The Pr–Ag–Al system

Journal of Alloys and Compounds 291 (1999) 175–180

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The Pr–Ag–Al system O.V. Zhak, Yu.B. Kuz’ma* Department of Analytical Chemistry, the Ivan Franko State University of Lviv, Kyryla and Mefodia Str. 6, 290005, Lviv, Ukraine Received 9 April 1999; received in revised form 18 May 1999; accepted 20 May 1999

Abstract The isothermal section of the Pr–Ag–Al phase diagram at 870 K over the whole concentration region has been determined using X-ray diffraction. The solubility of the third component in the binary compounds PrAg 2 , PrAg, Pr 3 Al 11 and PrAl 2 and the homogeneity ranges of the ternary phases Pr 2 (Ag,Al) 17 , Pr(Ag,Al) 5 , Pr(Ag,Al) 4 have been established. The existence of earlier known ternary compounds Pr 2 (Ag,Al) 17 (Th 2 Ni 17 and Th 2 Zn 17 -type structures), Pr(Ag,Al) 4 (BaAl 4 -type structure) and Pr(Ag,Al) 3 (PuNi 3 -type structure) has been confirmed and five new ternary compounds Pr(Ag 0.54 Al 0.46 ) 11 (BaCd 11 -type, a51.10262(3), c50.70979(2) nm, R50.057), Pr(Ag 0.70 Al 0.30 ) 5 (SmAg 3.5 Al 1.5 -type, a50.54498(8), c50.9332(2) nm), Pr(Ag 0.60 Al 0.40 ) 5 (DyAg 2.4 Al 2.6 -type, a50.9321(2), c5 0.9582(2) nm), Pr(Ag 0.45 – 0.56 Al 0.55 – 0.44 ) 5 (CaCu 5 -type, a50.5506(3), c50.4417(2) nm, R50.093), Pr(Ag 0.26 Al 0.74 ) 2 (AlB 2 -type, a50.4216(2), c50.42128(7) nm, R F 50.031) have been found. The atomic parameters and statistical distribution of silver and aluminum atoms in the Pr 2 (Ag 0.43 – 0.52 Al 0.57 – 0.48 ) 17 (Th 2 Ni 17 -type, R50.074) structure have been refined.  1999 Elsevier Science S.A. All rights reserved. Keywords: Phase diagram; Pr–Ag–Al system; Crystal structure

1. Introduction The Pr–Ag–Al system has not been studied over the whole concentration region. Partial investigations [1–5] have shown the formation of a few ternary compounds, but no homogeneity ranges, atomic coordinates and mode of Ag and Al atoms distribution in the crystal structures were reported. The lattice parameters for these compounds are listed in Table 1. Proceeding from the great similarity of the Ln–Ag–Al systems, where Ln represents a light rare earth metal, one can expect the formation of a number of new ternary compounds in the Pr–Ag–Al system like that for La, Ce, Nd and Sm containing systems. Because silver and aluminum have very close values of atomic radii (rAg 50.1445, rAl 50.1432 nm [6]) these atoms can be easily substituted one by the other in the crystal structures. This leads to the formation of solid solutions for the binary compounds and of considerable homogeneity ranges for the ternary compounds in the Ln–Ag–Al systems. Therefore, the study of the Pr–Ag–Al phase equilibrium diagram over the whole concentration range, synthesis and determination of the crystal structures and the limit compositions of the new ternary compounds were the main goals of our investigation. *Corresponding author.

The binary systems at the boundaries of the Pr–Ag–Al system have been studied widely. According to the phase diagram [7], the binary compound (d-phase) with the hexagonal Mg-type structure and a considerable homogeneity range exists in the Ag–Al system at 870 K. The intermediate b-phase with the cubic W-type structure decomposes eutectoidally at 867 K, and the m-phase (bMn type structure) exists only below 723 K. Therefore, the two last compounds (b- and m-phases) were not observed at the temperature of our investigation. The summarized Pr–Ag phase diagram is presented in Ref. [8]. The compound PrAg 5 with a constant composition is formed peritectically at 1103 K and, probably, has the CaCu 5 -type structure. The compound Pr 14 Ag 51 congruently melts at 1306 K; its structure belongs to the hexagonal Gd 14 Ag 51 -type and has a small homogeneity region. The compound PrAg 2 with a constant composition is formed peritectically at 1120 K. Above 890 K it crystallizes with the hexagonal AlB 2 -type structure and below 890 K — with the orthorhombic CeCu 2 -type structure. The compound PrAg (CsCl-type structure) is formed from the melt at 1205 K. According to the Pr–Al phase diagram [9], the compound PrAl 2 with the cubic MgCu 2 -type structure congruently melts at 1753 K, and the compound Pr 3 Al peritectoidally decomposes at 903 K. All other compounds

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O.V. Zhak, Y.B. Kuz’ ma / Journal of Alloys and Compounds 291 (1999) 175 – 180

are formed by peritectic reactions. The high temperature modification of Pr 3 Al 11 has the tetragonal BaAl 4 -type structure, which transforms to the orthorhombic La 3 Al 11 type structure below 1238 K. The compound PrAl 3 has an Mg 3 Cd-type structure and Pr 2 Al is of PbCl 2 -type structure. The compound PrAl has a CeAl-type structure above 973 K and DyAl-type structure below this temperature. Two allotropes of Pr 3 Al compound exist. The hexagonal Mg 3 Cd-type forms below 603 K and the cubic AuCu 3 -type forms above this temperature. Therefore, in the binary systems a great number of compounds are formed, and because of this we can expect a complex character of the interaction in the ternary Pr– Ag–Al system.

2. Experimental details In the Pr–Ag–Al system we have studied 90 alloys including the binary ones. The starting materials for the sample preparations were praseodymium (99.5 wt.% pure), silver and aluminum (both of 99.99 wt.% pure). Pieces of metals were arc melted using a tungsten electrode in an atmosphere of purified argon and then annealed in evacuated quartz ampoules at 870 K during 600 h. The samples containing more than 50 at.% Ag or 45 at.% Pr were heated for 1000 h. The annealed alloys were quenched in cold water without breaking the ampoules. A phase analysis was carried out using X-ray powder diffraction patterns obtained on a powder DRON 3M diffractometer in the continuous mode (Cu K a -radiation). The lattice parameters and crystal structure of the ternary

compounds were refined using diffraction data collected by a u –2u scan technique with steps of 0.058 in 2u and exposition times of 20–30 s at every point. The limits of the solid solutions were determined as described in Ref. [10]. All calculations were performed using a CSD software [11].

3. Results

3.1. Equilibrium diagram The phase diagram of the Pr–Ag–Al system at 870 K is shown in Fig. 1. All known binary compounds, except PrAg 5 , have been confirmed at the temperature investigated. Considerable solid solutions are formed for the binary compounds PrAg 2 , PrAg, Pr 3 Al 11 and PrAl 2 . Their limit compositions and the lattice parameters are listed in Table 1. All other binary compounds dissolve less then 5 at.% of the third component. At 870 K the existence of the earlier reported ternary compounds with crystal structures belonging to the Th 2 Ni 17 , Th 2 Zn 17 , BaAl 4 and PuNi 3 -types [1–5] has been confirmed. Moreover, for the Th 2 Ni 17 , Th 2 Zn 17 and BaAl 4 -type compounds the limit compositions of the homogeneity ranges have been established (Table 1). It was observed that lattice parameters of the ternary compounds are practically constant at the limits of the homogeneity ranges. The ternary compound with the AlB 2 -type structure was investigated using a single crystal selected from the melted

Fig. 1. Isothermal section of the Pr–Ag–Al system at 870 K.

O.V. Zhak, Y.B. Kuz’ ma / Journal of Alloys and Compounds 291 (1999) 175 – 180

177

Table 1 Crystallographic data for the composition limits of the solid solution of binary compounds and ternary compounds in the Pr–Ag–Al system N

Phase

a

– –a –a –a 1

Pr(Ag 0.68 Al 0.32 ) 2 PrAg 0.60 Al 0.40 Pr 3 (Ag 0.13 Al 0.87 ) 11 Pr(Ag 0.30 Al 0.70 ) 2 Pr(Ag 0.54 Al 0.46 ) 11

Structure type CeCu CsCl La 3 Al 11 MgCu 2 BaCd 11

Pr 1.6 Ag 7.5 Al 9.5 2

Th 2 Ni 17 Pr 1.6 (Ag 0.43 – 0.52 Al 0.57 – 0.48 ) 17 Pr 2 Ag 10 Al 7

3

Th 2 Zn 17 Pr 2 (Ag 0.57 – 0.68 Al 0.43 – 0.32 ) 17

4 5 6

Pr(Ag 0.70 Al 0.30 ) 5 Pr(Ag 0.60 Al 0.40 ) 5 Pr(Ag 0.45 – 0.56 Al 0.55 – 0.44 ) 5

SmAg 3.5 Al 1.5 DyAg 2.4 Al 2.6 CaCu 5

PrAg 0.7 Al 3.3 7

BaAl 4 Pr(Ag 0.20 – 0.23 Al 0.80 – 0.77 ) 4

–d

Pr 6 Ag 13 Al 10

Th 6 Mn 23

PrAgAl 2 8

Lattice parameters, nm

Ref.

a

b

c

0.4695(2) 0.3738(2) 0.4296(3) 0.80299(3) 1.10262(2)

0.7312(5)

0.7984(3)

1.3074(7)

1.0137(5) 0.70979(2)

–b –b –b –b –b

0.9357

0.9102

[1]

0.93484(2)c

0.91033(3)

–b

0.9417

1.3673

[2,3]

0.94129(5)c

1.3660(1)

–b

0.54498(8) 0.9321(2) 0.5506(3)c

0.9332(2) 0.9582(2) 0.4417(2)

–b –b –b

0.4303

1.1067

[4]

0.4315(6)c

1.0865(9)

–b

1.3271

[3]

0.5632

2.7045

[3,5]

0.56292(7)

2.6915(5)

–b

0.4216(1)

0.42128(7)

–b

PuNi 3 Pr(Ag 0.33 Al 0.67 ) 3

–d

Pr(Ag 0.26 Al 0.74 ) 2

AlB 2

a

The composition limits of the solid solutions on the base of binary compounds. Data of this work. c Lattice parameters are listed for the compositions: Pr 2 Ag 8.7 Al 8.3 , Pr 2 Ag 11.4 Al 5.6 , PrAg 2.42 Al 2.58 , PrAg 0.9 Al 3.1 . d Compound was not found at 870 K. b

sample. However, this compound was not observed in the annealed samples. Four new ternary compounds with the BaCd 11 , CaCu 5 , DyAg 2.4 Al 2.6 and SmAg 3.5 Al 1.5 -type structures have been obtained in the Pr–Ag–Al system at 870 K for the first time. A considerable homogeneity range was found to exist only for the compound of the CaCu 5 -type structure (Table 1).

3.2. Ternary compounds and their crystal structures The compound PrAg 5.9( 1) Al 5.1( 1) has a tetragonal structure of the BaCd 11 -type. The refinement of the atomic positional and thermal parameters using the powder full profile diffraction data in the range 2u 515–1308 (Cu K a -radiation), taking into the account texture (the texture parameter along [111] axis is equal to 0.63(3)), resulted in the residuals: R i 50.057, R p 50.113. Finally we have established the atomic distribution in this structure as shown in Table 2. The shortest interatomic distances (dPr – T3 50.3365(2), dT1 – T2 50.28958(2), dT2 – T3 5 0.2711(2) and dT3 – T3 50.2623(3) nm) are close to the corresponding sum of the classical metallic radii of the components.

By the analogy with the compound Ce 22x (Ag 0.46 Al 0.54 ) 17 Ag y (x 5 0.39, y 5 0.18) for the isotypic compound in the Pr–Ag–Al system the composition Pr 1.6 Ag 7.5 Al 9.5 was assigned [1]. Because of that we carried out the refinement of the atomic parameters and occupancies in the last structure which resulted in the residuals R i 50.074 and R p 50.139 (hexagonal structure of Th 2 Ni 17 -type, 2u 516–1308, texture parameter along [100] axis is equal to 0.86(2)). The final atomic positional and thermal parameters are listed in Table 2. In contrast to the isotypic compound Ce 1.6 (Ag,Al) 17 , in the praseodymium compound all crystallographic positions are fully occupied and its composition can be described by the formula Pr 2 Ag 8.7( 1) Al 8.3( 1) . The isotropic thermal parameter for the Pr atoms in the 2(b) position are considerably higher than for the Pr atoms in the 2(d) position. The shortest interatomic distances in this structure are dPr – T1 5 0.3220(5), dT1 – T1 50.2664(7) and dT4 – T4 50.2668(5) nm. The crystal structure of the compound Pr(Ag 0.70 Al 0.30 ) 5 which is isotypic with Sm(Ag 0.73 Al 0.27 ) 5 (new structure type) is the subject of a separate publication. Since the atomic coordinates and atomic distribution for the NdAg 3.26 Al 1.89 structure have been established using the X-ray single crystal method [14], we assumed these

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Table 2 Atomic coordinates and isotropic thermal parameters of the PrAg 5.9( 1 ) Al 5.1( 1 ) , Pr 2 Ag 8.7( 1 ) Al 8.3( 1 ) , PrAg 2.42( 6 ) Al 2.58( 6 ) and PrAg 0.52( 1 ) Al 1.48( 1 ) structures B 310 2 , nm 2

Compound composition, structure type and distribution of atoms

Space group and site positions

Coordinates x

y

z

PrAg 5.9( 1 ) Al 5.1( 1 ) , BaCd 11 4Pr 4T1(3.25(4)Ag10.48(4)Al) 8T2(3.80(7)Ag14.20(7)Al) 32T3(16.3(2)Ag115.7(2)Al)

I41 /amd 4(a) 4(b) 8(d) 32(i)

0 0 0 0.1190(2)

3/4 1/4 0 0.4554(2)

1/8 1/8 1/2 0.1730(3)

0.9(1) 0.8(1) 1.4(2) 1.1(1)

Pr 2 Ag 8.7( 1 ) Al 8.3( 1 ) , Th 2 Ni 17 2Pr1 2Pr2 4T1(2.00(4)Ag12.00(4)Al) 6T2(2.58(6)Ag13.42(6)Al) 12T3(6.23(8)Ag15.77(8)Al) 12T4(6.58(8)Ag15.42(8)Al)

P63 /mmc 2(b) 2(d) 4(f) 6(g) 12(j) 12(k)

0 1/3 1/3 1/2 0.3281(5) 0.1633(3)

0 2/3 2/3 0 0.9614(4) 0.3266(3)

1/4 3/4 0.1037(5) 0 1/4 0.9801(4)

1.4(1) 0.8(1) 1.2(1) 2.4(3) 2.2(2) 1.8(1)

PrAg 2.42( 6 ) Al 2.58( 6 ) , CaCu 5 Pr 2T1(2Ag) 3T2(0.42(6)Ag12.58(6)Al)

P6 /mmm 1(a) 2(c) 3(g)

0 1/2 1/3

0 0 2/3

0 1/2 0

0.9(1) 1.6(7) 1.5(1)

PrAg 0.52( 1 ) Al 1.48( 1 ) , AlB 2 Pr 2T(0.522(6)Ag11.478(6)Al)

P6 /mmm 1(a) 2(d)

0 2/3

0 1/3

0 1/2

0.621(6) 0.743(9)

parameters for the isotypic compound Pr(Ag 0.60 Ag 0.40 ) 5 without a complete crystal structure determination. The lattice parameters of the latter compound are listed in Table 1. The compound Pr(Ag,Al) 5 with the CaCu 5 -type structure has a variable composition. The refinement of the atomic parameters for 22 unique hkl reflections (the texture parameter along [111] axis is equal to 0.24(4)) resulted in a residual of R50.093. As it is shown in Table 2, in this structure a partially ordered distribution of Ag and Al atoms is observed. The shortest interatomic distances in this structure are dPr – T1 50.3179(3) and dT1 – T2 50.2721(3) nm. The crystal structure of the compound PrAg 0.52( 1) Al 1.48( 1) (AlB 2 -type structure) has been studied using X-ray single crystal diffraction (Laue method, rotation, De Jong Bouman; single crystal DARCH diffractometer, Mo K a -radiation, 132 unique hkl reflections with Fhkl . 4s Fhkl ; R F 5 0.031, R w 5 0.034). The final atomic positional and thermal parameters are listed in Table 2. The shortest interatomic distances in the PrAg 0.52 Al 1.48 structure are dPr – T 50.3219(1) and dT – T 50.2434(1) nm. Some information about the crystal structure determination of this compound has already been published in Ref. [12].

4. Discussion The present study of the interaction between the components in the Pr–Ag–Al system is a continuation of

systematic investigations of the Ln–Ag–Al systems, where Ln represents a light rare earth metal. The isothermal sections of the systems La–Ag–Al [13], Ce–Ag–Al [14], Pr–Ag–Al, Nd–Ag–Al [15] and Sm–Ag–Al [10] have been studied over the whole concentration region. The Eu–Ag–Al system was studied only for obtaining the ternary compounds of definite structure types. Summarized data concerning the number of ternary compounds in the above mentioned systems and their crystal structures are listed in Table 3. From six to nine ternary phases are known in each of the investigated Ln–Ag–Al systems, and the maximum number of compounds is found in the Pr–Ag–Al system. The compound PrAg 0.52 Al 1.48 probably exists at high temperature or it is a metastable phase. This conclusion is based on the fact that a sample of the corresponding composition, annealed at 870 K, contained a solid solution of silver in the binary compound PrAl 2 (MgCu 2 -type structure). In all of the Ln–Ag–Al systems (Ln — light rare earth metal) the binary compounds LnAg (CsCl-type structure), LnAg 2 (CeCu 2 -type structure), LnAl 2 (MgCu 2 -type structure) and Ln 3 Al 11 (La 3 Al 11 -type structure) formed solid solutions. The solubility of aluminum in the LnAg compounds (CsCl-type structure) reaches 20–30 at.% and increases from La to Nd. The solubility of Al in the binary compounds LnAg 2 (CeCu 2 -type structure) is also about 20–30 at.% of Al for systems containing La, Ce, Pr or Nd. Even though the binary compound with CeCu 2 -type structure does not exist in the Sm–Ag system, in the Sm–Ag– Al system a ternary compound with a considerable homo-

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179

Table 3 Structure types (ST) of the ternary compounds in the Ln–Ag–Al systems and coordination numbers (CN) of atoms in the crystal structures Ln ST BaHg 11 BaCd 11 Th 2 Ni 17 Th 2 Zn 17 SmAg 3.5 Al 1.5 DyAg 2.4 Al 2.6 CaCu 5 BaAl 4 Th 6 Mn 23 PuNi 3 CeCu AlB 2

La

Ce

Pr

Nd

Sm

Eu

a

[3] [1,3,14] a [1,2,14] a [2,3,14] a

[1,3,13] a [1,2,12] a [2,3,12] a

b

[1] a,b [2,3] a,b b

[15] a,b [3,13,16] [12] a [14] a

a

[14,16] [4,14] a

a

[3,5,15] a

b

[4] a,b [3] a [3,5] a,b b

[1,15] a [15] a [15] a [15,16] a [15] a [15,17] a

[1,10] a [10] a [10] a [10,16] a

[1] a ?c ?c ?c ?c [16] a

[13] a

[10] a [10] a

?c ?c

CN Ln

T

20 22 20 19 16 16 20 22 17 20,16 20 20

12,10 14,12 14,13,12 14,13,12 12 12,9 12 12,9 14,13,12 12 10 9

a

Compound exists, reference in square brackets. Data of this work also. c Data are absent. b

geneity range is formed. The solubility of Ag in the binary rare earth aluminides increases from lanthanum to samarium. The other binary compounds of the Ln–Ag and Ln–Al systems form insufficient solid solutions. Data concerning the extent of the solid solution ranges of the binary compounds are summarized in Fig. 2. All ternary compounds in the Ln–Ag–Al systems are formed in the concentration region of low rare earth content, not exceeding 33 at.%. In all crystal structures of the Ln–Ag–Al compounds the Ln atoms are characterized by larger coordination numbers (CNs) (from 16 to 22) compared to those for the Ag and Al atoms (or their statistical mixture). The latter atoms have CNs of 14, 13, 12, 10 or 9. The majority of structures with CNs of 12 for the smaller atoms belongs to the structure group with the icosahedral coordination. These structure types are as follows, BaCd 11 , Th 2 Ni 17 , Th 2 Zn 17 , CaCu 5 , SmAg 3.5 Al 1.5 , Th 6 Mn 23 and PuNi 3 . The structures of the BaHg 11 -type have coordination polyhedra (CPs) of the smaller atoms in the form of 10-vertices

polyhedra like those for Al atoms in the MnAl 6 structure. The CPs of Ag and Al atoms in the structures of the BaAl 4 -type are in the form of an Archemedean cube with one square face capped by an additional atom. In the structure types DyAg 2.4 Al 2.6 , CeCu 2 and AlB 2 the smaller size atoms center trigonal prisms with additional atoms outside the rectangular faces (CN 9) or outside the rectangular faces and one of the bases of the prisms (CN 10). In general, one can observe a tendency to decreasing CNs of the smaller atoms when the rare earth content in the ternary compound increases. The dependence of the unit cell volumes of the ternary Ln–Ag–Al compounds versus the atomic number of Ln opens the possibility to estimate the valence state of the rare earth atoms in the compounds. As one can see from Fig. 3, deviations from a linear dependence of the unit cell volumes are observed for europium compounds with BaCd 11 and CaCu 5 -type structures, which may indicate an intermediate valency of the europium atoms lying between 12 and 13. Less clear deviations to smaller values of the

Fig. 2. Extension of the solid solutions on the base of binary compounds with the structures of CsCl, CeCu 2 , MgCu 2 and La 3 Al 11 -types in the Ln–Ag–Al systems.

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Fig. 3. Unit cell volumes of the ternary lanthanoid silver aluminides (* — per one formula unit).

unit cell volumes for the cerium compounds (Th 2 Zn 17 and PuNi 3 -type structures) permit it to suppose that the intermediate valency of cerium atoms is between 14 and 13.

References [1] O.V. Denysyuk, B.M. Stel’makhovych, Yu.B. Kuz’ma, J. Solid State Chem. 109 (1994) 172.

¨ [2] G. Cordier, G. Dorsam, R. Henseleit, A. Mehner, S. Thies, C. Geibel, J. Alloys Comp. 186 (1992) 161. ¨ [3] G. Cordier, G. Dorsam, R. Kniep, J. Magn. Magn. Mater. 76–77 (1988) 653. ¨ [4] G. Cordier, G. Dorsam, C. Rohr, J. Less-Common Metals 166 (1990) 115. ¨ [5] G. Cordier, E. Czech, G. Dorsam, R. Henseleit, A. Mehner, S. Thies, C. Geibel, J. Less-Common Metals 169 (1991) 55. [6] N. Wiberg, in: Lehrbuch der Anorganischen Chemie, Walter de Gruyter, Berlin, 1995, pp. 1838–1841. [7] A.J. McAlister, Bull. Alloy Phase Diagr. 8 (1987) 526. [8] K.A. Gschneidner Jr., F.W. Calderwood, Bull. Alloy Phase Diagr. 6 (1985) 23. [9] K.A. Gschneidner Jr., F.W. Calderwood, Bull. Alloy Phase Diagr. 10 (1989) 31. [10] O.V. Zhak, B.M. Stel’makhovych, Yu.B. Kuz’ma, J. Alloys Comp. 237 (1996) 144. [11] L.G. Akselrud, Yu.N Gryn, P.Yu. Zavalij, V.K. Pecharsky, V.S. Fundamensky, in: Collected Abstracts 12th European Crystallographic Meeting, Moscow, August, 1989, Vol. 3, Izv. Acad. Nauk SSSR, Moscow, 1989, p. 155. [12] O.V. Zhak, B.M. Stel’makhovych, in: Red Book. Constitutional Data and Phase Diagrams of Metallic System, Vol. 41, Materials Science International Services GmbH, Stuttgart, Germany, 1999, pp. 629– 630. [13] Yu.B. Kuz’ma, O.V. Zhak, S.Yu. Shkolyk, Dopov. Acad. Nauk Ukr. 3 (1995) 101. [14] O.V. Zhak, B.M. Stel’makhovych, Yu.B. Kuz’ma, Russ. Metall. 6 (1995) 158. [15] Yu.B. Kuz’ma, O.V. Zhak, O.S. Sarapina, Russ. Metall. 2 (1997) 166. [16] O.V. Denysyuk, B.M. Stel’makhovych, Yu.B. Kuz’ma, Russ. Metall. 5 (1993) 213. [17] O.V. Denysyuk, B.M. Stel’makhovych, Yu.B. Kuz’ma, Visnyk L’viv Derzh. Univ. Ser. Khim. 33 (1994) 24.