Thermal expansion of LaB₆ from 298 to 998 K

We propose LaB₆ as a temperature calibration standard for high-temperature (HT) X-ray diffractometry owing to its high temperature stability. Such HT applications require a reliable HT lattice parameter or, equivalently, peak position data, which have not been readily accessible to the diffraction community to date. As such, the thermal expansion behavior of NIST SRM 660a LaB₆ was assessed in the temperature range 298-998 K using HT Bragg-Brentano parafocusing θ:θ X-ray diffractometry in conjunction with Rietveld analysis. Data were collected in the 2θ range 20-150° at a data collection rate of 0.5° θ min^-1 in air and at 1 atm. The temperature was stepped in 50 K increments. The cubic unit-cell lattice parameter [a(T)] of LaB₆ in Å was found to vary as a(T) = 4.15678 (±0.00001) + Ξ(T - 298 K) + Ψ(T - 298 K)², where Ξ = 2.4645 × 10^-5 (±4.8904 × 10^-8) Å K^-1 and Ψ = 1.0325 × 10^-8 (±6.7376 × 10^-11) Å K^-1. The isobaric volume thermal expansion coefficient (TEC) was obtained as α_V ^P  = (5.9291 × 10^-6) + (4.9680 × 10^-9)(T - 298 K) K^-1, from which the corresponding linear TEC was obtained as α_L ^P = (1.9764 × 10^-6) + (1.6560 × 10^-9)(T - 298 K) K^-1. The 3 × 3 matrix representations of the single-crystal isobaric linear TEC and the volume expansivity were obtained for the cubic crystal class to which LaB₆ belongs. Also, the temperature dependence of the lattice parameter data of this study was compared with past landmark studies on LaB₆ by Dutchak et al. [Inorg. Mater. (1972), 8, 1877-1880] and Aivazov et al. [Inorg. Mater. (1979), 15, 1015-1016].