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 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‐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 Pn21 a (non‐standard setting of Pna21), leading to the low‐temperature polymorph LT‐HP‐Tb3O5 by loss of a mirror plane (displacive phase transition).


Crystal structure of HT-HP-Tb3O5
As reported in the main article, the Tb1 ions are coordinated by eight oxygen anions with interatomic distances between 233 and 253 pm in form of a bi-capped trigonal prism. However, another oxygen atom at a distance of 332 pm caps the third face of the trigonal prism, therefore forming a tri-capped trigonal prismatic coordination of Tb1 (see Figure S1). For HT-HP-Tb3O5, this oxygen anion cannot be regarded coordinating Tb1 due to its large interatomic distance, though it is included in the coordination description of atoms at the Sb2 position of some intermetallics crystallizing in the β-Yb5Sb3 type structure, e.g.
Ti5Sb3. [2] This is due to the smaller relative interatomic distance deviation of the three capping atoms in Ti5Sb3 (335, 334, and 362 pm) in comparison to Tb3O5 (240, 233, and 332 pm). Similarly, the second crystallographic independent anions on the Sb1 position are ascribed to be coordinated by eight cations in form of a bi-capped distorted octahedron rather than seven as in HT-HP-Tb3O5 ( Figure S1). Furthermore, some of the square pyramids are transformed into octahedra by the approach of a sixth atom and the tetrahedra change into trigonal bipyramids in a similar way in for instance Ti5Sb3. [2] The elongated hexagons formed by the oxygen anions as the base planes of the hexagonal prisms in the crystal structure of HT-HP-Tb3O5 form a parquet-like patchwork, as is shown in Figure S3. This pattern is also found, more or less distorted, in the structure of Rh5Ge3 and all its derivative structure types. [3] We should note that this is the typical symmetry imposed by space group Pnma. Figure S1. Coordination of the terbium atoms in the crystal structure of HT-HP-Tb3O5, including one more non-coordinating oxygen atom each for comparison with isostructural compounds.
For LT-HP-Tb3O5, a value of 26,928 kJ mol -1 was obtained, to be compared with 27,132 kJ mol -1 (deviation: 0.75%). This validates the accuracy of the crystal structure solution of HT-HP-Tb3O5 and LT-HP-Tb3O5.

Further details on the charge distribution calculations
Comprehensive calculations and discussions regarding the valence state of the rare earth cations in various cerium oxides -namely the A-and C-type sesquioxides, the dioxide and some intermediate higher oxides  Table S6). This huge difference in mean interatomic distances accounts for the clearly unequal valence of the two terbium sites in HT-HP-Tb3O5, which could therefore be ascribed Class 1 according to Robin and Day [9] with the two distinct crystallographic sites having integral but unequal valence. Nevertheless, the results obtained by the CHARDI calculations, the lower coordination number of Tb2 of seven, and the fact that the HT-HP-Tb3O5 samples obviously show a strong optical absorption in the visible spectrum rather speak for a Class 2 inhomogeneous mixed valence of the terbium cations.

Powder X-ray diffraction
The experimental powder X-ray diffraction pattern of the reaction product ( Figure S5) is consistent with the theoretical powder pattern simulated from single-crystal X-ray diffraction data. Thus, the experimental powder pattern shows some additional reflections (marked with asterisks) assignable to Tb6O11 arising as a side product. By indexing the reflections of HT-HP-Tb3O5, we obtained the parameters a = 978.97(8), b = 670.36(4), c = 604.35(4) pm, and a volume of 0.39661(3) nm³. S11 Figure S5: Experimental powder X-ray diffraction pattern of HT-HP-Tb3O5 (top) in comparison to the theoretical powder pattern simulated from single-crystal X-ray diffraction data (bottom). Reflections marked with asterisks originate from Tb6O11.