Elastic and anelastic anomalies due to spin-state transitions in orthorhombic perovskite from isoelectronic behavior of Co3+ and Fe2+

Abstract

Elastic and anelastic anomalies associated with spin-state transitions of Co 3+ in NdCoO 3 and GdCoO 3 perovskites with the Pnma structure have been investigated using resonant ultrasound spectroscopy (RUS) at high frequencies (0.1-1.5 MHz) and dynamic mechanical analysis (DMA) at low frequencies (0.1-50 Hz). Analysis of spontaneous strains related to octahedral tilting transitions and calculated using lattice parameter data from the literature show that the sequence of changing spin states with increasing temperature, low spin → low spin + high-spin → intermediate spin, is accompanied by significant variations in shear strain due to changes in ionic radius of the Co 3+ ions. This implies significant spin state/strain coupling, which, in turn, leads to renormalization of the shear modulus. In NdCoO 3 the shear strains are small, so the coupling is weak and the shear modulus increases with falling temperature in a manner that scales semi-quantitatively with an empirical spin order parameter. In GdCoO 3 the shear strains are much greater and softening of the shear modulus by up to ~35% in the low-spin + high-spin field arises through the influence of both the spin state/strain coupling and the order parameter susceptibility. Enhanced acoustic dissipation is also observed in the low-spin + high-spin field and is tentatively attributed to mobility of transformation twin walls, which are likely to have structures modified by changes in local Co 3+ spin-state populations. Co 3+ is isoelectronic with Fe 2+ and, although the spin-state transitions observed in cobaltate and silicate perovskites are not the same, the large shear strains associated with octahedral tilting in (Mg,Fe)SiO 3 at high pressures suggest that the effects of a high-spin → intermediate-spin transition of Fe 2+ would be closely analogous to those shown by GdCoO 3 . A spin-state transition of Fe 3+ in silicate perovskite would have a similar influence, due to the change in radius and its influence on octahedral tilting via the Goldschmidt tolerance factor.

Static and dynamic strain coupling behaviour of ferroic and multiferroic perovskites from resonant ultrasound spectroscopy

“…The pattern of changes in the shear modulus of LaCoO 3 measured at RUS frequencies through the spin state transition temperatures of ∼100 and ∼500 K has indeed been found to reflect the pattern of changes of q spin estimated from average Co-O bond lengths [185]. The correlation between these parameters is even closer in NdCoO 3 [186], which has both R-point and M-point tilts (Pnma). In GdCoO 3 , the shear strains coupled to octahedral tilting are larger, with the result that an applied stress is more likely to induce a relaxation of the spin state to give softening of the form implied by equation ( 1), i.e.…”

“…with dependence on the order parameter susceptibility. Significant softening occurs below ∼900 K where the changes in shear strain associated with changing q spin are greatest [186].…”

Mechanical spectroscopy in twinned minerals: Simulation of resonance patterns at high frequencies

“…With the advent of (multi) ferroic materials in physics and material sciences, the same techniques, such as low-frequency DMA measurements and high-frequency resonant ultrasonic spectroscopy (RUS) have been used to characterize mechanical properties and yield significant advances in both experimental techniques and the interpretation of mechanical spectra (Salje and Schranz 2011;Carpenter and Salje 1998;Scott et al 2012). Recent work in the mineralogical literature such as Zhang et al (2012aZhang et al ( , 2012b, Prencipe et al (2009), Harrison et al (2003Harrison et al ( , 2004aHarrison et al ( , 2004b, Ohno et al (2000), Isaak and Ohno (2003), Isaak et al (2006), McKnight et al (2007), Carpenter et al (2007, and Salje et al (2011aSalje et al ( , 2011bSalje et al ( , 2011c use DMA and RUS methods to determine the temperature and pressure dependence of elastic moduli to understand by which mechanisms phase transitions in minerals occur. All ferroelastic phase transitions show strong temperature and pressure dependencies of the elastic moduli near the transition points [for a review Carpenter and Salje (1998) and Salje (2012)].…”