The High-Pressure Oxide Tb3O5 and its Non-Centrosymmetric Low-Temperature Polymorph-A Comprehensive Study
Résumé
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 8GPa and 1323K. Single-crystal X-ray structure determination showed that HT-HP-Tb3O5 crystallizes in the orthorhombic space group Pnma, isopointal to the beta-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 mu(eff) = 9.21(2) mu(B) per formula unit fits well to the calculated moment of mu(calc)=9.17(mu 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-Tb3O5 at low temperature, which was confirmed by synchrotron X-ray powder diffraction data recorded at 2K. The metastable high-pressure modification HT-HP-Tb3O5 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-Tb3O5 by loss of a mirror plane (displacive phase transition).
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