Understanding how ${\mathrm{ABO}}_4$ oxysalt frameworks respond to compression is essential for constraining the lattice-dynamical mechanisms that govern pressure-induced structural transitions. However, intrinsic lattice instabilities are often obscured during static compression by broad mixed-phase regions. Here we measured longitudinal sound velocities ($V_{\mathrm{P}}$) of barite (${\mathrm{BaSO}}_4$) and celestite (${\mathrm{SrSO}}_4$) under shock compression up to ∼10 GPa, and performed first-principles stress-strain calculations for ${\mathrm{BaSO}}_4$. Our results show that the $V_{\mathrm{P}}$ of celestite varies smoothly with pressure over the investigated range, whereas barite exhibits a ∼6% decrease in $V_{\mathrm{P}}$ between ∼5.2 and ∼7.0 GPa. This anomaly is consistent with the $Pnma\to P{2}_1{2}_1{2}_1$ phase transition in ${\mathrm{BaSO}}_4$ but occurs at substantially lower pressure and within a narrower pressure interval than under static compression. First-principles calculations show that the Pnma lattice becomes unstable at shear stresses of ∼1.7–3.5 GPa, close to the estimated shock-induced deviatoric stresses, and that the instability involves cooperative tilting of ${\mathrm{SO}}_4$ tetrahedra and distortion of Ba–O polyhedra. These results suggest that high strain-rate shock loading couples transient deviatoric stresses to lattice instabilities, enabling access to the transition prior to equilibrium relaxation. The observed $V_{\mathrm{P}}$ softening provides experimental support for an underlying lattice instability associated with the transition. Our results highlight that sound-velocity measurements under shock compression can provide a sensitive probe of incipient lattice instability in ${\mathrm{ABO}}_4$ frameworks and illustrate how strain-rate-assisted shear instabilities reshape phase-transition pathways in complex oxysalts.

Close

PrevNext

View figure in article

• High-pressure studies
• Phase transitions
• Shock waves
• Minerals
• First-principles calculations