Motivated by the recent synthesis of Ba${}_2$CuO${}_{3+\delta }$ (BCO), a high temperature superconducting cuprate with putative $d_{3z^2-r^2}$ ground state symmetry, we investigated its electronic structure by means of Cu $L_3$ x-ray absorption (XAS) and resonant inelastic x-ray scattering (RIXS) at the Cu $L_3$ edge on a polycrystalline sample. We show that the XAS profile of BCO is characterised by two peaks associated to inequivalent Cu sites, and that its RIXS response features a single, sharp peak associated to crystal-field excitations. We argue that these observations are only partially compatible with the previously proposed crystal structure of BCO. Based on our spectroscopic results and on previously published powder diffraction measurements, we propose a crystalline structure characterized by two inequivalent Cu sites located at alternated planes along the $c$ axis: nominally trivalent Cu(1) belonging to very short Cu-O chains, and divalent Cu(2) in the oxygen deficient CuO${}_{1.5}$ planes. We also analyze the low-energy region of the RIXS spectra to estimate the magnitude of the magnetic interactions in BCO and find that in-plane nearest neighbor superexchange exceeds 120 meV, similarly to that of other layered cuprates. Although these results do not support the pure $d_{3z^2-r^2}$ ground state scenario, they hint at a significant departure from the common quasi-2D electronic structure of superconducting cuprates of pure $d_{x^2-y^2}$ symmetry.

受最近合成钡的影响${}_2$氧化铜${}_{3+\delta }$ （BCO），推定的高温超导铜酸盐 $d_{3{ž}^2-{\mathrm{[R}}^2}$ 基态对称性，我们通过铜研究了其电子结构 ${\mathrm{大号}}_3$ 铜的X射线吸收（XAS）和共振非弹性X射线散射（RIXS） ${\mathrm{大号}}_3$多晶样品的边缘。我们表明，BCO的XAS谱图的特征在于与不等价的Cu位点相关的两个峰，并且其RIXS响应具有与晶体场激发相关的单个尖锐峰。我们认为这些观察仅与先前提出的BCO晶体结构部分兼容。根据我们的光谱学结果和先前发表的粉末衍射测量结果，我们提出了一种晶体结构，其特征是两个不等价的Cu位点位于沿着该峰的交替平面上$C$ 轴：名义上属于非常短的Cu-O链的三价Cu（1）和缺氧CuO中的二价Cu（2）${}_{1.5}$飞机。我们还分析了RIXS光谱的低能区域，以估计BCO中磁相互作用的强度，发现面内最近邻超交换超过120 meV，与其他层状铜酸盐相似。尽管这些结果不支持纯$d_{3{ž}^2-{\mathrm{[R}}^2}$ 在基态的情况下，他们暗示与纯超导铜酸盐的普通准二维电子结构有很大的不同 $d_{X^2-{ÿ}^2}$ 对称。