The Study Of The Peierls-like Phase Transition In The Spinel Sulphide CuIr₂S₄

In the spinel oxide MgTi₂O₄ and sulphide CuIr₂S₄,a metal-insulator transition, accompanied with the transition of magnetism and structure etc.,happens when cooling the temperature.This phase transition is called the Peierles-like phase transition.In fact,the traditional Peierls phase transition only occurs to the one-dimensional metallic chains due to the special energy band structure.But a Peierls-like phase transition can also happen to some two- or three-dimensional materials due to the complex interaction between the lattice,charge,orbital,spin.The spinel structure MgTi₂O₄ and Culr₂S₄ are the typical materials with the Peierls-like phase transition.There are abundant physical mechanisms in this kind of materials,such as the metal-insulator transition,the charge density wave,the orbital ordering,spin-dimerization,the magnetic ordering,etc.These mechanisms are hot topics of the present physical research.Besides,the investigation of the Peierls-like phase transition is help for the further understanding of the classic Peierls phase transition.In this dissertation,the author devoted his effort to the study of the Peierls-like phase transition,together with the charge ordering,the orbital ordering and the spin-dimerization.The whole dissertation consists of five chapters.In the first chapter,we give a brief overview of the progresses of the Peierls-like phase transition.Firstly,we introduce the conception of the classic Peierls phase transition in the one-dimensional metallic chains,and the charge ordering wave and orbital ordering wave.Secondly,we elucidate the research of the Peierls-like phase transition in the two-dimensional and three-dimensional system.Finally,we lay emphasis on the lattice structure,charge ordering,orbital ordering,spin-dimerization in the spinel structure MgTi₂O₄ and CuIr₂S₄.We can achieve the basic physical concepts about the Peierls-like phase transition and the orbital-induced Peierls state in this chapter.This part is helping to build up background knowledge for the research.In the second chapter,we obtain the lattice pressure released system through the substitution of Ag with big ionic radius for Cu in the A site,while achieve the lattice pressure enhanced system through the vacancies in the A site.We investigate the abnormal pressure effect by the comparison of the lattice pressure released system Cu_1-xAg_xIr₂S₄ and the lattice pressure enhanced system Cu_1-yIr₂S₄.It is found that the Peierls-like phase transition is suppressed by the releasing of lattice pressure while it is strengthened by the enhancement of the lattice pressure.Meanwhile,for the first time,the contributions of the Pauli paramagentism,Landau diamagnetism,Larmor diamagnetism and Curie paramagnetism are distinguished by fitting the magnetization, and we obtained the proportion of Ir ions which do not participate the spin-dimerization.In the third chapter,we study the pressure effect by means of applying direct hydrostatic pressures.It is found that the phase transition temperature moves to higher temperature with the increasing of the hydrostatic pressure.This is because that the application of hydrostatic pressure decreases the lattice,which favors the phase transition.In addition,we give an explanation for the origin of the different conductive mechanisms in the insulating phase,i.e.,the variable range hopping originate from the energy gap produced by the dimerized Ir-Ir chains,while the thermal activation originate from the non-dimerized Ir.In the fourth chapter,the doping of magnetic Fe in the A site Cu_1-xFe_xIr₂S₄ system is discussed.Many interesting phenomena are found in this system:â‘ the Fe ionexists in the A site with the valence of +1 as +2,and the phase transition is completely suppressed by gentle introduction of Fe;â‘¡a unexpected magnetic phase transition appears at T^*=110 K in the low doping system(x=0.01 and 0.025);â‘¢the magnetic state of this system evolve from ferromagnetic to paramagnetic,and back to ferromagnetic state with the increase of Fe,while the magnetic transition at T^* disappears gradually with the increase of Fe;â‘£a new magnetic transition presents at T^**= 100 K in the heavy doping system(x=0.2,0.3,0.4),and a spin-cluster transition appears at low temperatures.All these phenomenia can explain by the cluster model based on Fe polaring Ir.In the fifth chapter,the substitution in the B site CuIr_2-xM_xS₄(M=W/Mn) system is investigated.W acts as W⁴⁺ in the CuIr_2-xW_xS₄ system,while Mn exists as Mn²⁺in the CuIr_2-xMn_xS₄ system.In the CuIr_2-xW_xS₄ system,the phase transition is suppressed due to the random and the reduction of Ir⁴⁺ caused by the doping of W⁴⁺,and the Puli magnetism is also weakened.In the CuIr_2-xMn_xS₄ system,the random also introduced by the doping of Mn²⁺,which suppress the transition.On the other hand,the number of Ir⁴⁺ ions is increased owing to the valence balance,which favors the phase transition.Thus,the suppression of the phase transition is more effective for the W doping system than the Mn doping system.The magnetic moments of Mn produce a paramagnetic background in the CuIr_2-xMn_xS₄. This work is supported by the National Natural Science Foundation of China through Grant No.10334090,No.1054029,and the State Key Project of Fundamental Research,China No.2007CB925001.