Characteristic examples of three types of change of form of p–n relationships at 25°C, as the contents of elements alloying with palladium are gradually increased (4,37,70,74–83,95–98,104,108,153,154), are illustrated in Figure 9. As for the palladium-hydrogen system itself (4), such relationships have been determined from direct equilibrations with hydrogen gas, as well as being derived from measurements of electrode potential. Similarly as for palladium-hydrogen, temperature dependences of the p–n relationships have been used to supplement calorimetric data (110) in deriving heats and entropies of hydrogen absorption, both at initially low values (79–82,95,100,108) of hydrogen content (n →0) and over α ⇌ β transition regions (4,37,70,74–83,104,154,178).

The sequence of relationships in Figure 9(c) shows gradual increases with increasing rhodium content in the range of n over which α and β phases coexist. This sequence now seems unique to the palladium-rhodium-hydrogen system (45,75,153,154), since some suggestions of similarities of the palladium-manganese-hydrogen system (179) seem doubtful in view of possible incomplete equilibrations.

The remaining families of p–n isotherms for palladium alloy-hydrogen systems would thus seem to be divisible into two groups represented by those in Figures 9(a) and 9 (b). In both cases the ranges of n over α + β phase regions are successively shortened with increasing content of the alloying element, even in the cases of alloys with readily hydride-forming transition or inner transition elements such as vanadium (82) or cerium (76), but the corresponding values of equilibrium pressure are being successively increased (74,79,81,82), as in Figure 9(a), or decreased (37,70,75,80), Figure 9(b).

Decreases, in the range of n over α + β regions, with increasing amounts of alloying elements, have continued to be deduced from X-ray determinations of α and β phase lattice constants for additional palladium alloy-hydrogen systems (4,79–82,157,180–182).

Apart from the probable case of the palladium-nickel-hydrogen system (50), these general trends of decreasing extents of α, β coexistence with increasing contents of alloying element are similar to the effects of increasing temperature in the palladium-hydrogen system, being accompanied by decreases in the extents of hystereses of absorption and desorption relationships between hydrogen contents and other experimental parameters (4,69,96–98). This has been taken as complementary to decreases in the value of critical temperatures for two phase coexistence and, as discussed later, has importance in choices of alloy compositions as hydrogen permeation membranes.

In addition to the arguments concerning the solubility of hydrogen in the palladium-silver alloys, the extents of decreases of hydrogen solubilities with increasing contents of other alloying metals have been used (95,104,108) in support of an essentially protonic nature of the hydrogen entities. However, in order to allow for proposed levels of electron donation it has seemed necessary to postulate adoptions of oxidation states by the alloying elements which can be difficult to reconcile with their relative positions in sequence of electronegativities.

Moreover as for the palladium-hydrogen system, latterly there has been increasing emphasis on the account to be taken of lattice expansions accompanying hydrogen absorption (26,37,166) in palladium alloys. There has also been attention to the localised as well as to the continuing effects of the alloying metals as exemplified by appearance, potential measurements (183) and by Mössbauer Spectroscopy examinations (133,180,184,185). For example, evidence of preferential location of hydrogen around rhodium atoms (186) might be correlated with wider ranges of α and β phase coexistence with rhodium content in the palladium-rhodium-hydrogen system in Figure 9(c). A particular example of the significance of differences in the distribution of alloying atoms has been provided by measurements, made under high pressures of hydrogen, of changes of electrical resistivity of a palladium-iron alloy of approximate composition Pd₃Fe (187). Figure 10 illustrates the substantial differences between the ranges of pressures (hydrogen chemical potentials) over which evidence was found of significant hydrogen absorption in ordered and disordered structures of the alloy.

The two main families of p–n isotherms illustrated in Figures 9(a) and 9(b) also differ from one another in regard to the direction of changes of the heats of hydrogen absorption with alteration of alloying metal content (4,37,70,74–76,79–82,100,178), relative to heats of hydrogen absorption in pure palladium. Thus those alloys with the pattern of Figure 9(b) in general exhibit more exothermic heats of hydrogen absorption than palladium, both at infinite dilution (n → 0) and over regions of α and β coexistence, while the converse (less exothermic heats of absorption than palladium) appears generally true for alloy series associated with families of p–n isotherms as in Figure 9(a).

Attempts to provide underlying reasons have so far stressed two alternative ways by which the different series of alloys may themselves be divided. In one line of argument (100,178), emphasising differences in electronic structure, it has been pointed out that patterns similar to Figure 9(a) are followed by alloys formed by palladium with other transition elements, and those of Figure 9(b), accompanied by more exothermic heats of hydrogen absorption, tend to occur with alloys of non transition elements.

An alternative division (76, 82, 100) has been based on findings that the patterns of p–n relationships in Figure 9(b) are followed by alloys of palladium with elements which produce overall average increases of f.c.c. lattice constants (“expanded alloys”) and vice versa. Since additions of platinum to palladium produce very small increases of lattice constant (188), the palladium-platinum-hydrogen system constitutes a marginal exception to this latter division. However, it may be possible that differences of elastic constants provide a deciding feature in this case. In explanations based on size factors, it might be argued that atoms of alloying elements causing increases of lattice parameters in an elastic medium could produce overall expansions in the volumes of interstices which could result in lower energies being required to introduce hydrogen, as reflected by the high exothermic heats of hydrogen absorption. It is in any event interesting, as illustrated in Figure 11, that relative extents of increases of initial lattice parameter with contents of alloying element, quite closely parallel extents of decreases of isobaric (1 atm) hydrogen solubilities in such chemically diverse sub-stitutionally alloying metals as cerium (76), silver (70–72,86,95–98,104,160), lead (80) and with interstitially incorporated boron (37,189).

Occurrences of superconducting transitions at very high contents of hydrogen have been studied with several series of palladium alloys. In some series higher transition temperatures have been recorded than with pure palladium (46,143,190), and again slightly higher values have been found for corresponding deuterides (46). The highest transition temperature so far reported has been ∼17K with a Pd₅₅Cu₄₅ composition (46). With increasing content of alloying element in several alloy series the transition temperature passes through a maximum value and, as for PdH_n(D_n), plots of transition temperature against hydrogen content also exhibit maxima (46). Recent studies with palladium-silicon alloys (191) have shown that superconducting transitions also can be found with amorphous materials.

For many of the investigated palladium alloy-hydrogen systems, the alloy element can only be considered to be in solid solution up to relatively low limits of concentration (37,75, 80, 82, 85, 108, 133, 180, 181, 189, 190, 192, 193), and only a few studies have so far been reported with higher alloy percentage intermetallic phases (194,195).

However, palladium can form binary alloys with certain neighbouring elements also crystallising in the f.c.c. structure, which approximate to solid solutions over the entire composition range.

In principle these systems have offered possibilities of studying gradual changes in alloying behaviour from pure palladium to that of the pure alloying element. In practice this has required extension of studies into the range of high hydrogen pressures, which at upper limits correspond to particularly high fugacities (45,196). Here, difficulties of making direct measurements of hydrogen contents have so far required that these are inferred from changes in physical properties such as thermoelectric power (50,197) and electrical resistivity.

Examples of relationships derived between electrical resistivity and high hydrogen pressures obtained with three such series of alloys (45,50,68,69,154,196), combined with data previously derived over lower ranges of pressure (4,70,75), are illustrated in Figure 12.

The somewhat complex case of the palladium-silver-hydrogen system (70,196) in Figure 12(a) at 25°C illustrates, in keeping with the family of p-n isotherms in Figure 9(b), that initial increases of silver content are associated with pronounced changes of resistivity, complementary to regions of α → β transitions or quasi transition, at increasingly lower pressures than for the α → β transition for pure palladium. This trend may be seen to continue for compositions of up to approximately Pd₅₀Ag₅₀, but for still higher silver contents significant changes of resistivity only become perceptible at increasingly higher pressures, as shown by the examples of Pd₃₁Ag₆₉ and Pd₁₉Ag₈₁. These provide an indication that substantial absorption of hydrogen by pure silver should only occur at 25°C at hydrogen pressures in excess of 30 kilobars.

Trends in the form of relationships for the palladium-rhodium-hydrogen and palladium-platinum-hydrogen systems (50,68,69,154) in Figures 12(b) and (c) are somewhat more simply continuous in appearance.

In the case of the palladium-rhodium-hydrogen system (154), measurements of approximate hydrogen contents attained under high hydrogen pressures have endorsed indications from the persistence of the hysteresis effects illustrated in Figure 12(b) that α ⇌ β phase hydride transitions continued to occur for alloying contents up to 80 per cent rhodium. These findings suggested the possibility of the formation of a β -phase hydride by pure rhodium at still higher pressures, and subsequent analogous studies of resistivity changes at 250°C have since indicated the formation of such a rhodium hydride at pressures in the region of 40 kilobars (155).

Figure 12(c) illustrates, for the palladium-platinum series of alloys, that continuously increasing hydrogen pressures again are needed to effect significant hydrogen absorption as platinum contents are increased (68,69). This is consistent with trends in p-n relationships (4,74) similar to those in Figure 9(a) which also would be consistent with universal decreases of hydrogen solubility with increasing hydrogen content at any given reference of hydrogen pressure at 25°C.

In contrast to the pattern in Figure 12(b) for the palladium-rhodium-hydrogen system, Figure 12(c) also illustrates a gradual reduction of hysteretic differences between “absorption” and “desorption” relationships with increasing platinum content. Again this is consistent with indications from p-n relationships and X-ray studies (4,50,69,74) of complementary decreases of the critical temperature of α and β phase coexistence to around 25°C, for platinum contents of about 25 per cent. However the resistivity changes with hydrogen pressure in Figure 12(c) also provide evidence of a continuation of substantial hydrogen absorption by alloys with platinum contents much higher than 20 per cent, over what have been termed regions of quasi two phase coexistence (68). Even for an alloy containing 70 per cent platinum, the onset of what would seem likely to be a further such region of absorption has been detected at hydrogen pressures of around 30 kilobar (50,69). Extrapolations of available data have suggested that analogous absorptions of hydrogen by pure platinum could be expected to occur at hydrogen pressures of near 100 kilobar, at 25°C.

For the palladium-platinum alloys with ≥ 20 per cent platinum, the absence of hysteretic effects and accompanying lower levels of macroscopic deformation in cycles of absorption and desorption of hydrogen have been found to be of some practical advantage. Because of this, measurements of changes of the electrical resistivity of certain palladium-platinum and other suitable alloy compositions have been usefully employed for repetitive analytical determinations of the solubility of hydrogen in solutions (4,198), and for estimating high values of hydrogen chemical potentials in electrochemical studies (39,50,62).