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

Abstract

The Raman spectra of Ca1‐xSrxAl2Si2O8 (0 ≤ x ≤ 1) feldspars have been measured, to investigate at a short‐range scale the effect of changing composition on the P ➔ I and I ➔ 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 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. 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 and I2/c than between I and P. The absence of mechano‐luminescence in triclinic I compositions suggests that this property is enhanced in local P configurations at conditions close to the P to I transition. A possible mechanism for mechano‐luminescence involving a compression induced P to I transition in local Eu2+ bearing P configurations is proposed.