R Matvijishyn, P Demchenko, Y Gorelenko - Crystal structure and some magnetic properties - страница 1

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ВІСНИК ЛЬВІВ. УН-ТУ

Серія хім. 2008. Bun. 49. Ч. 1. С. 43-49

VISNYK LVIV UNIV. Ser. Khim. 2008. No 49. Part 1. P. 43-49

УДК 669.866.74.782:548.3:537.214

CRYSTAL STRUCTURE AND SOME MAGNETIC PROPERTIES OF Er1-x(Nia818Sia182)s+2x (x = 0.08) PHASE WITH TbCu7 STRUCTURE

R. Matvijishyn1, P. Demchenko1, Yu. Gorelenko1, V. Pavlyuk1,2, E. Rozycka-Sokolowska2, B. Marciniak2

1 Ivan Franko National University of Lviv, Kyryla & Mefodiya Str., 6, 79005 Lviv, Ukraine

2 Institute of Chemistry and Environment Protection, Jan Dlugosz University, al. Armii Krajowej, 13/15, 42-200 Czestochowa, Poland

The crystal structure of the new phase Er0 920(3)Ni422(2)Si0 94(2) has been determined by X-ray analysis of a single crystal that was extracted from the Er16Ni69Si15 alloy (Xcalibur3TM CCD diffractometer, 85 unique reflections, R1 = 0.0261, wR2 = 0.0655, S = 1.214). This phase crystallizes in the TbCu7 structure type: space group P6/mmm, hP8-1.92, with lattice parameters a = 4.843(1), c = 3.9897(12) A. For the alloy of Er16Ni69Si15 composition the paramagnetic susceptibility fits the Curie-Weiss law above 80 K with the Weiss parameter 6*p = 14.5 K and effective magnetic moment /4ff = 9.75(8) //B/Er-atom. The Curie point, found from the extrapolated /iS2(T) dependence, is equal

to 12.5 K.

Key words: transition metal alloys and compounds, crystal structure, magnetisation, magnetic measurements, X-ray diffraction.

Intermetallic compounds consisting rare earth and 3d-metals attract special attention due to their interesting properties: heavy fermions, Kondo systems, co-existence of magnetism and superconductivity, high coercivity etc. Magnetic intermetallic compounds of rare earth- and 3d-metals play an essential role in the power permanent magnets production. Presently, industry extensively produces permanent magnets based on SmCo5, Sm2Co17 and Nd2Fe14B. The binary phases RET5 (where RE is a rare earth metal and T is a 3d-metal) crystallize mostly in a hexagonal CaCu5 structure type [1]. This type is an 'archetype' for approximately 50 related structures [2] with general composition RE1-xT5+2x. Among these structures there are well-known types with uniaxial structures, e.g. Th2Ni17, Th2Zn17, ThMn12, TbCu7. It is known that the substitution of the T-atoms by atoms of the light _p-block elements (B, C, N, Al, Ga, Si) leads to different structural transformations and furthermore improve the magnetic characteristics. Thus, it is interesting to study, in structure and properties aspects, the influence of the third component on formation and properties of the phases.

Recently we reported on formation and crystal structure of a new Er0.85Co4.31Si compound [3, 4] with the TbCu7 structure type. It has been shown that the addition of silicon have leaded to a formation of this compound at 800°C while the binary phase ErCo5 (structure type CaCu5) was found in a limited temperature range. This phase forms peritectically at 1 340°C and decomposes eutectoidally at 1 240°C. According to the accepted phase diagram of the Er-Ni system [5] the hexagonal phase ErNi5 (CaCu5 structure

© Matvijishyn R., Demchenko P., Gorelenko Yu. et al., 2008

type [6]) melts congruently at 1 384°C and exists in a whole temperature range. Thus, it is ineteresting to study the silicon influence on the structure of ErNi5 compound. This work presents the data of the X-ray investigation of single crystal and some magnetic measure­ments data for Er16Ni69Si15 alloy and of the new phase Er1-x(Ni0818Si0182)5+2x (x = 0.08).

The prism-shaped single crystal was extracted from the alloy of Er16Ni69Si15composition. Sample was melted from weighted pieces of initial components of high purity (Er - 99.86%, Ni - 99.99%, Si - 99.999%) under an argon Ti-gettered atmo­sphere in an arc furnace with a water-cooled copper hearth and then annealed in an evacu­ated silica tube at 873 K for one month. The suitable for X-ray analysis single crystal has been preliminary investigated using the Laue and Weissenberg methods (RKW-86 and RGNS-2 cameras, MoK^-radiation), afterwards using the automatic single-crystal diffracto-meter Oxford Diffraction Xcalibur3TM CCD (MoKa-radiation, graphite monochromator, ft>scans). Processing of collection and reduction data was performed using CrysAlis CCD [7] and CrysAlis RED [8] programs. The solution and refinement of crystal structure were performed using SHELX-97 [9] program package. Standardization procedure was performed using program Structure Tidy [10]. Other experimental and crystallographic data are given in Table 1.

Table 1

Experimental and crystallographic data for Er1-x(Ni0818Si0182)5+2x (x = 0.08)

Compound

Er0.920(3)Ni4.22(2)Si0.94(2)

Structure type

TbCu7

Space group

P6/mmm

Z; Pearson symbol

1; hP8-1.92

Mr

428.04

Lattice parameters, A

a = 4.8433(11) c = 3.9897(12)

Crystal size, mm

0.14 x 0.12 x 0.015

Crystal color

metallic dark grey

Absorption coefficient, mm-1

47.651

Grange for data collection

4.86 to 33.04°

Limiting indices

-6 < h < 7, -6 <k <7, -6 < l <5

Reflections collected/unique

683 / 85 [R(int) = 0.0806]

Completeness to в

98.0 %

Absorption correction

Analytical

T   ■ T

1 max? 1 min

0.4887; 0.0018

Refinement method

Full-matrix least-squares on F2

Data/restraints/parameters

85 / 1 / 12

Goodness-of-fit on F2

1.214

Final R indices [I > 2sigma(I)]

R1 = 0.0261, wR2 = 0.0655

R indices (all data)

R1 = 0.0261, wR2 = 0.0655

Extinction coefficient

0.09(3)

Largest diffr. peak and hole

1.955 and -1.964 e/A3

Magnetic susceptibility of annealed Er16Ni69Si15 alloy was measured by the Faraday weighting in a vacuum at 80-500 K temperature range and varied from 3.1 to 10 kOe magnetic fields. The equipment was calibrated using the HgCo(NSC)4 susceptibility standard in the 80-340 K temperature range. The magnetisation curves were measured in the temperature range 1.9-400 K and in magnetic fields up to 50 kOe using a Superconducting Quantum Interference Design (SQUID) magnetometer at the Institute of Low Temperature and Structure Research, Polish Academy of Science in Wroclaw.

The structure solution of Er1.x(Ni0818Si0182)5+2x (x = 0.08) (refined composition Er0920(3)Ni422(2)Si094(2)) was performed by direct procedure after applying the analytical absorption correction. The preliminary solution of structure leads to the atomic positions of the CaCu5 structure type. But the analysis of differential Fourier map shows the additional electron density of about 9.64 e/A3 at coordinates x y z 0 0 0.297 that is typical for the TbCu7 structure type. After including this peak into atom list and applying the restrain G(Er)+G(Ni3) = 1 the atomic parameters were refined in the anisotropic approximation down to R1 = 0.0261 and the corresponding values are listed in Table 2. The refined composition of the single crystal corresponds to the Er15.1Ni69.4Si15.5 alloy that is very close to the composition of primary alloy. As one can see, the structure is characterized by disorder (about 41%), but it is typical for the representatives of TbCu7 structure type.

Table 2

Atomic positional and displacement parameters for Er1.x(Ni0818Si0 182)5+2x (x = 0.08)

Atom

Site

x

y

z

Displacement parameters*, A2 x103

G

 

 

 

 

 

 

U„        U22 U33

 

 

Er

1(a)

0

0

0

13.6(3)

13.0(4)   13.0(4) 14.9(5)

6.5(2)

0.9195(30)

Ni1

2(c)

1/3

2/3

0

15.5(5)

20.1(7) 20.1(7) 6.4(7)

10.0(3)

1

(Ni, Si)2

3(g)

1/2

0

1/2

11.3(5)

15.4(8) 7.0(8) 8.6(8)

3.5(4)

G(Ni) =

 

 

 

 

 

 

 

 

0.687(15)

 

 

 

 

 

 

 

 

G(Si) =

 

 

 

 

 

 

 

 

0.313(15)

Ni3

2(e)

0

0

0.292(7)

11.5(6)

 

- **

0.0805(30)

Ueq is defined as one third of the trace of the orthogonalized Uj tensor.

The anisotropic displacement factor exponent takes the form: -2я2 [h2a*2Un + ... + 2hka*b*U12]. **Anisotropic displacements were not refined.

Clinographic projection of the Er1-x(Ni0818Si0182)5+2x (x = 0.08) phase unit cell is shown in Fig. 1. According to the results of refinement of occupation factor G, 8% of Er atoms in this phase are replaced by Ni-Ni dumb.bells in position 2(e). The structure is formed by net 63 of Ni1 atoms in 2(c).site, by net 3636 (Kagome) of (Ni, Si)2 atoms in 3(g)-site (Fig. 2), and by net 36 of Er atoms in 1a-site or Ni3 atoms in 2(e)-site (Fig. 3). The interatomic distances are in a good agreement with the sums of atomic radii of elements [11] and are listed in Table 3. The shortest distance with the highest deviation (93.1 % of the sum of the atomic radii) is observed between Er and Ni1 atoms, with the Er-Ni distance of 2.7963(6) A.

Magnetization and magnetic susceptibility versus temperature of the Er16Ni69Si15 alloy are presented in Fig. 4 Above 80 K the reciprocal susceptibility fits the Curie-Weiss law with the Weiss parameter 14.5 K and effective magnetic moment of 9.75(8) juB. The Curie point, found from the extrapolated juS2(T) dependence, is equal to 12.5 K. A slight excess of the erbium atom magnetic moments in the paramagnetic temperature region in

Fig. 1. Clinographic projection of the Er1-x(Ni0818Si0182)5+2x (x = 0.08) unit cell (TbCu7-type structure) with displacement ellipsoids

Fig. 2. Net 63 formed by Ni1 atoms in 2(c)-site and net 3636 (kagome) formed by (Ni,Si)2 atoms in 3(g)-site

Fig. 3. Net 36 formed by Er atoms in 1(a)-site (G=0.92) or Ni3 atoms in 2(e)-site (G = 0.08)

compare with the Er3+ gJ [J(J+1)]1/2 calculated moments may be affected by the nearest Ni1 and Ni3 neighbor atoms enhancement, because the interatomic Er-Ni1 and Er-Ni3 distances in this crystal structure are shortened.

Table 3

Interatomic distances (d, A) for Er1-x(Ni0818Si0182)5+2x (x = 0.08)

Atoms

d, A

Atoms

d, A

Er     - 2Ni3

1.16(3)a

(Ni, Si)2 - 4(Ni, Si)2

2.4217(5)

- 6Ni1

2.7963(6)

- 4Ni1

2.4360(5)

- 2Ni3

2.82(3)

- 4Ni3

2.560(9)

- 12(Ni, Si)2

3.1375(6)

Ni3           - Er

1.16(3) *

- 2Er

3.9897(12)

- Ni3

1.66(5) *

Ni1   - 6(Ni, Si)2

2.4360(5)

- Ni3

2.33(5)

- 3Ni1

2.7963(6)

- 6(Ni, Si)2

2.560(8)

- 3Er

2.7963(6)

- Er

2.82(3)

 

 

- 6Ni1

3.029(11)

Distances do not realize physically.

і—■—і—■—і—■—і—■—і—■—і—■—і—■—і—■—і—■—і—r

0       50      100     150     200     250     300     350     400 450

Temperature (K)

Fig. 4. Magnetization in applied field H = 5 kOe (left) and reciprocal magnetic susceptibility in fields H = 3.1-10 kOe vs. temperature (right) for the Er16Ni69Si15 alloy, annealed at 873 K. In insert the field dependence of magnetization at 1.9 K temperature is plotted

The investigated ternary phase probably belongs to the solid solution based on the binary ErNi5 (CaCu5 structure type) compound with continuous structural transition from CaCu5-type to TbCu7-type, or can be separated by two-phase region, e.g. as in the Yb-Fe-Al system [12]. Therefore, it is necessary to investigate more precisely the probable homogeneity range, temperature interval of existence and structural changes for these hexagonal phases.

1. Stadelmaier H.H., Reinsch B. Magnetic Applications // Eds. J.H. Westbrook, R.L. Fleischer. Intermetallic Compounds: Principles and Practice, John Wiley & Sons: Chichester. 1995. Vol. 2. P. 303-322.

2. Parthe E., Gelato L., Chabot B., Penzo M., Cenzual K., Gladyshevskii R. TYPIX-Standardized Data and Crystal Chemical Characterization of Inorganic Structure Types // Gmelin Handbook of Inorganic and Organometallic Chemistry, 8th ed. Springer. Berlin, 1993-1994.

3. Demchenko P., Konczyk J., Bodak O., Matvijishyn R., Muratova L., Marciniak B. Single crystal investigation of new phase Er0.85Co4.31Si and CoSi // Collected Abstracts of the IX International Conference on Crystal Chemistry of Intermetallic Compounds Lviv (Ukraine). September 20-24. 2005. P. 78.

4. Demchenko P., Konczyk J., Bodak O., Matvijishyn R., Muratova L., Marciniak B. Single crystal investigation of new phase Er0.85Co4.31Si and CoSi // J. Alloys Comp. 2008. (in press).

5. Okamoto H. Er-Ni (Erbium-Nickel) // J. Phase Equilibria. 2001. Vol. 22 (5). P. 596-597.

6. Buschow H.J. Crystal structures, magnetic properties and phase relations of erbium-nickel intermetallic compounds // J. Less-Common Met. 1968. Vol. 16 (1). P. 45-53.

7. Oxford Diffraction, CrysAlis CCD. Version 1.170. Oxford Diffraction Ltd., Abingdon.

Oxford (England), 2004.

8. Oxford Diffraction, CrysAlis RED. Version 1.171. Oxford Diffraction Ltd., Abingdon.

Oxford (England), 2005.

9. Sheldrick G. M., SHELXS97 and SHELXL97 - WinGX Version. Release 97-2,

University of Gottingen. Germany, 1997.

10. Gelato L.M., Parthe E., STRUCTURE TIDY - a computer program to standardize crystal structure data // J. Appl. Cryst. 1987. Vol. 20. P. 139-143.

11. Emsley J. Die Elemente. Berlin-New-York: Walter de Gruyter, 1994.

12. Bodak O., Tokaychuk Ya., Manyako M., Pacheco V., Cerny R., Yvon K. Structural and magnetic properties of iron-rich compounds in the Yb-Fe-Al system // J. Alloys Comp. 2003. Vol. 354. P. L10-L15.

КРИСТАЛІЧНА СТРУКТУРА ТА МАГНІТНІ ВЛАСТИВОСТІ СПОЛУКИ Erlx(Nio,8i8Sio,i82)5+2x ЗІ СТРУКТУРОЮ ТИПУ TbCu

Р. Матвіїшин1, П. Демченко1, Ю. Гореленко1, В. Павлюк1, Е. Рожицька-Соколовська2, Б. Марцініак2

1 Львівський національний університет імені Івана Франка, вул. Кирила і Мефодія, 6, 79005 Львів, Україна

2 Інститут хімії та захисту довкілля, Університет Яна Длугоша, вул. Народної Армії, 13/15, 4-200 Ченстохова, Польща

Рентгенівським методом монокристала досліджено кристалічну структуру нової фази Er^920(3)Ni^22(2)Si^94(2), монокристал якої було вилучено із зразка складу Er16Ni69Si15 (Xcalibur3TM CCD дифрактометр, 85 незалежних рефлексів, R1 = 0,0261, wR2 = 0,0655, S = 1,214). Сполука кристалізується у структурному типі TbCu7: просторова група P6/mmm, hP8-1,92 з параметра­ми комірки a = 4,8433(11), c = 3,9897(12) A. Для зразка складу Er16Ni69Si15 парамагнітна сприйнятливість описується законом Кюрі-Вейса вище температури 80 K з параметром Вейса 0p = 14,5 K та ефективним магнітним моментом /ieff = 9,75(8) /VEr-атом. Температура Кюрі, визначена екстраполяцією залежності ///(T), становить 12,5 K.

Ключові слова: сплави та сполуки перехідних металів, кристалічна структура, магнетизм, магнітні вимірювання, рентгенівська дифракція.

Стаття надійшла до редколегії 04.10.2007 Прийнята до друку 03.01.2008

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