Manipulating Upconversion Emission and Thermochromic Properties of Ho 3+ /Yb 3+ -Codoped Al 2 Mo 3 O 12 Microparticles by Negative Lattice Expansion for Multimode Visual Optical Thermometry.

To ameliorate the inherent thermal quenching behaviors of upconverting materials, a series of Ho 3+ /Yb 3+ -codoped Al 2 Mo 3 O 12 (i.e., Al 2 Mo 3 O 12 :Ho 3+ /2 x Yb 3+ ) microparticles were developed. Upon excitation at 980 nm, intense upconversion (i.e., UC) emissions arising from Ho 3+ are observed, and their optimal states occur at x = 0.09. Besides, the UC mechanisms of these generated emissions from 5 F 4 / 5 S 2 and 5 F 5 levels all pertain to a two-photon absorption process. Furthermore, modified thermal quenching performances are realized in the resultant microparticles, in which the intensities of the UC emissions arising from 5 F 4 / 5 S 2 levels decrease as the temperature increases, while that of the UC emission from the 5 F 5 level increases and then decreases with the increase of temperature. The coexistence of nonradiative transition promoted crossrelaxation, and energy transfer routes can be responsible for the above phenomenon. By studying the diverse UC emission characteristics at high temperatures, we revealed the thermometric properties of Al 2 Mo 3 O 12 :Ho 3+ /2 x Yb 3+ microparticles, where their sensitivities can be regulated by selecting the spectral mode and dopant contents. According to the intensity ratio of the UC emissions originating from 5 F 5 → 5 I 8 to ( 5 F 4 , 5 S 2 ) → 5 I 7 transitions at different temperatures, one obtains that the relative and absolute sensitivities of the developed compounds reach up to 0.464% and 0.1739 K -1 , respectively. Additionally, by the analysis of the thermochromic performances of final products, their thermometric characteristics were also investigated. Note that the environmental temperature is able to be facilely read out by distinguishing the emitting color. These results verify that the Al 2 Mo 3 O 12 :Ho 3+ /2 x Yb 3+ microparticles are promising luminescent materials for multimode visual optical thermometry.