Compressibility of hibonite (CA6): a single crystal synchrotron radiation high-pressure study

The elasticity of hibonite (ideally CaAl12O19, space group P63/mmc) has been investigated by synchrotron radiation high-pressure single crystal X-ray diffraction, using a membrane-driven diamond anvil cell mounting Boehler-Almax design diamonds and methanol:ethanol:water (16:3:1) and helium as pressure-transmitting fluids, at the ID15B beamline of ESRF, Grenoble, (λ = 0.4111 Å) up to 19.5 GPa. We collected 26 pressure data points. On compressing, no displacive phase transition has been observed. We have fitted lattice volume data with a BM2 EoS, using EosFit7c [1], and obtain zero-pressure isothermal bulk modulus (K0T) of 204(1) GPa and a unit-cell volume of 590.44(14) Å3. Linearized EoS were fitted using cubed lattice parameters [1] and obtained zero-pressure isothermal bulk modulus (K0T) of 256(3) GPa for a lattice parameter and of 141(1) GPa for c lattice parameter. Therefore, the structure is significantly stiff, and strain develops predominantly on the c direction. Hibonite structure is based on the periodic repetition along [0001] of ten layers of approximately closest-packed oxygen atoms. The sequence can be expressed as (chhhcchhhc), where c and h symbolize cubic and hexagonal closest-packed layers, respectively. The cubic close-packed layers constitute blocks that have the spinel structure (S = [M6O8]2+) and are interlayered between blocks having the hexagonal close-packed character R (= [AM6O11]2-), resulting in a staking with a S′RSR′S′ sequence, where R' and S' are rotated 180° around c relative to R and S. Hibonite has the structural formula A[XII]M1[VI]M2[V]M32[IV]M42[VI]M56[VI]O19, where Ca is 12‐ fold coordinated at site A and Al3+ ions are distributed over three distinct octahedra [M1, M4 and M5], the M3 tetrahedron, and the unusual fivefold coordinated trigonal bipyramid M2. The hibonite sample comes from Sierra de Comechingones (Argentina) and has composition (Ca1.01Na0.01)1.02 (Al11.58V0.33Ti0.02Mg0.06Si0.01)1.00O19. Structure refinements yields V ordered in M2[V] M42[VI] sites with composition M2(Al0.94V0.06) and M4(Al1.73V0.27)2 whereas Mg orders in M32[IV] (with composition M3(Al1.94Mg0.06)2). Evolution of polyhedral volumes show that M2[V] M42[VI] and A show a decrease of ca. 10% whereas it is lesser for the rest of the sites. The M2[V] M42[VI] and A sites belong to the R-block, whereas M1[VI]M32[IV]M56[VI] constitute the S-block. Therefore, there is a heterogeneity of the strain along [0001], which alternates between the S-blocks and the more compliant R-block. Congruently, high-T studies [2] have observed that the R-block expands more than the S-block. It remains uncertain if this heterogeneity is due to compositional stain (V ordered at the R-block) or to intrinsic behavior of the hibonite structure.