Excitation power dependent optical temperature behaviors in Mn⁴⁺ doped oxyfluoride Na₂WO₂F₄

A Mn⁴⁺ doped Na₂WO₂F₄ phosphor was synthesized through a two-step wet chemical method. The relationship between crystal structure and luminescence properties is discussed and unusual strong intense zero phonon lines (ZPLs) have been found in a distorted octahedral environment. The power dependent luminescence spectra exhibit the existence of down conversion luminescence intensity saturation under a high pumping power limit. The fluorescence intensity ratios of anti-Stokes bands to the ZPL and Stokes bands reveal an obvious temperature dependent relationship based on thermal de-population from the low states to the upper states of an intrinsic Mn^{4+ 2}E_g → ⁴A_2g transition. The temperature dependent emission intensity of Mn⁴⁺ is investigated by changing the excitation power, and an optical temperature sensitivity as high as 0.00658 K^-1 is achieved at 193 K with the intensity ratio of anti-Stokes bands to the ZPL under 488 nm excitation by a Xenon lamp. This work presents a new method to realize optical thermometry at low temperature by controlling the intensity ratio of the anti-Stokes bands to the ZPL.