The first example of a ratiometric thermometer based on lanthanide phosphonate is reported, which operates down to the cryogenic temperature range with a maximum relative sensitivity of 3.9% K⁻¹.

A layered erbium(III) phosphonate compound, [Er(notpH₄)(H₂O)]ClO₄·3H₂O (1), in which the Er^III ion has a pseudo-D_5h symmetry exhibits field tunable multiple magnetic relaxation. The near-IR emission spectrum of 1, excited at 1064 nm (Nd:YAG laser), provides a direct probe of the crystal field splitting correlated to the magnetic data.