Magnetostriction in NiF₂ : Combined γ-Ray and Neutron Diffraction

Abstract

NiF₂ has a tetragonal rutile type structure with fluorine at (x,x,0). According to a prediction deduced from the Ewald-Born theory of structuralBiréfringence, a very small shift in the fluorine positional parameter x due to magnetic ordering may be the cause for a change in the opticalBiréfringence near the Néel temperature [T_N = 73.2 K]. The predicted value of Δx = x(l5 K) − x(298 K) = -4.5 × 10^-4 ≅ 2.96 × 10^-3 Å demands measurements of very high accuracy. The experiments were performed using the Berlin γ-ray diffractometer [γ = 0.0392 Å] and time-of-flight neutrons [γ = 0.5−3 Å] from the pulsed spallation source IPNS, Argonne. The same single crystal plate of about 3 × 3 × 1.5 mm³ was investigated by both methods. The results obtained from the neutrons are in good agreement with those from γ-ray diffraction which confirm the theoretical prediction:

298K 15K Δx

γ-ray diffraction x = 0.30371(9) x = 0.30325(7) -4.6(1.1) × 10^-4

Neutron diffraction x = 0.30366(4) x = 0.30333(3) -3.3(0.5) × 10^-4