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Home , Osmium, Platinum group, Ruthenium

Reactions between Some Alkali and

By 0. Loebich, Jr. and Ch. J. Raub Forschungsinstitut fur Edelmetalle und Metallchemie, Schwabisch Gmiind, West Germany

The present knowledge of reactions between the alkali metals, lithium, sodium, potassium and with the metals, platinum, , , , and , their reaction products, and the relevant binary diagrams, is surveyed. Information is included on the magnetic and electrical behaviour of some alloys, especially those with the higher platinum contents.

Although the alloying behaviour of the potassium or rubidium. The reactivity of the platinum group metals with the Group IB ele- alkali metals was manifest by the rapid corro- ments , and has been studied sion of many of the alloys on exposure to air. extensively, little attention has been given to Platinum or palladium alloys with less than 35 their alloys with the Group IA, alkali, metals. atomic per cent lithium could be handled in air To remedy this situation an investigation of without excessive oxidation, but at higher these systems was started at the Fors- lithium concentrations powdery chungsinstitut fiir Edelmetalle und Metall- products, white to grey-black in colour, were chemie, and while this work has been in formed. Platinum alloys were found to be more progress several publications have described the reactive than palladium ones. In general alloys structure of various intermediate phases of the heavier alkali metals tarnished more that appear in these systems. Work by K. H. J. rapidly on exposure to air, even at the lowest Buschow and J. H. N. Van Vucht (I, 12) and alkali concentrations, forming a wet W. Bronger and B. Nacken (4, 8) is particularly hydroxide layer. relevant, and some of their data are included in Table I. Preparation and Investigation During the preparation and investigation of Several methods were adopted for pre- these alloys difficulties arise due to the paration. These involved reacting mixtures of relatively low boiling points of the alkali metals, high purity elements under in the glove compared with the high melting temperatures box illustrated in Figure I. The general experi- of the platinum metals. For example these mental procedures were as follows: temperature differences range from (i) W'ith the arc furnace coupled directly to the 1330 to 1552OC for lithium-palladium to 688 glove box, palladium and platinum alloys with to 2500OC for the rubidium-ruthenium com- lithium were produced by arc melting on a bination. In addition, the chemical reactivity of water cooled copper hearth in a high purity the alkali metals increases with the heavier argon atmosphere. The furnace charges metals, but there is a decreasing tendency to weighed between I and 5 grams and consisted form intermediate phases with platinum group either of pressed tablets of lithium pieces and metals. Thus at least eight phases are found in platinum group metal sponge or powder, or the lithium-palladium system compared with alternatively of small pieces of both elements. possibly one in the lithium-ruthenium system, In the latter case only the platinum metal and none in any of the systems containing became molten, then reacted with the lithium.

Platinum Metals Rev., 1981, 25, (31, 113-120 113 Platinum

Compound Crystallographic Data Metals

Structure Structure Reo., Classifi- Classifi-

1981, cation cation Composition symbol Lattice pzrameter Composition symbol Lattice p:ramete

25, Compound t.% palladium (17) Type A see Reference Compound at.% platinum (17) Type A see Reference (3) LiPd, -8588 cF32 MSPt, a = 7.71-7.74 LiPt, -87.588 cF32 a = 7.763-7.75f Li Pd, - 6&7 5 mP8 - a = 5.35-5.41 LiPt, -6S73 cF32 a = 7.678-7.69: b = 2.72-2.73 c = 7.65-7.67 8=9&91.5

- 114 LiPd 45.5-51.8 hP2 LiRh a = 2.74-2.77 LiPt h P2 a = 2.73 c = 4.13-4.28 c = 4.23

Li,Pd, 39.5-41 cP2 CSCI? a = 2.97

Li,Pd - hP3 AIB, a = 4.23 Li,Pt - hP3 a=4.19 c = 2.73 c = 2.66 Li,Pd - cF16 BiF, a=6.19

Li15Pd, - c176 Cu,,Si, a = 10.69 Li,, Pt.,? Evidence by D.T.A. only, see Reference (7) see Reference Li,Pd - cF? a = 19.01-1 9.35 Li,Pt? 92x15 Li,Pt?

LiRh, 018 Ir,Li a = 2.659 Ir,Li 018 Ir,Li a = 2.672-2.672 (12) b = 8.602 b = 8.693-8.697 c = 4.657 c = 4.670-4.67 1

LiRh hP2 LiRh a = 2.649 lrLi hP2 LiRh a =2.649-2.651 (12) c = 4.359 c = 4.3974.399

NaPd,? non cubic NaPt, cF24 Cu,Mg a=7.50 (9) (ii) of specimens was undertaken in closed or cylinders, sealed by under high purity argon. Such specimens consisted either of pressed tablets or of capped tubes of palladium or platinum which completely enclosed the . (iii) In differential thermal analyses, molten alkali metals were reacted directly with platinum metal powders enclosed in or stainless steel tubes. After the tubes, measur- ing 4 mm in diameter and 15 mm long, had been filled they were closed by flattening the ends which were then folded twice and again compressed. The alloy samples were investigated by a variety of techniques including metallography, D.T.A., X-ray methods, and by magnetic and electrical measurements. Metallographic examination of prepared microsections was generally unsuccessful, since only the few low Fig. 1 Samples for examination were produced lithium content alloys of palladium and from alkali memls and platinum group metals of platinum which were stable in air gave informa- 99 and 99.9 atomic per cent purity, rc?spectively. tive results. X-ray powder diffraction This argon filled glove box was used both .for the prrparation of arc melted samples and to enable photographs were taken using CuKI,radiation. the consiitiwnt elements to be sealed into tubes or Despite attempts to inhibit the reaction by cylinders prior to the reaction processes coating the powder samples with vacuum grease before they were removed from the glove box, balance in fields up to I tesla. Even though by evacuating the body of the camera, or these measurements were made in a helium by attempting-unsuccessfully-to contain atmosphere, contact of the samples with air crushed samples in capillary tubes, it was not during mounting was unavoidable, and for this possible to investigate alloys containing high reason measurements made on sintered samples alkali metal concentrations without (tablets) with their larger surface area proved interference by the air. less reliable than those made on samples pre- For the determination of reactions, transfor- pared by melting, or on the cylindrical diffusion mations and melting temperatures, D.T.A. was samples (capped tubes). Electrical resistivity performed under a helium or argon atmosphere and electromotive force measurements were in the temperature range 50 to 1550°C~with made on a number of deformable heating and cooling rates of 5 to IOOC per lithium-platinum alloys. minute. In addition to the normal signals, the start of the reaction that produces an inter- Experimental Results mediate phase is indicated by an exothermic The reactions of the alkali metals with those signal during the first heating cycle. Problems of the platinum group metals are summarised. encountered included reactions between the samples and the container materials, and the I. Reactions with Lithium volatilisation of the alkali metals. Mainly on the basis of D.T.A. measurements, Measurements of , a phase diagram has been established for the and in some cases of its variation with lithium-palladium system, shown here as temperature, were carried out in a Faraday-type Figure 2 (2), and a partial phase diagram for

Platinum Metals Rev., 1981, 25, (3) 115 lithium-platinum, Figure 3 (3). Crystallogra- pletely at concentrations above 70 atomic per phic data for the phases present in these two cent ruthenium, in a slow single stage reaction. systems is given in Table I, together with such From this it is concluded that only one inter- information as is known for the other systems. mediate phase, near Ru,Li, exists in the system. Rhodium and lithium react at about 3ooOC Osmium does not react with lithium below in a two-stage reaction. When the rhodium con- 9oo0C. As the only thermal effects observed centration is >so atomic per cent, the solidifica- were those originating from pure lithium we tion of free lithium is not detected on cooling, expect no intermediate phases. At higher and it is concluded that complete reaction has temperatures reactions with the container occurred. materials, or volatilisation of the alkali metals, Iridium and lithium react at 500 and 54Ooc. started. No appreciable shift of the freezing The reaction at the lower temperature is more point of the lithium was detected, indicating vigorous for smaller iridium concentrations. virtually no . Lithium again reacts completely when the con- centration of the platinum metal is 250 atomic 2. Reactions with Sodium per cent. Palladium and sodium react at 3oo0C, com- It is concluded that the only intermediate plete absorption of the sodium being observed phases that exist in the two above systems are at palladium concentrations above 60 atomic RhLi and Rh,Li, and the analagous IrLi and per cent, and the phase formed decomposes Ir,Li phases, respectively (I I, 12). peritectically at 797 t 5OC. The X-ray pattern At 45ooC ruthenium and lithium react com- of the phase, which is assumed to be Pd,Na,

Platinum Metals Rev., 1981, 25, (3) 116 contains weak lines indicating a symmetry an appreciable solid solubility of the platinum lower than cubic. metals in sodium was observed. Platinum reacts with sodium at 300 to 370 3. Reactions with Potassium and at 350 to 4ooOC. Sodium is completely used The platinum metals do not appear to react up in alloys containing more than atomic per 50 with potassium. The behaviour of the cent platinum. The reaction was more potassium suggests a fairly easy and rapid reac- pronounced at the lower temperature when the tion with the tantalum and stainless steel con- platinum concentration was less than 50 atomic tainers, probably catalysed by small amounts of per cent. No melting points of alloys were hydroxides. In mixtures with platinum and observed below 90oOC. We assume two phases palladium, potassium is lost nearly of the approximate compositions PtNa and quantitatively on heating to 8ooOC. In alloying Pt,Na, the latter being known to exist possibly experiments with the other platinum metals as a cubic Laves phase. Its lattice constant was volatilisation was observed as low as 5oo0C. confirmed as a = 7.50 k (9). However, 0.01 A This may indicate a small solubility of the seven lines of the diffraction pattern that potassium in palladium and platinum. were observed did not yield conclusive evidence that the phase is in fact a Laves phase, see Pt,Li Magnetic and Electrical Properties (7). Magnetic measurements were made only on With the other platinum metals sodium does palladium and platinum. The samples were in not react below 700OC. At higher temperatures the form of sintered compacts, melted spheres sodium starts to escape from the reaction of approximately 5 mm diameter and, in the capsules, indicating no reduction in its vapour case of the heavier alkali metals, on cylindrical pressure. diffusion (capped tubes) samples heat treated Depending on the amount of palladium for I week at 300 to 400°C. Susceptibility data (platinum), reaction product and sodium are summarised in Table 11. Most values are present, interaction was found with tantalum extrapolated from data obtained in two phase containers above 75ooC, and with stainless steel regions. Because of experimental difficulties ones at approximately IOOO"C.No indication of during the preparation of lithium rich lithium-platinum alloys by melting, data in this composition range are relatively uncertain. For sodium, potassium and rubidium alloys measurements were made only on diffusion samples, where the alkali metal was initially encapsulated in a platinum metal cylinder (capped tube). The validity of the results obtained on the diffusion samples tended to be confirmed by the very small loss in weight during heat treatment, and by the fact that no free alkali metal was present after heating. Further confirmation was provided by chemical analyses, and it was concluded that the alkali metal had diffused completely into the sur- rounding platinum metal. The diffusion method of sample preparation, at temperatures slightly above the reaction temperature found by D.T.A., was found to be necessary because all the heavier alkali metals evaporated during preliminary melting trials.

Platinum Metals Rev., 1981, 25, (3) 117 = Table II Room Temperature Molar Susceptibilities of Palladium and Platinum Phases with Lithium and Sodium (yemu = 10-6em3)

Phase p em u/mol e Phase pemulmole

Palladium 556 Platinum 189

Lithium LiPd, 245 - 188 LiPt, 83 - 72 LiPd, 90 - 16 LiPt, 15- 11 LiPd 4- 0 LiPt 2+ 3 Li,Pd, o+ 2 Li,Pd 0 - 24 Li,Pt 22 2 Li15Pd4 6k 2 Li15Pt4 7+ 3 Li,Pd 28 2 2 Li,Pt 11 * 4 Li,Pt 16 k 3

Sodium NaPd, -240 NaPt, -50

Furthermore unacceptable uptake of cent. Potassium dissolves in solid palladium to molybdenum was found in potassium alloys about 0.4 atomic per cent, and in platinum to sintered in direct contact with molybdenum less than 0.1 atomic per cent, the limit of detec- capsules at temperatures above 600OC. tion, while rubidium has a solubility limit below An estimate of the solid solubility of the 0.1 atomic per cent in both palladium and alkali metals in platinum and palladium was platinum. In fact there is no deviation in the obtained by a graphical method. This implies susceptibilities measured from those expected that the molar reduction of the susceptibility by from the additively computed values for a dissolution of the heavier alkali metals is simple physical mixture. identical to that for lithium, that is by an The decrease of susceptibility with increasing amount typical for a single electron metal. In alkali metal content observed in binary systems this way the solid solubility of sodium in which form intermetallic compounds correlates palladium was estimated as z to 2.5 atomic per with the decrease in the variation with tempera- cent, and in platinum as about I atomic per ture of the susceptibility (as measured for PdLi

Table Ill Resistivity of Some Platinum Rich Lithium-Platinum Alloys

Composition, Resistivity, &'cm atomic per I cent lithium State I 77K I 273K I 373K

0.0 Homogeneous 1.84 9.85 13.7 0.6 4.84 13.3 17.5 1.3 6.97 16.1 20.5 1.6 9.21 18.7 23.2 2.7 Heterogeneous 7.91 18.7 20.8 5.2 6.89 15.5 19.4

Platinum Metals Rev., 1981, 25, (3) 118 and PtLi alloys (2, 7)) expected from the onset of pure Pauli spin in phases con- taining about 50 atomic per cent palladium or platinum. Wires and strips made from thin sheets of PtLi @latinurn-lithium) alloys containing up to 3 atomic per cent lithium were produced easily by cold working, and were used for electrical measurements. These were performed after a final heat-treatment of the samples, for 25 hours at 6oo0C, in vacuo. Between o and IOOOC no deviation in resistivity from linearity was found, however, values at -196'C are lower than if extrapolated from linear behaviour. Data are given in Table 111. Within the solubility range of 1.8 atomic per cent lithium, resistivity increases by 5.~0cm/at.!%Li and its temperature coefficient of resistance is lowered by I x ro-Wat.%Li. Corresponding values for a palladium-lithium sample containing 4.3 atomic per cent lithium are p,, = 26pR cm at zo°C and aloe = 18.7 x ro-4/K. Measurements are still being made on this binary system, including the fairly broad Lil'd, phase region. The thermoelectric power of the platinum- amount decreasing from lithium to potassium lithium alloys increases by 0.15mV/at.%Li as could be expected from their molar volumes. between o and IOOOC. Temperature variations Magnetic susceptibilities of platinum and for four alloys are shown in Figure 4. palladium are reduced by alloying with the alkali metals, which is typical for a monovalent Discussion metal. This is valid for solid solutions as well as Bearing in mind the experimental difficulties for the intermediate phases. From the magnetic mentioned, one can say that most probably no behaviour, no intermediate phases are expected intermetallic phases occur in the binary systems for binaries containing potassium or rubidium. of osmium with the alkali metals. No evidence Room temperature susceptibilities of was found for the existence of intermetallic platinum and palladium alloys with non- phases in the corresponding systems of magnetic metals plotted against electron con- ruthenium, iridium or rhodium with alkali centrations fall on the same curve (13, 15 and metals heavier than lithium, nor in platinum 16), see Figure 5. For the alloys containing and palladium with those heavier than sodium. more than 2 atomic per cent lithium there is an Mutual solid seem to be negligible in increasing deviation to smaller values near an most cases, exceptions again being palladium electron concentration of 0.02, the lines for the and platinum. Solution of palladium or lithium alloys following the initial slope up to platinum in lithium to a reduction of the Pt,Li, or the solid solution limit for the melting temperature by about 35 or IoOC, lithium-palladium alloys (6 atomic per cent respectively. No similar effect could be found lithium). In terms of the Friedel theory of band for any of the other systems examined. structure modification in the host metal, when Palladium, and to a minor extent platinum also, alloyed with electron donor metals (I 41, the dissolves small amounts of alkali metal, the smaller gradient of the reference curve is

Platinum Metals Rev., 1981, 25, (3) 119 understood as an increased broadening of the Acknowledgement d-band below the Fermi level, with increasing We wish to thank the Deutsche Forschungs- content of solute metal. In lithium alloys, gemeinschaft Bonn-Bad Godesberg for supporting possibly the very low mass of the donor metal our investigations. leads to a partial removal of the perturbation References centres by the lattice vibrations at room tern*- I J. H. N. Van Vucht and K. H. J. Buschow, 3. perature. As a result, the number of electrons Less-Common Met., 1976,48,(2), 345 necessary to fill the d-band remains small over a 2 0. Loebich and Ch. J. Raub, 3. Less-Common wide concentration range. Met., 1977,55,(11~67 3 R. Ferro, R. Capelli, S. Delfino, and G. Centineo, Structural determinations of the more reac- Atti Accad. Naz. Lincei, Cl. Sci. Fis., Mac. Nat., tive phases could be carried out by the method Rend., 1968~45,(6), 564 of Van Vucht and Buschow (I). Following 4 B. Nacken and W. Bronger, 3. Less-Common Met., 5% pulverisation in an argon filled glove box their ‘977, (21, 323 5 W. Bronger, Diss., University of Munster, I 961 samples were loaded into special holders, having 6 0.Loebich and W. Wopersnow, -7. Less-Common beryllium windows to permit X-ray diffraction Met., 1979,63, (2), Pf33- measurements to be made under vacuum. 7 0. Loebich and Ch. J. Raub, J. Less-Common Met., 1980,70, (I), P47 Sample preparation by chemical methods, for 8 W. Bronger, B. Nacken and K. Ploog, J. Less- example the reaction of alkali with Common Met., 1975,43, (1/2), 143 the platinum metals and-if necessary- 9 C. P. Nash, F. M. Boyden and L. D. Whittig, 3. Am. Chem. Sac., dehydrogenation of the ternary hydrides 1960,82,(23), 6203 10 W. Bronger and W. Klemm, Z. Anorg. Allg. formed (4, 8), probably yield samples which are Chem., 1962,319,(1/2), 58 more homogeneous than those produced by 11 H.G. Wheat, C.-Y. Cheng, R. J. Bayuzick, R. W. melting. This chemical method is particularly Sullivan and C. B. Magee, 3. Less-Common Met., suitable in critical cases when the temperature I978,58, (I), PI3 I2 H. C. Donkersloot and J. H. N. Van Vucht, 7. interval between the of the alkali Less-CommonMet., 1976,50,(2),279 metal and the of the platinum ‘3 H.J. Schaller, 2. Metallkd. 1979,70, (5), 318 group metal is wide. On the other hand low ‘4 J. Friedel, Phil. Mag., I952,43, 1533;Adv. Phys., 1954,3, (12), 466; Nuovo Cimento, 1958,7,287 temperature diffusion in sealed capsules also ‘5 I. R. Harris and M. Norman, J. Less-Common appears to be a promising method of prepara- Met., 1968,15,(3), 285 tion. Since the reactivity of intermetallic phases 16 R. G. Jordan and 0. Loebich, 3. Less-Common with high alkali metal concentrations is higher Met., ‘975,399 (I),55 ‘7 W. B. Pearson, “Handbook of Lattice Spacings than that of the alkali metal alone, a dry inert and Structures of Metals and Alloys”, Vol. 2, atmosphere is essential during investigations. Pergamon Press, Oxford, I 967

Platinum Metals Rev., 1981, 25, (3) 120