Quantitative analysis of Eu (II)/Eu (III) ratio in alkaline-earth silicate phosphors by 151Eu Mössbauer spectroscopy

Calcium orthosilicate phosphors activated with Eu ions were prepared by solid-state reaction under various reducing conditions of a double crucible method. The Eu(II)/Eu(III) ratio in the Ca1.9Eu0.1SiO4+δ phosphors obtained was determined by 151Eu Mossbauer measurement to investigate the effect of preparation conditions on the valence states and photoluminescence properties. The fraction of Eu2+ increased monotonically with increasing amount of carbon powders used in the outer alumina crucible. The highest Eu2+ fraction was 88 % for the sample that was prepared by using a carbon crucible and the largest amount of carbon powder outside. From XRD measurements, crystallinities of the phosphors became better with increase of the reducing power, i.e., Eu(II)/Eu(III) ratio. Photoluminescence band due to Eu2+: d-f transition peaked at 504 nm was dominantly observed in all the samples, while that due to Eu3+: f-f transition could not be excited even by 394nm or 464nm except in the sample containing more than 99% Eu3+ fraction. From precise evaluation with an integrating sphere, the quantum yield of photoluminescence was improved drastically with increase of Eu2+ fraction, especially above 70%. Quantitative analysis by the 151Eu Mossbauer measurements demonstrated clear correlation between the valence states and the luminescent properties of the Eu2+ phosphors.