Observation of multiferroic properties in pyroxene NaFeGe2O6

A pyroxene-based quantum magnet with multiple magnetization plateaus

Pyroxenes (AMX2O6) consisting of infinite one-dimensional edge-sharing MO6 chains and bridging XO4 tetrahedra are fertile ground for finding quantum materials. Thus, here, we have studied calcium cobalt germanate (CaCoGe2O6) and calcium cobalt silicate (CaCoSi2O6) crystals in depth. Heat capacity data show that the spins in both compounds are dominantly Ising-like, even after being manipulated by high magnetic fields. On cooling below the Néel temperatures, a sharp field-induced transition in magnetization is observed for CaCoGe2O6, while multiple magnetization plateaus beneath the full saturation moment are spotted for CaCoSi2O6. Our analysis shows that these contrasting behaviors potentially arise from the different electron configurations of germanium and silicon, in which the 3d orbitals are filled in the former but empty in the latter, enabling electron hopping. Thus, silicate tetrahedra can aid the interchain superexchange pathway between cobalt(II) ion centers, while germanate ones tend to block it during magnetization.

Three different Ge environments in a new Sr5CuGe9O24 phase synthesized at high pressure and high temperature

In the framework of expanding the range of copper-based compounds in the pyroxene family, we have synthesized at high pressure and high temperature a powder containing a mixture of a new phase with stoichiometry Sr₅CuGe₉O₂₄ having two identified impurity phases. Electron crystallography showed that the new phase crystallizes in the monoclinic space group P2/c, with unit-cell parameters a = 11.8 Å, b = 8.1 Å, c = 10.3 Å and β = 101.3°. We applied the recently developed low-dose electron diffraction tomography method to solve the structure by direct methods. The obtained structure model contains all 9 independent cation positions and all 13 oxygen positions. A subsequent refinement against powder X-ray diffraction data ascertained the high quality of the structure solution, in particular, the unusual structural arrangement that there are three different environments for Ge in this phase.