The influence of an ultra-small amount of heterovalent Y³⁺ activator ions in aqueous solution on the defect formation of different growth sectors of α-Ni²⁺SO₄·6H₂O crystals

Materials based on α-NiSO₄·6H₂O (NSH) (sp. gr. P4₁2₁2, Z = 4) are used for lithium-ion batteries and UV filters for the solar-blind range. Their performance characteristics can be varied by introducing activator ions М. The behaviour of M in NSH depends on differences in crystallochemical characteristics of Mⁿ⁺ and Ni²⁺ ions, the way of introducing M ions into the solution, and the growth sectors of the crystals. The goal of this work is to establish the distribution of Y³⁺ ions over the growth sectors of NSH:Y crystals when they are introduced into a solution in extremely small quantities (c = 10 mM).

Two crystals were grown on NSH (001) plates: NSH:Y-1—perpendicular to <001> (32 g; growth duration 144 h); NSH:Y-2—parallel to <001> (269 g; growth duration 336 h). The habit (mmm) was found for NSH:Y-1, faceted by {001}, {101}, {011}, {012}, and {110} with face areas (S_face{hkl}) {101}>{011}>{012}>{001}>{110}. In the habit (mm2) of NSH:Y-2 crystals, new faces ((102), (12), and (1 )) with S_face(hkl) (101)>(012)>(102)>(011)>(001)>(1 2)>(1 )>(110) also appeared. The NSH:Y-2 crystal, by its (00) face, was at the bottom of the crystallizer during growth (uneven supply of the solution).

The c, Å tetragonal crystal parameter (X-ray diffraction of powdered crystals) decreased with the decrease in Y³⁺ content (с_м, ppm—mass spectrometry), c{101}>c{102}>c{001}, consistent with crystal morphology. Defect formation can be described by the quasi-chemical reaction 0V_Ni^n’+Y_i^m· and the composition (Ni²⁺_1-x[])(Y³⁺_i(x))SO₄]. In the {101}-{102}-{001} series, a decrease in Y³⁺ content is accompanied by a decrease in the Y³⁺ interstitials (Y_i^m·) and vacancies in the Ni²⁺ site (V_Ni^n’).

The compositions of the NSH:Y growth sectors, on which the functional properties depend, are important for the application of the material in the form of plates.

Funding: Ministry of Science and Higher Education of the Russian Federation, grant No. FSFZ-2024-0026.