Lanthanide stannate pyrochlores (Ln₂Sn₂O₇; Ln  =  Nd, Gd, Er) at high pressure

Lanthanide stannate pyrochlores (Ln₂Sn₂O₇; Ln  =  Nd, Gd, and Er) were investigated in situ to 50 GPa in order to determine their structural response to compression and compare their response to that of lanthanide titanate, zirconate, and hafnate pyrochlores. The cation radius ratio of A³⁺/B⁴⁺ in pyrochlore oxides (A₂B₂O₇) is thought to be the dominant feature that influences their response on compression. The ionic radius of Sn⁴⁺ is intermediate to that of Ti⁴⁺, Zr⁴⁺, and Hf⁴⁺, but the 〈Sn-O〉 bond in stannate pyrochlore is more covalent than the 〈B-O〉 bonds in titanates, zirconate, and hafnates. In stannates, based on in situ Raman spectroscopy, pyrochlore cation and anion sublattices begin to disorder with the onset of compression, first measured at 0.3 GPa. The extent of sublattice disorder versus pressure is greater in stannates with a smaller Ln³⁺ cation. Stannate pyrochlores (Fd-3m) begin a sluggish transformation to an orthorhombic, cotunnite-like structure at ~28 GPa; similar transitions have been observed in titanate, zirconate, and hafnate pyrochlores at varying pressures (18-40 GPa) with cation radius ratio. The extent of the phase transition versus pressure varies directly with the size of the Ln³⁺ cation. Post-decompression from ~50 GPa, Er₂Sn₂O₇ and Gd₂Sn₂O₇ adopt a pyrochlore structure, rather than the multi-scale defect-fluorite  +  weberite-type structure adopted by Nd₂Sn₂O₇ that is characteristic of titanate, zirconate, and hafnate pyrochlores under similar conditions. Like pyrochlore titanates, zirconates, and hafnates, the bulk modulus, B ₀, of stannates varies linearly and inversely with cation radius ratio from 1 1 1 GPa (Nd₂Sn₂O₇) to 251 GPa (Er₂Sn₂O₇). The trends of bulk moduli in stannates in this study are in excellent agreement with previous experimental studies on stannates and suggest that the size of the Ln³⁺ cation is the primary determining factor of B ₀. Additionally, when normalized to r _A/r _B, the bulk moduli of stannates are comparable to those of zirconates and hafnates, which vary from titanates. Our results suggest that the cation radius ratio strongly influences the bulk moduli of stannates, as well as their overall compression response.