The High-Pressure Oxide Tb O 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 O has been prepared from an equimolar mixture of Tb O and Tb O under reaction conditions of 8 GPa and 1323 K. Single-crystal X-ray structure determination showed that HT-HP-Tb O crystallizes in the orthorhombic space group Pnma, isopointal to the β-Yb Sb -type structure. Temperature-dependent measurements of the magnetic susceptibility showed that HT-HP-Tb O is a Curie-Weiss paramagnet. The observed effective magnetic moment of μ =9.21(2) μ per formula unit fits well to the calculated moment of μ =9.17 μ . Low-field measurements revealed antiferromagnetic ordering at T =3.6(1) K. Heat capacity measurements indicated an intrinsic structural phase transition of HT-HP-Tb O 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 O undergoes a translationengleiche transition from space group Pnma to Pn2 a (non-standard setting of Pna2 ), leading to the low-temperature polymorph LT-HP-Tb O by loss of a mirror plane (displacive phase transition).