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

Spectroscopic properties of a series of (Sr_0.98-xBa_xEu_0.02)Si₂O₂N₂ (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 (Sr_0.98-xBa_xEu_0.02)Si₂O₂N₂ (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 Eu²⁺ luminescence reveals abrupt changes under pressure (decay time, energy and shape) which indicate the variation of the local symmetry and crystal field strength in Eu²⁺ sites. These changes are attributed to the reversible pressure-induced structural phase transitions of triclinic (Sr_0.98-xBa_xEu_0.02)Si₂O₂N₂ into orthorhombic structure. Pressure in which phase transition occurs decreases linearly with increasing of Ba composition in (Sr_0.98-xBa_xEu_0.02)Si₂O₂N₂ series. Additionally, very different pressure shifts of the Eu²⁺ luminescence in different phases of (Sr0.98-xBaxEu0.02)Si₂O₂N₂:Eu from -40 cm^-1/kbar to 0 cm^-1/kbar have been observed. This effect is explained by different interaction of the Eu²⁺ 5d electron with the second coordination sphere around the impurity cations.