Phosphides and polyphosphides

Preparation methods of phosphides and polyphosphides have been systematically described and discussed by von Schnering and Honle (1988). The following techniques and remarks, which are also of general interest, may be quoted. 

In the review by Kanatzidis et al. (2005), the preparation by the tin-flux method is mentioned also for several ternary phosphides and polyphosphides of rare-earth and transition metals. Typically the components (R metal, T metal, P and Sn in an atomic ratio of about 1 4 20 50) in sealed silica tubes were slowly heated, to avoid violent reactions, up to 800°C, annealed at that temperature for 1 week and slowly (2 K/h) cooled to ambient temperature. The tin-rich matrix was dissolved in diluted hydrochloric acid. The authors described the preparation of compounds corresponding for instance to the formula MeT4P12 (Me = heavy rare-earth metals and Th and U, T = Fe, Ru, etc.) and to the series of phases MeT2P2 (Me is a lanthanide or an actinide and T a late transition metal) having a structure related to the BaAl4 or ThCr2Si2 types. 

Among other methods, however generally resulting in not very high-purity, replacement reactions (metathesis reactions) have been used in a number of cases to prepare stable phosphides, which will be unaffected by a subsequent dissolution process of the by-products, for instance  

Electrolytic or chemical (by carbon and hydrogen) reduction processes have been employed in a number of cases to obtain phosphides from phosphates. 

It may finally be mentioned that molecular beam methods have been used for the preparation of semiconductors as GaP and of thin films of higher polyphosphides. 

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A number of binary phosphides and polyphosphides (compounds containing P—P bonds), for instance those of Mn, Tc, Re, Fe, Ru, Os can be prepared, often in well crystallized form, by the tin-flux technique. The mixture generally containing an excess of P (red P) and a high excess of tin is heated, possibly at a slow rate, to the required temperature (600°-1000°C) and maintained at that temperature for several days and then slowly cooled. In several cases the products may be recovered by dissolving the tin-rich residue in hydrochloric acid. The preparation of several ternary phosphides and of arsenides and antimonides has also been described (see 6.11.3). 

Almost all metals form phosphides, and over 200 different binary compounds are now known. In addition, there are many ternary mixed-metal phosphides. These phosphides consist of metal cations and phosphide anions. In addition to some simple anions (P3-, P -, P ), there are many polyphosphide anions that exist in the form of rings, cages, and chains, as shown in Fig. 15.3.4. 

In some metal phosphides, the polyphosphide anions constitute infinite chains and sheets, as shown in Fig. 15.3.5. 

The hydrogen of this phosphide appears to have a slight acidic character, since the phosphide dissolves in alcoholic alkalies giving deep red solutions which contain polyphosphides, similar to those which are formed by the action of alcoholic alkali on finely divided scarlet phosphorus. These compounds are easily hydrolysed by dilution, or by the addition of acids, with the precipitation of a yellow or reddish mixture of solid hydrogen phosphide and scarlet phosphorus (which possibly contains a suboxide or P4H.OH8,4). 

Molecular beam methods are now widely used for the preparation of common semiconductors such as GaP and their intergrowth with other compounds. Thin films of iron phosphides of 0.5 to 25 J,m thickness can be electrodeposited from sulfate solutions. Depending on the deposition conditions (time, pH, temperature), the phosphides FeP, Fe2P, and/or FesP occur. The preparation of higher polyphosphides, for example, KP15, as thin films demonstrates the range of still yet unexplored preparative methods. This is also valid for the electrolysis of phosphates. The latter techniques is mostly suited for metal-rich phosphides. ... 

Table 6 gives a survey on the range of M-P distances. The upper limits are uncertain because the selected distances are only compared with atomic radii and take not into account the topology (convex polyhedra procedure). The M-P coordination in sohd phosphides demonstrates both coordination and donor functions in isolated complex compounds (cf the (E15) complexes, stabihzed in the coordination sphere of transition metals). This is especially true with the polyphosphides. An extreme is CU2P3I2 (see Section 6.5.7), which is an adduct of Cul and elemental phosphorus (charge transfer complex). Chains of polycychc... 

The diversity in structure and bonding possible for phosphides is effectively demonstrated by the monophosphides. Monophosphides MP of the group 1 and 2 elements (El, E2) are polyphosphides with i(P ) chains and P2" dumbbells, respectively. Ell and E12 monophosphides are not known. The E3 and E13 monophosphides are the so-called normal compounds with 3x = (M) (see Section 2). With El3, they form the zinc blende structure with tetrahedral heteroatomic bonds. Ternary derivatives such as MgGeP2 and CuSi2P3 have a random distribution of the M atoms, whereas CdGeP2, crystallizes in the ordered chalcopyrite type with a TO[GeP4/2] tetrahedral net (see Section 6.4). The E3 monophosphides form the NaCl structure. CeP is remarkable because of its physical properties (metal-semiconductor transition heavy-fermion behavior). The E14 monophosphides show the break usually observed when passing the Zintl border. Binary lead phosphides are not known SiP and GeP... 

Crystalline [([18]crown-6)K][P(CN)2] is the preferred material for further reactions. In the P4-disproportionation reaction, crown ether potassium polyphosphides are formed as the second product. Their composition and solubility depend on the stoichiometry used. The intense red color of the solution in early stages of the reaction is caused by an intermediate high concentration of soluble polyphosphides. The final equilibrium concentration of these phosphides can contaminate and color the isolated dicyanophosphide. To avoid this contamination, the remaining soluble polyphosphides are converted to insoluble ones by an excess of white phosphorus. 

The preparation and structure of magnesium polyphosphide, MgP4, have been described. The compound was prepared by the reaction of gaseous phosphorus with the phosphide MgaP2 at 600 °C in a sealed silica tube. Evidence for a primitive monoclinic cell was obtained from electron microdiffraction. Refinement of X-ray powder diffraction data showed that the compound is isostructural with... 

The ability of phosphorus to exist as isolated anions or larger anionic polyphosphide networks with P-P bonds enables possible formation of transition metal phosphides [14], which are an important class of binary metal/non-metal compounds with a wide variety of structures, compositions and properties. For example, phosphorus-to-metal ratios in these compounds have a wide range, from metal-rich (MPj where v < 1) to monophosphides (MP) and phosphorus-rich polyphosphides (MPj where x > 1) [93]. 

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