Crystal and magnetic structure evolution in mercury containing perovskites

Wei-Tin Chen

Center for Condensed Matter Sciences and Center of Atomic Initiative for New Materials, National Taiwan University, Taipei, Taiwan

 


Abstract:

AMn7O12 manganite belongs AA3B4O12 quadruple perovskite family, where MnA’ MnB cations occupy square-planar and heavily tilted octahedral sites, respectively. Novel quadruple perovskite AMn7O12 (A = Hg, HMO) was prepared by high pressure high temperature (HPHT) synthesis techniques and exhibit multiferroic property at low temperature. [1] In addition to the charge ordering phase transition from cubic to rhombohedral phases observed at its analogue cousin CaMn7O12, HMO experiences a further symmetry-lowering phase transition to Pnn2 orthorhombic phase close to room temperature ~260 K. It was revealed from symmetry element analysis that the improper ferroelectric polarisation in HMO originates from the lattice instabilities directly linked to charge and orbital degrees of freedom. On the other hand, AMn7O12 solid solution (A = La1−xCax and Na1−xCax) series were also realised by HPHT techniques. [2] Detailed crystallographic study revealed the evolution of tetragonality and the transformation of orbital ordering distortion modes in the solid solution. An alternating ordered-insulating and disordered-conducting electron stripes arrangement was observed at particular doping region, showing a new state of matter. From these studies, it was demonstrated that while the A’-site is rather robust, the charge and orbital ordering and their intrinsic coupling are very sensitive to the A cation oxidation state and consequently B cation valence. In this report, the rhombohedral to orthorhombic phase transition observed in HMO is further investigated with chemical pressure, electron- and hole- doping, and the crystal structure evolution of HMO pristine material are discussed. Furthermore, mercury-containing double perovskites A2MnTeO6 series was also synthesised with HPHT conditions. The mercury and calcium parent compounds adapt rhombohedral and monoclinic structure at room temperature, respectively. The neutron diffraction studies show the propagation vector and the magnetic structure of the parent compounds, and a novel magnetic structure with incommensurate propagation vector at low temperature was revealed for the mercury double perovskite. Further results will be discussed in this report.


 


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