Gd5Si4-x Bi x Structures: Novel Slab Sequences Achieved by Turning off the Directionality of Nearest-Slab Interactions

Substitution of Bi for Si leads to the complete cleavage of the interslab dimers T-T in the Gd(5)Si(4-x)Bi(x) system with x = 1.58 - 2.42 (T is a mixture of Si and Bi). Equivalence of the interslab T...T contacts, achieved through combination of the electronic and geometrical parameters, removes directionality of nearest-slab interactions and allows for a novel slab stacking. Two new slab sequences, ABCDABCD (x = 2.07, I4(1)/acd space group) and ABADABAD (x = 2.42, P4(2)bc), have been discovered in Gd(5)Si(4-x)Bi(x) in addition to the known one, ABAB, that is dominant among the RE(5)X(4) phases (RE is a rare-earth element, X is a p-element). The slab stacking for x = 2.07 and x = 2.42 is dictated by the second-nearest slab interactions which promote an origin shift either for the entire slab sequence as in ABCDABCD or for every other second-nearest slab pair as in ABADABAD. The loss of the directionality of the nearest-slab bonding allows for extensive stacking faults and leads to diffuse scattering.