Thermal expansion of LaB₆ from 298 to 998K

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 298-998K 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°θmin-1 in air and at 1atm. The temperature was stepped in 50K 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-298K)+Ψ(T - 298K)2, where Ξ = 2.4645 × 10-5 (±4.8904 × 10-8) ÅK-1 and Ψ = 1.0325 × 10-8 (±6.7376 × 10-11)ÅK-1. The isobaric volume thermal expansion coefficient (TEC) was obtained as αVP=(5.9291×10-6)+(4.9680×10-9)(T-298K)K-1, from which the corresponding linear TEC was obtained as αLP=(1.9764×10-6)+ (1.6560×10-9)(T-298K)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].