Gd<sub>5</sub>Si<sub>4-<i>x</i></sub>Bi<sub><i>x</i></sub> Structures: Novel Slab Sequences Achieved by Turning off the Directionality of Nearest-Slab Interactions
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Substitution of Bi for Si leads to the complete cleavage of the interslab dimers T−T in the Gd5Si4-xBix 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, I41/acd space group) and ABADABAD (x = 2.42, P42bc), have been discovered in Gd5Si4-xBix in addition to the known one, ABAB, that is dominant among the RE5X4 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.
