  We report the synthesis of new equimolar high-entropy alloys (HEAs) formed
from five or six 4d/5d transition metals that are each from a different Group
of the Periodic Table. These include MoReRuRhPt and MoReRuIrPt, which have a
valence electron count (VEC) of 8 and crystallise in the hexagonal close-packed
(hcp) structure, making them isoelectronic and isostructural to the elements Ru
and Os and the high-pressure phase of the well-known HEA CrMnFeCoNi. Analogous
HEAs with VECs of 7, 6 and 5 crystallise with multiple phases, which we
rationalise using the atomic size difference, ${\delta}$, and the
electronegativity difference, ${\Delta}$${\chi}$, of the constituent elements.
Finally, we find our hcp and mixed hcp-${\sigma}$ phase HEAs to be
superconductors, and compare their critical temperatures to those of
isostructural elements and other HEAs that have recently been reported.
