Characterisation of tungstate and molybdate crystals ABO(4) (A = Ca, Sr, Zn, Cd; B = W, Mo) for luminescence lifetime cryothermometry

Luminescence lifetime thermometry for remote temperature monitoring of cryogenic objects requires materials that exhibit a suitably large change of the luminescence kinetics at low temperatures. Results of systematic studies of the temperature-induced changes in the luminescence of tungstates and molybdates with the general formula ABO(4) (A = Ca, Sr, Zn, Cd; B = W, Mo) over the 4.5-300 K temperature range are summarized. It is shown through analysing changes of the emission and excitation spectra, as well as the decay kinetics, that in these materials the luminescence is due to the emission of self-trapped excitons, a process that exhibits strong temperature dependence. The main emphasis of the study is on establishing the factors that determine the character of the temperature dependence of the luminescence decay time constant. We discuss our findings in terms of a model that analyses the dynamics of radiative and non-radiative transitions between the excited and ground states of the emission center. Two thermally activated processes drive the observed changes. The first is the non-radiative decay of excited states, resulting in a decrease of the luminescence decay time constant at high temperatures. Additionally it is demonstrated that in molybdates and tungstates the fine splitting of the excited state facilitates a second mechanism for thermally activated exchange of charged carriers between the two split levels. This has a noticeable effect on the dynamics of the radiative decay at low temperatures. We established that the sensitivity of the luminescence lifetime to temperature changes can be estimated by using information on the energy structure of materials. It is concluded that within tungstates and molybdates under study SrWO4 is the most promising material for application in luminescence lifetime cryothermometry. (C) 2018 Acta Materialia Inc. Published by Elsevier Ltd. This is an open access article under the CC BY license. (http://creativecommons.org/licenses/by/4.0/)