Double pyrovanadates CaMgV <sub>2</sub> O <sub>7</sub> : Formation mechanism, phase structure, and thermodynamic properties
Guishang Pei, Junyi Xiang, Qingyun Huang, Xuewei Lv
Abstract
Abstract A double pyrovanadate CaMgV 2 O 7 sample was synthesized via a facile solid‐state route under an air atmosphere. The nonequilibrium formation pathways of the CaMgV 2 O 7 were investigated via powder X‐ray diffraction. A multistep reactions path (metavanadates–pyrovanadates–double pyrovanadate CaMgV 2 O 7 ) was proposed to describe the formation of the CaMgV 2 O 7 considering the thermodynamic and kinetic factors. The cell unit parameters of the CaMgV 2 O 7 sample indicated the crystallization according to a monoclinic system with space group P 12 /c 1(14), and the lattice parameters of a = 6.756 Å, b = 14.495 Å, c = 11.253 Å, β = 99.12, and V = 108.806 Å 3 . X‐ray photoelectron spectroscopy also confirmed the +5 oxidation state vanadium in CaMgV 2 O 7 . The endothermic effects at 1033 and 1143 K were related to the incongruent melting and liquidus temperatures of CaMgV 2 O 7 , respectively. The comprehensive thermodynamic properties of CaMgV 2 O 7 were established in both low‐ and high‐temperature regions, utilizing a physical property measurement system and multi‐high‐temperature calorimetry (96 lines). The heat capacity (200 J mol K −1 ) and entropy (198 J mol K −1 ) at 298.15 K were computed based on the low‐temperature heat capacity values, and the enthalpy of formation at 298.15 K was also estimated. The fitted high‐temperature capacity can be used to obtain the changes in the enthalpy, entropy, and Gibbs free energy. This study is part of building a reliable thermodynamic database of the CaO–MgO–V 2 O 5 system.