Detection by double-modulation ESR spectroscopy of millisecond-range polarization fluctuations and Vogel-Fulcher behavior in partially deuterated potassium dihydrogen phosphate.

Measurements of polarization fluctuations at time scales longer than ${10}^{\mathrm{\ensuremath{-}}6}$ s have not been feasible via electron spin resonance techniques. Using partially deuterated ${\mathrm{KH}}_{2}$${\mathrm{PO}}_{4}$ (KDP) as an example, this study shows that the double-modulation electron spin resonance technique enables the detection of polarization fluctuations on the millisecond time scale. The measured correlation times for 73% deuterated KDP exhibit Vogel-Fulcher behavior, indicating that this lattice may exhibit structural glass behavior, with a glass formation temperature of 138\ifmmode\pm\else\textpm\fi{}10 K.