Structural phase transitions and photoluminescence properties of oxonitridosilicate phosphors under high hydrostatic pressure.

Spectroscopic properties of a series of (SrBaEu)SiON (0 ≤ x ≤ 0.98) compounds has been studied under high hydrostatic pressure applied in a diamond anvil cell up to 200 kbar. At ambient pressure the crystal structures of (SrBaEu)SiON (0 ≤ x ≤ 0.98) are related to the ratio of strontium to barium and three different phases exists: orthorhombic Pbcn(0.78 ≤ x ≤ 0.98), triclinic P1 (0 < x ≤ 0.65) and triclinic P1 (0.65 < x < 0.78). It was found that Eu luminescence reveals abrupt changes under pressure (decay time, energy and shape) which indicate the variation of the local symmetry and crystal field strength in Eu sites. These changes are attributed to the reversible pressure-induced structural phase transitions of triclinic (SrBaEu)SiON into orthorhombic structure. Pressure in which phase transition occurs decreases linearly with increasing of Ba composition in (SrBaEu)SiON series. Additionally, very different pressure shifts of the Eu luminescence in different phases of (Sr0.98-xBaxEu0.02)SiON:Eu from -40 cm/kbar to 0 cm/kbar have been observed. This effect is explained by different interaction of the Eu 5d electron with the second coordination sphere around the impurity cations.