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Electrochemical deposition of gold–tin alloy from ethylene glycol electrolyte
Surface & Coatings Technology 204 (2010) 1314-1318
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Surface & Coatings Technology j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / s u r f c o a t
Electrochemical deposition of gold–tin alloy from ethylene glycol electrolyte T.N. Vorobyova, O.N. Vrublevskaya ⁎ Research Institute of Physico-Chemical Problems, Belarusian State University, Leningradskaya St. 14, Minsk 220030, Belarus
a r t i c l e
i n f o
Article history: Received 20 March 2009 Accepted in revised form 8 October 2009 Available online 28 October 2009 Keywords: Gold–tin alloy Electroplating Ethylene glycol electrolyte Phase composition
a b s t r a c t The possibility of Au–Sn alloy deposition from the ethylene glycol electrolyte has been shown. Deposited alloys contain 27–54 at.% of tin and include AuSn2, Au5Sn, AuSn crystalline phases together with amorphous gold. The coatings consist of tightly packed submicron grains grown up into agglomerates 2–4 μm in diameter. The rate of the alloy deposition can be varied from 1 to 5 μm h− 1 at current density 5–50 mA cm− 2. The process of Au–Sn electroplating is characterized by the absence of noticeable cathode passivation, diffusion limitations and hydrogen reduction. The rate of electrodeposition is greatly dependent on Au(III) and Sn(IV) concentration in solution that allows to control Au:Sn ratio in the alloy. © 2009 Elsevier B.V. All rights reserved.
1. Introduction White gold Au–Sn alloys are of great interest in jewellery and electronics for their mechanical properties, corrosion stability, color, solderability and the reduced cost [1–13]. The alloy containing 30 at.% of tin and 70at.% of gold is traditionally applied as the solder that can be produced in a form of a paste which consists of Au–Sn powder and organic binder [1], as a coating obtained by electron beam evaporation [2,3], by thermal evaporation [4], in the result of electrodeposition [5–21]. The lacks of the paste usage are fast oxidation of metal particles and the necessity to bind them with the help of an organic substance which diminishes the solderability. Vapor or electron beam deposition of a solder is characterized by high cost of the process. One of the methods of Au–Sn eutectic solder electrochemical deposition is the sequential plating of gold and tin layers [6,7]. In such case the thickness of gold and tin layers and the regime of final thermal treatment needed for the alloying are the main factors which influence the composition and the structure of the resulting solder. From the analysis of literature data concerning to the electrochemical deposition of Au–Sn alloy from the solutions containing gold an tin salts simultaneously it is possible to pick out three types of the proper electrolyte: a) acid solutions with tetracyanoaurate(III) and Sn (IV)-containing ions as the gold and tin precursors [14,16,18]; b) slightly acid electrolytes on the base of tetrachloroaurate(III) and Sn (II)-containing ions [5,8–12,16,19]; c) alkaline electrolytes on the base of dicyanoaurates(I) and stannates(IV) [14,15,17–20]. It is a known fact that slightly acid electrolytes on the base of tetrachloroaurate(III) and Sn(II)-containing ions for gold–tin alloy electrodeposition have a lot of lacks. Tin compounds are easily hydrolyzed; gold can be reduced in the solution bulk (that is inherent ⁎ Corresponding author. E-mail address: [email protected] (O.N. Vrublevskaya). 0257-8972/$ – see front matter © 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.surfcoat.2009.10.010
especially to the electrolytes containing noncyanide gold compounds). The proceeding of side processes makes the elemental and phase composition of the alloy uncontrollable. Authors of a series of articles tried to solve the problem of solutions' stability by the usage of different additives and deposition regimes [5,8–12,16,19]. So, the search of stable electrolytes providing deposition of the Au–Sn alloy with a controlled elemental and phase composition does not lost its actuality. Acid or alkaline solutions are rather stable but they can irritate the substrate. Organic electrolytes have a row of advantages in comparison with the aqueous baths, such as deposition of the metals which cannot be reduced in aqueous solutions; the stability of the most of electrolytes with organic solvents to redox reactions in the bulk and to hydrolysis that excludes the necessity to use special ligands [21]. Nevertheless, the electrolytes based on organic solvents provide a low rate of coating growth connected with an insufficient solution conductivity and high viscosity. In this work we have chosen the ethylene glycol (EG) as the solvent in the electrolyte for gold–tin alloy electrodeposition. EG solutions are known to be used for tin electrodeposition [22]. EG forms stable complexes with some metals [23]. The purpose of this work was to determine the possibility of Au–Sn alloy electrodeposition from the EG solutions, to study the factors affecting the rate of the alloy deposition, its composition and morphology and to analyze the peculiarities of gold and tin codeposition in the EG solutions. 2. Experimental The solutions on the base of EG (99.95 wt.%) as the solvent containing Sn(IV) chloride pentahydrate and potassium tetracyanoaurate(III) complex were investigated. The electrolytes contained 4.9–12.0 wt.% of water because tin compound was used in the form of crystalline hydrates. Tin and gold compound concentrations were varied in the range of 0.75–
T.N. Vorobyova, O.N. Vrublevskaya / Surface & Coatings Technology 204 (2010) 1314-1318
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2.80 and 0.05–0.10mole dm− 3, accordingly. Electroplating was conducted in galvanostatic conditions (direct current source B4-49, Russia) in polypropylene tank at constant stirring. Platinum foil (99.99 wt.%) and copper foil (99.99 wt.%) were used as the anode and cathode, respectively. Metal film thickness was analyzed gravimetrically. The elemental composition of the alloy was established by EDX method (Roentex equipment to the scanning electron microscope) and by X-ray fluorescence analysis with the usage of “Spectroscan Maks GF-2E” (Russia, St. Petersburg). The morphology of the alloy deposits was examined by scanning electron microscope LEO 1420 (Germany). Electrochemical measurements were carried out in a home-made three electrode cell with a platinum counter electrode and a silver wire (99.9 wt.%) as the pseudo reference electrode. Gold (99.9 wt.%) or copper (99.9 wt.%) foil 1 cm2 in the area were used as the working electrodes. The choice of a silver wire as the pseudo reference electrode based on the recommendations given for non-aqueous electrolytes in the review [24]. Its advantages are the absence of a water–organic liquid boundary resulting in the unknown junction potential and of a water diffusion into the electrolyte. Cyclic voltammograms (CV) were recorded at the potential sweep rate of 20 mVs− 1 by means of PI 50-1 potentiostat and KSP-4 recorder (Russia). All the experiments were carried out at a temperature 20 ± 2 °C. Potential sweep ranged from −5.0 to +0.4 V. In this window of electrochemistry ethylene glycol was not reduced or oxidized according to CVs obtained for copper electrode immersed into NaCl solution in EG. Phase analysis of the alloy films was carried out by means of X-ray diffractometer DRON-3 (Russia) using copper radiation. Identification of the compounds was carried out by using the data of the JSPDS data files [25]. The crystallite sizes were calculated using a series of reflections. 3. Results and discussion 3.1. Electrochemical measurements The CV curves illustrating cathodic processes on copper and gold electrodes in solution containing SnCl4 or KAu(CN)4 or both of these substances are presented in Fig. 1. It is important to note the unusual shape of the CVs having almost a straight form with a very small hysteresis between the forward (cathodic) and the reverse sweep. These lines have no plateau or peaks up to −5.0 V. Au(III) reduction begins at more positive potential (+0.2 V on copper and gold) than the reduction of Sn(IV) (−0.25 V on copper and 0.0 V on gold) occurs. The data show that potential of Sn(IV) reduction beginning is shifted to more positive side on the gold electrode comparing to the copper cathode (Table 1). Simultaneous gold and tin reduction begins at the potential +0.1 V on copper and − 0.2 V on gold (Fig. 1). The slight shift of the potential of tin reduction beginning to positive values at simultaneous tin and gold electrodeposition can be connected with the process of tin cementation on the gold reduced. Current density of gold reduction on the both electrodes is much less comparing to the current of tin deposition (Fig. 1, Table 1) that correlates with the difference in Sn(IV) and Au(III) concentrations in the solution. The electrode material has a great effect on the CV curves. Gold or tin reduction and codeposition of these metals proceed at a higher current density on copper than on gold. The rate of tin deposition is less sensitive to the substrate nature than the rate of gold deposition is. The current density of Au–Sn alloy deposition on gold is 3.3 times smaller than on copper cathode. The effect of the electrode material on the current density is well pronounced on the whole length of the CVs that can be easily explained because the thickness of films deposited during voltammetric investigations did not exceed 50 nm. The difference in current density of the alloy deposition on copper and gold can be attributed to formation of
Fig. 1. CV curves illustrating cathodic deposition of tin (curve 1), gold (curve 2), Au–Sn alloy (curve 3) on copper (a) and on gold (b); hydrogen evolving on copper (c). Tin is reduced from 0.75 M SnCl4 solution in EG, gold is reduced from 0.05 M KAu(CN)4 solution; simultaneous Au(III) and Sn(IV) reduction occurs from the EG solution containing 0.75 M SnCl4 and 0.05 M KAu(CN)4. Hydrogen is evolving from 0.1 M NaCl EG solution containing 6.7 wt.% of H2O.
Table 1 Potentials of the beginning of Au(III) and Sn(IV) reduction on copper and gold working electrodes. Metal
Electrode Potentials of the beginning Current density at the of metal reduction, E, V potential −3.0 V, mAcm− 2
Sn(IV) Sn(IV) Au(III) Au(III) Sn(IV) and Au(III) codeposition Sn(IV) and Au(III) codeposition
Cu Au Cu Au Cu
− 0.25 0 + 0.2 + 0.2 + 0.1
7.1 5.7 2.1 0.2 10.0
Au
− 0.2
3.0
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dissimilar phases that is discussed in Section 3.3. Here we note that Au5Sn phase is preferably forming on the surface of gold electrode at the beginning of deposition. AuSn phase is more inherent to deposition on copper. It was important to analyze the possibility of hydrogen reduction from the water introduced into the EG with crystalline hydrates. For this purpose the CV curve was measured for copper electrode in 0.1 M NaCl solution in the EG containing 6.7 wt.% of water (Fig. 1c). It is seen that hydrogen is reduced from the water present the EG beginning from − 0.45 V and the corresponding current in the absence of gold and tin compounds in the bath is rather high. It equals to 8 mA cm− 2 at −1.0 V. The evolving of hydrogen almost does not proceed in the presence of tin and gold compounds in the EG. So, the current density at −1.0 V does not exceed 2 mA cm− 2 and belongs mostly to the process of Au–Sn deposition. The alloying is confirmed by X-ray diffraction data and it proceeds in the absence of gas bubbles. The suppressed hydrogen reduction from the electrolyte containing Sn (IV) and Au(III) compounds can be explained by binding the water molecules in aqua complexes with metal ions.
demands many specific conditions. We tried to analyze the factors which influence tin content in the alloy. In water solutions the content of the negative metal can be enlarged by the growth of its ion concentration in an electrolyte, by deposition of gold in conditions of the limited diffusion current, by over polarization of gold reduction and depolarization of tin reduction. Often it is achieved at high tin concentration and current density. The analysis of the data in Table 2 shows that the alloys with the highest tin content equal to 50 ± 4 at.% are obtained at the moderate current density 17–30 mA cm− 2 from solutions no. 1, 3, 5 with low Au (III) concentration about 0.05 mole dm− 3 and rather high Sn(IV) concentration in the range of 0.75–2.8 moledm− 3. The lowest tin content in the alloy not larger than 27–38 at.% occurs when coatings are deposited from solutions with high Au(III) concentration equal to 0.1 mole dm− 3 independently on Sn(IV):Au(III) ratio in the bath (solutions no. 2, 4). Current efficiency of the plating process was determined on the base of gravimetric data and the content of tin and gold in the coatings. It did not exceed 30%. 3.3. Phase composition and chemical composition of the alloy
3.2. Deposition rate and atomic composition of Au–Sn alloy On purpose to find the optimal bath composition and conditions for Au–Sn alloy electrodeposition we varied Sn(IV) and Au(III) concentration in solutions from 2.8 to 0.75 mole dm− 3 and from 0.1 to 0.05 mole dm− 3, accordingly. The Sn(IV):Au(III) ratio was changed in the range of 56:1, 28:1, 15:1, 14:1 (solutions no. 1–5, Table 2). The results of the investigation have shown that the alloy deposition from EG solutions occurs at current densities between 10 and 50 mA cm− 2 set by the direct current source with a quite satisfactory rate equal to 0.9–5.2 µm h− 1. This fact gives evidence that metal ions are rather mobile in the suggested electrolytes and the rates of film growth in aqueous and EG solutions have rather close values. The rate of the alloy film deposition is dependent on the Sn(IV):Au(III) ratio and the concentration of metal compounds. At the mole ratio equal or less than 28:1 and Sn(IV) concentration 0.75–1.4 mole dm− 3 the deposition rate grows up with current density (Table 2). At higher Sn(IV) concentrations the proportionality disappears and the rate of Au–Sn alloy deposition does not exceed 2.4 μm h− 1 at current densities up to 50 mA cm− 2 that can point to the existence of some diffusion retardation. The highest rate of the Au–Sn deposition equal to 5.2 μm h− 1 is observed at Sn(IV):Au(III) ratio in the bath being 15:1 and at the lowest concentration of components (solution no. 5, Table 2). Unfortunately, tin content in the alloy obtained in this solution is not high. Tin content in the deposited alloys is varied from 27 to 54 at.% (Table 2). That gives evidence to effective tin codeposition with the noble metal. Such effective deposition of Sn with Au from the water solutions
It is known from the literature that Au–Sn alloy on the base of intermetallic ξ-Au5Sn and δ-AuSn compounds has the best soldering ability [26,27]. The melting point of their eutectic is 280 °C, while δ-AuSn phase has a melting point at 419.3 °C and represents a subtractional solid solution containing 50.0–50.5 at.% of tin. The ξ-Au5Sn phase includes 16.7 at.% of tin. This phase can be defined by a unite cell containing 15 atoms of gold and 3 atoms of tin. The results of X-ray diffraction analysis have shown that crystalline phases in all the coatings obtained are represented by intermetallic compounds such as AuSn, Au5Sn and AuSn2 (Fig. 2, Table 2). The deviations of experimental 2-theta angles from the data in the Powder Diffraction Files do not exceed 0.05o and appear to be as to the higher (AuSn2 and Au5Sn phases) so to the lower 2-theta values (AuSn). The existence of these deviations can be obliged to not stoichiometric composition of solid phases. The peaks in diffractograms are rather narrow and their half width varies from 0.2 to 0.4° (in 2-theta angles). The calculations of the crystallite sizes have shown that the smallest of them in AuSn phase are of 22–33 nm. In AuSn2 phase the crystallite sizes amount to 31– 34 nm and in the sample containing AuSn together with Au5Sn phases these sizes are of 31–41 nm. The most intensive peaks inherent to phases under discussion and specified in the Powder Diffraction Files are present in the experimental diffractograms. The less intensive peaks are not observed that can be connected with the small thickness of the films under investigation. There are some distinctions in the height of peaks presented in the experimental diffractograms and in the Powder Diffraction Files. This
Table 2 The dependence of the alloy deposition rate, atomic and phase composition on the Sn(IV) and Au(III) content in the electrolyte and current density. No. Solution composition, moledm− 3 Sn(IV):Au(III), mole ratio Current density, mAcm− 2 Deposition rate, μ m h− 1 Elemental alloy composition, at.% Phase composition 1
Sn(IV) – 2.8 Au(III) – 0.05
56:1
2
Sn(IV) – 2.8 Au(III) – 0.1
28:1
3
Sn(IV) – 1.4 Au(III) – 0.05
28:1
4
Sn(IV) – 1.4 Au(III) – 0.1
14:1
5
Sn(IV) – 0.75 Au(III) – 0.05
15:1
17 27 30 10 24 49 5 15 21 7 15 25 5 15 21
1.8 1.7 1.8 2.4 1.7 0.9 1.2 1.3 1.9 1.8 2.4 3.5 2.6 4.5 5.2
Au Au Au Au Au Au Au Au Au Au Au Au Au Au Au
– 47; Sn – – 67; Sn – – 53; Sn – – 71; Sn – – 62; Sn – – 70; Sn – – 59; Sn – – 63; Sn – – 47; Sn – – 71; Sn – – 69; Sn – – 73; Sn – – 63; Sn – – 61; Sn – – 72; Sn –
53 33 47 29 38 30 41 37 54 29 31 27 37 39 28
AuSn2 – – – – – AuSn, Au5Sn – – AuSn – – AuSn – –
T.N. Vorobyova, O.N. Vrublevskaya / Surface & Coatings Technology 204 (2010) 1314-1318
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It is important to note that crystalline phases of metallic tin and gold are absent in all the alloys obtained. The formation of crystalline intermetallic compounds means that alloying occurs during deposition from EG solutions and needs no special heating. The comparison of the experimental data gives evidence that the atomic Sn(IV):Au(III) ratio in the determined intermetallic compounds is not equal to this ratio in coatings (Table 2). Total gold content in the most of the alloys is higher than it is in the discovered intermetallic compounds that shows the presence of non detected Xray amorphous phase rich in gold. The exception are coatings obtained from the electrolyte no. 3 characterized by the presence of two crystalline phases one of which is rich in gold (Table 2). This electrolyte provides the formation of an alloy famous for its soldering ability according to [26,27]. 3.4. Morphology of Au–Sn alloys SEM photos of Au–Sn coatings about 1 μm thick (Fig. 3) show their uniform surface formed by tightly packed grains 0.2–0.8 μm in size. These initial grains are grown up together with the formation of agglomerates different in size (1.5–10 μm) and a form depending on tin and gold concentration in the solution, current density and deposition rate. It can be concluded that these agglomerates are flat because their size exceeds the coating thickness. The agglomerates are almost absent in coatings deposited at low current densities (5–7 mA cm− 2). The current density enlargement to 10–20 mA cm− 2 promotes the agglomerate formation and at further increase in current density the sizes of agglomerates become constant or slightly growing. The largest sizes of agglomerates are inherent to the alloys deposited from the most diluted solution no. 5 at high current density (Fig. 3d.). The tendency of agglomerate size increase with the current density is not in contradiction with the traditional grain size diminution with the current growth. The explanation has to be done that we discuss here the appearance of the secondary structures and the enhanced growth of agglomerates is provoked by intensification of the initial grain appearance. It is interesting to note that the alloys rich in gold are more finegrained (Fig. 3c). The sizes of agglomerates in the coatings containing 73 at.% of Au do not exceed 2–3 μ m while they mount to 3–4 μm in the samples containing 46 at.% of gold. Mean agglomerate sizes in these samples are equal to 1.3 and 2.2 μm, accordingly. It is important to underline non porous structure of the coatings under investigation. 4. Conclusions
Fig. 2. Data on X-ray phase analysis of Au–Sn alloy coatings 1 μm thick: a — solution 1 (Table 1), current density 17A cm− 2, b — solution 4, current density 7 A cm− 2, с — solution 3 at current density 5 A cm− 2. Interpretation was based on the data from the work [25].
fact can be attributed to a texture which often appears in thin coatings when crystallites grow in the preferential direction. SEM data allow us to observe needle-like grains, the grains of oval and lamellar form (Fig. 3). The AuSn2 phase which is most rich in tin but not interesting for soldering is deposited only from the bath with Sn(IV):Au(III) ratio equal to 56:1 (solution no. 1, Table 2). δ-AuSn phase is deposited from the bath at Sn(IV):Au(III) ratio varied from 28:1 to 14:1 while the components concentrations are 1.4–0.75 mole dm–3 for Sn(IV) and 0.05–0.10 mole dm− 3 for Au(III) (solutions no. 3–5). The Au5Sn phase enriched in gold is obtained at a small current density not exceeding 15 A cm− 2 and moderate Sn(IV):Au(III) ratio about 28:1 (solution no. 3). The most often it is formed on the gold substrate and especially at the beginning of the film growth (1–2 μ m thick).
1. It is shown that EG electrolytes on the base of Sn(IV) chloride and potassium tetracyanoaurate(III) can be used for Au–Sn alloy electroplating at tin content equal to 27–54 at.%. The alloys most rich in tin are deposited from solutions with small Au(III) concentration equal to 0.05 mole dm− 3 and Sn(IV) concentration varying from 0.75 to 2.8 mole dm− 3. 2. Au–Sn alloy coatings deposited from the EG solution include AuSn2, AuSn and Au5Sn phases and an amorphous phase rich in gold. 3. The alloy containing the both Au5Sn and AuSn crystalline phases are deposited from 1.4 M solution of Sn(IV) in EG at a current density equal to 15–21 mA cm− 2 and a rate of 1.3–1.9 μm h− 1. Two times diminution in Sn(IV) content in such an electrolyte provides the possibility to enlarge the rate of an alloy deposition up to 2.6– 5.2 μ m h− 1 at small current density 5–21 mA cm− 2 and to obtain the alloy on the base of AuSn intermetallic compound. 4. Voltammentric investigations have shown the absence of the electrode passivation during Au(III) and Sn(IV) reduction in the EG electrolyte, the proportionality of the current density to the set potential in a broad electrochemical window and its high dependence on Au(III) and Sn(IV) concentration in the solution. This fact
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Fig. 3. SEM photographs of Au–Sn coatings 1 μ m thick: a and b — coatings rich in tin deposited from the solution no. 3 (Table 2); c and d — coatings rich in gold deposited from solutions no. 4 and 5, accordingly; a, c — current density 15 A cm− 2; b, d — current density 21 A cm− 2; the deposition rate increases in the row a, b, c, d from 1.3 to 5.2 µm h− 1.
allowed us to determine the conditions of Au and Sn alloying with the proper Au:Sn ratio in the coatings equal to 28:1. 5. Au–Sn coatings deposited from the EG electrolyte are non porous, fine grained and consist of tightly packed grains 0.2–0.8 μm in size forming agglomerates with mean sizes 2–4 μm. References [1] A.R. Mickelson, N.R. Basavanhally, I.C. Lee, Optoelectronic Packing, John Willey & Sons Inc, New York, 1997. [2] C.C. Lee, C.Y. Wang, G.S. Matijasevic, IEEE Trans. Components Hybrids Manuf. Technol. 14 (1991) 407. [3] A. Katz, H. Lee, K.L. Tai, Mater. Chem. Phys. 37 (1994) 303. [4] G.R. Dohle, J.J. Callahan, K.P. Martin, T.J. Drabic, IEEE Trans. Compon. Packag. Manuf. Technol. Part B, Adv. Packag. 19 (1996) 57. [5] W. Sun, D.G. Ivey, J. Mater. Sci. 36 (2001) 757. [6] J.W. Yoon, H.S. Chun, Mater. Eng. A 473 (2008) 119. [7] J.W. Yoon, H.S. Chun, Microsyst. Technol. 13 (2007) 1463. [8] J. Doesburg, D.G. Ivey, Plating Surf. Finish. 88 (4) (2001) 78. [9] G.H. Jeong, J.H. Kim, D Lee, S.J. Suh, Mater. Res. Soc. Symp. Proc. 894 (2006) 85. [10] A. He, Q. Liu, D.G. Ivey, J. Mater. Sci. 17 (1) (2006) 63.
[11] Y. Zhang, D.G. Ivey, Plating Surf. Finish. 91 (2) (2004) 28. [12] Y. Funaoka, S. Arai, N. Kaneko, Electrochemistry 72 (2) (2004) 98. [13] B. Bozzini, A. Fanigliulo, G. Giovanenelli, S. Natali, C. Mele, J. Appl. Electrochem. 33 (2003) 747. [14] B. Bozzini, G. Giovanenelli, S. Natali, M. Serra, A. Fanigliulo, J. Appl. Electrochem. 33 (2002) 165. [15] W. Kuhn, W Zilske, US Pat. 4634505, 1987. [16] D.G. Ivey, W. Sun, US Pat. 6245208, 2001. [17] P. Stevens, J.M. Deuber, K.R. Rosikiewisz, US Pat. 4013523, 1977. [18] E. Uchida, T. Okada, US Pat. 6544398, 2003. [19] B. Djurfos, D.G. Ivey, Mater. Sci. Eng., B, Solid-State Mater. Adv. Technol. 90 (2002) 309. [20] B. Bozzini, G. Giovannelli, S. Natali, M. Serra, A. Fanigliulo, J. Appl. Electrochem. 32 (2002) 165. [21] Electrochemistry in Nonaqueous Solutions By Kosuke Izutsu (Matsumoto, Japan), Wiley-VCH, Weinheim. ISBN 3-527-30516-5, 2002 xiv + 346 pp. [22] A.N. Susoev, N.N. Gavurina, Russ. J. Appl. Chem. 34 (9) (1961) 2001. [23] D. Knetsch, W.L. Groeneveld, Inorg. Chim. Acta 7 (1973) 81. [24] A.W. Bott, Curr. Sep. 14 (2) (1995) 64. [25] Powder Diffraction File JCPSD Int. Center for Diffraction Data Swarthnore. Cards # 8– 463; 28–440, 31–568. [26] J. Ciulik, M.R. Notis, J. Alloys Compos. 191 (1993) 71. [27] G.S. Matijasevic, C.C. Lee, C.Y. Wang, Thin Solid Films 223 (1993) 276.
Contents lists available at ScienceDirect
Surface & Coatings Technology j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / s u r f c o a t
Electrochemical deposition of gold–tin alloy from ethylene glycol electrolyte T.N. Vorobyova, O.N. Vrublevskaya ⁎ Research Institute of Physico-Chemical Problems, Belarusian State University, Leningradskaya St. 14, Minsk 220030, Belarus
a r t i c l e
i n f o
Article history: Received 20 March 2009 Accepted in revised form 8 October 2009 Available online 28 October 2009 Keywords: Gold–tin alloy Electroplating Ethylene glycol electrolyte Phase composition
a b s t r a c t The possibility of Au–Sn alloy deposition from the ethylene glycol electrolyte has been shown. Deposited alloys contain 27–54 at.% of tin and include AuSn2, Au5Sn, AuSn crystalline phases together with amorphous gold. The coatings consist of tightly packed submicron grains grown up into agglomerates 2–4 μm in diameter. The rate of the alloy deposition can be varied from 1 to 5 μm h− 1 at current density 5–50 mA cm− 2. The process of Au–Sn electroplating is characterized by the absence of noticeable cathode passivation, diffusion limitations and hydrogen reduction. The rate of electrodeposition is greatly dependent on Au(III) and Sn(IV) concentration in solution that allows to control Au:Sn ratio in the alloy. © 2009 Elsevier B.V. All rights reserved.
1. Introduction White gold Au–Sn alloys are of great interest in jewellery and electronics for their mechanical properties, corrosion stability, color, solderability and the reduced cost [1–13]. The alloy containing 30 at.% of tin and 70at.% of gold is traditionally applied as the solder that can be produced in a form of a paste which consists of Au–Sn powder and organic binder [1], as a coating obtained by electron beam evaporation [2,3], by thermal evaporation [4], in the result of electrodeposition [5–21]. The lacks of the paste usage are fast oxidation of metal particles and the necessity to bind them with the help of an organic substance which diminishes the solderability. Vapor or electron beam deposition of a solder is characterized by high cost of the process. One of the methods of Au–Sn eutectic solder electrochemical deposition is the sequential plating of gold and tin layers [6,7]. In such case the thickness of gold and tin layers and the regime of final thermal treatment needed for the alloying are the main factors which influence the composition and the structure of the resulting solder. From the analysis of literature data concerning to the electrochemical deposition of Au–Sn alloy from the solutions containing gold an tin salts simultaneously it is possible to pick out three types of the proper electrolyte: a) acid solutions with tetracyanoaurate(III) and Sn (IV)-containing ions as the gold and tin precursors [14,16,18]; b) slightly acid electrolytes on the base of tetrachloroaurate(III) and Sn (II)-containing ions [5,8–12,16,19]; c) alkaline electrolytes on the base of dicyanoaurates(I) and stannates(IV) [14,15,17–20]. It is a known fact that slightly acid electrolytes on the base of tetrachloroaurate(III) and Sn(II)-containing ions for gold–tin alloy electrodeposition have a lot of lacks. Tin compounds are easily hydrolyzed; gold can be reduced in the solution bulk (that is inherent ⁎ Corresponding author. E-mail address: [email protected] (O.N. Vrublevskaya). 0257-8972/$ – see front matter © 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.surfcoat.2009.10.010
especially to the electrolytes containing noncyanide gold compounds). The proceeding of side processes makes the elemental and phase composition of the alloy uncontrollable. Authors of a series of articles tried to solve the problem of solutions' stability by the usage of different additives and deposition regimes [5,8–12,16,19]. So, the search of stable electrolytes providing deposition of the Au–Sn alloy with a controlled elemental and phase composition does not lost its actuality. Acid or alkaline solutions are rather stable but they can irritate the substrate. Organic electrolytes have a row of advantages in comparison with the aqueous baths, such as deposition of the metals which cannot be reduced in aqueous solutions; the stability of the most of electrolytes with organic solvents to redox reactions in the bulk and to hydrolysis that excludes the necessity to use special ligands [21]. Nevertheless, the electrolytes based on organic solvents provide a low rate of coating growth connected with an insufficient solution conductivity and high viscosity. In this work we have chosen the ethylene glycol (EG) as the solvent in the electrolyte for gold–tin alloy electrodeposition. EG solutions are known to be used for tin electrodeposition [22]. EG forms stable complexes with some metals [23]. The purpose of this work was to determine the possibility of Au–Sn alloy electrodeposition from the EG solutions, to study the factors affecting the rate of the alloy deposition, its composition and morphology and to analyze the peculiarities of gold and tin codeposition in the EG solutions. 2. Experimental The solutions on the base of EG (99.95 wt.%) as the solvent containing Sn(IV) chloride pentahydrate and potassium tetracyanoaurate(III) complex were investigated. The electrolytes contained 4.9–12.0 wt.% of water because tin compound was used in the form of crystalline hydrates. Tin and gold compound concentrations were varied in the range of 0.75–
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2.80 and 0.05–0.10mole dm− 3, accordingly. Electroplating was conducted in galvanostatic conditions (direct current source B4-49, Russia) in polypropylene tank at constant stirring. Platinum foil (99.99 wt.%) and copper foil (99.99 wt.%) were used as the anode and cathode, respectively. Metal film thickness was analyzed gravimetrically. The elemental composition of the alloy was established by EDX method (Roentex equipment to the scanning electron microscope) and by X-ray fluorescence analysis with the usage of “Spectroscan Maks GF-2E” (Russia, St. Petersburg). The morphology of the alloy deposits was examined by scanning electron microscope LEO 1420 (Germany). Electrochemical measurements were carried out in a home-made three electrode cell with a platinum counter electrode and a silver wire (99.9 wt.%) as the pseudo reference electrode. Gold (99.9 wt.%) or copper (99.9 wt.%) foil 1 cm2 in the area were used as the working electrodes. The choice of a silver wire as the pseudo reference electrode based on the recommendations given for non-aqueous electrolytes in the review [24]. Its advantages are the absence of a water–organic liquid boundary resulting in the unknown junction potential and of a water diffusion into the electrolyte. Cyclic voltammograms (CV) were recorded at the potential sweep rate of 20 mVs− 1 by means of PI 50-1 potentiostat and KSP-4 recorder (Russia). All the experiments were carried out at a temperature 20 ± 2 °C. Potential sweep ranged from −5.0 to +0.4 V. In this window of electrochemistry ethylene glycol was not reduced or oxidized according to CVs obtained for copper electrode immersed into NaCl solution in EG. Phase analysis of the alloy films was carried out by means of X-ray diffractometer DRON-3 (Russia) using copper radiation. Identification of the compounds was carried out by using the data of the JSPDS data files [25]. The crystallite sizes were calculated using a series of reflections. 3. Results and discussion 3.1. Electrochemical measurements The CV curves illustrating cathodic processes on copper and gold electrodes in solution containing SnCl4 or KAu(CN)4 or both of these substances are presented in Fig. 1. It is important to note the unusual shape of the CVs having almost a straight form with a very small hysteresis between the forward (cathodic) and the reverse sweep. These lines have no plateau or peaks up to −5.0 V. Au(III) reduction begins at more positive potential (+0.2 V on copper and gold) than the reduction of Sn(IV) (−0.25 V on copper and 0.0 V on gold) occurs. The data show that potential of Sn(IV) reduction beginning is shifted to more positive side on the gold electrode comparing to the copper cathode (Table 1). Simultaneous gold and tin reduction begins at the potential +0.1 V on copper and − 0.2 V on gold (Fig. 1). The slight shift of the potential of tin reduction beginning to positive values at simultaneous tin and gold electrodeposition can be connected with the process of tin cementation on the gold reduced. Current density of gold reduction on the both electrodes is much less comparing to the current of tin deposition (Fig. 1, Table 1) that correlates with the difference in Sn(IV) and Au(III) concentrations in the solution. The electrode material has a great effect on the CV curves. Gold or tin reduction and codeposition of these metals proceed at a higher current density on copper than on gold. The rate of tin deposition is less sensitive to the substrate nature than the rate of gold deposition is. The current density of Au–Sn alloy deposition on gold is 3.3 times smaller than on copper cathode. The effect of the electrode material on the current density is well pronounced on the whole length of the CVs that can be easily explained because the thickness of films deposited during voltammetric investigations did not exceed 50 nm. The difference in current density of the alloy deposition on copper and gold can be attributed to formation of
Fig. 1. CV curves illustrating cathodic deposition of tin (curve 1), gold (curve 2), Au–Sn alloy (curve 3) on copper (a) and on gold (b); hydrogen evolving on copper (c). Tin is reduced from 0.75 M SnCl4 solution in EG, gold is reduced from 0.05 M KAu(CN)4 solution; simultaneous Au(III) and Sn(IV) reduction occurs from the EG solution containing 0.75 M SnCl4 and 0.05 M KAu(CN)4. Hydrogen is evolving from 0.1 M NaCl EG solution containing 6.7 wt.% of H2O.
Table 1 Potentials of the beginning of Au(III) and Sn(IV) reduction on copper and gold working electrodes. Metal
Electrode Potentials of the beginning Current density at the of metal reduction, E, V potential −3.0 V, mAcm− 2
Sn(IV) Sn(IV) Au(III) Au(III) Sn(IV) and Au(III) codeposition Sn(IV) and Au(III) codeposition
Cu Au Cu Au Cu
− 0.25 0 + 0.2 + 0.2 + 0.1
7.1 5.7 2.1 0.2 10.0
Au
− 0.2
3.0
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dissimilar phases that is discussed in Section 3.3. Here we note that Au5Sn phase is preferably forming on the surface of gold electrode at the beginning of deposition. AuSn phase is more inherent to deposition on copper. It was important to analyze the possibility of hydrogen reduction from the water introduced into the EG with crystalline hydrates. For this purpose the CV curve was measured for copper electrode in 0.1 M NaCl solution in the EG containing 6.7 wt.% of water (Fig. 1c). It is seen that hydrogen is reduced from the water present the EG beginning from − 0.45 V and the corresponding current in the absence of gold and tin compounds in the bath is rather high. It equals to 8 mA cm− 2 at −1.0 V. The evolving of hydrogen almost does not proceed in the presence of tin and gold compounds in the EG. So, the current density at −1.0 V does not exceed 2 mA cm− 2 and belongs mostly to the process of Au–Sn deposition. The alloying is confirmed by X-ray diffraction data and it proceeds in the absence of gas bubbles. The suppressed hydrogen reduction from the electrolyte containing Sn (IV) and Au(III) compounds can be explained by binding the water molecules in aqua complexes with metal ions.
demands many specific conditions. We tried to analyze the factors which influence tin content in the alloy. In water solutions the content of the negative metal can be enlarged by the growth of its ion concentration in an electrolyte, by deposition of gold in conditions of the limited diffusion current, by over polarization of gold reduction and depolarization of tin reduction. Often it is achieved at high tin concentration and current density. The analysis of the data in Table 2 shows that the alloys with the highest tin content equal to 50 ± 4 at.% are obtained at the moderate current density 17–30 mA cm− 2 from solutions no. 1, 3, 5 with low Au (III) concentration about 0.05 mole dm− 3 and rather high Sn(IV) concentration in the range of 0.75–2.8 moledm− 3. The lowest tin content in the alloy not larger than 27–38 at.% occurs when coatings are deposited from solutions with high Au(III) concentration equal to 0.1 mole dm− 3 independently on Sn(IV):Au(III) ratio in the bath (solutions no. 2, 4). Current efficiency of the plating process was determined on the base of gravimetric data and the content of tin and gold in the coatings. It did not exceed 30%. 3.3. Phase composition and chemical composition of the alloy
3.2. Deposition rate and atomic composition of Au–Sn alloy On purpose to find the optimal bath composition and conditions for Au–Sn alloy electrodeposition we varied Sn(IV) and Au(III) concentration in solutions from 2.8 to 0.75 mole dm− 3 and from 0.1 to 0.05 mole dm− 3, accordingly. The Sn(IV):Au(III) ratio was changed in the range of 56:1, 28:1, 15:1, 14:1 (solutions no. 1–5, Table 2). The results of the investigation have shown that the alloy deposition from EG solutions occurs at current densities between 10 and 50 mA cm− 2 set by the direct current source with a quite satisfactory rate equal to 0.9–5.2 µm h− 1. This fact gives evidence that metal ions are rather mobile in the suggested electrolytes and the rates of film growth in aqueous and EG solutions have rather close values. The rate of the alloy film deposition is dependent on the Sn(IV):Au(III) ratio and the concentration of metal compounds. At the mole ratio equal or less than 28:1 and Sn(IV) concentration 0.75–1.4 mole dm− 3 the deposition rate grows up with current density (Table 2). At higher Sn(IV) concentrations the proportionality disappears and the rate of Au–Sn alloy deposition does not exceed 2.4 μm h− 1 at current densities up to 50 mA cm− 2 that can point to the existence of some diffusion retardation. The highest rate of the Au–Sn deposition equal to 5.2 μm h− 1 is observed at Sn(IV):Au(III) ratio in the bath being 15:1 and at the lowest concentration of components (solution no. 5, Table 2). Unfortunately, tin content in the alloy obtained in this solution is not high. Tin content in the deposited alloys is varied from 27 to 54 at.% (Table 2). That gives evidence to effective tin codeposition with the noble metal. Such effective deposition of Sn with Au from the water solutions
It is known from the literature that Au–Sn alloy on the base of intermetallic ξ-Au5Sn and δ-AuSn compounds has the best soldering ability [26,27]. The melting point of their eutectic is 280 °C, while δ-AuSn phase has a melting point at 419.3 °C and represents a subtractional solid solution containing 50.0–50.5 at.% of tin. The ξ-Au5Sn phase includes 16.7 at.% of tin. This phase can be defined by a unite cell containing 15 atoms of gold and 3 atoms of tin. The results of X-ray diffraction analysis have shown that crystalline phases in all the coatings obtained are represented by intermetallic compounds such as AuSn, Au5Sn and AuSn2 (Fig. 2, Table 2). The deviations of experimental 2-theta angles from the data in the Powder Diffraction Files do not exceed 0.05o and appear to be as to the higher (AuSn2 and Au5Sn phases) so to the lower 2-theta values (AuSn). The existence of these deviations can be obliged to not stoichiometric composition of solid phases. The peaks in diffractograms are rather narrow and their half width varies from 0.2 to 0.4° (in 2-theta angles). The calculations of the crystallite sizes have shown that the smallest of them in AuSn phase are of 22–33 nm. In AuSn2 phase the crystallite sizes amount to 31– 34 nm and in the sample containing AuSn together with Au5Sn phases these sizes are of 31–41 nm. The most intensive peaks inherent to phases under discussion and specified in the Powder Diffraction Files are present in the experimental diffractograms. The less intensive peaks are not observed that can be connected with the small thickness of the films under investigation. There are some distinctions in the height of peaks presented in the experimental diffractograms and in the Powder Diffraction Files. This
Table 2 The dependence of the alloy deposition rate, atomic and phase composition on the Sn(IV) and Au(III) content in the electrolyte and current density. No. Solution composition, moledm− 3 Sn(IV):Au(III), mole ratio Current density, mAcm− 2 Deposition rate, μ m h− 1 Elemental alloy composition, at.% Phase composition 1
Sn(IV) – 2.8 Au(III) – 0.05
56:1
2
Sn(IV) – 2.8 Au(III) – 0.1
28:1
3
Sn(IV) – 1.4 Au(III) – 0.05
28:1
4
Sn(IV) – 1.4 Au(III) – 0.1
14:1
5
Sn(IV) – 0.75 Au(III) – 0.05
15:1
17 27 30 10 24 49 5 15 21 7 15 25 5 15 21
1.8 1.7 1.8 2.4 1.7 0.9 1.2 1.3 1.9 1.8 2.4 3.5 2.6 4.5 5.2
Au Au Au Au Au Au Au Au Au Au Au Au Au Au Au
– 47; Sn – – 67; Sn – – 53; Sn – – 71; Sn – – 62; Sn – – 70; Sn – – 59; Sn – – 63; Sn – – 47; Sn – – 71; Sn – – 69; Sn – – 73; Sn – – 63; Sn – – 61; Sn – – 72; Sn –
53 33 47 29 38 30 41 37 54 29 31 27 37 39 28
AuSn2 – – – – – AuSn, Au5Sn – – AuSn – – AuSn – –
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It is important to note that crystalline phases of metallic tin and gold are absent in all the alloys obtained. The formation of crystalline intermetallic compounds means that alloying occurs during deposition from EG solutions and needs no special heating. The comparison of the experimental data gives evidence that the atomic Sn(IV):Au(III) ratio in the determined intermetallic compounds is not equal to this ratio in coatings (Table 2). Total gold content in the most of the alloys is higher than it is in the discovered intermetallic compounds that shows the presence of non detected Xray amorphous phase rich in gold. The exception are coatings obtained from the electrolyte no. 3 characterized by the presence of two crystalline phases one of which is rich in gold (Table 2). This electrolyte provides the formation of an alloy famous for its soldering ability according to [26,27]. 3.4. Morphology of Au–Sn alloys SEM photos of Au–Sn coatings about 1 μm thick (Fig. 3) show their uniform surface formed by tightly packed grains 0.2–0.8 μm in size. These initial grains are grown up together with the formation of agglomerates different in size (1.5–10 μm) and a form depending on tin and gold concentration in the solution, current density and deposition rate. It can be concluded that these agglomerates are flat because their size exceeds the coating thickness. The agglomerates are almost absent in coatings deposited at low current densities (5–7 mA cm− 2). The current density enlargement to 10–20 mA cm− 2 promotes the agglomerate formation and at further increase in current density the sizes of agglomerates become constant or slightly growing. The largest sizes of agglomerates are inherent to the alloys deposited from the most diluted solution no. 5 at high current density (Fig. 3d.). The tendency of agglomerate size increase with the current density is not in contradiction with the traditional grain size diminution with the current growth. The explanation has to be done that we discuss here the appearance of the secondary structures and the enhanced growth of agglomerates is provoked by intensification of the initial grain appearance. It is interesting to note that the alloys rich in gold are more finegrained (Fig. 3c). The sizes of agglomerates in the coatings containing 73 at.% of Au do not exceed 2–3 μ m while they mount to 3–4 μm in the samples containing 46 at.% of gold. Mean agglomerate sizes in these samples are equal to 1.3 and 2.2 μm, accordingly. It is important to underline non porous structure of the coatings under investigation. 4. Conclusions
Fig. 2. Data on X-ray phase analysis of Au–Sn alloy coatings 1 μm thick: a — solution 1 (Table 1), current density 17A cm− 2, b — solution 4, current density 7 A cm− 2, с — solution 3 at current density 5 A cm− 2. Interpretation was based on the data from the work [25].
fact can be attributed to a texture which often appears in thin coatings when crystallites grow in the preferential direction. SEM data allow us to observe needle-like grains, the grains of oval and lamellar form (Fig. 3). The AuSn2 phase which is most rich in tin but not interesting for soldering is deposited only from the bath with Sn(IV):Au(III) ratio equal to 56:1 (solution no. 1, Table 2). δ-AuSn phase is deposited from the bath at Sn(IV):Au(III) ratio varied from 28:1 to 14:1 while the components concentrations are 1.4–0.75 mole dm–3 for Sn(IV) and 0.05–0.10 mole dm− 3 for Au(III) (solutions no. 3–5). The Au5Sn phase enriched in gold is obtained at a small current density not exceeding 15 A cm− 2 and moderate Sn(IV):Au(III) ratio about 28:1 (solution no. 3). The most often it is formed on the gold substrate and especially at the beginning of the film growth (1–2 μ m thick).
1. It is shown that EG electrolytes on the base of Sn(IV) chloride and potassium tetracyanoaurate(III) can be used for Au–Sn alloy electroplating at tin content equal to 27–54 at.%. The alloys most rich in tin are deposited from solutions with small Au(III) concentration equal to 0.05 mole dm− 3 and Sn(IV) concentration varying from 0.75 to 2.8 mole dm− 3. 2. Au–Sn alloy coatings deposited from the EG solution include AuSn2, AuSn and Au5Sn phases and an amorphous phase rich in gold. 3. The alloy containing the both Au5Sn and AuSn crystalline phases are deposited from 1.4 M solution of Sn(IV) in EG at a current density equal to 15–21 mA cm− 2 and a rate of 1.3–1.9 μm h− 1. Two times diminution in Sn(IV) content in such an electrolyte provides the possibility to enlarge the rate of an alloy deposition up to 2.6– 5.2 μ m h− 1 at small current density 5–21 mA cm− 2 and to obtain the alloy on the base of AuSn intermetallic compound. 4. Voltammentric investigations have shown the absence of the electrode passivation during Au(III) and Sn(IV) reduction in the EG electrolyte, the proportionality of the current density to the set potential in a broad electrochemical window and its high dependence on Au(III) and Sn(IV) concentration in the solution. This fact
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Fig. 3. SEM photographs of Au–Sn coatings 1 μ m thick: a and b — coatings rich in tin deposited from the solution no. 3 (Table 2); c and d — coatings rich in gold deposited from solutions no. 4 and 5, accordingly; a, c — current density 15 A cm− 2; b, d — current density 21 A cm− 2; the deposition rate increases in the row a, b, c, d from 1.3 to 5.2 µm h− 1.
allowed us to determine the conditions of Au and Sn alloying with the proper Au:Sn ratio in the coatings equal to 28:1. 5. Au–Sn coatings deposited from the EG electrolyte are non porous, fine grained and consist of tightly packed grains 0.2–0.8 μm in size forming agglomerates with mean sizes 2–4 μm. References [1] A.R. Mickelson, N.R. Basavanhally, I.C. Lee, Optoelectronic Packing, John Willey & Sons Inc, New York, 1997. [2] C.C. Lee, C.Y. Wang, G.S. Matijasevic, IEEE Trans. Components Hybrids Manuf. Technol. 14 (1991) 407. [3] A. Katz, H. Lee, K.L. Tai, Mater. Chem. Phys. 37 (1994) 303. [4] G.R. Dohle, J.J. Callahan, K.P. Martin, T.J. Drabic, IEEE Trans. Compon. Packag. Manuf. Technol. Part B, Adv. Packag. 19 (1996) 57. [5] W. Sun, D.G. Ivey, J. Mater. Sci. 36 (2001) 757. [6] J.W. Yoon, H.S. Chun, Mater. Eng. A 473 (2008) 119. [7] J.W. Yoon, H.S. Chun, Microsyst. Technol. 13 (2007) 1463. [8] J. Doesburg, D.G. Ivey, Plating Surf. Finish. 88 (4) (2001) 78. [9] G.H. Jeong, J.H. Kim, D Lee, S.J. Suh, Mater. Res. Soc. Symp. Proc. 894 (2006) 85. [10] A. He, Q. Liu, D.G. Ivey, J. Mater. Sci. 17 (1) (2006) 63.
[11] Y. Zhang, D.G. Ivey, Plating Surf. Finish. 91 (2) (2004) 28. [12] Y. Funaoka, S. Arai, N. Kaneko, Electrochemistry 72 (2) (2004) 98. [13] B. Bozzini, A. Fanigliulo, G. Giovanenelli, S. Natali, C. Mele, J. Appl. Electrochem. 33 (2003) 747. [14] B. Bozzini, G. Giovanenelli, S. Natali, M. Serra, A. Fanigliulo, J. Appl. Electrochem. 33 (2002) 165. [15] W. Kuhn, W Zilske, US Pat. 4634505, 1987. [16] D.G. Ivey, W. Sun, US Pat. 6245208, 2001. [17] P. Stevens, J.M. Deuber, K.R. Rosikiewisz, US Pat. 4013523, 1977. [18] E. Uchida, T. Okada, US Pat. 6544398, 2003. [19] B. Djurfos, D.G. Ivey, Mater. Sci. Eng., B, Solid-State Mater. Adv. Technol. 90 (2002) 309. [20] B. Bozzini, G. Giovannelli, S. Natali, M. Serra, A. Fanigliulo, J. Appl. Electrochem. 32 (2002) 165. [21] Electrochemistry in Nonaqueous Solutions By Kosuke Izutsu (Matsumoto, Japan), Wiley-VCH, Weinheim. ISBN 3-527-30516-5, 2002 xiv + 346 pp. [22] A.N. Susoev, N.N. Gavurina, Russ. J. Appl. Chem. 34 (9) (1961) 2001. [23] D. Knetsch, W.L. Groeneveld, Inorg. Chim. Acta 7 (1973) 81. [24] A.W. Bott, Curr. Sep. 14 (2) (1995) 64. [25] Powder Diffraction File JCPSD Int. Center for Diffraction Data Swarthnore. Cards # 8– 463; 28–440, 31–568. [26] J. Ciulik, M.R. Notis, J. Alloys Compos. 191 (1993) 71. [27] G.S. Matijasevic, C.C. Lee, C.Y. Wang, Thin Solid Films 223 (1993) 276.