| In recently years, the theory and synthesized experiment research of BC2N– a kind of expected new super-hard materials, have made significant achievements.In theoretical aspects: Cohen proposed three possible atomic array model of BC2N in 1989. Tateyama calculated the possible transition from h- BC2N to c BC2N under high temperature and high pressure conditions. Mattesini and S.F.Mater using first-principles pseudopotential method to further explore the possibility existence of cubic BC2N structure. They made two structural model, one is similar to the cubic diamond structure; another is similar to the hexagonal diamond structure.In experimental aspects: Sasaki and Bartlett used BCl3,CH3CH as raw materials and made the access to the component of h-BC2N product through the CVD device; Qinghua W synthesized the high-purity h-BC2N under the conditions of 5.55 G Pa and 1500°C. Vladimr. L. Solozhenko using laser heating produced the cubic phase of the BC2N from h-BC2N in the diamond anvil; Yanshan University in cooperation with the State Key Laboratory of super-hard materials of Jilin University used Ca3B2N2 as catalyzer to get the hexagonal phase and the cubic phase BCN compounds. The lattice constant of the hexagonal phase are: a=0.2506 nm; c= 0.6657nm, the cubic lattice constant is 0.3596 nm.However, in the present study, the most phases of synthesized BC2N compounds are limited in hexagonal and cubic structure. The purpose of the present work is to clarify the crystal structure of the high-pressure phase formed by the room-temperature compression of BC2N.In our experiment, we find a phase transition from h- BC2N to the wurtzite structure of BC2N. The transition is incompletely. h-BC2N transform to wurtzite structure occurs at the pressure between 11.636GPa and 14.903GPa based on the analysis of the corresponding XRD spectrum. In addition, we calculate the lattice parameters, the equation of state and the compressibility of the c-axis for both phases. Also we use Brich-Murnaghan equation of state to calculate the bulk modulus K0 of wBC2N.We have noticed that the h (002) which is the strongest line and h(101), the second strongest line at the pressure of 1.4410GPa still exist when the wurtzite phase come out and we could not get a pure diffraction profile of the high-pressure phase, this may be due to the incomplete transformation to w-BC2N in the experiment. It may have a highly imperfect structure and consists of very small crystals. Also, it is impure due to contamination by other polymorphs. In our work, the peaks corresponding to the two phase are indexed completely as 100, 002, 110 and 103 lines in the XRD spectrum at the pressure of 30.030GPaIt is obvious to find out from the compressibility of the c-axis that they exhibit different pressure dependence. The h-BC2N is somewhat more compressible than w-BC2N, mainly because of the low-density structure of the former and the close-packed structure of the latter. As it follows from the pressure dependence of c/a ratio, the w-BC2N structure is more compressible along the c axis (c/a) decreases from an initial value of 1.6559 at the pressure of 14.903GPa toward 1.6505 at 30.030GPa). Thus, with increasing pressure (under 30.030GPa in our experiment) the value of is c/a gradually close to the ideal ratio of 1.633 for a hexagonal close-packed structure, this should be accompanied by the increase in the phase stability of wurtzitie structure. From the analysis of the XRD spectrum, the ratio of c/a and the value of d-spacing we get the conclusion that the wurtzite phase gradually becomes stable as the pressure increased in the range of 30.030GPa.We use Brich's finite-strain formalism to fit the elastic pressure-volume data to a Brich-Murnaghan equation of state which expressed pressure (P) as an expansion in strain (f): P=3K0 f (1+2f)5/2[1+ af+ bf2]The parameters of the fit is K0=275±26GPa. The value of bulk modulus calculated for w-BC2N is very close to c- BC2N (K 0=282±15GPa).The result indicates this phase has the same property in compressibility, which also means that BC2N will have a very important apply in many domains as the third-generation super-hard materials.
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