Abstract: Using the mean-field theory, numerical calculation expressions for the internal energy and specific heat were obtained, within the BC model considering the transverse high spin S=2 ferromagnetic system. The thermodynamic properties were investigated mainly nearby the phase transition points of the system, especially nearby the first-order phase transition points. The longitudinal crystal field dependence of the thermodynamic properties in the system was explored. It is found in the system that there is latent heat of phase transition only in very small crystal field range and the corresponding specific heat decrease suddenly, namely the first-order phase transition appears when the transverse field remains the same value. Then with the increase of crystal field, there is no hidden heat of phase transition in the system, the internal energy changes continuously and the corresponding specific heat suddenly decreases, namely the second-order phase transition appears. When the first order-order phase transition appears specific heat increases suddenly and when the second-order phase transition appears specific heat decreases suddenly. With the decrease of crystal field the internal energy increases and the specific heat decreases within the second-order phase transition range. The internal energy and the specific heat vary complexly within the first-order phase transition range.

Key words: Mean-field theory , Internal energy , Latent heats of phase transition , Specific heat , First-order phase transition 

摘要： 在BC模型框架下考虑横向磁场作用的高自旋S=2的铁磁自旋系统,采用平均场理论得到系统的内
能和比热的数值计算公式,重点研究了系统在相变点附近的热力学性质,尤其是一级相变的热力学性质,还考察了
纵向晶场D 对自旋系统热力学性质的影响。研究发现,当有相同的横场作用时,系统在较小的晶场范围内才会出现
相变潜热,对应的比热发生跃变,即发生一级相变现象,此后随着晶场的增加系统不再出现相变潜热,内能连续变
化,对应的比热发生跃变,即发生二级相变现象。系统发生一级有序-有序相变时,比热跃增;系统发生二级相变
时,比热跃减。在二级相变区域内能随晶场的减小而增加,比热随晶场的减小而减小;在一级相变区域,内能和比热
的变化比较复杂。

关键词: 平均场理论 , 内能 , 相变潜热 , 比热 , 一级相变