Low temperature thermal expansion of CsNiCl3 and RbNiCl3

Abstract
The thermal expansion of the linear‐chain antiferromagnets CsNiCl3 and RbNiCl3 has been measured from 1.5K to 90K. Anomalies in the linear expansion coefficients of CsNiCl3 near the spin‐rearrangement and Néel temperatures show large differences due to the anisotropic spin‐phonon interaction. The magnetic Grüneisen parameter for the associated volume coefficient is similar to that observed in other systems. For the one‐dimensional ordering the corresponding Grüneisen parameter is inconsistent with that predicted from ultrasonic data. The thermal expansion behavior of RbNiCl3 near its Néel temperature differs markedly from that of CsNiCl3. Possible reasons for the difference are discussed.