Structural Description of Pressure-Induced Amorphization in ${\mathrm{ZrW}}_2{\mathrm{O}}_8$

${\mathrm{ZrW}}_2{\mathrm{O}}_8$ undergoes a high-pressure amorphization transition above 1.5 GPa to a phase which is recoverable to ambient conditions. Reverse Monte Carlo modeling of neutron and x-ray total scattering data from ${\mathrm{ZrW}}_2{\mathrm{O}}_8$ recovered from $\sim 4\mathrm{GPa}$ shows that the large increase in density on pressurizing ${\mathrm{ZrW}}_2{\mathrm{O}}_8$ is accommodated within the structure by increased bonding between the ${\mathrm{WO}}_4$ tetrahedra. This increases the tungsten coordination; changes to the ${\mathrm{ZrO}}_6$ octahedral environment are not required. This densified crystal-based model, which contains significant local disorder within a distorted periodic structure, is also in reasonable agreement with x-ray and neutron total scattering data measured in situ at high pressure.

Figure 1

(a) Neutron and (b) x-ray weighted pair distribution functions $G(r)$ from recovered amorphous ${\mathrm{ZrW}}_2{\mathrm{O}}_8$ (thin blue lines) together with the equivalent functions calculated from refined RMC model B (thick red lines). Also shown are the partial pair distribution functions, $g_{ij}(r)$ (c) and a comparison with $G^N(r)$ from crystalline $\alpha \mathrm{\text{−}}{\mathrm{ZrW}}_2{\mathrm{O}}_8$ [17] measured at room temperature and pressure (inset).

Figure 2

A fragment of the $\alpha \mathrm{\text{−}}{\mathrm{ZrW}}_2{\mathrm{O}}_8$ crystal structure showing which additional bonds are formed between ${\mathrm{WO}}_4$ tetrahedra (red) and ${\mathrm{ZrO}}_6$ octahedra (blue) to create the starting models for the amorphous structure. The green bond (W1 to O3) is formed in all models; the light blue bond (Zr to O4) in model A; a proportion of yellow bonds (W2 to O4) in model B (see text for details). Atom labels refer to 2.

Figure 3

(a) Neutron and (b) x-ray structure factor data from recovered amorphous ${\mathrm{ZrW}}_2{\mathrm{O}}_8$ (thin blue lines) and the equivalent functions obtained from refined RMC model B (thick red lines). The insets show comparisons with data from amorphous ${\mathrm{ZrW}}_2{\mathrm{O}}_8$ measured at $\simeq 3\mathrm{GPa}$.

Figure 4

Polyhedral representation of a section of (a) $\alpha \mathrm{\text{−}}{\mathrm{ZrW}}_2{\mathrm{O}}_8$ and (b) amorphous ${\mathrm{ZrW}}_2{\mathrm{O}}_8$ from RMC refinement of model B. Green filled and unfilled polyhedra correspond to $\mathrm{Zr}–\mathrm{O}$ and $\mathrm{W}–\mathrm{O}$ polyhedra; $\mathrm{Zr}–\mathrm{O}$ and $\mathrm{W}–\mathrm{O}$ bonds are defined as atoms less than 2.3 and 2.4 Å apart, respectively.