This work demonstrates a direct density functional description of the finite-temperature thermodynamic properties of solids exhibiting phase transitions through positional and spin symmetry breaking degrees of freedom. A classic example addressed here is the rare-earth (R) nickelates RNiO₃ where the ground state is characterized by crystallographic and magnetic (e.g., antiferromagnetic) long-range order (LRO), whereas the higher temperature paramagnetic phase manifests a range of local spin and positional symmetry breaking motifs with short-range order (SRO). Unlike time-dependent simulations of spin and positional degrees of freedom, in the present work, phases are described via a superposition of static configurations constructed by populating a periodic base lattice supercell allowing for the formation of energy lowing distribution of positional and spin local motifs. The thermal populations of the configurations in such a superposition phase are obtained from the energy-minimized Density Functional Theory (DFT)-calculated partition functions at different temperatures. This approach offers flexible inclusion of different physical contributions to the free energy, such as elastic, electronic and phonon free energies, all obtained from the same underlying DFT total energy calculations of periodic structures. The thermodynamic and magnetic properties of both LRO and SRO crystallographic and spin phases, including antiferromagnetic (AFM) to paramagnetic (PM) Néel phase transition in YNiO₃ are studied. Including spin and phonon contributions, we find a DFT-calculated Néel temperature to be 144 K in satisfactory agreement with the experimental value of 145 K; whereas omitting the phonon contribution, one obtains a Néel temperature of 81 K. We present phonon contributions to the DFT-calculated temperature-dependent SRO, heat capacities, and the polymorphous distribution of nonzero local magnetic moments in the PM phase. This approach thus extends to finite temperatures the symmetry-broken DFT description of both the AFM and PM phases, demonstrating that a thermodynamic superposition approach based on symmetry broken configurations evaluated by a mean-field like DFT is sufficient to obtain a consistent description of the thermal physics of the AFM, PM phases and their interconversion in 3d oxides illustrated by YNiO₃.

这项工作证明了固体的有限温度热力学性质的直接密度泛函描述，固体通过位置和自旋对称破坏自由度表现出相变。这里讨论的一个典型例子是稀土 (R) 镍酸盐 RNiO ₃其中基态的特征在于晶体学和磁性（例如，反铁磁）长程有序（LRO），而较高温度的顺磁相表现出一系列具有短程有序（SRO）的局部自旋和位置对称破坏基序。与自旋和位置自由度的时间相关模拟不同，在目前的工作中，通过填充周期性基础晶格超晶胞构建的静态配置的叠加来描述相位，从而形成位置和自旋局部图案的能量降低分布。这种叠加相中的配置的热种群是从能量最小化密度泛函理论 (DFT) 计算的不同温度下的配分函数中获得的。这种方法灵活地包含了对自由能的不同物理贡献，例如弹性、电子和声子自由能，所有这些都是从周期性结构的相同基础 DFT 总能量计算中获得的。LRO 和 SRO 结晶相和自旋相的热力学和磁性，包括 YNiO 中的反铁磁 (AFM) 到顺磁 (PM) Néel 相变₃被研究。包括自旋和声子贡献，我们发现 DFT 计算的 Néel 温度为 144 K，与 145 K 的实验值一致；而忽略声子贡献，一个获得 81 K 的 Néel 温度。我们提出声子对 DFT 计算的温度相关 SRO、热容量和 PM 相中非零局部磁矩的多态分布的贡献。因此，这种方法将 AFM 和 PM 相的对称破坏 DFT 描述扩展到有限温度，证明了基于由平均场（如 DFT）评估的对称破坏配置的热力学叠加方法足以获得对热的一致描述YNiO 说明的 AFM、PM 相及其在 3d 氧化物中的相互转化的物理特性₃ .