Mn2+ doped Zn3(PO4)2 phosphors: Irreversible thermochromic materials useful as thermal sensors

• Mn2+ doped Zn3(PO4)2 compounds exhibit various structural forms depending the temperature.
• The emission spectra of the various phases are different (spectral modifications).
• The compounds can be used as irreversible thermographic sensors.

Mn2+ doped Zn3(PO4)2 phosphate exhibits a rich thermal history with numerous events. Following the dehydration of Zn3(PO4)2⋅4H2O hopeite at temperatures up to T = 400 °C, various allotropic forms α/δ → γ → β can be stabilized with irreversible transitions for a 600 °C ⩽ T ⩽ 1000 °C temperature range. Furthermore, in these different networks, a transition metal can occupy distorted tetrahedral (α/δ forms) and pentahedral sites as well as octahedral sites for the high-temperature γ and β forms that correspond to non-centrosymmetric environments. Depending on the thermal history of the samples, two different types of emission spectra were obtained. By increasing the annealing temperature, the green emission associated with Mn2+ in a Td environment progressively disappears in favor of the red one that corresponds to Mn2+ stabilized in fivefold and sixfold coordination sites. There is also a dependence of the emission color on the excitation wavelength arising from the differences in the excitation spectra of Mn2+ in these two types of sites. Excitations in the oxygen-manganese charge transfer band at 250 nm or in the 3d–3d absorption region at 420 nm give rise to the red or green emission at 420 nm, the green one being enhanced for an excitation at 420 nm. The α/δ → γ transition at 600 °C ⩽ T ⩽ 900 °C can easily be monitored in the emission spectra obtained after an excitation at 250 nm with a rather good chromatic contrast. The γ → β high-temperature transition at 900 °C ⩽ T ⩽ 1000 °C can be more easily followed by in the emission spectra obtained after an excitation at 420 nm. By taking into account both these emission spectra, various trichromatic coordinates can be calculated and allow subsequent phase transitions. The chromaticity of Zn3(PO4)2: Mn2+ will strongly change for 600 °C ⩽ T ⩽ 1000 °C, and one can consider this phosphate as an interesting thermal sensor.

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