Modified two-sublattice pseudospin lattice coupled-mode (PLCM) model Hamiltonian by adding third- and fourth-order anharmonic interactions, extra spin–lattice interactions, direct spin–spin interaction as well as external applied electric field has been considered for \(\mathrm{CsH}_{2}\mathrm{PO}_{4}\) (abbreviated CDP) and \(\mathrm{CsD}_{2}\mathrm{PO}_{4}\) (abbreviated DCDP) crystals. By using double-time temperature-dependent Green’s function method and Dyson’s equation, the expressions for the shift, width, soft mode frequency, dielectric constant and loss tangent have been evaluated. By fitting the model values of various physical parameters in the expressions, the thermal variations in soft mode frequency, dielectric constant and loss tangent have been calculated. Our theoretical results show good agreement with the experimental data of Levstik et al. CDP undergoes a ferroelectric phase transition at \(T_{c}=153\)K, whereas the ferroelectric phase transition in isomorphous DCDP is \(T_{c}=267\)K, showing a large isotope effect explains the importance of the O–H–O bonds in the phase transition mechanism.

通过考虑三阶和四阶非调和相互作用，额外的自旋-晶格相互作用，直接自旋-自旋相互作用以及外部施加的电场，修改了两子格伪自旋晶格耦合模式（PLCM）模型哈密顿量。 mathrm {CsH} _ {2} \ mathrm {PO} _ {4} \）（缩写为CDP）和\（\ mathrm {CsD} _ {2} \ mathrm {PO} _ {4} \）（缩写为DCDP）晶体。通过使用与温度相关的两次格林函数方法和戴森方程，对位移，宽度，软模频率，介电常数和损耗角正切的表达式进行了评估。通过在表达式中拟合各种物理参数的模型值，已计算出软模频率，介电常数和损耗角正切的热变化。我们的理论结果与Levstik等人的实验数据吻合良好。CDP在\（T_ {c} = 153 \） K处经历铁电相变，而同构DCDP中的铁电相变为\（T_ {c} = 267 \） K，表明较大的同位素效应解释了CDP的重要性。 O–H–O键处于相变机制中。