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

Abstract

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).