Effect of Alkali and Trivalent Metal Ions on the High-Pressure Phase Transition of [C₂H₅NH₃]M^I_0.5M^III_0.5(HCOO)₃ (M^I = Na, K and M^III = Cr, Al) Heterometallic Perovskites

Abstract

We report the high-pressure behavior of two perovskite-like metal formate frameworks with the ethylammonium cation (EtAKCr and EtANaAl) and compare them to previously reported data for EtANaCr. High-pressure single-crystal X-ray diffraction and Raman data for EtAKCr show the occurrence of two high-pressure phase transitions observed at 0.75(16) and 2.4(2) GPa. The first phase transition involves strong compression and distortion of the KO 6 subnetwork followed by rearrangement of the −CH 2 CH 3 groups from the ethylammonium cations, while the second involves octahedral tilting to further reduce pore volume, accompanied by further configurational changes of the alkyl chains. Both transitions retain the ambient P 2 1 / n symmetry. We also correlate and discuss the influence of structural properties (distortion parameters, bulk modulus, tolerance factors, and compressibility) and parameters calculated by using density functional theory (vibrational entropy, site-projected phonon density of states, and hydrogen bonding energy) on the occurrence and properties of structural phase transitions observed in this class of metal formates.

Influence of Metal Defects on the Mechanical Properties of ABX3 Perovskite-Type Metal-formate Frameworks

“…Compared to other formate-based perovskites, this value is slightly lower than for dimethylammonium metal formates ( B 0 = 21.3 GPa for M = Mn II and 26.3 GPa for M = Co II ), ,, but similar to that of [NH 2 NH 3 ]­Zn­(HCOO) 3 with B 0 = 19 GPa . The defective GuaFe 2/3 □ 1/3 features a lower bulk modulus of 14.3(2) GPa, which is comparable to the behavior of the heterometallic [C 2 H 5 NH 3 ]­K 0.5 Al 0.5 (HCOO) 3 . The difference in bulk moduli of GuaMn and defective GuaFe 2/3 □ 1/3 highlights how defects increase the mechanical compliance of the framework.…”

Memory Effect on the Synthesis of Perovskite-Type Li-Ion Conductor LixLa2/3–x/3TiO3 (LLTO)

“…In this structure, the entropic contributions from different configurations can arise from the Li + located on the 1b site along with the La 3+ ions. The configurational entropy calculated for the Pm-3m structure is relatively low among the various polymorphs, at 15.80 μeV atom −1 K −1 for the cubic Pm-3m structure as reported by Sotomayor et al, 25 or at 22.03 μeV atom −1 K −1 in the structure reported by Jimenez et al 31 This means that the entropic contributions stabilizing Pm-3m require other forms of entropy beyond configurational, such as vibrational entropy that has been shown to affect the onset of temperature-or pressuredependent phase transitions, 51 or configurational entropy of tilting. Although we do not calculate these other entropic contributions here, based on the experimental observation of the reversible phase transformation to Pm-3m above 1300 °C, we can anticipate that this is the equilibrium phase at high temperatures.…”