Study of the pressure-induced phase transition in paratellurite (TeO2)

X-ray measurements have been carried out on paratellurite (${\mathrm{TeO}}_2$) at pressures up to 89 ± 1 kbar. Based on both single-crystal and polycrystalline data, a phase transformation from the tetragonal $P{4}_1{2}_{1}2 \left(D_4^4\right)$ space group to the orthorhombic $P{2}_1{2}_1{2}_1 \left(D_2^4\right)$ space group is observed in the vicinity of 8 kbar; no other pressure-induced transitions have been found. These results are consistent with recent neutron-scattering measurements, but at variance with the only previously reported high-pressure x-ray study of this material. In addition, ambient-pressure single-crystal measurements were performed to temperatures below 10 K with no indication of a low-temperature structural transformation. The pressure dependences of the unit-cell parameters have been determined both above and below the critical pressure $P_c$. In the orthorhombic phase, the $b$ axis exhibits a negative linear compressibility and the quantity $\frac{{\mathrm{}(b-a)\mathrm{}}^2}{\mathrm{ab}}$ is found to vary linearly with pressure, extrapolating to zero at 8 ± 1 kbar. Formal theoretical considerations are given for determining the order parameter of this transition and the essential equivalence of other published theoretical treatments is demonstrated. Single-crystal oscillation photographs indicate that in a hydrostatic environment, a domain structure will exist above $P_c$ with strained regions at the domain interfaces which appear to fracture at about 12 kbar. Direct evidence is presented for pressure-dependent atomic motion in the orthorhombic phase. These data can be most easily interpreted in terms of small atomic perturbations in a direction commensurate with the acoustic phonon mode known to be associated with this transition.