  A new type of compositionally complex (medium- or high-entropy) M5Si3
silicides is synthesized. Both (V1/5Cr1/5Nb1/5Ta1/5W1/5)5Si3 and
(Ti1/5Zr1/5Nb1/5Mo1/5Hf1/5)5Si3 form single-phase homogenous solid solutions.
Notably, (V1/5Cr1/5Nb1/5Ta1/5W1/5)5Si3 forms the hexagonal gamma (D88) phase,
while all its five constituent binary silicides, V5Si3, Cr5Si3, Nb5Si3, Ta5Si3,
and W5Si3, are stable in the tetragonal alpha (D8l) or beta (D8m) phases.
Annealing at 1600C demonstrates that this hexagonal gamma phase is stable.
Comparison of the experimental and calculated X-ray diffraction patterns,
Rietveld refinements, and analysis of aberration-corrected scanning
transmission electron microscopy high-angle annular dark-field images suggest
cation ordering, which reduces the configurational entropy. This work expands
the field of high-entropy and compositional complex ceramics by not only
discovering a new compositional complex silicide phase but also demonstrating
the cation ordering and unusual phase stability. These compositionally complex
silicides can be combined with refractory high-entropy alloys to make the
high-entropy counterparts to the Nb-silicide and Mo-Si-B composites.
