Anharmonic Modification Of The Thermofunction Of Be

As a kind of Alkaline Earth Metal,beryllium is located in Group IIA in the Periodic Table.Beryllium is a little scarce,with the content in the earth to be 0.001%. Beryllium has the ability of against corrosion from liquid metals,so is often used as neutron moderator in the nuclear reactor;with beryllium's low density, high elastic modulus and good thermal stability,beryllium copper alloy is also used to manufacture the tools who won't produce sparks when be used, such as the key parts of aircraft engines,precision instruments etc. For the low storage in the earth and the important role in the nuclear reactors,aviation and other fields,we must use them effectively and efficiently,and this requires a better understanding of beryllium's structural properties.Although only four electrons out of the nucleus of beryllium,the basic research about beryllium's structures is still not enouth.Under the room pressure and room temperature,the most stable structure of beryllium is a hexagonal-close-packed phase (hcp,with space group P63/MMC),which will transforms into a body-centered-cubic phase(bcc,with space group IM3M) at 1530±10K and melts at 1560 K. However,it is failed to give a reasonable explanation of the problem based on the quasi-harmonic approximation or harmonic approximation calculations. Researchers generally believe that under the low-pressure high-temperature conditions,the contribution of the anharmonic effect on the free energy of bcc is much larger than the hcp,and results in the phase transition eventually at about 1530K.Essentially,the phase transition reflects the stability of diffenent structures.The stability includes the thermodynamic stability and dynamical stability. In order to decribe the dynamical stability, people use the method of harmonic approximation to calculate the phonon spetrum generally.By the harmonic approximation,the bcc phase of many elements and compounds gives phonon spectrum with imaginary frequency in some Brillouin zone,so such a structure is generally considered unstable. In fact there are many elements and compounds of bcc phase don't show imaginary requency under certain temperature in the experiments, and even the thermodynamic stability is better than the hcp structure. However, as is relates to the contribution of anharmonic term, the conflict has long not been well handled. To address this issue, P.souvtzis invented the self-consistent ab initio molecular dynamics (SCAILD)method.By introducing anharmonic term, the treatment of many materials with SCAILD method have received very good effect of correction. In this paper,we treat the phase transition of beryllium with this approach,and have get the phonon spectrum of bcc-Be with no imaginary frequency.Based on this fact,together with introducing the quasi-harmonic approximation (QHA) and trying Wallace intrinsic anharmonic correction, we have finally get the conclusion been consistent with the experiments qualitatively.