  Ho2Ir3Si5 belongs to the family of three-dimensional (3D) R2Ir3Si5 (R = Lu,
Er and Ho) compounds that exhibit a colossal first-order charge density wave
(CDW) transition where there is a strong orthorhombic-to-triclinic distortion
of the lattice accompanied by superlattice reflections. The analysis by
single-crystal X-ray diffraction (SXRD) has revealed that the Ir-Ir zigzag
chains along c are responsible for the CDW in all three compounds. The
replacement of the rare earth element from non-magnetic Lu to magnetic Er or Ho
lowers TCDW, where TCDWLu = 200 K, TCDWEr = 150 K and TCDWHo = 90 K. Out of the
three compounds, Ho2Ir3Si5 is the only system where second-order superlattice
reflections could be observed, indicative of an anharmonic shape of the
modulation wave. The CDW transition is observed as anomalies in the temperature
dependencies of the specific heat, electrical conductivity and magnetic
susceptibility, which includes a large hysteresis of 90 to 130 K for all
measured properties, thus corroborating the SXRD measurements. Similar to
previously reported Er2Ir3Si5, there appears to be a coupling between CDW and
magnetism such that the Ho3+ magnetic moments are influenced by the CDW
transition, even in the paramagnetic state. Moreover, earlier investigations on
polycrystalline material revealed antiferromagnetic (AFM) ordering at TN = 5.1
K, whereas AFM order is suppressed and only the CDW is present in our highly
ordered single-crystal. First-principles calculations predict Ho2Ir3Si5 to be a
metal with coexisting electron and hole pockets at the Fermi level. The Ho and
Ir atoms have spherically symmetric metallic-type charge density distributions
that are prone to CDW distortion. Phonon calculations affirm that the Ir atoms
are primarily responsible for the CDW distortion, which is in agreement with
the experiment.
