The High‐Pressure Oxide Tb 3 O 5 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‐Tb 3 O 5 has been prepared from an equimolar mixture of Tb 4 O 7 and Tb 2 O 3 under reaction conditions of 8 GPa and 1323 K. Single‐crystal X‐ray structure determination showed that HT‐HP‐Tb 3 O 5 crystallizes in the orthorhombic space group Pnma , isopointal to the β‐Yb 5 Sb 3 ‐type structure. Temperature‐dependent measurements of the magnetic susceptibility showed that HT‐HP‐Tb 3 O 5 is a Curie–Weiss paramagnet. The observed effective magnetic moment of μ eff =9.21(2)  μ B per formula unit fits well to the calculated moment of μ calc =9.17  μ B . Low‐field measurements revealed antiferromagnetic ordering at T N =3.6(1) K. Heat capacity measurements indicated an intrinsic structural phase transition of HT‐HP‐Tb 3 O 5 at low temperature, which was confirmed by synchrotron X‐ray powder diffraction data recorded at 2 K. The metastable high‐pressure modification HT‐HP‐Tb 3 O 5 undergoes a translationengleiche transition from space group Pnma to Pn 2 1 a (non‐standard setting of Pna 2 1 ), leading to the low‐temperature polymorph LT‐HP‐Tb 3 O 5 by loss of a mirror plane (displacive phase transition).