A soft chemistry approach to the synthesis of single crystalline and highly pure (NH4)CoF3 for optical and magnetic investigations.

Stefanie Siebeneichler,Anja-Verena Mudring,Martin Valldor,Volodymyr Smetana,Katharina V Dorn

doi:10.1063/5.0023343

Stefanie Siebeneichler, Anja-Verena Mudring + Show 3 more

Open Access

https://doi.org/10.1063/5.0023343

Copy DOI

Abstract

A new ionothermal synthesis utilizing 1-alkyl-pyridinium hexafluorophosphates [CxPy][PF6] (x = 2, 4, 6) led to the formation of highly crystalline single-phase ammonium cobalt trifluoride, (NH4)CoF3. Although ammonium transition-metal fluorides have been extensively studied with respect to their structural and magnetic properties, multiple aspects remain unclear. For that reason, the obtained (NH4)CoF3 has been investigated over a broad temperature range by means of single-crystal and powder x-ray diffraction as well as magnetization and specific heat measurements. In addition, energy-dispersive x-ray and vibrational spectroscopy as well as thermal analysis measurements were undertaken. (NH4)CoF3 crystallizes in the cubic perovskite structure and undergoes a structural distortion to a tetragonal phase at 127.7 K, which also is observable in the magnetic susceptibility measurements, which has not been observed before. A second magnetic phase transition occurring at 116.9 K is of second-order character. The bifurcation of the susceptibility curves indicates a canted antiferromagnetic ordering. At 2.5 K, susceptibility measurements point to a third phase change for (NH4)CoF3.

Highlights

Following Powder x-ray diffraction (PXRD) studies gave evidence of tetragonal distortion for (NH4)CoF3 as well as (NH4)MnF3 and (NH4)ZnF3.47 It was proposed that (NH4)CoF3 crystallizes in its low temperature phase in the highest symmetry tetragonal subgroup P4/mmm, and atomic coordinates for the hydrogen positions were suggested based on geometric criteria.[47]
Ammonium cobalt trifluoride (NH4)CoF3 was synthesized via a new ionothermal approach in which 1-alkyl-pyridinium hexafluorophosphate [CxPy][PF6] (x = 2, 4, 6) was applied as both the solvent and the reactant
Thermal decomposition and structural phase transition were investigated via TG/differential thermal analysis (DTA) and Differential scanning calorimetry (DSC) methods. (NH4)CoF3 decomposes above 523 K and experiences a structural change at 123.7 K upon cooling and at 129.8 K upon heating

Summary

Introduction

The family of ammonium metal trifluorides with the formula (NH4)MF3 has been intensively studied for several decades.[1–16] The most popular synthetic approach for the (NH4)MF3 series (M = Mg, Mn, Fe, Co, Ni, Cu, Zn, and Cd) was precipitation in methanolic solution,[1,2,3,5,7,8,9,11,14] while syntheses in agar–agar gel,[4,8,9,14] aqueous solution,[12] HF solution,[15] hydrothermal syntheses,[11,13,14] and high temperature solid-state reactions[10] were successful for selected systems. In contrast to conventional solid-state or solvothermal syntheses, ionothermal reactions[19] with task specific ionic liquids (ILs)[20] combine several advantages: The IL can serve as the reaction and crystallization medium, as the element source (be the reaction partner), and as the mineralizer. Ionothermal approaches bear the advantage over conventional organic solvents and water that the chemical and physicochemical characteristics can be readily tuned through the choice of cations and anions.[21,22] The application of ILs has been found advantageous in the syntheses of open-framework materials and modification of their composition and structural architectures as templating agents.[23–35]

Methods

Results

Conclusion