AB-INITIO PSEUDOPOTENTIAL CALCULATIONS OF THE ATOMIC AND ELECTRONIC-STRUCTURE OF THE TA(100) AND TA(110) SURFACES

We have performed ab initio pseudopotential plane-wave calculations of both the atomic and electronic structure of the Ta (100) and (110) su rfaces. The atomic relaxation of Ta (110) is calculated while our resu lts for the structure of Ta (100) agree well with previous experimenta l reports. We find that the topmost interlayer separations contract re lative to their bulk values by (12+/-1)% and (2+/-1)% for Ta (100) and (110), respectively. The changes in the second and deeper interlayer separations differ in sign. The calculated surface formation and relax ation energies are 1.92 eV/atom and -0.17 eV/atom for the relaxed Ta ( 100) surface and 1.18 eV/atom and -0.03 eV/atom for the relaxed (110) surface. We have also calculated the work functions for both surfaces and find values of 4.0+/-0.1 eV for (100) and 4.9+/-0.1 eV for (110), which are both in good agreement with experiment. Our study of the res pective surface states and their variation as a function of the relaxa tion indicates that, in contradiction to a previous report, the surfac e states do not provide the driving force for the relaxations. We repo rt our calculation of the [110] zone boundary phonon frequencies for b ulk Ta. Lastly, this work demonstrates that the pseudopotential method can be applied accurately and efficiently even in the case of a colum n V transition metal such as Ta.