Role of carbon in AlCNi3 and GaCNi3: A density functional theory study

In this paper we report first-principles calculations on the ternary antiperovskites AlCNi3 and GaCNi3. We find that in contrast to recent experimental results AlCNi3 and GaCNi3 are nonmagnetic metals. Taking into account data from the literature as well as our own calculations, we have evidence that some of the recent experiments on AlCNi3 and GaCNi3 may have been performed on samples with carbon deficiencies. We believe that the conclusions which have been drawn from these experiments caused several misinterpretations. First, neither in our density functional theory (DFT) calculations for AlCNi3 nor in those for GaCNi3 do we find indications for a magnetic ground state (AlCNi3) or the presence of strong electron-electron correlations (GaCNi3). Second, we find that the local density approximation (LDA) and in particular the generalized gradient approximation (GGA) are well suited for a proper description of these compounds. The unusually large volume differences between the LDA calculations and some experiments are thus rather due to a comparison with substoichiometric samples. By employing accurate full-potential DFT calculations using both the LDA and the GGA we performed fixed spin moment calculations in order to estimate the Stoner enhancement factors S. In addition we investigated TC0.5Ni3 with T=Al and Ga and found that the density of states at the Fermi energy is expected to increase about linearly upon lowering the carbon concentration. This explains the tendencies toward magnetism found in experiment, since for the carbon-free border compounds AlNi3 and GaNi3 their proximity to ferromagnetic order is well established.