Preparation And High-pressure Physical Properties Of Tetragonal Calcium Tetraboride Single Crystal

The development of society and the progress of science and technology are closely related to the design and synthesis of new materials,and hard materials have played an indispensable and important role in the development of new technologies,national defense construction,aerospace and other fields.Diamond is a typical representative of superhard material.Its hardness is 80-120 GPa,which is the highest hardness material at present.However,the low thermal stability,poor toughness,and extremely poor conductivity of diamonds greatly limit their application range.Therefore,finding a new hard material with high hardness,high conductivity and high chemical stability has always been one of the hot research issues in the material field.The strong covalent bond system of B-C-N-O is another important component of superhard material.These elements are very close to the atomic radius of carbon element,so they can form a variety of hybrid orbits,which is very likely to form diamond like structures with extremely high hardness.Boron is one of the important elements in the system and can form a large number of complex compounds,therefore boron rich materials have also attracted widespread attention.Binary metal boride MB_n（n=1,2,4,6,12...）has always been considered as a kind of potential hard material with a variety of excellent properties.It has excellent properties such as high strength,good conductivity,flame retardancy,wear resistance and light weight,and has broad application prospects in industry.Among them,rare earth and alkaline earth metal boride often have some special properties.For example,Mg B₂ with simple structure has excellent superconductivity,and the superconducting transition temperature reaches 39 K.Its biggest advantage is that it does not need expensive liquid helium refrigeration,and can work in the environment directly cooled by a refrigerator.It is a new kind of low-cost and high-performance superconductor.Similarly,La B₆,which has excellent thermal ion emission properties,is used as a cathode emission material in high-power electron microscopes.Tetraborate in MB_n is mainly composed of trivalent rare earth boride with Th B₄ type crystal structure.In this tetragonal boron cage structure,Ca²⁺was once considered to be the ion with the largest radius that can be accommodated.However,there has always been controversy over whether Ca B₄ can be successfully synthesized.As early as the 1960s,RW Johnson obtained a powder like compound resembling Ca B₄,which has the same crystal structure as La B₄.However,the theoretical calculation of the electronic structure of tetra boride shows that pure Ca B₄is impossible to exist.It was not until Tian et al.successfully synthesized Ca B₄ with a tetragonal structure through a special solid-liquid reaction under high pressure that controversy gradually dissipated.However,there are still few reports on the research of tetragonal Ca B₄,especially its mechanical properties and physical properties under high pressure,and a large number of theoretical predictions indicate that calcium boron compounds are a potential high-quality material with ultra-light,high conductivity,and high hardness.On this basis,we successfully synthesized pure Ca B₄ single crystals under high temperature and pressure,and conducted detailed tests on their structure and properties under normal pressure.Furthermore,we explored the changes in their microstructure with pressure through high-pressure in-situ Raman testing.This article systematically studies the formation laws and physical properties of Ca B₄ crystals.We have successfully prepared Ca B₄ crystals using a hexahedral top large cavity press and high-temperature high-pressure method.Under high-temperature and high-pressure conditions,Ca B₄ crystals are formed through solid-liquid reactions.Boron elements first dissolve into molten calcium,and then react with calcium to form Ca B₄ crystals before precipitation.The electrical transport performance test of the crystal found that its resistivity gradually decreases with the decrease of temperature,consistent with the conductive mechanism of alloy properties,and its remaining resistivity isρ_{（2 K）}=4.2μΩ·cm.The microhardness test results of each crystal plane show that the hardness values of different crystal planes are basically the same,with a hardness value of H_v=22 GPa,indicating that Ca B₄ crystal is a hard material with isotropic hardness.The theoretical calculation results show that its high hardness mainly comes from the short and strong B-B covalent bond in the B network.High voltage in situ electrical transport properties measurements have shown that Ca B₄crystals maintain metal conductivity throughout the range of 0-99.6 GPa.Within 10GPa,the sample resistance first decreases with the increase of pressure.When the pressure reaches 10 GPa,the resistance abnormally increases,which is related to the equistructural phase transition of the crystal.To investigate the changes in its microstructure,we conducted in-situ high-pressure Raman spectroscopy tests on different crystal planes of Ca B₄ crystal.As the pressure increased,all Raman peaks on different crystal planes shifted blue and gradually widened,indicating that the lattice was compressed under pressure,the bond length was shortened,and the structural symmetry was reduced.When the pressure reaches 12 GPa,the shift rate of Raman vibration peaks in the range of 300-700 cm^-1 changes,indicating that the Ca B₄ crystal undergoes an isostructural phase transition.Based on first principles calculations,we investigated the electronic properties,mechanical properties,and structural changes of Ca B₄ under high pressure,and provided four types of Raman scattering activated vibration modes(A_1g,B_1g,B_2g,and E_g)in the lattice vibration of Ca B₄.