Third time's the charm: intricate non-centrosymmetric polymorphism in LnSiP3 (Ln = La and Ce) induced by distortions of phosphorus square layers

Abstract

Complex polymorphic relationships in the LnSiP₃ (Ln = La and Ce) family of compounds are reported. An innovative synthetic method was developed to overcome differences in the reactivities of the rare-earth metal and refractory silicon with phosphorus. Reactions of atomically mixed Ln + Si with P allowed for selective control over the reaction outcomes resulting in targeted isolation of three new polymorphs of LaSiP₃ and two polymorphs of CeSiP₃. In situ X-ray diffraction studies revealed that the developed method bypasses formation of the thermodynamic dead-end, the binary SiP. Careful re-determination of the crystal structure ruled out the previously reported ordered centrosymmetric structure of CeSiP₃ and showed that the main LnSiP₃ polymorphs crystallize in the non-centrosymmetric Pna2₁ and Aea2 space groups featuring distinct distortions of the regular P square net to yield either cis–trans 1D phosphorus chains (Pna2₁) or disordered-2D phosphorus layers (Aea2). The disordered 2D nature of the P layers in the Aea2 LaSiP₃ polymorph was confirmed by Raman spectroscopy. A unique centrosymmetric P2₁/c polymorph was observed for LaSiP₃ and has a completely different crystal structure lacking P layers. Consecutive polymorphic transformations at increasing temperatures for LaSiP₃(Pna2₁ → P2₁/c → Aea2) were derived from optimized synthetic profiles and confirmed by a combination of phonon computations and experimental in situ and ex situ annealings. Crystal structures of the LaSiP₃ polymorphs were verified via advanced solid state NMR analysis using ³¹P MAS and ³¹P{¹³⁹La} double resonance techniques. A combination of phonon and electronic structure calculations, NMR T₁ relaxation times, UV/Vis/NIR spectroscopy, and resistivity measurements revealed that all the reported polymorphs are semiconductors with resistivities and thermal conductivities strongly dependent on the degree of distortion of P square layers in the crystal structure. Reported here, non-centrosymmetric LnSiP₃ polymorphs with tunable resistivity and thermal conductivity provide a platform for the development of novel functional materials with a wide range of applications.