Unified model for the transverse electric susceptibility in KH$_2$PO$_4$ and NH$_4$PO$_4$ -type crystals « Theoretical and experimental studies of the transverse dielectric properties of KH $_{2}$ PO $_{4}$ » Protonic E-mode fluctuations as the dominant mechanism for electric field gradients in KH$_2$AsO$_4$-type crystals