Crystal plumbides

Herein we review the crystal chemistry, physical properties and structure-property relationships of known plumbides in the R-T-Pb systems. The latter have not been investigated as detailed as the tin based systems. So far, the studies mainly focussed on the 3d transition metals. 

A method that has so far only scarcely been used for the synthesis of the RxTyPbz plumbides is the lead flux. The latter has successfully been used for the growth of phosphide crystals (Kanatzidis et al., 2005). This technique has certainly a great potential, especially for lead-rich R TyPbz compounds. 

In the following section we list and discuss the crystal chemical data of many RxTyPbz plumbides. The basic crystallographic data, lattice parameters, space... 

The plumbides R(Ag, Pb)3 with R = Y, Sm, Gd-Tm crystallize with a very simple structure type, i.e. Q13AU, an ordered version of the cubic close packing. The rare earth atoms fill Wyckoff position la (the origin of the unit cell, see Figure 19), while the silver and lead atoms show random distribution on the 3c site. The phase analytical investigations reveal that up to 78% lead can occupy that site. Both sites have cuboctahedral coordination (CN 12). 

Some of the plumbides within the Yb3Rh4Sni3 family show complex superstructures. They have the compositions R- Rh Pb ip (R = Y, Pr, Nd, Sm, Yb) (Venturini et al., 1986) and the structure is closely related with the Eri tSnIEr4Rhfl Snis structure (Hodeau et al., 1984 Vandenberg, 1980). This complex structure type crystallizes in space group I4i/acd. All investigated stannide crystals are twinned by recticular pseudomerohedry. So far, the corresponding plumbides have only been studied by X-ray powder diffraction. 

Most of the R2Ni2Pb plumbides have been grown in the form of single crystal platelets either via a lead flux or by special annealing procedures (Chinchure et al., 2003). The surfaces of such crystals show terrace-like structures. The single crystals then allow direction dependent magnetic measurements. 

So far more than 180 rare earth-transition metal-plumbides have been reported. They crystallize with 23 different structure types. Apart from the few lead rich plumbides with YbsRlpSnis and related structures, only plumbides with 33 at% or even lower lead content have been reported. Some ternary systems exhibit large liquidus ranges in the lead rich regions at 870 °C. Through phase analytical investigations at lower temperatures one will certainly get access to new lead rich phases. In view of the more than 500 and 850 rare earth-transition metal-stannides and indides, respectively, the lead based systems certainly have a great potential for many more phases to be discovered. 

Most plumbides are available only in the form of microcrystalline powders. In contrast to the situation for stannides, it is difficult to grow small single crystals suitable for X-ray investigation. Furthermore the plumbides do not resist the humidity of the air and must therefore be kept under inert conditions. This chemical behavior is similar to the europium-based plumbides (Pottgen and Johrendt 2000). The structural details of the YbrPb phases are given in table 8. 

The silver plumbide was first reported with Cahti2-type structure with a statistical Ag/Pb occupancy (Mazzone et al. 1983). Recent single-crystal data, however, have revealed that the unit cell is tripled and not doubled. YbAgPb crystallizes with its own structure type (Merlo et al. 1996), space group P62m (fig. 11). It contains planar and puckered AgsPbs layers. This structure was already discussed in sect. 3.6 for isotypic YbAgSn. 

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