MgCuA[ 2 type - SRPd 0.8 ~12RAu - Handbook on the Physics and Chemistry of Rare Earths

SRPd 0.8 ~12RAu

YNiAI 2 MgCuA[ 2 type

Re3B type derivative

oC16, Cmcm 0=4.07, b=lO.lò, c=7.06 Ä

B - - ' x Æ ~ N

f / / I L ~ / - . . /

/ \ / ' ~ t F ~ D " / ', ~ I ~ -

\ r k . 3 " ~ k _ ) - t s.. k_) ~ k_) ~ /

/ x x I i / ~ \ / x I . / \ . J

q " ) f - - \ ~ C " " / f - \ ~ "

,0 ,«' O:i.© '.«' O.i~

/ \i /,.~ I r"~ 7 \~ f'N ~ F"~ " [ X t I " ' k . . J r ' " ' k J k J k_) u ~ ,x, /

f I f ;

Sc3N i4Ge 4 Gd3Cu4Ge 4 type

0[22, Tmmm

a=5.908, b=6.598, c=12.910 Ä

Fig, 29. The crystal structures o f YNiA12 and Sc3Ni4Ge 4.

The Sc3Ni4Ge 4, structure can also be compared with the 6360: Sc3Co2Si 3 structure.

If one slices the Sc3Ni4Ge 4 structure parallel to (010) at y = 0 and y = ½, separates the slabs slightly, completes the atom halves (R3/2T2M 2 4-R3/2M = R3T2M3), and shifts the new slabs alternatively by c/2, an idealized model o f the S c 3 C o 2 S i 3 s t r u c t u r e

is obtained (see fig. 13).

It was shown above that the Sc3Ni4Ge4 structure consists o f trigonal prisms and sheets of face-joined R4T2M 2 columns. It is o f interest to find out if a structure exists with only face-joined R 4 T 2 M 2 columns. Such a structure would have the composition 7533:RT2M and should have the MgCuAI2-type, a Re3B-type derivative. N o 7533 compound of MgCuA12-type exists but the aluminide 7567:YNiAl» shown in the left-hand part of fig. 29, has this structure type. In this compound the T and M atoms in the characteristic columns have been interchanged. Here we have one other example of the unusual behaviour of A1 as compared, for example, to Si and Ge in these compounds. Another example o f site interchange has been discussed before with the ZrNiAl-type (see 6750:ScRuGe).

It should be mentioned that silicides and germanides with composition 7533 are

C R Y S T A L S T R U C T U R E S A N D C R Y S T A L C H E M I S T R Y 175

known. They crystallize with the YPd2Si-type, a Fe3C-type derivative. Geometrically, both binary types, Fe3 C and Re3B, are closely related. Using the concept of periodic unit cell twinning they can be derived from base structures with different stackings (Chabot and Parthé, 1978).

7350 La3Rh4Ge4 o122 a = 4.1746 HoKP, 82

or Immm b = 4.2412

La3Rh[2 6' P]Rh~ 4tl ~ [Ge[2 6' p¿]Ge 2 c = 25.234 U3Ni4Si4-type (YaAGFG, 79)

Isotypic compounds:

Ce3Ni4Si4?: Ce6Ni6Si 7 has been reported with the same space group (BOG, 69).

The similarity of composition and lattice parameters makes the isotypy with La3Rh4Ge4 likely.

La3Rh4Ge4: HoKP, 82

The structure of La3Rh4Ge4, of U3Ni4Si4-type , shown in fig. 30, can be considered as a periodic intergrowth of two kinds of slabs. One of these slabs with composition LaRhE«P1Ge t6'pl is a segment of a ternary ordered A1B2-type derivative structure [see 67(75):Ce(Ni.25Si.75)2] and the second with composition LaRh~4tlGe2 is a segment of the ThCr2Si2 structure type, a ternary ordered BaA14-type derivative structure (see 8050:CeNi2Si2). The compound LaRhGe itself has not been studied, but LaRh2Ge2 crystallizes with the ThCr2Si:-type with lattice constants as given in fig. 30 (Ho, 82).

According to Yarmolyuk et al. (YaAGFG, 79) the U3Ni4Si4-type structure together with the 7567:CeNiSi2 and 7780:Ce3Ni2Si8 structures, the last two shown in fig. 31, are members of a structural series of formula Rù

+mT2nM4m.

The parameter n is the number of ordered A1B2 and m the number of ordered BaA14 structure segments. The structural data for the members of this structural series and their n and m values are listed in table 10. We note that the two end-members of this

TABLE 10

Structural data for the members o f the structural series Rn+ù,T2ùM~, proposed by Yarmolyuk et al. (YaAGFG, 79).

n m Code C o m p o s i t i o n Structure Pearson's Space

type classif, group

symbol

0 6700 R T 2 A1B 2 hP3 P6/mmm

2 1 7350 R3T4M « U3Ni4Si 4 oi22 Immm

1 1 7567 R T M E CeNiSi 2 oC16 Cmcm

1 2 7780 R3T2M 8 Ce3Ni2Si 8 oC26 Cmmm

0 ~ 80100 R M 4 BaA14 tI 10 I4/mmm

/ - - \ S ~ ' , f ~ \

f l - - - - ( I-'-'- - -~ i - - - - - ' I

ù]-:

\ \ C ) /

Y',,9/

O X } -

,1 ' 0 , T ,o,, T

I . . . . f '~ /

/ \ / . . . . \ / J

"l-',,o,'T',,o,,"l-

C»:./--O-~L :uO

.'" :', 0 ,_ c,, 0 ,'- ~', '~

, 0 ,_, 0 ,_, 0 ,_ ~ # ~

\ / , / /

% _ I . k g , > - { k g

,;-.1

" ' , 9 . U. ',o.,"2

- A " " ," ~ ~

' / , W ( " ; " ~ J . ~ m n-"

~ t , J

n«

~ ~

n.-

: / x

ù - " , 0 , ' - ' , 0 Y',

_ , 0 , - ; : , 0 , _ ; . , 0 ,_ ~ z o.,

x / ,. i i ~ \ i

- ' , 0 / , ' £ ' , \ 0 i'~ß.. "~` 0 ', I " - I~ ~

ù C D , , C D . . ,

, ; , ~ t ; ^. ^. ^. ^. ^. ^. ^.

1- o,1

__1

.> Q» r--

õ . ~ ~ o

. - LO

"u ~ d d

~ > ,

0 ~:m

0 ( _\ _{/ /}\ / ~ ~ _~. ~-~

~ © ~

-1- j I ~ (.9

.ê

E $ #

n - u~ e4 ~

ro . ~ oJ" " .~

o z ~

,=. .~

o II ~

C R Y S T A L S T R U C T U R E S A N D C R Y S T A L C H E M I S T R Y 177

(L..~

B a A [ 4

m=2

A[B 2 RM 2

i n=l

i i

I i

S J-,

" i

I t

I I

BoA[4 I AIB2

R2T2MzM 4 I RM 2

m=2 ~~ n:l

I I

BaAI. 4 R2T2M2M4

m=2

CesNi2Si 8 CesNi2Si 8 type oC26, Cmmrn

a=4.085, b=25.9558, c=4.1786 Ä

Height R T M

ù~ 0 0 o

I i i i

I ~ t-" I / \,

1 ~ ~ t ~ I ~

" Y 0 "

Y ~ @ @

©, (._)i»-~

.... ; ' ' [ ... ~ ( 7 " L T " ... -

t ' ;

i 0..,_

/ \ ~ i «

A[B 2 RM z

n = l

BaA[ 4

R T z M 2 m = l

AtB 2

R M 2 n = l

BaA[ 4 RTzM z

m = l

AIB 2 RM z

n = l

t I I

I fL.

L x / - - f - " '~ \»

; ~ : ~ , ,.

o IC),,,

,~..»-,~-L.21 ' ~ A .L T b o ' N :

¢,-KA,, w--~«

i ' i k..2 ~

, k._.) ~«k._.J, .~ .«

t .es ,(%"

/ v \ /.. v l ~ J ' "

I I \ /

I I T

I I

iCeMg2Si21 ALB 2 'tCeMg2Si z

I t R M z I RT2M 2

I R T a M 2 I I

i i

I m=l i n=l I1 m=l

/ ~1 ~ ~~ L .

. / i "f \

..,0: o . , ~ : , o ,O"

~ 0 , o .~~ ',~

\ y L " " "T ~L''-,

~" o ',Q.~.

D., o :, :--(

I i I I

~ T j

~qL

//~'U

" I

A[B 2 CeMg2Si 2

R M 2 R T 2 M 2

n:1 m='l

CeNiSi 2 NdNiGa z

CeNiSi 2 type

oC16, Cmcm oC16, Cmmm

a=4.141, b=16.418, c=4.068 Ä a=4.192, b=17.564, c=4.155 Ä

Fig. 31. The Ce3Ni2Si 8 and CeNiSi2 structures together with the N d N i G a 2 structure closely related to the latter.

structural series are RT2 with A1B2-type (n = ~ , m = 0) and RM 4 with BaA14-type (n = 0, m = ~ ) . Thus one might expect that in the structures of the members of this structural series all A1B: slabs have composition RT2 and all BaA14 slabs composition RM 4. This is, however, not the case. In CeNiSi» for example, the A1B2 slabs have composition R M » Similarly the A1 sites in the BaA14 slabs are not occupied by M atoms alone, but in the different structures by M and T atoms in different ratios.

Nevertheless, in spite of these different site occupations, the overall chemical formula is correctly given by the structure series formula. We note that the trigonal rare earth prisms in La3Rh4Ge4 are compressed (a < b) but stretched in CeNiSiz (a > c) and Ce3Ni2Si8 (c > a). In the first case the prisms are centred by T and M atoms, in the last two compounds by M atoms only. It is known from studies of relative prism dimensions in binary RT and RM compounds that the prisms in the first are always compressed and in the latter always stretched.

For a more general discussion of structures which can be considered as an intergrowth of BaAI4 segments and segments of other simple structure types, see 8050: CeNi2Si:.

7364 Sc4Mn4Si7* tI60 a = 13.06 KoBK, 80

or I4/mmm c = 5.227

Sc4Mn~6olSi6Si[8, a]

Zr4Co4Ge7-type (Je, 69) or V-phase type

*In the publication on Sc4Mn4Si7 the Wyckoff position of the Si(2) is incorrectly given as 8j) instead of 8i). This probably explains why the R value would not go below 0.142.

Isotypic compounds:

8c4T48i7: T = Mn a~, Fe b)

a)KoBK, 80 b~ß. Chabot, unpublished results [a = 13.099(4), c = 5.075(3)/~]

The Sc4Mn4Si 7 structure with Zr4Co4Ge7-type together with the structures of Sc2Cr4Si» ScFeSi2 and ZrMnSi2, the last two having the same crystal-chemical formula, are shown in fig. 32.

All structures are characterized by two construction elements:

-parallel infinite rectilinear columns (perpendicular to the plane of projection) of Si-centred antiprisms,

-parallel infinite rectilinear columns (perpendicular to the plane of projection) of Cr, Mn, Fe-centred face-joined (deformed) octahedra formed by Si atoms.

The T atoms in the octahedron centres interact strongly with each other (more or less depending on the T element) through the common octahedron faces. The corresponding T - T distances are shorter than the sum of the metallic radii of the elements.

C R Y S T A L S T R U C T U R E S A N D C R Y S T A L C H E M I S T R Y 179

TABLE 11

Structural d a t a for crystal structures characterized by infinite rectilinear c o l u m n s of M - c e n t r e d a n t i p r i s m s a n d T - c e n t r e d M octahedra.

Code R - T - M Structure Pearson's Space Antiprism Octahedron Crystal-chemical

compound type classif, group formula

symbol

7364 Sc4Mn4Si 7 ZhCo4Si 7 tl60 I4/mmm R8/2 M[Sa] + 4M6/4T [6°] ~ R4T~6°IMöM [8,al (7567) ZrMnSi 2 0148 Immm R6/2T2/2M[(6, 2)"1 q_ 2M4/4M2/2T[6Ol + M---+ R3T~6°IMsM [C6' 2)al 7567 ScFeSi2 ZrFeSi2 0C96 Cmca R6/2T2/2M[(6, 2)"1 nu 2M4/4M2/2TE6Ol + M-~' R3T~6°IMsM [(6' 2)"1 8256 Sc2Cr4Si s Nb2Cr4Sis, 0144 Ibam R4/2T4/2M[(4, 4)a] "t- 2M4/4M2/2T t6°l ___, R2T2Tf6OlM4M[(4, 4)ù]

V6Sis-type derivative

The structural data and crystal-chemical formulae for all structures having those structural characteristics are given in table 11. The structures differ in the R/T ratio of the atoms which form the M-centred antiprisms and in the type of linkage of the centred octahedron columns with other octahedron columns (linkage by means of common octahedron edges). In ScFeSi2 and ZrMnSi» to complete the octahedral surrounding of those Fe or Mn atoms which participate in the formation of the antiprisms, extra Si atoms are positioned between the antiprism columns. Also the 7567:ScMnSi2 structure with TiMnSi2-type is built up of M-centred antiprism columns and T-centred M octahedron columns (see fig. 44); however, the columns are not infinite; only column fragments of three joined polyhedra occur.

With the exception of Sc2Cr4Si 5 all T atoms which participate in the formation of the antiprisms are always octahedrally coordinated by Si atoms. This applies also to 8063 : Sc2Fe3Si 5 (fig. 54).

7388 Y3ReB7 oC44 a = 3 . 5 2 5 KuM, 76

or Cmcm b = 15.80

Y3Re ~ (B6 Br6'pl) c = 9.366

Isotypic compounds: All references KuM, 76 R3ReB7: R = Gd, Tb, Dy, Ho, Er, Tm, Y

The structure of Y3ReB7, shown in fig. 33, is characterized by a folded boron-boron chain, which extends infinitely in the [001] direction. The chain consists of fragments of a double chain connected by single boron atoms.

B / ,-: ~ _1 .~(

©

', ,

, , f , , :

,~-j_

B ~ ' 7

^{'7, :} ^J ^, ^,,

OJ rO

co ~ ~

iii ~.."

Ii 0

CO~ ~ IE~5 i ~" ~ Il (-9

0 (_)

+.~

E--

O 0

ù.Z, O v

ra~

c.i

C R Y S T A L S T R U C T U R E S A N D C R Y S T A L C H E M I S T R Y 181

« \ / , i \ J ~ ,

~ . ) - - . 7

o~. o Q o ~ . o q , ,

! .... / t }

( O Q " "

j O

^'Æ-'

ùc fr')

E ~

c ~ b - :

g

~ ~ "

O 0 o 4

~.6 ~

o ~ 8

~,

, < ! c ( ")',,'C-, , : ' , ' ( 5'w"

ù,-" : " ~ , ' ,'~[-".~J-;."-i ,~' . " : " - " ' x " i ' r : " , < - - J :'-,- L ( ) ,b;«) '.». I ( I .., ;~. < . -

:ù 3.~(;<~.., ~, :_., ~,ù ~ i" , ~ : ? ' : . . . ~ , _ ,

f I - - - 4 I ~ I 1 . . . . I I

/ ::~ ' : ~ L ., i ~ ~ \ / ~

ù ) 6 ~ ( - ,

I - - - ~ ß f " N ~ : \ /

ù,ù,' 0 ' . , ' - - ~ - ' _ - ~ . ü ~ . ' 0 < :-.~.-'-t" ,~k--t ;

"-.../:~. _ ù ~ - 7 - ~ " ? - C " , ,, ' - - : - - ~ v "'t"

ù .. 'j . k l ~~i.. £ . )_r/ \ ~ .

t / 0 /

Y s R e B 7 oC44, Cmcm a=5.525, b=15.80, c=9.566 Ä

Height t / 2 0

y Q

~x Re

0 ' i ;

B 0 (-';

Fig. 33. The structure of Y3ReB7 with an infinite folded boron-boron chain.

7471 Se«Co4Si~0 tP38 a = 12.01

or P4/mbm c = 3.936

ScsCo4Si 8 ~ [Si[26, p]]

BrYB, 80

Isotypic compounds:

RsCo4Sil0:

R5Rh4Sil0:

RsIr4Sil0:

R»Os4Gelo:

RslhGe~o:

R = L u «), S c ~) R = L u «), S c a)

R = Dy b), H o b), Er b), T m b), Lu ó), Yo), SC 'b) R = 5(")

R = y m

~)BrYB, 80 b)BrS, 81 «)Br, 82

The ScsCo4Silo structure and the geometrically related 7578:La3CozSn 7 structure are shown in fig. 34. Both structures are built up o f intergrown Cu3Au-type segments and 7567:CeNiSi2-type segments (see fig. 31). We note that the CeNiSi2 structure itself is built up of BaAl4-type and A1B2-type segments. In the La3Co2Sn 7 structure

CRYSTAL STRUCTURES A N D CRYSTAL CHEMISTRY 183

. . . t \

t i t ~ « I '~~

"7- ~ CO

_ ~ _ ~

( ~ I I 0

t /

2 \ )

. . . C'~ ':7, ~ 2 - - " m

? . . . . --..~t

-, " J ). ~-J

, ~ -, i ~ ~

. j . ~ . . . . . S I . 3 I

I /

o N

ù ~

0 t'M . o

_.j ~

~ 0 "

o ~ ~

03 E m (ù) o_

1=3

rj 0

there are intergrown slabs, in the 5c5C04Sii0 structure, however, intergrown columns.

The composition of La3Co2Sn 7 can be rationalized according to 2RTM2 (CeNiSi2-type slab) + RM3 (Cu3Au-type slab) = R3T2M7. For S%C04Silo we find RM2 (A1B2-type column) + 4 R3/4TM»/4 (BaAl4-type column, edge-connected to four other columns) + RM3 (Cu3Au-type column) = R»T4Mlo.

7511 Ce3CosSi hP24 a = 4.960 Bo, 71

P63/mmc c = 16.450 CeNi3-type (CrO, 59) derivative

Isotypic compounds:

R-Fe-AI: R = TN )*

R3Ni8AI:' R = Pr b'i), Nd «'i), Sm i), Gd a'i), Tb «'i~, Dy ~'°, Ho 0, Er/), Tm 0, Lu i), YY'i) R3C08Si: R = Ce a'h)

*Composition is given as TbFe2AI.

a)BoG, 70 b)RyZK, 78 «)RyZY, 79 d~RyZM, 78 «)RyZM, 80 J)RyZ, 77 g)Oe, 75 h)Bo, 71 i)Ry, 78

The Ce3Co8Si with CeNi3 derivative structure is a member of the R2+ùT3 + 5nM structural series (discussed below) with n = 1. It is composed of CaCu»-type slabs and ordered ternary Laves-type slabs as found in 6725 : Sc2Co3Si with Mg2Cu3Si-type (fig.

17). The atom arrangement in Ce3CosSi can be conveniently compared with that of 7522:Dy3Ni7B» shown in fig. 38, another ternary ordered CeNi3 derivative structure which belongs to another structural series R2+nT4+3nM2ù with n = 1. The CeNi3, Ce3C08Si and Dy3Ni7B2 structures all have the same structure sites, but they differ in their occupation. In CeNi3, one finds intergrown CaCu» and binary Laves-type slabs. In Ce3Co•Si , the Laves-type slab is replaced by an ordered ternary Laves-type slab; however, in Dy3NiTB2 it is the CaCus-type slab which is replaced by a ternary ordered CaCus-type slab, i.e. a CeCo3B2-type slab.

For a better understanding of the Ce3C08Si structure and the large group of ternary structures, all to be interpreted as intergrown (binary and/or ternary) Laves-type and/or CaCu»-type slabs, it is useful to discuss first the corresponding binary structures.

In binary systems R - T several intermetallic compounds with composition between RT2 and RT» are formed. These can be grouped in the structure series R2m +nT4m + 5n"

The parameter m indicates the number of Laves-type slabs R2T 4 and n the number of CaCus-type slabs RT». The structures can be conveniently presented if the hexagonal structure blocks are referred to orthohexagonal axes and projected along

CRYSTAL STRUCTURES AND CRYSTAL CHEMISTRY 185 the orthohexagonal axis (perpendicular to the (11~0) plane of the hexagonal ceU) as shown in fig. 35 (Parthé and Lemaire, 1975). The members of the R2m+ùT4m+5ù structural series are obtained by stacking n RTs blocks on top of m R2T4 blocks.

However, so far only structures with m = 1 or oo are known and the formula of the series simplifies to R2+ùT4+sn. One can distinguish two subseries depending on whether subsequent R2T4 are stacked as in the hexagonal Laves phase (hexagonal subseries where all structures have space group P63/mmc) or as in the cubic Laves phase (rhombohedral subseries with space group R~m). The block stacking for both binary subseries is shown in fig. 36.

All known ternary structures are derivatives of the structures of the hexagonal subseries. In the ternary structures we also find R2T4 and RT» blocks. However, some R2T 4 blocks may be replaced by R2T3M blocks and some RT5 blocks by RT3M2 blocks.

The RzT3M block is identical to the R2T 4 block except that one T atom is replaced by an M atom. The atom arrangement in the R2T3M block, which corresponds to the Mg2Cu3Si-type, can be studied in the left hand part of fig. 17, a drawing of the 6725:Sc2C03Si structure with two R2T3M slabs per unit cell.

Height 1 112 1/4,3/4 0

The CaCu 5 and Laves phase R ( ~ ~~-'~

structure blocks ^{' ~ ' J} ^{\ -} /

T 0 0 0 '~; ~

l ~ "{"Q""i/~\

0....-o...-~.o.~0 O ~ 0 : ~ .

O2 ~~ ¢O ~ O L _

I \ ~ '"'., ( ) ,.'"" I

© ó o ó ~ 0 ,

l I I I, I

I I I

I . . . 1 / " 1 ... I._

'-" T O ' , O 0 . . O : O

O - - o - - , j - - o o T • ,_..,-- ~ _,_.ô."--,

t" : " / - ' ~ S ' f ' ~ ' - , r - ~ " : ' ~ 'c ,kJ,~.k,._)._,k.J, .J Cc°co~ ~/zCMoz~ "( "~ ~ ~J~"'-'k ) ~ ' " ~ ) ' " ~ " k ) ' - ' ( ' ~ ' J " "

... i / ~, ...

oo~p , , o o z ùL O 0 0 O O ... --- ... O

RT 5 block R2T 4 block

Fig. 35. The RT 5 and R2T 4 structure blocks used in the binary R2+nT4+sn structural series, after Parthé and Lemaire (1975). The upper part shows the CaCus structure and one half of the unit cell of the MgZn 2 structure in a projection along their hexagonal c axes. The lower part shows the structure blocks derived from the corresponding orthohexagonal unit cells projected along a short orthohexagonal axis. The RT»

block is a rectangular prism [x~33 a(CaCus) x a(CaCus) x c(CaCus)], the R2T 4 block is a parallelepiped with two right angles [base: ~ a(MgZn2)x a(MgZnz), height: ½c(MgZnz)].

R T z R T 3

n:O n:l

Hexagona[ subseries (P6z/mmc)

MgZn 2 CeNi 5

R~ R~9 RT~

n:2 n:3 n:4

Ce2Ni 7 Sm5Co19

Rhombohedm[ subseries (R3m)

[ . . . .

, ... " ~

MgCu 2 PuNi 5 Gd2Co 7 Ce5Co19

Fig. 36. Block stacking for the hexagonal and rhombohedral R2+ùT4+5~ structural series, after Parthé and Lemaire (1975). 24 denotes a R2T4 block as found in the Laves phase structures and 15 a RT5 block with an atom arrangement as in the CaCu»-type.

The RT3M 2 block is identical to the RT5 block, except that two T atoms are replaced by two M atoms, which leads, in the case of borides, to a small reduction of the height o f the block. The RT3M2 block can be seen in the last drawing of fig.

37. It corresponds to the 8340:CeC%B2 structure.

Slabs made of these different structure blocks can be intergrown in different ways, giving rise to three different ternary structural series:

(a) A

Rm+ùTsm+3ùM2ù

structural series with intergrown CaCu»-type and CeCo3B2-type slabs. The structures listed in table 12 and shown in fig. 37 all have composition code 83 .. and can be obtained by stacking m CaCus-type slabs on top of n CeCo3B2-type slabs. All structures are CaCu5-type derivative structures.

CRYSTAL STRUCTURES AND CRYSTAL CHEMISTRY 187

TABLE 12

Structural data of the R m +ùTs,ù + 3nM2n structural series, where the structures are built up of m CaCus-type slabs and n CeCo3B2-type slabs. All structures are CaCus-type derivatives.

m n Code Structure Pearson's Space

type classif, group

symbol

oo 0 8300 CaCu 5 hP6 P6/mmm

2 1 8 3 1 3 Nd3Nil3B / hP 18 P6/mmm

1 1 8 3 2 0 CeCo4B hP12 P6/mmm

1 2 8 3 2 7 C e 3 C o l l B 4 bP18 P6/mmm

1 3 8 3 3 0 Ce2Co7B 3 hP24 P6/mmm

0 oo 8 3 4 0 CeCo3B z hP6 P6/mmm

TABLE 13

Structural data for the binary structural series R2+ùT4+5ù (Mg2Zn 4 + n CaCu»), the ternary structural seiles R2+ùT4+3ùM2ù (Mg2Zu4+ n CeCo3B2) and ternary structural seiles R2+ùT3+5ùM (Mg2Cu3Si + n CaCu»). The classification symbol after Pearson and the space group are the same for corresponding

structures of the binary and ternary structural seiles.

n I{2 + ùT4 + sù R2 + nT4 + 3ùM2ù R2 + nT3 + 5ù M

Structure Pearson's Space Code Structure Code Structure

type classif, group type type

symbol

0 MgZn 2 hPl2 P63/mmc 6700 MgZn 2 6 7 2 5 Sc2Co3Si

1 CeNi 3 hP24 P6Jmmc 7 5 2 2 Dy3NiTB 2 7 5 1 1 Ce3Co8Si

2 Ce2Ni 7 hP36 P63/mmc 7 8 2 9 C e 2 C % B 2 7807 R4TI3M

3 Sm»Col9 hP48 P63/mmc 7 9 3 2 R s T I 3 M 6

4 RT4 bP56 P6Jmmc 8 0 3 3 R3TsM4

oo CaCu» bP6 P6/mmm 8 3 4 0 C e C o 3 B z 8300 CaCu»

(b) A R2+nT4+3ùM2n structural series with intergrown MgZn 2 and CeCo3B2-type slabs. In the structures listed in the middle of table 13 and shown in fig. 38 there are, stacked on top of every MgZn2-type slab, n CeCo3B2-type slabs. All structures are ternary derivatives of the structures of the binary R 2 +nT4+ 5n structural series.

(c) A R2+nT3+5ùM structural series with intergrown Mg2Cu3Si-type slabs and CaCu5-type slabs. The structures listed in the right-hand part of table 13 are composed of one ordered ternary Laves-type (i.e. Mg2Cu3Si-type) slab and n CaCus-type slabs. All structures are ternary derivatives of the structures of the binary

R2 +nT4 + 5n structural series.

In fig. 39 an enlargement of the eomposition triangle shown in figs. 1 and 2 is given together with the cornposition lines of the one binary and three ternary structural series.

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CRYSTAL STRUCTURES AND CRYSTAL CHEMISTRY 191

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7522 Dy3Ni7B2 hP24 a = 5.078 KuC, 80

P63/mmc c = 14.331 CeNi3-type (CrO, 59) derivative

Isotypic compounds:

R3Ni7B2: R = N & ), Sm b), G d a'b), Tb a,b), D y Œ»), H o a»), Er «,b), T m a,b), LU a'b), ya, b)

")KuBMSC, 79 b)KuC, 80

The Dy3Ni7B 2 structure, shown in fig. 38, is a m e m b e r of the

R2+nT4+3nM2n

structural series with n = 1 and is built up of Laves-type slabs and CeCo3B2-type slabs. It is discussed with the 7511 :Ce3C08Si structure. The tatter structure is also a CeNi3-type derivative but constructed o f intergrown Mg2Cu3Si-type slabs and CaCu»-type slabs.

Three structure types are found with composition 7533: the PrCo2Ga-type, oP8, the YPd2Si-type, oP16, and the MnCu2A1 (Heusler phase)-type, cF16, discussed with YPd2Sn.

7533 PrCo2Ga oP8 a = 5.021 Y a K , 76

or P m m a b = 4.043

PrCo2Ga t'6pl c = 6.860

Isotypic compounds: All references Y a K , 76 RCo2Ga: R = La, Pr

The PrCo2Ga structure, shown in fig. 40, can be considered as an intergrowth o f 8340:CeCo3B2-type slabs (see fig. 58) and CsCl-type slabs according to

PrCo3Ga2 + P r C o = 2 PrCo2Ga.

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Dalam dokumen Handbook on the Physics and Chemistry of Rare Earths (Halaman 178-196)