Atomic structure and formation mechanism of a newly discovered charge density wave in the m=2 monophosphate tungsten bronze

The $m$=2 member of the monophosphate tungsten bronze family has been considered the only one in the family without an electronic instability at low temperature. In this paper, we report the discovery of a charge density wave phase in this compound, with a transition temperature of 290 K and an incommensurate modulation vector $\mathbf{\text{q}}$=0.245$\mathbf{\text{b*}}$+ $\text{upxi}$$\mathbf{\text{c*}}$. The presence of this new phase is confirmed by diffraction and resistivity measurements. Pre-transitional dynamics are investigated using diffuse and inelastic x-ray scattering, revealing a clear Kohn anomaly. We analyze both structural and electronic contributions to the phase transition, providing a comprehensive picture of the mechanism driving this newly identified instability.