  Using single crystal neutron and x-ray diffraction, we discovered a charge
density wave (CDW) below 320 K, which accounts for the long-sought origin of
the heat capacity and resistivity anomalies in UPt2Si2. The modulation
wavevector, Qmod, is intriguingly similar to the Fermi surface nesting
wavevector of URu2Si2. Qmod shows an unusual temperature dependence, shifting
from commensurate to incommensurate position upon cooling and becoming locked
at ~ (0.42 0 0) near 180 K. Bulk measurements indicate a cross-over toward a
correlated coherent state around the same temperature, suggesting an interplay
between the CDW and Kondo-lattice-like coherence before coexisting
antiferromagnetic order sets in at TN = 35 K.
