Hydrated and fluorinated potassium scandium phosphates: topology symmetry analysis and structure prediction

Abstract

Two centrosymmetric modifications of a hydrated phosphate KSc(HPO₄)₂ (Pnam and P12₁/c1) and two polar modifications of a fluorinated phosphate K₃Sc₃(P₃O₁₀F₂)F₅ (Cmc2₁ and I1a1) were synthesized hydrothermally in the temperature range of 280–290 °C and in the pressure range of 80–100 atm. Phosphate KSc(HPO₄)₂ demonstrates phase transition from unknown orthorhombic at room temperature to known monoclinic at low temperature isostructural to several Sc-phosphates with Rb, Cs, and NH₄. Orthorhombic K₃Sc₃(P₃O₁₀F₂)F₅ is isostructural to K₃Fe₃(P₃O₁₀F₂)F₅, monoclinic – resembles a recently investigated compound. Detailed topology symmetry analysis using OD-theory carried out for polar fluorinated Sc-phosphates allowed separating a common layer with a local trigonal symmetry P31m. Close polytypic relations are described and a series of hypothetical structures in the unit family are predicted. Under Nd:YAG laser illumination, powders of acentric fluorophosphates demonstrate second harmonic light emission (SHG). Determination of optical nonlinearity was fulfilled for orthorhombic and monoclinic fluorine-phosphate modifications using SHG from extremely dispersed powders with grains 3–5 mcm. The second harmonic outputs 2.5 and 1.2 compared to those of the α-quartz powder under the same conditions correspond to space-averaged nonlinear optical coefficients 〈d〉 = 0.55 and 0.38 pm V⁻¹ for these two acentric modifications, respectively.