Raman spectroscopy, phase transitions, microstructures, and mechano-luminescence in Ca-Sr feldspars (CaAl2Si2O8-SrAl2Si2O8)

Abstract

The Raman spectra of Ca_1-xSr_xAl₂Si₂O₈ (0 ≤ x ≤ 1) feldspars have been measured, to investigate at a short-range scale the effect of changing composition on the P $\overline{1}$ ➔ I $\overline{1}$ and I $\overline{1}$ ➔ I2/c phase transitions and how phase transitions affect the mechano-luminescence found in these feldspars. Transmission electron microscopy (TEM) and X-ray diffraction (XRD) were done on the same samples, to pinpoint the transitions, by the analysis of selected area diffraction patterns (SAED) and of the spontaneous strain. Natural anorthite (CaAl₂Si₂O₈) shows a large number of well-resolved peaks, together with sharp h + k even, l odd diffractions (c-type) in SAED patterns, indicative of a P $\overline{1}$ symmetry. In synthetic anorthite, the Raman spectrum is similar to that of natural anorthite, but the peaks are broader, and c-type reflections are elongated. As Sr increases, the c-type reflections become more diffuse and elongated, as the Raman peaks do. At x > 0.6, new peaks appear in Raman spectra, and other peaks show a change in their wavenumber. Moreover, the c-type reflections disappear completely, and the lattice becomes I-centred. Between x ~ 0.6 and x = 0.86(3), the XRD patterns prompt for a triclinic structure, with S.G. I $\overline{1}$. The transition to the monoclinic I2/c structure is well constrained by the ferroelastic strain, whereas the Raman spectra are more similar between the monoclinic and triclinic I $\overline{1}$ and I2/c than between I $\overline{1}$ and P $\overline{1}$. The absence of mechano-luminescence in triclinic I $\overline{1}$ compositions suggests that this property is enhanced in local P $\overline{1}$ configurations at conditions close to the P $\overline{1}$ to I $\overline{1}$ transition. A possible mechanism for mechano-luminescence involving a compression induced P $\overline{1}$ to I $\overline{1}$ transition in local Eu²⁺ bearing P $\overline{1}$ configurations is proposed.