Crystal growth and structure of a high temperature polymorph of Sr2TiO4 with tetrahedral Ti-coordination, and transition to the Ruddlesden–Popper tetragonal phase

Abstract

Single crystals of transition metal oxides forming the Ruddlesden–Popper series are necessary for complete studies of their often exciting physical properties. In the strontium titanate family, crystal growth of all compounds apart from SrTiO₃ has been elusive so far. We have successfully grown crystals of the high-temperature polymorph of Sr₂TiO₄, by using a floating-zone melt growth followed by a rapid cooling procedure. We report the crystal structure of the new modification, which is isostructural to the orthorhombic Sr₂VO₄ and Sr₂CrO₄. This structure hosts an uncommon layered sub-lattice of TiO₄ tetrahedra and transforms into the tetragonal low-temperature polymorph via a complex reconstructive transition. The transformation mechanism between the two phases was studied and explains the reasons for the unsuccessful growth of tetragonal Sr₂TiO₄. The orthorhombic Sr₂TiO₄ is an insulator, with a band gap of 3.9 eV. It has a large (≈40) dielectric constant, but despite its polar structure does not show any signatures of a ferroelectric order.