Crystal Chemistry of ABO4 Compounds - ACS Symposium Series

18 Crystal Chemistry of ABO Compounds 4

A. T. ALDRED

Downloaded by UNIV OF SYDNEY on January 10, 2018 | http://pubs.acs.org Publication Date: March 8, 1984 | doi: 10.1021/bk-1984-0246.ch018

Argonne National Laboratory, Materials Science and Technology Division, Argonne,IL60439

To evaluate the factors affecting the structural stability of some crystalline materials that are potential hosts for radioactive wastes, the crystal structures of a series of A P5+1-xV5+xO compounds, where A is lanthanum or a member of the rare-earth series, were determined. The end-member phosphates (APO ) have the monoclinic Monazite structure (P2 /n) for A = La, Ce-Gd, and the tetragonal Zircon structure (I4 /amd) for A = Tb - Lu. The corresponding vanadates have the Monazite structure only for LaVO , and the Zircon structure for A = Ce - Lu. When the end members are isostructural, e.g., LaPO /LaVO , Monazite, YbPO /YbVO , Zircon, complete solid-solution behavior is observed, and a plot of the unit cell volume against x shows that Végard's Law is followed. When the end members are not isostructural, a systematic change in the solubility range in both structures is found as A is varied, and the data have been systematized in terms of a simple, potentially predictive, structure-field map. The pervasive polymorphism of these ABO compounds, involving both reconstructive and displacive transformations and metastable structures produced by different sample preparation methods, indicates that the crystal structural stability of substituted compounds needs to be carefully evaluated as a function of temperature to assess the structural integrity of waste-form materials. 3+

4

4

1

1

4

4

4

4

4

4

The nature of bonding and i t s r e l a t i o n s h i p t o s t r u c t u r e i n a c t i n i d e - c o n t a i n i n g m a t e r i a l s i s a subject of continuing i n t e r e s t . From the perspective of nuclear waste i s o l a t i o n , i t i s important t o understand a l l the f a c t o r s a f f e c t i n g the l o n g term s t a b i l i t y of a c t i n i d e ions i n p o t e n t i a l host m a t e r i a l s , be

0097-6156/84/0246-0305S06.00/0 © 1984 American Chemical Society

Barney et al.; Geochemical Behavior of Disposed Radioactive Waste ACS Symposium Series; American Chemical Society: Washington, DC, 1984.

306

GEOCHEMICAL BEHAVIOR OF RADIOACTIVE WASTE

they amorphous or c r y s t a l l i n e . The present work forms part of a systematic study to evaluate f a c t o r s a f f e c t i n g s t r u c t u r a l s t a b i l i t y , and therefore i n t e g r i t y , of some c r y s t a l l i n e materials that are e i t h e r p o t e n t i a l hosts, or may y i e l d i n s i g h t i n t o p o s s i b l e hosts, f o r nuclear waste i s o l a t i o n . Data from t h i s work may a l s o be used to a s c e r t a i n s t r u c t u r a l changes that may occur when s u f f i c i e n t a c t i n i d e or f i s s i o n - p r o d u c t l a n t h a nide ions i n the host m a t e r i a l have decayed. The p a r t i c u l a r group of ABO4 compounds was chosen because the f o u r major s t r u c t u r e types that occur, namely; Monazite (monoclinic, P22/n, prototype CeP0 ), Z i r c o n ( t e t r a g o n a l , I^/amd, prototype Z r S i 0 ) , S c h e e l i t e ( t e t r a g o n a l , I 4 j / a , prototype CaW0 ), and Fergusonite (monoclinic, I2/c, prototype YNb0 ), show a number of s t r u c t u r a l s i m i l a r i t i e s . They are c h a r a c t e r i z e d by strong t e t r a h e d r a l bonding of oxygen atoms around the Β atoms, with the tetrahedra w e l l i s o l a t e d from one another, and e i g h t - f o l d ( n i n e - f o l d f o r monazite) c o o r d i n a t i o n around the A atoms. The s t r u c t u r e s w i l l accommodate a range of valences at both A and Β s i t e s as long as o v e r a l l charge balance i s maintained. There i s extensive polymorphism i n compounds having these s t r u c t u r e s , i n v o l v i n g both r e c o n s t r u c t i v e and d i s p l a c i v e transformations. The pathways from one s t r u c t u r e to another include temperature, pressure, p r e p a r a t i o n techniques, and small changes i n atom s i z e (Figure 1). The existence of polymorphism suggests that the free-energy d i f f e r e n c e s among the s t r u c t u r e s are s m a l l , and therefore, i n d i c a t e s an enhanced p o s s i b i l i t y that multicomponent s u b s t i t u t i o n s , of the type necessary i n a nuclear waste host, may promote phase transformations and s t r u c t u r a l changes over the long term. 4

4

4


4

The concept of a s t r u c t u r e - f i e l d map (1) has proven u s e f u l i n systematizing the occurrence of d i f f e r e n t s t r u c t u r e s among a range of f i x e d - s t o i c h i o m e t r y compounds of the ^ B y O ™ type studied here. A binary phase diagram i s constructed i n which the axes represent the c r y s t a l r a d i i (2) of the A and Β i o n s , r and rjj, f o r the appropriate near-neighbor c o n f i g u r a t i o n . The published 03) s t r u c t u r e - f i e l d map (SFM) f o r the A B 0 compounds does show regular regions of s t a b i l i t y f o r the s t r u c t u r e s studied here, and i t i s evident that the r a r e - e a r t h s e r i e s provides a f i n e g r i d s i z e i n terms of v a r i a t i o n s of r . However, the lack of a p p r o p r i a t e l y - s i z e d B ions produces wide gaps i n the p l o t . In an attempt t o remedy t h i s , we have prepared and c h a r a c t e r i z e d a s e r i e s of s u b s t i t u t e d compounds of the form Α 3 ( Β Ι _ Β ) 0 · If we presume that a compound of t h i s form has a mean B-ion radius ^ r = (1 - x) ^r« + χ r f , then we can produce a more d e t a i l e d and p r e c i s e l y defined SFM. To evaluate the concept of a mean i o n i c radius, we proceed to analyse l i t e r a t u r e data with respect to A s i t e s u b s t i t u t i o n i n A B-> 0 compounds. Schwartz (4) has reported that comA

3 +

5 +

4

5 +

A

+

5 +

Χ

χ

4

B

3+

f i

+

4


18.

Crystal Chemistry of ABO Compounds

ALDRED

307

4

3 +

4 +

2 +

pounds prepared i n which A i s replaced by 0.5 T h +0.5 A (A = Ca, Sr, Ba, Cd, Pb) and B i s P, As, or V have e i t h e r the Monazite or Z i r c o n s t r u c t u r e s . I f we take a mean radius f o r t h i s composite A i o n r (the s u p e r s c r i p t 8 denotes the nearneighbor c o n f i g u r a t i o n ) based on the tabulated r a d i i f o r the t e t r a v a l e n t and d i v a l e n t ions i n e i g h t - f o l d c o o r d i n a t i o n ( 2 ) , then Schwartz's r e s u l t s can be added to the SFM. This i s shown i n the l e f t hand s i d e of F i g u r e 2, and the data do indeed f i t the o r i g i n a l systematics, apart from T h qPbQ^VO^ and ^ 0.5^ 0.5^4 ^ i c h e the Z i r c o n s t r u c t u r e * r a t h e r than the a n t i c i p a t e d Monazite s t r u c t u r e . Figure 2 a l s o i n c l u d e s the r e s u l t s of Fonteneau, et a l . (5) on the corresponding niobate compounds ( B ^ = Nb) c o n t a i n i n g e i t h e r T h ^ or which have the Fergusonite s t r u c t u r e , and of Davis et a l . ( 6 ) on ^ 0.5^0.5^4 which has the Monazite s t r u c t u r e . To push the a n a l y s i s one stage f u r t h e r , the u n i t c e l l volumes V of the compounds studied by Schwartz have been c a l c u l a t e d , and are p l o t t e d as V*' vs ° r i n the r i g h t hand side (rhs) of Figure 2. Again, the data of Fonteneau et a l . ( 5 ) , who produced a s i m i l a r , but more r e s t r i c t e d , p l o t , are i n c l u d e d . The s o l i d l i n e s represent the r e s u l t s of l i n e a r l e a s t squares of V / vs 5 +

8

A

Q

n

r

n a v


+

+

a

3

A

1

r

3

f o r the A P O 4 (Monazite), A P O 4 (Zircon), A A S O 4 (Zircon), ( Z i r c o n ) and A N D O 4 (Fergusonite) compounds where A i s a rare e a r t h (Ce-Lu). The values f o r a l l the s e r i e s except A A S O 4 were determined i n t h i s Laboratory (7)· The r e s u l t s f o r A A S O 4 were obtained from N a t i o n a l Bureau of Standards reports ( 8 ) . The l i n e a r r e l a t i o n s h i p between u n i t c e l l volume and the cube of the i o n i c radius among a s e r i e s of i s o s t r u c t u r a l compounds has been emphasized by Shannon and Prewitt (2) as a powerful means of s y s t e m a t i z i n g c r y s t a l l o g r a p h i c r e s u l t s . The data of Schwartz and Fonteneau et a l . (rhs Figure 2) are consistent with the u n s u b s t i t u t e d A B 04 r e s u l t s and thus support the concept of a mean radius r. and by analogy r as a p r e d i c t o r , i n combination with the appropriate SFM, of the occurrence of p a r t i c u l a r s t r u c t u r e types. A

A V O 4

3+

5+

f i

Experimental Samples were prepared from s t a r t i n g m a t e r i a l s at l e a s t 99.9% pure i n the form of the normal s e s q u i - and pentoxides except f o r (NH4) HP04. Lanthanum sesquioxide was obtained by f r e s h decomposition of lanthanum oxalate at 1000°C. Conventional s o l i d - s t a t e s i n t e r i n g techniques were used, and care was taken to mix and g r i n d samples under an i n e r t atmosphere (argon) and heat t r e a t them i n pure dry oxygen. Repeated f i r i n g ( i n the temperature range 1100-1400°C, as a p p r o p r i a t e ) , g r i n d i n g and mixing cycles were used u n t i l the x-ray d i f f T a c t o m e t e r scans showed no change and no evidence of the s t a r t i n g m a t e r i a l s . The s e n s i t i v i t y l e v e l of the d i f f T a c t o m e t e r t r a c e s , based on 2


GEOCHEMICAL BEHAVIOR O F RADIOACTIVE WASTE

FERGUSONITE

MONAZITE CeP0

YNb0

4

2V6 ,3 /5 ,4 /4 +

+

+

\

+

3 /5 +

+

Γ

INCREASING

1

INCREASING

y

INCREASING


y INCREASING

INCREASING Τ

I

SCHEELITE CoW0

4

TETRAGONAL 14,/omd 3V5 ,4 /4 +

F i g u r e 1.

+

ι ι

ZIRCON ZrSi0

4

MONOCLINIC I2/C

MONOCLINIC

+

4

TETRAGONAL 14,/a INCREASING—- l / 7 , 2 V 6 , 3 / 5 , 4 / 4 Ρ +

+

+

+

+

+

+

+

P o s s i b l e pathways between c r y s t a l s t r u c t u r e s b c u r r i n g among Α3+β5+θ4 compounds.

ο -Lu φ

Η 1.2

3

θ MONAZITE (M) • ZIRCON (Ζ) Δ FERGUSONITE (F)

5

A+B+0| 4

1.0

As 0.3

Nb

VI I il

ι I

0.5 4r (Â)

0.7

F i g u r e 2.

6.6

6.8 V (Â)

7.0

I/3

B

L e f t hand s i d e : P a r t i a l S t r u c t u r e f i e l d map f o r Α β 5 θ 4 compounds. Right hand s i d e : Corresponding v a r i a t i o n o f the cube r o o t o f the u n i t c e l l volume w i t h the S i z e o f the A ion r . 3 +

+

8

A


18.


ALDRED

4

r e l a t i v e i n t e n s i t i e s , i s -1%. A d d i t i o n a l l y , l i n e widths were required to be comparable t o the instrumental r e s o l u t i o n and the l i n e widths of standard samples (Ce0 , Si0 )« Samples that meet t h i s c r i t e r i o n should be homogeneous and the s u b s t i t u t e d ions should t h e r e f o r e , be d i s t r i b u t e d randomly and uniformly throughout the m a t e r i a l . Several samples were examined by scanning e l e c t r o n microscopy, p r i m a r i l y to check compositions by means of the EDX technique. Good agreement was found between nominal and measured compositions, and so nominal compositions are used i n the rest of the paper. Phase i d e n t i f i c a t i o n was done on the basis of both d-spacing and the peak height i n t e n s i t y of a l l the x-ray l i n e s . These values were compared with values obtained f o r the end-member (unsubstituted) compounds and a l s o c a l c u l a t e d by means of the Lazy-Pulverix computer program ( 9 ) . P r e c i s i o n l a t t i c e parameters were obtained by the Debye-Scherrer method with a 114.6 mm d i a . camera and f i l t e r e d Cr K r a d i a t i o n ; standard least-squares methods were used.


2

2

a

Results The p h a s e - i d e n t i f i c a t i o n s t u d i e s of the present work are sum marized i n Figure 3, which c o n s i s t s of a d e t a i l e d SFM f o r the A ( i- V ) 0 s e r i e s where A i s e i t h e r La or a rare e a r t h . The s a l i e n t f e a t u r e s of t h i s p l o t are as f o l l o w s : When the two end-member compounds are i s o s t r u c t u r a l , e.g., LaPO^/LaVO^ (monazite) or H 0 P O 4 / H 0 V O 4 ( z i r c o n ) , complete s o l i d - s o l u t i o n behavior i s observed. For those Α Ρ ν θ 4 s e r i e s where the end members are not i s o s t r u c t u r a l , the width of the two-phase f i e l d i s a systematic f u n c t i o n of r . At l a r g e r (Ce, Pr) the two-phase f i e l d i s narrow and dominated by a monazite s o l i d solution f i e l d . For intermediate r (Nd, Sm), the two-phase f i e l d i s broad with l i t t l e s o l u b i l i t y i n e i t h e r the monazite or z i r c o n s t r u c t u r e s . At small r (Gd), the two-phase f i e l d i s again narrow with extensive s o l i d s o l u b i l i t y i n the z i r c o n phase. The r e l a t i v e amounts of both phases i n any two-phase sample was estimated on the basis of r e l a t i v e x-ray i n t e n s i t i e s over the whole p a t t e r n . The composition of the phase boun daries i n any A P i _ V 0 4 s e r i e s were then determined by the l e v e r r u l e . The s o l i d l i n e representing the boundary of the Z i r c o n and Monazite two-phase f i e l d i n Figure 3 was drawn on t h i s b a s i s . Some r e s u l t s are a l s o i n c l u d e d i n Figure 3 f o r the systems Gd _ Tb P04, l-x x 4 r e s p e c t i v e end members have the Monazite and Z i r c o n s t r u c t u r e s . Based on room temperature x-ray r e s u l t s , there i s , e s s e n t i a l l y , no s o l u b i l i t y of Tb i n the Monazite s t r u c t u r e of GdP04 and no s o l u b i l i t y of Gd i n the Z i r c o n s t r u c t u r e of T D P O 4 . On the other hand, at l e a s t 50% of the La ions i n LaV04 may be replaced by Ce with r e t e n t i o n of the Monazite s t r u c t u r e and at l e a s t 20% of the Ce p

+

x

x

4

1 - χ

χ

8

A

A

8

A

8

A

x

x

a

1

x

n

d

L a

C e

V 0

w

h

e

r

e

t h e

x


309

310

GEOCHEMICAL BEHAVIOR OF RADIOACTIVE WASTE

ions i n CeV0 may be replaced by La ions with r e t e n t i o n of the Z i r c o n s t r u c t u r e . The two-phase f i e l d i s thus very narrow. Some representative l a t t i c e constant data w i l l now be pre sented i n g r a p h i c a l form. Figure 4 shows the composition dependence of the f o u r l a t t i c e constants of the monoclinic monazite s t r u c t u r e f o r L a P i _ V 0 4 ( c i r c l e s ) , where complete s o l i d s o l u b i l i t y i s observed, and f o r CePi ν 0 where there i s s u b s t a n t i a l but not complete s o l i d s o l u b i l i t y . In the case of LaPj_ V 0 , a l l the l a t t i c e parameters vary l i n e a r l y with χ f o r 0 < χ < 1 i n d i c a t i n g that Végards Law i s observed. F o r l-x x°4> l a t t i c e parameters again vary l i n e a r l y f o r 0 < χ < 0.7 and are p a r a l l e l to the L a - s e r i e s r e s u l t s . The l a t t i c e constants f o r the χ = 0.8 sample, which c o n s i s t s of a major Monazite and a minor Z i r c o n phase, represent values at the phase boundary, determined i n the manner described e a r l i e r as χ = 0.78 and shown by the v e r t i c a l dashed l i n e s i n Figure 4. T r a n s p o s i t i o n of these l a t t i c e parameters to χ = 0.78 shows that they a l s o f i t on the s t r a i g h t l i n e d e s c r i b i n g the r e s u l t s f o r lower x. E x t r a p o l a t i o n of the r e s u l t s f o r C e P i _ V 0 i n Figure 4 to χ = 1 y i e l d s the f o l l o w i n g l a t t i c e constants f o r h y p o t h e t i c a l (non-equilibrium) CeV0 with the Monazite s t r u c ture: a - 7.00 A, b - 7.24 A, c = 6.69 A , 3 = 105.1°. Yoshimura and Sata (10) reported the existence of a monoclinic form of CeV0 prepared by o x i d a t i o n of CeV03 at temperatures below 400°C. The s t r u c t u r e was metastable and decomposed i r r e v e r s i b l y (and e x o t h e r m i c a l l y ) on heating above 400°C. The reported l a t t i c e parameters: a = 6.98 A , b = 7.22 A , c = 6.76 A, 3 = 105.0°, are i n good agreement with the e x t r a p o l a t e d values l i s t e d above. Figure 5 shows the composition dependence of the l a t t i c e constants f o r H o P ] _ V 0 and G d P i _ V 0 compounds with the t e t r a g o n a l Z i r c o n s t r u c t u r e . Again, the l i n e a r v a r i a t i o n shows that Végard's Law i s obeyed f o r the Ho-based system where there i s complete s o l u b i l i t y i n the Z i r c o n s t r u c t u r e . The l i n e a r v a r i a t i o n i n the Gd-based system p a r a l l e l s that of the Ho system over the s o l u b i l i t y range i n the Z i r c o n s t r u c t u r e . The composition of the phase boundary, determined i n the manner described e a r l i e r , i s at χ = 0.30. E x t r a p o l a t i o n of the data f o r Gd P i - V 0 to χ = 0.0 y i e l d s parameters of a = 6.093 (3) A and c = 6.970 (3) A f o r h y p o t h e t i c a l GdP0 with the Z i r c o n structure. 4

x

x

χ

x


C e P

x

χ

4

4

v

t

n

e

x

x

4

4

4

x

x

x

x

4

x

x

4

4

4

Discussion The l a t t i c e parameter r e s u l t s f o r a l l the Α Ρ ι _ ν 0 χ

χ

4

series 1

J

studied here are brought together i n Figure 6 where V ' is p l o t t e d against concentration (and rjp. The l i n e a r v a r i a t i o n i n the complete s o l i d - s o l u t i o n s e r i e s L a P i _ V 0 (Monazite), ° l - x x ° 4 ( Z i r c o n ) , and Y b P i - V 0 ( Z i r c o n ) again shows that x

H

p

x

4

v

x

x

4


Crystal Chemistry of AB0 Compounds 4


ALDRED

F i g u r e 4.

V a r i a t i o n of the monoclinic l a t t i c e constants w i t h composition f o r compounds i n the LaPi_ V 0 and CeP-^ V 0 systems. x

x

4

-x

x

4


312

GEOCHEMICAL BEHAVIOR OF RADIOACTIVE WASTE 4


0-32

Γβ (A) 0,40

0.36

0.44

0,48

χ

F i g u r e 5· V a r i a t i o n of the t e t r a g o n a l l a t t i c e constants w i t h com p o s i t i o n f o r compounds i n the HoP, V 0 and GdP-, __V 0 systems. A

r (Â) 0.40

4

0.32

0.36

Λ

B

0.44

0.48

χ

F i g u r e 6. V a r i a t i o n of the cube root of the u n i t c e l l volume with composition f o r the AP^ ° series. ν

χ

χ

4


18.

ALDRED


313

4

Végard's Law i s obeyed and that the concept of a mean radius of the Β i o n i s a v a l i d one. It thus provides a d d i t i o n a l j u s t i f i c a t i o n f o r the use of the s t r u c t u r e f i e l d map as shown i n Figure 3 to p r e d i c t s t r u c t u r a l existence and s t a b i l i t y i n a p a r t i a l l y - s u b s t i t u t e d compound. Even i n the cases of p a r t i a l s o l i d s o l u b i l i t y , the composi t i o n dependence of V / i s p a r a l l e l w i t h i n a given c r y s t a l s t r u c t u r e as demonstrated i n Table I, which gives the r e s u l t s of l i n e a r least-square a n a l y s i s of the data of Figure 6· The f i n a l column of Table I shows the r e l a t i v e change i n V ' with composition determined i n t h i s a n a l y s i s normalized by the r e l a t i v e change i n the radius of the Β i o n Δ r = * r - * r = 0.185 Â. For the Monazite s t r u c t u r e , t h i s r a t i o i s e s s e n t i a l l y 1, i . e . the l a t t i c e i s expanding/contracting, on average, e x a c t l y enough to compensate f o r the d i f f e r e n c e i n s i z e of the two Β i o n s . On the other hand, t h i s r a t i o i s s i g n i f i c a n t l y l a r g e r (1.3) f o r the Z i r c o n s t r u c t u r e . This dichotomy i s p a r a l l e l e d i n the response of V*' to changes i n the s i z e of the A i o n i n the A P O 4 and A V O 4 s e r i e s (7) and r e f l e c t s a d i f f e r e n c e between the close-packed low-symmetry Monazite s t r u c t u r e and the more open higher-symmetry Z i r c o n s t r u c t u r e . In cases where polymor phism between these two s t r u c t u r e s (Figure 2) i s known t o occur, e.g. ThSi04 (11) > there i s a d i f f e r e n c e of ~8% i n the u n i t c e l l volumes of the two s t r u c t u r e s . This observation may have s i g n i f i c a n c e to the problem of accommodation of a range of ions of d i f f e r e n t s i z e s i n t o a nuclear waste i s o l a t i o n host. It i s l i k e l y that t r a n s u r a n i c ions w i l l be incorporated i n t o such materials i n the t e t r a v a l e n t form. Given the tabulated r a d i i of the t e t r a v a l e n t t r a n s u r a n i c ions (2), which are smaller than l r , s u b s t i t u t i o n of these ions by appropriate valence compensation with a d i v a l e n t i o n would y i e l d a composite A i o n small enough to p o s s i b l y s t a b i l i z e the Z i r c o n s t r u c t u r e as opposed to the Monazite s t r u c t u r e i n the A P O 4 s e r i e s (Figure 1). A r e c o n s t r u c t i v e transformation i n v o l v i n g a s i g n i f i c a n t volume change such as t h i s could have a severe i n f l u e n c e on s t r u c t u r a l i n t e g r i t y . In summary, we have demonstrated that the concept of a s t r u c t u r e - f i e l d map i s a u s e f u l one i n systematizing the occur rence of c r y s t a l s t r u c t u r e s i n a s e r i e s of i s o - s t o i c h i o m e t r i c compounds. In a d d i t i o n , the concept of a weighted mean radius of an i o n at a p a r t i c u l a r s i t e i n a s u b s t i t u t e d compound has been found to be a v a l i d one. The use of a SFM to p r e d i c t s t r u c t u r a l s t a b i l i t y and provide warnings about p o s s i b l e p o l y morphism (and so s t r u c t u r a l i n t e g r i t y ) i n a complex multicomponent s u b s t i t u t e d system could be a u s e f u l t o o l i n a s s e s s i n g p o t e n t i a l hosts f o r nuclear waste i s o l a t i o n . 1

3

1


fi

v

3

p

3

+


314

G E O C H E M I C A L BEHAVIOR

Table 1.

WASTE

Results of L i n e a r Least-square f i t s t o V l / 3 = Β + Cx f o r AP- V 0 4 Compounds X


O F RADIOACTIVE

X

A_

Structure

X

La Ce Pr

Monazite tl

0-1.0 0-0.78 0-0.44

Pr Nd Sm Eu Gd Ho Yb

Zircon

M

it II

π It II tl

0.95-1.0 0,88-1.0 0.765-1.0 0.61-1.0 0.30-1.0 0-1.0 0-1.0

4

C

C/A -r

B

0.191(1) 0.193(2) 0.181(4)

1.03(1) 1.04(1) 0.98(2)

M).2 ^0.24 0.249(5) 0.234(6) 0.247(2) 0.244(1) 0.244(1)

vL.l

Crystal Chemistry of ABO4 Compounds - ACS Symposium Series

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