Broken Hexagonal Symmetry in the Incommensurate Charge-Density Wave Structure of $2H$-Ta${\mathrm{Se}}_2$

High-resolution x-ray scattering studies show a new charge-density wave (CDW) structure on warming through the commensurate-incommensurate transition in $2H$-Ta${\mathrm{Se}}_2$ at 93 K. In contrast to the fully incommensurate CDW structure seen on cooling, hexagonal symmetry is broken in the new phase and the triple-$\overrightarrow{\mathrm{q}}$ CDW has one commensurate and two incommensurate wave vectors. At 112 K (warming) the CDW transforms to the fully incommensurate structure.