Studies Of Superconductivity In IVB Metal Polyhydrides And Ti Element

Superconductivity possesses potential applications in both scientific and industrial fields due to its zero electrical resistance and complete diamagnetic effect.However,most of the current applications of superconducting materials require the support of low-temperature technology,which limits the wide application of superconductivity.Scientists have been tirelessly pursuing the further improvement to achieve room temperature superconducting.In this dissertation,we utilized a self-developed diamond anvil cell device combing in-situ laser heating and synchrotron X-ray technology（SXRD）to explore a new state of matter under high pressure.We have made some breakthroughs about element superconductors and new metal polyhydrides.Considering that IVB metals Zr and Hf are adjacent to rare earth metals Y and La,which might be good electron donor.Meanwhile,they are prone to undergo s-d orbital energy level conversion under pressure,creating the possibility for high temperature superconductivity.We reported the synthesis of zirconium polyhydrides and hafnium polyhydrides under high pressures and high temperature.The dense Ti exhibits a superconducting transition with the maximum T_c^onset above 26.2 K.The innovative results obtained are mainly as follows:（1）Obtained the high T_c superconductivity in zirconium polyhydrides with T_c^onsetabove 71 K at 220 GPa and～2000 K.The resistance shows a rapid drop at 71 K and approaches to zero at 63 K.Theμ₀H_c2（0）is estimated to be～36 T from measuring the shift of T_c as function of applied magnetic field and the superconducting coherent length is calculated to be about 30?based on Ginzburg Landau model.It is theoretically predicted that the Cmc2₁-Zr H₆ is stable within the 150-275 GPa,and T_c is estimated to be 70 K at 215 GPa,that in principle matches with our experimental results,Hence the observed superconductivity here might arise from the Cmc2₁-Zr H₆.Zirconium polyhydrides are the first high T_c polyhydride for 4d transition metal hydrides.The discovery of high T_c superconductivity in zirconium polyhydrides sheds light to search for new superconductors in transition metal hydrides.To study whether the high temperature superconductivity of zirconium polyhydrides can be maintained after the synthetic pressure is unloaded,which is an important research direction of high pressure,we carried out the electrical experiments under unloading process.It was found that T_cgradually decreased as the pressure decreased,when the pressure reached to 135 GPa,the T_c of 62 K was still maintained.（2）Synthesized a new hafnium superhydride superconductor of C2/m-Hf H₁₄phase at 206 GPa and～2000 K,the T_c^onset above 83 K and zero resistance T_c^zero at 78 K are observed by in-situ high pressure high-pressure electromagnetic transport comprehensive measurements.The upper critical fieldμ₀H_c2（0）was estimated to be 24T by GL theory and the consequent superconducting coherent lengthξto be 37?.The high-pressure synchrotron X-ray technology（SXRD）experiments shows that the majority of diffraction peaks are contributed by C2/m-Hf H₁₄.The refinement generates the lattice parameters of a=4.72?,b=3.33?,c=4.7?,β=90.27°.The slope of d H_c/d T can be obtained to be k=-0.55 T/K for the criteria of T_c^95%.The slope of d H_c/d T is one fourth of Ca Hn（k=-1.97 T/K,T_c～210 K）,almost half of Zr H_n（k=-0.99 T/K,T_c～70 K）,which suggests that the magnetic vortex pinning force in hafnium polyhydride is very weak.The schematic view of C2/m-Hf H₁₄ crystal structure shows that most of the hydrogen atoms are bonded with the bond length less than 0.9?.That is why the T_c of Hf H₁₄ is lower than that for Ca H₆（H-H length～1.24?）and La H₁₀（H-H length～1.1?）,although its hydrogen content is higher.In generally,the longer of the bond length between hydrogen atoms,the weaker the intermolecular forces,and the larger the corresponding debye frequencyω_D,which makes it more possible to achieve high-temperature superconductivity.Therefore,despite its high hydrogen content in Hf H₁₄has a lower T_c due to the absence of H-H molecules.The discovery of C2/m-Hf H₁₄with T_c～83 K is the first 5d transition metal polyhydride superconductor with T_c above the liquid nitrogen temperature.In order to deeply research the stability of hafnium polyhydrides under lower pressure,we conducted the process of releasing pressure about C2/m-Hf H₁₄.As the pressure decreasing,the normal state resistance gradually increased,and the corresponding superconducting T_c decreased from 83 K at 243 GPa to 80 K at 200 GPa,and finally reached to 40 K at 101 GPa.（3）Reported record high T_c element superconductivity achieved in titanium under high pressures,which breaks up the record of elemental superconductivity lasting for16 years.It is challenging to search for high T_c superconductivity（SC）in transition metal elements wherein d electrons are usually not favored by conventional BCS theory.We demonstrate that dense Ti exhibits a superconducting transition with the maximum T_c^onset above 26.2 K and zero resistance T_c^zero of 21 K,which are record high values hitherto achieved among element superconductors through detailed in-situ high pressure electrical transport measurements.Theμ₀H_c2（0）is estimated to be～32 Tesla with coherence length 32?.Furthermore,a more specific and comprehensive superconducting phase diagram of dense titanium is constructed within 18-310 GPa.Reported the continuous superconductive transition up to 310 GPa,which is highest pressures to maintain superconductivity so far.The s-d electron transfer and d electron dominance are suggested to play a key role in the extraordinarily high T_c in Ti element and maintaining the phenomenon of robust T_c above 20 K values extended over a wide pressure range.For the IIIB group metals of Y and La,pressure induced s-d electrons transfer is suggested to play an important role in the high temperature SC of their metal polyhydrides.The s-d electrons transfer under high pressure would increase the structure instability and tend to enhance the electron-phonon coupling and thus lead to high T_c.Therefore,the vicinity of the region of IIA and IIIB groups should be an important reason for the high T_c SC observed in Zirconium polyhydride&hafnium polyhydride,considering which possesses the valence electron configuration of 4d²5s²and 5d²6s²,respectively.In terms of the superconducting characteristics of metal elements K,Ca,Sc,Ti and V in the fourth period.Element K is not a superconductor,and the condition of elements Ca,Sc,Ti and V to transform into superconductors is gradually decreasing.This phenomenon shows that having a moderate number of d electrons is beneficial for increasing of T_c.The pressure effect will lead to a moderate transfer of s-d electrons,which will increase the contribution of d electrons to N（E_f）,while excessive d electrons will weaken superconductivity due to the local spin.The discovery of high superconductivity in Ti element,raises the possibility of finding more higher T_cmaterials via pressure driven correlation effects stemming from the contributions of d electrons,including elemental solids and their alloys.