In Situ Studies and Magnetic Properties of the Cmcm Polymorph of LiCoPO4 with a Hierarchical Dumbbell-Like Morphology Synthesized by Easy Single-Step Polyol Synthesis

Abstract

LiCoPO 4 (LCP) exists in three different structural modifications: LCP-Pnma (olivine structure), LCP-Pn2 1 a (KNiPO 4 structure type), and LCP-Cmcm (Na 2 CrO 4 structure type). The synthesis of the LCP-Cmcm polymorph has been reported via high pressure/temperature solid-state methods and by microwave-assisted solvothermal synthesis. Phase transitions from both LCP-Pn2 1 a and LCP-Cmcm to LCP-Pnma upon heating indicates a metastable behavior. However, a precise study of the structural changes during the heating process and the magnetic properties of LCP-Cmcm are hitherto unknown. Herein, we present the synthesis and characterization of LCP-Cmcm via a rapid and facile soft-chemistry approach using two different kinetically controlled pathways, solvothermal and polyol syntheses, both of which only require relatively low temperatures (~200 • C). Additionally, by polyol, method a dumbbell-like morphology is obtained without the use of any additional surfactant or template. A temperature-dependent in situ powder XRD shows a transition from LCP-Cmcm at room temperature to LCP-Pnma and finally to LCP-Pn2 1 a at 575 and 725 • C, respectively. In addition to that, the determination of the magnetic susceptibility as a function of temperature indicates a long-range antiferromagnetic order below T N = 11 K at 10 kOe and 9.1 K at 25 kOe. The magnetization curves suggests the presence of a metamagnetic transition.

Direct synthesis and characterization of mixed-valent Li0.5−δCoPO4, a Li-deficient derivative of the Cmcm polymorph of LiCoPO4

“…Cmcm-type LiCoPO 4 samples with varied Li contents were obtained from a polyol process as described in our previous work 21 using LiCH 3 COO (ChemPur, 99+%), (Co(CH 3 COO) 2 $4H 2 O, Merck, 99.99%), and H 3 PO 4 (Merck, 85 wt%) as precursors, and tetraethylene glycol (TTEG, Merck, $99.0%) as solvent. For the synthesis of Li 1Àg CoPO 4 (Cmcm), a Li : Co : P molar ratio of 3 : 1 : 10 was used, whereas Li 0.5Àd CoPO 4 (Cmcm) was obtained from a modied process using a ratio of 1 : 1 : 10.…”

“…The Jana2006 soware 22 was used for the structure renement by the Rietveld method, using the recently reported structure solution of LiCoPO 4 (Cmcm; ICSD no. 432186) 21 as a starting model. Details on the Rietveld renement strategy and parameters used can be found in our previous work.…”

“…19 A thorough investigation of the thermal stability revealed that LiCoPO 4 (Pna2 1 ) converts to the olivine LiCoPO 4 (Pnma) modication at 527 C. 19 Interestingly, the Pna2 1 structure re-emerges as a stable high-temperature phase above 800 C. 19 The LiCoPO 4 (Cmcm) polymorph was rst reported by Amador et al 20 using a high-pressure, high-temperature synthesis route (6 GPa, 900 C). Alternative pathways using lowtemperature procedures such as microwave-assisted solvothermal 16 and polyol 21 synthesis have been demonstrated recently. The structure is built from [CoO 6 ] and [PO 4 ] units, with Li + ions occupying tetrahedral sites.…”

“…16 The magnetic characterization suggested a longrange antiferromagnetic order below T N ¼ 11 K at low elds (10 kOe) and the presence of a metamagnetic transition. 21 Investigations on the thermal stability showed that the structure is metastable and transforms to olivine-type LiCoPO 4 (Pnma) at 575 C, which then transforms to the Pna2 1 modication at 675 C. The thermodynamically stable Pnma-LiCoPO 4 phase was obtained aer cooling. 21 Based on our previous work on the three LiCoPO 4 polymorphs, 17,19,21 we herein present the rst Li-decient structural derivative of the Cmcm modication with the nominal composition Li 0.5Àd CoPO 4 .…”

“…21 Investigations on the thermal stability showed that the structure is metastable and transforms to olivine-type LiCoPO 4 (Pnma) at 575 C, which then transforms to the Pna2 1 modication at 675 C. The thermodynamically stable Pnma-LiCoPO 4 phase was obtained aer cooling. 21 Based on our previous work on the three LiCoPO 4 polymorphs, 17,19,21 we herein present the rst Li-decient structural derivative of the Cmcm modication with the nominal composition Li 0.5Àd CoPO 4 . To the best of our knowledge, this is the rst time that a sub-stoichiometric Li x CoPO 4 phase has been synthesized directly (bottom-up) by a so-chemical polyol approach as opposed to electrochemical or electrochemical Li extraction (top-down) techniques described in the literature.…”