Magnetic Properties of the Quasi-1D Magnesium Lanthanide Borates Mg$Ln$B$_5$O$_{10}$

TL;DR

MgLnB5O10 compounds form quasi-1D Ln chains within a monoclinic framework, enabling low-dimensional magnetic phenomena. The authors synthesize nine MgLnB5O10 members via a solution-based Pechini method and perform comprehensive structural, magnetic, and thermodynamic characterization, including magnetocaloric analysis. Magnetic behavior ranges from van Vleck-dominated Eu/Sm to Pr indicating possible singlet states, to Nd, Tb, Dy, Ho, Er showing Ising-like anisotropy and Gd displaying Heisenberg spins; TbB5O10 shows ordering at ~2.25 K, while PrB5O10 hints at a singlet ground state. MgGdB5O10 emerges as the strongest magnetocaloric candidate at low and high fields, highlighting potential for cryogenic refrigeration, whereas the series as a whole supports quasi-1D magnetism and invites further neutron scattering studies to elucidate magnetic structures and interactions.

Abstract

Lanthanide borates are widely studied for their optical and magnetic properties. A wide variety of structures are known with 3, 2, 1 and 0 dimensional connectivity of lanthanide ions. Here, we explore Mg$Ln$B$_5$O$_{10}$, with a quasi-1D arrangement of the $Ln$ ions. Polycrystalline samples of Mg$Ln$B$_5$O$_{10}$ ($Ln$ = La, Pr, Nd, Sm--Er) were synthesised with high purity via a sol-gel method. Powder X-ray diffraction data confirmed the reported monoclinic space group ($P2_1/c$). The magnetic $Ln^{3+}$ ions in Mg$Ln$B$_5$O$_{10}$ form relatively isolated zig-zag chains parallel to the $b$ axis. Magnetic susceptibility and isothermal magnetisation were measured: all samples except $Ln=$ (Eu, Sm) fit the Curie-Weiss Law in isothermal magnetisation at high temperatures, in broad agreement with theoretical expectations. $Ln =$ (Nd, Tb, Dy, Ho) exhibit signatures characteristic of Ising spin saturation, implying single ion anisotropy, while Gd exhibits characteristics of Heisenberg spins. Estimation of magnetic interactions suggests that Mg$Ln$B$_5$O$_{10}$ are candidate materials for quasi-1D magnetism. The magnetocaloric entropy change was also calculated, with MgGdB$_5$O$_{10}$ showing promise for application to solid-state refrigeration at liquid helium temperatures.

Figures (11)

• Figure 1: Crystal structure of MgLaB5O10 in space group $P2_1/c$: (a) all atoms; (b) only the lanthanum ions. The La$^{3+}$ ions form zig-zag chains in the $b$ direction and the distances $R_\mathrm{intra}$ and $R_\mathrm{inter}$ are indicated. Image produced using VESTA. momma_vesta_2011
• Figure 2: Rietveld refinement of synchrotron PXRD data Diamond for MgHoB5O10. Measured intensity is in black, calculated intensity in red and the difference offset below in grey. Inset: the $(100)$ and $(10\bar{2})$ reflections, illustrating the difficulty in achieving accurate fitted intensities. Refinement performed using TOPAS Software. coelho_topas_2018
• Figure 3: MgLnB5O10 lattice parameters (obtained from Rietveld refinement of synchrotron PXRD data) Diamond plotted against effective ionic radii for Ln = (La,Pr,Nd,Sm--Er). Radii are for 9-coordinate $Ln^{3+}$ ions Shannon1976. (Error bars are smaller than data points.)
• Figure 4: Magnetic susceptibility $\chi$ vs T (blue), and $1/(\chi-\chi_0)$ vs T. ($1.8\leq T(\mathrm{K}) \leq 300$; data in red, Curie-Weiss fit in orange.) Linear fits were obtained down to $\sim$80 K (and lower for Gd). Sm and Eu did not obey the Curie-Weiss Law.
• Figure 5: $d\chi/dT$ vs temperature for MgPrB5O10 (left) and MgTbB5O10 (right). Sharp changes in $d\chi/dT$ at $\sim$10 and 2.25 K respectively indicate the onset of an ordering transition.