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Platinum Metals Rev., 1984, 28, (2), 63

Superconductivity of the Platinum Metals and Their Alloys

A Review Summarising the Published Scientific Data

  • By Ch. J. Raub
  • Forschungsinstitut für Edelmetalle und Metallchemie, Schwäbisch Gmünd, West Germany
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Article Synopsis

Continuing investigation of superconductivity, particularly of platinum metal compounds, has resulted in a growing understanding of the phenomenon. Accumulating data contain some remarkable experimental results which in time must surely find both theoretical explanation and industrial application.

The electrical and magnetic behaviour of the platinum metals and their compounds has fascinated solid state chemists and physicists for many years. Together with iron, cobalt and nickel the platinum metals belong to Group VIII of the Periodic Table, and one of the earliest problems was to consider why they did not show any magnetic ordering, in contrast to iron, cobalt and nickel. On the other hand, when in the form of solid solutions or compounds they exhibited very interesting magnetic behaviour, for example localised moments and spin glass behaviour. The magnetic strengths of cobalt-platinum alloys even found industrial application.

With the discovery of “superconductivity” by Kammerlingh-Onnes and the hypothesis of B. T. Matthias and others that superconductivity—that is the loss of any electrical resistance below a critical temperature, Tc—and ferromagnetism exclude each other even though they are based on similar phenomena, the investigation of the low temperature electrical and magnetic behaviour of the platinum metals and their compounds became of special interest.

In the course of this research many new and unexpected phenomena were discovered in platinum metal alloys, namely superconductivity of hydrides, and superconductivity/ferro- or antiferromagnetism in ternary compounds, topics which subsequently developed into specialised research areas of their own. Even now it is impossible to predict the Tc or Tm (Tm = magnetic ordering temperature) of an alloy. Indirectly this work has led to new approaches in other areas, for example the growth and study of single crystals.

Up to now superconductivity has found only limited application in high technology areas for example in magnets for fusion and atomic research at cern and Culham, and for electrical motors and generators. However, one can expect that the platinum metal compounds discussed here, with their unique properties, will in time find applications which cannot be envisaged at present, perhaps especially for solid state devices.

It has been known for some time that three of the six platinum group metals in high purity form become superconducting, these being ruthenium, osmium and iridium. In addition, many years ago, it was predicted from investigations made of rhodium alloy systems that the element rhodium might also become a superconductor at a temperature around 0.2 mK (1), and this has recently been established (2, 2). The superconductivity of the individual metals will now be considered.

Ruthenium


The superconducting transition temperature of ruthenium has been placed between 0.40 and 0.509 ± 0.002 K (4, 5, 6). The generally accepted values for its superconducting properties are: transition temperature Tc = 0.49 ± 0.015 K, critical field Hc = 69 ± 2 Oe, Debye temperature θ
D = 580 K, and the coefficient of low temperature specific heat γ = 2.8 mJ/mol/K (7). The dependence of critical temperature on its atomic mass M (isotope effect) is relatively small. For powdered and arc furnace melted samples of Ru99 and Ru104 it was reported that the critical temperature coincides within the accuracy of measurements (8) and similar results were obtained later (9, 10). Measurements made on samples with an average atomic mass of 99.2, 101.1 and 103.9 in magnetic fields up to 70 Oe gave the dependence: Tc ≈ M-1/8. At pressures up to 30,000 atmospheres the critical temperature is not affected by the applied pressure (11), while its volume dependence δln(Tc/θ
D)/δlnV= - 16 (12).

Osmium


The critical temperature of osmium as given in the literature ranges from 0.4 to 0.71 K (13). An arc furnace melted sample of 99.9 per cent purity had a resistance ratio of ρ
o/ρ
273 = 0.4 and Tc = 0.71 K, at a width of transition of 0.02 K (14). Today generally accepted values for the superconducting properties are Tc = 0.66 ± 0.03 K, Hc = 70 Oe, θ
D = 500 K and γ = 2.35 mJ/mol/K (7). Isotope effect measurements do not agree, even when apparently identical samples are checked (10, 13). According to Geballe and Matthias (10) osmium has no isotope effect, while Hein and Gibson (15) found Tc ≈ M-0.2. The critical field is 80.9 Oe when = 187.4, and 80.2 Oe when = 192.0 (15). At a pressure of 30,000 atm the critical temperature of osmium is reduced by 0.05 K (11). In a review paper considering the influence of the purity of samples on their critical temperature it was shown that the sample with the highest resistance ratio, which was ρ
273/ρ
o = 500, had Tc = 0.638 K (16).

Iridium


By suitable heat treatment in an arc melting furnace it has been possible to remove from iridium samples, by evaporation, ferromagnetic impurities such as iron, when a Tc = 0.14 K is obtained for the iridium (17). However, for samples containing 50 ppm of iron Tc = 0.10-0.11 K (18). Iridium purified by electron beam melting until the total impurity content is below 10 ppm has a critical temperature curve with a mid point at 0.1125 ± 0.005 K.

Furthermore its transition curve is characterised by an uncommon hysteresis; the transition from the normal to the superconducting state occurs discontinuously but from the superconducting to the normal state it is continuous. Additionally, it is influenced by the applied magnetic field. The coherence length is 4.4 × 10-4 cm, and the penetration depth 3.9 × 10-6 cm. Small impurity concentrations reduce the critical temperature of iridium and make it a superconductor of the second kind. Accepted values are now: Tc = 0.1125 ± 0.001 K, Hc(T = 0) = 16 ± 0.05 Oe, θ
D = 425 K and γ = 3.19 mJ/mol/K (13); the pressure dependence of Tc is -(5.1 ± 0.9) × 10-4K/atm (19).

Rhodium


In the purities generally available commercially, rhodium is not superconducting at 0.086 K (13) nor at 0.003 K (1). On solid solution samples of the alloy systems rhodium-osmium and rhodium-iridium the superconductivity was measured at compositions of up to 80 atomic per cent rhodium and an estimate then made of the behaviour of rhodium as an element (1). X-ray analysis suggested the samples were homogeneous, but microprobe investigations of the rhodium-iridium alloys showed some slight coring, but for the rhodium-osmium alloys the concentration varied by up to 20 atomic per cent. In order to avoid the introduction of impurities and segregation, the alloys had not been heat treated and therefore showed relatively broad transitions. The superconducting transitions for the rhodium-iridium alloys were spread over a width of 1 mK while for the rhodium-osmium alloys, at the higher temperatures, the spread was very wide and sometimes incomplete. However, as mentioned earlier, it was deduced from these results that rhodium would become a superconductor at a temperature of 0.2 × 10-3 K, a temperature predicted to be “at least a decade away” (1). With significantly improved refrigeration facilities it has recently been established that polycrystalline rhodium is in fact superconducting at a critical temperature of 325 μ K (2, 3).

Palladium and Platinum


Even extra purified palladium and platinum are not superconducting. The lowest test temperature published is 0.1 K (7, 13,20) but according to a private communication from A. C. Mota neither palladium nor platinum are superconducting, even if checked at temperatures as low as 10 × 10-3 K.

Alloys of Platinum Group Metals: with Hydrogen


The discovery of superconductivity in palladium-hydrogen samples, loaded electrolytically up to H:Pd ratios >0.8, started an intensive investigation of binary and ternary palladium-hydrogen alloys with regard to superconductivity (21). By using ion-implantation techniques and low temperature electrolysis, H:Pd ratios >1.0 were obtained with critical temperatures of 8.8K for palladium-hydrogen and ∽10.7 K for palladium-deuterium samples (22). Ternary palladium-copper-hydrogen samples exhibited a maximum critical temperature at H:Pd55 Cu45 ≈ 0.8 of about 17 K. The values in the system palladium-silver-hydrogen are: H:Pd70Ag30 ≈ 0.8 and Tc ≈ 16K and in the system palladium-gold-hydrogen they are: H:Pd84Au16 ≈ 0.9 and Tc = 13.6 K (23). In these alloys the maximum critical temperature shows up at lower H:Pd ratios and this effect is explained by the reduction of the critical temperature lessening “spin” fluctuations (24). Apparently the disappearance of the strong paramagnetism of palladium is a prerequisite for the occurrence of superconductivity, as has been shown earlier for palladium-molybdenum and palladium-tungsten solid solutions (24). Ion-implantation experiments on Pd1-xAlx and Pd1-xTix alloys (H:Pd ≤ 0.15) showed a reduction in the maximum critical temperature with increasing aluminium and titanium concentration (25, p. 236). Palladium with ion-implanted lithium, boron, carbon or nitrogen showed superconductivity only for the palladium-boron-hydrogen and palladium-carbon-hydrogen alloys at 3.8 and 1.3 K. The lithium and nitrogen containing samples were not superconducting above 0.2 K (25, p. 237). The effect of replacing part of the palladium by nickel, platinum or rhodium and silver in ion-implanted and pressure loaded palladium-hydrogen and palladium-deuterium alloys has been considered (25, p. 243; 26, 27, 28). Up to 10 atomic per cent nickel suppresses the critical temperature rather strongly, but platinum has little influence on critical temperature at concentrations up to 25 atomic per cent (25, p. 253).

There is no relationship between critical temperature, susceptibility and the coefficient of the low temperature specific heat for the rhodium-palladium-silver and nickel-palladium-platinum alloys investigated (25, p. 254).

The pressure coefficient of palladium-hydrogen alloys δlnTc/δP is -0.01 K/bar (25, p. 253).

For samples containing identical hydrogen and deuterium concentrations, the isotope effect results in a Tc,D/Tc,H in the range 1 to 2.2 (29). In thin palladium films it was observed that the behaviour corresponds to that of bulk palladium at thicknesses above 2000 Å. Films with thicknesses of 320 Å exhibit no superconductivity at up to 1 K (30). In very fine palladium-hydrogen powder samples, produced by electrolytic co-deposition at -30°C, which X-rays showed to be amorphous, no superconductivity was observed above 1.9 K even if the palladium:hydrogen ratio was varied (29), confirming observations by Raub and Loebich (31).

Alkaline Metals


None of the alloys of the platinum metals with lithium, sodium or potassium is superconducting above 2 K. This is valid for the solid solutions, as well as for the compounds at present known (7, 13, 32). Electrolytically loading palladium-lithium solid solution samples with hydrogen did not produce superconductivity above 1.9 K (33).

Copper, Gold and Silver


None of the alloys of the platinum metals with copper, gold or silver is superconducting above 2 K. Rhodium-copper samples, in the solid solution range, are not superconducting down to 0.3 K (33). The ternary phase Ag2Pd3S, A13-type, a = 7.235 Å, has Tc = 1.13 K (34).

Group II Elements


Barium, Beryllium, Calcium, Magnesium, and Strontium


Beryllium forms superconducting phases with four of the platinum metals but not with palladium and platinum; however sometimes it is culear to which phases superconductivity must be attributed (7, 13, 35). Be13Ru and Be2Ru show critical temperatures of 1.3 and 1.35 K, respectively (35). Of the many beryllium-rhodium compounds only Be2Rh is superconducting at 1.37 K (35). The transition temperatures of the beryllium compounds increase with beryllium content: for Be5Os Tc = 9.2, for Be2Os Tc=3.07 and for Be0.95Os0.05 Tc = 0.57 K (7, 35). Among the beryllium phases with iridium only Be5Ir is superconducting, with Tc = 1.5 K (35). The transition temperatures of the calcium compounds with C15-type Laves phase structure are for CaRh2 Tc = 6.4 and for CaIr2 Tc = 4 to 6.15 K; for the corresponding strontium and barium phases SrRh2 Tc = 6.2, SrIr2 Tc = 5.7, SrPt2 Tc = 0.7, and BaRh2 Tc = 6.0 K (36, 36).

Cadmium, Mercury and Zinc


None of these three are known to form superconducting phases with the platinum metals, which is at least partially explained by the problems concerned with the formation of superconducting shields and the experimental difficulties due to the wide range of vapour pressures among the components (37).

Group III Elements


Aluminium, Boron, Gallium, Indium and Thallium


The compound B3Ru is superconducting at 2.58 (7) and B2Ru at 1.6 K (38,39). None of the binary borides of platinum or palladium shows superconductivity above 1.2 K (32, 32). Superconducting ternary palladium-hydrogenboron alloys can be prepared by ion-implantation techniques (25). OsB2 has a critical temperature of about 2 K (39), but superconducting binary iridium borides are unknown. Ternary iridium-boron phases having an orthorhombic structure are B2IrMo2 with Tc = 1.25, B2Ir1.2V1.8 Tc = 1.25 and B2IrV2 Tc = 1.12 K. The corresponding ruthenium compounds are B2RuMo2 with Tc = 2.48 to 2.57, B2RuW1.5 Tc = 2.83 to 3.28, B2RuW2 Tc = 3.32 to 3.42 K (39). The hexagonal phases Ba0.67Pt3B2, Sr0.67Pt3B2 and Ca0.67Pt3B2 are superconductors below 5.6, 2.7 and 1.57 K, respectively (40).

Among osmium-aluminium phases OsAl and Os4Al13 become superconducting at 0.9 and 3.3 K, respectively (32, 32). The critical temperature of 5.9 K for OsAl3 is probably the same for Os4Al13 (41, 41).

Lanthanum, Scandium, Yttrium and the Rare Earth Metals


With the exception of palladium and platinum, the platinum metals together with lanthanum, scandium, yttrium and the rare earth elements form many superconductors (7, 13, 32, 43, 44, 51, 46, 47). It was first shown by V. B. Compton and B. T. Matthias (48) that the C14- and C15-type Laves phases of the platinum metals and the rare earths, and the Group III metals without free 4f electrons, exhibit superconductivity, while those with free 4f electrons, that is the elements from praseodymium to ytterbium exhibit ferromagnetism, an exception being CeRu2 (C15-type) for which Tc = 4.9 K (32, 48). The simultaneous occurrence of superconductivity and ferromagnetism in pseudo-binary systems has been investigated in a series of papers on C15-type YOs2-GdOs2, CeRu2-PrRu2 and CeRu2-GdRu2 alloys (4451).

Among the C15 phases of the noble metals with Group III elements, CeRu2 and LaOs2 exhibit the highest critical temperature, this being 6.5 K. Among C14-type phases they are YOs2 and LuOs2 with Tc = 4.7 and 3.49 K, respectively. Among the Os2-rare earth compounds the Curie temperature is highest at OsGd2 (80 K). It drops down evenly to near 0 K at OsLu2 (46). The magnetic moments show two maxima at Ru2Pr and Ru2Dy and a minimum for europium compounds. This behaviour is equalled by that of the iridium and osmium rare earth C15-type phases (52).

In the system iridium-yttrium a maximum critical temperature of 0.5 K is found at about 50 atomic per cent. Most likely this is caused by shields of Ir3Y or Ir4Y. Ir2Y3 is superconducting at 1.61 K and Ir2Y is superconducting in its homogeneity range, from 0.88 to 2.18 K. The composition of the superconducting phase richest in iridium is near Ir4Y (53). Nevertheless the behaviour is still somewhat unclear. Scandium-rich rhodium alloys show superconductivity beginning (partial superconductivity) near 0.9 K (53).

In the systems consisting of platinum metals with yttrium, scandium and lanthanum superconductivity is explained by T. H. Geballe as most likely being of a filamentary nature (54). In the lanthanum-rhodium system approximately 1 per cent lanthanum in the melt is enough to make the solidified alloy appear fully superconducting. Even 0.1 per cent lanthanum gave a smeared transition (54).

Ternary Compounds of the Type RERh4B4


The ternary borides of the rare earth metals belong to a group of new superconducting compounds, which was discovered by B. T. Matthias and collaborators in 1977 (55). The compounds are tetragonal in structure, of the CeCo4B4 type (55, 56). As with the binary C15 phases of the rare earths, elements with an unfilled 4f shell, namely neodymium, samarium, erbium, thulium and lutetium, show superconductivity; those with the filled shell, that is gadolinium, terbium, dysprosium and holmium are ferromagnetic (57). Up to now, it is impossible to prepare compounds with lanthanum, cerium, europium and ytterbium. All superconducting phases exhibit long range magnetic ordering at a temperature Tm which is below the superconducting transition temperature Tc. The superconducting transition temperatures and the ferromagnetic Curie temperatures of the RERh4B4 compounds are shown in Figure 1 (55).

Fig. 1

Superconducting critical (Tc), Néel (TN) and Curie (Tm) temperatures of RERh4B4 compounds, after reference (55). ∘ Tc, ▪ Tm, ▭ TN

Superconducting critical (Tc), Néel (TN) and Curie (Tm) temperatures of RERh4B4 compounds, after reference (55). ∘ Tc, ▪ Tm, ▭ TN

A wide range of pseudo-binary compounds of these phases was investigated for the interaction between superconductivity, magnetic ordering and “crystalline field splitting”, for example Er1-xHoxRh4B4 (58) and Sm1-xErx Rh4B4 (55). Rhodium can be replaced at least partially by ruthenium (59) or iridium (60). For the phase Ho(Ir0.6Rh0.4)4B4 co-existence between superconductivity and antiferromagnetism was observed, the Néel temperature being higher than the critical temperature (60). Crystallographic, magnetic and superconducting properties of the system YRh4B4-LuRh4B4-ThRh4B4 lead the authors to conclude that the lattice volume has relatively little influence on the electronic density of states.

Several reviews of the numerous investigations carried out on ternary borides have already been presented (5964). The structure of RERh4B4 is shown in Figure 2 (64).

Fig. 2

The primitive tetragonal crystal structure of the rare earth rhodium borides RERh4B4 with the outline of the unit cell indicated by broken lines. For clarity, the Rh4B4-clusters are not drawn to scale, after reference (64)

The primitive tetragonal crystal structure of the rare earth rhodium borides RERh4B4 with the outline of the unit cell indicated by broken lines. For clarity, the Rh4B4-clusters are not drawn to scale, after reference (64)

Group IV Elements


Germanium, Lead, Silicon and Tin


Among these compounds the only superconductors are PdSi, where Tc = 0.93 K; PtSi, Tc = 0.8 K; Rh5Ge3, Tc = 2.12 K (7, 7, 7); together with RhGe, Tc = 0.96 K (65); IrGe, Tc = 4.7 K (66); PtGe, Tc = 0.4 K (32); and RhSn2, Tc = 0.60 K (67). Ternary superconductors are the ruthenium and osmium germanides Y3Ru4Ge13 and Y3Os4Ge13, having a Yb3Rh4Sn3 type structure, see Tables I and II (68, 69, 70). Ternary silicides and germanides with ruthenium, rhodium and osmium and the rare earths, of the Sc5Co4Si10 type of structure, show a similar behaviour (68, 69, 70). The superconducting transition temperatures of the RE3T4Ge13 phases are shown in Table I and the magnetic ordering temperatures in Table II (68, 68, 68).


Table I

Superconducting Transition Temperatures (Tc) of Some RE3T4Ge13 Compounds

(RE is Yttrium or Lutetium; T is Ruthenium or Osmium)

CompoundTc ± 0.1K
Y3Ru4Ge13 1.7 -1.4
Y3Os4Ge13 3.9-3.7
Lu3Ru4Ge13 2.3-2.2
Lu3Os4Ge13 3.6-3.1

Table II

Magnetic Ordering Temperatures (Tm) for Some RE3T1Ge13 Compounds

Osmium compoundsTm ± 0.1 KRuthenium compoundsTm ± 0.1 K
Ce3Os4Ge13 6.1 Ce3Ru4Ge13 6.7
Pr3Os4Ge13 16.0 Pr3Ru4Ge13 14.2
Nd3Os4Ge13 1.9
Eu3Os4Ge13 10.1
Tb3Os4Ge13 14.1
Dy3Os4Ge13 2.1
Er3Os4Ge13 1.9 Er3Ru4Ge13

The superconducting transition and magnetic ordering temperatures of Sc5Co4Si10 type phases, having a primitive tetragonal lattice of a new structure type, are listed in Table III (68, 68, 70).


Table III

Superconducting Transition Temperatures (Tc) and Magnetic Ordering Temperatures (Tm) of Sc5Co4Si10 Type Compounds

CompoundTcKTmK
Sc5Co4Si10 5.0 - 4.8
Sc5Rh4Si10 8.54 - 8.45
Sc5Ir4Si10 8.46 - 8.38
Y5Ir4Si10 3.0 - 2.3
Lu5Ir4Si10 3.76 - 3.72
Tm5Ir4Si10 1.0
Er5Ir4Si10 2.3
Ho5Ir4Si10 1.5
Dy5Ir4Si10 5.0
Y5Ir4Si10 2.62 - 2.58
Y5Os4Ge10 8.68 - 8.41

Similar superconductivity/magnetic ordering behaviour is shown by ternary stannides RERh1.1Sn3.6 and REOsxSny, respectively (7181). Both ErOsxSny and ErRh1.1Sn3.6 for example show re-entrant superconductivity (7281). The phases in the system Me-tin show at least four different structures: I, I', II, and III depending on the kind of Me (80). Phase I compounds are cubic, space group Pm3n with a lattice constant a = 9.7 Å; I' compounds are distorted cubic; II are tetragonal with aII = 13.75 and cII = 27.4 Å while III are cubic with aIII = 13.7 Å (80). Phases I and II with Me being rare earth elements from the series lanthanum to gadolinium, and ytterbium, calcium, strontium and thorium are superconducting between 8.7 and 1.9 K. Phase II of the erbium compound exhibits re-entrant superconductivity (where Tc = 0.97 K and Tm = 0.57 K). Phase II of yttrium and scandium are superconducting at 3.2 and 4.5 K, respectively (80).

Ternary compounds of the MnCu2Al (Heusler) type YPd2M (where M is tin, lead, indium and antimony) are superconducting. Superconducting transition temperatures are YPd2Sn 3.72 YPd2Pb 4.76 and YPd2In 0.85 K (82).

The phases RhPb2 and PdPb2, being C16- or CuAl2-type are superconducting at 2.66 and 2.95 K, respectively (83).

Titanium, Zirconium, Hafnium and Thorium


Many alloys of titanium, zirconium and hafnium with even fairly small additions of other transition elements show a big increase in the transition temperature of their hexagonal close-packed a -phase (43, 84), see Figure 3. Most of these effects are caused by superconducting filaments or shields of the ß -phase or a/ß transition intermediate phases, as was at first assumed (85) and recently discussed (85). In these systems, a very careful definition of the state of the sample is necessary in order to obtain reliable results on their superconductivity. For example oxygen impurities may play a role which is shown by the superconductivity of the ternary EO3-type phases Ti0.573Rh0.287O0.14 and Ti0.573Ir0.287O0.14 (Tc = 3.37 and 5.5 K, respectively), also Zr0.61Rh0.285O0.105, Zr0.61Pd0.285O0.105 and Zr0.65Ir0.265O0.085 (Tc = 11.8, 2.09 and 2.30 K, respectively) (32). The A15 phases Ti3Ir and Ti3Pt become superconducting at 5.4 and 0.58 K, respectively (32). Among the C14-type phases HfOs2 and ZrOs2 (Tc = 2.69 and 3.00 K respectively) show superconductivity at higher temperatures than the C15 phases ZrIr2 and ThIr2 (Tc = 4.10 and 6.5 K, respectively) (32). Data for the D102-type phases are for Th7Rh3 Tc = 2.15, for Th7Os3 Tc = 1.51 and for Th7Ir3 Tc = 1.52 K. Th7Pt3 is not superconducting above 1.02 K (32). In the system titanium-platinum, depending on sample preparation, a critical temperature between 0.4 and 4.5 K is measured. Ternary superconducting solid solutions based on body-centred cubic titanium-rhodium and zirconium-rhodium have been investigated (43, 84); in all cases the critical temperature was reduced by the presence of platinum group metals.

Fig. 3

Transition temperature versus concentration curves for titanium-iridium and titanium-rhodium alloys, after reference (43)

Transition temperature versus concentration curves for titanium-iridium and titanium-rhodium alloys, after reference (43)

Group V Elements


Phosphorus, Arsenic, Antimony and Bismuth


Superconductivity data for binary phosphides and arsenides of the platinum metals are listed in Table IV. It is interesting to note the difference between the critical temperature of the high and low temperature forms of Pd7P3 and Pd2As, where the high temperature form always exhibits a slightly higher critical temperature. Among the antimonides only the B81-type phases PdSb and PtSb are superconducting with Tc = 1.5 and 2.1 K, respectively (88). Among the bismuthides, ß -PdBi2 has a higher critical temperature than a -PdBi2, Tc = 4.2 and 1.7 K, respectively (7, 13). A review on the superconductivity of binary compounds of the platinum metals with elements of Groups IV, V and VI has been published elsewhere (89).


Table IV

Superconducting Transition Temperatures and Structures of Binary Phosphides and Arsenides of Rhodium and Palladium

CompoundTcKStructure
Rh2P 1.3 cubic (CaF2)
Rh4P3 2.5 orthorhombic
RhAs 0.58 orthorhombic (MnP)
Rh1.4As 0.56 hexagonal
Pd3P 0.75 orthorhombic (Fe3C)
Pd7P3 (h.t.) 1.0 rhombohedral
Pd7P3 (l.t.) 0.7 ?
Pd5As2 0.46 ?
Pd2As (h.t.) 1.7 hexagonal (Fe2P)
Pd2As (l.t.) 0.6 hexagonal

Ternary hexagonal phosphides and arsenides of ruthenium and osmium constitute a new class of ternary superconductors discovered in 1980 (90). Transition temperatures and lattice parameters of the known superconducting ternary phosphides are listed in Table V and of the corresponding arsenides in Table VI (90, 91). The proposed structure of the hexagonal ternary phosphide ZrRuP is shown as Figure 4 (90).


Table V

Superconducting Ternary Phosphides of Ruthenium and Osmium

CompoundTcKStructureLattice constants Å
a ± 0.006c ± 0.004
TiRuP 1.33 ZrRuSi 6.325 3.542
ZrRuP 12.34 – 10.56 ZrRuSi 6.459 3.778
HfRuP 12.70 – 11.08 ZrRuSi 6.414 3.753
TiOsP < 1.2 ZrRuSi 6.285 3.625
ZrOsP 7.44 – 7.1 ZrRuSi 6.460 3.842
HfOsP 6.10 – 4.96 ZrRuSi 6.417 3.792

Table VI

Superconducting Ternary Arsenides of Ruthenium and Osmium

CompoundTcKStructureLattice constants Å
abc
TiRuAs <0.35 TiFeSi 7.287 11.227 6.515
ZrRuAs (900°C) 11.90 – 10.03 ZrRuSi 6.586 3.891
ZrRuAs (flux) 3.2 ZrRuSi 6.624 3.823
HfRuAs 4.93 – 4.37 ZrRuSi 6.568 3.842
ZrOsAs 8.0 ZrRuSi 6.602 3.794
HfOsAs 3.2 ZrRuSi 6.569 3.808

Fig. 4

The proposed structure of the hexagonal lernary phosphide ZrRuP. Lines have been drawn to indicate the Ru3 two-dimensional triangular clusters and also the linking of these clusters through zirconium-ruthenium bonds, after reference (90)

The proposed structure of the hexagonal lernary phosphide ZrRuP. Lines have been drawn to indicate the Ru3 two-dimensional triangular clusters and also the linking of these clusters through zirconium-ruthenium bonds, after reference (90)

Depending on their preparation and heat treatment, some of the superconducting phosphides and arsenides show high or low temperature modifications, which in most cases have now been fully identified and reported (91).

No superconductivity was observed for the LaFe4P12 type compounds RERuP12 (RE = cerium, praseodymium or neodymium). The influence of oxygen on the critical temperature of ZrRuP has been investigated (92). Oxygen is not taken into the lattice, but reduces the phosphorus concentration, which itself determines critical temperature. By specific heat measurements, the following critical temperature data were obtained: for ZrRuP Tc = 13, for HfRuP Tc = 10.8 and for TiRuP Tc = 1.2 K. The relatively low critical temperature of TiRuP may be caused by some kind of magnetic interaction (93).

Vanadium, Niobium and Tantalum


Solid solutions of elements of Groups V and VI in the Periodic Table, when present in ruthenium and osmium reduce their critical temperatures (43). In vanadium-rich solutions the presence of 10 atomic per cent palladium or platinum reduces the critical temperature of vanadium to below 1.5 K (94).

Intermetallic Phases


Since the last review article on the superconductivity of the platinum metals and their alloys practically no new binary superconductors have been found (43). Some work has been concerned with the influence of additions on the behaviour of pseudo-binary phases (7, 13, 32, 43, 100). Critical temperatures of the δ-phases in the systems niobium-rhodium and tantalum-rhodium have been shown to be 2.95 K for the homogeneous phase Nb0.652Rh0.348, and 424 K for the heterogeneous sample Nb0.637Rh0.363 (96). This may be caused by a very narrow homogeneity range.

The δ-phase in the tantalum-rhodium system is not superconducting, however (96). A15-phases with the highest transition temperatures among these compounds are Nb3Pt and Mo0.74Ir0.76 with Tc = 9.3 and 9.05 K, respectively. Special attention should be given to the peculiar A15-type phase Os0.5V0.5 which has a critical temperature of 5.15 K (7, 13, 32, 43, 97).

Group VI Elements


Sulphur, Selenium and Tellurium


A review of the superconductivity of sulphur, selenium and tellurium compounds has been presented (98). Superconductivity was discovered only among the selenides of palladium (for Pd6-7Se Tc = 0.66 and for Pd4Se Tc = 0.42 K) and the tellurides of palladium, platinum and iridium. Data on the latter are given in Table VII.


Table VII

Superconducting Tellurides of the Platinum Metals

CompoundTcKStructure
Pd3Te 0.76
Pd1.1Te 4.07 hexagonal (B8.)
PdTe 3.85 hexagonal (B8.)
PdTe1.02 2.56 hexagonal (B8.)
PdTe1.08 1.88 hexagonal (B8.)
PdTe2 1.69 trigonal (C6.)
PtTe 0.59 orthorhombic
IrTe3 1.18 cubic (C2.)

The decrease of the critical temperature of the B8 phase PdTe with tellurium concentration within its homogeneity range, is peculiar. The coefficient of the electronic specific heat γ drops from 13.5 to 9.9 cal/mol/degree2, while θ
D, the Debye temperature, is reduced from 277 to 239 K in the same range (98).

Apparently there is a sharp peak in the density of states and energy at the stoichiometric composition.

Chromium, Molybdenum and Tungsten


In the system ruthenium-chromium the critical temperature of the 20 atomic per cent chromium alloy is 0.3 K, while at 30 atomic per cent it is higher at 1.65 K (99). Both RuCr2 and RuCr3 are superconducting, at 2.0 and 3.3 K, respectively (99).

Solid solutions of molybdenum and tungsten with ruthenium show an increase in the critical temperature with increasing ruthenium content (43); the critical temperature of the δ-phase with 50 atomic per cent molybdenum is 10.6 K (100).

Further work on superconductivity in the ruthenium-molybdenum system has previously been summarised (44). The δ-phase Ru0.4W0.60 is superconducting in the range 4.67 to 5.2 K (32). Among osmium-chromium alloys the δ-phase and also the A15-phase OsCr3 are superconductors with critical temperatures of 1.03 and 4.03 K, respectively (7, 13). The δ-phases of osmium with molybdenum and tungsten have critical temperatures of 5.65 and 3.81 K, respectively (101). Superconducting iridium-chromium alloys are IrCr3, of ß -W type, with Tc = 0.75; IrMo3 with critical temperatures in the range of 8.17 to 8.8 K, depending on ordering and the δ-phase Ir0.28Mo0.72 with a critical temperature of 4.46 K (7, 13, 43). The A12-type compound Nb0.60Pd0.40exhibits superconductivity at 2.04 to 2.47 K (101).

Group VII Elements


No superconducting phases are known between the platinum group metals and Group VII elements. Only small amounts of rhenium in iridium reduce its critical temperature, while a few atomic per cent iridium in rhenium seem to have little influence on its critical temperature (102, 103).

Group VIII Elements


Iron, cobalt and nickel, when present in solid solution, all considerably reduce the critical temperature of the superconducting platinum metals. The influence on non-superconducting platinum and palladium is less (102, 103). Superconducting compounds are unknown.

Summary


The platinum metals and their compounds exhibit an extremely wide variety of behaviour at low temperatures. This ranges from the four elements which are superconducting to new classes of superconducting platinum metal hydrides, borides, silicides, stannides, phosphides and arsenides. Since some of the phases show the co-existence of magnetism and superconductivity, their investigation has resulted in a wide interest in low temperature materials research. In addition to increasing the understanding of the phenomenon of superconductivity, these investigations will no doubt result in time in applications which are not yet visualised.

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