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 Pn2(1) a (non‐standard setting of Pna2(1)), leading to the low‐temperature polymorph LT‐HP‐Tb(3)O(5) by loss of a mirror plane (displacive phase transition).