Mössbauer study of synthetic Mg 0.22 Fe 0.78 SiO 3 clinopyroxene

Abstract

Transmission Mossbauer spectra of synthetic Ca-free P21/c Mg0.22Fe0.78SiO3 clinopyroxene were collected at temperatures in the range 4.2 to 745 K and in an external magnetic field of 60 kOe at 180 K. The magnetic order-disorder transition temperature was determined by Mossbauer thermoscanning to be 21 ± 3 K. Above this temperature, all Mossbauer spectra consist of a superposition of two doublets, respectively produced by Fe2+ ions at an almost regular octahedral M1 site and at a more distorted octahedral M2 site. The temperature variation of the Fe2+ center shifts were analyzed using the Debye model for the lattice vibrations. The characteristic Mossbauer temperatures were found to be 356 K ± 35 K for M1 and 333 K ± 25 K for M2. From the external field (60 kOe) Mossbauer spectrum recorded at 180 K, the principal component Vzz of the electric field gradient (EFG) was determined to be positive for both sites but precise values for the magnitudes of the asymmetry parameters η of the EFG could not be determined. The temperature variations of the M1 and M2 quadrupole splittings ΔEQ(T ) are consistent with the higher distortion of the M2 octahedra. Using the crystal-field model to interpret ΔEQ(T ), the energy gaps δ1 and δ2 of the first excited electronic states within the 5D orbital term were estimated to be 410 ± 50 cm−1 and 730 ± 50 cm−1 for M1, and δ1 = 1050 ± 75 cm−1 for M2.