Effects of volume mismatch and electronic structure on the decomposition of ScAlN and TiAlN solid solutions

Thin solid films of metastable rocksalt structure (c-) Sc(1-x)Al(x)N and Ti(1-x)Al(x)N were employed as model systems to investigate the relative influence of volume mismatch and electronic structure driving forces for phase separation. Reactive dual magnetron sputtering was used to deposit stoichiometric Sc(0.57)Al(0.43)N(111) and Ti(0.51)Al(0.49)N(111) thin films, at 675 degrees C and 600 degrees C, respectively, followed by stepwise annealing to a maximum temperature of 1100 degrees C. Phase transformations during growth and annealing were followed in situ using x-ray scattering. The results show that the as-deposited Sc(0.57)Al(0.43)N films phase separate at 1000-1100 degrees C into nonisostructural c-ScN and wurtzite structure (w-) AlN, via nucleation and growth at domain boundaries. Ti(0.51)Al(0.49)N, however, exhibits spinodal decomposition into isostructural coherent c-TiN and c-AlN, in the temperature interval of 800-1000 degrees C. X-ray pole figures show the coherency between c-ScN and w- AlN, with AlN (0001) parallel to ScN(001) and AlN < 0 (1) over bar 10 > parallel to ScN <(1) over bar 10 >. First-principles calculations of mixing energy-lattice spacing curves explain the results on a fundamental physics level and open a route for design of novel metastable pseudobinary phases for hard coatings and electronic materials.