The effect of TiO₂ crystalline phase on microstructure and optical features of Zn₂TiO₄ doped with Mn

The influence of crystalline form of TiO2 (rutile and anatase) on microstructure and optical properties of Zn2TiO4 ceramics doped with manganese has been investigated by X-ray diffraction, Scanning electron microscopy, as well as by optical and electron paramagnetic resonance (EPR) methods. The ceramics was produced by sintering in the air at temperatures in the range of 900-1200 degrees C of a mixture of ZnO and TiO2 powders. Both types of ceramics became denser and demonstrated growth of grains in size with the increase of annealing temperature. Meanwhile, the ceramics made from anatase show abnormal grain growth and formation of preferred crystallographic grain orientation. The grain size of the ceramics made from anatase was larger for all annealing temperatures and increased from submicron up to tens of microns at 1100 and 1200 degrees C. This is attributed to known stimulating role of anatase in the formation of cubic Zn2TiO4 phase and/or anatase-to-rutile transformation. The concurrent optical absorption, photoluminescence (PL) and EPR studies of the ceramics revealed Mn2+ and Mn4+ ions in the Zn2TiO4 crystal phase. For all annealing temperatures the Zn2TiO4 made from anatase have larger content of Mn2+, while the Zn2TiO4 made from rutile show larger content of Mn4+ and about twice higher intensity of red PL caused by E-2 ->(4)A(2) spin forbidden transitions of Mn4+. The PL intensity decreased in several times at high annealing temperatures starting from 1100 degrees C for the ceramics made of anatase, whereas from 1200 degrees C for that made of rutile. This effect strongly correlates with densification of ceramic's microstructure and is ascribed mainly to the reduction of Mn4+ to Mn2+. It is concluded that rutile is preferable for creating Mn4+ centers in the Zn2TiO4 and making Mn4+-activated red phosphor.