An accurate X‐ray diffraction study of (Y_0.95Bi_0.05)Fe₃(BO₃)₄ single crystals in the temperature range 90–500 K was performed on a laboratory diffractometer and used synchrotron radiation. It was established that the crystal undergoes a diffuse structural phase transition in the temperature range 350–380 K. The complexity of localization of such a transition over temperature was overcome by means of special analysis of systematic extinction reflections by symmetry. The transition temperature can be considered to be T_str ≃ 370 K. The crystal has a trigonal structure in the space group P3₁21 at temperatures of 90–370 K, and it has a trigonal structure in the space group R32 at 375–500 K. There is one type of chain formed by the FeO₆ octahedra along the c axis in the R32 phase. When going into the P3₁21 phase, two types of nonequivalent chains arise, in which Fe atoms are separated from the Y atoms by a different distance. Upon lowering the temperature from 500 to 90 K, a distortion of the Y(Bi)O₆, FeO₆, B(2,3)O₃ coordination polyhedra is observed. The distances between atoms in helical Fe chains and Fe—O—Fe angles change non‐uniformly. A sharp jump in the equivalent isotropic displacement parameters of O1 and O2 atoms within the Fe—Fe chains and fluctuations of the equivalent isotropic displacement parameters of B2 and B3 atoms were observed in the region of structural transition as well as noticeable elongation of O1, O2, B2, B3, Fe1, Fe2 atomic displacement ellipsoids. It was established that the helices of electron density formed by Fe, O1 and O2 atoms may be structural elements determining chirality, optical activity and multiferroicity of rare‐earth iron borates. Compression and stretching of these helices account for the symmetry change and for the manifestation of a number of properties, whose geometry is controlled by an indirect exchange interaction between iron cations that compete with the thermal motion of atoms in the structure. Structural analysis detected these changes as variations of a number of structural characteristics in the c unit‐cell direction, that is, the direction of the helices. Structural results for the local surrounding of the atoms in (Y_0.95Bi_0.05)Fe₃(BO₃)₄ were confirmed by EXAFS and Mössbauer spectroscopies.

中文翻译：

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硼酸铁（Y0.95Bi0.05）Fe3（BO3）4在90-500 K的温度范围内的晶体结构，相变和结构变形。

在实验室衍射仪上使用同步辐射对90-500 K温度范围内的（Y _0.95 Bi _0.05）Fe ₃（BO ₃）₄单晶进行了精确的X射线衍射研究。可以确定的是，晶体在350-380 K的温度范围内经历了扩散结构的相变。通过对对称的系统消光反射进行特殊分析，克服了这种转变在温度范围内的局限性。相变温度可以被看作是Ť _STR ≃370 K的晶体具有在空间群的三角结构P 3 ₁21在90-370的K的温度，并且它具有在空间群的三角结构ř在375-500 K.有由FeO的形成的一个类型链的32 ₆沿着八面体Ç轴在ř 32相。当进入P 3 ₁ 21相时，会出现两种不等价的链，其中Fe原子与Y原子分开的距离不同。将温度从500 K降低到90 K后，Y（Bi）O ₆，FeO ₆，B（2,3）O ₃变形观察到配位多面体。螺旋铁链中原子之间的距离和Fe-O-Fe角的变化不均匀。在Fe-Fe链中，O1和O2原子的各向同性等效位移参数急剧上升，并且在结构转变区域观察到了B2和B3原子的等效各向同性位移参数以及O1，O2的明显伸长，B2，B3，Fe1，Fe2原子位移椭球。已经确定，由Fe，O1和O2原子形成的电子密度螺旋可能是决定稀土硼酸铁的手性，旋光性和多铁性的结构元素。这些螺旋的压缩和拉伸是对称性变化和许多特性的体现，其几何形状受与结构中原子的热运动竞争的铁阳离子之间的间接交换相互作用控制。结构分析将这些变化检测为结构中许多结构特征的变化。c单元格方向，即螺旋方向。通过EXAFS和Mössbauer光谱证实了（Y _0.95 Bi _0.05）Fe ₃（BO ₃）₄中原子局部周围的结构结果。