Tourmaline is a kind of borosilicate mineral with complex structure and composition, which is used in the fields of functional materials, environmental protection and healthcare. Since the positive and negative charge centers of tourmaline do not coincide, tourmaline has a natural spontaneous polarization that is one of the most important properties, having its application in various fields. In previous studies, the changes of element occupancy, valence proportion of key elements and crystal structure parameters in tourmaline cause the lattice distortion, resulting in a change of spontaneous polarization intensity. It is thus of great significance to reveal the relationship among the composition, structure and property of tourmaline. In this paper, the control path along iron speciation-microstructure-spontaneous polarization of Fe-Mg tourmaline was investigated to clarify the structure-activity relationship dominated by cation occupancy in Y-site of tourmaline and propose the optimal configuration of tourmaline with the excellent performance.

Six single crystals of tourmaline samples with different Fe contents were collected from different locations in China (i.e., Hebei province, Xinjiang province, Yunnan province and Guangxi province), respectively. After digestion, the chemical compositions of the samples were determined by ICP. The natural tourmaline was crushed by a model RH-800 high-speed multifunctional pulverizer (Zhejiang Ronghao Industry and Trade, China), and ground by a model XPM-Φ100×4 planetary muller (Wuhan Prospecting Machinery Factory, China). The composition of tourmaline was determined by a model 7700x inductively coupled plasma optical emission spectrometry (ICP-OES, Agilent Co., USA). The crystallographic structure of tourmaline was examined by a model X'Pert PRO X-ray diffractometer (XRD, PANalytical B.V., the Netherlands) with Cu K_α radiation over a 2θ range between 3° and 80°. The occupancy of Fe²⁺ and Fe³⁺ at Y and Z sites of tourmaline was characterized by a model MVT-1000Mossbauer spectrometer (Wissel Co., Germany). The crystal structure of tourmaline was characterized by a model Smart APEX-CCD X-ray single-crystal diffractometer (Bruker Co., Germany) with Mo Kα radiation, and the structure refinement was completed by a crystallography software named SHELXS-97. The spontaneous polarization intensity was measured quantitatively and indirectly by an alkali solution method.

The structure difference due to different composition of tourmaline is mainly caused by the element change in Y site, in which Fe element plays a crucial role. Based on the calculation of intrinsic electric dipole moment, the spontaneous polarization intensity of tourmaline along the c-axis is more intense than that in other directions (cylinder), and the T polyhedron is the largest contributor to the intrinsic electric dipole moment of tourmaline. The intrinsic electric dipole moments generated by each polyhedron along the a and b axes cancel each other due to the toroidal symmetrical structure of tourmaline, resulting in a small total intrinsic electric dipole moment.

The intrinsic electric dipole moment and the spontaneous polarization intensity of the six tourmaline samples are in the order GX-1>YN-1>XJ-1>HB-1>XJ-2>HB-2. The more intense the polyhedral distortion is, and the larger the intrinsic electric dipole moment and the more intense the spontaneous polarization will be. For tourmalines with Fe content that is higher than Mg content, the polyhedron distortion of tourmaline and the intrinsic electric dipole moment increase with the increase of Fe²⁺ content and iron atom in Z site. For tourmalines with Fe content that is lower than Mg content, the structure distortion and the intrinsic electric dipole moment decrease with the increase of Fe content, and increases with the increase of Fe²⁺ content.

For the Fe-Mg tourmalines, the Y site is mainly occupied by Fe atoms and Mg atoms, and the change of Mg/Fe molar ratio directly affects the expansion and deformation of Y polyhedron, resulting in the change of tourmaline structure. After the intrinsic electric dipole moment of tourmaline is analyzed at different Mg/Fe ratios, the intrinsic electric dipole moment of tourmaline decreases with the decrease of Fe content when the molar ratio of Mg/Fe is less than 1. This is due to the decrease of Fe²⁺ content, leading to the weakening of structural distortion. The intrinsic electric dipole moment of tourmaline increases with the decrease of Fe content when the Mg/Fe molar ratio is greater than 1, which is consistent with the previous results. For tourmaline with Fe content that is lower than Mg content, the lower the Fe content is, and the more intense the structural distortion and the greater the inherent electric dipole moment will be. When the Mg/Fe molar ratio is greater than 2, the intrinsic electric dipole moment and the spontaneous polarization intensity of tourmaline at different Mg/Fe molar ratios become stable.

For tourmalines with Fe content that was higher than Mg content, the cell volume increased with the increase of magnesium content, and the polyhedron distortion of tourmaline and the intrinsic electric dipole moment increased with the increase of Fe²⁺ content and iron atom in Z site. For tourmalines with Fe content that was lower than Mg content, the cell volume increased with the increase of Fe content, the structural distortion and the intrinsic electric dipole moment decreased with the increase of Fe content, and increased with the increase of Fe²⁺ content. The results of spontaneous polarization test showed that the characteristic properties of tourmaline were consistent with the intrinsic electric dipole moment. The configuration structure-function relationship of tourmaline with Fe as a main factor was revealed via analyzing the variation trend of intrinsic electric dipole moment under the dominance of Y-end positive ion. The performance of tourmaline decreased with the decrease of Fe content when the Mg/Fe molar ratio was less than 1. The performance of tourmaline increased with the decrease of Fe content when the Mg/Fe molar ratio was greater than 1. The spontaneous polarization increased when the Mg/Fe molar ratio was greater than 2.