N Muts, M Manyako, W Lasocha - The tb-hf-si system at 873 k - страница 1

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Chemistry of Metals and Alloys

Chem. Met. Alloys 2 (2009) 187-193 Ivan Franko National University of Lviv www. chemetal-j ournal. org

The Tb-Hf-Si system at 873 K

Nataliya MUTS1*, Mykola MANYAKO1, Wieslaw LASOCHA2, Roman GLADYSHEVSKII1

1 Department of Inorganic Chemistry, Ivan Franko National University of Lviv,

Kyryla i Mefodiya St. 6, 79005 Lviv, Ukraine

2 Faculty of Chemistry, Jagiellonian University,

Ingardena St. 3, 30060 Krakow, Poland * Corresponding author. E-mail: NataliyaMuts@org.lviv.net

Received December 4, 2009; accepted December 23, 2009; available on-line April 27, 2010

The isothermal cross-section of the phase diagram of the ternary Tb-Hf-Si system at 873 K was constructed. The formation of two compounds was established: Tb2Hf3Si4 (Sc2Re3Si4 type, ^4^2, tP36, Z = 4; a = 0.72057(8), c = 1.3199(2) nm) and (Tb^Hf^Si (CrB type, Cmcm, oS8, Z = 4; a = 0.42241(8), b = 1.0483(2), c = 0.38227(7) nm).

Terbium / Hafnium / Silicon / Phase diagram / Crystal structure

Introduction

The purpose of this work was to study the phase equilibria in the Tb-Hf-Si system at 873 K and to determine the crystal structure of new compounds. The binary Tb-Si and Hf-Si systems which limit the investigated ternary system have been well studied and the phase diagrams over the whole concentration region have been constructed in [1] and [2], respectively. No investigation of the binary Tb-Hf system has been published. Crystallographic data of the binary compounds reported in the Tb-Si and Hf-Si systems are listed in Table 1.

Experimental details

Six binary and 41 ternary alloys were prepared by arc-melting the elements under a purified argon atmosphere. Elements of the following purities were used: Tb, 99.9%; Hf, 99.9%; and Si, 99.999%. The samples were annealed at 873 K for 1000 h in evacuated quartz tubes and subsequently quenched in cold water. The mass of each sample was 1 g. Phase analysis was carried out using X-ray powder diffraction with Debye-Scherrer technique (non-filtered Cr K radiation). The programs LATCON [19] and PowderCell-2.4 [20] were used for calculations. The crystal structures were refined from X-ray powder diffraction patterns, recorded with a DRON-2.0 M (Fe Ka radiation), HZG-4a (Cu Ka), or XPERT PRO (Cu Ka radiation) diffractometer using the program DBWS-9807 [21].

Results and discussion

During the investigation of the ternary Tb-Hf-Si system at 873 K we confirmed the existence and structure type of the following binary compounds: Tb5Si3 (structure type Mn5Si3), Tb5Si4 (Sm5Ge4), TbSi (FeB), TbSi2-x (AlB2), TbSi2-y (<x-GdSi2), Hf2Si (CuAl2), Hf5Si3 (Mn5Si3), Hf3Si2 (U3Si2), Hf5Si4 (zr5Si4), HfSi (FeB), and HfSi2 (ZrSi2).

The isothermal cross-section of the Tb-Hf-Si system at 873 K is shown in Fig. 1. The formation of two new compounds was established and their crystal structures were refined: Tb2Hf3Si4 (structure type

Sc2Re3Si4)   and   (Tb0.7Hf0.3)Si   (CrB type).

Crystallographic data for the new ternary compounds are given in Table 2.

The structures of Tb2Hf3Si4 and (Tb07Hf03)Si were refined on diffraction data from polycrystalline samples. The refinements were carried out with the full-profile Rietveld method. Cell parameters and atomic coordinates for the initial model were taken from the compounds Sc2Re3Si4 and CrB [22]. The final refinements included scale factors, zero point, cell parameters, atomic coordinates, displacement parameters, pseudo-Voigt peak profile parameters, texture parameters. The atomic coordinates for Tb2Hf3Si4 and (Tb0.7Hf0.3)Si are presented in Tables 3 and 4. Powder diagrams for some samples are shown in Figs. 2-4.

The solubility of the third component in the binary compounds of the Tb-Si and Hf-Si systems was determined. At 873 K the solid solutions based on the binary compounds Hf5Si3 and Tb5Si3 with hexagonal

Table 1 Crystallographic parameters of the binary compounds in the Tb-Si and Hf-Si systems.

Compound

Structure type

Pearson symbol

Space group

Cell parameters, nm

Reference

 

 

 

 

a

b

c

 

TbsSi3

Mn5Si3

hP16

P63/mmc

0.843

-

0.630

[1,3,4]

Tb5Si4

Sm5Ge4

oP36

Pnma

0.741

1.458

0.769

[1,5]

TbSi

FeB

oP8

Pnma

0.7919

0.3833

0.5703

[1,6,7]

Tb2Si3

V2B3

oS20

Cmcm

0.42178

2.3912

0.38230

[8]

TbSi2-x

AlB2

hP3

P6/mmm

0.3846

-

0.4143

[1,9-11]

(TbSiL67)

 

 

 

 

 

 

 

TbSi2-y

oc-GdSi2

oI12

Imma

0.398

0.407

1.337

[1,6,10,12]

(TbSi2)

 

 

 

 

 

 

 

Hf2Si

CuAl2

tI12

I4/mcm

0.6544

-

0.5173

[13,14]

Hf5Si3

Mn5Si3

hP16

P63/mcm

0.7840

-

0.5496

[14-16]

Hf3Si2

U3Si2

tP10

P4/mbm

0.6983

-

0.3672

[14,17]

Hf5Si4

Zr5Si4

tP36

P41212

0.7030

-

1.2804

[14]

HfSi

FeB

oP8

Pnma

0.6855

0.3700

0.5220

[14]

HfSi2

ZrSi2

oS12

Cmcm

0.3677

1.4550

0.3649

[14,18]

Table 2 Crystallographic parameters of the ternary compounds in the Tb-Hf-Si system.

No.

Compound

Structure

Pearson

Space

Cell parameters, nm

 

 

type

symbol

group

a

b

c

1

2

Tb2Hf3Si4

(Tb0.7Hf0.3)Si

Sc2Re3Si4 CrB

tP36 oS8

P41212 Cmcm

0.72057(8) 0.42241(8)

1.0483(2)

1.3199(2) 0.38227(7)

Table 3 Atomic coordinates for Tb2Hf3Si4 (structure type Sc2Re3Si4, Pearson symbol tP36, space group P41212, a = 0.72057(8), c = 1.3199(2) nm, Z = 4; RB = 0.079).

Atom

Wyckoff position

x

y

z

Biso,

10-2 nm2

Tb

8b

0.000(2)

0.342(2)

0.2122(7)

0.6(1)

Hf1

8b

0.156(1)

0.003(2)

0.375(2)

0.4(2)

Hf2

4a

0.167(2)

0.167(2)

0

0.4(2)

Si1

8b

0.279(7)

0.013(7)

0.177(4)

0.8(3)

Si2

8b

0.389(8)

0.324(9)

0.314(4)

0.8(3)

Si

Fig. 1 Isothermal cross-section of the phase diagram of the ternary Tb-Hf-Si system at 873 K: 1 - Tb2Hf3Si4,

2 - (Tb0.7Hf0.3)Si.

Hf5Si3

(type Mn5Si3)

Hf5Si4

(type Zr5Si4)

a = 0.69067(9), b = 0.37710(4),

c = 0.52454(6) nm; RB = 0.078 a = 0.77937(8), c = 0.55324(6) nm;

RB = 0.081

a = 0.70806(9), c = 1.2906(2) nm;

RB = 0.096

20.00 40.00

S0

00                     80.00                      100.0

Hf5Si3

a

= 0.77847(8), c = 0.55230(9) nm;

(type Mn5Si3)

R B

= 0.177

TbSiL67

a

= 0.38382(2), c = 0.41263(3) nm;

(type AlB2)

R B

= 0.146

HfSi

a

= 0.69030(8), b = 0.37661(4),

(type FeB)

c =

0.52448(5) nm; RB = 0.178

 

 

b

Fig. 2 X-ray diffraction powder patterns for samples Tb5Hf50Si45 (Fe Ka radiation, Rp = 0.071, Rwp = 0.093) (a) and Tb^Hf40Si45 (Cu Ka radiation, Rp = 0.027, Rwp = 0.035) (b).

a

HfSi2

(type ZrSi2)

TbSi1.82

(type a-GdSi2)

a =0.68966(8), b = 0.37706(4), c = 0.52391(5) nm; RB = 0.096

a = 0.36782(6), b = 1.4584(3),

c = 0.36416(6) nm; RB = 0.126 a = 0.39610(5), b = 0.40238(5),

c = 1.3367(2) nm; RB = 0.095

Hf5Si3

(type Mn5Si3)

Tb5Si3

(type Mn5Si3)

a = 0.7868(1), c = 0.5544(1) nm;

RB = 0.079

a = 0.8389(2), c = 0.6289(2) nm; RB = 0.203 b

Fig. 3 X-ray diffraction powder patterns for samples Tb20Hf20Si60 (Cu Ka radiation, Rp = 0.044, Rwp = 0.058) (a) and Tb20Hf42.5Si37.5 (Cu Ka radiation, Rp = 0.086, Rwp = 0.112) (b).

a

N. Muts etal., The Tb-Hf-Si system at 873 K

Tb0.7Hf0.3Si

(type CrB) TbSi^

(type AlB2)

Tb2Hf3Si4

(type Sc2Re3Si4)

a = 0.42192(8), b = 1.0477(2), c = 0.38254(7) nm; RB = 0.086

a = 0.38435(5), c = 0.41343(7) nm; RB = 0.051

a = 0.72057(8), c = 1.3199(2) nm; RB = 0.079

Fig. 4 X-ray diffraction powder patterns for sample Tb30Hf20Si50 (Fe Ka radiation, Rp = 0.056, Rwp = 0.071).

Table 4 Atomic coordinates for (Tb07Hf03)Si (structure type CrB, Pearson symbol oS8, space group Cmcm, a = 0.42241(8), b = 1.0483(2), c = 0.38227(7) nm, Z = 4; RB = 0.083).

Atom

Wyckoff position

x

y

z

Biso,>

10-2 nm2

Tb0.7Hf0.3

Si

4c 4c

0 0

0.3574(6) 0.083(2)

1/4 1/4

0.5(1) 1.2(5)

Mn5Si3 structure type extend up to 12 at.% Tb and 6 at.% Hf, respectively, whereas the binary compounds HfSi and TbSi with orthorhombic FeB structure type dissolve not more than 5 at.% of the third component. The unit-cell parameters of the solid solutions based on the compounds Hf5Si3 and Tb5Si3 with structure type Mn5Si3 are shown in Fig. 5. It should be noticed that, according to the literature [16], the compound Hf5Si5 with structure type Mn5Si3 occurs only when stabilized by oxygen, nitrogen or carbon. The other compounds do not dissolve significant amounts of the third component. It should be emphasized, that there are no continuous solid solutions between the binary compounds TbSi and HfSi with orthorhombic FeB structure type, or Tb5Si3 and Hf5Si3 with hexagonal Mn5Si3 structure type.

The structure type Sc2Re3Si4 is an ordered substitution variant of the Zr5Si4 type. The binary structure type Zr5Si4 is represented by a compound with composition R5M4 in such systems as {La,Ce,Pr,Nd}-Si and {Ti,Zr,Hf}-Si [23]. The superstructure Sc2Re3Si4 is realized in the systems Sc-{V,Cr,Re}-Si and {Gd,Tb,Dy,Ho,Er}-Ti-Si [24].

The structure type CrB is one of the most common structure types of inorganic compounds. Two Wyckoff positions 4c of the space group Cmcm are occupied by larger and smaller atoms, respectively. This structure type is represented by binary (for example, {Eu,Dy,Ho,Er,Tm,Yb,Lu}Si or ZrSi       [23]),       ternary       (for example,

{La,Ce,Pr,Nd,Sm,Gd}(Al0.5Si0.5), Tb(Al0.15Si0.85)

[25-27]), and quaternary compounds (for example, (Tb0.70Zr0.30)(Al0.17Si0.83) [27]). We cannot exclude the existence of a small homogeneity range for the compound (Tb0.7Hf0.3)Si with CrB structure type.

One of the interesting features of the Tb-Hf-Si system is the coexistence of the binary compound Hf5Si4 with Zr5Si4 type and of the ternary compound

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