Evidence for an electronically-driven CDW in a 1D metallic MOF

Charge density wave (CDW) phases are unconventional quantum states that often arise in low-dimensional metallic systems and are themselves found alongside other exotic phenomena. Finding such states in porous materials is exceedingly rare. In fact, [Ln(NO3)1-x]3(HOTP)2 (Ln = La, Nd; H6HOTP = 2,3,6,7,10,11- hexahydroxytriphenylene; LnHOTP) is the only example of a porous material wherein a CDW state has been proposed on the basis of a structural modulation. However, whether the modulation is trivial, and thus purely structural in nature, or it stems from a CDW, and thus has electronic origin, remains unknown. Here, low-temperature and high-pressure crystallography provide evidence for an electronic origin of the CDW phase in a series of LnHOTP (Ln = La, Ce, Pr, Nd, and Sm) MOFs, including the original La and Nd materials. We show that the modulation affects the relative rotation of neighboring HOTP ligands, and that the magnitude of the wavevector that defines the modulation, q, is sensitive to pressure. Importantly, the wavevector exhibits commensurability lock-in at one third of the c unit cell parameter, q = 1/3 c, providing key evidence for energetic stabilization of the CDW phase.