Photoluminescent Thermometer Based on a Phase-Transition Lanthanide Silicate with Unusual Structural Disorder

The hydrothermal synthesis of the
novel Na­[LnSiO4] (Ln
= Gd, Eu, Tb) disordered orthorhombic system is reported. At 100 K,
and above, these materials are best described in the centrosymmetric
orthorhombic Pnma space group. At lower temperatures
(structure solved at 30 K) the unit cell changes to body-centered
with Imma symmetry. The materials exhibit unique
photophysical properties, arising from both, this phase transformation,
and the disorder of the Ln3+ ions, located at a site with D2d point symmetry. Na­[(Gd0.8Eu0.1Tb0.1)­SiO4] is an
unprecedented case of a luminescent ratiometric thermometer based
on a very stable silicate matrix. Moreover, it is the first example
of an optical thermometer whose performance (viz., excellent sensitivity
at cryogenic temperatures <100 K) is determined mainly by a structural
transition, opening up new opportunities for designing such devices.