Lattice dicarbides

The salt-like carbides. Among these are aluminium tricarbide imethanide) AI4C3 (containing essentially C ions) in the crystal lattice and the rather more common dicarbides containing the C ion, for example calcium dicarbide CaCjt these carbides are hydrolysed by water yielding methane and ethyne respectively ... 

The C-C distance in CaC2 is close to that in ethyne (120.5 pm) and it has been suggested that the observed increase in the lanthanoid and actin-oid carbides results from a partial localization of the supernumerary electron in the antibonding orbital of the ethynide ion [C=C] (see p. 932). The effect is noticeably less in the sesquicarbides than in the dicarbides. The compounds EuC2 and YbC2 differ in their lattice parameters and hydrolysis behaviour from the other LnC2 and this may be related to the relative stability of Eu and Yb (p. 1237). 

Eu—G system.—The black dicarbide, EuC2, was prepared by GEBELTand Eick [231] by the reaction of europium metal with graphite (1 2 ratio) in a steel bomb at about 1050° G. The compound has a body-centered tetragonal structure with the lattice parameters a = 4.045 and c 6.645 A. The lattice constants compare reasonably well with SrC2 (a = 4.11, c = 6.68 A). 

Thorium dicarbide has three polymorphs. Because of their range of homogeneity, unique transformation temperatures cannot be given those quoted below are for the phase in equilibrium with carbon. The low temperature form, stable below (1690 + 40) K, is monoclinic, space group C2/c (CuO type) with a = (6.692 + 0.003), b= (4.223 +0.003), c = (6.744 + 0.003) A, / = (103.12 + 0.11)° [1968BOW/KRI] at room temperature, as determined by neutron diffraction. These results confirmed the lattice parameters and space group established earlier by Hunt and Rundle... 

In these systems, two types of rare earth carbides have been confirmed to exist the rare earth dicarbides and the rare earth sesquicarbides. The existence of the monocarbides of cerium and praseodymium (Warf 1955, Brewer and Krikorian 1956, Dancy et al. 1962) has been discredited. It was found on the basis of an X-ray study (Spedding et al. 1958) that the earlier reported cerium monocarbide was most probably a solid solution of carbon in cerium the lattice parameter reported by Brewer and Krikorian (1956) for the cerium monocarbide was identical with that of cerium metal saturated with carbon. 

Other interatomic distances in the rare earth dicarbides show unusual features. Pauling s bond number (Pauling 1960) for the C-R and R-R bonds in these dicarbides increases roughly with the atomic number of the rare earth atom 0.5 (C-La) to 0.9 (C Lu) (Atoji 1961) for the nearest C-R distance 0.2 to 0.35 for the next nearest C-4R distances 0.1 to 0.2 for the nearest R-R distances which are equal to the lattice parameters 3 to 5 and 3.5 to 4.5, respectively, for the total bond numbers of the carbon and rare earth atoms. The bond numbers for YC2 fall between Ho and Lu, indicating that Y in YC2 behaves as a heavy lanthanide (Atoji 1961, 1962). 

The shortest R-R distances in the rare earth sesquicarbides, Rq-3Ri, are nearly 10% shorter than the shortest R R distances in the dicarbides, which are equal to their lattice parameters. Other R0-2R2 and R0-6R3 distances are also shorter than this value, indicating that there are stronger R-R interactions in the sesquicarbides than in the dicarbides. The interatomic distances in Ce2C3, particularly the Ce-Ce distances are markedly smaller than the expected values for CcjCj with the pure trivalent Ce atoms. This is in accordance with the result obtained from the paramagnetic scattering analysis (Atoji and Williams 1967). 

In general, the decrease in lattice parameters of the rare earth carbides from La to Lu obeys the lanthanide contraction law, e.g. the plots of the lattice parameter against the rare earth ion radius for the dicarbides (tetragonal) and the hypocarbides (cubic) show a linear relationship, as shown in figs. 11 and 12. 

Fig. 11. The a and c lattice parameters of the dicarbides as a function of the ionic radius (Schwetz et al. 1979).

Lancasta microindentation tester, 50 Lanthanide dicarbides pendulum hardness, 295 Vickers hardness, 304 Lanthanum boride, LaB , 297 Lanthanum oxide (1 03) effect on silica hardness, 238 as network modifier, 238 Lanthanum silicate (La2 i207) hardness, 242 precipitate in silica, 238 Lateral vent crack, 55, 126, 148, 154, 158-161, 235 analysis of, 159-161 circular contours of, 159 critical flaw size, 154 length as a function of load, 161 and surface distortion, 159 at thin film interfaces, 205 Lattice energy and fracture energy, 198 and hardness, 22, 24 of silicon, 24... 

Total and partial DOSs of ScCj.o are shown in Fig. 2.15. The DOS of YCi 0 differs from that for ScC 10 only in a small broadening of the bands in the region of metal states. Fig. 2.16 gives the DOS for YC2 dicarbide. The crystal structure of such dicarbides can be represented as a NaCl-type structure containing, instead of isolated C atoms, pairs of C atoms along the c axis, resulting in an extension of the MC2 crystal lattice in this direction. The presence of such atom pairs determines the specific peculiarities of the dicarbide electronic spectra. 

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