Structure transformation and remarkable site-distribution modulation of Eu3+ ions in CaMoO4 : Eu3+ nanocrystals under high pressure

Abstract

High-pressure behaviors of scheelite-type CaMoO₄ : Eu³⁺ nanocrystals with an average size of 30 nm have been investigated by using Raman and luminescence spectroscopy at pressures of up to 21 GPa in a diamond anvil cell (DAC). Under the loading of pressure, the softening of the external T(B_g) mode before 10.8 GPa and the appearance of new Raman peaks suggest a phase transformation around this pressure from scheelite to fergusonite structure. When the pressure is released, the scheelite phase recovers due to the small difference in bond strength between scheelite and fergusonite structures. The transformation pressure is slightly enhanced in nanosized CaMoO₄ : Eu³⁺ as compared to the bulk due to the difference in surface energy between the two samples of different sizes. Moreover, it is identified that the Eu³⁺ ions occupy both the bulk and the surface sites in CaMoO₄ : Eu³⁺ nanocrystals at ambient pressure by the site-selective excitation, emission and lifetime spectra. As an effective site probe, the red-to-orange luminescence intensity ratio of Eu³⁺ ions, I(⁵D₀ → ⁷F₂)/I(⁵D₀ → ⁷F₁), is found to exhibit remarkable changes with pressure, which indicates a large variation of the distribution and the local symmetry of Eu³⁺ ions.