Layered Monophosphate Tungsten Bronzes [Ba(PO 4 ) 2 ]W m O 3 m −3 : 2D Metals with Locked Charge‐Density‐Wave Instabilities

Phosphate tungsten and molybenum bronzes represent an outstanding class of materials displaying textbook examples of charge‐density‐wave (CDW) physics among other fundamental properties. Here we report on the existence of a novel structural branch with the general formula [Ba(PO 4 ) 2 ][W m O 3 m −3 ] ( m =3, 4 and 5) denominated ′ layered monophosphate tungsten bronzes ′ (L‐MPTB). It results from thick [Ba(PO 4 ) 2 ] 4− spacer layers disrupting the cationic metal‐oxide 2D units and enforcing an overall trigonal structure. Their symmetries are preserved down to 1.8 K and the compounds show metallic behaviour with no clear anomaly as a function of temperature. However, their electronic structure displays the characteristic Fermi surface of previous bronzes derived from 5 d W states with hidden nesting properties. By analogy with previous bronzes, such a Fermi surface should result into CDW order. Evidence of CDW order was only indirectly observed in the low‐temperature specific heat, giving an exotic context at the crossover between stable 2D metals and CDW order.