Unconventional Thermal Expansion in quasi-one-dimensional monoclinic BaIrO$_3$

Abstract

We have investigated the temperature dependence of the crystal structure of quasi-one-dimensional monoclinic BaIrO$_3$ using X-ray diffraction. Diffraction patterns were measured across a temperature range from 13 K to 300 K, with 5-degree steps, and Rietveld refinements were performed to extract the relevant lattice parameters. The resulting cell volumes exhibit a significant deviation from the Debye model predictions for lattice-specific heat within a reasonable range of the Debye temperature, Gr{ü}neisen parameter, and bulk modulus. This suggests an invar-like, unconventional thermal expansion behavior. The deviation begins near the weak ferromagnetic transition temperature, indicating a strong correlation with changes in the electronic and magnetic structure of monoclinic BaIrO$_3$.

Figures (6)

• Figure 1: Crystal structure of 9R-type monoclinic (C2/m) BaIrO$_3$. Upper panel is b-axis view and lower a-axis view. Ir1 - Ir4 denote crystallographically distinct Ir sites in a unit cell. Solid parallelogram denotes the unit cell.
• Figure 2: X-ray diffraction patterns of monoclinic BaIrO$_3$ at the temperature range from 13 to 300 K. The data were obtained at every 5 K step, but displayed at $\sim$20 K step for clarity. The patterns for phase boundary are displayed in black color. (Inset) Resistivity vs. temperature curve.
• Figure 3: Rietveld refinement result for the 13 K XRD pattern of BaIrO$_3$.
• Figure 4: Temperature dependence of the Ir-Ir cation distances for each bonding type, based on the structure refinements.
• Figure 5: Temperature dependence of the lattice constants and the lattice parameter angle of monoclinic BaIrO$_3$. (Inset) Schematic monoclinic unit cell for high/low temperature range.