${\mathrm{LaPt}}_2{\mathrm{Si}}_2$ in a single-crystalline form was subjected to structure, thermodynamic, thermal, and electron transport studies with a special emphasis on the structure phase transition appearing at $T=85$ K. X-ray diffraction proves the noncentrosymmetric tetragonal structure of ${\mathrm{CaBe}}_2{\mathrm{Ge}}_2$-type (space group P4/nmm). The transition manifests as a small step of opposite sign in both lattice parameters, leaving almost no volume change. $c$ decreases and $a$ increases in the low-$T$ phase, but the change of the $c/a$ ratio does not exceed $0.1\%$. Additional periodicity, related to the supposed charge density wave (CDW) state, can be related to satellites corresponding to the wave vector $q\approx (0.36,0,0)$, which start to grow with temperature decreasing below 175 K and almost vanish (or relocate from the investigated $ab$ plane) below the 85 K transition. Electrical resistivity reveals that the 85 K transition is hysteretic in temperature, with the difference between heating and cooling being almost 10 K, proving the first-order type of the transition. The transition dramatically enhances resistivity in the low-$T$ state, pointing to a formation of a pseudogap. This, however, does allow a superconducting state, arising below $T_c=1.6$ K. The temperature dependence up upper critical field is not compatible with the weak coupling BCS theory. Strong anisotropy of electronic structure and its dramatic changes at the structure transition are manifest also in thermal expansion and thermoelectric power. The Sommerfeld coefficient $\gamma =7.8\mathrm{mJ}{\mathrm{mol}}^{-1}{\mathrm{K}}^{-2}$ and Debye temperature of 205 K could be derived at low temperatures, but the specific heat has a strongly non-Debye like $T$ dependence, which can be ascribed to a low-energy Einstein mode.

• Received 15 March 2019
• Revised 2 August 2019

DOI:https://doi.org/10.1103/PhysRevB.100.064103