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-Tb3 O5 has been prepared from an equimolar mixture of Tb4 O7 and Tb2 O3 under reaction conditions of 8 GPa and 1323 K. Single-crystal X-ray structure determination showed that HT-HP-Tb3 O5 crystallizes in the orthorhombic space group Pnma, isopointal to the β-Yb5 Sb3 -type structure. Temperature-dependent measurements of the magnetic susceptibility showed that HT-HP-Tb3 O5 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-Tb3 O5 at low temperature, which was confirmed by synchrotron X-ray powder diffraction data recorded at 2 K. The metastable high-pressure modification HT-HP-Tb3 O5 undergoes a translationengleiche transition from space group Pnma to Pn21 a (non-standard setting of Pna21 ), leading to the low-temperature polymorph LT-HP-Tb3 O5 by loss of a mirror plane (displacive phase transition).