Crystalline vanadium (II) fluoride, VF2. Preparation, structure, heat capacity from 5 to 300 °K and magnetic ordering

Abstract

Crystalline VF2 was prepared by the reduction of VF3 in an atmosphere of 3 H2:1 HF at 1100°C. The VF2 formed deep blue crystalline needles. The crystal structure is of the rutile type, space group P42/mnm. Parameters determined by x-ray diffraction are a=4.804±0.005 Å, c=3.237±0.005 Å, u=0.306±0.005. The low temperature heat capacity of VF2 is reported from 5 to 300 °K. A sharp peak in heat capacity associated with the development of long-range magnetic order is observed at 7 °K. Smoothed values of the total heat capacity, entropy, enthalpy, and Gibbs energy are tabulated between 5 and 300 °K. Values at 298.15 °K are: C °P=15.10 cal °K−1 mole−1, S °=18.217 cal °K−1 mole−1, H°−H°0=2661.0 cal mole−1. By a corresponding states approximation the magnetic contributions to the heat capacity and entropy are calculated. The magnetic heat capacity has a gradual maximum at 26.8±1 °K. The heat capacity curve agrees with that calculated for a one-dimensional chain with antiferromagnetic Heisenberg interactions. A value of the exchange constant, J/k=−9.4±0.4 °K is obtained.