A₂TiO₅ (A = Dy, Gd, Er, Yb) at High Pressure

Abstract

The structural evolution of lanthanide ATiO (A = Dy, Gd, Yb, Er) at high pressure is investigated using synchrotron X-ray diffraction. The effects of A-site cation size and of the initial structure are systematically examined by varying the composition of the isostructural lanthanide titanates and the structure of dysprosium titanate polymorphs (orthorhombic, hexagonal, and cubic), respectively. All samples undergo irreversible high-pressure phase transformations, but with different onset pressures depending on the initial structure. While each individual phase exhibits different phase transformation histories, all samples commonly experience a sluggish transformation to a defect cotunnite-like (Pnma) phase for a certain pressure range. Orthorhombic DyTiO and GdTiO form P2am at pressures below 9 GPa and Pnma above 13 GPa. Pyrochlore-type DyTiO and ErTiO as well as defect-fluorite-type YbTiO form Pnma at ∼21 GPa, followed by Im3̅m. Hexagonal DyTiO forms Pnma directly, although a small amount of remnants of hexagonal DyTiO is observed even at the highest pressure (∼55 GPa) reached, indicating kinetic limitations in the hexagonal DyTiO phase transformations at high pressure. Decompression of these materials leads to different metastable phases. Most interestingly, a high-pressure cubic X-type phase (Im3̅m) is confirmed using high-resolution transmission electron microscopy on recovered pyrochlore-type ErTiO. The kinetic constraints on this metastable phase yield a mixture of both the X-type phase and amorphous domains upon pressure release. This is the first observation of an X-type phase for an ABO composition at high pressure.

Firing salts method for the synthesis of orthorhombic Gd2TiO5: experimental characterization supported by DFT first principles calculations

“…On the other hand, there are four Ti 4+ cations with five-fold oxygen coordination, giving rise to an offset square based pyramidal polyhedral. The orthorhombic RE 2 TiO 5 compound have no mixed occupancy in the different 4c Wyckoff symmetry sites [3][4][5]. These changes allow them to have a wide range of important applications from high permeability dielectrics used in memory devices [6], biomedical applications [7], luminescence [8,9], nuclear reactor control rod materials or as a neutron absorber [3,10,11].…”

Re2TiO5: Special dielectric titanates exhibiting abnormally low permittivity and a positive coefficient of resonance frequency

“…12 For example, ALa 4 Ti 4 O 15 (A = Ba, Sr, and Ca) dielectric ceramics prepared by Tohdo et al have been proven to have ε r = 41.4-44.4, Q × f = 41 008-50 246 GHz, and τ f = −26.0 to −8.4 ppm/ • C. 13 Similarly, rare-earth titanates in the binary Re 2 O 3 -TiO 2 offer a series of excellent physical and chemical properties encompassing high mechanical strength, good thermal stability, refractoriness, and special optical properties. [14][15][16][17][18] The most representative compounds are Re 2 Ti 2 O 7 and Re 2 TiO 5 , which can be synthesized by all rare-earth elements from La to Lu and Y, Sc. 19,20 The Re 2 Ti 2 O 7 compounds with a pyrochlore structure possess competitive dielectric properties.…”

“…14,19 In general, when accommodating large-sized Re elements (e.g., La, Nd, and Sm), an orthogonal structure is preferred, whereas Re 2 TiO 5 tends to adopt a cubic structure for small-sized Re elements (e.g., Yb and Lu). 16,24 On the other hand, the middle-sized elements (e.g., Gd, Dy, and Ho) may assume either the orthogonal, hexagonal, or cubic structure depending on the condition of crystallization. 14,25 Structural transformation is expected to undergo either by constructing solid solutions or by varying the sintering condition.…”

“…Of particular interest is that Re 2 TiO 5 crystallizes in a variety of crystallographic symmetries (orthogonal, hexagonal, and cubic structure) depending on the ionic radius of Re, sintering condition, ambient pressure, and so forth 14,19 . In general, when accommodating large‐sized Re elements (e.g., La, Nd, and Sm), an orthogonal structure is preferred, whereas Re 2 TiO 5 tends to adopt a cubic structure for small‐sized Re elements (e.g., Yb and Lu) 16,24 . On the other hand, the middle‐sized elements (e.g., Gd, Dy, and Ho) may assume either the orthogonal, hexagonal, or cubic structure depending on the condition of crystallization 14,25 .…”

Novel complex ceramic oxides, Ln2TiO5 (Ln = La, Sm, Gd, Tb, Dy, Ho, Er, and Yb), for polyphase nuclear waste‐forms

“…It is also possible to achieve high‐pressure assisted phase transitions that deviate from those attained via high temperature sintering. Park et al found that Dy 2 TiO 5 , Er 2 TiO 5, and Yb 2 TiO 5 with cubic symmetry took on the orthorhombic Pnma form at 21 GPa 23 . Another significant difference between the pressure and the temperature induced phase transition is the irreversible transition found when using pressure.…”