Antiferromagnetic Spin Correlations Between Corner-Shared [FeO5]7– and [FeO6]9– Units, in the Novel Iron-Based Compound: BaYFeO4

A novel quaternary compound in the Ba–Y–Fe-O phase diagram was synthesized by solid-state reaction and its crystal structure was characterized using powder X-ray diffraction. The crystal structure of BaYFeO4 consists of a unique arrangement of Fe3+ magnetic ions, which is based on alternate corner-shared units of [FeO5]7– square pyramids and [FeO6]9– octahedra. This results in the formation of stairwise channels of FeO polyhedra along the b crystallographic axis. The structure is described in an orthorhombic crystal system in the space group Pnma with lattice parameters a = 13.14455(1) Å, b = 5.694960(5) Å, and c = 10.247630(9) Å. The temperature-dependent magnetic susceptibility data reveal two antiferromagnetic (AFM) transitions at 33 and 48 K. An upturn in the magnetic susceptibility data above these transitions is observed, which does not reach its maximum even at 390 K. The field-dependent magnetization data at both 2 and 300 K show a nearly linear dependence and do not exhibit significant hysteresis. Heat capacity measurements between 2 and 200 K reveal only a broad anomaly without any indication of long-range ordering. The latter data set is not in good agreement with the magnetic susceptibility data, which makes it difficult to exactly determine the magnetic ground state of BaYFeO4. Accordingly, a temperature-dependent neutron diffraction study is in order, which will enable resolving this issue. The theoretical study of the relative strengths of magnetic exchange interactions along various possible pathways, using extended Hückel spin dimer analysis, shows that only interactions between square pyramidal and octahedral centers are significant, and among them, the intrachannel correlations are stronger than interchannel interactions. This is the first physical property study in such a magnetic ion substructure.

通过固相反应法合成了Ba-Y-Fe-O相图中的新型四元化合物，并采用粉末X射线衍射对其晶体结构进行了表征。BaYFeO4的晶体结构呈现出独特的Fe³+磁性离子排布模式，该排布以交替出现的[FeO5]^7-四方锥与[FeO6]^9-八面体的共角结构单元为基础，沿晶体学b轴方向形成了由FeO多面体构成的阶梯状通道。该晶体属于正交晶系，空间群为Pnma，晶格参数为a=13.14455(1) 埃，b=5.694960(5) 埃，c=10.247630(9) 埃。变温磁化率测试数据显示，该化合物在33 K与48 K处存在两次反铁磁（antiferromagnetic, AFM）转变；在这两个转变温度以上，磁化率出现抬升现象，且该抬升即使在390 K时仍未达到峰值。在2 K与300 K下测得的场变磁化数据均呈现近乎线性的变化趋势，未表现出明显的磁滞现象。在2 K至200 K区间内开展的热容测试仅观测到一处宽化的反常峰，未发现长程磁有序的迹象。上述热容数据与磁化率数据的吻合度欠佳，致使精准确定BaYFeO4的磁基态颇具挑战。因此，开展变温中子衍射研究以厘清该问题实为必要之举。采用推广休克尔自旋二聚体分析（extended Hückel spin dimer analysis）对多种可能路径下的磁交换相互作用相对强度开展的理论研究表明，仅四方锥与八面体中心之间的相互作用具有显著意义，且其中通道内的关联强于通道间的相互作用。这是针对此类磁性离子亚结构开展的首次物性研究。