Low temperature study of cobalt based pyroxene

We propose to re-investigate the low temperature structure of the CaCoSi2O6 pyroxene and a possible structural response to the magnetic transition at 40 K. Our main study aims to understand the nature of an unusual plateau observed on the magnetization curve between 20 K and 40 K. Preliminary analysis points towards a possible, not reported earlier, spin crossover transition (SCO) of Co(II) between a high-spin S = 3/2 and a low-spin S = ½ states. A structural study with neutron powder diffraction did not reveal any decrease of the unit cell usually associated with the SCO effect. However, our low temperature IR spectroscopy and XAFS Co K study suggest weak distortion of the average crystal structure. We require a high resolution structural study to reveal weak symmetry breaking. Additionally, our neutron powder diffraction study in the magnetically ordered phase, revealed new peaks, which cannot be associated with the current model of the magnetic or nuclear structure.