Journal Archive

Platinum Metals Rev., 2013, 57, (2), 87
doi: 10.1595/147106713X663780

Platinum-Based and Platinum-Doped Layered Superconducting Materials: Synthesis, Properties and Simulation

Experimental and theoretical results for newest group of high-temperature superconductors

  • Alexander L. Ivanovskii
  • Institute of Solid State Chemistry, Ural Branch of the Russian Academy of Sciences,
  • 620990 Ekaterinburg, Russia
    • Email: ivanovskii@ihim.uran.ru

Article Synopsis

In 2011, the newest group of layered high-temperature superconductors were discovered: platinum-based quaternary 10-4-8 (Ca10(Pt4As8)(Fe2As2)5) and 10-3-8 ((CaFe1–xPtxAs)10Pt3As8) phases with superconducting transition temperatures (TC) up to 35–38 K. Intensive studies have been carried out to investigate their preparation and properties. This finding stimulated much activity in search of related materials and has attracted increased attention to platinum as a component of layered superconductors. This review presents experimental and theoretical results devoted to two main groups of superconducting materials with platinum: Pt-based materials (where Pt forms individual sub-lattices inside building blocks of corresponding phases such as SrPtAs, SrPt2As2 and LaPt2B2C) and Pt-containing materials, where Pt acts as a dopant. Synthesis, basic properties and simulation of these materials are covered.

 

1. Introduction

Platinum and a rich series of Pt-based alloys and compounds (as bulk, films or nanostructured species) are well known as critical materials for many applications (besides jewellery and investment) – for example they are excellent catalysts for chemical processing, and have many uses in the automotive industry (for example, in catalytic converters, spark plugs and sensors), in electronics (for high-temperature and non-corrosive wires and contacts), in petroleum refining, and also in medicine, electrochemistry and fuel cells. However, the participation of Pt in the formation of superconducting materials is much less well known (1–3). Superconductors find use in applications such as magnetic levitation (‘maglev’) trains, magnetic resonance imaging (MRI) scanners and particle accelerators and have further potential for more efficient electricity generation and distribution as well as fast computing applications.

The face-centred cubic (fcc)-Pt metal remains non-superconducting (1) even at the lowest accessible temperatures of solid matter, T ∼1.5 μK (4, 5). It is believed that one of the obstacles to a possible superconducting transition is the purity of the metal, especially with regard to the concentration of magnetic impurities (6). Strong electron-phonon coupling, favourable for the formation of Cooper pairs in fcc-Pt, may also be a factor. Enhanced electronic susceptibility and the Sommerfeld coefficient (owing to low-dispersive near-Fermi bands and high carrier concentration) bring Pt close to magnetic instability (Stoner factor ∼4 (7)), when spin fluctuations may completely suppress superconductivity in this metal (4). A very low-temperature superconducting transition (at TC ∼1.9 mK) was observed for compacted high-purity Pt powder with average grain sizes of ∼2 μm (6, 7); for Pt powders with nanosized grains (∼100–300 nm) TC increases to ∼20 mK (8, 9). It is supposed that the granular structure and the lattice strains related to local inhomogeneity (which is incommensurate with the Fermi surface nesting vectors (10)) are the key factors for the occurrence of inter- and intragranular superconductivity in granular Pt (8–10). In any case, ‘pure’ Pt as a superconductor seems unlikely.

However, a new set of Pt-based alloys and compounds represent very attractive groups of modern superconducting materials, and these have become the subjects of much research interest, particularly owing to clear evidence of unconventional pairing mechanisms for these systems.

Traditional John Bardeen, Leon Cooper and Robert Schrieffer (BCS)-like theories of superconductivity hold that pairs of electrons within nonmagnetic materials are coupled to phonons. In the case of unconventional superconductors, various mechanisms without phonons are suggested. For example, the unusual properties of UPt3 (11) including a heavy fermion state below T = 20 K, dynamic antiferromagnetism (AFM) with onset at magnetic transition temperature, TN = 6 K, and an anisotropic superconducting state with three distinct superconducting phases, provide strong evidence for unconventional spin-triplet superconductivity. In turn, CePt3Si is the first heavy-fermion superconductor without inversion symmetry, and its discovery (12) has initiated widespread research activity in the field of so-called noncentrosymmetric superconductors (13–15). Recently such superconductors lacking a lattice inversion centre have been investigated for the possibility of spin-triplet dominated pairing symmetry. Related Pt-based noncentrosymmetric superconductors are also known: BaPtSi3 (16), Li2Pt3B (17) and LaPt3Si (18).

Another exciting material is platinum hydride (PtH) (19–21), for which the superconducting transition was predicted at TC ∼12 K (19) – the highest superconducting transition temperature among known metal hydrides – at pressure P ∼90 GPa. Recent theoretical estimates confirm that the critical temperature of the two high-pressure phases of PtH correlates with electron-phonon coupling (19). Another group of low-TC (< 8.5 K) superconductors include germanium-platinum compounds with the skutterudite-like crystal structure MPt4Ge12 (where M are alkaline earth metals (strontium or barium), rare earth metals, thorium or uranium) (23–26).The majority of the listed Pt-based materials (a) belong to three-dimensional (3D)-like crystals; and (b) adopt low-temperature superconductivity.

One of the most remarkable achievements in physics and materials sciences was the discovery of high-temperature superconductors with TC values equal to or above the historical limit of TC ∼23 K for niobium-germanium (Nb3Ge). Starting with the discovery of the superconducting transition at TC = 35 K in Ba-doped La2CuO4 in 1986 (27), several exciting families of high-TC materials were subsequently found. Among these are the discoveries of superconductivity in layered materials: MgB2 (TC ∼39 K) in 2000 (28) and fluorine-doped LaFeAsO (TC ∼26 K) in 2008 (29). These discoveries have inspired worldwide research efforts and have been the subject of many reviews (30–51). Most recently, phases with considerably increased values of TC ∼56 K were synthesised (Gd1–xThxFeAsO (52), Sr1–xSmxFeAsF (53) and Ca1–xNd1–xFeAsF (54)), and these form a new class of so-called iron-based high-temperature superconductors. The unconventional superconductivity of these materials, including various types of pairing and the coexistence of superconductivity with magnetism, has been widely discussed.

Several groups in this class of Fe-based superconductors are now known. The majority of them are iron-pnictide (Pn) (or chalcogenide (Ch)) phases (Fe-Pn or Fe-Ch, respectively). These materials can be categorised into the following major groups. From the chemical point of view, the simplest of them are binaries: 11-like phases (such as FeSex (45, 55)); ternary 111-like (such as AFeAs, where A are alkali metals (56)) and 122-like (such as BFe2Pn2, where B are alkali earth metals (57), or AxFe2–yCh2 (58)) materials; and a wide group of quaternary 1111-like superconductors including pnictide oxides or pnictide fluorides such as RFeAsO (R are rare earth metals) and BFeAsF.

Recently, more complex materials such as B3Sc2Fe2As2O5 (32225 phases) (59) and B4M2Fe2Pn2O6 (M are d block metals) (42226 (or 21113) phases) were proposed (60, 61) as parent phases for new high-TC superconductors (46). For some of these, relatively high transition temperatures were established, for example TC ∼17 K for Sr4Sc2Fe2P2O6 (60) and TC ∼37 K for Sr4V2Fe2P2O6 (61). This family was further expanded when new pnictide oxides such as Can+2(Al, Ti)nFe2As2Oy (n = 2, 3, 4) (62), Ca4Al2Fe2(P,As)2O6–y (63), Sr4(Sc,Ti)3Fe2As2O8, Ba4Sc3Fe2As2O7.5, Ba3Sc2Fe2As2O5 (64), Ca4(Mg,Ti)3Fe2As2Oy (65), Sr4MgTiFe2Pn2O6 (66, 67), and Ba4Sc2Fe2As2O6 (68) were successfully prepared and studied (69–77).

For all the listed iron-based superconducting materials:

  1. The crystal structure includes two-dimensional (2D)-like (Fe-Pn) (or Fe-Ch) blocks, which are separated by A or B atomic sheets (for 111- and 122-like phases, respectively) or by (RO), (B3M2O5) or (B2MO3) blocks for more complex 1111, 32225 or 21113 phases; the simplest 11-like binaries consist of stacked (Fe-Ch) blocks;

  2. The electronic bands in the window around the Fermi level are formed mainly by the states of (Fe-Pn) (or Fe-Ch) blocks, which are responsible for superconductivity, whereas the A and B atomic sheets or oxide blocks, which are often termed also as spacer layers, serve as insulating ‘charge reservoirs’; and

  3. These materials have high chemical flexibility to a large variety of constituent elements together with high structural flexibility, and atomic substitution inside the blocks (electron or hole doping) is one of the main strategies for designing new superconducting systems with desirable properties (30–51).

The next promising step in expanding of this family of high-temperature superconductors was made in 2011, when a unique group of Pt-based materials: 10-4-8 (Ca10(Pt4As8)(Fe2As2)5) and 10-3-8 ((CaFe1–xPtxAs)10Pt3As8) phases was discovered (78–80) and intensive studies of their properties were initiated (81–85). For these materials superconductivity has been detected up to TC ∼35–38 K, which is probably induced either by Pt doping of the blocks (FeAs) in the 10-3-8 phase or by indirect electron doping in the 10-4-8 phase owing to additional Pt2+ in the platinum arsenide blocks (78–80). Thus, the role of Pt in the formation of superconducting materials becomes very intriguing.

Pt as a component of layered superconducting materials has been investigated for a long time, and Pt has been found to play a triple role: (a) as a dopant, (b) as a component of non-superconducting blocks (spacer layers), and (c) as a component of superconducting blocks. Thus, all superconductors with Pt can be divided onto two groups: Pt-based materials (where Pt forms individual sub-lattices inside blocks) and Pt-containing materials, where Pt acts as a dopant.

The following sections will focus on the above mentioned materials to cover the basic issues of their synthesis, main properties and simulation.

2. Pt-Based Superconducting Materials

Besides the 10-3-8 and 10-4-8 phases, some other Pt-based superconductors are known, such as SrPtAs (86), SrPt2As2 (87) and RPt2B2C (where R are rare earth metals or Th) (88–94), see Table I.

Table I

Pt-Based Layered Superconducting Materials: Structural Properties and Critical Temperatures, TC

Type Material Space group Lattice constants, Å
 TC, K Refs.
a b c
1221 YPt2B2C I4/mmm – – – 10–11 (88, 89)
LaPt2B2C I4/mmm 3.875 3.875 10.705 10.5–11 (88)
PrPt2B2C I4/mmm 3.837 3.837 10.761 6–6.5 (88, 89)
NdPt2B2C I4/mmm 3.826 3.826 10.732 2.5 (90, 91)
ThPt2B2C I4/mmm 3.83 3.83 10.853 6.7–7 (92–94)

111 SrPtAs P63/mmc 4.244 4.244 8.989 4.2 (86)

122 SrPt2As2 P4/nmm 4.46 4.51 9.81 5.2 (87)

10-4-8 (CaFe1–xPtxAs)10Pt4–yAs8; α-phase P4/n 8.716 8.716 10.462 ∼11–31 (80)
(CaFe1–xPtxAs)10Pt4–yAs8; β-phase, x ∼0.13 P1 8.7282 8.7287 11.049 ∼30 (80)
(CaFe1–xPtxAs)10Pt4–yAs8; x ∼0.36 P1 8.719 8.727 11.161 32.7–38 (88)

10-3-8 (CaFe1–xPtxAs)10Pt3As8; x ∼0.05 P1 8.776 8.781 10.689 ∼11–35 (80)
(CaFe1–xPtxAs)10Pt3As8; x ∼0.16 P1 8.795 8.789 21.008 13.7 (83)

2.1 1221 Phases (Borocarbides)

Historically, the systematic study of layered Pt-based superconducting materials began with borocarbides RPt2B2C (1221 phases) in the mid-1990s and was continued in the new millennium (87–99). These phases crystallise in the tetragonal LuNi2B2C-type structure, which is an interstitial modification of the ThCr2Si2-type, and attract attention mainly because of the coexistence of various types of magnetic ordering and superconductivity. Since data about the properties of these materials are discussed in detail in a set of available reviews (100–104), here only the structural and superconducting parameters for known Pt-based superconductors are listed (Table I). All these materials belong to the class of low TC superconductors.

2.2 SrPtAs

In 2011, the hexagonal phase SrPtAs was discovered (86) as a new low-temperature superconductor with TC ∼4.2 K. Polycrystalline samples of SrPtAs were prepared (86) by a solid-state reaction with PtAs2 as a precursor mixed with Sr and Pt powders using several steps of heating. SrPtAs adopts a hexagonal structure (space group P63/mmc, #194) derived from the well-known AlB2-type structure and can be schematically described as a sequence of two honeycomb planar sheets, where one plane is formed by Sr atoms, and the other (PtAs) by hexagonal Pt3As3, see Figure 1. The atomic coordinates are Sr: 2a (0;0;0), Pt: 2c (⅓;⅔;¼), and As: 2d (⅔;⅓;¼), the lattice constants are a = 4.244 Å and c = 8.989 Å (86, 105).

Fig. 1.

Crystal structures of: (a) AlB2 and (b) SrPtAs. The structure of SrPtAs can be described as an ordered variant of the AlB2-type structure, where the Al sites are occupied by Sr and the boron sites are occupied either by Pt or As atoms so that they alternate in the honeycomb layer as well as along the c-axis (86)

 

Some theoretical efforts have been undertaken to predict the electronic and some other properties of SrPtAs (106–108). It is thought that this material should be characterised as a quasi-2D ionic metal (106), which consists of metallic-like (PtAs) sheets alternating with Sr atomic sheets coupled by ionic interactions. The near-Fermi valence bands are derived from the Pt 5d states with an admixture of the As 4p states. The Fermi surface of SrPtAs is formed by two quasi-2D (cylindrical-like) sheets parallel to the kz direction (along the Γ-A direction) and by two sheets at the zone corners (around K-H). All the Fermi surfaces are hole-like. A very small closed electronic-like pocket was found around K, see Figure 2. Taking into account the relativistic effects, this small electronic-like pocket disappears (102), and the Fermi surface of SrPtAs becomes fully hole-like. This feature distinguishes SrPtAs from other layered pnictogen-containing superconductors. It was also pointed out that SrPtAs provides a prime example of a superconductor with locally broken inversion symmetry (107). The calculated anisotropy in Fermi velocity, conductivity and plasma frequency related to the layered structure were found to be enhanced owing to spin-orbit coupling; further, it was predicted that electron doping would be favourable for an increase in TC (108). Finally, SrPtAs was found (106) as a mechanically stable and soft material with high compressibility lying on the border of brittle/ductile behaviour, and the parameter limiting its mechanical stability is the shear modulus G, Table II.

Fig. 2.

(a) Fermi surface; and (b) Electronic bands of SrPtAs (106)

 

Table II

Calculated Bulk Modulus (B, in GPa), Compressibility (β, in GPa-1), Shear Modulus (G, in GPa), and Pugh's Indicator (G/B) for SrPtAs (106) and SrPt2As2 (113)

Phase/parameter SrPtAs SrPt2As2b SrPt2As2c
BV,R,VRHa 79/10/44.5 101/99/100 71/71/71
β 0.023 0.010 0.014
GV,R,RVHa 30/15/22.5 27/25/26 29/5/17
G/B 0.51 0.26 0.24

[i] a B(G)V,R,RVH as calculated within Voigt (V)/Reuss (R)/Voigt-Reuss-Hill (VRH) approximations, see for example (109)

[ii] b For SrPt2As2 polymorphs of CaBe2Ge2-type

[iii] c For SrPt2As2 polymorphs of ThCr2Si2-type

2.3 SrPt2As2

For the chemically similar phase SrPt2As2 (110), low-TC superconductivity (∼5.2 K) has also been found (87), and this phase seems very attractive as the first superconductor from the wide family of related Pt-containing 122-like materials: for example, ThPt2Si2, YbPt2Si2, UPt2Si2, RPt2Si2 (R = La, Nd, Er, Dy, Ce), ThPt2Ge2, YbPt2Si2, UPt2Ge2 and RPt2Ge2 (112).

Polycrystalline samples of SrPt2As2 were synthesised using stoichiometric amounts of Sr, PtAs2 and Pt powders by a solid-state reaction (87). SrPt2As2 adopts a tetragonal CaBe2Ge2-type structure (space group P4/nmm, #129) (87, 110, 111). The atomic positions are Sr: 2c (¼, ¼, zSr); 2a (¾, ¼, 0); Pt2: 2c (¼, ¼, zPt); As1: 2b (¾, ¼, ½); and As2: 2c (¼, ¼, zAs), where zSr,Pt,As are the internal coordinates. The lattice parameters are listed in Table I. This structure can be schematically described as a sequence of Sr sheets and [Pt2As2] and [As2Pt2] blocks consisting of {PtAs4} and {AsPt4} tetrahedrons: …[Pt2As2]/Sr/[As2Pt2]/Sr/[Pt2As2]/Sr/[As2Pt2]… as shown in Figure 3.

Fig. 3.

Left: Crystal structures of: (a) SrPt2As2 with CaBe2Ge2-type; and (b) ThCr2Si2-type structures (87) and the corresponding Fermi surfaces (113). Right: Charge density maps of SrPt2As2 polymorphs illustrating the formation of directional “inter-block” covalent bonds: (c) As-Pt bonds for CaBe2Ge2-type; and (d) As-As bonds for ThCr2Si2-type structures (113)

 

For SrPt2As2, superconductivity coexists with the charge density wave (CDW) state (87) and this material exhibits a CDW transition at about 470 K (110).

Theoretical probes (113, 114) predict that SrPt2As2 is essentially a multiple-band system, with the Fermi level (EF) crossed by Pt 5d states with a rather strong admixture of As 4p states, Figure 4. It was found (113) that CaBe2Ge2-type SrPt2As2 is a unique system with an ‘intermediate’-type Fermi surface (Figure 3), which consists of electronic pockets having a cylinder-like (2D) topology (typical of 122 FeAs phases) together with 3D-like electronic and hole pockets. The latest are characteristic of ThCr2Si2-like iron-free low-TC superconductors. The non-monotonic behaviour of the density of states (DOS, see Figure 4) near the EF suggests the possibility of significant changes of TC due to various (electron or hole) doping.

Fig. 4.

Total and partial densities of states (DOSs) of SrPt2As2 polymorphs with structures of: (a) CaBe2Ge2-type; and (b) ThCr2Si2-type (113)

 

Analysis (113) revealed that other features of CaBe2Ge2-like SrPt2As2 are as follows:

  1. Essential differences in contributions to the near-Fermi region from the [Pt2As2] and [As2Pt2] blocks when conduction is anisotropic and occurs mainly in [Pt2As2] blocks;

  2. Formation of a 3D system of strong covalent Pt-As bonds (inside and between [Pt2As2]/[As2Pt2] blocks, see Figure 3), which is responsible for enhanced stability of this polymorph – in comparison with the competing ThCr2Si2-like phase; and

  3. Essential charge anisotropy between adjacent [Pt2As2] and [As2Pt2] blocks.

It has also been predicted that CaBe2Ge2-like SrPt2As2 will be a mechanically stable and relatively soft material with high compressibility, which will behave in a ductile manner, Table II. However, the ThCr2Si2-type SrPt2As2 polymorph, which contains only [Pt2As2] blocks, is less stable and will be a ductile material with high elastic anisotropy.

A family of higher-order polytypes has been proposed (113), which can be formed as a result of various stacking arrangements of the two main types of building blocks ([Pt2As2] and [As2Pt2]) in different combinations along the z axis. This may provide an interesting platform for further theoretical and experimental work in the search for new Pt-based superconducting materials.

In 2012, a new family of related ternary platinum phosphides APt3P (A = calcium (Ca), strontium (Sr) or lanthanum (La)) was discovered (115). These phases crystallise in a tetragonal structure, where the anti-perovskite units Pt6P are placed between Sr sheets. All three materials showed low-temperature superconductivity. The highest TC ∼8.4 K was found for SrPt3P. Local-density approximation (LDA) calculations (116) reveal the 3D-like multiple band structure of APt3P phases. The increase of TC for SrPt3P with hole doping (for example, by partial replacement of Sr with potassium (K), rubidium (Rb) or caesium (Cs)) was predicted.

2.4 Quaternary 10-4-8 and 10-3-8 Superconducting Phases

In 2011, superconductivity with TC ∼25 K was reported for the tetragonal phase Ca10(Pt4As8)(Fe2As2)5 formed in the quaternary Ca-Pt-Fe-As system (76). Very soon, additional reports (78, 80) became available, where the related Ca-Pt-Fe-As systems are examined and enhanced superconductivity with transition temperatures up to TC ∼38 K, achieved by substitution of Pt for Fe in the (Fe2As2) blocks, is reported.

One of the most intriguing features of these new Pt-based materials (78–85) is the presence of (Fe2As2) blocks, which are typical of the family of Fe-Pn superconductors, together with oxygen-free blocks [PtnAsm].

Based on Zintl's chemical concept of ion electron counting, it was proposed (79, 80) that [Pt4As8] and [Fe2As2] blocks in the Ca10(Pt4As8)(Fe2As2)5 phase are metallic-like (i.e. both blocks will give appreciable contributions to the density of states at EF) leading to enhanced inter-block coupling and thus to an enhanced transition temperature of this system. It has also been suggested that similar phases with additional metallic-like blocks might provide an interesting platform for the discovery of novel high-TC superconducting materials.

Single crystals of Ca10(PtnAs8)(Fe2–xPtxAs2)5 were grown (78) by heating a mixture of Ca, FeAs, Pt and As powders. The mixture was placed in an alumina crucible, sealed in an evacuated quartz tube, and heated in one of two ways. Heating at 700°C for 3 h and then at 1000°C for 72 h followed by slow cooling to room temperature yielded an α-phase with TC ∼38 K, whereas heating at 1100°C and slow cooling to 1050°C for 40 h yielded a β-phase with TC ∼13 K (78).

The atomic structures of the α-phase Ca10(Pt4As8)(Fe2–xPtxAs2)5 (termed also as 10-4-8 phase) and the β-phase Ca10(Pt3As8)(Fe2–xPtxAs2)5 (10-3-8 phase) are depicted in Figure 5; the lattice parameters are listed in Table I.

Fig. 5.

Crystal structures of: (a) 10-4-8 phase (α-phase Ca10(Pt4As8)(Fe2–xPtxAs2)5); (b) 10-3-8 phase (β-phase Ca10(Pt3As8)(Fe2–xPtxAs2)5) (83); and (c) Experimentally-derived Fermi surface for the β-phase (117)

 

These structures can be schematically described as a sequence of 2D-like [Pt4As8]([Pt3As8]) and [Fe2As2]5 blocks separated by Ca sheets; in turn, platinum-arsenide blocks [Pt4As8]([Pt3As8]) are formed by corner-shared {PtAs4} squares, whereas iron-arsenide blocks consist of {FeAs4} tetrahedrons. In both cases [Pt4As8]([Pt3As8]) and [Fe2As2]5 blocks contain a set of non-equivalent types of Fe, Pt and As atoms (78–85).

Further studies of superconducting gap anisotropy (82), low energy electronic structure, and Fermi surface topology (using angle resolved photoemission spectroscopy, see Figure 5) (117), the critical magnetic fields (118), and some transport properties (84, 119) together with theoretical calculations of the electronic band structure and parameters of interatomic bonds (80, 81) reveal some interesting features of these materials. In particular, Pt doping into FeAs blocks was found to play a critical role for the occurrence of superconductivity. This doping-dependent evolution of the superconducting state is illustrated in Figure 6, where the electronic phase diagram for Ca10(Pt3As8)(Fe2–xPtxAs2)5 is depicted. About 2 wt% Pt doping produces superconductivity, and the superconducting transition temperature reaches its maximum TC ∼13.6 K in the doping range 0.050 < x < 0.065. With further Pt doping, TC slowly decreases.

Fig. 6.

Electronic phase diagram for Ca10(Pt3As8)(Fe2–xPtxAs2)5 (84) which illustrates the doping-dependent formation of semiconducting, metallic-like, and superconducting states for this material

 

The first studies of electronic properties and interatomic bonding (80, 81) reveal that for Ca10(Pt4As8)(Fe2As2)5:

  1. The electronic bands in the window around the Fermi level are formed mainly by the Fe 3d states of [Fe2As2]5 blocks;

  2. The [Pt4As8] blocks will behave as semi-metals with very low densities of states at the Fermi level;

  3. The near-Fermi bands adopt a ‘mixed’ character: simultaneously with quasi-flat bands, a series of high-dispersive bands which intersect the Fermi level was found; and

  4. The chemical bonding in Ca10(Pt4As8)(Fe2As2)5 is complicated and includes an anisotropic mixture of covalent, metallic and ionic interatomic and inter-block interactions, see Figure 7.

Fig. 7.

(a) Partial density of states; and (b) Crystal orbital Hamilton population (COHP) of the Pt-As bonds (80); and (c) Charge density map for Ca10(Pt4As8)(Fe2As2)5 phase, which illustrates the formation of directional covalent As-As bonds inside (Pt4As8) blocks (81)

 

Inside [Fe2As2]5 blocks covalent Fe-As and metallic-like Fe-Fe bonds take place, whereas inside [Pt4As8] blocks a system of covalent Pt-As and As-As bonds emerges. Further, inside these blocks interatomic ionic interactions occur owing to charge transfer Fe → As and Pt → As. The inter-block charge transfer occurs from the electropositive Ca ions to [Pt4As8] and [Fe2As2]5 blocks. It is important that the charge transfer Ca10 → [Pt4As8] is much greater than the transfer Ca10 → [Fe2As2]5, i.e. in contrast to the majority of known superconducting Fe-containing materials (38–43, 51), the new phase Ca10(Pt4As8)(Fe2As2)5 includes two negatively charged blocks, where the charge of the conducting [Fe2As2]5 blocks is much smaller than for the Pt-As blocks. The chemical modification of PtnAs8 blocks may lead to the discovery of similar materials with increased TC (83).

3. Platinum-Doped Layered Superconducting Materials

The first attempts to investigate Pt as a dopant which can optimise the properties of layered superconductors were undertaken as early as the 1990s when the high-temperature superconductor cuprates were examined (120, 121). Next, the effects induced by Pt doping of 1221 phases (borocarbides), which exhibit a rich variety of phenomena associated with superconductivity, magnetism, and their interplay, were studied (122–131).

For non-magnetic superconductors such as YNi2B2C and LuNi2B2C, the introduction of Pt atoms at the nickel sites leads to modifications of their superconducting properties. For series of single crystals of YNi2–xPtxB2C (x = 0.02, 0.06, 0.1, 0.14 and 0.2), which were grown by the travelling solvent floating zone method (125), with an increase in the Pt content the critical temperature decreases from TC ∼15.9 K to TC ∼13 K for x = 0.14, Figure 8. The results were explained (125) assuming the increase in inter-band scattering in the multi-band superconductor YNi2B2C.

Fig. 8.

(a) Normalised real part of alternating current (AC) susceptibility as a function of temperature in YNi2–xPtxB2C (125); and (b) Superconducting transition temperature (TC) and magnetic transition temperature (TN) in Er(Ni1–xPtx)2B2C as functions of the Pt concentration, x (131)

 

Pseudo-quaternary samples Y(Pd1–xPtx)2B2C were prepared by mechanical alloying followed by a thermal treatment (126). It was found that Pt stabilises the formation of the tetragonal superconducting phase YPd2B2C (which adopts the highest TC ∼23 K among the borocarbides), when an almost single-phase material with TC near 15 K for Y(Pd0.8Pt0.2)2B2C was formed after annealing at 1273 K.

For magnetic superconductors such as ErNi2B2C, the introduction of Pt atoms influences both TC and TN (129–131). For example the measurements for Er(Ni1–xPtx)2B2C (polycrystalline samples with Pt content x = 0.0, 0.05, 0.10, 0.15 and 0.20 were synthesised by standard arc melting under protective argon atmosphere (131)) reveal that the variation of TC as a function of x contains two intervals, see Figure 8. At the first step, a strong decrease in TC in the range 0 ≤ x < 0.10 occurs, whereas a much weaker drop of TC was observed with a further increase of x (131). The value of TN, by contrast, decreases almost monotonically. Thus, the Pt impurities in superconducting 1221 borocarbides usually lead to reduction of TC. The explanation of the observed effects requires further studies.

A different effect accompanies the introduction of Pt inside layered 122-like Fe-based pnictides such as BFe2Pn2 (132–136). It is known that ‘pure’ BFe2Pn2 phases (parent materials for Fe-based superconductors) are located on the border of magnetic instability and commonly exhibit temperature-dependent structural and magnetic transitions with the formation of collinear AFM spin ordering, whereas superconductivity emerges either as a result of hole or electron doping into these parent compounds (38–43, 47). Accordingly, this effect was observed for some Pt-doped 122-like phases. Polycrystalline samples of SrFe2–xPtxAs2 (0 ≤ x ≤ 0.4) were prepared by a solid-state reaction method using SrAs, FeAs and metallic powders of Fe and Pt as reagents. The mixture was pressed into a Ta capsule, sealed in an evacuated quartz tube, and heated at 1000°C for 48 h. The measurements demonstrated that as a result of Pt doping, the magnetic order of the parent phase SrFe2As2 is suppressed, superconductivity for SrFe2–xPtxAs2 emerges at approximately x = 0.15, and TC reaches a maximum of 16 K at x = 0.2 (132).

A similar effect was detected for the related system BaFe2–xPtxAs2 (133), where at the doping level of x ∼0.1 the maximum transition temperature TC ∼25 K was achieved. This situation is well illustrated in Figure 9, where the electronic phase diagram of BaFe2–xPtxAs2 for the doping range x = 0–0.25 is depicted. In a simplified way, these effects can be interpreted in terms of the difference in the number of valence electrons between the doped transition metal (Pt) and iron, i.e. the chemical scaling of the electronic phase diagram (137, 138).

Fig. 9.

Phase diagram of BaFe2–xPtxAs2 for the doping level x = 0–0.25 (133). At x < 0.02 a magnetic state with AF spin fluctuations exists. Superconductivity appears at x = ∼0.02, and TC reaches its maximum value (25 K) at x = 0.1

 

However, some exceptions can exist here: for the related system CaFe2–xPtxAs2 it was established (134) that the substitution of Pt is ineffective in the reduction of AFM ordering as well as for inducing of superconductivity up to a solubility limit at x ∼0.08. This challenge calls for further studies.

4. Conclusions

This overview has covered the relatively little-known role of platinum in design and modification of modern superconducting materials. The main goal was to highlight recent experimental and theoretical results that may give an insight into the current status and possible development of layered superconducting materials with Pt.

To date, two types of such materials have been discovered: Pt-based materials (where Pt forms individual sub-lattices inside building blocks of corresponding phases such as SrPtAs, SrPt2As2, LaPt2B2C and (CaFe1–xPtxAs)10Pt3As8) and Pt-containing materials (such as Y(Pd1–xPtx)2B2C or SrFe2–xPtxAs2), where Pt acts as a dopant. The role of Pt can be radically different. For example, the Pt impurity in superconducting borocarbides usually leads to a reduction of TC; whereas the introduction of Pt inside layered Fe-based pnictides such as BFe2Pn2 leads to the occurrence of superconductivity (with high transition temperatures to TC ∼25 K) in these non-superconducting parent materials. A very promising step in expanding the family of superconducting materials with Pt was made in 2011, when the unique quaternary phases: 10-4-8 (Ca10(Pt4As8)(Fe2As2)5) and 10-3-8 ((CaFe1–xPtxAs)10Pt3As8) with highest TC ∼35–38 K were discovered.

The author hopes that this overview will be useful as a compendium to guide further research into layered superconducting materials with Pt, which seem interesting and challenging systems for providing new and promising superconductors.

Acknowledgements

Financial support from the Russian Foundation for Basic Research (RFBR) (Grant 12-03-00038-a) is gratefully acknowledged.

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The Author

Alexander L. Ivanovskii completed his PhD in 1979 at the Institute of Solid State Chemistry in Ekaterinburg, Russia, and accomplished his habilitation in Chemistry at the same institute in 1988. He was promoted to Professor in 1992 and since 1994 he has been head of the Laboratory of Quantum Chemistry and Spectroscopy at the Institute of Solid State Chemistry at the Ural Branch of the Russian Academy of Sciences. Professor Ivanovskii is the author or coauthor of more than 470 research articles and 12 monographs. His main research interests are focused on the theory of electronic structure, chemical bonds, and computational materials science of superconductors, superhard materials and inorganic nanostructures.

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