Distortion mode anomalies in bulk PrNiO3: Illustrating the potential of symmetry-adapted distortion mode analysis for the study of phase transitions

The origin of the metal-to-insulator transition (MIT) in RNiO3 perovskites with R = trivalent 4 f ion has challenged the condensed matter research community for almost three decades. A drawback for progress in this direction has been the lack of studies combining physical properties and accurate structural data covering the full nickelate phase diagram. Here we focus on a small region close to the itinerant limit (R = Pr, 1.5 K < T < 300 K), where we investigate the gap opening and the simultaneous emergence of charge order in PrNiO3. We combine electrical resistivity, magnetization, and heat capacity measurements with high-resolution neutron and synchrotron x-ray powder diffraction data that, in contrast to previous studies, we analyze in terms of symmetry-adapted distortion modes. Such analysis allow us to identify the contribution of the different modes to the global distortion in a broad temperature range. Moreover, it shows that the structural changes at the MIT, traditionally described in terms of the evolution of the interatomic distances and angles, appear as abrupt increases of all nonzero mode amplitudes at TMIT = TN ∼ 130 K accompanied by the appearance of new modes below this temperature. A further interesting observation is the existence of a nearly perfect linear correlation between the amplitude of the breathing mode associated to the charge order and the staggered magnetization below the MIT. Our data also uncover a previously unnoticed anomaly at T ∗ ∼ 60 K (∼0.4 × TMIT), clearly visible in the electrical resistivity, lattice parameters and some mode amplitudes. Since phase coexistence is only observed in a small temperature region around TMIT (∼±10 K), these observations suggest the existence of a hidden symmetry in the insulating phase. We discuss some possible origins, among them the theoretically predicted existence of polar distortions induced by the noncentrosymmetric magnetic order [Perez-Mato et al., J. Phys.: Condens. Matter 28, 286001 (2016); Giovanetti et al., Phys. Rev. Lett. 103, 156401 (2009)].

金属-绝缘体转变（MIT）的起源问题，在具有三价4f离子的RNiO3钙钛矿中，已对凝聚态物质研究界构成了近三十年的挑战。在此领域进展的障碍之一，是缺乏结合物理性质和准确的结构数据，以覆盖整个镍酸盐相图的研究。在本研究中，我们聚焦于巡游极限附近的一个小区域（R = Pr，1.5 K < T < 300 K），在该区域中，我们探究了PrNiO3中的能隙开启与电荷序的同步出现。我们将电导率、磁化率和热容量测量与高分辨率中子及同步辐射X射线粉末衍射数据相结合，与之前的研究不同，我们采用对称适应畸变模式对其进行分析。这种分析使我们能够在广泛的温度范围内识别不同模式对全局畸变的贡献。此外，它表明，传统上用原子间距离和角度的演变来描述的MIT结构变化，表现为在TMIT = TN ∼ 130 K时所有非零模式幅度的急剧增加，并伴随着在此温度以下新模式的产生。一个有趣的观察是，与电荷序相关的呼吸模式幅度与MIT以下的双折射磁化之间存在近乎完美的线性相关性。我们的数据还揭示了一个在T * ∼ 60 K（∼0.4 × TMIT）处之前未曾注意到的异常，这在电导率、晶格参数和一些模式幅度中表现得非常明显。由于相共存仅在TMIT（∼±10 K）附近的小温度区域内观察到，这些观察结果表明在绝缘相中存在一种隐藏的对称性。我们讨论了可能的起源，其中包括非中心对称磁性秩序引起的极性畸变的理论预测[佩雷斯-马托等，《物理：凝聚态物质》28，286001（2016）；乔凡内蒂等，《物理评论快报》103，156401（2009）]。