Synthesis, structures, and magnetic properties of alkali metal manganese(iii) fluorophosphates containing low-dimensional S = 2 spin structures

Abstract

Five alkali metal manganese(III) fluorophosphates, K₂Mn(PO₃F)F₃ (I), Rb₂Mn(PO₃F)F₃ (II), Rb₂Mn₂(PO₃F)(PO₄)F₃ (III), Rb₃Mn₃(PO₃F)₂(PO₄)F₅ (IV), and CsMn(PO₃F)F₂ (V), were successfully synthesized using a hydrothermal method. The monofluorophosphate anion (PO₃F)²⁻ groups work as “chemical scissors” to promote low-dimensional spin structures with the aid of alkali metal cations. I and II had an S = 2 uniform chain structure formed by corner-sharing trans-MnO₂F₄ octahedra. III contained a quasi one-dimensional S = 2 spin chain composed of trans-MnO₄F₂ octahedra and trans-MnO₂F₄ octahedra through vertex-sharing fluorine atoms running along the [1−10] direction, whereas V exhibited a zigzag S = 2 spin chain built from the facial-MnO₃F₃ octahedron. In contrast, IV constitutes weakly coupled S = 2 spin trimers, where the three-dimensional (3D) framework contains two-dimensional {Mn₃O₄F₄(PO₃F)₂}²⁻ layers formed by trinuclear Mn-octahedral units via corner-sharing fluorine atoms separated by PO₃F tetrahedra. Magnetic measurements confirmed that I and II possess antiferromagnetic (AFM) long-range ordering at low temperature, with intrachain exchange couplings J_intra/k_B = −12.1(1) K for I and J_intra/k_B = −8.1(2) K for II. The magnetic behaviour of IV was very complex, with three successive magnetic transitions owing to weak AFM coupling among S = 2 spin trimers. V shows an alternating S = 2 one-dimensional (1D) chain with canted AFM ordering at 16.5 K.