Superconductivity of the Pla tinurn
Metals and Their Alloys
A REVIEW SUMMARISING THE PUBLISHED SCIENTIFIC DATA
By Ch. J. Raub
Forsrhungsinstitut fur Edelrnetalle und Metallrhernie, Schwabisrh Grnund, West German!
Continuing investigation 0.f superconductivity, particularly o~fplatinum
metal compounds, has resulted in a growing undwstanding o,f 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. M a t t h i a s a n d o t h e r s t h a t
superconductivity-that
is the loss of any
electrical resistance below a critical
temperature, T,- 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 subse-
Platinum Metals Rev., 1984, 28, (2), 63-75
quently developed into specialised research
areas of their own. Even now it is impossible to
predict the T, or T, (T,= 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
Table I
Superconducting Transition
Temperatures (TJ of Some
RE,T,Ge,, Compounds
(RE is Yttrium or Lutetium;
T is Rutheniumor Osmium)
Lu,Ru,Ge,,
63
3.9- 3.7
2.3 - 2.2
3.6- 3.1
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 (I), and this has recently been
established (2, 3). The superconductivity of the
individual metals will now be considered.
( 1 0 )osmium
has no isotope effect, while Hein
and Gibson ( I 5 ) found T , z M-".*. The critical
field is 80.9 Oe when
= 187.4, and 80.2 Oe
= I 92.0 ( I 5). At a pressure of 30,000
when
atm the critical temperature of osmium is
reduced by 0.05 K ( I I). 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
= 500, had T, = 0.638 K ( I 6).
Ruthenium
The superconducting transition temperature
of ruthenium has been placed between 0.40 and
0.509 k 0.002 K (4, 5 , 6). The generally
accepted values for its superconducting
properties
are:
transition
temperature
T, = 0.49 t 0.01 5 K,
critical
field
Debye
temperature
H, = 69 5 2 Oe,
O1,= 580K, and the coefficient of low
temperature specific heat 1' = 2.8 mJ/moVK (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 RuIo4 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
of 99.2, 1 0 1 . 1 and 103.9 in magnetic
mass
fields up to 70 Oe gave the dependence:
Tc --M-l'R. At pressures up to 30,000
atmospheres the critical temperature is not
affected by the applied pressure ( I I), while its
volume dependence Gln(TJOD)/GlnV = - I 6 ( I 2).
a
Osmium
The critical temperature of osmium as given
in the literature ranges from 0.4 to 0.7 I K ( I 3).
An arc furnace melted sample of 99.9 per cent
purity had a resistance ratio of pu/p27)= 0.4
and T, = 0.71 K, at a width of transition of
0.02 K (14). Today generally accepted values
for the superconducting properties are
T, = 0.66 & 0.03 K, H,= 70 Oe, O D = 500 K
and y = 2.35 mJ/mol/K (7). Isotope effect
measurements do not agree, even when
apparently identical samples are checked
( I 0 , I 5). According to Geballe and Matthias
Platinum Metals Rev., 1984, 28, ( 2 )
64
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 TL= 0. I 4 K is
obtained for the iridium (17). However, for
50 ppm
of
iron
samples
containing
T, = 0. I 0-0. I I K ( I 8). Iridium purified by
electron beam melting until the total impurity
content is below 1 0 ppm has a critical
temperature curve with a mid point at
0. I I 25 t 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 x Io-lcm, and the penetration depth
3.9 x 1 0 'cm. Small impurity concentrations
reduce the critical temperature of iridium and
make it a superconductor of the second kind.
Accepted values are now: T, = 0.1 125 & 0.001
K, H,(T = 0 ) = 1 6 i 0.05 Oe, OD = 425 K and
1' = 3. I 9 mJ/mol/K ( I 3); the pressure depend0.9) x ro-'K/atm(rg).
enceofT,is-(5.1
*
Rhodium
In the purities generally available commercially, rhodium is not superconducting at
0.086 K ( I 3) nor at 0.003 K ( I ) . On solid solution samples of the alloy systems rhodiumosmium 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 ( I ) . X-ray analysis
suggested the samples were homogeneous, but
microprobe investigations of the rhodiumiridium 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 I 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 x lo-) K, a temperature predicted to be "at
least a decade away" ( I ) . With significantly
improved refrigeration facilities it has recently
been established that polycrystalline rliodium is
in fact superconducting at a critical
temperarure of 325 pK (2, 3).
C U Jz~ 0.8 of about 17 K. The values in the
system
palladium-silver-hydrogen
are:
H:Pd7"Ag3"z 0.8 and T, z I 6 K and in the
system palladium-gold-hydrogen they are:
H:PdsrAuI6=: 0.9 and T,= 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 Pd I-~AI,and
Pd I-xTixalloys (H:Pd 0.I 5 ) 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 palladiumboron-hydrogen
and
palladium-carbonhydrogen 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 ionimplanted and pressure loaded palladiumhydrogen 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.
<
Palladium and Platinum
Even extra purified palladium and platinum
are not superconducting. The lowest test
temperature published is 0.1 K (7, 1 3 , 2 0 ) but
according to a private communication from A.
C. Mota neither palladium nor platinum are
superconducting, even if checked at
temperatures as low as I o x I o-) K.
Alloys of Platinum Group Metals:
with Hydrogen
The discovery of superconductivity in
palladium-hydrogen
samples, l o a d e d
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 ionimplantation techniques and low temperature
electrolysis, H:Pd ratios > I .o were obtained
with critical temperatures of 8.8 K for
palladium-hydrogen
and
- I 0.7 K
for
palladium-deuterium samples (22). Ternary
palladium-copper-hydrogen samples exhibited a
maximum critical temperature at H:Pdsr
Platinum Metals Rev., 1984, 28, ( 2 )
65
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-palladiumplatinum alloys investigated (25, p. 254).
The pressure coefficient of palladiumhydrogen alloys 61nTJ6P is -0.01 W a r (25, p.
253).
For samples containing identical hydrogen
and deuterium concentrations, the isotope
in the range I to 2.2
effect results in a Tc,l)/Tc,H
(29). In thin palladium films it was observed
that the behaviour corresponds to that of bulk
palladium at thicknesses above 2000 A. Films
with thicknesses of 320 A exhibit no
superconductivity at up to I K (30). In very fine
palladium-hydrogen powder samples, produced
by electrolytic co-deposition at -3o"C, which
X-rays showed to be amorphous, no superconductivity was observed above 1.9 K even if the
pa1ladium:hydrogen ratio was varied (29), confirming observations by Raub and Loebich (3 I).
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 I .9 K (33).
are for CaRh2T, = 6.4 and for CaIr2 T, = 4 to
6. I 5 K; for the corresponding strontium and
barium phases SrRh2 T, = 6.2, SrIr2 -Ic= 5.7,
SrPt?T, = 0.7, and BaRhz T, = 6.0 K (36, 37).
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 t o the wide range of vapour
pressures among the components (37).
Croup I11 Elements
:II u in in i 11m, Horon. (;a I I iu m. I nd i uni
and Thalliuni
The compound BlRu is superconducting at
2.58 (7) and B2Ru at 1.6 K (38,39). None of the
binary borides of platinum or palladium shows
Copper, Cold and Silver
I .2 K
(32, 39).
superconductivity above
None of the alloys of the platinum metals Superconducting ternary palladium-hydrogenwith copper, gold or silver is superconducting boron alloys can be prepared by ionabove 2 K. Rhodium-copper samples, in the implantation techniques (25). OsB2 has a critical
solid solution range, are not superconducting temperature of about 2 K (39), but
down to 0.3 K (33). The ternary phase Ag2Pd3S, superconducting binary iridium borides are
unknown. Ternary iridium-boron phases
A I 3-type, a = 7.235 A, has Tc= I . I 3 K (34).
having an orthorhombic structure are B21rMo2
Group I I Elements
T,= 1.25 and B21rV2
with T, = 1.25, B21rI.2VI.X
Barium, Beryllium, Calcium. Magnesium. T, = I .I 2 K. The corresponding ruthenium
and Strontium
compounds are BzRuMo2 with T,= 2.48 to
Beryllium forms superconducting phases 2.57, B ~ R u W5 I T, = 2.83 to 3.28, BzRuWz
with four of the platinum metals but not with TL= 3.32 to 3.42 K (39). The hexagonal phases
palladium and platinum; however sometimes it Bao67PtlB2, Sr0.67Pt3B2 and Ca067Pt$2 are
is unclear to which phases superconductivity superconductors below 5.6, 2.7 and 1.57 K,
must be attributed (7, 13, 35). Be13Ru and respectively (40).
Among osmium-aluminium phases OsAl and
Be2Ru show critical temperatures of I .3 and
1.35 K, respectively (35). Of the many Os4Alli become superconducting at 0.9 and
beryllium-rhodium compounds only Be2Rh is 3.3 K, respectively (32,41). The critical
superconducting at I .37 K (35). The transition temperature of 5.9 K for OsA13is probably the
temperatures of the beryllium compounds same for Os4AlI l (4 I , 42).
increase with beryllium content: for BerOs
T , = 9.2, for Be20s T,= 3.07 and for Lanthanum. Srandium. I ttrium
BeoscOsoo~
T,= 0.57 K (7, 35). Among the and the Hare Earth I\.It.tals
beryllium phases with iridium only Bedr is
With the exception of palladium and
superconducting, with T,= 1.5 K (35). The platinum, the platinum metals together with
transition temperatures of the calcium com- lanthanum, scandium, yttrium and the rare
pounds with Cr 5-type Laves phase structure earth elements form many superconductors (7,
Platinum Metals Rev., 1984, 28, ( 2 )
66
and ferromagnetism in pseudo-binary systems
has been investigated in a series of papers on
1
4
1
C I 5-type YOs2-GdOs2, CeRuz-PrRu2 and
CeRu2-GdRuz alloys (44-5 I).
Among the C I S phases of the noble metals
with Group I11 elements, CeRuz and Laosz
exhibit the highest critical temperature, this
being 6.5 K. Among C I ~ - t y p ephases they are
YOSZ and LUOSZwith T,=4.7 and 3.49K,
respectively. Among the Os2-rare earth compounds the Curie temperature is highest at
OsGd2 (80 K). It drops down evenly to near o K
0
at OSLUZ
(46). The magnetic moments show two
0
0
o
maxima at RuzPr and RuzDy and a minimum
I
. . . . . . , .
U Ce Pr Nd Rn Srn Eu Gd Tb Dy Ho E r Trn Yb Lu
for europium compounds. This behaviour is
RARE EARTH ELEMENT
equalled by that of the iridium and osmium rare
I i g . I Superronducting critical (Tc).
earth C I 5-type phases (52).
YVPel ( T h ) and Curie (T,,,) temperatures
In the system iridium-yttrium a maximum
of HERh J5 I compounds, after rdt.renre
critical temperature of 0.5 K is found at about
( * 5 5 ) - 0‘1’#, mT,,,. oT1
50 atomic per cent. Most likely this is caused by
shields of Ir3Y or Ir4Y. IrZY, is superconducting
at 1.61 K and Ir2Y is superconducting in its
I 3, 32,43,44,45,46,47). It was first shown by
V. B. Compton and B. T. Matthias (48) that the homogeneity range, from 0.88 to 2. I 8 K. The
C14- and CIS-type Laves phases of the composition of the superconducting phase
platinum metals and the rare earths, and the richest in iridium is near Ir4Y (53). Nevertheless
Group 111 metals without free 4f electrons, the behaviour is still somewhat unclear.
rhodium alloys show
exhibit superconductivity, while those with free Scandium-rich
beginning
(partial
4f electrons, that is the elements from superconductivity
praseodymium to ytterbium exhibit ferromag- superconductivity) near 0.9 K (53).
netism, an exception being CeRuz (C I ytype)
In the systems consisting of platinum metals
for which T,= 4.9 K (32, 48). The with yttrium, scandium and lanthanum
simultaneous occurrence of superconductivity superconductivity is explained by T. H. Geballe
-
\
Y
r
.
.
T
.
Fig. 2 The primitive tetragonal crystal
structure of the rare earth rhodium
borides RERh.&with the outline of the
unit cell indicated by broken lines. For
clarity, the RhaI-clusters are not
drawn t o scale, after reference (64)
Platinum Metals Rev., 1984, 28, ( 2 )
67
as most likely being of a filamentary nature
(54). In the lanthanum-rhodium system
approximately I 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 RERh J3 I
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 CeCo4B4type (55, 56). As with the binary
Cr 5 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 T, which is
below the superconducting transition
temperature T,. The superconducting transition temperatures and the ferromagnetic Curie
temperatures of the RERh4B4compounds are
shown in Figure I (55).
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
Erl-,Ho,Rh4B4 (58) and Sml-,Er, Rh4B4(55).
Rhodium can be replaced at least partially by
ruthenium (59) or iridium (60). For the phase
Ho(Ir06Rho4)JB,
co-existence
between
superconductivity and antiferromagnetism was
observed, the Nee1 temperature being higher
than the critical temperature (60). Crystallographic, magnetic and superconducting properties
of the system Y Rh 4B4-LuRh +B4-ThRh+B 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 (59-64). The structure
of RERhJB4is shown in Figure 2 (64).
Group IV Elements
Germanium. Lead, Silicon a n d ‘Tin
Among these compounds the only
superconductors are PdSi, where T, = 0.93 K;
PtSi, T,= 0.8 K; Rh5Ge3, ? I c = 2.12 K (7, 13,
32); together with RhGe, T c = 0.96 K (65);
IrGe, T,= 4.7 K (66); PtGe, T,= 0.4 K (32);
and RhSn2, T,= 0.60 K (67). Ternary
superconductors are the ruthenium and
osmium germanides Y1RuGel3 and Y3OsGe13,
having a Yb1Rh4Sn,type structure, see Tables I
and I1 (68, 69, 70). Ternary silicides and germanides with ruthenium, rhodium and osmium
and the rare earths, of the S C + ~ O $ ~type
I O of
structure, show a similar behaviour (68, 69, 70).
The superconducting transition temperatures
of the R E I T J G c 1 phases
3
are shown in Table I
and the magnetic ordering temperatures in
Table I1 (68,69, 70).
Table II
Magnetic Ordering 'Temperatures ( T,,,) for Some itb;:5'l ,Gr
Osmium compounds
Ce,Os,Ge,,
Pr,Os,Ge,,
Nd,Os,Ge,,
Eu,Os,Ge,,
Tb,Os,Ge,,
DySOs,Ge
Er,Os,Ge,,
,,
Platinum Metals Rev., 1984, 28, ( 2 )
T, 2 0.1 K
Ruthenium compounds
Ce,Ru,Ge,,
Pr,Ru,Ge,,
6.1
16.0
1.9
10.1
14.1
2.1
1.9
Er,Ru,Ge,,
68
(:om~ioiinds
T,+0.1
6.7
14.2
K
Table Ill
S u p e r c o n d u c ting T r a n s i t i o n
T e m p e r a t u r e s (T,.) a n d Magnetic
O r d e r i n g T e m p e r a t u r e s (T",)of
Sc$o &Si T y p e C o m p o u n d s
,,,
~~
Cornpound
TCK
Sc,Co,Si,,
5.0 -4.8
Sc,Rh,Si,,
8.54- 8.45
Sc,l r,Si
8.46- 8.38
Y,lr,Si ,o
3.0 -2.3
Lu,lr,Si,,
3.76- 3.72
Tm ,I r,Si
T, = 0.57 K). Phase I1 of yttrium and scandium
are superconducting at 3.2 and 4.5 K,
respectively (80).
Ternary compounds of the MnCu2Al
(Heusler) type YPdZM (where M is tin, lead,
indium and antimony) are superconducting.
Superconducting transition temperatures are
YPd2Sn 3.72 YPd2Pb 4.76 and YPdlIn 0.85 K
(82).
The phases RhPb2 and PdPb2, being Cr 6- or
CuAlz-type are superconducting at 2.66 and
2.95 K, respectively (83).
Titanium. Zirconium, H a f n i u m
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 tr-phase (43, 84), see Figure 3.
Most of these effects are caused by
superconducting filaments or shields of the bphase or (r//l 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 EOrtype
,
Er51r4Silo
Ho,l r,Si
Dy51r,Sil,
Y51r4Silo
2.62- 2.58
Y5°s4Ge10
8.68- 8.41
The superconducting transition and magnetic ordering temperatures of Sc&o4Si lo type
phases, having a primitive tetragonal lattice of a
new structure type, are listed in Table 111 (68,
69,701.
Similar superconductivity/magneticordering
behaviour is shown by ternary stannides
RERhl.lSn3.6 and REOs,Sn,,
respectively
(71-81). Both ErOsSn, and ErRhl 1Snj,6 for
example show reentrant superconductivity
(72-81). The phases in the system Me-tin
show at least four different structures: I, 1', 11,
and I11 depending on the kind of Me (80). Phase
I compounds are cubic, space group Pm3n with
a lattice constant a = 9.7 A; I' compounds are
distorted cubic; I1 are tetragonal with
all = I 3.75 and C I I = 27.4 A while 111 are cubic
with alII= 13.7 A (80). Phases I and I1 with Me
being rare earth elements from the series
lanthanum to gadolinium, and ytterbium,
calcium, strontium and thorium are
superconducting between 8.7 and I .9 K. Phase
I1 of the erbium compound exhibits re-entrant
superconductivity (where T, = 0.97 K and
Platinum Metals Rev., 1984, 28, ( 2 )
Table I V
Superconducting Transition
T e m p e r a t u r e s and S t r u c t u r e s of
Binary Phosphides a n d Arsenides
of R h o d i u m a n d t'alladium
Cornpound
Rh,P
Rh4P3
RhAs
Rh,.,As
Pd,P
Pd,P, (h.t.)
Pd,P, (l.t.1
Pd,As,
Pd,As (h.t.)
Pd,As (1.t.)
69
TCK
Structure
1.3
2.5
0.58
0.56
0.75
cubic (CaF,)
orthorhombic
orthorhombic (MnP)
hexagonal
orthorhombic (Fe,C)
rhombohedra1
?
?
hexagonal (Fe,P)
hexagonal
1 .o
0.7
0.46
1.7
0.6
-
b-ig
;i T r a n s i t i o n
l e nipera Iure versus w n centration
curbes
Cor
t i Ia n iu ni ir id iti ni
a n rl
titanium-rhodium
alloys,
after rrfcrcnce ( X i )
-
4.
5
2
6
8
10
12
ATOMIC PER CENT RHODIUM. I R I D I U M
4
14
1'6
rhodium and zirconium-rhodium have been
phases Ti0 mRh0 28700.14 and Tin.mIr0.2~700.1~
(T, = 3.37 and 5.5 K, respectively), also investigated (43, 87); in all cases the critical
was reduced by the presence of
ZrclhlRh,).?HS00.101, Z ~ O . ~ I P ~ ~IOS
~ . ~ Xand
S O I temperature
I
Z r l 1 6 S I r l l . ~ a (T,=
~ O ~ .I~1.8,
s ~ 2.09 and 2.30 K, platinum group metals.
respectively) (32). The A15 phases T i J r and
Ti3Pt become superconducting at 5.4 and Group V Elements
0.58 K, respectively (32). Among the (214-type Phosphorus, Arsenic. A n t inion)
phases HfOs2 and ZrOs2 (T,= 2.69 and 3.00 K and Bismuth
respectively) show superconductivity at higher
Superconductivity data for binary
temperatures than the C15 phases ZrIr2 and phosphides and arsenides of the platinum
ThIr2 (T, = 4.10 and 6.5 K, respectively) (32). metals are listed in Table IV. It is interesting to
Data for the DIo2-type phases are for Th7Rh3 note the difference between the critical
T,= 2.15, for Th70sj T,= 1.51 and for ThJr3 temperature of the high and low temperature
T, = 1.52 K. Th,Pt, is not superconducting forms of Pd7P3 and PdlAs, where the high
above 1.02 K (32). In the system titanium- temperature form always exhibits a slightly
platinum, depending on sample preparation, a higher critical temperature. Among the
critical temperature between 0.4 and 4.5 K is antirnonides only the B8 l-type phases PdSb and
measured. Ternary superconducting solid solu- PtSb are superconducting with T , = 1.5 and
tions based on body-centred cubic titanium- 2 . 1 K, respectively (88). Among the
+
Fig.
C lhe propod
strocturr of the hexagonal
trrnar) phosphicle ZrHuP.
Lincs h a w been drawn to
indiratr
the
1tu.t t w o d inie n s i o n a I Ir ia rig u I a r
clusters and also the linking
of thvse clusters through
z i r c o n i u m r u t h c n i u ni
bonds, after reference ( 9 0 )
-
Platinum Metals Rev., 1984, 28, ( 2 )
70
Tabla V
Superconducting Ternary Phosphides of Ruthenium and Osmium
Lattice constants 8,
Compound
1
Structure
ZrRuSi
ZrRuSi
ZrRuSi
ZrRuSi
ZrRuSi
ZrRuSi
1.33
12.34- 10.56
12.70- 1 1.08
(1.2
7.44- 7.1
6.10- 4.96
TiRuP
ZrRuP
HfRuP
TiOsP
ZrOsP
HfOsP
bismuthides, P-PdBi2 has a higher critical
temperature than tr-PdBi2, T, = 4.2 and I .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).
Ternary hexagonal phosphides and arsenides
of ruthenium and osmium constitute a new
class of ternary superconductors discovered in
I 980 (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).
Depending on their preparation and heat
treatment, some of the superconducting
phosphides and arsenides show high or low
temperature modifications, which in most cases
I
6.325
6.459
6.414
6.285
6.460
6.417
3.542
3.778
3.753
3.625
3.842
3.792
have now been fully identified and reported (9 I ) .
No superconductivity was observed for the
LaFejPlz
type
compounds
RERuPI2
(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 T,= 13, for HfRuP T,= 10.8 and for
TiRuP T, = I .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
Table V I
Superconducting Ternary Arsenides of Ruthenium and Osmium
I
Compound
TiRuAs
ZrRuAs (900OC)
ZrRuAs (flux)
Hf RuAs
ZrOsAs
HfOsAs
(0.35
1 1.90- 10.03
3.2
4.93 - 4.37
8.0
3.2
Platinum Metals Rev., 1984, 28, ( 2 )
Lattice constants
Structure
Ti FeSi
ZrRuSi
ZrRuSi
ZrRuSi
ZrRuSi
ZrRuSi
71
A
t-ll7.287
6.586
6.624
6.568
6.602
6.569
1 1.227
6.515
3.891
3.823
3.842
3.794
3.808
temperatures (43). In vanadium-rich solutions
the presence of I o atomic per cent palladium or
platinum reduces the critical temperature of
vanadium to below I .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 &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 Nbo.637Rh0.363(96). This
may be caused by a very narrow homogeneity
range.
The &phase in the tantalum-rhodium system
is not superconducting, however (96). A I yphases
with the highest transition temperatures among
these compounds are Nb3Pt and MOn.7JTo.76 with
T, = 9.3 and 9.05 K, respectively. Special attention should be given to the peculiar A15-type
phase Oso.rVo5 which has a critical temperature
of 5 . 1 5 K (7, 13, 32, 43,97).
Croup VI Elements
has been presented (98). Superconductivity was
discovered only among the selenides of
palladium (for Pd6-7Se T, = 0.66 and for P d S e
T, = 0.42 K) and the tellurides of palladium,
platinum and iridium. Data on the latter are
given in Table VII.
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 1’
drops from 13.5 to 9.9 callmolldegree’, while
0th 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 a n d Tungsten
In the system ruthenium-chromium the
critical temperature of the 2 0 atomic per cent
chromium alloy is 0.3 K, while at 30 atomic per
cent it is higher at I .65 K (99). Both RuCr2 and
RuCrl 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 &phase
with 50 atomic per cent molybdenum is 10.6 K
( 100).
Sulphur, Selenium and Tellurium
A review of the superconductivity of
sulphur, selenium and tellurium compounds
Further work on superconductivity in the
ruthenium-molybdenum system has previously
been summarised (44). The &phase RUo4Wo6niS
superconducting in the range 4.67 to 5.2 K (32).
Among osmium-chromium alloys the &phase
and
also the Aig-phase OsCr3 are
superconductors with critical temperatures of
1.03 and 4.03 K, respectively (7, 13). The 6phases of osmium with molybdenum and tungsten have critical temperatures of 5.65 and
3.8 I K, respectively ( I o I ). Superconducting
iridium-chromium alloys are IrCr 3,ofP-W type,
with
= 0.75; IrMol with critical temperatures
in the range of 8.17 to 8.8 K, depending on
ordering and the &phase IrozlrMoo72with a
critical temperature of 4.46 K (7, I 3, 43). The
A I 2-type compound Nbo6J’dOJoexhibits superconductivity at 2.04 to 2.47 K (101).
Table V I I
Superconducting Tellurides
of the Platinum Metals
Compound
Pd 3Te
Pd, lTe
PdTe
PdTe 1 02
PdTe 08
PdTe
PtTe
IrTe
Structure
0.76
4.07
3.85
2.56
1 .88
1.69
0.59
1.18
hexagonal (88.1
hexagonal (88.1
hexagonal (88.1
hexagonal (B8.1
trigonal (C6.1
orthorhombic
cubic (C2.1
Platinum Metals Rev., 1984, 28, ( 2 )
72
Group VII Elements
Summary
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 ( I 02, I 03).
Group VIII Elements
Iron, cobalt and nickel, when present in solid
solution, all considerably reduce the critical
temperature of the superconducting platinum
metals. T h e influence o n non-superconducting
platinum and palladium is less (102, 103).
Superconducting compounds are unknown.
T h e 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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Sputtered Cobalt-Platinum Thin Magnetic Films
It has long been known that certain cobaltplatinum alloys possess useful magnetic
properties which can, to some extent, be
selected by choosing an appropriate alloy composition and varying the heat treatment given.
In the annealed condition these alloys can be
machined or rolled and drawn to size without
difficulty. As a result they are used for a number of very specialised applications where
worthwhile economic advantage can result from
harnessing the powerful magnetic properties of
these intrinsically valuable alloys.
A thin magnetic film possessing high
coercivity and remanence and with high
hysteresis loop squareness could find application
as a high density magnetic recording medium,
and in many magnetic devices. However, in
contrast to bulk cobalt-platinum, the coercivity
of electrodeposited cobalt-platinum films is too
low to be useful for such purposes. Now
Masahiro Kitada and Noboru Shimizu of the
Central Research Laboratory of Hitachi
Limited have reported on the development of
r.f. sputtered cobalt-platinum thin films which
Appl.
exhibit good magnetic properties
P ~ Y s . I, 983, 54, ( 1 2), 7089-7094).
T h e films were prepared using a conventional
r.f. sputtering system in which the target was
either a sintered cobalt-platinum disc or a
cobalt disc upon which a calculated amount of
platinum was placed, so enabling the composition of the target to be changed readily.
The magnetic properties and microstructure
of the sputtered films, and the influence of
sputtering conditions upon these were studied.
High coercivity cobalt-platinum films were
obtained with compositions in the range 5-35
atomic per cent platinum. A maximum
coercivity of 1 8 0 0 Oe was obtained for a
cobalt-20 atomic per cent platinum alloy,
which had a remanence of about 9500 G.
The coercivity and remanence of alloy films
with a thickness of 8ooA are shown here in the
Figure; vacuum pressure prior to introduction
of the argon sputtering gas and argon pressure
during sputtering were 2 x I O - ~and 5 x
Torr, respectively.
u.
Platinum Metals Rev., 1984, 28, ( 2 )
75