The High‐Pressure Oxide Tb3O5 and its Non‐Centrosymmetric Low‐Temperature Polymorph–A Comprehensive Study

In this article, the first thoroughly characterized mixed‐valent binary rare earth oxide synthesized under high‐pressure/high‐temperature conditions, and its low‐temperature polymorph are reported. Crystalline HT‐HP‐Tb3O5 has been prepared from an equimolar mixture of Tb4O7 and Tb2O3 under reaction conditions of 8 GPa and 1323 K. Single‐crystal X‐ray structure determination showed that HT‐HP‐Tb3O5 crystallizes in the orthorhombic space group Pnma, isopointal to the β‐Yb5Sb3‐type structure. Temperature‐dependent measurements of the magnetic susceptibility showed that HT‐HP‐Tb3O5 is a Curie–Weiss paramagnet. The observed effective magnetic moment of μeff=9.21(2) μB per formula unit fits well to the calculated moment of μcalc=9.17 μB. Low‐field measurements revealed antiferromagnetic ordering at TN=3.6(1) K. Heat capacity measurements indicated an intrinsic structural phase transition of HT‐HP‐Tb3O5 at low temperature, which was confirmed by synchrotron X‐ray powder diffraction data recorded at 2 K. The metastable high‐pressure modification HT‐HP‐Tb3O5 undergoes a translationengleiche transition from space group Pnma to Pn21a (non‐standard setting of Pna21), leading to the low‐temperature polymorph LT‐HP‐Tb3O5 by loss of a mirror plane (displacive phase transition). Under pressure: A displacive phase transition at low temperature was investigated for the mixed‐valent binary rare earth oxide HP‐Tb3O5. This metastable high‐pressure phase thereby undergoes a translationengleiche transition from space group Pnma to Pn21a (non‐standard setting of Pna21) leading to the low‐temperature polymorph LT‐HP‐Tb3O5 by loss of a mirror plane. The crystal structures and physical properties of both phases are reported in detail.