Synthesis, crystal structure and magnetic properties of the two polymorphs of novel S=1 osmate; Li4MgOsO6

Abstract

Li4MgOsO6 was synthesized by two different solid-state reaction procedures. The crystal structures were determined by X-ray powder diffraction technique and it was revealed that Li4MgOsO6 crystallizes in two different crystal symmetries in ordered rock salt structure type, namely monoclinic C2/m and orthorhombic Fddd. The unit cell constants for the monoclinic system are a=5.1074(4) Å, b=8.8182(4)Å, c=5.0902(2)Å, and β=109.845(4)° and those of the orthorhombic structure are a=5.8485(1)Å, b=8.3821(1)Å, and c=17.6212(3)Å. In both systems, Os6+ ions reside exclusively in a specific crystallographic position while Li+ and Mg2+ ions exhibit mix occupancy. The temperature dependent magnetic susceptibility data for both S=1 osmate systems do not support the occurrence of any magnetic transition down to 2K. The Curie–Weiss fit to the paramagnetic regime of the magnetic susceptibility data reveal highly negative θ value (−114.81K and −121.87K for C2/m and for Fddd systems, respectively), which are indicative of predominant antiferromagnetic (AFM) interactions in both systems. The experimental effective magnetic moment (μeff) value for the monoclinic phase is 2.13μB and that of the orthorhombic system is 2.34μB. Due to the rather strong AFM interactions and lack of magnetic transition down to 2K, both of these novel osmates are placed in the class of highly frustrated magnets. Low temperature magnetic susceptibility (below 2K) and dynamic magnetic properties studies (μsr studies) are in order to better understand the magnetic ground states of these two polymorphs of Li4MgOsO6.