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

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.

Order-to-Disorder Transition and Hydrogen Bonding in the Jahn–Teller Active NH4CrF3 Fluoroperovskite

“…The crystal structure of NH 4 CrF 3 has the same arrangement of the long Jahn–Teller axis as the d -type polymorph of KCuF 3 , corresponding to the same antiferrodistortive orbital ordering of 3 d 3 x 2 − r 2 and 3 d 3 y 2 − r 2 in the ab -plane and C-type orbital ordering. The structure also stands out among the ammonium fluoroperovskites as it displays ammonium ordering at room temperature, in contrast to, e.g., NH 4 MnF 3 , NH 4 CoF 3 , and NH 4 CuF 3 . − The ordering temperature of 405 K is significantly higher than that of NH 4 MnF 3 and NH 4 CoF 3 , of 182 and 127.7 K, respectively, , indicating that the hydrogen bonding is particularly strong in NH 4 CrF 3 .…”

“…The structure also stands out among the ammonium fluoroperovskites as it displays ammonium ordering at room temperature, in contrast to, e.g., NH 4 MnF 3 , NH 4 CoF 3 , and NH 4 CuF 3 . 29−32 The ordering temperature of 405 K is significantly higher than that of NH 4 MnF 3 and NH 4 CoF 3 , of 182 and 127.7 K, respectively, 29,31 indicating that the hydrogen bonding is particularly strong in NH 4 CrF 3 .…”