell containing four units of M 0 . Table I lists the lattice parameters for these substances. The over-all arrangement of atoms is shown abstractly i n Figure 1. T h e cross-hatched squares represent metal atoms coordinated w i t h eight oxygen atoms at the corners of a regular cube. The layers above and below are shifted i n such a w a y that the metal-filled cubes fill the positions that are vacant i n the plane shown i n the diagram. I n this arrangement the M 0 coordination cubes are stacked so that each edge is shared w i t h a neighboring M 0 coordination group. A n alternative representation is to show the stacking order of the layers of atoms normal to the body diagonal (a threefold axis) of these cubes (Figure 2) (4). I n this figure the metal atom layers are designated b y a capital letter and the oxygen layers b y a lower case. In any layer all the atoms are the same and are i n a closest packed arangement but not at closest packed distances. T h e hexagons at the left of the figure show the relative position of the atoms i n the A , B and C layers. 2 0 0

l e 5 4

2

2

2

8

8

7



Vs

/ /

//


Figure 1.

//

// //

Elevation

Structure of Pr0

// //

Fluorite type M O .

View

Figure 2.

// //

//

V, / / // V, // V, Top

49

Rare Earth Oxides

View

viewed as a series of atom layers along C

2

llGX

axis

C e 0 is pale yellow, P r 0 is black, and T b 0 is dark brown. A light-colored P r 0 has not been observed, even when treated w i t h molecular oxygen at 600 atm. The dark colors of the P r 0 and T b 0 crystals suggest a deviation from stoichi ometry, although, at least i n the case of P r 0 , the deviation has not been detected by weighing. 2

2

2

2

2

2

2


ADVANCES IN CHEMISTRY SERIES


50

A l l the sesquioxides of the rare earths are very light colored and a l l the inter mediate oxides of C e , P r , and T b are dark. Sesquioxides. T h e sesquioxides exist i n one or more of the three forms desig nated as A , B , and C types. T h e A type has a hexagonal unit cell, the Β type is monoclinic, a n d the C form is body-centered cubic. Figure 3 indicates roughly the fields of stability of the various types. Transformations from the C form to either the A or Β form occur for all oxides u p to holmium. T h e accepted transition temperatures (when the C form is heated) w i l l , no doubt, be lowered when very long annealing times are used. Figure 3 is idealized a n d extended from one given b y R o t h and Schneider (18), who have pointed out that the temperature and the size of the ions suffice to determine the type of sesquioxide. Transforma tions from the A or Β form to the C form have been reported b y Warshaw and Roy (19) i n some cases.

MELTING POINT

2500

Γ 1

/ /

I

2000

/

ι

A 1500

/

!

C

/

B

I

/

/

1

/

.

l

1000

/

/

I

/

/

/

y — '

500

0 La

ι Ce

1 ι ι 1 1 ι Pr Nd Pm Sm Eu Gd Tb

1 1 Dy Ho

i 1 1 Er Tm Yb UJ

Figure 3. Fields of stability for M 0 2

;{

In experiments recently performed by the authors quenched sesquioxides were prepared by sprinkling a fine powder through an argon plasma jet. Material of appropriate mesh size forms perfect transparent spheres about 0.01 to 0.02 m m . in diameter. Powder diagrams were taken of the material so treated a n d it was observed that N d 0 was A form; S m 0 , T b O , and D y 0 were Β form; H o 0 and Y 0 were C form. These observations fix the intersections of the curves i n Figure 3 with the liquidus line, if it is assumed that no transition occurs during cooling. L o w e r Oxides. T h e question of the incorporation of oxygen into the rare earth metal lattice and the extent of oxide formation between the metal a n d the sesquioxide has not been systematically studied. T h e lower oxides reported i n Table I were prepared b y distilling the rare earth metals i n a system of l o w oxygen partial pressure ( 9 ) . This is one of the great unexplored regions of the rare earth oxide systems. Ordered Intermediate Phases. Between the dioxides of C e , P r , and T b a n d the sesquioxides discussed above is a sequence of oxides of intermediate composi tion. T h e best established of these are listed i n Table I and shown diagrammatically i n Figure 4. T h e existence of these stable intermediate oxides is indicated 2

2

3

2

3

2

a

2

3

3


2

3


Rare Earth Oxides

51

by their x-ray diffraction patterns (2-5, 10), kinetics of oxygen transport (16), (P,X) isotherms (12, 13), electrical measurements (10), a n d weight change isotherms (7,11). E a c h measurement, sensitive to the existence of ordering, agrees on the main features of the phase relationships suggested below for each system. Table I should be consulted for the lattice parameters for each sequence. M0 . The dashed line i n Figure 4 at this composition should be interpreted to indicate C-type oxides between the composition M 0 and M O w h i c h have been reported i n each system or are implied b y the isotherms. T h e real nature of these phases and the range of their composition are not known. T

1 < 6 5

1 > 5


Tb

1 > 7 0

ι C,B

C

R

Τ *R

F

F(?)

C,A

C-b. c.c, _J

Α-hex agonal, B-monocWnic, I I I

R-rhombohedral, I I

Lattice-composition

F-fluorite 1

1.8

1.7

Figure 4.

T-trteHnic, I

representation

of most stable MO phases n

M0 . A n upper limit to a C-type phase is indicated at this composition i n x-ray diagrams i n both the P r O ^ a n d CeO* systems and is shown graphically i n the isotherms i n the P r O ^ system (12). M0 . A phase of narrow composition limits and w i t h ideal composition M 0 is stable over a w i d e range of temperature and oxygen pressure for each system. T h e strong lines i n powder diagrams of this phase are characteristic of the fluorite phase. However, some of them are split into several components and, i n addition, there are many weak lines. T h e splitting of the strong lines and a l l the weak lines are explained on the basis of a rhombohedral cell containing seven M atoms and twelve Ο atoms w i t h a = 6.750 A . and a = 99°33' (for P r 0 ) . T h e contents of the cell, if it were fully oxidized, w o u l d be M 0 . T h e removal of the two oxygen atoms along the threefold axis w o u l d give the correct stoichiometry, destroy the cubic symmetry, and allow the rhombohedral distortion to occur. It is believed that this phase is the same i n a l l three systems. T h e proposed structure (2, 10) has the interesting feature of sequences of M 0 octahedra, one above the other along the threefold axis to give chains running the full length of the crystal. This feature also occurs i n the C-type M 0 and is discussed later. Surrounding these M O threads are sheaths of M 0 polyhedra similar to those groups i n the A and Β structure of the sesquioxides. 1

7 0

1 > 7 1 4

7

1 2

7

7

1 2

1 4

6

2

e


7

3

52

ADVANCES IN

CHEMISTRY SERIES

M0 . A distinct phase of relatively narrow composition range occurs as Pr0 . The splitting of the strong lines in the powder pattern can be accounted for i n terms of a face-centered rhombohedral unit cell. The true unit cell for this phase is not known. Bevan (3) reports a rhomobohedral phase of narrow composition range at this stoichiometry (γ phase). It was indexed w i t h a hex agonal unit cell. PrO . Between P r O and P r O the phase diagram has not been re solved. Tensiometric measurements suggest a stable phase at P r O at low tem perature. X - r a y diffraction patterns usually show broad lines w h i c h are definitely complex. It is believed that there may be some stable phases in this region w h i c h have not yet been isolated. M0 . I n the T b O ^ and C e O ^ systems this phase is of striking stability. The powder patterns have been differently indexed, as may be seen from Table I. A miscibility gap exists in the C e O ^ system between χ = 1.81 and 2.00. Tb0 . A phase i n the T b O ^ system of composition greater than T b 0 has been observed i n samples treated at 600-atm. pressure of molecular oxygen. ΡΓΟΙ.833· One of the most dramatic features of the entire praseodymium oxide system is the phase P r O , w h i c h has a narrow range of composition over wide variation of temperature and pressure. This fact accounts for the observance of this phase when samples of heated oxides are cooled slowly in air. The structure of this phase is generally credited in the literature as being fluorite, w i t h a = 5.468 A . However, recent diffractometer traces of some samples of this composition show about a dozen extra very weak lines. A unit cell w h i c h w o u l d also account for these lines has not been discovered. Since there is a miscibility gap between the composition P r 0 and Pr0 .oo> it w o u l d not be surprising to find that they have different structures. l i 7 S

1 > 7 8

1 < 8 0

1 > 7 0

] > 8 3

1 - 8 0


1 - 8 1

1 > 8 1

1 < 8 3

6

n

1 > 8 3

2

Discussion Coordination i n M 0 and M 0 Phases. In the several rare earth oxides the configuration of oxygen atoms around the metal atom is found to be one of several different types. In the fluorite structure each metal atom is surrounded by eight oxygen atoms at the corners of a cube; all the metal-oxygen distances are the same (Table I I ) . If the same symbolic representation were made for the C-type oxide as i n Figure 1 for the dioxide, it w o u l d look m u c h the same. T h e metal positions re main almost unchanged, while the M 0 cubes become M 0 coordination groups w i t h oxygen atoms missing i n one of two ways, across the face diagonal (an u n usual arrangement) or along the body diagonal. The cubes with oxygen missing along the body diagonal lie i n straight lines along a line through that body diagonal. L o o k e d at from the point of view of atom layers, the stacking arrangement is the same as i n M 0 . However, one fourth of the oxygen positions are vacant i n each plane and these vacancies are i n groups of four arranged i n the form of a Y with threefold symmetry. If the A form is reduced to the same representation as for the C form and fluorite lattice, Figure 5 results. Some of the cations have moved into interstitial positions. One may view the pattern as consisting of repeated strips of the dioxide, two cubes thick, remaining after the structure collapses on itself i n such a way that the cubes share faces along a line of shear. In the layer above, each cube simply shifts down a space and the shear line becomes a shear plane. O f course, the metal atoms and the oxygen atoms have shifted slightly from their ideal posi tion, as indicated below. 2

3

2

8

6

2


4. ΕΥ RING AND HOLMBERG Table II.

Metal-Oxygen Distances in Coordination Polyhedra

Oxide

M0 M0 ,

3

C

Pr 0

3

Β

Sm 0

3

G

Sm 0

3

2


Α. A.

(i ih

PrO> Λ Pr 0

2

Μ0, , M0 7 Ί

M 0

A. A.

8

, A.

8-2.335

2

2

53

Rare Earth Oxides

3-2.66 3-2.32 1-2.40 (1-3.60) 6-2.40 and 2-1.96 2-2.50 2-2.76 2-2.32 2-2.49 1-2.38 1-2.29 1-2.76 (1-3.80) and 2-2.29 2-2.56 1-2.49 1-2.25 1-2.71 (1-3.60)

2-2.28 2-2.37 1-2.31 1-2.26 (1-3.12) (1-3.60)

6-2.36 and 2-1.92 2-2.45 2-2.71

M e t a l - o x y g e n distances are presented b y or calculated f r o m K o e h l e r K o e h l e r (14), o r C r o m e r (6).

\\

\\ /

/

/

/ /

/

/

/

\\

/

/

/

/

/

/

/

/ /

\ \ /

( 75),

\\

\\

\\

/

/

/

and Wollan

/

/

/

/

\\

/

\\

\ \ \\ / / \ \ \\

\ \ \\

/ /

/

Figure 5. A type

/

M0 2

Seeing this relationship between the A form and the dioxide structure, one can imagine a homologous series where the M 0 slabs simply vary i n thickness, but are joined together in the same way as in the A form, by sharing faces. T h e next member of such a series, having the general formula M 0 ^ _ i , is shown i n Figure 6. T h e only two members of the series known at present are the A form, 2

w

M 0 , and fluorite, M 0 2

3

2

2

.

In the sesquioxides the hexagonal A form has a seven coordination about the metal atoms. T h e coordination group M 0 can be c escribed as an octahedron 7




Figure 6. Hypothetical (n = 3)

Figure 8. Β type

Mn0 -i 2n

Sm O 2

s


4. EYRING AND HOLMBERG

Rare Earth Oxides

55

w i t h an additional oxygen atom above the center of one of the faces. Actually there is also an oxygen atom below the opposite face but at a m u c h greater distance from the metal atom. This last oxygen atom, together w i t h the seven i n the co ordination group, forms a distorted cube. These distorted cubes form the basis of the representation given above for the A type lattice. T h e monoclinic Β form has a structure closely related to that of the A form. B y choosing a monoclinic unit cell for the A form (taking as the new axis a — a — b + 2c , h — — a —b and c — a — b — c ) the relationship between them is easily seen. Figures 7 and 8 show these similar structures projected along the b axis. T h e metal i n the Β form is both six and seven coordinated. T h e M O group is similar to that i n the A form and has a distant oxygen neighbor i n the same way as the M O group i n the A form. T h e M 0 group is an octahedron w i t h two distant oxygen neighbors at opposite faces. The metal-oxygen distances i n A P r 0 and Β S m 0 are shown i n Table II. Figures 9, 10, and 11 illustrate the similarities and differences i n coordination between the two forms. T h e oxygen atom ar rangement i n the A and Β forms is almost the same as in the dioxide. However, the metal atom arrangement is very different. T h e coordination cubes i n the dioxides are joined by sharing edges, but i n the A form the distorted cubes of oxygen atoms are joined b y sharing edges and faces. Nonstoichiometry i n Rare E a r t h Oxide Systems. As more work is done on the intermediate oxides at l o w temperatures, regions earlier thought to be non stoichiometric are resolved into phases of narrow composition limit. It is possible that for carefully annealed specimens the entire range of composition w i l l be re solved into definite compounds of narrow composition limits separated b y t w o phase regions. This, of course, presumes that equilibrium can be achieved. One small single crystal fragment, black i n color, whose composition was about PrO} gave a very interesting diffraction pattern (10). It is a superposi tion of the pattern for P r O - ( A type) and the single ciystal pattern observed for P r O j . T h e reflections w h i c h w o u l d be common to the J?r0 ( A type) and Pr0 i n the zero layer (hkO) precession diagram were enhanced and i n the shape of X's. The same reflections i n the upper levels were separated into multiple reflections. A precession diagram taken w i t h the precession axis normal to the hexagonal axis showed the superposition of patterns from unit cells w i t h c axes of 6.0 and 9.5 A . F r o m the relationships shown i n Figure 2, it is not surprising that a single crystal could consist of parts w i t h the hexagonal A type and parts w i t h the fluorite-type structure, both oriented with common threefold axes. Some precession diagrams from single crystal preparations of compositions ΡΓΟΙ.ΤΙ d PrO are best explained in terms of multiple twinning i n the C direction. Parts of the crystal have the A B C A B C A . . . arrangement and other parts the A C B A C B A . . . order. It is clear that many interesting nonequilibrium states consisting of stacking faults or jumbled arrangements of coordination poly hedra may complicate the entire picture. h

h

h

h

h

h

h

h

T


T

6

2

2

3

3

δ 3

T

15

7 1

1

7 1

a

n

x

7 0

Above the critical temperature a single disordered phase seems to exist. Per haps i n these cases the coordination polyhedra are disordered to some extent. In these " c u b i c " systems where such a variety of coordination seems possible and where slip may occur in three directions, critical temperatures may be lower than would otherwise be observed, In the bixbyite structure ( C form) the six-coordinated oxygen atoms are of two arrangements and the oxygen positions are sufficiently irregular to suggest anion vacancies. In the intermediate oxides the structures may be considered to




Figure 11. C type

MO 2

s



Rare Earth Oxides

57

be related to or derived from the C type and the fluorite type, and the oxygen coordination polyhedra sufficiently irregular to justify speaking of anion vacancies. However, a new series of oxides may be realized having the general composi tion M 0 „ _ i , w h i c h are related to the A , B , and fluorite type oxides i n w h i c h the coordinated oxygen atoms are more regularly arranged, and may be considered to have metal interstitial atoms. Actually, in these systems, speaking either of anion vacancies or metal interstitials is probably imposing a nomenclature w h i c h has little relevance. n

2


Acknowledgment The authors express appreciation to Bruce H y d e and A . D . Wadsley for many stimulating discussions. Literature

Cited

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Ordered Phases and Nonstoichiometry in the Rare Earth Oxide System

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