Carbide halides

Perovskites — are a very large family of compounds with the crystal structure similar to that of (Ca, Sr) TiC>3 mineral, named after the Russian mineralogist T. A. Per-ovski (1792-1856). This structure is often called an inorganic chameleon due to the diversity of chemical compositions, structural modifications, and properties. Although most members of this family are oxidic compounds, some nitrides, carbides, halides, and hydrides also have this structure. 

The most numerous and most interesting compounds with the perovskite structure are oxides. Some hydrides, carbides, halides, and nitrides also crystallize with this structure (4). This review will refer only to the study of oxides and their behavior in the gas-solid interface and in heterogeneous catalysis. It will not cover, however, electric, magnetic, and optical properties of perovskites. Comprehensive studies on these... 

In particular, the previously described phases Tb6Br7 and Ergl7 (Berroth et al. 1980) are carbide halides. The structurally characterized compounds are summarized in table 5. 

In all cases the tetrahedral and/or octahedral voids in the metal sublattices are filled with interstitial atoms. To cover the wealth of existing compounds, a subdivision into carbide halides and hydride halides has been made in the following discussion. 

R2X2C and R2X2C2. Figure 29 shows the different structures of rare earth metal carbide halides based on planar four-layer slabs. 

At the end of the description of the rare earth metal carbide halides it seems worthwhile to summarize some facts. These compounds contain single C atoms, or Cj entities with C-C single and double bonds. The kind of species seems entirely related to the number of residual valence electrons at the metal site. As we are dealing with electropositive metals, these electrons will be transferred to MOs of the unit and it is the number of vacant antibonding MOs which determines the kind of carbido species. Thus, the ideas of Atoji (1961) concerning binary carbides can be extended to the rare earth metal carbide halides. A more detailed discussion of the bonding will be given in sect. 3. 

The cause for the martensitic-type transition that occurs when RXH phases are hydrogenated to RXH2 is easily understood along the lines of arguments presented for the layered carbide halides. In the structure of RXH (x < 1) only tetrahedral voids between the metal atom layers are occupied by H atoms. Therefore, positions near the octahedral voids are electrostatically favorable for the X atoms. In RXH2 both the tetrahedral voids and the octahedral voids are occupied by H atoms. As the number of... 

Investigations of the electrical and magnetic properties of the metal-rich rare earth halides have focussed on the Gd halide hydrides (deuterides) and carbides, and the Tb halide hydrides (deuterides). Table 10 summarizes some significant electrical and magnetic data of Gd, Tb, Sc and Y compounds. The binary compounds, the carbide halides with cluster chains or planes, and the hydride halides are discussed in detail. 

The most metal-rich carbide halides have the formula R2XC and are hitherto only known for R = Gd. Their crystal structures can either be described as condensed layers of edge-sharing octahedra of Gd atoms separated by X atom layers, or they can be derived from the Gd2C structure by interleaving neighboring Gd atom bilayers... 

Gd6Cl5C3+j seems to represent the only example for a R carbide halide containing C and C2 units simultaneously. This feature might explain the dependence of the... 

Ihble 5-8. Gd carbide halides. Crystallographically different C-C distances deviate by less than 5 pm from the given values. The number of electrons z (per formula unit) in M-M bonding states is derived via the ionic limit on the basis of the experimental C-C distances ((C2) and (C2) corresponding to 130 and 145 pm, respectively). 

The transition from Gd2Q3 to Gd2Q3N sketched above is an inconvincingly formal one since a topochemical reaction of this kind is not possible. The difference between the black Gd2Q3 and the colorless Gd2Q3N is too large to help decipher the details of what happens when the borderline between cluster systems and simple salts is crossed. On the other hand, the multitude of known carbide halides of the lanthanides allows smaller steps to be observed. 

The Structures of the layered carbide halides RE2X2C2 are closely related to those of the metallic hydride halides discussed in the previous subsection, except for the fact that Q units occupy the centers of the metal atom octahedra and the halogen atoms take different positions above and below the metal atom bilayers. Regardless, the strictly two dimensional character is preserved. 

Volume III Halides, Hydroxides, Oxides. (1997) ISBN 0-9622097-2-4 (descriptions of 628 mineral species, including antimonates, antimonites, arsenites, carbides, halides, hydroxides, nitrides, oxides, phosphides, silicides, vanadium oxysalts). 

Equal four-layer slabs are also observed for carbide halides CR2 X2 and (C2)R2 X2 with single carbon atoms and dicarbon units, respectively, residing in all of the octahedral interstices between the metal double layers. There are also three-layer slab structures of the composition CR2)X in which one halide layer is missing such that the sequence is of the RRX type. In principle, they may be obtained with the same rare earth elements as the hydride halides. 

Different kinds of non-oxide perovskite-type compounds have been known in carbides, halides, nitrides, and hydrides [2], Conjecturing from the oxide ion conduction in ABO3, it would be possible to expect anionic conduction, such as halide ionic or nitride ionic, in these non-oxide perovskite compounds ABX3. 

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