How to separate two Ts in a pod: Classifying T- and T′-type Ruddlesden-Popper cuprates by machine learning

Abstract

The first-order Ruddlesden-Popper (RP) phases A₂BX₄ adopt three structure types that differ in coordination geometry around the B site: T-type (octahedral) and T′-type (square planar), which are most common, and T∗-type (square pyramidal), which is rare. Especially for RP cuprates A₂CuO_4–δ, it is not intuitively obvious which structure is preferred depending on the combination of cations occupying the A site. Machine learning models were developed that can separate the T- and T′-type structures among these cuprates with an accuracy of >90 %, provided that the T∗-type does not form and the phases can be synthesized. Based on these models, structures were predicted for solid solutions (A′, A″, A‴)₂CuO_4–δ containing a complex mixture of A cations (A′, A″, A‴ = Sr, La, Gd, Ho, In, Bi). The predictions were tested by targeting various members of these solid solutions through high-temperature reactions followed by slow cooling. Three samples contained pure RP phases which were confirmed to adopt the predicted structures: T-type for Sr_0.4La_1.5Ho_0.1CuO_3.8, and T′-type for Gd_1.7Ho_0.2Bi_0.1CuO₄ and La_0.4Gd_1.2Ho_0.4CuO₄. Five other samples were mixtures that contained RP phases whose structures (when not T∗-type) were correctly identified by a slightly better performing model based on extra randomized trees classifier.