Magnetovolume-driven zero (ZTE) and negative (NTE) thermal expansion in Tb2Fe17 with uncommon rhombohedral crystal structure

Zero thermal expansion (ZTE) and negative thermal expansion (NTE) are uncommon phenomena in which materials exhibit negligible dimensional change or lattice contraction upon heating. R 2 Fe 17 alloys (R = rare earth) are well-known for displaying these anomalous behaviors, particularly within magnetically ordered states where strong coupling between magnetic and lattice degrees of freedom governs the thermal response. By combining temperature-dependent neutron diffraction (ND) experiments with mean-field (MF) and density functional theory (DFT) calculations, we have investigated the structure stability, electronic and magnetic properties, and magnetovolume effects of the Tb 2 Fe 17 bulk alloy with the unconventional rhombohedral R 3 ̄ m crystal structure. DFT calculations indicate that the formation energies of the hexagonal and rhombohedral phases are nearly identical, suggesting that both crystal structures are energetically stable. The rhombohedral Tb 2 Fe 17 alloy exhibits ferrimagnetic ordering with a Curie temperature T C = 408 K. The experimental temperature dependence of the atomic magnetic moments on the Fe and Tb sublattices is well described within a MF model including crystal field effects, with the calculated values being consistent with those obtained from ND data. A key finding is that the temperature dependence of the unit cell volume of Tb 2 Fe 17 displays two notable anomalies below T C : a near ZTE region at low temperatures followed by NTE occurring within the temperature range between 320 and 410 K, arising from the anomalous temperature dependence of the basal and uniaxial lattice parameters. Additionally, a significant spontaneous magnetostriction ( ω s = 1 . 38 × 1 0 − 2 at 6 K) and the strong pressure-driven decrease of T C d T C / d P ≈ − 46 K / G P a further evidence the strong magnetoelastic coupling governing the magnetic and structural stability of the system. Altogether, these results provide new insights into the interplay between the crystal structure and the magnetic properties in Tb 2 Fe 17 , positioning it as a compelling candidate for applications requiring exceptional dimensional stability under thermal shocks. • First magnetic structure determination of rhombohedral Tb 2 Fe 17 . • DFT shows rhombohedral and hexagonal Tb 2 Fe 17 are energetically equivalent. • Mean-field model reproduces Fe and Tb magnetic-moment evolution vs temperature. • Zero and negative thermal expansion below the Curie temperature. • Curie temperature decreases linearly with pressure (d T C /d P = − 46 K/GPa).