T Krachan, B Stelmakhovych - Phase equilibria and crystal structure of the ternary compounds of the la-ag-ga system in the region up to 50 0 at of la - страница 1

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Chemistry of Metals and Alloys

Chem. Met. Alloys 3 (2010) 24-28 Ivan Franko National University of Lviv www.chemetal-j ournal. org

Phase equilibria and crystal structure of the ternary compounds of the La-Ag-Ga system in the region up to 50.0 at.% of La

Tetyana KRACHAN1, Bogdan STELMAKHOVYCH2*

1 Department of Chemistry, Faculty of Biotechnology, State Agrarian and Engineering University in Podillya,

Shevchenka St. 13, Kamyanets-Podilskyy, Ukraine

2 Department of Analytical Chemistry, Ivan Franko National University of Lviv,

Kyryla i Mefodiya St. 6, 79005 Lviv, Ukraine * Corresponding author. E-mail: bstelmakhovych@gmail.com

Received April 4, 2010; accepted June 29, 2010; available on-line November 5, 2010

Phase diagrams of the La-Ag-Ga system at 500°С (La content 0-50.0 at.%) and 300°С (La content 0-33.3 at.%) have been built using X-ray structure and phase analysis. The existence of the ternary gallide LaAg10-05Ga3 0-3 5 (BaAl4-type) has been confirmed and the atomic coordinates, displacement parameters and site occupancies have been determined. A new ternary compound, La(Ag051Ga049)11 with BaHg11-type structure (space group Pm3m, a = 0.87619(1) nm, wRp = 0.072), has been discovered at 300°С.

La-Ag-Ga alloys / Phase equilibria / Ternary intermetallic compounds / Crystal structure / X-ray diffraction

1. Introduction

Phase diagrams have been built (entirely or partially) for the RE-Ag-Ga systems where RE = Y [1], Ce [2], Tb [3], the Ce-Ag-Ga system being investigated only in the Ga-rich region. Other RE-Ag-Ga systems have been studied only with the purpose of discovering new ternary gallides of certain structure types: BaAl4, La3Aln, P-YbAgGa2, CaIn2, KHg2. None of the discovered ternary gallides contains more than 33.3 at.% of the rare earth element (RE).

A compound with variable composition LaAg0 55-0675Ga3 45-3 325 and BaAl4-type structure (a = 0.4375-0.4356, c = 1.0655-1.0729 nm) is known in the La-Ag-Ga system [4]. Taking into consideration that all the ternary compounds in the RE-Ag-Ga systems form at a RE content of up to 33.3 at.%, we carried out investigations of the La-Ag-Ga system in this region. Since in the RE-Ag-Ga systems the character of the phase equilibria varies with temperature, the experiment was carried out at two temperatures: 500°С and 300°С.

2. Experimental

Samples for the investigation were prepared by arc melting of elements with the following purity: La (99.5 wt.%), Ag (99.95 wt.%), Ga (99.99 wt.%) in purified argon atmosphere. The alloys were annealed in evacuated quartz ampoules at 500°С for 600-900 hours or at 300°С for 1200 hours and then quenched in cold water without breaking the ampoules. Samples with high Ga content were placed in molybdenum crucibles. Phase analysis was carried out using either X-ray powder diffraction patterns obtained by the Debye-Scherrer technique with nonfiltered Cr-K radiation in cameras of 57.3 mm diameter, or X-ray powder diffraction patterns recorded on a DRON-3M diffractometer in the continuous mode (Cu-Ka radiation) using the 6-26 scan technique with steps of 0.02° (26max = 120°) and exposure time of 5-30 s at every point. All calculations were performed using

CSD [5] and PC-GSAS [6] software.

3. Binary systems

According to the phase diagram of the La-Ag system, two compounds with point composition: LaAg5 (exists in two modifications with the temperature of polymorphic transformation being ~540°С) and LaAg2 form peritectically. The compounds La14Ag51 and LaAg form congruently and the former has a certain homogeneity region [7].

A generalized version of the phase diagram of the La-Ga system shows the existence of five compounds, one of which, LaGa2, forms congruently at 1450°С and has a wide homogeneity region (18-33 at.% La). LaGa6 forms peritectically at 477°С. The other compounds that exist in the system are beyond the investigated region [9].

In the Ag-Ga system a compound with 27-32 at.% of    Ga     forms     peritectically     at 609°С.

Table 1 Crystallographic data of the compounds in the binary systems La-Ag, La-Ga and Ag-Ga.

Compounds

Space group

Structure type

Lattice parameters, nm

Ref.

 

 

 

a

b

c

 

a-LaAg5

P63/mmc

MgZn2

0.55690

-

0.90775

[7]

LaAg5

F43m

AuBe5

0.796

-

-

[8]

La14Ag51

P6/m

Gd14Ag51

1.2955

-

0.9525

[7]

 

 

 

1.2935(3)

 

0.9523(4)

*

LaAg2

Imma

KHg2

0.4825

0.7287

0.8196

[7]

 

 

 

0.4832(3)

0.7285(5)

0.8204(5)

*

LaAg

Pmbm

CsCl

0.378-0.380

-

-

[7]

 

 

 

0.3811(7)

 

 

*

LaGa6

P4/nbm

PuGa6

0.6101

-

0.7696

[12]

LaGa2+x

P6/mmm

AlB2

0.4324

-

0.4410

[12]

 

 

 

0.4317(1)

 

0.4418(2)

*

Z-Ag2Ga(H)

P63/mmc

Mg

0.28869

-

0.46753

[13]

Z'-Ag2Ga(L)

P62m

Mg2In

0.77460

-

0.28704

[13]

 

 

 

0.7763(2)

 

0.2881(2)

*

AgGa

Pmbm

CsCl

0.3171

-

-

[11]

* our results

Table 2 Crystallographic data of some La-Ag-Ga ternary alloys annealed at 500°C.

Composition, at.%

Phase analysis

Lattice parameters, nm

La

Ag

Ga

 

a

b

c

10

45

45

La(Ag,Ga)4

0.4383(3)

-

1.0612(7)

 

 

 

Ag2Ga

0.7764(5)

-

0.2872(3)

20

10

70

La(Ag,Ga)4

0.43817(1)

-

1.06104(5)

20

15

65

La(Ag,Ga)4

0.43517(3)

-

1.0738(1)

20

20

60

La(Ag,Ga)4

0.43543(1)

-

1.07734(1)

25

50

25

La(Ag,Ga)4

0.4348(1)

-

1.0765(1)

 

 

 

La(Ag,Ga)2

0.492(1)

0.721(1)

0.813(3)

33.3

6

60.7

La(Ga,Ag)2

0.43158(7)

-

0.4423(1)

33.3

20

46.7

La(Ag,Ga)2

0.4721(4)

0.7298(5)

0.8107(6)

 

 

 

La(Ga,Ag)2

0.43161(6)

-

0.4426(2)

33.3

30.7

36

La(Ag,Ga)2

0.4723(2)

0.7302(4)

0.8112(5)

33.3

35.7

31

La(Ag,Ga)2

0.4753(4)

0.7312(2)

0.8107(3)

33.3

48.7

18

La(Ag,Ga)2

0.4822(2)

0.7349(2)

0.8143(2)

33.3

56.7

10

La(Ag,Ga)2

0.4795(5)

0.7435(8)

0.8113(8)

The compound undergoes a polymorphic transformation at 432°С. Another compound, AgGa, forms peritectically at 302°С. The maximum solubility of Ga in Ag is ~18 at.% at 380°С and decreases down to 11.8 at.% Ga at 200°С [10,11].

Summarized data on the crystal structure of the binary compounds in the systems that bound the investigated La-Ag-Ga system (Ag-LaAg2-LaGa2-Ga region) are shown in Table 1. Existence of the compound AgGa was not confirmed at the investigated temperatures.

4. Results and discussion 4.1 Phase equilibria

The phase diagram of the investigated part of the La-Ag-Ga system is shown in Fig. 1(a,b). At 500°С we found a considerable solid solution based on the LaAg2 compound, which reaches the composition of LaAg1.05Ga0.95. Lattice parameters of some typical samples containing the solid solution LaAg2-xGax are shown in Table 2. The solubility of gallium in the LaAg compound does not exceed ~22 at.%. The composition limit of the solid solution is defined as LaAg0.56Ga0.44 (a = 0.3792(5) nm). The unit cell volume of the solid solution decreases with increasing substitution of Ga for Ag, which is in agreement with the atomic dimensions of the two elements. The other binary compounds do not dissolve noticeable amounts of the third element.

The ternary gallide La(Ag,Ga)4 has a homogeneity region (LaAg1.0-0.5Ga3.0-3.5) that agrees well with previously obtained data [4]. At 500°С the compound La(Ag,Ga)4 is in equilibrium with all the binary

Fig. 1 Part of the phase diagram isothermal section of the of the La-Ag-Ga system at 300°C and 500°C.

compounds of the investigated part of the system (Fig. 1(a)), which may indicate its congruent formation.

Phase analysis of the Ga-rich part of the system at 300°С (Fig. 1(b)) revealed that, with the decrease of temperature, one more compound, La(Ag,Ga)11 with a cubic structure of the BaHg11-type (a = 0.87649(2) nm) and a small homogeneity region, forms.

4.2 Crystal structure of La(Ag018Ga0.82)4

It is known that the La(Ag,Ga)4 compound has a BaAl4-type structure [4]. A refinement of the atomic coordinates and distribution of the atoms in the structure was carried out. These data are shown in Table 3, and the interatomic distances in the structure are shown in Table 4.

The BaAl4-type belongs to the class of structures with tetragonal-antiprismatic coordination of the smallest atoms. In the structure of La(Ag,Ga)4 these atoms are Ga. The tetragonal antiprisms are formed by La atoms and a statistical mixture of smaller atoms (Ag+Ga). The interatomic distances in the structure are   close   to   the   sum   of  the   atomic radii: rLa = 0.1877 nm, rGa = 0.139 nm, rAg = 0.1444 nm [14]. Slight shortening of the interatomic distances between Ga atoms in 4(e) position (10 %) can indicate the appearance of partially covalent interaction between these atoms. In the structure of La(Ag,Ga)4 one can observe partial ordering of Ag and Ga atoms, since only Ga atoms center antiprisms, forming Ga2 pairs.

4.3 Crystal structure of the new compound La(AgQ.51Ga0.49)n

The compound La(Ag0.51Ga0.49)11 was detected only in samples that were annealed at 300°С: It has a point composition with almost equal amounts of Ag and Ga. The structure of the compound belongs to the BaHg11-type. The conditions under which the structure was determined, atomic coordinates and the distribution of the atoms are shown in Table 5, and the interatomic distances in Table 6.

The BaHg11-type belongs to the class of structures with coordination polyhedra (СР) similar to the 10-vertex polyhedron characteristic of the MnAl6-type (Т2 atoms) [15]. The deformed icosahedron in the structure of La(Ag0.51Ga0.49)11 corresponds to the position  8(g).  Such coordination  is typical for

Table 3 Refinement of the crystal structure of La(Ag0.18Ga0.82)4.

Composition Structure type Space group Lattice parameters, nm

Cell volume, nm3 Number of atoms in cell Calculated density, g/cm3 Radiation, nm Mode of refinement

26    , °

max

Scale factor

R

La(Ag0.18Ga0.82)4

BaAl4

I4/mmm (No. 139)

a = 0.43824(1), c = 1.06099(4) 0.20377(2) 10

7.256(1)

Cu-Ka, 0.154185 Full-profile (CSD) 120

0.6797(8)

RI = 0.055, Rp = 0.095

Atoms

WP

x

y

z

Biso-102, nm2

La

2(a)

0

0

0

0.48(5)

T1(2.56(3)Ga+1.44(3)Ag)

4(d)

0

1/2

1/4

0.56(5)

Ga

4(e)

0

0

0.3834(3)

0.85(8)

Table 4 Interatomic distances (5) for La(Ag018Ga082)4.

Atoms

La

-8 Ga

-8 T1 -4 Ga

-4 T1

-4 La -1 Ga

-4 T1 -4 La

5, nm 0.3337(1) 0.3440(1) 0.2609(2)

0.3099(1)

0.3440(1) 0.2474(5)

0.2609(2)

0.3337(1)

Table 5 Refinement of the crystal structure of new ternary compound La(Ag0.51Ga0.49)11.

Composition Structure type Space group Lattice parameter, nm Cell volume, nm3 Number of atoms in cell Calculated density, g/cm3 Radiation, nm Mode of refinement

26    , °

max

Scale factor

_R_

Atoms

La

T1(0.37(2)Ag+0.63(2)Ga)

Ag

T2(0.83(3)Ag+11.17(3)Ga) T3(7.71(3)Ag+4.29(3)Ga)

WP

3(d) 1(b) 8(g) 12(i)

12( j )

La(Ag0.51Ga0.49)n

BaHg11

Pm3m (No. 221) a = 0.87649(1)

0.6734(1) 36

8.293(1)

Cu-Ka, 0.154185

Full-profile (PC-GSAS)

120

2.9542

x

y

z

Biso-102, nm2

1/2

0

0

0.51(2)

1/2

1/2

1/2

0.51(3)

0.1755(2)

x

x

0.99(3)

0

0.3490(3)

y

0.69(3)

1/2

0.2671(3)

y

0.58(3)

Т. Krachan, В. Stelmakhovych, Phase equilibria and crystal structure of the ternary compounds ...

Table 6 Interatomic distances (5) for La(Ag0.5iGa0.49)n.

Atoms

5, nm

La

-4 Т3

0.3311(3)

 

-8 Т2

0.3333(3)

 

-8 Ag

0.3581(2)

Т1

-12 Т3

0.2887(3)

Ag

-3 Т2

0.2644(3)

 

-3 Т3

0.3062(2)

 

-3 Ag

0.3076(3)

 

-3 La

0.3581(4)

Т2

-2 Ag

0.2644(3)

 

-2 Т2

0.2647(4)

 

-4 Т3

0.2783(3)

 

-2 La

0.3333(3)

Т3

-4 Т2

0.2783(3)

 

-1 Т1

0.2887(3)

 

-4 Т3

0.2887(3)

 

-2 Ag

0.3062(2)

 

-1 La

0.3311(3)

transition metal, Cu, and Ag atoms in compounds with rare earth elements. In the investigated structure this position is the only one, which is occupied solely by Ag atoms, whereas the other positions are occupied by a statistical mixture of Ag and Ga atoms.

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T Krachan, B Stelmakhovych - Phase equilibria and crystal structure of the ternary compounds of the la-ag-ga system in the region up to 50 0 at of la