Impact of grain size, Pr3+ concentration and host composition on non-contact temperature sensing abilities of polyphosphate nano- and microcrystals

In this article, varied praseodymium polyphosphate hosts: M-I(Li, Na K)Pr(PO3)(4) microcrystals and LiLa1-xPrx(PO3)(4) (x = 0.01-1) nanocrystals were successfully synthesized by the flux method and the coprecipitation technique, respectively. The size of stoichiometric nanocrystals of LiPr(PO3)(4) was tuned by the temperature of thermal treatment in range of 35-145 nm. In order to determine the most suitable material for the non-contact optical thermometric applications, the temperature sensing measurements were carried out by using luminescence intensity ratio (LIR) of emission bands corresponding to the P-3(1) -> H-3(5) and P-3(0) -> H-3(5) electronic transitions of Pr3+ ions into the 123-423 K temperature range. The influence of the host material composition of M-I(Li, Na, K)Pr(PO3)(4) microcrystals on the sensitivity of luminescent thermometers was studied. It is found that the sensitivity of lithium praseodymium polyphosphate is the highest of all micropowders under investigation. Moreover, it is found that the nanocrystals reveal much higher relative sensitivity in respect to the microcrystalline counterparts. The highest sensitivity of LIR temperature sensing is found for LiPr(PO3)(4) nanocrystals (35 nm grain size) in the whole temperature range, reaching 0.283%/K at 164 K. The impact of the average grain size on the sensitivity of LIR based thermometers of LiPr(PO3)(4) nanocrystals was investigated. It is found that the reduction of the grain size from 145 to 35 nm results in the enhancement of the relative sensitivity from 0.156 to 0.240%/K at 223 K. Additionally it is found that the high dopant concentration possesses favorable influence on the relative sensitivity of LiLa1-xPrx(PO3)(4) nanocrystalline luminescent thermometers. (C) 2019 Chinese Society of Rare Earths. Published by Elsevier B.V. All rights reserved.