Fig. 2: Dielectric spectra of confined water.

The dielectric constant ε′ (a) and loss ε″ (b) of water confined in MFU-4l-HCOO (where MFU stands for “Metal-Organic Framework Ulm-University”) are shown versus frequency ν for selected temperatures. The lines are simultaneous fits of the real and imaginary parts employing Cole–Cole functions for the three observed relaxation processes and a power law for a phenomenological description of the strong low-frequency increase. This power law is defined by ε″ = a ν^s−1 and ε′ = a tan(sπ/2) ν^s−1 with an exponent parameter s < 1 and a denoting a prefactor. The numbers in the figure indicate the different processes (1–3 are indicated for the 210 K spectrum; four denotes the fastest process, detected at low temperatures only). For comparison, the plusses show results for the pure metal-organic framework without water (“empty”) at 188 K.