Stability of (Cs, K)Al₄Be₅B₁₁O₂₈ (londonite) at high pressure and high temperature: a potential neutron absorber material

Abstract

The stability and the thermo-elastic behaviour of a natural londonite

$$ [^{{1a}} \left( {Cs_{{0.36}} K_{{0.34}} Rb_{{0.15}} Ca_{{0.04}} Na_{{0.02}} } \right)_{\Sigma 0.91}{}^{{4e}} \left( {Al_{{3.82}} Li_{{0.05}} Fe_{{0.02}} } \right)_{{\Sigma 3.89}}{}^{{4e}} \left( {Be_{{3.82}} B_{{0.18}} } \right)_{{\Sigma 4}}{}^{{12h}} \left( {B_{{10.97}} Be_{1} Si_{{0.01}} } \right)_{{\Sigma 11.98}} O_{{28}}] $$

has been investigated up to 1,273(3) K (at 0.0001 GPa) and up to 4.85(5) GPa (at 298 K) by means of in situ X-ray powder diffraction. Up to 973 K, no evidence of phase transition or anomalous thermo-elastic behaviour was observed. At T > 973 K, londonite shows the first evidence of an irreversible structure destabilisation. The volume thermal expansion coefficient between 298 and 973 K is α₀ = 2.38(6)·10⁻⁵ K⁻¹. Londonite shows an elastic behaviour up to 4.85 GPa. No phase transition has been observed within the pressure range investigated. P–V data fitted with a second-order Birch–Murnaghan equation of state give V ₀ = 389.1(1)ų and K _T0 = 280(12) GPa. On the basis of the good thermo-elastic behaviour, substantiated by the significantly low compressibility, the modest thermal expansion up to 1,000 K and the significantly high amount of boron (B₂O₃ ~ 50wt%), londonite-type materials could be considered as potential inorganic host for ¹⁰B in composite neutron-absorbing materials.